Design and Construction of Carbon NanoTubes (CNTs) Triggered 
Pseudospark Switch 
 
by 
 
Haitao Zhao 
 
 
 
 
A dissertation submitted to the Graduate Faculty of 
Auburn University 
in partial fulfillment of the 
requirements for the Degree of 
Doctor of Philosophy 
 
Auburn, Alabama 
August 4, 2012 
 
 
 
 
Keywords: Pseudospark switch, carbon nanotubes, hollow cathode effect, 
 field emission, pulsed power  
 
 
Copyright 2012 by Haitao Zhao 
 
Approved by 
 
Hulya Kirkici, Chair, Professor of Electrical and Computer Engineering 
Bogdan Wilamowski, Professor of Electrical and Computer Engineering 
 Stuart Wentworth, Associate Professor of Electrical and Computer Engineering  
 
 
 
 
 
 
ii 
 
Abstract 
 
 
Pseudospark switches have been used as fast closing switches in pulsed power 
systems. Advantages include high hold off voltage of 10s kV, high conduction current of 
~104 A, fast current rise time of 1011 A/s, low delay and jitter time approaching ns, and 
long lifetime. In general, pseudospark switches have hollow anode and hollow cathode 
geometry and can be triggered by several methods that increase the charge carrier density 
in the hollow cathode region. The efficiency of these charge carrier accumulation 
determines the performance of switch operation. 
In the electron injection triggering where electrons emitted from cold cathode 
emitter, are accelerated in the electric field and initiate the switch closing. Therefore, cold 
cathode material is critical to this trigger method. Carbon nanotubes (CNTs) are known 
for their excellent field emission characteristics. This property makes them a good 
candidate for the cold cathode material in this application.  
In this research work, pseudospark switch triggered by carbon nanotubes coated 
electrode is designed and constructed. The trigger electrode is fabricated by coating 
CNTs, either randomly aligned CNTs, vertically aligned CNTs or trench wall patterned 
CNTS on silicon in chemical vapor deposition method (CVD). The CNTs are synthesized 
under different growth conditions with varying sputtering time, oxidation time, and 
growth time. The patterned CNTs are fabricated using monolithic IC fabrication 
processes. The trigger electrode is then assembled and tested in a pseudospark switch 
ii 
 
constructed in-house with a quartz vacuum tube, a hollow cathode and a planar anode. 
The switch test experiments are carried out in different background pressures, different 
trigger voltages, and trigger electrodes with different types of CNTs.  Delay and jitter 
time of the pseudospark switch under different working conditions are studied. Hold off 
voltage, conduction current and current rise time are presented, and optimizations of the 
switch operating characteristics are discussed.  
ii 
 
Acknowledgments 
 
 
     The author would like to express his special thanks to his advisor Dr. Hulya 
Kirkici for her commitment to this project, technical advice, and support through the 
duration of this study. It would have been impossible to complete this work without her 
support. He also appreciates her support not only in academic work but also in his life. 
She is more than an advisor and like a mother in life. 
The author wishes to thank Dr. Bogdan Wilamowski, Dr. Stuart Wentworth for 
serving on his committee and Dr. Park for being the outside reader. Special thanks are 
extended to technical staff Linda Barresi and Calvin Cutshaw for the material support 
throughout the experiment. 
The author wishes to thank Dr. Yu-chun (Brad) Chen for valuable training and 
help in his research and to thank the fellow lab mates Shaomao Li, Mark Lipham, 
Ramesh Bokka, Fang Li, Zhenhong Li, Huirong Li, Rogger Tsai, Ming Zhang, and Rujun 
Bai, and to thank Department of Electrical and Computer Engineering for providing 
support and a friendly environment. 
     Special acknowledgement and heartfelt gratitude must be given to author?s 
parents, wife, and son for their love, support, and encourage. 
  
 
 
? Work supported by a grant from AFOSR with a contract # AF-FA9550-08-10051 
 iii 
Style manual or journal used Graduate School: Guide to preparation and submission of 
thesis and dissertations 
 Computer software used Microsoft Office 2007 
 iv
Table of Contents 
 
 
Abstract ............................................................................................................................... ii 
Acknowledgments.............................................................................................................. iv 
List of Tables ...................................................................................................................... x  
List of Figures .................................................................................................................... xi 
Chapter 1 Introduction  ........................................................................................................1 
Chapter 2 Background .........................................................................................................4 
2.1 Pulsed Power Engineering  ............................................................................4 
2.2 Pseudospark Switches  ..................................................................................7 
2.2.1 Gaseous breakdown and glow discharges  .........................................10 
2.2.2 Hollow cathode discharges  ................................................................11 
2.2.3 The hollow cathode effect  .................................................................11 
2.2.4 Triggering mechanisms  .....................................................................13 
2.2.4.1 Surface discharge  .....................................................................13 
2.2.4.2 Pulsed low-current glow discharge  ..........................................14 
2.2.4.3 Optical triggering  .....................................................................15 
2.2.4.4 Electron injection  .....................................................................16 
2.3 Limitations of Pseudospark Switches  .........................................................16 
2.3.1 Life time limit  ....................................................................................16 
           2.3.2 Hold off voltage limit ........................................................................17
ii 
 
           2.3.3 Conduction current limit  ...................................................................18 
2.3.4 Delay and jitter time limit  .................................................................18 
Chapter 3 Carbon Nanotubes  ............................................................................................20 
3.1 Introduction  ................................................................................................21 
3.2 Structure of CNTs .......................................................................................21 
3.2.1 Single wall carbon nanotubes (SWCNTs)  .........................................21 
3.2.2 Multi wall carbon nanotubes (MWCNTs) ..........................................23 
3.2.3 Random and aligned oriented CNTs   ................................................25 
3.3 Properties of CNTs  .....................................................................................25 
3.3.1 Electrical property  .............................................................................26 
3.3.2 Chemical property  .............................................................................26 
3.3.3 Mechanical property  ..........................................................................28 
3.4 Synthesis of CNTs .......................................................................................29 
3.4.1 Growth mechanism  ............................................................................29 
3.4.2 Synthesis methods  .............................................................................33 
3.4.2.1 Arc-discharge method  ..............................................................33 
3.4.2.2 Laser ablation method  ..............................................................35 
3.4.2.3 Chemical vapor deposition (CVD) method  ..............................36 
3.4.2.4 Plasma enhanced chemical vapor deposition (PEVCD)  
method  ......................................................................................37 
3.5 Characterization Methods ............................................................................39 
Chapter 4 Carbon Nanotubes Fabrication and Optimization .............................................41 
4.1 Synthesis of Randomly Oriented CNTs   ....................................................42 
4.1.1 Silicon wafer preparation  ..................................................................42 
 iii 
4.1.2 Catalyst sputtering deposition  ...........................................................43 
4.1.3 Chemical vapor deposition  ................................................................45 
4.2 Optimization of CNT Synthesis Parameters ...............................................48 
4.3 Aligned CNT Synthesis  ..............................................................................52 
4.4 Patterned CNT Synthesis  ............................................................................57 
    4.5 Electrical Properties of CNTs  .....................................................................61 
4.5.1 Electron field emission  ......................................................................61 
4.5.2 Turn-on field and life time measurements  ........................................65 
Chapter 5 Triggered Pseudospark Switch Construction and Test  ....................................71 
5.1 Switch Body Construction ...........................................................................71 
5.2 Test Experimental Setup  ............................................................................73 
5.3 Switch Hold-off Voltage Measurements  ....................................................75 
5.4 Switch Operation-Trigger Mode  ................................................................76 
5.4.1 Delay time measurements and analysis  .............................................80 
5.4.2 Conduction current and current rise time  ..........................................83 
5.4.3 Jitter time calculation  ........................................................................85 
5.5 Switch Operation under Different CNTs as Trigger Electrode  ..................86 
Chapter 6 Summary and Conclusion  ................................................................................91 
References  .........................................................................................................................95
ii 
 
List of Tables 
 
 
Table 3.1 Turn-on field of typical electron emitter materials ............................................20 
Table 3.2 Young?s modulus, tensile strength, and density of carbon nanotubes  
compared with other materials ..........................................................................28 
Table 4.1 Turn-on field of the tested CNT samples...........................................................69 
Table 5.1 SEM images of tested samples ..........................................................................77 
Table 5.2 The jitter variation  ............................................................................................86 
Table 5.3 Random and aligned samples  ...........................................................................90 
 iii 
List of Figures 
 
 
Figure 2.1 General scheme of a pulsed power system  ........................................................5 
Figure 2.2 A typical pulse shape  .........................................................................................5 
Figure 2.3 Typical operation regions of gaseous switch in relation to Paschen curve of 
breakdown  .........................................................................................................7 
 
Figure 2.4 Schematic of pseudospark switch .......................................................................8 
Figure 2.5 Phases of pseudospark discharge .....................................................................10 
Figure 2.6 Movements of pendulum electrons ..................................................................12 
Figure 2.7 Formation of the cathode fall  ..........................................................................13 
Figure 2.8 (a) Schematic drawing of the pseudospark switch triggered by surface    
 discharge (b) enlarge view of the trigger electrode ..........................................14 
Figure 2.9 Pulsed low-current glow discharge triggered pseudospark switch ..................15 
Figure 2.10 Back lighted thyratron ....................................................................................16 
Figure 2.11 Paschen curve   ...............................................................................................18 
Figure 3.1 The formation of a SWCNT .............................................................................22 
Figure 3.2 Classification of CNTs: (a) armchair (b) zigzag (c) chiral nanotubes ..............23 
Figure 3.3 Structure of the Multi wall carbon nanotubes (MWCNTs) ..............................24 
Figure 3.4 TEM pictures of MWCNTs ..............................................................................24 
Figure 3.5 (a) aligned CNTs and (b) random CNTs ..........................................................25 
Figure 3.6 CNT growth stages (a) root growth (b) tip growth...........................................30 
Figure 3.7 Schematic representation of two types of CNT growth mechanism ................31
ii 
 
