Defect Complexes and Non-Equilibrium Processes Underlying the P-Type Doping of GaN
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ABSTRACT It is well known that hydrogen plays a key role in p-type doping of GaN. It is believed that H passivates substitutional Mg during growth by forming a Mgs-N-Hi complex; in subsequent annealing, H is removed, resulting in p-type doping. Several open questions have remained, however, such as experimental evidence for other complexes involving Mg and H and difficulties in accounting for the relatively high-temperature anneal needed to remove H. We present first principles calculations in terms of which we show that the doping process is in fact significantly more complex. In particular, interstitial Mg plays a major role in limiting p-type doping. Overall, several substitutional/interstitial complexes form and can bind H, with vibrational frequencies that account for hitherto unidentified observed lines. We predict that these defects, which limit doping efficiency, can be eliminated by annealing in an atmosphere of H and N prior to the final anneal that removes H. INTRODUCTION GaN is the most promising wide-gap material for blue-green optoelectronics, but further improvements are needed to enhance performance and reliability for commercial applications. A major issue is to increase p-type conductivity. It has been established that H enhances the incorporation of dopants such as Mg, but it must be removed by post-growth annealing to activate the dopants.[1,2] In general, only a relatively small fraction of the total Mg is activated.[3,4] Theory has offered a simple account of the process[5,6]: In the absence of H, Mg shallow acceptors are compensated by N vacancies (VN+) and Ga interstitials (Gai+++), both of which are donors and have low formation energies in p-type material.[7] When H is present, it passivates substitutional Mg (MgGa-) by forming a MgGa-N-H complex.[5] The Fermi level rises to the midgap region and the formation of VN+ and Gai+++ is suppressed. Subsequent annealing removes H and activates MgGa-. Theory predicted[5] the vibrational frequency of the MgGa-N-H complex at 3360 cm-1 and, experiments[4] have since found a line at 3125 cm-1 whose intensity decreases during annealing. Nevertheless, there are unambiguous indications that the doping process is more complex. The temperature needed to remove H (~700oC) is much higher than expected from the calculated energy to break the MgGa-N-H bond (~1.5 eV).[5] Photoluminescence, infrared, and Raman data show clearly that other Mg-related defects are present.[1,3,4] In particular, in material grown by molecular beam epitaxy (MBE), where typically only 10% of Mg is electrically active, infrared and Raman lines in the 2000-cm-1 range have been attributed to direct MgH bonds.[3] Theory so far has not offered any potential candidates for these complexes. Here, we report first-principles calculations of key defect reactions and show that p-type doping is in fact a significantly more complex process. In particular, we show that, in addition to MgGa-N-H, interstitial Mg (Mgi) and its complexes play a major role in controlling the process. In Ga-rich growth c
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