Doping of AlGaN Alloys
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Nitride-based device structures for electronic and optoelectronic applications usually incorporate layers of AlhGalj.N, and n- and p-type doping of these alloys is typically required. Experimental results indicate that doping efficiencies in AlGal-,N are lower than in GaN. We address the cause of these doping difficulties, based on results from first-principles densityfunctional-pseudopotential calculations. For n-type doping we will discuss doping with oxygen, the most common unintentional donor, and with silicon. For oxygen, a DX transition occurs which converts the shallow donor into a negatively charged deep level. We present experimental evidence that oxygen is a DX center in AiGal-,N for x>-0.3. For p-type doping, we find that compensation by nitrogen vacancies becomes increasingly important as the Al content is increased. We also find that the ionization energy of the Mg acceptor increases with alloy composition x. To address the limitations on p-type doping we have performed a comprehensive investigation of alternative acceptor impurities; none of the candidates exhibits characteristics that surpass those of Mg in all respects. INTRODUCTION
Significant theoretical and experimental attention has been devoted to doping of GaN: which dopants to use, how to increase doping efficiency, what sources of compensation may occur, etc. For practical electronic and optoelectronic devices, however, it is essential to be able to control not just doping of GaN, but also of AIGaN alloys. For instance, AIGaN alloys form the thick cladding layers in nitride-based injection lasers, and the resistivity of these layers plays a major role in the device characteristics. Most research to date has indicated that AIGaN alloys are more difficult to dope than pure GaN, and the ability to dope AIGaN alloys significantly decreases with increasing Al content. Several experimental studies have indicated a decrease in n-type conductivity of AlGal- N with increasing x. Koide et al.1 reported a decline in free electron concentration for x>0.2. For unintentionally n-type doped AIGaN, Lee et al.2 reported a rapid decrease in conductivity for x>0.4. McCluskey et al.3 found a significant decrease in conductivity for x>0.3 in unintentionally doped AIGaN samples; they were able to attribute the unintentional conductivity to oxygen. McCluskey et al. also found that intentional doping with silicon produced highly conductive material for x=0.44. Bremser et al. 4' 5also achieved intentional n-type doping with silicon up to x=0.42, but for x>0.42 addition of Si resulted in highly resistive films. a Present address: Department of Physics, Washington State University, P.O. Box 642814, Pullman, WA 99164-
2814 G 10.4 Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Research Society
The decrease in doping efficiency with increasing Al content is even more dramatic for ptype AIGaN. While p-type doping of pure GaN was originally a problem, those difficulties have largely been overcome due to the use of the Mg acceptor and the understanding of the role
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