Growth of InGaN Films by MBE at the Growth Temperature of GaN
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
In this paper we report on the growth and characterization of InGaN alloys grown by the MBE method in the temperature range of 725-800 °C which coincides with the optimum temperature of GaN film growth. EXPERIMENTAL METHODS Three series of films were grown by the ECR assisted MBE method. Details of this growth method were reported previously and in the present paper only a brief summary is provided [7,8]. C-plane (0001) sapphire substrates were first subjected to the nitridation process. This leads to a thin atomically smooth AIN layer as revealed by the RHEED pattern. In the second stage these substrates were coated with approximately a 300 A GaN buffer grown at 550 °C, which was also found to be atomically smooth [8]. The first series of InGaN alloys were grown directly onto a low temperature GaN buffer layer and were approximately I ýtm thick. The growth of the second series of InGaN alloy films was preceded by the growth of a few thousand angstroms of GaN film. The thickness of the InGaN films were between 3000-4000 A. The third series consists of a double heterostructure with the InGaN film sandwiched between two GaN films. All the films were grown at growth rates between 10-30 A/min. X-ray diffraction (XRD) measurements were carried out in a fourcircle, double crystal X-ray diffractometer with Cu-Kca radiation as the excitation source. The radiation was monochromated by a curved graphite crystal. The indium concentration in the films was determined by computing the relative shift of the InGaN Bragg peak with respect to the GaN peak and applying Vegard's law. Photoluminescence measurements were carried out with a 10mW He-Cd laser. A 0.5m grating spectrometer with a holographically blazed grating (1800 grooves/mm) was used to disperse the collected light. Absorption data was obtained from transmission measurements where a tungsten lamp was used as the source. EXPERIMENTAL RESULTS AND DISCUSSION Thick InGaN grown on a thin GaN buffer Fig. 1 shows the XRD data from thick InGaN films grown directly on a 300 A GaN buffer. The thickness of the InGaN films is 1 ýtm and the growth rate was determined to be 26 A/min. for the In0 .09Ga 0 .8 IN film and 30 A/min. for the In0 .21Ga0 .79N. The FWHM of the XRD peak was greater for InGaN films as compared to the GaN film which can be attributed to strain or domain size effects. No attempt was made to incorporate more indium under these conditions of growth. Fig. 2 shows the optical absorption and the PL spectrum of the In0 .zlGa0 .79 N sample. The energy gap, as determined from the optical absorption data, is 2.65 eV. However, the peak of the photoluminescence spectrum occurs at 2.32 eV which is 330 meV less than the bandgap. The FWHM for the PL is 220 meV. It can be seen from the absorption spectrum that the absorption edge is not sharp but has a tail implying the presence of optically active states present in the bandgap. It is speculated that these deeper states could be due to impurities, although de
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