Facets Formation Mechanism of GaN Hexagonal Pyramids on Dot-Patterns via Selective MOVPE
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A
(a)
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Figure 1. Schematic diagram of the mask with dot-patterns of hexagons arranged at width of 5gm and spacing of lOgim..
(b)
(c) Figure 2. SEM image of the typical GaN facet structure.
Figure 3. SEM images of GaN grown on the dot-patterned windows for different growth times of (a) 2, (b) 5 and (c) 7min. The 0 growth temperature was 1,025 C and the flow rate of TMG was 91.61 jimol/min.
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spacing of 10Igm as shown in Fig. 1. The MOVPE reactor was operated at atmospheric pressure. The source gases were TMG and ammonia (2.Oslm) with hydrogen as the carrier gas (2.Oslm). RESULTS Figure 2 shows the GaN three-dimensional structure grown on the dot-patterned windows. Any GaN deposition is not observed on the SiO 2 mask, indicating the excellent selectivity of GaN growth. The facet structure of GaN is composed of a hexagonal pyramid covered with six {1101} facets on the side or a frustum of a hexagonal pyramid having a (0001) facet on the top. The facet formation mechanism has been investigated by observing change in the facet structure with the growth time. Figures 3 (a), (b), and (c) show SEM images of GaN grown on the dotpatterned windows for different growth times of 2, 5, and 7min. The growth temperature was 1,0251C and the flow rate of TMG was 91.61 gmol/min. Figure 4 and 5 indicate the height and widths (top and bottom) of the facet structure as a function of the growth time. As clearly shown in Fig. 5, the {1iOI } facets does not almost grow and are stable. Figs. 3 and 4 show, however, the (0001) facet growth is dominant at the initial growth stage but almost stops at growth time around 7min and the facet structure is maintained. These results show that the self-limited stable facet structure is obtained after the certain growth time. Dependence of the facet structure on the growth conditions is summarized in Fig. 6. These samples were grown for enough time longer than the growth time to obtain a self-limited stable facet. The width of the top (0001) facet is shown as a function of the growth temperature for different TMG flow rates. For the same TMG flow rate the width of the (0001) face increases linearly with increasing the growth temperature. The width changes from 0 to 1.6gm at the TMG flow rate of 91.61gmol/min. For the same growth temperature the width of the (0001) facet increases with decreasing the TMG flow rate. The width changes from 0 to 0.7gm at the 1,000 0 C. Thus these results indicate that the change in the growth temperature or the TMG flow rate enables us to control the top (0001) facet in the three-dimensional microstructure of GaN.
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Figure 4. Height of the facet structure as a function of the growth time.
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