High Pressure Study of III-Nitrides and Related Heterostructures
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termined their pressure coefficients. Our results provide a direct measure of the pressure dependence of the direct band gap at F point of the samples. We found the pressure coefficients for the InxGa,-xN and AlxGa,-xN alloys to be slightly dependent on the alloy compositions. EXPERIMENT All samples used in this work were nominally undoped single-crystal epitaxial films grown by MOCVD using (0001) sapphire as substrates. The GaN samples were grown at a temperature around 1040 0 C with 50-nm AIN buffers. The InxGal-xN alloy layers were deposited at a temperature around 800 0 C on 2-jim thick GaN layers which were deposited on sapphire substrates before the growth of the alloys. The thickness of the InxGa,.N layers is typically around a few thousand angstroms (A). The AlxGa,-,N alloys were directly grown on sapphire substrates. The alloy compositions for the samples were determined by X-ray diffraction and optical transmission measurements. The pressure-dependent photoluminescence (PL) measurements were carried out using gasketed diamond anvil cells (DAC). The pressure medium is a 4:1 methanol/ethanol mixture. In order to accommodate the limited dimensions of the DAC sample space, small sample chips with sizes of -200x2O0x30 jtm 3 were prepared by mechanical polishing and cutting. For the measurements performed at low temperatures, the cells were mounted onto the cold finger of a closed cycle refrigerator and cooled down to desired temperatures. PL spectra were measured using an experimental setup consisting of either a HeCd laser (325 nm) or a frequency-doubled Ar÷ laser (244 nm) as an excitation source and a 1-m double-grating monochromator connected to a photon-counting system. The applied pressures were determined by the standard method of monitoring the shift of the ruby RI line. RESULTS AND DISCUSSION Pressure dependence of exciton emissions and the broad yellow emission band in GaN The PL spectra of the GaN samples used in this work at atmospheric pressure were dominated by two sharp and strong near-band-edge exciton emission luminescence lines. A weak and broad emission band in the yellow spectral region could be also observed. Under applied pressure, these exciton spectral features and the broadband emission in the yellow spectral region were found to shift towards higher energy as the GaN band gap increases. Typical PL spectra of the GaN measured for three different pressures at 10 K is shown in Fig. 1. The inset of the figure details the spectral features of excitons associated with the direct band-gap -C-Fv transitions in GaN. Temperature-dependent PL measurements performed on the sample suggest that the strongest emission line at 3.4838 eV (BX) is due to the radiative recombination of excitons bound to neutral donors and the one at 3.4905 eV (FXA), together with the weak emission shoulder (FXB) on the higher energy side, can be attributed to the intrinsic free-exciton emission. The intensity of BX emission was found to decrease much faster with increasing the sample temperature than that of the other two s
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