Temperature Dependence of the Absorption Band Gap Edge of GaN
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temperature. It is found that the fitting parameters were sample-dependent, suggesting that the variation of the band edge is influenced by the presence of defects and dislocations. EXPERIMENT
The GaN layers were nominally undoped and grown by MOCVD and MBE techniques on c-plane, i.e. (0001), oriented sapphire substrates. The optical absorption spectra were recorded, using CARY 05E spectrometer in conjunction with a closed cycle refrigerator. The temperature was controlled between 13 and 300 K to within + 1K. The optical absorption spectra recorded at 13 K for two samples are shown in Fig. 1. Spectra (a) and (b) were taken for two samples grown by MOCVD and MBE, respectively. An exciton sharp line is observed in spectrum (a) at 353.556 nm (3.507eV). This exciton line was observed in the entire temperature range, but its full width half maximum is increased as the temperature increases. The MBE sample, on the other hand, exhibits a spectrum with a broad exciton line at 356.33 nm (3.479 eV). Spectrum (c) was obtained for a GaN sample grown by sputtering technique and plotted for comparison. The band edge absorption is almost washed out in this sample. DISCUSSIONS The band edge absorption of GaN samples is studied as a function of temperature. The result is shown in Fig. 2. It is well known that the band edge of any III-V semiconductor becomes less steep (less abrupt) as the temperature is increased. This phenomena causes a difficulty in 553 Mat. Res. Soc. Symp. Proc. Vol. 395 0 1996 Materials Research Society
(0) (Jiown by MOCVD (b) Grown by MII.(c) Grown by spatterinig I= 13K
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Wavelength (nni) Fig. 1. The absorbanceas afunction of photon energy at temperature = 13 K. Spectrum (a), (b), and (c) were taken for samples grown by MOCVD, MBE, and sputtering techniques, respectively. The absorptionpeak in spectrum (a) at 353.556 nm is due to an exciton line in the MOCVD GaN Layer. determining the band edge as a function of temperature, especially above 50 K where the exciton lines are usually washed out. However, we found, in the present study, that the exciton line in the 0 MOCVD GaN samples is still present, even at room temperature in agreement with Amato et al.1 We defined the band gap of this sample by the exciton peak energy. The result is shown in Fig. 2 as the solid circles. However, since the exciton line is not that sharp in the MBE GaN sample [Fig. 1, spectrum (b)], the band gap of this sample is defined in a different method in order to minimize the error in the band gap energy. This method is based on the calculation of the first derivatives of the spectra recorded as a function of temperature. Two spectra recorded at 13 and 300 K (lower panel) and their derivatives (upper panel) were selected and plotted in Fig. 3. It
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Fig. 2. The band gap energy variation as a function of temperature. Solid circles and squares were takenfor MOCVD and MBE samples, respectively. The solid
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