Radiation Hardness Study of AlGaN/GaN High Electron Mobility Transistors (HEMTs)
Type of DegreePhD Dissertation
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Gallium nitride (GaN) has unique inherent properties such as ionic-covalent bond, large direct bandgap, excellent thermal stability, high threshold displacement energy and higher break-down field. Also, the relatively low phonon loss and a high threshold for electron-hole pair gen-eration upon ionizing radiation make GaN and its alloys a prominent candidate for the applica-tions in a high radiation environment. Aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure system possesses a unique interface driven characteristic with high electron densi-ty and high mobility at the interface which gives additional radiation hardness to the Al-GaN/GaN system. The study of gamma-ray and proton irradiation provides better insight into device response and defect creation for practical applications of AlGaN/GaN HEMTs in radia-tion environments. In this work, a detailed investigation was performed on the direct-current (dc) electrical performance and optical characteristics of pristine and irradiated AlGaN/GaN HEMTs with 120 MRad dose of 60Co-gamma-rays (γ-rays) in one experiment, and 100 keV protons with fluences 1×1010, 1×1012, and 1×1014 protons/cm2 in another. A slight degradation of dc characteristics was observed for the devices fabricated on gamma-ray irradiated HEMT epi-layers, indicating the presence of radiation-induced defects. No additional irradiation induced strain was detected from comparing the Raman peak frequency position of pristine and irradiated samples. However, full-width-at-half-maximum (FWHM) of the Raman and near-band-edge PL peaks increased af-ter irradiation, which suggests the degradation of crystal quality. The spectroscopic photocurrent-voltage (SPIV) study with sub-bandgap and above bandgap illumination confirmed the pre-existence of sub-bandgap defects in the heterostructure and revealed the possibility of their re-arrangement or the introduction of new defects after gamma-ray irradiation. Proton irradiation-induced effects on HEMTs was studied by emulating a certain space radiation environment using relatively low energy (100 keV) proton beam. Proton irradiation-induced sub-gap traps were detected by SPIV measurements. Raman study revealed that proton irradiation had induced strain relaxation on the HEMTs epi-layers. No substantial change in the crystal quality of epi-layers was indicated from Raman and PL studies. Charge carrier density was increased for the samples irradiated with 1×1012 and 1×1014 protons/cm2 fluences, estimated via Raman spectroscopy and the charge-control model analysis. The magnitude and direction of transistor threshold voltage shift were also dependent on proton fluence. Overall, degradation of transistor output characteristics of the fabricated HEMTs was observed as the proton fluence in-creased. Based on the level of performance of the irradiated devices, it was concluded that Al-GaN/GaN HEMTs is relatively resistant to high dose (120 MRad) gamma-ray irradiation, but it can introduce additional traps or re-configure the pre-existing traps, and affect the electrical and optical characteristics of HEMTs. Additionally, the relative degree of influence on the materi-al/device characteristics by 100 keV protons was not severe. Therefore, it can be suggested that the AlGaN/GaN HEMTs have high endurance for exposure to relatively high fluences of low-energy proton beams.