Silicon Carbide/Aluminum Oxide Field-Effect Transistors
Type of DegreePhD Dissertation
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4H-Silicon Carbide is a wide band gap semiconductor with attractive physical properties for high power, high-frequency devices, and electronics that operate under harsh environments that are not accessible to conventional silicon devices. A significant challenge in 4H-SiC metal oxide semiconductor field-effect transistors (MOSFETs) is the poor channel conductance due to the trapping of carriers by high-density of near-interface traps at the 4H-SiC/SiO2 interface. Many trap passivation methods have been researched as a solution and currently, nitridation of the interface via processes such as nitric oxide (NO) annealing is the most reliable approach. The nitrided SiO2/4H-SiC interfaces are still far from ideal and alternate approaches are desirable. One such approach is the deposition of high-k dielectrics which has added advantages over conventional SiO2. Among these alternative dielectrics, atomic layer deposition (ALD) of Al2O3 has shown promising results in the literature. The focus of this thesis is the study of ALD Al2O3/4H-SiC interfaces using systematic process variations, device fabrication and electrical characterization. Among the electrical characterization methods used to evaluate interface trap densities (Dit), constant capacitance deep level transient spectroscopy (CCDLTS) is a method capable of differentiating trap types in MOS devices. Two signature near interface oxide traps named O1 and O2 are typically detected for the SiO2/4H-SiC interfaces by CCDLTS. In this work, for the first time, it was found that such traps are absent in Al2O3/4H-SiC interfaces formed on the H-terminated 4H-SiC surface. This strongly indicates that the O1 and O2 traps are inherent to SiO2/4H-SiC interfaces. This result motivated further investigation where a systematic study of the effect of 4H-SiC surface treatments prior to the deposition of Al2O3 by ALD was conducted. This is the first comprehensive study where Al2O3/4H-SiC MOSFETs were fabricated to analyze the dependence of the channel mobility on the surface treatments. Among the studied surface treatments, H2 annealing at high temperature prior to ALD was found to result in reduced Dit and impressive channel mobility along with improved stability for the Al2O3/4H-SiC MOS devices. The most likely reason for this is that H2 annealing results in a Si-H terminated 4H-SiC surface, which in turn leads to a more uniform nucleation of the ALD Al2O3 thin film, resulting in a better SiC/Al2O3 interface. In addition, it was also found that similar to SiO2/4H-SiC, nitridation of the Al2O3/4H-SiC interface results in trap passivation and possible surface doping by N. In this work, interfacial nitridation was carried out by performing sacrificial oxidation of SiC in NO. This results in a sub-nm thick SiON layer on top of SiC and subsequent H2 annealing of this surface prior to Al2O3 deposition leads to further improvements. Al2O3/4H-SiC MOSFETs fabricated with this process resulted in peak field-effect mobility of 52 cm2/Vs which is 2x higher channel electron mobility compared to conventional nitrided SiO2/4H-SiC MOSFETs. The channel mobility results in this work are very encouraging for the application of ALD Al2O3 on SiC from the point of view from channel conductance. However, it was observed that the large dielectric leakage currents associated with the defects in the bulk of the ALD thin films is the biggest challenge that need to be overcome. In this work, it was conclusively demonstrated that 4H-SiC surface treatments prior to ALD are key to the optimization of MOS interfaces using deposited dielectrics. The Hydrogen and Nitrogen based surface treatments developed in this work can be applied to other deposited dielectrics in the future as well.