Figure 3.8 Schematic of CNTs growth mechanisms (a) saturation of catalytic     
                    nanoparticle with carbon (b) carbon precipitation process (c) tube  
                    growth process  ..............................................................................................33 
Figure 3.9 Schematic diagram of the Arc apparatus  .........................................................34 
Figure 3.10 Schematic of laser ablation method ................................................................35 
Figure 3.11 Schematic of CVD apparatus .........................................................................37 
Figure 3.12 Schematic of PECVD .....................................................................................38 
Figure 4.1 Schematic of ion sputtering system ..................................................................44 
Figure 4.2 Fe sputtered silicon substrate  ...........................................................................45 
Figure 4.3 Schematic of CVD reactor................................................................................46 
Figure 4.4 SEM pictures of CNTs  ....................................................................................47 
Figure 4.5 CNTs with different C2H2 concentration  .........................................................49 
Figure 4.6 CNTs with different operation temperature  ....................................................50 
Figure 4.7 CNTs with different catalyst thickness.............................................................51 
Figure 4.8 An optimized CNT sample  ..............................................................................52 
Figure 4.9 Aligned CNT samples (a) 1min carbon pretreatment (b) 5min carbon 
pretreatment (c) 20min carbon pretreatment ....................................................54 
Figure 4.10 Mechanism of aligned CNT synthesis   ..........................................................56 
Figure 4.11 CNT samples with different gap distance  .....................................................56 
Figure 4.12 Dielectric strength test of SiO2 (x axis is the applied voltage and the 
                   y axis is the emitting current) ..........................................................................57 
Figure 4.13 Masks of photolithography, mask1 has 0.2 inch square holes and  
                   mask2 has 0.15 inch square holes ...................................................................58 
Figure 4.14 Patterned substrate  .........................................................................................58 
Figure 4.15 Flow chart of the operation process ...............................................................59 
Figure 4.16 Cross sectional view of one pattern hole  .......................................................60 
 iii 
Figure 4.17 Profile of patterned hold edge ........................................................................60 
Figure 4.18 Patterned CNTs (a) top view of the patterned CNTs (b) enlarge view  
                   of the CNTs inside the pattern ........................................................................61 
Figure 4.19 Schematic of the field emission test system   .................................................62 
Figure 4.20 Interface of the test program ...........................................................................64 
Figure 4.21 Field emission characteristics of random CNTs and aligned CNTs samples  
 with 40 min growth time   ................................................................................65 
Figure 4.22 SEM images of CNTs samples under 30k magnifications  ............................67 
Figure 4.23 Field emission characteristics of CNTs samples ............................................68 
Figure 4.24 Turn on field vs. Growth time ........................................................................69 
Figure 4.25 CNTs life time test..........................................................................................70 
Figure 5.1 Sketch of pseudospark switch   ........................................................................72 
Figure 5.2 Sample holder as part of the trigger electrode  .................................................72 
Figure 5.3 Assembled PSS Switch Chamber   ...................................................................73 
Figure 5.4 Schematic of the test experimental setup with diagnostics  .............................74 
Figure 5.5 Hollow cathode breakdown event  ...................................................................75 
Figure 5.6 Self breakdown voltage of non-triggered switch as a function of pressure .....76 
Figure 5.7 Trigger pulse shape  .........................................................................................78 
Figure 5.8 Triggered breakdown event   ............................................................................79 
Figure 5.9 Triggered breakdown waveforms  ....................................................................79 
Figure 5.10 SEM images of CNTs used as the trigger sample   ........................................80 
Figure 5.11 Delay time   ....................................................................................................82 
Figure 5.12 Delay time variation  ......................................................................................82 
Figure 5.13 Field emission characteristics of random-40 CNT sample   ...........................83 
 iv
Figure 5.14 Screen shot of current waveform at switch closing ........................................84 
Figure 5.15 Current raise time variation as function of trigger pulse voltage ...................85 
Figure 5.16 Statistics of delay time  ..................................................................................86 
Figure 5.17 Delay time of different CNT samples ............................................................87 
Figure 5.18 Field emission characteristics of random CNT samples ................................88 
Figure 5.19 Field emission results of random and aligned CNTs in partial vacuum  .......89 
Figure 5.20 Delay time for random and aligned CNTs with 40 min growth time   ...........90 
 
 
 
 
1 
 
Chapter 1  
Introduction  
 
 
In the modern pulsed power field, the power switch technology is challenged by 
new applications, such as electron/ion beam generators which require a high current 
density of 104 A/cm2, pulsed modulators which generate a peak current up to 19 kA, and 
excimer-lasers which reach a maximum voltage of 32 kV [1].  The capability of high 
power up to 1012 W and low jitter time of 10-9 s is identified as a significant parameter in 
a switch design and fabrication for pulsed power applications. The conventional switches, 
for example high-voltage switch gears, are no longer sufficient to meet these stringent 
requirements. Other switch theories, such as the gaseous spark discharge and hollow 
cathode effect, as well as the hardware innovation are some of the attracting research 
topics.  
The pseudospark switch, also known as cold-cathode thyratron, is one of the most 
studied devices to control high voltage and high current pulse discharges. It is 
characterized by a very rapid breakdown phase with high hold off voltages of several tens 
of kV and peak currents of 10s kA with pulse repetition of up to several kHz. These are 
attributed to the ?hollow cathode effect? that allow glow discharge to operate at relatively 
lower voltage and higher current than traditional glow discharges. The discharge can be 
2 
 
operated at 100% current reversal without turning into a constricted arc [2]. In a complete 
pseudospark switch, the trigger mechanism that supplies the seed electrons for a 
breakdown is critical because it greatly affects the switch performance. Several trigger 
mechanisms have been reported in the literature [3, 4]. The first one is the surface 
discharge triggering that employs a dielectric material as part of the trigger. But the 
lifetime of the trigger section is rather short because of the surface erosion of the 
dielectric material. The second trigger method is pulsed low current glow discharge 
triggering. The disadvantage is that a fairly high pulsed voltage is needed to generate the 
glow for trigger. The third method is optical triggering, where stringent requirements on 
the light source (laser beam or UV flashlight) to generate photoelectrons for trigger, 
makes them remain in advanced research stage. Another problem of optical triggering is 
the difficulty in large scale development where another setup for the laser or UV-flash 
light system is needed. The last method is electron beam injection, or field emission 
triggering. One of the disadvantages of this method is that a third triggering electrode is 
needed, which greatly increased the hardware complexity. And also the performance of 
the switch is highly dependent on the emitter materials. Traditionally, a metal electrode is 
used for this but high turn-on electric field of these materials, to some extent, has limited 
the development of the pseudospark switch for compact pulsed power applications. 
Therefore, it is important to study and achieve innovations on both switch body design 
and emitter materials. 
The primary objective of this work is to design and construct a pseudospark 
switch that is easy-to-construct, compact, and has the potential of use with high hold off 
voltage and low delay time with nanosecond jitter. Instead of a traditional metal electrode, 
3 
 
CNT coated cold electron emitters are considered. The switch is triggered by electron 
injection mechanism in which the CNTs coated silicon electrode is used as the trigger 
electrode. The performance of the switch is tested under different operating conditions 
and the switch optimization parameters are reported. 
      
 4
Chapter 2  
Background 
 
2.1 Pulsed Power Engineering 
Pulsed power engineering is the science and technology of storing energy over a 
relatively long period of time (usually seconds or minutes) and releasing it to a load as 
electrical energy over a much shorter time (usually microseconds or nanoseconds). 
Normally, a pulsed power system consists of an energy storage stage, a pulse forming 
stage, and a load as shown in Figure 2.1. Energy can be stored in several forms: 
chemically, mechanically or electrically. By using appropriate switches, closing switch or 
opening switch, the high energy is transferred from energy storage stage to the load 
through pulse forming stage. Depending on the energy storage and load requirements, 
switches can be closing switch (capacitive) or opening switch (inductive). Highest energy 
and power that can be achieved in a single pulse are in the order of 100 MJ and 10 TW 
respectively [5]. The corresponding voltage is from 104 ? 107 V, and the current is in the 
range of 103 to 107 A. Figure 2.1 shows a sketch of a typical pulse forming network 
representing a complete pulse power system, and Figure 2.2 shows a typical pulse shape 
across a load. 
 
 5
 
Figure 2.1 General scheme of a pulsed power system 
 
Figure 2.2 A typical pulse shape [5] 
 
The important parameters of a pulsed power switch that delivers a pulse are listed 
and defined as follow: 
Hold off Voltage: The maximum static voltage that can be applied to the switch before 
breakdown is triggered. 
Rise Time: The time it takes for the voltage to rise from 10% to 90% of its peak voltage 
 6
Voltage Fall Time: After breakdown is initiated, the time interval during which the 
voltage drops from the value of hold off to that of the conduction voltage. 
Pulse During: The full width between rise and decay times at half maximum of the pulse 
amplitude (FWHM). 
Recover Time: The time interval during which the voltage reverses its polarity. 
Repetition Rate: The rate at which the switch can be closed and opened without 
degradation of performance. 
Conduction Drop: The voltage drop across the switch impedance during the conduction. 
Peak Power: The maximum value of the product of voltage and current pulses which 
occur at the same time across the load. 
Transfer Energy: The time integral of the product of voltage and current pulses. 
Life Time: Under normal operating conditions, the total number of switching operations 
beyond which the switch can no longer function properly. 
Reliability: A measure of the ratio between the numbers of success of operations to the 
total number of operations within the life time of the switch. 
Delay time: The time interval between the trigger voltage and the switch breakdown. 
Jitter time: The statistic standard deviation of delay times over successive 
closing/opening. 
Requirements for the pulse length, average and peak energies, and the repetition 
rate are highly application specific. However, compactness, and portability are common 
driving forces in today?s pulsed power systems [6]. Delay time and jitter are the most 
important parameters in terms of how efficiently the energy is transferred to the load. 
 7
2.2 Pseudospark Switches 
Switches are important part of pulsed power system and stringent requirements 
imposed on them depending on applications. Based on the pressure, switches are 
classified into high pressure switch, such as spark gap, partial vacuum switch, such as 
thyratron and ignitron, and vacuum switch, such as vacuum spark gap, as shown in 
Figure 2.3.  
  
    Figure 2.3  Typical operation regions of gaseous switch in  relation to  
Paschen curve of breakdown  [5] 
 
Pseudospark switches, also known as cold cathode thyratron, are one type of 
partial vacuum switches with significant application prospect. It was first developed by 
Christiansen and Schultheib in 1978 as low-pressure, high-voltage plasma devices [7]. 
Great progress has been made in this area since then, including novel fabrication 
 8
techniques and advanced applications. Nowadays, pseudospark switch has been proven to 
be one of the most efficient closing switches in pulsed power system. It is capable of high 
power, high repetition rate and fast recovery time. The pseudospark switch has a special 
hollow cathode and hollow anode geometry. Typical operation pressure is around 1 to 103 
mTorr. The geometry of pseudospark switch is very simple and suffers less electrode 
wear compared to other high pressure switches such as sparkgaps. Figure 2.4 is a basic 
schematic of the pseudospark switch. 
 
Figure 2.4 Schematic of pseudospark switch [1] 
 
There are four phases in the pseudospark discharge: predischarge, hollow cathode 
discharge, super glow discharge, and decay of the discharge plasma [2]. The features for 
each phase are listed below and the first three phases are shown in Figure 2.5. 
 9
Predischarge (ignition of the pseudospark):  
? Electrons are released from walls of cathode hole 
? High avalanche occurs in cathode backspace (high ?/p) 
? Dense plasma forms in cathode hole and backspace 
Hollow cathode discharge phase: 
? Electrons from cathode backspace are extracted into electrode gap by virtual 
anode, and accelerated with almost no collisions 
? Plasma goes into and shortens the electrode gap (2?106 m/s), then breakdown 
occurs. 
Superdense glow discharge phase: 
? Electron emission from backspace is limited 
? Ions are accelerated by cathode fall (106 V/cm), collide with  gas particle, 
bombard cathode surface, and induce field-enhanced thermionic electron emission 
(3000-4000 K in 30-100 ns) 
Decay of the discharge plasma phase: 
? Determined by external circuit 
? No quenching phenomenon during current carrying phase 
? Regain hold-off capability during current zero 
 10
 
Figure 2.5 Phases of pseudospark discharge [2] 
 
