InAs is the only binary III-V compound semiconductor that exhibits a natural surface accumulation due to the high density of donor surface states. The Fermi level is pinned at any surface of an InAs wafer, regardless of orientation. It is therefore very likely that an accumulation layer is present at both the top and bottom surface or interface of a thin InAs epilayer with an intermediate bulk-like region between them. Epitaxial layers of InAs sandwiched between two 30 nm thick layers of In0.8Al0.2As or In0.52Al0.48As were grown on InP substrates by solid-source molecular beam epitaxy. Their static and dynamic properties were determined by means of gated Hall, resistivity and C-V measurements using a three-layer model to account for interface accumulation as well as the residual bulk-like intermediate region. The InAs/In0.8Al0.2As heterojunction interface has a significantly lower density of interface states than that of the In0.52Al0.48 As/InAs interface. It is possible to drive such a structure from accumulation through flat band into depletion by means of moderate negative gate voltages. Using similar measurements, the effect of the thickness of the InAs layer as well as the presence or absence of a step-graded buffer on the density of surface states was determined.
soource:Iopscience
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