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T. Strasser, W. Schattke
Gallium nitride has attracted considerable interest because of its applicability for blue light emitting diodes and lasers. To improve their technological potential a detailed understanding of its electronic and geometric surface structure is required. In 1997 Dhesi et al. [1] presented an extensive experimental examination of the electronic structure of the GaN(0001) surface by angle resolved photoelectron spectroscopy. The sample were grown using molecular-beam epitaxy obtaining a surface with a sharp 1x1 low-energy electron diffraction pattern (LEED). In the spectra a non dispersive feature was identified as emission from a surface state near the valence-band maximum. Using scanning tunneling microscopy and total energy calculations within the local density functional theory, Smith et al. [2] predicted an Ga adlayer structure for the 1x1 surface. Using this model as a starting point, we calculated the photocurrent in normal and off-normal emission within the one-step model. By comparing our detailed interpretation with the experiment by Dhesi et al. we could connect the geometric and electronic structure to the measured photocurrent [3]. Especially, the surface state pointed out near the upper valence band edge could be identified in theory and experiment, which underlines the plausibility of the surface model by Smith et al. Later, a second experimental examination by angular resolved photoemission by Chao et al. [4] yielded rather different spectra. The GaN thin films where grown by metal-organic chemical-vapor deposition, which also show a very sharp 1x1 LEED pattern. Two new surface bands were measured, one of them being highly dispersive throughout most of the valence band. Wang et al. [5] performed a systematic ab initio examination of the formation energy and band structure of several clean and Ga and N covered 1x1 GaN(0001) and GaN(000-1) surface configurations. Only the surface band structure of the clean nitrogen terminated GaN(000-1)-(1x1) is in favorable agreement with the measured data. But still theory and experiment show larger deviations, such as near the K point, where the measurements show a strong dispersing band which has no counterpart in theory. Therefore it seems necessary to use the potentials of one-step photoemission calculations to clarify this deviations. Starting from the ab initio surface models by Wang et al. we calculate and analyze the photocurrent in the sense of matrix elements, density of states and final band structure and compare it with the measured data going beyond a simple comparison between experimental and theoretical surface band structures. [1] S.S. Dhesi, C.B. Stagarescu, K.E. Smith, D. Doppalapudi, R. Singh and T.D. Moustakas; Phys. Rev. B 56, 10271 (1997). [2] A.R. Smith, R.M. Feenstra, D.W. Greve, J. Neugebauer and J.E. Northrup; Phys. Rev Lett. 79, 3934 (1997). [3] T. Strasser, C. Solterbeck, F. Starrost, and W. Schattke; Phys. Rev. B 60, 11577 (1999). [4] Y.-C. Chao, C.B. Stagarescu, J.E. Downes, P. Ryan, K.E. Smith, D. Hanser, M.D. Bremser, and R.F. Davis; Phys. Rev. B 59, R15586 (1999). [5] Fu-He Wang, Peter Krueger, and Johannes Pollmann; Phys. Rev. B 64, 035305 (2001). |