Surface-Related Photoluminescence Effects in GaN
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Surface-Related Photoluminescence Effects in GaN M. A. Reshchikov, M. Zafar Iqbal,a D. Huang, L. He, and H. Morkoç Virginia Commonwealth University, Richmond, VA 23284, U.S.A. a On leave in VCU from the Quaid-i-Azam University, Islamabad, Pakistan ABSTRACT Photoluminescence (PL) from GaN epilayers is found to be sensitive to the ambient atmosphere and length of UV exposure. We studied the effect of UV illumination in different ambients including air, oxygen, nitrogen and hydrogen gases on room-temperature PL of GaN grown on sapphire by molecular beam epitaxy. In some samples the PL intensity increased markedly in vacuum as compared to excitation in air, whereas in others it decreased appreciably. While air and oxygen showed strong reversible variation of the PL intensity as compared to vacuum, nitrogen and hydrogen atmospheres led to a very small change. In some samples we observed a shift of the yellow luminescence band with change of ambient, in others no shift was detected. PL intensity also changed during UV irradiation when the sample was in air ambient. Possible reasons for our observations are discussed. INTRODUCTION Surface quality and surface-related effects are of obvious interest for semiconductor device applications. Photoluminescence (PL) from GaN has been reported to show strong dependence on the ambient atmosphere around the samples during excitation. Air evacuation, for instance, is known to cause a strong enhancement of the luminescence and a large decrease in photoreflectance as compared to excitation in air [1,2]. On the other hand, exposure to air at room temperature for several days led to reduction of leakage currents, enhancement of barrier heights, and improving ideality factors in AlGaN Schottky diodes [3]. These effects are believed to be related to the surface states. It is known that due to spontaneous polarization a strong upward band bending is expected at the Ga-face, whereas at N-face surface positive ionized donors largely compensate the downward band bending in n-type GaN [4,5]. The band bending is also affected by surface structural defects, broken bonds, oxidation, formation of overlayers of Ga atoms at the surface [6], and presence of adsorbates. Nienhaus et al. [7] have demonstrated that adsorption of oxygen increases the ionization energy and electron affinity of GaN and AlxGa1-xN surfaces. Furthermore, Bermudez [8] has found that oxygen removes surface states, resulting in reducing the band bending compared to clean GaN surface. As a result, band bending at the surface is reduced from 0.9 to 0.5 eV. In case of the practical surface, the band bending was found to be about 0.4 eV [8]. Oxidation, adsorption of foreign atoms, existence of broken bonds, etc. depend on the growth conditions and surface treatment. Thus we may expect different band bending near the surface, as well as existence of different surface states in GaN prepared in different conditions. In this letter, we report detailed PL study of GaN under different ambient conditions, including air, vacuum, oxyge
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