2.2.1 Gaseous breakdown and glow discharges  
The glow discharge is a luminous low pressure discharge with applied voltage at 
100 V to several kV through a gas. The simplest type of glow discharge is a direct-
current glow discharge which consists of two electrodes in a cell held at low pressure 
about 0.1-10 Torr.  If the applied voltage is increased to breakdown voltage, the gas will 
be ionized and a breakdown of the gas is observed, reaching the glow discharge regime. 
The glow discharge is a diffuse, steady discharge but if the applied voltage continues to 
increase, abnormal glow discharge begins. As the voltage is further increased to the point 
that the cathode glow covers the entire cathode, the current increases sharply and arc 
discharge begins. 
The breakdown voltage of glow discharge is determined by the Paschen Law. It is 
a well-known plasma-physics law stating that the amount of potential difference required 
for a gas to breakdown is a function of pressure and gap distance. In the pseudospark 
discharge, it starts in the hollow cathode behind the borehole and expands to electrode 
gap. 
 11
2.2.2. Hollow cathode discharges  
If a single plane cathode in glow discharge is replaced by a cathode with hollow 
structure, the current will be orders of magnitude larger at the same discharge voltage. 
The negative glow region of the discharge moves into the hollow structure resulting in 
higher ionization and excitation processes.  This is called the hollow cathode effect and 
the hollow cathode structure is an important feature of the pseudospark switch [8]. The 
operation region of the hollow cathode discharge is 1 torr-cm < pD < 10 torr-cm, where p 
is pressure and D is a combination of electrode gap distance, hollow cathode diameter, 
and depth of the cathode cavity [9]. The pd in the Paschen breakdown curves determines 
that the switch operates at the ?left side of the Paschen Curve? shown in Figure 2.3, 
whereas the PD determines the hollow cathode discharge development. 
2.2.3 The hollow cathode effect 
In a traditional high current pulsed power switch, an externally heated cathode 
must be used [2, 10]. The cathode needs to be preheated before the switch operation, in 
order to get sufficient electron emission. Thus external circuit unavoidably limits the 
reduction of the switch size where compact switches are needed. This led to the 
development of the pseudospark switches employing hollow cathode geometry. Due to 
the hollow cathode effect, the electron emission has been characterized as a super-
emissive mode in which a quasi-uniform thermionic or field-enhanced thermionic 
emission dominates [11, 12]. Therefore, the sufficient electron emission is assured and 
the external circuit to heat the cathode can be removed. 
The hollow cathode effect is mainly caused by pendulum electrons [13]. These 
special electrons emitted from the cathode surface float in the cathode cavity, reflecting 
 12
back and forth between the cathode sheaths, as shown in Figure 2.6. The movements of 
the pendulum electrons result in increased rate of ionizing collisions. The low weight 
electrons move fast towards the anode and the heavier ions stay in the cathode cavity, 
forming a virtual anode. Therefore, the electron emission rate increases due to the virtual 
anode. The cathode fall, defined as the distance between the virtual anode and the 
cathode sheaths, can be significantly thin in hollow cathode geometries leading to a 
extremely high electric field. Thus, the ion velocities and secondary electron emissions 
increase. Moreover, the newly emitted electrons further ionize the medium gas, 
generating more ions. The cathode fall becomes even thinner because of the expansion of 
the virtual cathode. In this way, the hollow cathode structure increases the probability of 
multiple processes and collisions for all particles, creating high current density plasma 
inside the hollow cathode cavity. Figure 2.7 indicates the formation of the cathode fall. 
 
Figure 2.6 Movements of pendulum elecrons 
 13
 
Figure 2.7 Formation of the cathode fall 
2.2.4 Triggering mechanisms 
Triggering system is the most important region of the pseudospark switch. At 
least 109 electrons are needed to initiate a low timing jitter breakdown [11]. Delay and 
jitter time are directly determined by trigger mechanisms. Four mechanisms, namely, 
surface discharge, low current glow discharge, optical triggering, and electron injection, 
are extensively studied in the literature and widely used in applications [1, 2, 10]. 
2.2.4.1 Surface discharge [10, 14, 15] 
In the surface discharge triggering method, a trigger electrode located between 
two insulator discs, is inserted into the hollow cathode region of the main switch, as 
shown in Figure 2.8. A high voltage pulse is then applied to the trigger electrode for 
surface flashover discharge. This method is based on generating seed electron during the 
flashover across the insulator surface. Therefore, insulators are required to have low 
surface breakdown voltages and long lifetimes. High peak current can be provided in this 
method but the repeatability is low due to the limited life time of the insulator materials. 
 14
 
                                                                    a 
                               
                                                                    b 
Figure 2.8 (a) Schematic drawing of pseudospark switch triggered by surface discharge 
(b) enlarge view of the trigger electrode [16] 
 
2.2.4.2 Pulsed low-current glow discharge 
In this method, the trigger module and main switch are separated by a third stage, 
normally a cylindrical cage forming another hollow cathode, as shown in Figure 2.9.  A 
negative DC voltage is applied to the trigger electrode to generate a pulsed hollow 
cathode discharge inside the trigger module. The electrons generated here are driven to 
the main cathode by the help of auxiliary electrodes, and the breakdwon is initiated in the 
main switch by these electrons propogating through the small holes on the cage [5]. 
 15
 
Figure 2.9 Pulsed low-current glow discharge triggered pseudospark switch [17] 
 
2.2.4.3 Optical triggering 
Pseudospark switch can also be triggered optically. A beam of light, either from a 
flash lamp, a radiation from a spark discharge, or a UV laser, such as XeCl at 308 nm and 
KrCl, impinges onto the cathode back space through a glass window and initiate the 
switch breakdown.  This type of pseudospark switch is also called back lighted thyratron 
(BLT) and the structure can be super compact because the trigger region has been 
completely separated from the switch body. The operating characteristics of a BLT with a 
minimum dimension of approximately 20 mm with capability of 4 kA conduction current 
and 20 kV hold off voltage have been reported in [18] ( see Figure 2.10 for a schematic 
of a BLT). 
 
 16
 
Figure 2.10 Back Lighted Thyratron [19] 
 
2.2.4.4 Electron injection 
Electron injection triggering operates very reliable with low jitter and delay time 
[20]. Electrons, normally from an electron beam or generated from a metal through field 
emission, enter the cathode cavity with relatively high energy, ionize the background gas 
by collisions, and initiate the switch breakdown. This method can be very efficient due to 
the fast velocity of electrons. But the electron source, for example the cold cathode 
electron emitter, is a key factor. It is necessary to use an emitter material with low turn-on 
field and high emission current. Electron beam system usually requires more complicated 
structure in peripheral circuitry. 
2.3 Limitations of Pseudospark Switches 
2.3.1 Life time limit 
Although the life time and repetition rate of the pseudospark switch are 
dramatically improved, they still suffers from the electrode erosion because pseudospark 
 17
switches operate in high current mode [21]. During the superemissive phase, the face of 
the cathode, traditionally made from copper, is heated due to the high power density of 
the electron beam (109-1012 W/cm3) which can evaporate the cathode material and cause 
rapid electrode degradation [22]. The erosion effect can be reduced by using other 
materials with higher melting point, such as molybdenum. The difficulties of the 
electrode machining and material cost are some of the limitations of these switches.  
2.3.2 Hold off voltage limit.  
The hold off voltage of pseudospark switch is the maximum voltage that the 
switch can withstand. As mentioned earlier, the pseudospark switches operate on the left 
side of Paschen curve, where ?gap-distance? is much less than the mean free path as 
shown in Figure 2.11. Experiments have shown that the hold off voltage for a single gap 
pseudospark switch cannot comfortably exceed 32 kV without impairing the switch 
lifetime [23]. Attempts to overcome this limited hold off voltage led to the development 
of multiple-gap switch structures by stacking up multiple single-gap switches [23, 24]. 
For a reliable hold off voltage of 70 kV, at least three gaps are necessary. But two 
problems limit these structures. First, the switch size cannot be made as compact with 
multi-gap structure; Second, simultaneous triggering issue is challenging and impairing 
the delay time. 
 18
 
Figure 2.11 Paschen curve [25] 
 
2.3.3 Conduction current limit 
Pseudospark switches are designed for high conduction current. But, if the current 
exceeds 45 kA, interactions between cathode spots are enhanced, leading to buildup of 
high-erosive cathode spots and evaporate the cathode material rapidly. The resulting high 
electrode erosion therefore limits the switch applications as either a low current or a 
short?life device [26]. Multichannel operation is a way to solve this problem and has 
been widely studied [27, 28]. Multi-channels are distributed around a common hollow 
cathode. By this way, conduction current up to several 100 kA can be obtained. The 
limitations in this method are the complexity of the cathode structure and the cost of the 
materials. 
2.3.4 Delay and jitter time limit 
Delay time and jitter are determined by trigger mechanisms mentioned above. 
Each trigger method has its own advantages and disadvantages. For example, in the 
electron injection method, 109 to 1010 electrons are need for a reliable triggering. A lower 
 19
number of electrons results in high delay and jitter values or cannot initiate the discharge 
at all [29]. Therefore, the efficiency of trigger mechanism is another important factor that 
imposes limitation. 
 20
Chapter 3  
Carbon Nanotubes 
 
In order to improve the performance of the pseudospark switch, a proper cold 
cathode emitter material must be used to provide the seed electrons needed to initiate the 
hollow cathode discharge. Materials with high electric field emission rate are desired for 
a low delay and jitter time in a switch operation. Many materials have been studied and 
the results are reported in terms of turn-on field and emission current [30 - 34]. The turn-
on field varies from 3 V/?m to 12 V/?m, and the emission current is around a few ?A. 
Some high performance materials, such as carbon nanotubes (CNTs), rough polished 
copper, polycrystalline diamond and ZnO, have been tested and the turn-on fields are 
listed in Table 3.1 [35]. The results clearly show that CNTs have the lowest turn-on field. 
This property of carbon nanotubes makes them a good candidate for cold cathode emitter 
materials. 
Table 3.1 Turn-on field of typical electron emitter materials 
Material  CNTs   Copper  Diamond  ZnO  
Turn-on field   0.9 V/?m  5 V/?m  2.2 V/?m  >10 V/?m  
Saturation current density 1 mA/cm2 0.1 mA/cm2 0.1 mA/cm2 1 mA/cm2 
 
 21
3.1 Introduction  
Carbon nanotubes are one of the pure forms of the carbon and have been of 
greatest interest since their discovery in 1991 by Iijima [36]. The name of the nanotube is 
derived from their size as the diameter of nanotube is in the order of nanometers and their 
lengths could be of several micrometers. CNTs are members of the fullerene structural 
family where the molecule is composed entirely of carbon in the forms of hollow spheres, 
ellipsoids, or tubes. CNTs have unique nano-structures with remarkable electrical, 
mechanical and chemical properties. They are one of the most diverse nano-scale 
materials leading to implementation of practical and commercial products in many 
aspects. For example, the high performance field effect transistors [37-39], single-
electron transistors [40, 41], atomic force microscope tips [42], field emitters [43], and 
chemical/biochemical sensors [44] 
3.2 Structure of CNTs 
CNTs can be classified into many types: single wall carbon nanotubes (SWCNTs), 
multiwall carbon nanotubes (MWCNTs), randomly oriented CNTs and vertically aligned 
CNTs. Structures and properties vary from one type to another. 
3.2.1 Single wall carbon nanotubes (SWCNTs) 
A single wall carbon nanotube, nearly a one-dimension structure, is formed by 
rolling up a graphene sheet, which is assumed to be a single layer from a 3D graphite 
crystal, into a cylinder capping each end of the cylinder with half of fullerene molecule, 
as illustrated in Figure 3.1.  
 22
 
Figure 3.1 The formation of a SWCNT [45] 
 
The structure of the SWCNTs is differentiated into two regions with different 
physical and chemical properties. The first one is the sidewall and the second one is the 
end cap of the tube. The symmetry and the electronic structures of the nanotubes change 
with directions, diameter and rolling of the graphene sheet. Figure 3.2 shows three 
different types of the SWCNTs structure.  
 23
 
Figure 3.2 Classification of CNTs: (a) armchair (b) zigzag (c) chiral nanotubes [46] 
 
The definition of each type can be explained from Figure 3.3. (n, m) is the chiral 
vector where integers n and m denote the number of unit vectors along the directions in 
the honeycomb crystal lattice of the grapheme [47].  
3.2.2 Multi wall carbon nanotubes (MWCNTs) 
Multi wall carbon nanotubes (MWCNTs) can be considered as a collection of 
concentric SWCNTs with different diameters, illustrated in Figure 3.3. Normally, the 
inner diameter is approximately 1-3 nm and the outer diameter is approximately 10 nm. 
Figure 3.4 shows TEM pictures of functional MWCNTs samples showing  multi tube-
walls with gap distance around 3.4 nm. 
 
