J. M. Worlock

Optical spectroscopy of ultrasmall structures etched from quantum wells

We have studied the photoluminescence and photoexcitation spectra of ultrasmall structures, of approximately 500 A in dimension, which we refer to as quantum ribbons and quantum disks. These are fabricated from GaAs‐AlGaAs quantum wells grown by molecular beam epitaxy and patterned by electron beam lithography. Contrary to our expectation, photoluminescence from these structures is extremely efficient. The excitation spectra of the two types of small structures differ greatly from each other and from that of the as‐grown quantum wells. These differences may be a result of the confinement of the carriers in these small structures.

Strain-induced lateral confinement of excitons in GaAs-AlGaAs quantum well microstructures

We report evidence for lateral confinement of excitons within a continuous two‐dimensional GaAs‐AlGaAs quantum well. The confinement to ‘‘wires’’ within the well was produced by partially etching a pattern through the upper AlGaAs barrier. We propose a new mechanism, that of patterned strain, for lateral quantum confinement of carriers in semiconductor microstructures, to explain our results.

Determination of nonradiative surface layer thickness in quantum dots etched from single quantum well GaAs/AlGaAs

Low‐temperature cathodoluminescence spectroscopy was used to investigate the luminescence efficiency of reactive ion etched quantum dots, varying in diameter from 200 μm down to 60 nm. The luminescence efficiency was found to be degraded both with decreasing nanostructure size and with increasing etch depth. A solution to the standard model for diffusion and recombination was applied to the data to determine the surface recombination velocity S. We found that for dots smaller than the diffusion length, the standard diffusion model becomes insensitive to the value of S and fails to predict that there is a size of dot in which the luminescence is completely extinguished. To understand qualitatively the luminescence degradation in etched nanostructures we describe a damage layer thickness ξ. The value of ξ determines the smallest quantum structure that will still emit light. We show that ξ increases with increasing etch depth and is therefore dependent on etching conditions.

Strain‐induced confinement of carriers to quantum wires and dots within an InGaAs‐InP quantum well

We describe a novel method of confining carriers by deliberately creating large inhomogeneous strain patterns in a quantum well. The strain modulates the band gap to provide lateral quantum confinement for excitons. Here, we generate strain confinement in an InGaAs quantum well by reactive ion beam assisted etching through an overlying compressed pseudomorphic quaternary layer using etch masks patterned by electron beam lithography. Photoluminescence spectra of arrays of wires and dots show red‐shifted band gaps in direct evidence of lateral confinement. We compare our results to finite element calculations of the inhomogeneous strain in an InP substrate from a compressed overlayer patterned into rectangular wires.

Optical investigation of atomic steps in ultrathin InGaAs/InP quantum wells grown by vapor levitation epitaxy

Ultrathin InGaAs/InP single quantum well structures, grown by chloride transport vapor levitation epitaxy, have been investigated by low‐temperature photoluminescence (PL). Well‐resolved multiple peaks are observed in the PL spectra, instead of an expected single peak. We attribute this to monolayer (a0/2=2.93 A) variations in quantum well (QW) thickness. Separate peak positions for QW thicknesses corresponding to 2–6 monolayers have been determined, providing an unambiguous thickness calibration for spectral shifts due to quantum confinement. The PL peak corresponding to two monolayers occurs at 1.314 eV, corresponding to an energy shift of 524 meV. Experimental data agree very well with a simple effective mass theory.

Optical properties of quantum wires produced by strain patterning of GaAsAlGaAs quantum wells

Abstract We have confined excitons to wires within continuous GaAsAlGaAs quantum wells. The confinement is produced by inhomogeneous strain created by patterning and etching a compressively stressed overlayer of amorphous carbon. Potential wells for excitons beneath 400 nm wide wires are 31 meV, as measured by the red-shift of the exciton emission. We compare our results to expectations based upon finite-element calculations of the strain tensor, and discuss the complicated effect of the inhomogeneous strain on valence-band structure.

Excitation spectroscopy of strain-patterned semiconductor wires

Abstract We show here the first excitation spectroscopy of semiconductor wires produced by strain patterning. We observe efficient trapping of laterally diffusing excitons into the wires. In addition, we report a strong anisotropy in the optical selection rules that results from the mixing of the light and heavy hole by the anisotropic strain.

Intrinsic bistability in an optically pumped quantum well structure

We describe a new semiconductor heterostructure, configured into a bistable device that can be switched either optically or electrically. The two states between which switching occurs involve very different levels of charge accumulation in a quantum well channel, affecting both the photoluminescence spectrum and the vertical photocurrent. The agents in the switching mechanism appear to be hot carriers.