Role of Hydrogen and Hydrogen Complexes in Doping of GaN
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ABSTRACT We have calculated electronic structure, energetics and migration for hydrogen and hydrogen complexes in GaN employing state-of-the-art first-principles calculations. Using these results in combination with previous detailed investigations about native defects we have calculated the concentration of hydrogen and dopants for different growth conditions. Our results reveal a fundamental difference in the behavior of hydrogen in p-type and ntype material. In particular, we explain why hydrogen has little effect on donor impurities and why H concentrations are low in n-type GaN. We discuss why hydrogen is beneficial for acceptor incorporation in GaN, and identify the limitations of this process. INTRODUCTION Hydrogen is a well known impurity which plays an important role in semiconductors by compensating or passivating native defects and impurities, saturating dangling bonds or forming complexes. In high-temperature growth techniques (such as metalorganic vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE)] which are commonly employed for GaN growth hydrogen is highly abundant. Several experimental investigations indicate that hydrogen plays an important role in the doping of GaN: Nakamura et al. demonstrated that Mg-doped and initially semiinsulating GaN can be made p-type by thermal annealing in vacuum and nitrogen ambient but not in NH 3 or H ambient.[1] Furthermore, Nakamura et al. showed that the activation of the Mg acceptors can be reversed by annealing in a hydrogen ambient,[1] revealing the crucial role played by hydrogen. Based on these observations Van Vechten et al. suggested that hydrogen enables p-type doping by suppressing compensation by native defects.[2] These authors went on to propose the incorporation (and subsequent removal) of hydrogen as a general method for improving p-type as well as n-type doping of wide-band-gap semiconductors. The Van Vechten model highlights the important role of hydrogen, but leaves various issues unexplained, such as the lack of hydrogen incorporation in n-type GaN, and the success of p-type doping (without post-growth treatments) in MBE (molecular-beam epitaxy). In the present paper we summarize the properties H exhibits in GaN and discuss how H affects doping in GaN. In particular, we explain why H is beneficial for p-type doping but has little effect on donor impurities. Based on these results we identify the specific conditions under which H is beneficial and discuss the limitations of this process.
619 Mat. Res. Soc. Symp. Proc. Vol. 423 ©1996 Materials Research Society
METHOD Employing first-principles density-functional theory we have calculated the total energy surface, electronic structure and atomic geometry for hydrogen in all relevant charge states (H+, H0 , H-).[3] 32-atom supercells were used and atomic relaxation is fully taken into account. The calculations were performed for cubic GaN which has a higher symmetry than the wurtzite structure. As shown in Ref. [4] the wurtzite and the cubic phase show nearly equivalent formation ener
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