 24
 
Figure 3.3 Structure of the Multi wall carbon nanotubes (MWCNTs) [48] 
 
Figure 3.4 TEM pictures of MWCNTs [49] 
 25
3.2.3 Random and aligned oriented CNTs 
From the orientation point of view, the CNTs can be divided into random CNTs 
and aligned CNTs.  Aligned CNTs exhibit a unique feature that all tubes orient vertically 
in one direction. An ideal aligned CNT is just like a straight line with zero curvature. 
While in the random CNTs, tubes twine up freely. Aligned CNTs, with tube density 
around 109 tube/cm2, are much denser than random CNTs. Both of them can be 
synthesized easily by chemical vapor deposition method (CVD). Figure 3.5 shows the 
SEM pictures of aligned and random CNTs. 
 
Figure 3.5 (a) aligned CNTs and (b) random CNTs [50] 
 
3.3 Properties of CNTs 
CNTs have many exceptional properties which motivated extensive research 
interests on their manufacturing and applications. They possess high intrinsic strength, 
stiffness, flexibility, high thermal and electrical conductivity. Furthermore, novel 
polymer composites are invented by incorporating CNTs to alter and improve the 
 26
material properties [51]. Three properties of CNTs, electrical, chemical, and mechanical, 
are the most important and have been widely studied in applications. 
 3.3.1 Electrical property 
CNTs exhibit excellent electrical properties. Tested with four-probe measurement, 
the resistivity of different MWCNTs ranges from 5.1x10-6 to 1.2x10-4 cm? [52], and the 
resistivity of SWCNTs range from 3.4x10-7 to 1x10-6 cm?  [53]. Both results are very 
low compared to other materials, showing that CNTs have unique electronic properties. 
 Due to the low resistivity and high thermal conductivity, CNTs have attracted 
interest for their applicability as very-large-scale integration (VLSI) interconnects [53]. 
Also, an isolated CNT can carry current densities in excess of 1000 MA/cm2 without 
damage even at an elevated temperature of 250?C [54]. Recent modeling work has shown 
that the CNT bundle interconnects can potentially offer advantages over copper 
interconnect [55]. 
 Recently, semiconducting CNTs have been used to fabricate field effect 
transistors (CNTFETs), which show great promise due to their superior electrical 
characteristics over silicon based MOSFETs. This device has been found to be superior in 
terms of subthreshold slope, a very important property for low power applications [56]. 
3.3.2 Chemical property 
The Chemical property of CNTs is attributed to the curvature-induced local strain, 
which arises from pyramidalization of the sp2-hybridlized carbon atoms and 
misalignment of ??or-bitals [57]. SWCNTs behave chemically inert due to the difficult 
covalent attachment of molecular species to fully sp2-bonded carbon atoms on the 
 27
nanotube sidewalls. Adsorbing molecules to nanotubes via non-covalent forces, however, 
turns out to be facile and important, which leads to many potential applications [58]. 
CNTs also have promising applications in drug delivery and cancer therapy. 
Systems being used currently for drug delivery include dendrimers, polymers, and 
liposomes, but carbon nanotubes present the opportunity to work with effective structures 
that have high drug loading capacities and good cell penetration qualities. These 
nanotubes function with a larger inner volume to be used as the drug container, large 
aspect ratios for numerous functionalization attachments, and the ability to be readily 
taken up by cells [59]. Selective cancer cell destruction is achieved by functionalization 
of SWNT with a folate moiety, selective internalization of SWNTs inside cells labeled 
with folate receptor tumor markers, and NIR-triggered cell death, without harming 
receptor-free normal cells [60]. 
 Biosensors are another application of CNTs. Carbon nanotube-plasma polymer-
based amperometric biosensors for ultrasensitive glucose detection have been fabricated. 
The glucose biosensor showed ultrasensitivity (a sensitivity of 40 ?A mM-1 cm-2, a 
correlation coefficient of 0.992, a linear response range of 0.025 ?1.9 mM, a detection 
limit of 6.2 ?M at S/N = 3, +0.8V vs Ag/AgCl), and a rapid response time (<4 seconds in 
reaching 95% of maximum response) [61]. The aligned carbon nanotube ultrasensitive 
biosensor for DNA detection is also reported [62]. Hybridization kinetics between 
complementary and target ssDNA nucleotide base pairs result in a local charge 
generation between base pairs that is injected into the SWCNTs resulting in a detectable 
change in SWCNT electrical conductance. This conductance change is amplified 
electrically and recorded. Based on previous Langmuir DNA kinetics calculations, the 
 28
projected sensitivity level of the SWCNT-DNA sensor is considerably higher than 
traditional fluorescent and hybridization assays [62]. 
3.3.3 Mechanical property 
Carbon nanotube, with Young?s modulus on the order of Tera-Pa and the tensile 
strength of 100 Giga-Pa, is one of the strongest materials in nature. Table 3.2 compares 
CNTs with other material. The defect free nanotubes are much stronger than the graphite 
because of the axial component of ? bonding. This component is greatly increased with 
rolling of graphite sheet into a seamless cylinder structure. The Young?s modulus of 
MWCNTs is large compared to SWCNTs typically in the range of 1.1 to 1.3 TPa. This is 
because MWCNTs consist multiple layers of SWCNTs, and the highest Young?s 
modulus of SWCNTs is considered along with the coaxial intertube coupling of van der 
walls force [63]. 
Table 3.2 Young?s modulus, tensile strength, and density of carbon nantotubes 
compared with other materials reported in [64]. 
 
 
 
 29
3.4 Synthesis of CNTs 
3.4.1 Growth mechanism 
Several CNTs growth mechanisms have been proposed, regarding to the 
generation of free carbon atoms and the precipitation of dissolved carbon from the 
catalyst particles. However, the growth mechanism is still not well understood, due to the 
diverse fabrication methods [65]. In general, the growth of CNTs is believed to be due to 
the decomposition of hydrocarbon gas mixtures, like acetylene and methane, with the 
presentence of catalyst, such as iron, molybdenum. Regardless of what mechanism occurs 
in the growth of CNTs, there are three stages common in the growth process as shown in 
Figure 3.6. 
1. Metal catalyst particles formation on the substrate: this process is done normally by ion 
sputtering or e-beam evaporation. Different metals are used as catalyst based on the 
growth method such Fe, Co. 
2. Free carbon atom formation and diffusion after catalyst formation: Free carbon atoms 
are generated in the hydrocarbon deposition process in high temperature and diffuse into 
the catalyst particles.  
3. Carbon tubes growth:  Rod-shaped carbon tubes grow out of the catalyst particles 
rapidly.  
 30
 
 
Figure 3.6 CNT growth stages (a) root growth (b) tip growth 
 
There are two major theories of growth process, one is proposed by Baker and 
coworkers where the hydrocarbon is decomposed into C and H on the top surface of the 
metal catalyst [66]; then CNTs are grown when these carbon fragments diffuse into the 
metal particle and precipitate at the other end. The catalyst particle remains on the tip of 
the tube during the growth process until the leading catalytic particles is neutralized or 
the reaction is stopped due to the encapsulation of metal catalyst by a layer of carbon [66]. 
This is shown schematically in Figure 3.6 (a). 
The other theory states that the catalytic metal particle always remains at the 
bottom of the filament or tube during the growth process [67]. The rapid diffusion of 
carbon species through the catalyst results in the growth of the filament from their bases, 
unlike the first model where the tubes are grown from their tips. CNTs based on these 
two theories is illustrated in Figure 3.7 
 31
 
Figure 3.7 Schematic representation of two types of CNT growth mechanism [4] 
 
A third theory, combing these two models together, has been proposed by 
Louchev [68]. Two important characteristic times in the diffusion process of carbon 
species are defined to explain the CNT growth mechanism. There are: 
(1) The characteristic diffusion time of carbon through the catalytic metal nano-partcle to 
the nanotube growing point is defined as 
bpd DRt /
2=  
where pR is the radius of catalytic metal particle and bD is the diffusion coefficient. 
(2) The surface saturation time of order is defined as  
22 / QDCt
bs =  
 32
Where C is the saturation concentration of carbon and Q is the impinging carbon flux. 
The surface saturation is defined as the increase of the carbon content to the saturation 
concentration level, which is responsible for triggering the carbon precipitation directly 
on the outer surface of the metal nanoparticles. 
 When dt << st , the carbon diffuses through the metal particle body much faster 
researching the saturation point. Therefore, the carbon is precipitated at the bottom of the 
catalytic particles, lifting the particles during the growth process. This type of growth is 
defined as tip-growth. When dt >> st , the carbon diffuses through the metal particle 
surface much faster researching the saturation point. The carbon is precipitated from the 
surface of the catalytic particles. Therefore, the tubes are grown out from the catalytic 
particle surface, leaving the catalytic particles at the bottom. This type of growth is 
defined as bottom-growth. This process is illustrated in Figure 3.8 [68]. 
 33
 
Figure 3.8 Schematic of CNTs growth mechanisms (a) saturation of catalytic nanoparticle 
with carbon, (b) carbon precipitation process, left: when dt << st  , right: dt >> st , (c) tube 
growth process,  left: tip-growth, right: root growth [68] 
 
3.4.2 Synthesis methods 
 There are many methods of synthesizing controllable CNTs. The arc-discharge, 
laser ablation, and the chemical vapor deposition (CVD), which also includes the plasma 
enhanced chemical vapor deposition (PECVD), have been the main methods used for 
CNTs synthesis. 
3.4.2.1 Arc-discharge method 
 CNTs are first produced by arc-discharge method in 1991 [36]. This method 
creates CNTs through arc-vaporization of two graphite rod placed end to end, separated 
 34
by approximately 1mm, in an enclosure that is usually filled with inert gas, normally 
helium and argon, at low pressure. The hardware setup is shown in Figure 3.9. During the 
arc-discharge, a deposit of randomly oriented MWCNTs with some by-products such as 
polyhedral particles and other graphitic particles forms on the cathode. The deposit 
growth rate is around 1~2 mm/min. 
 
Figure 3.9 Schematic diagram of the Arc apparatus [69] 
   
 Large-scale synthesis of MWCNTs by arc-discharge has been achieved in helium 
atmosphere [70, 71]. It is found that the methane is the best gas for the synthesis of 
MWCNTs, due to the thermal decomposition of methane producing hydrogen that 
achieves higher temperature and activity compared to inert gases such as He or Ar [72]. 
The drawback of arc-discharge method is that the discharge conditions have to be 
 35
carefully controlled. Also, the CNTs produced with this method need to be purified 
before applications. 
3.4.2.2 Laser ablation method 
Carbon nonotubes can be synthesized by laser vaporization apparatus with an 
ablated pure graphite target located in an oven. This method is originally used as a source 
of metal clusters and ultrafine particles [73, 74], and then developed for fullerene and 
CNT production. Figure 3.10 shows a schematic of experimental setup of laser ablation 
method [75]. 
 
