Electronic and Optical Properties of Energetic Particle-Irradiated In-rich InGaN
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Electronic and Optical Properties of Energetic Particle-Irradiated In-rich InGaN S.X. Li,1,2 K.M. Yu,1 R.E. Jones,1,2 J. Wu,1 W. Walukiewicz,1 J.W. Ager III,1 W. Shan,1 E.E. Haller,1,2 Hai Lu,3 William J. Schaff, 3 and W. Kemp4 1
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA 94720 3 Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853 4 Air Force Research Laboratory, Kirtland Air Force Base, Kirtland AFB, NM 87117 2
ABSTRACT We have carried out a systematic study of the effects of irradiation on the electronic and optical properties of InGaN alloys over the entire composition range. High energy electrons, protons, and 4He+ were used to produce displacement damage doses (Dd) spanning over five orders of magnitude. The free electron concentrations in InN and In-rich InGaN increase with Dd and finally saturate after a sufficiently high Dd. The saturation of carrier density is attributed to the formation of native donors and the Fermi level pinning at the Fermi Stabilization Energy (EFS), as predicted by the amphoteric native defect model. Electrochemical capacitance-voltage (ECV) measurements reveal a surface electron accumulation whose concentration is determined by pinning at EFS. INTRODUCTION Among the group III-nitrides, InN, a narrow bandgap semiconductor [1, 2], is certainly the least studied. Even basic parameters, such as its direct bandgap value, were the subject of some controversy [3-8], it is now clear that InN a direct gap of of ~0.7eV, in contrast to the previously believed 1.9eV. In many instances the larger reported bandgap values can be understood by a significant Burstein-Moss shift due to the high free electron concentration in the samples [9]. The narrow bandgap of InN has opened up many new possible applications, such as InGaN tandem solar cells, inspired from the almost perfect match of InGaN bandgap span with the solar spectrum [10]. To test the irradiation hardness of InGaN, we have carried out a systematic study of the effects of irradiation on the electronic and optical properties of InGaN alloys over the entire composition range from InN to GaN. The irradiation, which produces mostly native point defects, also reveals the origin of the tendency of InN to be n-type, as predicted by the amphoteric defect model [11, 12]. EXPERIMENTAL DETAILS The epitaxial InN and In1-xGaxN thin films (310-2700 nm thick) used in this study were grown on c-sapphire substrates by molecular beam epitaxy (MBE) with GaN as the buffer layer [13]. The initial free electron concentrations in these samples range from the low 1018 cm-3 to low 1017 cm-3 and the mobility ranged from 7 cm2/V·s (x = 0.76) to above 1500 cm2/Vs (x = 0). In addition, a GaN sample (3 µm thick with an electron concentration ~7.74×1017 cm-3) and a
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GaAs sample (~10µm thick with an electron concentration ~8×1016 cm-3) were also included in this study. The sam
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