Serpentine superlattice: concept and first results

Abstract

We present an epitaxial growth technique for vicinal substrates which avoids the geometric tilt sensitivity inherent to the cyclic growth of such structures as tilted superlattices. This cyclical deposition and growth technique produces a serpentine superlattice (SSL) (M.S. Miller et al., in: Proc. 20th Intern. Conf. on Physics of Semiconductors, Thessaloniki, August 1990). Purposefully varying the per-cycle growth rate produces a lateral superlattice with a variable, meandering tilt, with 1 D, microscopically confined electronic states formed where the superlattice bends over. The quantum wire electronic confinement is principally determined by the SSL shape near these widest regions, near places with a vertical tangent. This shape is accurately established through the smooth adjustment of the fractional monolayer growth rates. If a large enough range of growth rates is spanned, then a particular confinement structure is guaranteed: With a SSL, quantum wire arrays with particular energy levels may be precisely realized. Not only is this true at a particular place on the substrate, but the growth rate variations across a substrate are automatically accommodated and uniform wires are realized over the entire wafer. Calculations of the two-dimensional subband structure were performed for (Ga,Al)As SSLs to confirm the 1D electronic states, determine optimal structure parameters and explain experimental results. Such (Ga,Al)As SSLs have been prepared by MBE and characterized with photoluminescence. We find emitted light in several samples that is polarized parallel to the direction of the vicinal substrate's steps, which we interpret to be consistent with quasi-1D confinement.