Doping of AlGaN Alloys
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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G10.4 (1999) ABSTRACT Nitride-based device structures for electronic and optoelectronic applications usually incorporate layers of AlxGa1-xN, and n- and p-type doping of these alloys is typically required. Experimental results indicate that doping efficiencies in AlxGa1-xN 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 AlxGa1-xN 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 AlGaN alloys. For instance, AlGaN 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 AlGaN alloys are more difficult to dope than pure GaN, and the ability to dope AlGaN alloys significantly decreases with increasing Al content. Several experimental studies have indicated a decrease in n-type conductivity of AlxGa1-xN with increasing x. Koide et al.1 reported a decline in free electron concentration for x>0.2. For unintentionally n-type doped AlGaN, 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 AlGaN 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 991642814
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