The Influence of Hydrogen Plasma Passivation on Electrical and Optical Properties of Algan Samples Grown on Sapphire
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THE INFLUENCE OF HYDROGEN PLASMA PASSIVATION ON ELECTRICAL AND OPTICAL PROPERTIES OF AIGaN SAMPLES GROWN ON SAPPHIRE A.Y. POLYAKOV, M. SHIN, S.J. PEARTON*, M. SKOWRONSKI, D.W. GREVE**, J.A. FREITAS*** Department of Materials Science & Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, [email protected] * Department of Materials Science and Engineering, University of Florida, 132 Rhines Hall, Gainesville, Florida, FL 32611-6400 ** Department of Electronic and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890 *** Sach/Freeman Assoc. Inc. Landover, MD 20785, Naval Research Lab.: Contract # N0001493-C-2227 ABSTRACT Hydrogen plasma passivation effects are studied for undoped AlGaN layers grown by MOCVD. Hydrogen treatment at 200 0 C for lh led to a substantial decrease in carrier concentration accompanied by an increase in electron mobility. The magnitude of the near-band edge absorption tails also dramatically decreased after hydrogen passivation. The effect is explained by pairing of negatively charged hydrogen acceptors with positively charged native donors commonly believed to be related to nitrogen vacancies. The bond strength in such hydrogen complexes increases as the composition of AlGaN moves towards AIN, as revealed by results of post hydrogenation annealing. INTRODUCTION The material system of A1GaN/GaN/InGaN has some very interesting properties, such as wide bandgap, high saturation velocity, and high thermal conductivity, which make it very attractive for applications in optoelectronics and high frequency/high power/high temperature electronics. Nowadays most of the groups are growing AlGaN films by metalorganic chemical vapor deposition (MOCVD), predominantly on lattice mismatched sapphire substrates. Due to some relatively recent advances in growth involving deposition of low temperature AIN or GaN thin buffer layers the crystalline and electronic quality of GaN films grown by MOCVD has remarkably improved. The electron concentration decreased from 1020 cm-3 to about 1016 cm- 3 or below, the electron mobilities increased from 10 cm 2/Vs to about 600-800 cm 2 /VNs ( a comprehensive review and relevant references can be found in Gaskill et al.[l]). Improvement of the quality of AlGaN layers has also been achieved, and several groups reported having been able to grow films with low carrier concentrations of about 1017 cm- 3 [2,3] (or even semi-insulating [4]) and respectable mobilities on the order of 100 cm 2 /V.s. However, frequently undoped AlxGal-xN films with x
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