Effect of Carrier Gas on the Surface Morphology and Mosaic Dispersion for GaN Films by Low-Pressure MOCVD
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were approximately 20 nm and 50 nm for hydrogen and nitrogen carrier gases, respectively. The surface morphology of these films was studied by atomic force microscopy (AFM), using a Nanoscope III instrument. Both contact and tapping mode methods were used to obtain the AFM topographs. Contact mode micrographs were acquired employing commercial 100 lgm long triangular cantilevers with etched Si 3 N4 tips. Imaging was carried out after the film surfaces were cleaned by dipping each sample in 1M hot HCI for 10 minutes, followed by repeated rinses in ultrapure water and reagent grade methanol, and subsequent drying under a stream of N2 . Cantilevers used for tapping mode images were 125 gm long diving board type with Si tips. Typical operating parameters for the tapping mode images were 300 kHz drive frequency at 120 mV drive amplitude. The out-of-plane [{0 0 . 1 }GaN//{00 . 1}sapphirel structural coherence was ascertained from conventional 0/20 X-ray diffraction scans collected on a Phillips APD powder diffractometer employing graphite-monochromatized CuKoa radiation. The outof-plane mosaic spread for these films were measured in reflection from movingcrystal/fixed-counter 0 scans (rocking curves), employing a high-precision X-ray spectrometer equipped with a channel-cut, four-bounce germanium monochromator to provide a CuKai beam of high spectral purity and resolution. Additionally, the inplane [{10.0}GaN// {11.0}sapphire] scattering from these films were examined in transmission using a Buerger precession camera and Zr-filtered MoKa radiation. RESULTS AND DISCUSSION Atomic Force Microscopy: Surface Imaging Unnucleated Growth. As expected for a three-dimensional island growth mode [8-10], deposition of GaN onto unnucleated (00.1) sapphire yields large hexagonal prisms (Figure 1) with either hydrogen or nitrogen as the carrier gas. In the films grown in hydrogen, Figure 1 (A), these prisms range 50 to 100 prm in diameter, with a mean value clustered about 60-70 gIm. Moreover, these prism were typically 1.5 to > 4 gim in
Figure 1. AFM images (100 gm x 100 gm) for unnucleated growth in: (A) hydrogen as
the carrier gas and (B) nitrogen as the carrier gas. 262
height, but many were outside the maximum vertical and horizontal scan limits (120 gim x 4.5 gim) of the atomic force microscope. Smaller subunits on prismatic faces appear to be layered (like stacked sand dollars), with an average height of tens of nm high and on average about 400 nm across. Unnucleated films grown in nitrogen as the carrier gas also exhibit [Figure 1 (B)] large hexagonal prisms, but in this case the prisms are only 10 to 20 jim across the base and 200 to 400 nm high. Even the crystallites that appear flat have a shallow pyramidal top. Here, the smaller subunits are 120 to 145 nm across and are more crystalline in appearance that those for unnucleated growth in hydrogen and the prisms also appear to be less closely packed. Self-Unnucleated Growth. The pseudo two dimensional growth surfaces of the selfnucleated GaN films [Figure 2 (A)and (B)] are
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