Figure 3.10 Schematic of laser ablation method [75] 
 
The basic principle of this method is carbon target vaporization by laser ablation. 
A laser beam is introduced onto the carbon based target located in the centre of a quartz 
tube furnace. The target is vaporized in high-temperature argon atmosphere. The CNTs 
produced in the carbon vaporization process are conveyed by the gas to a special 
 36
collector. This method has several advantages, including high tube quality, and high yield. 
However, the large scale production of CNTs using this method is not as impressive as 
arc-discharge method. 
3.4.2.3 Chemical vapor deposition (CVD) method 
Chemical vapor deposition (CVD) is another widely used method to synthesize 
CNTs. It uses hydrocarbon vapor, such as methane, thermally decomposed in the 
presence of a metal catalyst. Chemical vapor deposition has several advantages, including 
simplicity, economic and controllable growth. 
The CNTs synthesis process with CVD method essentially involves two steps: (1) 
catalyst preparation and (2) carbon nanotubes growth. In the first step, a transition metal 
such as Ni, Fe and Co is sputtered on to the substrate material. Then the catalyst coated 
substrate is subjected to either thermal annealing or chemical etching to induce catalyst 
particle nucleation. Nanotubes are achieved by changing the carbon source molecules 
into gas phase and depositing on the transition metals if the proper parameters are 
maintained. Parameters, such as type of hydrocarbon, type of catalyst, growth pressure, 
and temperature, are important for CNT growth in CVD method. Figure 3.11 shows a 
schematic of the CVD apparatus used in [76]. 
 37
 
 
Figure 3.11 Schematic of CVD apparatus [76] 
 
Apart from large scale production, CVD also offers the possibility of producing 
single CNT for use as probe tips in atomic force microscopy (AFM). The tips produced 
are smaller than mechanically assembled ones, thus significantly improving AFM 
resolution [77]. 
3.4.2.4 Plasma Enhanced Chemical vapor deposition (PECVD) method 
 Plasma enhanced chemical vapor deposition (PEVCD) is a type of CVD where 
chemical reactions involved in the deposition process occur after generation of plasma of 
the reacting gas through interactions in the plasma. Electrical (DC or RF) discharge is 
used to create the glow discharge which contains radicals, electrons, and ions, instead of 
the thermal lamp in the traditional CVD method. In a PECVD chamber, two parallel 
electrodes are mounted with substrate on the grounded one, as shown in Figure 3.12.  The 
distance between two electrodes is adjusted in order to sustain the discharge.  A separate 
heat source may be used to maintain the desired temperature of the substrate to enhance 
the nucleation density.  
 
 38
 
Figure 3.12 Schematic of PECVD [47] 
 
CNTs are synthesized by the glow discharge in the chamber by applying a high 
frequency high voltage to electrodes. Sources such as DC, RF (13.56 MHz) or 
Microwave (2.45 GHz) are used to generate the plasma. The reaction gas, either C2H2, 
CH4, C2H4, C2H6, or CO diluted with inert gas, is fed with operating pressure of 1 to 20 
mTorr. The source carbon molecules are broken into reactive carbon atoms by the energy 
provided from the plasma. The carbon nanotubes are grown on metal catalyst particles 
which are already prepared on the substrate. The local electric field formed between the 
 39
plasma and the substrate holder allows CNTs to be grown extremely vertical with high 
density. Because heat is not needed for chemical reactions, this process is achieved at low 
temperatures. 
The low operating temperature during the growth mechanism of carbon nanotubes 
is another advantage, which make it attractive for the integrating carbon nanotubes in 
semiconductor device fabrications. 
3.5 Characterization Methods 
 In order to better understand the electronic and mechanical properties of CNTs, 
the morphological and structural characterizations need to be examined. There are many 
methods used for the characterization of materials depending on the desired property. 
To test for defects, treatments, such as oxidation with nitric acids, can be 
performed to remove the caps at both ends [78]. This method reveals defects like 
carboxylic acid groups [78]. The determination of the concentration of these defects can 
be done by various methods. Calibrated energy dispersive X-ray and titration techniques 
with sodium bicarbonate are two methods which have been widely used [79]. 
In order to observe the photoluminescence phenomenon, the CNT bundles must 
be separated into individual tubes. Ultrasonication treatment with surfactants or using 
individual CNTs grown in channels of zeolite [80] is a good way to get individual tubes. 
X-ray photoelectron spectroscopy (XPS) can be used to determine the chemical 
structure of CNTs. The scanning tunneling microscopy (STM) provides three-
dimensional morphology information of samples. To use STM, CNTs must be deposited 
on a flat conducting substrate such as highly ordered pyrolytic graphite (HOPG) or Au. 
Infrared spectroscopy is often used to determine residual impurities from synthesis or 
 40
molecules capped on the CNT surface. It exhibits all the modification of the structure and 
reveals the nature of compounds added to the NTs and the catalytic properties [81]. 
The Raman spectroscopy is one of the most commonly used tools for structural 
characterization of CNTs. It is fast and also non-destructive [82]. The Raman line shape 
differs between metallic and semiconductor CNTs thus distinguishing these two types in 
a sample [83]. Many other characterization methods are also being studied. More 
information can be found in [84] 
 41
Chapter 4  
Carbon Nanotubes Fabrication and Optimization 
 
The primary objective of this research is to design and construct a pseudospark 
switch triggered by CNTs acting as cold cathode emitters. As mentioned, an appropriate 
trigger mechanism is very critical in pulsed power switches. Previous work has shown 
that CNTs have excellent field emission properties and can be used as electron emitter 
material in applications [35]. The high quality CNTs can emit milliamp current without 
degradation of field emission property in higher pressure than vacuum [86]. Therefore, if 
they are used in a plasma switch, they may offer improved switch performance. As a 
result, CNTs synthesis and optimization of the properties are the first step toward to the 
final goal of using them as trigger electrode in pseudospark switches.   
In this research, CNTs, both randomly oriented, and vertically aligned, are 
synthesized with Chemical Vapor Deposition method (CVD). The CVD method has 
advantages of simple equipment setup, and excellent deposition uniformity over large 
area. Silicon wafers are used as the substrate for CNTs growth and iron (Fe) is coated on 
the silicon substrate as the catalyst using an ion sputtering system. Then the CNTs are 
synthesized in a CVD furnace under temperature range of 650?C to 700?C. The quality of 
CNTs is examined under scanning electron microscopy (SEM) and the electrical property 
is characterized with field emission measurements in vacuum. Synthesis parameters are 
 42
optimized by testing different CNT synthesis approaches. Three types of CNTs, 
randomly oriented, vertically aligned, and patterned are fabricated and tested. 
4.1 Synthesis of Randomly Oriented CNTs 
 The synthesis process of CNTs mainly includes wafer preparation, catalyst 
sputtering deposition, and CNT growth in a CVD chamber. 
4.1.1 Silicon wafer preparation  
The silicon wafer preparation consists of wafer clean and wafer baking. Wafer is 
cleaned in the clean room and baked in vacuum. The cleaning process removes solvent, 
organic/ionic contamination, and hydrous oxidation from the surface. Baking process 
removes the moisture.  
Details of the cleaning process are listed as below: 
A. Solvent removal 
1. Immerse in boiling trichloroethylene (TCE) for 3 min. 
2. Immerse in boiling acetone for 3 min. 
3. Immerse in boiling methyl alcohol for 3 min. 
4. Wash in DI water for 3 min. 
B. Removal of residual organic/ionic contamination 
1. Immerse in a (5:1:1) solution of H2O-NH4OH-H2O2; Heat solution to 75-80?C 
and hold for 10 min. 
2. Quench the solution under running DI water for 1 min. 
C. Hydrous oxide removal 
1. Immerse in a (1:50) solution of HF-H2O for 15 sec. 
2. Wash in running DI water with agitation for 30 sec. 
 43
The cleaned wafer is then transferred into a vacuum chamber with a heater 
attached on the top of the wafer holder for baking. The purpose of baking is to increase 
the adhesion between silicon and catalyst by removing the moisture on the wafer surface.  
The baking procedure is listed as follows: 
1. Bring the baking chamber to a vacuum of 1x10-6 Torr with mechanical and turbo 
pumps. 
2. Turn-on the heater with 15% of the total power. Heater has controls to regulate the 
temperature increase rate. 
3. Keep the wafer heated at 100?C for 5 min. 
4. Cool down the wafer to room temperature and de-vacuum the chamber. 
4.1.2 Catalyst sputtering deposition  
The wafer is loaded into an ion sputtering deposition chamber when the wafer 
preparation process is finished. The ion sputtering system used in this research is 
schematically shown in Figure 4.1. The wafer substrate is mounted 15 cm above the 
plasma gun, also called as ion gun, is a filamentless ion source based on a microwave 
plasma discharge. A two-inch diameter Fe target, attached directly on the plasma gun, is 
used as the catalyst source. Argon is fed as the background gas and the pressure is 
maintained at 20 mTorr.  
 44
 
Figure 4.1 Schematic of ion sputtering system 
 
Once the DC/RF power is applied to the plasma gun, plasma is generated. The 
ions are accelerated in the electric and magnetic field and bombard the target surface. The 
Fe atoms sputtered from the surface float to the wafer substrate and attach to the wafer 
surface. Eventually, a layer of Fe is deposited on the wafer surface. The typical sputtering 
times vary from 1min to 20 min, depending on the desired catalyst thickness. In order to 
examine the uniformity of the Fe distribution on silicon surface, a 800?C annealing 
process is done to break the Fe thin film into small ?islands? before the SEM. Figure 4.2 
are SEM pictures of silicon with Fe particles uniformly deposited on the surface. 
 45
 
Figure 4.2 Fe sputtered silicon substrate. 
 
4.1.3 Chemical vapor deposition 
 After the Fe catalyst layer is coated on the silicon, the silicon substrate is then 
transferred to the thermal CVD reactor. The quartz tube is the main chamber of the 
system, as shown in the Figure 4.3. A controllable resistive heater is used to heat the 
chamber to a desired reactive temperature. A thermocouple placed on the wafer holder is 
used to measure the process temperature. The flow of the gas into the chamber and the 
pressure are electrically controlled by flow meters and a set of valves between the pump 
and the chamber.  
 46
 
Figure 4.3 Schematic of CVD reactor [85] 
 
Mixture of Acetylene (C2H2) and Argon (Ar) with flow rate of 20 and 75 sccm 
respectively, are used as the reaction gas maintaining a growth pressure of 70 Torr in the 
chamber. The CNTs are grown at a temperature of 650~750?C and the growth time is 
typically varied between 10~240 min depending on the desired tube density and tube 
length. Figure 4.4 shows the SEM pictures of a representative set of CNTs samples 
fabricated in this research. 
 47
 
Figure 4.4 SEM pictures of CNTs ( a: loose CNTs with good quality, b: dense CNTs with 
good quality, c: dense CNTs with bad quality, d: loose CNTs with bad quality) 
 
As seen, the CNTs in Figure 4.4 vary dramatically. Some samples are good in 
term of tube length and tube density with very few carbon clusters and some are not. 
Some samples have clean tubes but low density. Some other samples contain very few 
tubes where the carbon clusters are dominant. Therefore, the CNT synthesis parameters 
are optimized in order to get high quality carbon nanotubes without clusters. 
a 
b 
c 
d 
 48
4.2 Optimization of CNT Synthesis Parameters 
As stated earlier, high quality CNTs with low turn-on field is one of the important 
parameters of this research. Therefore, in this step, CNTs synthesis conditions using 
CVD method are thoroughly studied and optimized.  
The previous work reveals that three variables, C2H2 concentration, CVD 
temperature, and catalyst thickness, are important for the CNTs synthesis [86, 87]. 
Therefore, different growth conditions are tested and the qualities in terms of tube length 
and density of resulting CNTs are investigated with SEM imaging system. The deposition 
procedure of the CNT synthesis is carried out firmly as outlined in section 4.1.3 Chemical 
vapor deposition. The growth conditions and the SEM pictures of resulting CNTs are 
listed below. 
Ratio of C2H2 to Ar:  
Four different flow rate ratios of C2H2 to Ar, namely 10:75, 20:75, 30:75, 40:75, 
are used while keeping the CVD temperature of 700?C and catalyst thickness of 13 nm 
fixed. The SEM pictures of synthesized samples are shown in Figure 4.5. The sample 
with a ratio of 20:75 is concluded to have better morphology. Although carbon clusters 
present, the tube density is the highest. 
 49
 
