High-Quality III-V Nitrides Grown by Metalorganic Chemical Vapor Deposition
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183 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
-
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-250
0
250
50o
Angle (arc sec)
Figure 1: Five-crystal (0002) X-ray rocking curves for various GaN epitaxial layers grown by MOCVD on (0001) A12 0 3 substrates. 200 1801 S
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0.7
0.8
0.9
Film Thickness (pm)
Figure 2:
X-ray rocking curve FWHM values of GaN/A12 0 3 heteroepitaxial films vs. layer thickness.
The crystalline structure of selected films is also analyzed using high-resolution triple-axis Xray diffractometry. Shown in Figure 3 are the iso-intensity contours (on a log scale) of the full 0) vs. co-20 scans for one of the GaN/sapphire films. Note the presence of contour lines corresponding to the Pendelltisung fringes of Figure 2. These contour fringes are shown more dramatically in the intensity vs. angle form as shown in Figure 4a. The FWHM of this curve is -35 arc-s. The diffracted intensity along the horizontal (o0-20) axis is indicative of the variation of the lattice constant perpendicular to the diffraction planes. Figure 4a shows the distribution of the Bragg diffraction intensity as a function of angle, and the width of the curve is related to the distribution of the (0001) interplanar separations. The extent of the "mosaic tilt" of the GaN film can be related to the diffracted intensity along the vertical (0w)axis, displayed in the intensity plot of
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Figure 4b. The narrow FWHM of -11 arc-s for the scan along the 0)-axis indicates a low value of mosaicity in the heteroepitaxial GaN layer. The relatively broad 0o-20 curve compared to the narrower 0) curve is consistent with notion that low dislocation densities should be associated with less mosaic structure and that mosaicity plays a strong role in thermal stress relaxation. Dmega
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Figure 3:
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m/2Th
Triple-axis X-ray scan for a GaN/sapphire epitaxial layer grown by MOCVD.
Figure 4a: X-ray intensity along the 0o-20 axis at the 0)=0.0 plane. Pendellosung fringes and the FWHM value of -35 arc-s.
185
Note the well-defined
i
90
16'. 95
17' 700
17'. 05
17. 1i Dmeg, [degrees]
Figure 4b: X-ray intensity along the c0axis at the w)-20=0.00 plane; the FWHM - 11 arc-s. Photoluminescence (PL) spectra taken at 300K for the GaN films typically show an intense near-bandedge peak near X -370 nm and weak "defect-related" emission at -550 nm. Figure 5 (upper curve) displays the 300K PL spectrum of a -1.2 jim-thick GaN/A12 0 3 film. The high integrated intensity of the near-band-edge peak at -367 nm and the high intensity of this peak compared to the deep-level ("yellow") emission at longer wavelengths further indicate the samples data obtained from a high-quality to be of good optical quality. The lower curve shows similar PL "undoped" MOCVD-grown GaN/A120 3 film (-2.0 gm thick)7 . We note that the "undoped" film has a lower i
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