Traps and Defects in AlGaN-GaN High Electron Mobility Transistors on Semi-Insulating SiC Substrates
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Traps and Defects in AlGaN-GaN High Electron Mobility Transistors on Semi-Insulating SiC Substrates Yongkun Sin, Erica DeIonno, Brendan Foran, and Nathan Presser Electronics and Photonics Laboratory The Aerospace Corporation El Segundo, CA 90245 ABSTRACT High electron mobility transistors (HEMTs) based on AlGaN-GaN hetero-structures are promising for high power, high speed, and high temperature operation. Especially, AlGaN-GaN HEMTs grown on semi-insulating (SI) SiC substrates are the most promising for both military and commercial applications. High performance characteristics from these devices are possible in part due to the presence of high two-dimensional electron gas charge sheet density maintaining a high Hall mobility at the AlGaN barrier-GaN buffer hetero-interface and in part due to high thermal conductivity of the SiC substrates. However, long-term reliability of these devices still remains a major concern because of the large number of traps and defects present both in the bulk as well as at the surface leading to undesirable characteristics including current collapse. We report on the study of traps and defects in two MOCVD-grown structures: Al0.27Ga0.73N HEMTs on SI SiC substrates and Al0.27Ga0.73N Schottky diodes on conducting SiC substrates. Our HEMT structures consisting of undoped AlGaN barrier and GaN buffer layers grown on an AlN nucleation layer show a charge sheet density of ~1013/cm2 and a Hall mobility of ~1500cm2/Vâ‹…sec. Deep level transient spectroscopy (DLTS) was employed to study traps in AlGaN Schottky diodes and HEMTs fabricated with different Schottky contacts consisting of PtAu and Ni-Au. Focused ion beam was employed to prepare both cross-sectional and plan view TEM samples for defect analysis using a high resolution TEM. INTRODUCTION AlGaN-GaN HEMT technology is an ideal technology for RF and microwave power amplifiers and for high voltage switches due to the high breakdown fields and excellent electron transport characteristics in GaN [1-3]. Electron drift velocity is lower in GaN under low electric fields compared to that in GaAs, but peak and saturation electron velocities are higher in GaN under high electric fields compared to those in GaAs [1]. Although significant progress has been made in performance characteristics of AlGaN-GaN HEMTs grown on SiC substrates over the last decade, long-term reliability of these devices under high electric field operation has not been reported yet. Rather, poor reliability often observed from AlGaN HEMTs is not fully understood at this time, but the presence of point defects and dislocations including threading dislocations due to a huge lattice mismatch between SiC substrates and GaN-based materials as well as traps in AlGaN barrier and GaN buffer layers is believed to play a critical role in electrical degradation of the devices [4-6]. In our efforts to understand the root causes responsible for poor reliability of these devices, we employed DLTS to investigate traps in AlGaN Schottky diodes and HEMTs as well as TEM to investigat
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