MAN-MADE LOW-DIMENSIONAL SOLIDS: NEW CHALLENGES IN MICROSTRUCTURE MATERIALS SCIENCE

Abstract

Size quantization in man-made semiconductor structures of less than three-dimensions leads to exciting new electronic properties which are important for fundamental physics and for development of novel device concepts. Fundamental research as well as device applications based on these low-dimensional semiconductor structures require methods to fabricate the structures and to control their geometrical size on the nanometer scale in a reproducible manner. We introduce a new concept to directly synthesize III-V semiconductor quantum-wire and quantum-dot structures. The concept is based on the evolution of well ordered macrosteps (facets) on non-(100)-oriented GaAs surfaces during molecular beam epitaxy which allows us to produce arrays of alternating narrow and wide regions of GaAs in an AIAs matrix. This arrangement forms symmetric and asymmetric GaAs quantum-dot structures on (111) and (211) surfaces, respectively, and quantum-wire structures on (311) substrates. The accumulation of steps by step bunching on (210) GaAs makes possible the fabrication of mesoscopic step arrays in GaAs/AlAs multilayer structures having a periodicity of 230 Å. This periodicity is comparable to the exciton Bohr radius and thus of particular importance for the modulation of the electronic properties of GaAs based heterostructures. The existence of all these quantum-wire and quantum-dot structures is confirmed by high-resolution transmission-electron microscopy and atomic-force microscopy. The quantum confinement of carriers is revealed by the distinct electronic properties.