Correlation of surface morphology and optical properties of GaN by conventional and selective-area MOCVD
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
temperature reached -I020 0 C when the GaN growth took place. Film morphology was characterized by scanning electron microscopy (SEM). The optical properties were examined by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. PL measurements were taken using a 325 nm HeCd laser as the excitation source and detected by a GaAs PMT. The CL was collected by a parabolic mirror, and detected by a GaAs PMT. RESULTS AND DISCUSSIONS
1. PL, CL and CL WI study of GaNfilms grown on different buffer layers Under the experimental conditions described in detail in our previous paper,2 a single buffer layer of GaN leads to a opaque (hazy) surface with many hexagonal pit-like defects of sub-plm size distributed uniformly across the whole surface, as shown by the scanning electron microscopy (SEM) image in Fig. Ia. A single buffer layer of AIN yields an extremely non-
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Fig.l SEM images (60 gtm x 60 ptm) for a 0.7 grm thick GaN film grown on (a) an optimized single GaN buffer layer and (b) a double buffer layer consisting of GaN and AIN. The SEM image of the GaN film grown on a single AIN buffer layer was similar to (a) in the center area and (b) in the outer area. uniform surface, with morphology ranging from defect-free in the outer areas to hexagonal defects in the center. In contrast, the double buffer layer structure, consisting of 200 A of GaN and 500 A of AIN, leads to near featureless SEM image (Fig. Ib). When the thickness is increased to 3 p•n, the SEM image becomes entirely featureless. In order to study the correlation of morphology with emission centers microscopically, we have mapped out the GaN films using cathodoluminescence wavelength imaging (CLWI). To our knowledge, this is the first report on CLWI of GaN films. In CLWI, the wavelength at which there is a peak in the intensity of a luminescence spectrum, is determined as a function of spatial position, and a color or gray scale image directly mapping these wavelengths is generated.4 CLWI has been found particularly useful for the characterization of strained-layer systems. '6 . We have performed preliminary CLWI study of our GaN films grown on sapphire substrates. In general, inhomogeneous patterns are observed using CLWI for GaN films including the ones grown on the double buffer layer, indicating the optical non-uniformity of the film. Shown in Fig. 2a and Fig. 2b are the CL images taken at the near-band-edge emission wavelength (370 nm) for the films in Fig. la and Fig. Ib, respectively. As can be seen that the inhomogeneity, evidenced as intensity contrast in the image, increases as the film surface morphology becomes rougher. The dark spots
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in Fig.3a correspond to the pits in the SEM images while no obvious correlation can be found for the less bright spots in Fig. 2b. Fig. 2 (a) and (b) are CL images (45 jm x 45 prm) taken at the near-bandemission edge wavelength (370 nm) for the films in Fig. la and Fig. lb. respectively.
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We have also performed
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