Characterization of Gallium Nitride Grown on (0001) Sapphire by Plasma-Enhanced Atomic Layer Epitaxy
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TABLE I
Typical growth condition for PEALE
substrate pretreatment temperature GaN buffer layer thickness GaN buffer layer growth temperature GaN top layer growth temperature TMG flow rate N2 flow rate H2 flow rate chamber pressure typical film growth rates overall film thickness
650 or 780°C 250-500A 430-580°C 580-750°C 6-14 sccm 250-400 sccm 5-100 sccm 5 torr 1250A/hr 0.5 gm.
A Siemens D5000RA X-ray diffractometer was used to determine the structure (wurtzite or zinc blende) of the grown films. The 0-20 scan indicates that the thin film plane is parallel to the substrate surface since both the film peak and the substrate peak appear in the same scan. The same diffractometer was also used to perform a phi scan to identify the thin film orientation with respect to the substrate by examining asymmetric planes such as (1014). In our samples, we have chosen the (1014 ) GaN planes and (101 10 ) A12 0 3 planes for the phi scan. An X-ray rocking curve method was also used to further characterize the overall film quality using a Double Crystal Diffractometer (DCD), equipped with a "four bounce" Si-Ge Bartels type monochromator. The full width at half maximum (FWHM) of the (0002) GaN Bragg peak was obtained which provides a quantitative assessment of the film quality. A Topcon 002B Transmission Electron Microscope equipped with LaB6 filament and operated at 200 keV was also utilized to analyze the GaN films. The samples were examined in both plane and cross sectional view. RESULTS AND DISCUSSION A. Polycrystalline films Based on the X-ray 0-20 scan, we have found that all of the GaN thin films grown by the PEALE method on sapphire (0001) have the wurtzite structure with its c-axis perpendicular to the substrate surface. Because this scan alone can not give us enough information to identify whether the film is a highly oriented polycrystalline or a single crystal, the X-ray phi scan was also used. Most of the films we have grown are polycrystalline and exhibit an in-plane rotation with up to three different orientations. Since oc-GaN has a hexagonal structure, six peaks will appear at regular intervals of 60' corresponding to the six I 1014 ) planes in a 360°-phi-scan. In Figure la, there are three six-peak sets within the 360' scan. Each set of peaks is approximately 20° apart. This indicates that the highly c-oriented polycrystalline GaN films have grains with three different in-plane orientations. Also shown at the bottom of the same figure is the phi scan of the substrate which contains only three peaks within the 360' scan. The grains with the a orientation are rotated 30' with respect to the substrate and have the orientation relationship [0001]GaN//[0001]A120 3 , [1100]GaN//[1210]A1 2 0 3 and [2110]GaN//[1100]A1 20 3. When we increased the pretreatment temperature from 650°C to 780'C, the intensity of peak set a started to increase. The result of the phi scan is illustrated in Figure lb. The intensities of peak sets b and c become smaller, which indicates that the volume fraction of grains with these two orientation
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