AFM Study of High-Electron Mobility InSb Materials

InSb Multiple Quantum Wells

Currently the growth of InSb/Al_xIn_1-xSb Multiple Quantum Well (MQW) systems on buffered GaAs substrates incorporate a large number of defects which may compromise their electronic and optical properties. The defects initiate mainly at the GaAs/buffer interface and are created to relieve the strain induced by the mismatch in lattice constants between the growing layers and the substrate. The defects propagate and are destroyed as the thickness of the buffer increases and/or is modified through various compositional changes. The MQW structure, deposited after the buffer layer, inherits the remaining defects, mainly in the form of screw or threading dislocations which can be examined at the surface using Atomic Force Microscopy (AFM). This technique provides us with atomic resolution of the dislocations along the verticle axis and definitively shows the spiraling staircase of screw dislocations.
In addition to the standard dislocations, we also see Oriented Abrupt Steps or OAS's. These steps occur predominately along the axis, but may also be present along the [110] axis of the material. Contrary to the screw dislocations the OAS's have steep side wall angles of 5 to 15° which typically terminates at the peak of a screw dislocation. The OAS could then significantly alter the quantum well properties. If the MQW structures follow the surface topology, which is extremely probable, the quantum wells will become 'pinched' at an OAS, reducing the well spacing, which will effectively change the electrical and optical properties of the system.
In this work, we are using undoped MQW structures to correlate the structure of these defects, as viewed by AFM, with the observed optical (exitionic) transitions using Fourier transform infrared spectroscopy (FTIR). Additional use of X-Ray Diffraction (XRD) in this work helps determine the amount strain as well as the alloy compostion, x, of the AlxIn1-xSb barrier layers in the system.

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Publications

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1. "Charaterization of InSb Quantum Wells Containing High-Mobility Electron Systems", S.J. Cheung, S.C. Lindstrom, N. Dai, K.J. Goldammer, F. Brown, M.B. Johnson, R.E. Doezema, and M.B. Santos, J. Vac. Sci. Technol. B 17, 1151 (1999).
2. "Electrical Properties of InSb Quantum Wells Remotely-Doped with Si," K.J. Goldammer, W.K. Liu, G.A. Khodaparast, S.C. Lindstrom, M.B. Johnson, R.E. Doezema and M.B. Santos, J. Vac. Sci. Technol. B 16, 1367 (1998).