Growth and fabrication of quantum wells, wires, and dots for optical applications

Abstract

Although molecular beam and vapor phase epitaxial growth techniques can control layer thicknesses and uniformity at the atomic monolayer level of precision, control of quantum structure dimensions in lateral directions parallel to epitaxial layers is more difficult, with lithographic and focused beam techniques difficult to control below dimensions of order 100 nm. This is larger than the optimum maximum size for quantum wire and dot applications, which is of order 10 nm, as required for wire and dot devices to operate in their lowest quantum state/and not to be restricted to cryogenic temperature operation. An alternate, but challenging, means of defining small lateral dimensions is the use of the periodicity of atomic terrace steps on substrate crystals that are slightly misoriented with respect to principal crystal 1 For small misorientation of a crystal surface by a radians, the distance L5 between step edges is c/a, where c is the spacing between planes of substrate atoms. For a GaAs crystal misoriented two degrees from a (1 10) direction, L5 = 8 nm and thus is of the order of magnitude needed for quantum wire devices. Lateral periodicity of a quantum structure can be produced by growth of alternate half atomic layer coverage depositions in a step flow crystal growth mode. Structures showing this lateral periodicity have previously been grown both by molecular beam epitaxy and by metal-organic chemical vapor deposition.2'3 Deviations from a perfectly ordered lateral superlattice with layers perpendicular to the surface are important and are expected when the starting surface is not smooth with regular steps, when the crystal grows in an island growth mode, when there are kinks in the step edges, and when each pair of depositions deviates from an atomic monolayer total deposition. In recent work, we have measured surface smoothness and step regularity by in situ observations of the crystal growth surface by measurement of the widths of the specularly reflected beams in reflection electron diffraction during epitaxial molecular beam growth. From these measurements, an optimum temperature (of approximately 600 °C for GaAs and AlAs depositions with arsenic rich As4 growth conditions) has been determined, above which surface roughness on the growth terraces is minimized.4 Smoothing of the surfaces and development of a periodic array of steps is observed during buffer layer growth by measurement of narrowing of the specularly reflected beam for glancing incidence beams traveling in a direction normal to the terrace step edges. The regularization of step edge spacing is a result of the step growth process of crystal growth in which atomic migration to and bonding at step edges is accompanied by a repulsion which tends to prevent atoms from moving down a step to a lower terrace.5 Vertical superlattices formed in the above manner have been observed by transmission electron microscope observations of cross sections of GaAs/AlAs structures. Further evidence of the anisotropic optical properties expected from the vertical superlattices has been found in the anisotropy of photoluminescence excitation spectra.6 The technique has been used in conjunction with adjacent quantum well layer structures to produce quantum wire lasers operating in the lowest quantum wire quantum state. Most recently, the surfaces of GaAs/AlAs vertical superlattices grown on off axis substrates have been examined directly by atomic force microscopy.7 When the surface of a vertical superlattice is oxidized in