In situ preparation of the semiconductor-superconductor interface: Nb contacts on InAs quantum wells

In this article we describe an in situ, low damage, process sequence design ed to provide a more reliable, higher quality metal (superconductor)-semico nductor interface. We produce discrete Nb contacts on an InAs quantum well (125 Angstrom) that is embedded within thick (similar to 3000 Angstrom) Al0 .5Ga0.5Sb barrier layers by utilizing a multichamber ultrahigh vacuum syste m, in which two Varian GenII, Molecular Beam Epitaxy chambers are directly connected to a high vacuum etch station. The principal process steps includ e (1) a short, "selective" thermal Cl-2 etch of the top barrier layer to ex pose and define the InAs contact region, (2) a post-etch anneal (under As-2 flux) to desorb residual etch products (i.e., InCl3) and restore a clean a nd well-ordered InAs (2x4) surface, (3) a Sb capping/passivation layer on t he treated surface, (4) atmospheric transfer of the capped sample to a remo te Nb deposition chamber, and (5) a post-anneal (in vacuum) to desorb the S b cap immediately followed by Nb deposition. The priority throughout is min imizing damage and impurity exposure to the InAs surface. Morphology and ro ughness of the etched surface are explicitly considered in terms of the (1) initial native oxide removal: short Ar+ sputter at room temperature versus thermal desorption at 530 degrees C under Sb-2 flux, (2) thermal Cl-2 etch temperature (180-260 degrees C), and (3) annealing temperature for Sb deso rption (430-530 degrees C). (C) 1999 American Vacuum Society. [S0734-211X(9 9)02005-3].