Cubic InGaN Grown by MOCVD
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indium source, respectively. The reaction chamber maintains low pressure (76 torr) during growth. A GaN buffer layer was first deposited at 550 °C for 2 min, then the GaN epilayer with a thickness of 0.7 p.m was deposited at a temperature of 850 TC. Finally, the InGaN layer with a thickness of 0.3 ýtm was deposited on the GaN layer. X-ray diffraction (XRD) were used to determine the lattice constant and In content of InGaN films. The XRD measurements are performed using a synchrotron radiation with a wavelength of 1.535 A. Room-temperature photoluminescence (PL) measurements were performed on all samples using a computercontrolled PL system. The excitation source is a He-Cd laser which operates at a wavelength of 325 nm and produces 25 mw of output power. A setup of a monochromator and a photomultiplier is used for PL measurements. Result and Discussion
Figure 1 shows XRD profiles for several samples (a'20 mode). The positions of GaN (002) and (004) peaks are at 19.98 0 and 42.92 o, respectively. The InGaN diffraction peaks of (002) and (004) are separated clearly from GaN diffraction peaks of (002) and (004). It indicates the cubic phase InGaN films have been achieved. No diffraction peaks correspond to hexagonal phase GaN and InGaN are observed. Different diffraction-peak positions of InGaN indicate different indium composition x for InGaN films. Using Vegard's law and assuming a lattice constant of InN to be 0.498 nm, we calculated the indium alloy composition x from the lattice constant of InGaN crystals determined by XRD. The x values for different samples are indicated in Fig. 1. The highest In composition we have achieved here is 17%. The InGaN films show mirror-like surfaces.
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Figure 1. X-ray diffraction profiles (di20 mode) for cubic InGaN films.
Figure 2 shows the room temperature PL spectra for the samples shown in Fig. 1. Except the cubic GaN band edge emission at 387 nm, in all the spectrum, there is a strong peak at 427 nm to 450 nm which we assign to cubic InGaN band edge emission. The FWHM for PL peaks of our InGaN films range from 153 mev to 220 mev, with emission wavelength range from 427nm to 450nm. The InGaN peaks in the spectra are quite narrow even compared with hexagonal InGaN. The GaN peak is relatively weak reflecting a strong absorption from the 0.3 Atm thick InGaN film. The yellow band in our samples are also very weak. The quality of the films becomes better while the indium composition x decreased as indicated by the changes of FWHM.
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Wave length (angstrom) Figure 2. PL spectra of InGaN samples. We have plotted the PL peak energies of our InGaN samples as a function of the In composition, as shown in Fig. 3. The dashed lines in the figure are linear connections between the two points corresponding to GaN (3.2eV) and InGaN (
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