Temperature Dependence of the Optical Pro perties for InN Films Grown by RF-MBE
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Temperature Dependence of the Optical Properties for InN Films Grown by RF-MBE Y.Ishitani1 2 3, K.Xu3, W.Terashima1, H.Masuyama1, M.Yoshitani1, N.Hashimoto1, S.B.Che1 2 3, and A.Yoshikawa1 2 3 Dep. of Electronics and Mechanical Engineering Chiba Univ. 1, VBL Chiba Univ. 2, Japan Science and Technology Agency3 1-33 Yayoicho, Inageku, Chiba, 263-8522, Japan
ABSTRACT InN epitaxial layers are grown on sapphire substrates. The investigated samples have electron concentration in a range of 1.8 ×1018 − 1.1 ×1019 cm-3. Optical reflection and transmission measurements are performed. The plasma edge energy position in the spectra is constant in a measurement temperature range of 5 − 300 K. The reflection and transmission spectra are calculated on the basis of the LO phonon-electron coupling scheme and non-parabolic conduction band structure. From this analysis we find that the observed absorption edge is attributed to valence band to conduction band transition rather than the valence band to defect (impurity) band transition, and the intrinsic bandgap energy of 0.64 (±0.03) eV. energy increases by 40 – 50 meV as the temperature decreases from 295 to 10 K.
This bandgap
INTRODUCTION Recently, the bandgap energy (Eg) of InN was found to be less than 1 eV.[1] The applicable optical device wavelength range of InN-related alloy was proposed to cover a wide range of infrared to ultraviolet. As for physical properties Wu et al. reported the effective electron mass as a function of electron concentration Ne, and estimated the value of 0.07m0 for the intrinsic material (m0 : free electron mass ).[2] Their estimation was based on the infrared (IR) reflectance spectra around the plasma edge for the film thickness of 0.2 – 7.5 µm. Several researchers analyzed the spectra by a phonon-plasmon coupling scheme, and reported the effective mass values for various Ne.[3] [4] It was found that for samples with small thickness such as less than 1µm the effect of substrates on the spectra can not be neglected. We reported reflectance spectra at room temperature and 5 K for samples with Ne of 1.1×1019 − 4×1018 cm-3
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and the thickness of 2 – 5.5 µm. Plasma edge energies of these samples were found to be independent of temperature.[4] The Eg value of 0.64 eV for a sample with Ne of 3.5×1017 cm-3 was reported by Wu et al. from transmission measurement.[5] The increase in the transition energy with Ne was attributed to Burstein-Moss effect. However there is no report for the validity that the observed absorption edge originates from the valence to conduction band transition. There are a few studies for the possibility that the observed absorption edge is caused by the valence band to the impurity or defect band transition. The origin of the absorption edge remains unclear in several points of view such as the transition probability, density of states, and chemical potentials. Some researchers suggested the superiority of InN in a view point of the application to optical devices on the basis of the independence of PL peak energy on
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