Electron Transport Properties of InN
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Electron Transport Properties of InN R.E. Jones1,2, H.C.M. van Genuchten2, S.X. Li1,2, L. Hsu3, K.M. Yu1, W. Walukiewicz1, J.W. Ager III1, E.E. Haller1,2, H. Lu4 and W. J. Schaff4 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 2 Department of Materials Science and Engineering, University of California, Berkeley, CA 3 General College, University of Minnesota, Minneapolis, MN 4 Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY ABSTRACT High-energy particle irradiation has been used to control the free electron concentration and electron mobility in InN by introducing native point defects that act as donors. A direct comparison between theoretical calculations and the experimental electron mobility suggests that scattering by triply-charged donor defects limits the mobility in irradiated samples across the entire range of electron concentrations studied. Thermal annealing of irradiated films in the temperature range 425oC to 475oC results in large increases in the electron mobility that approach the values predicted for singly-ionized donor defect scattering. It is suggested that the radiation-induced donor defects are stable, singly-charged nitrogen vacancies, and triply-charged, relaxed indium vacancy complexes that are removed by the annealing. INTRODUCTION InN has generated significant attention in recent years because of its potential to extend the bandgap range of the III-nitride alloys into the infrared, which is attractive for electronic and optoelectronic applications such as high-efficiency photovoltaic cells. However, many essential properties of InN are still unknown; in particular, the role of native defects in determining the material’s transport properties is not yet well-characterized. It has been shown recently that native defects are the source of the inherent n-type conductivity of InN [1]. According to the amphoteric defect model (ADM), the formation energy of acceptor or donor defects depends on the position of the Fermi level (EF) with respect to the Fermi stabilization energy (EFS), which is the average energy level of native defects. The formation energy of donor (acceptor)-like defects in a semiconductor is greatly reduced for EF < EFS (EF > EFS) [2]. Since EFS is located high in the conduction band in InN [3], predominantly donor-like native defects are formed during high-energy particle irradiation of films with free electron concentrations (n) below mid-1020 cm-3. It can be argued, by the analogy with the GaAs case [2], that the donor defects include nitrogen vacancies (VN), and relaxed In vacancies (VIn). To form a relaxed VIn, one of the nearest neighbor N atoms moves towards the In vacancy site and transforms the VIn acceptor into a relaxed NIn + VN donor complex. The strong proclivity for forming donor-like defects in InN allows for the use of highenergy particle irradiation to achieve predictable, n-type doping by the controlled introduction of native defects. By using different doses of 2 MeV H+ and He+
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