Large Lattice Mismatch Epitaxy
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Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
LAYER MORPHOLOGY AT EARLY STAGES OF HETEROEPITAXIAL GROWTH Three examples have been chosen to illustrate the layer morphology which develops during the early stages of heteroepitaxial growth. The mismatch in the three cases ranges from nearly zero to -6%. Figure 1 taken from the study of Pashley and Li (1995) shows situations which are observed using STM after the deposition of 2 monolayers (ML) of ZnSe on Se-terminated, (2x 1) GaAs (001) surface by MBE at 300°C; the mismatch in this case is nearly zero. It is evident from Fig. 1(a) that the step edges on the GaAs surface are fairly jagged and growth nucleates in the form of islands along the edges. As shown in Fig. 1(c), the islands are in many cases two or more monolayers high.
0 100 200 300 400 500 600 700 i~iPoeson (A)
Figure 1
STM images showing the growth of ZnSe on the GaAs (001): Se-(2x 1) surface: (a) an area of 560 x 500 (Ac) 2 prior to the deposition of ZnSe, (b) an area of 410 x 240 (Ac) 2 after depositing two monolayers of ZnSe and (c) a line scan through the image shown in (b). After Pashley and Li (1995).
Figure 2 shows weak-beam, dark-field images of Ge islands observed after their deposition on (001) Si by MBE (Eaglesham and Hull 1995). In this case the mismatch is -4.25%. The presence of black/white lobes in Fig. 2 implies the presence of strains at the substrate/island
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interfaces. Since misfit dislocations are not observed at this stage, islands must be coherent with the underlying substrate.
Figure 2 Dark-field image of coherent Ge islands deposited on silicon substrate by MBE. After Eaglesham and Hull (1995). Figure 3 shows a situation observed after the deposition of 3 nm of GaSb on (001) GaAs using MBE (Kang et al. 1994). The growth also nucleates in the form of islands and edges of islands are aligned along directions. Misfit in this case is -6% and appears to be relaxed by the formation of two sets of dislocations that are also parallel to directions lying in the (001) plane.
Figure 3
Double weak-beam image of 30 A thick GaSb deposit showing two sets of 900 misfit dislocation arrays. After Kang et al. (1994).
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The preceding observations on island formation can be rationalized in a qualitative manner if it is assumed that the interfacial energy (E) is made up of two components: chemical (E,) and structural (Es). Ec accounts for interfacial chemistry and bonding, whereas Es may refer to the energy expended in creating misfit dislocations or coherent strain accommodation. If E is high for a heteroepitaxial system, the overgrowth will nucleate in the form of islands because the formation of a continuous substrate/layer interface is avoided. Since misfit is nearly zero for the GaAs/ZnSe system, it is suggested that Ec must be fairly high. On the other hand, both Ec and Es must contribute to E in the other two cases. FORMATION OF MISFIT AND THREADING DISLOCATIONS When misfit between the overgrowth and the underlying substrate exceeds a critical value, misfit di
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