Figure 4.5 CNTs with different C2H2 concentration 
 
Operation temperature:  
Two temperatures of 700?C and 750?C are tested twice on different C2H2 to Ar 
ratio.  First, 20:75 ratio is kept fixed and then the 30:75 ratio is kept fixed. The SEM 
pictures are shown in Figure 4.6 for these combinations. From these images, it is 
concluded that the samples grown in 700?C are better than the ones in 750?C. 
 50
 
Figure 4.6 CNTs with different operation temperature (The upper two samples are grown 
at 20:75 ratio of C2H2 to Ar and the bottom two samples are grown at 30:75 ratio) 
 
Catalyst (Fe) thickness:  
Two catalyst thicknesses of 13 nm and 60 nm are also examined. First the C2H2 to 
Ar flow rate ratio of 20:75 is used and then the ratio of 30:75 is used for each substrate 
with different catalyst thickness. Temperature is set at 700?C for these growth processes. 
The SEM pictures are shown in Figure 4.7. The 13 nm catalyst thickness obviously 
results in growing better CNTs than the one with 60 nm thickness. 
 51
 
Figure 4.7 CNTs with different catalyst thickness (The upper two samples are grown at 
20:75 ratio of C2H2 to Ar and the bottom two samples are grown at 30:75 ratio) 
 
From these pictures, we see that some parameter combination produces CNTs 
with better morphology. Based on a series of fabrications, we set the optimized CNT 
synthesis parameters as, C2H2 Concentration of 20:75 ratio, CVD temperature of 700?C, 
and Fe thickness of 13nm. The first optimized and repeatable CNT sample is fabricated 
with this set of parameters and is shown in Figure 4.8.        
 52
  
Figure 4.8 CNT sample fabricated with optimized CVD parameters 
 
4.3 Aligned CNTs Synthesis 
The aligned CNTs synthesis is also proposed and conducted using the knowledge 
of random CNTs growth. In the vertically aligned CNT synthesis, each tube needs the 
support from the surrounding tubes during the growth process. Two parameters that are 
important for the growth of vertically aligned CNTs are the uniformity and density of the 
catalytic Fe particle distribution on the substrate, and the catalytic Fe particle must be 
saturated with carbon to form a growth point. 
In this research, the catalytic Fe particles are coated by the ion sputtering 
deposition described before. This method shows a very good uniformity and density of 
 53
the catalytic layer. The carbon saturation is achieved by a carbon treatment process in 
which carbon atoms are sputtered and attached to catalyst Fe particles on the silicon 
substrate. The carbon sputtering time is critical because the Fe particles cannot be 
saturated if sputtered too short, or the Fe particles get oversaturated by carbon atoms, 
leaving no seed for CNT growth, if sputtered too long.  
In order to study the aligned CNT synthesis mechanism, four carbon sputtering 
time, 1 min, 5 min, 20 min, and 40 min, are tested. The substrate is cleaned and Fe 
sputtering deposited the same way as stated in the previous section. All CNTs are grown 
in the CVD furnace for 40 min. The CNT samples are shown in Figure 4.9. 
 
   
a 
 54
  
 
Figure 4.9 Aligned CNT samples (a) 1 min carbon pretreatment (b) 5 min carbon 
pretreatment (c) 20 min carbon pretreatment  
b 
c 
 55
The CNTs in sample (a) and sample (b) are well aligned with a relatively good 
tube density. Samples (c) looks more like the randomly oriented CNT. No CNTs are 
grown on the 40 min carbon sputtering deposited sample. This confirms that the catalyst 
gets oversaturated and no seed Fe catalytic particles exposed on the surface to support 
CNT growth.  
Based on the results above, we consider the aligned CNT synthesis mechanism is 
illustrated in Figure 4.10. We believe that, if a Fe particle is saturated enough by carbon, 
a CNT growth point forms, which is represented with a red ball in the Figure 4.10. When 
the growth points are dense and uniform on top of the substrate, each tube receives 
supporting force from neighboring tubes because of proximity to each other. This allows 
all tubes grow in one direction, resulting in aligned CNT growth. When some of the 
growth points are not well formed, represented with black balls in the Figure 4.10, no 
tube grows on that spot, leaving a large gap distance. As a result, No support force exits 
in that area and the surrounding tubes have freedom to grow in random directions. This 
leads to more randomly oriented CNTs. When the Fe particles are oversaturated by 
carbon completely, no growth points form on the surface. Therefore, carbon clusters are 
formed on the surface instead of CNTs. This mechanism is confirmed with CNT samples 
synthesized in our lab (see Figure 4.11).  
 56
 
Figure 4.10 Mechanism of aligned CNT synthesis 
 
 
Figure 4.11 CNT samples with different growth point distance 
 
 57
4.4 Patterned CNT Synthesis 
CNTs grown on a patterned silicon substrate with SiO2 on top has also been 
performed using the photolithography technique adopted by the semiconductor 
microfabrication industry.  
First, a 0.5 ?m thick silicon dioxide (SiO2) layer, acting as the dielectric spacer, is 
formed on the top of the silicon substrate. The dielectric strength of SiO2 layer has been 
tested both in vacuum and atmosphere, and maximum of 400 V can be applied without 
shorting the circuit. The test result is shown in Figure 4.12. 
 
Figure 4.12 Dielectric strength test of SiO2 (x axis is the applied voltage and the y axis is 
the emitting current) 
 
Second, a set of 0.5 cm by 0.5 cm square holes with a depth of 60 ?m is etched in 
the substrate by using wet and dry etching. Two masks are used in this step as shown in 
Figure 4.13. While the mask 1 is used to etch holes, the mask 2 is used to limit the 
catalyst area in the sputtering process. This is to restrict the CNT growth area small in the 
bottom of the holes, so the CNTs can grow without reaching the pattern edge. In this way, 
CNTs are only grown at the center bottom of the pattern and no tubes exit on the side 
wall. Desire not to have tubes on the sidewall is important because those tubes may cause 
 58
short circuits to the cathode body during testing the samples in the trigger electrode. 
Figure 4.14 shows a patterned square array on the substrate 
 
Figure 4.13 Masks of Photolithography, mask1 has 0.5 cm square holes and mask 2 has 
0.35 cm square holes. 
 
 
Figure 4.14 Patterned substrate 
 
 59
Last, the patterned substrate goes through the catalyst coating and CVD process. 
Similar to the one outlined earlier. A processing flow chart of the patterned CNT growth 
is shown in Figure 4.15. 
 
Figure 4.15 Flow chart of the patterned CNT fabrication process 
 
Figure 4.16 shows the cross sectional view of one pattern hole. The profile of the 
etched wall is measured to be 60 ?m and is shown in Figure 4.17. Figure 4.18 shows the 
CNTs inside the pattern, showing that randomly oriented CNTs have been successfully 
synthesized inside the silicon pattern.  
 
 
 
 60
 
 
 
Figure 4.16 Cross sectional view of one pattern hole (a) one pattern square (b) enlarge 
view of the square edge 
 
 
Figure 4.17 Profile of the pattern hole edge 
a b 
 61
  
(a)                                                                    (b) 
Figure 4.18 Patterned CNTs (a) top view of the patterned CNTs (b) enlarge view of the 
CNTs inside the pattern 
 
4.5 Electrical Properties of CNTs 
4.5.1 Electron field emission 
Electrical properties of CNTs are characterized through electron field emission 
measurements in vacuum. Electron field emission is the extraction of electrons from a 
solid, by tunneling through the surface potential barrier when a strong electric field is 
applied. The emission current, formed by the extracted electrons, represents the electrical 
ability of the material being tested.  
 In order to test the field emission properties of fabricated CNTs, a vacuum 
chamber equipped with ports for electrodes, pressure gauges, and other associate circuits 
is used in the experiment. The schematic test circuit is shown in Figure 4.19.  
 62
 
Figure 4.19 Schematic of the field emission test system [88] 
 
By using the mechanical and turbo molecular pump, the pressure of the chamber 
is maintained at vacuum pressure of less than 1x10-6 Torr. The glass spacer placed 
between cathode and anode is 140 ?m in thickness and the current limiting resistor is 3 
M?. The glass spacer has two functions in this test. First, it works like a dielectric layer 
between anode and cathode. Second, it defines the electron emission area on the CNT 
surface. With the glass spacer, electric field and emission current density are calculated 
from the raw I-V data acquired from the oscilloscope. The applied voltages, ranging from 
0~1200 V from a 5 kV high voltage power supply, are divided into three zones with three 
different increments. The first zone and the third zone are corresponding to the dark 
current emission stage, where electrons are not started to emit, and the saturated current 
emission stage, where the current stays at the maxim emission mode.  The current in 
these two stages does not change much and the voltage increment is set at 80 V. In the 
second zone, the emission current experiences the transient from dark current to the 
saturated current, providing the turn-on field information. The increment in the second 
zone is set at 10 V so that more detailed information can be obtained.  
 63
When testing, a set of increasing biased voltages (0~1200 V) is applied between 
the cathode (the CNTs sample holder) and the anode (the electron collector) with 
controlled increments mentioned above. This results in an ?up? I-V curve. Then, voltage 
gradually decreased back to zero following the same increments to get a ?down? I-V 
curve. This process is repeated several times until the ?up? and ?down? I-V curves 
overlap. Current data are acquired from a pico-ammeter through the GPIB card and 
transferred to computer. Both the electric field and the current density data are calculated 
with the following equations,  
S
IJ
d
IRVE
=
?=
 
where R is the resistance of the current limiting resistor with a value of 3 M?, d is the 
gap distance of 140 ?m, and S is the emission area of 0.0316 cm2. The test data are 
displayed in a MATLAB based program. For each applied voltage, five current data with 
1 second interval are recorded to assure the test accuracy. Figure 4.20 shows the 
MATLAB program interface and Figure 4.21 is a measured field emission characteristic 
of random CNT sample with 40 min growth time (a) and aligned CNT sample with 40 
min growth time (b). 
 64
 
Figure 4.20 Interface of the test program 
 
(a) 
 65
 
(b) 
Figure 4.21 Field emission characteristic of random CNTs and aligned CNTs sample  
with 40 min growth time, (a) Random CNTs, (b) Aligned CNTs 
 
 In Figure 4.21, before the applied electric field reaches a certain point, the current 
density remains at the dark current level of 10-11 A/cm2 for random CNTs testing and 10-9 
A/cm2 for aligned CNTs testing depends on the current noise level. This threshold electric 
field is termed as the ?turn-on field? and is defined as the applied electric field where the 
current density is at least one order higher than the dark current density. For the data 
shown in Figure 4.21, the turn-on field is around 0.857 V/?m for random CNTs and 1 
V/?m for aligned CNTs. 
4.5.2 Turn-on filed and life time measurements 
Compared to other electron emitter materials, CNTs exhibit low turn-on field and 
high saturation current density. There are very few studies interpreting the relationship 
between the low turn-on field and the length and density of the CNTs, which is related to 
the growth time in CVD. Understanding of this relationship is important in the 
 66
optimization of synthesis of CNTs and improves the cold-cathode emitter?s performance.   
For this, the CNTs are grown based on different growth time durations, namely, 
10 min, 20 min, 40 min, 80min 120 min and 240 min, respectively. SEM imaging is used 
to examine the tube length and density. Field emission results are analyzed to investigate 
the CNTs growth time dependency of the turn-on field. Also, the CNTs life time is tested 
by repeating 100 consecutive runs of field emission tests. 
Figure 4.22 shows the SEM images of the six CNTs samples mentioned above. 
Randomly oriented CNTs grow in all directions and tubes twine up, which leads to the 
difficulties on quantitative measurements of CNTs density. But, it can be qualitatively 
seen from the SEM images that the tube length and tube density increases with the 
growth time. Comparing the sample growth time of 240 min to 10 min sample, the 240 
min sample clearly contains more tubes. Figure 4.22 also shows that all of CNTs samples 
used in this study have good quality with very few clusters. 
 67
 
