Optical Band Gap Measurements of InN Films in the Strong Degeneracy Limit
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L11.22.1
Optical Band Gap Measurements of InN Films in the Strong Degeneracy Limit D.B. Haddad,1 J.S. Thakur,2 V.M. Naik,3 G.W. Auner,2 R. Naik1 and L.E. Wenger1 1 Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201 2 Department of Electrical and Computer Engineering, Wayne State University, Detroit, Michigan 48202 3 Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, Michigan 48128
ABSTRACT The optical properties of InN thin films (0.5 µm thick) grown on sapphire substrates by plasma source molecular beam epitaxy deposition have been measured in order to study the effect of electron degeneracy on the band gap measurement. X-ray diffraction measurements show that the films are wurtzite polycrystalline at a growth temperature of 325 oC, whereas a completely c-axis textured growth at a temperature of 475 oC. The Raman bands A1 (LO) and E2 are rather broad indicating the presence of a large number of structural defects. Hall effect measurements show that both the films are n-type with carrier concentrations of (8.0 ± 1.6) x 1020 cm−3 and (3 ± 0.6) x 1020 cm−3, respectively. The optical absorption data on these samples show n dependent band gap edge and a peak corresponding to plasmon due to strong electron degeneracy. The band gap absorption data were analyzed assuming a direct band gap and incorporating the Moss-Burstein shift effect. By taking into account the non-parabolic dispersion and the band-renormalization effects for the conduction band of InN, the calculated true band gap (0.7 eV) agrees with other recent measurements on high quality InN films.
INTRODUCTION Recently, the value of the optical band gap energy of InN has come under intense reinvestigation due to a disagreement between the generally accepted value of 1.9 eV 1-3 and recently reported values in the range of 0.7-1.0 eV.4-8 The smaller reported band gap values (< 1 eV) are thought to be associated with improvements in the thin film fabrication techniques leading to higher quality InN films with relatively less disorder, and are more consistent with values predicted by theoretical calculations.9 One possible hypothesis for the larger band gap energies measured in earlier InN films is that higher levels of donor impurities existed, which led to an increased concentration of electrons in the conduction band causing a strong electron degeneracy. In addition, a small effective electron mass in this system would promote a larger electron degeneracy.10 Since the optical absorption, which is traditionally used to determine the band gap value, is sensitive to both electron degeneracy and the existence of impurities, these effects on the band gap determinations must be considered in order to determine the true band gap energy. In this paper, we show that band gap determination using optical absorption data even on polycrystalline InN films yields 0.7 eV in agreement with the most recent values, after accounting for electron degeneracy and band-renormalization effects.11
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EXPERIMENT Thin InN
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