B. P. Van der Gaag

Fabrication of a Two-Dimensional Phased Array of Vertical-Cavity Surface-Emitting Lasers

We report the successful fabrication of a two‐dimensional phase‐locked array of vertical‐cavity surface‐emitting lasers. The array was comprised of more than 160 vertical‐cavity surface‐emitting lasers of 1.3 μm diameter with a separation of less than 0.1 μm between each lasing element. The array had a 25 μm diameter and each of the elemental lasers was located on a two‐dimensional rectangular lattice. The threshold current of the two‐dimensional array 45 mA yields a threshold current of 280 μA for an elemental laser. The far‐field beam angle of the array was as narrow as 7°, and the spectral purity was found to be good enough to allow for a clear holographic image reconstruction of a holographic memory.

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.

Microfabrication below 10 nm

We describe a new method of producing ultrasmall structures on thick substrates with electron beam lithography. Using an innovative exposure technique, we obtain features with lateral sizes smaller than the incident beam diameter. These patterns are transferred into GaAs/AlGaAs quantum well heterostructures using chemically assisted ion beam etching, and uniform arrays of structures with lateral dimensions below 10 nm are produced. We employ reflection electron microscopy measurements to correlate the structure size with the exposure and development conditions for this fabrication scheme.

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.

Growth of heavy carbon‐doped GaInAs lattice matched to InP by chemical beam epitaxy

Successful growth of p‐type (∼1×1020 holes/cm−3) C‐doped lattice matched GaInAs on InP(100) has been demonstrated using chemical beam epitaxy. Carbon tetrachloride was used as the C‐dopant gas and p‐type GaInAs was grown by chemical beam epitaxy using trimethylindium, triethylgallium and cracked arsine. Combinations of elemental and organometallic group‐III sources also resulted in p‐type layers. High performance C‐doped base InP/InGaAs heterojunction bipolar transistors were fabricated using chemical beam epitaxy grown material.

Assessing thermal Cl2 etching and regrowth as methods for surface passivation

The damage layer that results from ion beam processing ultimately limits the smallest size structure which will emit light and hence prevents the realization of low‐dimensional quantum size effects. Effective surface passivation of etched structures requires both layer removal and compensation of surface states. Ultimately these two processes must be made compatible. To address this problem, we have coupled an ultrahigh‐vacuum (UHV) dry‐etching system to a molecular beam epitaxy (MBE) growth chamber. Material is patterned and masked outside of the combined etching/regrowth system using electron beam lithography. Samples are then loaded into the combined UHV system and vertical structures are produced by chemically assisted ion beam etching (CAIBE). These structures are then subjected to a thermal Cl2 gas etch to remove the damage layer created by the ion beam etching. Samples are subsequently transferred directly into the MBE growth chamber, where a cladding layer of Al0.3Ga0.7As is grown over the etched ...

Lateral band‐gap patterning and carrier confinement in InGaAs/GaAs quantum wells grown by molecular beam epitaxy on nonplanar dot patterns

Two‐dimensional (2D) lateral band‐gap patterning and carrier confinement were observed in InGaAs/GaAs patterned quantum‐well (QWL) dots grown by molecular beam epitaxy on prepatterned, nonplanar GaAs substrates. Growth of the strained QWL layers on craters of 0.2–5 μm diameters etched onto the substrate results in 2D lateral potential wells formed at the inside and outside regions of the craters due to preferential migration of Ga and In adatoms. Evidence for lateral carrier confinement and efficient radiative recombination in the resulting dot‐ and ring‐shaped potential wells was provided by low‐temperature cathodoluminescence spectroscopy and imaging. The results of this study indicate the feasibility of producing damage‐free quantum dot and quantum ring heterostructures by growth of QWLs on patterned, nonplanar substrates.

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.