Novel semiconductor materials and structures produced by MBE and MOCVD

Abstract

The development of new epitaxial techniques has given rise to a variety of new material combinations. Pseudomorphic combinations where the partners have lattice constants which differ by more than 1% are currently being extensively studied. The built-in strain can alter the symmetry of magnitudes of the band concerned. Interesting examples of systems currently being investigated are strained layer superlattices based on Si/Si1-xGex, GaAs/InAs and InSb/InSb1-xAsx.Two narrow-gap systems are being explored in separate MBE programmes at the London University Interdisciplinary Research Centre as components for strained layer structures. The first is based on InSb, InAs and alloys of these two compounds. The second concerns heterostructures of alpha tin and InSb. Highlights of these two programmes are the following: the first observation of RHEED oscillations in InSb, attainment of record mobilities in thick film InAs (⩾ 200,000 cm2/Vs), the control of silicon and beryllium doping to very high concentrations enabling the fabrication of spike-doped and n-i-p-i structures in both materials with minimal dopant diffusion and highly non-linear optical absorption and the observation of striking Quantum Hall Effect. One of the most interesting and surprising results concerns the observation of a strong Shubnikov-de Haas effect and mobilities close to the bulk value in ultrathin films of weakly doped InSb grown on mismatched GaAs substrates. This result demonstrates that the dislocations found in the interface region are only weakly charged and therefore do not scatter the carriers significantly.The alloy system InAsxSb1-x is prone to metallurgical problems such as ordering and phase separation in the mid alloy range but high mobility samples have been grown. Other alloy systems, e.g., Al1-xInxAs, are currently of great interest and are also prone to similar problems.The strain associated with the small but significant mismatch between α-Sn and InSb stabilises the alpha phase up to 100°C and opens up an energy gap of 0.2eV. The first observation of the Shubnikov-de Haas effect with this heterostructure system demonstrates the presence of a high density two-dimensional electron gas at the interface. The carrier density is too high to arise solely from the band offsets. A complicated cyclotron resonance spectrum is observed.