Defects in GaN Pyramids Grown on Si(111) Substrates by Selective Lateral Overgrowth
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center pitch were formed using photolithography and reactive ion etching (RIE). The wafer was reloaded into the MOCVD system for the lateral overgrowth. The system pressure was 76 Torr. The growth started as soon as the susceptor temperature reached 1050 'C, by flowing ammonia and TEG into the reactor. The total time for the lateral overgrowth was 3 hours. The cross-sectional specimen was prepared using the usual 'sandwich' techniques: two pieces of the sample with the same orientation were glued face-to-face with epoxy. After mechanical grinding and dimpling of the cross-sectional specimen to a thickness of -10 pim, the specimen was further thinned by ion-beam milling to electron transparency. The specimens were then examined in a Philips CM30 TEM operating at 300 kV. The analysis of the composition was carried out using an Edax PV9900 XEDS (X-ray energy dispersive spectrum) system attached to the CM30 TEM. EXPERIMENTAL RESULTS AND DISCUSSION Observation by scanning electron microscopy (SEM) indicates that each pyramid has predominantly six { 10-1 facets [4]. These facets are identical to those of the pyramids grown on sapphire and SiC substrates [3]. Visible-light microscopy (VLM) of these specimens[4] can reveal the interface between the pyramids and the substrate, and show some fine structures in the center of the pyramids. There is a difference in the contrast between the center part and the lateral part of the pyramids. This difference could be associated with local differences in the defect density and growth mechanism. The height and width of the pyramids are -15 gtm.
4 2 gm
Substrate (Si)
Figure 1 A low-magnification bright-field (BF) image of the cross-sectional specimen showing a pyramid. g 0002.
Figure 1 is a low-magnification bright-field (BF) image of the cross-sectional specimen. A selectively grown GaN pyramid on Si substrate was clearly revealed in this image. In the center of the GaN pyramid (at and above the window area), dislocations thread through the pyramid nearly
perpendicular to the interface plane and the dislocation density is quite high. In the lateral growth part of the pyramid, the dislocations thread through the pyramid parallel to the interface plane. It is apparent that the density of the defects in the lateral part is lower than the core part (at and above the window area) of the pyramid and decreases with the distance away from the interface. Planar defects, usually parallel to the interface plane, were also observed near the interface. The mask (having a low average atomic number, Z) is revealed by the bright line between the pyramid and the substrate in the mass-thickness contrast TEM image of the cross-sectional specimen in figure 2. The size of the window in the mask is -5 gtm. The base of the pyramid is 15 jim wide from this image, consistent with the observations of VLM. The GaN pyramids clearly extend laterally well beyond the window. The contrast in the image also confirms that there is a layer between the Si 3N 4 mask and the substrate. At high magnifications, this
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