Figure 4.22 SEM images of CNTs samples under 30k magnifications 
 
 Field emission characteristics of these six CNT samples are tested and the results 
are shown in Figure 4.23.  As seen in this figure, field emission characteristic curves are 
orderly arranged in a way that as the growth time increases, the turn-on field decreases. 
Thus indicates that the CNTs sample with longer growth time has lower turn-on field. 
The lowest turn-on field is 0.5 V/?m belongs to the 240 min growth time sample and the 
highest turn-on field is 1.02 V/?m belongs to the 10 min growth time sample. 
 68
 
Figure 4.23 Field emission characteristics of CNTs samples 
 
 The turn-on fields for all six samples are listed in Table 4.1 and plotted as a 
function of growth time in Figure 4.24. An exponentially decreasing curve is observed. 
The turn-on field drops dramatically at the beginning, and stabilizes between 0.5~0.6 
V/m after the growth time is longer than 80 min. Increasing the growth time further does 
not decrease the turn-on field substantially.  
When the growth time is short, i.e. 10 min, the CNTs are short in length and loose 
in density, which means there are less electron emitting spots. As the growth time 
increases, CNTs become longer and denser, raising more electron emitting spots. As a 
result, the turn-on field decreased. However, increasing the tube length and tube density 
reaches a limit of the turn-on field. In this case, the field enhancement on the surface is 
 69
not as efficient. When the CNT density is high, the tubes form almost a flat surface and 
?Screen Effect? occurs. The screening effect is due to the close proximity of the tubes 
and it is known that it could seriously affect the emission current [89]. This leads to the 
fact that the current density remains almost the same no matter how long and how dense 
the tubes are.  
Table 4.1 Turn-on field of the tested CNT samples  
 
Growth time 
(min) 
10 20 40 80 120 240 
Turn-on 
field(V/?m) 
1.02 0.91 0.78 0.6 0.53 0.5 
 
  
Figure 4.24 Turn-on field vs. growth time  
 
 
Life time test of CNTs as cold cathode emitter is also examined by repeating 100 
consecutive field emission tests for samples with 10 min and 240 min growth time only. 
The turn-on fields are shown in Figure 4.25. For the 10 min growth time CNT sample, 
the turn-on field starts at 1.02 V/?m at first and stabilizes at 1.14 V/?m after 30th run. The 
 70
turn-on field of 240 min growth time sample starts at 0.5 V/?m and stabilizes at 0.57 
V/?m after 50th run. Both turn-on fields increase about 0.1 V/?m for repeating the test for 
100 times.  This data show a good working life time for both samples after several 
conditioning runs. Comparing these two samples, 240 min growth time sample needs 
more conditioning runs, which is reasonable because 240 min sample emits more 
electrons, which generates more heat. We speculate that the heat accumulates in the 
CNTs and damages weak tubes until a balance is reached. 
 
  
Figure 4.25 CNTs life time test 
 
 
 
 71
Chapter 5  
Triggered Pseudospark Switch Construction and Test 
 
5.1 Switch Body Construction 
The hollow cathode effect is a crucial point in the pseudospark switch design. In 
this project the hollow cathode effect is implemented in the switch hardware 
configuration shown in Figure 5.1. The designed pseudospark switch consists of a U-
shaped hollow cathode, planar anode, CNT coated trigger electrode, and periphery 
circuitry. A quartz tube, 8 cm long and 2 cm in diameter, is used as the switch enclosure 
forming the vacuum chamber. The U-shape electrode is pre-machined with a 2 mm 
diameter bore hole drilled on the front surface, and the planar anode, made out of a 
circular copper plate with a diameter of 1.8 cm, is placed 3 mm away from the cathode 
front surface. The CNTs electron emitter coated trigger electrode is held on a specially 
machined alumina holder, and placed at the open-end side of U-shape cathode, where the 
CNTs are fully exposed to the electric field. The overall switch is shown in Figure 5.1 
and the CNTs holder is shown in Figure 5.2. The switch is well sealed in a vacuum 
chamber that can be kept in vacuum of 1x10-6 Torr when not operated. 
 
 72
 
Figure 5.1 Sketch of pseudospark switch  
 
Figure 5.2 Sample holder serving as part of the trigger electrode  
 
The switch is designed to operate from a few mTorr to 100s of mTorr pressure by 
using a combination of a mechanical pump and a turbo pump. A gas inlet system is 
connected to the switch enclosure to which different inert gas, such as Helium, Argon, or 
Nitrogen, is selectively fed. The anode is connected to a 35 kV DC power supply through 
a 22 M? current limiting resistor, and the hollow cathode is grounded. The trigger 
electrode is connected to a negative pulse generator where negative pulse up to 1000 V 
 73
and current up to 50 A can be generated. In a single pulse, the trigger pulse rise time is 
less than 100 ns with a pulse width in the range of 0.5 to 1 ms. Different pulse width can 
be obtained by changing the resistance in the discharging circuit of the pulser. The 
assembled switch chamber is shown in Figure 5.3. 
 
Figure 5.3 Assembled PSS Switch Chamber 
 
5.2 Test Experimental Setup 
In the experiments, hold-off voltage, trigger delay, and jitter are measured. Figure 
5.4 shows the schematic of the experimental setup with the diagnostics. The hold-off 
voltage is measured through a high-voltage probe with 2000 attenuation, and the pulse 
voltage is measured with another high-voltage probe with 1000 attenuation. The current 
data is measured through a Rogowski current transformer in which the current is 
converted into voltage. Both voltage and current data are collected by an oscilloscope 
through a USB cable and recorded by a computer. Data is analyzed once each experiment 
set is completed. Two small holes on the cathode wall are made and a high speed camera 
is set to capture the visual information of the plasma formation in the cathode cavity. 
 74
Consecutive frames of the video images can be obtained to examine the transit plasma 
formation. 
 
Figure 5.4 Schematic of the test experimental setup with diagnostics 
 
The switch is first tested without triggering to determine the maximum 
breakdown voltage at a given background pressure. This determines the hold off voltage 
of the switch. Then it is tested with CNTs triggering to analyze the switch performance, 
such as the delay time and jitter. Five trigger electrodes are fabricated with different types 
of CNTs. The switch operating characteristics are analyzed for each trigger electrode 
having different CNTs. Switch performance comparisons are made based on these trigger 
electrodes. The test procedure is listed as follow: 
1. Hold off voltage test in different pressures 
2. Trigger delay time test with different trigger voltages 
3. Conduction current and current rise time test with different trigger voltages 
4. Jitter calculations based on delay tests. 
 75
5. Repeat of trigger delay test time for the remaining trigger electrodes having 
different CNTs, namely, random CNTs, aligned CNTs, and CNTs with different growth 
time 
5.3 Switch Hold-off Measurements 
The switch is first tested without triggering to determine the maximum self-
breakdown voltage by gradually increasing the anode voltage until a breakdown event is 
captured and the breakdown voltage is recorded. The Plasma formation is inspected 
visually by a video camera. One frame-image is shown in Figure 5.5. It clearly shows that 
the plasma is formed inside of the hollow cavity of the cathode and bridges the two 
electrodes through bright plasma.  
 
Figure 5.5 Hollow cathode breakdown event 
 
In order to get the breakdown statistic for different pressure, a pressure sweep test 
is carried out. Helium is fed as the background gas and the pressure varied from 5 mTorr 
to 100 mTorr for the fixed electrode geometry, as described in Figure 5.1. All breakdown 
voltages are collected and plotted in Figure 5.6. The breakdown voltage is around 24 kV 
at 5 mTorr and exponentially decreased to 15 kV at 100 mTorr, showing the switch is 
operating in the ?left side of the Paschen curve? which is the traditional operating 
pressure region for pseudospark switches [2]. Based on our experimental conditions, 100 
 76
mTorr is selected as the switch operation pressure and 12 kV is set as the switch hold off 
voltage to test the triggered mode operation. Higher hold off voltage can be obtained by 
decreasing the switch pressure.  
 
Figure 5.6 Self breakdown voltages of non-triggered switch as a function of pressure 
 
5.4 Switch Operation-Trigger Mode 
Once the operation pressure and the hold off voltage are determined, the switch is 
then tested under trigger mode. Five trigger electrodes based on different types of CNTs 
are used. These different CNTs are one vertically aligned and four randomly oriented 
samples fabricated under different growth conditions. The SEM images are shown in 
Table 5.1 and named as: 
Aligned-40:  vertically aligned CNTs with 40 min growth time 
Random-20:  randomly oriented CNTs with 20 min growth time 
Random-40:  randomly oriented CNTs with 40 min growth time 
Random-80:  randomly oriented CNTs with 80 min growth time 
Random-120:  randomly oriented CNTs with 120 min growth time 
 77
Table 5.1 SEM images of tested samples 
 
 
When triggering, the anode is set to 12 kV by the main power supply, and the 
cathode is grounded as shown in Figure 5.1. The operation pressure is fixed at 100 mTorr. 
A set of negative trigger pulses, ranging from 200 V to 800V is used to provide the 
negative bias on the CNTs where the seed electrons are emitted. In a single pulse, the 
trigger pulse rise time is less than 100 ns with a pulse width in the range of 0.5 to 1 ms. A 
recorded pulse shape is shown in Figure 5.7. 
 78
 
Figure 5.7 Trigger pulse shape 
 
Once the trigger voltage is applied, the plasma forms in the hollow cathode 
instantly, filing the cathode cavity with bright light. Meanwhile, it propagates towards the 
anode rapidly, bridging the two electrodes in a very short time. The image of the 
triggered breakdown event is shown in Figure 5.8, and the triggered breakdown 
waveforms are shown in Figure 5.9. The upper curve is the hold off/applied voltage, and 
the bottom curve is the trigger voltage. It can be seen that the applied voltage drops 
dramatically and almost simultaneously when the pulse trigger applies. The voltage 
across the cathode and anode gradually recovers to where plasma is formed momentarily 
within the switch, and to the value to maintain glow-discharge mode for a very short time. 
The pseudospark switch is designed to operate in a non-self-sustaining mode so the 
whole discharge and plasma event takes place in about 3 to 4 ms. Once the breakdown 
occurs, the stored energy is fully discharged until the external circuit is charged again 
 79
bringing the two main electrodes back to the hold off voltage through the 22 M? current 
limiting resistor.  
It should be noted that the emphasis in this dissertation is put on the initiation of 
the plasma and the operating characteristics of CNTs field emitted electron triggering 
information, such as the delay time and jitter. Other information, such as the discharge 
operation, and maximum peak power are not in the scope of this work. 
 
Figure 5.8 Triggered breakdown event 
 
Figure 5.9 Triggered breakdown waveforms 
 
 80
5.4.1 Delay time measurements and analysis 
Second step of the experiments is to examine the delay time variation as a 
function of the trigger voltage for a fixed trigger electrode. Five trigger pulse voltages, 
namely 200 V, 400 V, 600 V, 700 V and 800 V, are used in this test. The trigger sample 
is selected and fixed at the random CNTs with 40 min growth time. The SEM image of 
this sample is shown in Figure 5.10. 
 
