Triple-Period (Tp)-A and Cupt-A Type Ordering in Al 0.5 In 0.5 As Grown by Metalorganic-Vapor-Phase-Epitaxy
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grown on a (001) InP substrate. MOVPE growth was carried out at 70 Torr in a vertical reactor.8 The following three group-III source gases with their respective flow rates were used: TMAI at a flow rate of 0.0227 cc/min, TEGa at 0.0872 cc/min, and TMIn at 0.09 cc/min. V/III ratios for Alo.51n0 .5As were 199 (AsH 3: 27 cc/min) for layers LI, L2, and L3, and 396 (AsH3: 50 cc/min) for L4. Growth temperatures are shown in Fig. 1. Layer L4 was grown at 550TC. The growth rate was 3.8A/sec for A10.51n 0.5As, 3.5A/sec for the Gao.5 1n. 5As marker layer. All epitaxial layers were not intentionally doped. The TEM cross-section samples were prepared by using a dimple-grinder and by ion- milling after cleavage and polishing. A 200-kV TEM (TOPCON EM002B) was used. RESULTS AND DISCUSSIONS As shown in Fig. 1, layers L3 and L4 showed a quasi-periodic contrast modulation with a
S300A period. This is probably due to the composition modulation, which has been previously 9
observed by several researchers. The cross-section [110]-zone transmission electron diffraction (TED) patterns for Layer L3 showed diffractions from the CuPt-B type ordering (Fig. 2(a)). The same type of ordering in MOVPE grown A10.5 1no.5As was first reported at a higher growth temperature of 600TC.' 0 Diffractions from TP-A type ordering in Layer L3 are observed at 1/3 and 2/3 positions in the [-I-i 1] and [III] directions (two A-directions), although the diffractions are quite weak and diffusive, as shown in Fig.2(b). Figure 3 shows a [-110]-zone TEM lattice image of a region in Layer L3, located close to the Ga0 .51n0 .5As marker layer L3-B. We see both triple-period and double-period superlattice in this region; there is a domain of TP-A type ordering with a superlattice periodicity of 10.2 A in the region indicated by white arrows. The inset shows the Fourier transformation of the TEM image indicated by the white circle. We can see an intensity peak at the - 1/3, T 1/3,+± 1/3 positions that are consistent with triple-periodicity. We can also see TP-A type ordering in other places in this region, although the domains are also relatively small (20-30 A). CuPt-A type ordering was also observed in this region, an example of which is shown by the black arrows. Its domain was also small (several tens of angstroms). The intensity of TED in the case of CuPt-B was much stronger than that in TP-A, as shown in Figs. 2 (a) and (b). The co-existence of TP-A and CuPt-A type ordering indicates that, while the (2xl) surface reconstruction dominates the surface during A10.5 1n0.As MOVPE-growth, (2x3) and (1x2) domains also co-exist. The co-existence of TP-A and CuPt-A has also previously been observed in MBE-grown 5 Al0.51n 0 A As. TEM examination of the other A10.51n0.As layers (L 1, L2, and L4) revealed that there was still A-type ordering, but the domain densities were even much lower. It was difficult to identify the order of density among them.
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Fig. 2 (a) [110]-zone and (b) [-1 10]-zone TEM diffraction patterns for Layer L3 indicated in Figure 1. 268
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