K. Kash

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.

Luminescence characteristics of quantum wires grown by organometallic chemical vapor deposition on nonplanar substrates

Luminescence properties of GaAs/AlGaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on V‐grooved substrates are reported. A model of the crescent‐shaped wires yields parabolic QWR potential wells with subbands separated by 21.7, 3.9, and 16.7 meV for electrons, heavy holes, and light holes and effective width of 16 nm for the ground electron state. Spectrally and spatially resolved cathodoluminescence images reveal highly uniform emission from the QWR regions. Photoluminescence excitation spectra exhibit enhanced absorption at the QWR subbands, with subband separations in good agreement with the model.

Generation of macroscopic steps on patterned (100) vicinal GaAs surfaces

We show that macroscopic, as opposed to microscopic, steps can be obtained on a semiconductor vicinal surface when a perturbation has been ‘‘printed’’ on it, prior to epitaxial growth. This generic crystal growth concept has been studied here with the GaAs/AlGaAs system using organometallic chemical vapor deposition. The details of step formation, stabilization, and subsequent propagation have been investigated with scanning electron microscopy. Regular, sawtooth‐like growth patterns have been obtained, with periodic growth rate differences at the step edges. This novel lateral patterning technique was employed to fabricate arrays of quantum wire‐like heterostructures.

Observation of quantum dot levels produced by strain modulation of GaAs–AlGaAs quantum wells

We present the first observation of resolved quantum dot levels produced by strain modulation of a semiconductor. Excitons are confined in lateral potential wells of up to 60 meV, the largest yet reported for strain‐induced quantum wire or dot structures. The dependence of the quantum dot level separation on dot size is in agreement with calculations of the strain‐induced band edge modulation. For 200 nm wide carbon dot stressors the level separations are approximately 2 meV. Frustration of acceptor recombination by three‐dimensional localization of carriers in the strain‐induced potential wells is clearly observed. Finally, we detect luminescence from a single quantum dot at low excitation intensity, a result of the frustration of nonradiative recombination by localization.

Low‐loss multiple quantum well GaInAs/InP optical waveguides

Propagation losses as low as 0.24±0.06 dB/cm are demonstrated for single‐mode, GaInAs/InP multiple quantum well rib waveguides at 1.52 μm wavelength. We show that reproducibly low losses (≤0.6 dB/cm for rib widths ≥3 μm) can be maintained over a large chip area (9.4×5 mm2). Origins of the loss are discussed.

Formation of laterally propagating supersteps of InP/InGaAs on vicinal wafers

We propose and demonstrate a technique for artifically bunching the atomic steps on a vicinal substrate to form supersteps of almost arbitrary height. The process involves the etching of a grating of parallel grooves on the surface of a vicinal substrate followed by epitaxial growth that fills the grooves. Steps of height proportional to the period of the grating and the substrate misorientation angle are formed. The technique is demonstrated on a macroscopic scale for the InP/InGaAs material system using chloride transport vapor levitation epitaxy, resulting in InGaAs wire‐like structures confined both horizontally and vertically by InP. The growth of quantum wires and laterally periodic superlattices should be possible using this technique with proper scaling of parameters.

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

Abstract We have confined excitons to wires within continuous GaAsue5f8AlGaAs 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.

Strain‐induced lateral carrier confinement in quantum wells grafted onto nonplanar substrates

We remove a thin semiconductor film from its growth substrate and reattach it to a nonplanar host substrate. The film is under a large, localized bending stress. In a GaAs/AlGaAs film with a quantum well near one surface where the bending strain is greatest, carriers are laterally confined by the strain to regions where the band gap is red‐shifted by up to 62 meV.

Optical nonlinearity in GaAs quantum dots

We have measured the optical saturation intensity of GaAs quantum dots and have found it to be 50u2009W/cm2; more than an order of magnitude smaller than that reported for GaAs quantum wells. Compared to such quantum wells, our quantum dots also show a larger amount of saturation, again by more than an order of magnitude. We find that the saturation intensity of our quantum dots depends exponentially on the photoexcitation energy, with greater intensities required for photon energies closer to the bottom of the quantum dot confinement potential.