Figure 5.10 SEM image of CNTs used as the trigger electrode 
 
In this dissertation, the delay time is defined as the time difference between the 
initial drop points of both the trigger and the hold off voltage, as illustrated in Figure 5.11. 
For each trigger voltage, the delay time is recorded and calculated as defined. This 
process is repeated for several times and the delay time is averaged for each designated 
trigger voltage. The overall delay time as a function of trigger voltage for Random-40 
CNTs sample is shown in Figure 5.12. As can be seen, the lowest delay time is for 800 V 
 81
trigger pulse voltage with 3 ?s, where the longest delay is for 400 V trigger pulse voltage 
with 9 ?s. The trigger pulse voltage of 200 V did not trigger the switch. This result can be 
seen by the field emission data of this CNT sample shown in Figure 5.13. The 200 V 
triggering voltage is below the turn-on voltage of the CNTs where no emission current is 
detected. As a result, the switch failed to trigger. The trigger pulse voltage of 400 V is 
above the turn on voltage of the CNTs and produces almost 2 ?A current. Although the 
switch can be triggered by the field emitted electrons, the delay time is relatively long 
due to the low level of the current emission. When the trigger voltage is above 600 V, the 
emission current reaches 10~100 ?A, which results in a steady decrease in the delay time 
from the previous case. Figure 5.13 also shows that the field emission current continues 
to increase for trigger pulse voltage of 700 V and 800 V. The corresponding delay times 
of these two voltages triggering decrease as the trigger pulse voltage increase. Based on 
this trend, shorter delay time is possible for larger trigger voltage. This phenomenon can 
be explained that when the trigger voltage increases, the electric field on the CNTs 
increases as well. As a result, electron emission increases. Moreover, the emitted 
electrons are further accelerated in the cathode cavity by the higher electric field and 
plasma can be initiated easier with shorter delay time. 
 82
 
Figure 5.11 Delay time 
 
Figure 5.12 Delay time variation 
 83
 
Figure 5.13 Field emission characteristics of Random-40 CNT sample 
 
5.4.2 Conduction current and current rise time 
The conduction current is measured through a Rogowski current transformer. 
Figure 5.14 is a representative of the current waveform of a breakdown event from these 
measurements. The current rise time for each trigger is calculated. The peak current in 
these experiments ranges from 0.6 A to 1 A depending on the applied trigger pulse 
voltage. Although pseudospark switches are operated at kA ranges, in our experiments, 
we are only interested in the feasibility of triggering by use of CNTs. Therefore, a current 
limiting resistor is used to limit the conduction current in the order of 1 A range in the 
switch.    
 84
 
Figure 5.14 Screen shot of current waveform at switch closing  
 
Similar to the definition of the delay time, the current rise time is measured as the 
time difference between the trigger voltage drop and the maximum peak current. Shorter 
current rise time is a desired parameter in a switch operation.  The test results for 
different triggering voltages are plotted and shown in Figure 5.15. Similar to the delay 
time, the current rise time is also inversely proportional to the applied trigger pulse 
voltage. The current rise time is 520 ns for trigger pulse voltage of 400 V and decreases 
to 240 ns for trigger pulse voltage of 800 V.  
 
 
 85
 
Figure 5.15 Current rise time variation as a function of trigger pulse voltage 
 
5.4.3 Jitter time calculation 
In this dissertation, the jitter time is defined as the standard deviation of delay 
time for the successive switch closing and is calculated using Equation (1) based on 
twenty repetitions of the switch trigger.  
n
xxi 2)(? ?=s
                                                 (1)
 
where the x  is the average value of the twenty delay times, n is the total repetition 
number. Experimental conditions are kept identical throughout repetitions and the delay 
times for four different trigger voltages for number of tests are plotted in Figure 5.16. The 
calculated jitter time is shown in Table 5.2. The results show that hundreds of ns jitter 
time is achieved in this triggering method. 
 86
 
Figure 5.16 Statistics of delay time 
Table 5.2 The jitter variation 
Trigger voltage 400V 600V 700V 800V 
Jitter 314ns 442ns 238ns 236ns 
  
5.5 Switch Operation under Different CNTs as Trigger Electrode 
The switch operating characteristics are examined based on the remaining four 
different types of CNTs trigger samples and the switch performance comparisons are 
made in terms of delay time. The field emission data of all the randomly oriented CNTs 
is shown in Figure 5.9, whereas the field emission data of vertically aligned sample 
shown previously in Figure 4.21 (b). The delay time for the random CNT samples are 
processed as outlined in the previous section and presented in Figure 5.17. The random-
20 min sample has the longest delay time, while the random-120 min sample has the 
 87
shortest delay time, for all trigger voltages. Similar to the previous case, the trigger pulse 
of 200 V pulse can not trigger the switch except for the random-120 min sample. The 
data is consistent with the electron field emission abilities of each sample.  
As introduced earlier, the delay time of the switch depends on the performance of 
the trigger material. Therefore, the CNT electrical property, characterized as the electron 
field emission characteristics, play a significant role in the switch operation and the delay 
time. If the CNT sample has longer and denser tubes as a result of longer growth time, 
the turn-on field is decreased. Therefore, the delay time is predicted to be shorter. This is 
confirmed by the field emission data in Figure 5.18, where the random CNTs with 20 min 
growth time have a turn-on field of 0.89 V/?m while the random CNTs with 120 min 
growth time have a turn-on field of 0.56 V/?m. As a result, random-120 sample has the 
lowest delay time over all, while random-20 sample has the longest delay time for all the 
different trigger pulse voltages.  
 
Figure 5.17 Delay time of different CNTs samples 
 
 88
 
Figure 5.18 Field emission characteristics of random CNT samples 
 
The aligned CNT sample with 40 min growth time is also used to trigger the 
switch. Figure 5.19 compares the turn-on field of both aligned and random CNTs samples 
in different pressure. In both case, the turn-on field of the samples increases with the 
pressure. In vacuum, the turn-on field is around 0.98 V/?m for the aligned-40 sample, 
which is larger than the turn-on field of random CNT sample with 40 min growth time in 
the same vacuum condition. On the other hand, the turn on fields of random-40 and 
aligned-40 are 2.5 V/?m and 3.8 V/?m in 20 mTorr pressure, respectively. 
 89
 
Figure 5.19 Field emission results of rand and aligned CNTs in partial vacuum 
 
Figure 5.20 compares the delay time for aligned and random CNT samples in the 
condition of 40 min growth time. The result shows that the delay times triggered by these 
two samples are comparable and the delay time based on the aligned CNT triggering is a 
little bit smaller, which seems to conflict with the field emission test results mentioned 
above. A further study reveals that these two samples have different emission area. The 
aligned CNT sample has a large area, which compensates the disadvantage of higher 
turn-on field. The turn-on fields and the sample area are listed in Table 5.3.  
 90
 
Figure 5.20 Delay time for random and aligned CNTs with 40 min growth time 
Table 5.3 Random and aligned samples 
Growth time (min) Rand(40) Align(40) 
Turn-on field(V/?m) 0.78 0.98 
Turn-on field(V/?m) in 20 mTorr 2.5 3.8 
Area(cm2) 1.17 1.26 
 
 
 91
Chapter 6  
Summary and Conclusions 
 
Triggering system is one of the important parts of a pseudospark switch. At least 
109 seed electrons are needed to initiate a breakdown for switch closing with low jitter. 
Several mechanisms can provide these seed electrons, such as thermionic emission, field 
emission, or photoemission. In this research, the electron field emission is selected as the 
trigger method. The efficiency of the electron field emission determines how well the 
plasma switch operates. 
This project presents the design and construction of a CNT triggered pseudospark 
switch that is easy-to-construct, compact, and has the ability of over 10 kV hold off 
voltage and nanosecond jitter time. The switch is triggered by electron field emission 
mechanism in which the CNTs coated cold cathode is used as the trigger electrode.  
The designed pseudospark switch is constructed with a hollow cathode, a planar 
anode, a trigger electrode, and periphery circuits. A quartz tube, 8 cm long and 2 cm in 
diameter, is used as the switch enclosure (the vacuum chamber). The u shape electrode, 
located in the center of the chamber, is pre-machined and works as the hollow cathode. A 
cupper planar anode is placed 3 mm away from the cathode. Helium at 100 mTorr 
pressure is used as background gas in the chamber. CNTs are used as the cold cathode 
 92
electron emitter material. Electrons are emitted from the CNTs surface, get accelerated in 
the electric field and initiate the breakdown.   
CNT synthesis mechanism and property optimization have been thoroughly 
studied. Random, aligned, patterned CNTs have been synthesized with high quality and 
the electrical properties have been tested in term of the field emission ability. CNT 
samples with turn-on fields as low as 0.5 V/?m are obtained for the 240 min growth. 
The CNT synthesis study also reveals the relationship between turn-on field and 
the sample growth time. The turn-on field decreases exponentially with the sample 
growth time, and eventually stabilizes at certain points. Increasing the growth time 
further does not decrease the turn-one field substantially. When the growth time is short, 
the CNTs are short in length and loose in density, which means there are less electron 
emitting spots. As the growth time increases, CNTs become longer and denser, raising 
more electron emitting spots. As a result, the turn-one field decreased. However, the 
screen effect prevents a further decrease no matter how long and how dense the tubes are. 
Understanding of this relationship benefits the optimization of synthesis process of CNTs 
and improves the cold-cathode emitter performance. 
The lifetime of the trigger sample is also tested by repeating 100 consecutive field 
emission tests. Test results show a relatively good life time of CNT samples after several 
conditioning runs without obvious degradation is. The sample life time and the stability 
are very important as in many applications.  
The designed pseudospark switch, triggered with CNTs, is tested in different 
conditions and results are presented. The switch operates at 100 mTorr pressure with a 
 93
hold off voltage of 12 kV. The applied trigger voltage varies from 200 V to 800 V. The 
current rise time is about hundreds of nanosecond. 
The delay time for a set of trigger voltages, namely 400 V, 600 V, 700 V, 800 V, 
ranges from 2.97 to 8.32 ?s. A decreasing trend of the delay time with trigger voltage is 
observed. When the trigger voltage increases, the electric field on the CNT samples 
increases. As a result, more electron emission takes place from the sample surface. 
Moreover, the emitted electrons get further accelerated in the higher electric field and 
plasma can be initiated easier. Therefore, higher trigger voltage has a shorter delay time. 
The jitter time is expressed as the standard deviation of delay time for the successive 
switch closing. Hundred of nanosecond jitter time has been obtained. 
The triggering tests based on different types of CNT samples confirm that the 
delay time depends on the performance of the trigger material, represented as the electron 
field emission ability. The random 20 min sample with a largest turn-on field of 0.89 
V/?m, has the longest delay time, while the random 120 min sample with a lowest turn-
on field of 0.56 V/?m, has the shortest delay time, for all trigger voltages. The data is 
consistent with respect to the electron field emission characteristics of each sample. 
The switch is triggered when a trigger voltage as low as 400 V applied to the CNT 
cold cathode emitter. This result is one of the differentiating characteristic of our switch 
to the ones in the literature.  That means it provides several advantages in the switch 
design and fabrication. Low trigger voltage can be sassily supplied with less isolation 
requirements, leading to a possibility of switch size reduction and simpler peripheral 
circuitry. By use of lower than 1 kV trigger voltage, one can avoid unwanted discharge in 
the switch chamber and subsequent reduction in wear and increase switch stability. 
 94
In this project, the pseudospark switch has been successfully triggered with 
carbon nanotubes. This is the first reported results in the literature. But, several issues 
still need to be investigated, for example, electron emission mechanism, the transient 
plasma property, and the variation of the switch performance in different background 
conditions and the electrode materials. Further studies, including the switch optimization, 
trigger under different conditions, such as pressure, trigger pulse shape, and CNTs with 
different characteristics, need to be investigated to better understand the exact behavior of 
CNT triggered pseudospark switches.  
 
95 
 
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