A Study of Defects and Surface Properties in AlGaN/GaN High Electron Mobility Transistors (HEMTs) Grown by MOCVD on Semi
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0955-I11-02
A Study of Defects and Surface Properties in AlGaN/GaN High Electron Mobility Transistors (HEMTs) Grown by MOCVD on Semi-Insulating SiC Substrates Yongkun Sin1, Hyun I Kim2, Paul Adams2, and Gary Stupian1 1 Electronics and Photonics Laboratory, The Aerospace Corporation, El Segundo, CA, 90245 2 Space Materials Laboratory, The Aerospace Corporation, El Segundo, CA, 90245 ABSTRACT AlGaN/GaN HEMTs (High Electron Mobility Transistors) grown on semi-insulating (SI) SiC substrates are very promising for high power, high speed, and high temperature operation with great potential for both military and commercial applications. These high performance characteristics are possible due to presence of high two-dimensional electron gas (2 DEG) charge sheet density maintaining a high Hall mobility at the AlGaN barrier/GaN buffer hetero-interface. However, reliability of AlGaN HEMTs still remains a major concern because of the large number of defects and traps present both in the bulk as well as at the surface leading to current collapse. We report on the study of defects and surface properties in MOCVD-grown Al0.27Ga0.73N HEMT structures on SI SiC substrates. Our HEMT structures consist of a 25nm thick undoped AlGaN barrier layer and a 3µm thick undoped GaN buffer layer grown on a 100nm thick AlN nucleation layer. Hall measurements showed a charge sheet density of ~1013/cm2 and a Hall mobility of ~1500cm2/V⋅sec. Both cross-sectional and plan view TEMs were employed to study defects in the heterostructures and XPS (X-ray Photoelectron Spectroscopy) and AES (Auger Electron Spectroscopy) employed to study surface properties in both GaN and AlGaN layers. DC characterization results from AlGaN Schottky diodes with Pt/Au Schottky contacts are also reported along with results from AlGaN/GaN HEMT devices. INTRODUCTION AlGaN/GaN HEMT technology has been identified to become an ideal technology for highpower and high-speed operation for both military and commercial applications 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 AlGaN/GaN HEMT technology grown on SiC substrates in recent years, this technology is still facing a number of issues: (i) there is a huge lattice mismatch between SiC substrates and GaN-based materials that generates a large number of threading dislocations, (ii) DC and RF device characteristics are very sensitive to the presence of traps at the surface as well as in the bulk [4], and (iii) poor understanding of reliability. In this paper, we report our investigation on defects and surface properties of (Al)GaN materials as well as characterization results from AlGaN Schottky diodes and HEMT devices. EXPERIMENTAL METHODS AND RESULTS Poor reliability often observed from AlGaN HEMT devices is not well unders
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