(a) When a GaAs layer is grown on a planar SiGe layer or a large SiGe island with limited or no compliance, the GaAs epilayer is deformed or strained, whereas the SiGe layer is relaxed. In this case, as shown in (b), a high level of strain energy is stored in the GaAs epilayer as lateral compression and vertical tension. (c) The substrate compliance effect in SiGe nanowire structure enables both the nanowire and the epilayer to be deformed. The mismatched strain energy is thus distributed between the epilayer and nanowire, as shown in (d). The reduced strain energy accumulated in the epilayer suppresses the formation of defects.
Finite element simulation of the (a) lateral strain and (b) vertical strain as a function of depth from the GaAs surface in a heterostucture formed on . The thicknesses of the GaAs and SiGe are 20 and 100 nm. The width of GaAs/SiGe nanostructure is varied (100 nm, 200 nm, 500 nm, and ). The strain in GaAs is significantly reduced for narrower structures.
(a) SEM image showing the top view of a layer of GaAs grown on planar SiGe-on-insulator structure. Island formation for stress relief results in a rough GaAs surface. The cross-sectional TEM image in (b) is a zoomed-in view of a region in (c) which shows nucleation of GaAs islands on the SiGe surface. Defects such as stacking faults and dislocations are clearly observed in these GaAs islands and at the interface between GaAs and SiGe. These defects relieve the stress due to lattice mismatch at the GaAs/SiGe heterojunction.
(a) SEM image showing the top view of a nanowire with a width of 75 nm and a GaAs layer grown on it. Cross-sectional TEM micrographs in (b) and (c) show that the GaAs layer is pseudomorphically grown on SiGe. The GaAs lattice is well-aligned to the lattice and with no observable defects such as APDs or stacking faults.
Room temperature PL spectrum of GaAs on SiGe nanowire. Interference fringes can be observed, and are attributed to multiple reflections within multilayer structure, indicating abrupt and flat interface.
Micro-Raman spectra of GaAs grown on planar SiGe-on-insulator structure and SiGe nanowire structure. The redshift in the Si–Si, Si–Ge, and Ge–Ge mode phonons in the nanowire structure, as compared to planar structure, indicates that the SiGe nanowire is under tensile strain. The distribution of strain between the GaAs epilayer and the SiGe nanowire template is important for the suppression of defect formation.
AES spectra at five locations as shown in SEM micrograph in the inset. Both and were detected in the nanowire regions (locations 1, 2, and 3), confirming the existence of GaAs on the nanowire. Neither Ga nor As was detected in the regions (locations 4 and 5), indicating the high selectivity of the MEE method.
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