ALBERT

Notes: We report the growth of high-quality As-based ternary and quaternary alloys lattice matched to InP using a valved arsenic source that can post-heat the As beam after evaporation. We find that the optimum group-V-to-group-III beam-equivalent pressure ratio for growth of (In,Ga)As alloys using this source is considerably lower than values reported previously for growth using conventional As4 sources. Consequently, high-quality (In,Ga)As, (In,Al)As, and (In,Al,Ga)As alloys (and quantum wells made from these alloys) can be grown under the same growth conditions, i.e., substrate temperatures between about 525 °C and 540 °C and V/III pressure ratios between 10:1 and 15:1. Thick-film alloys and multiple-quantum-well structures grown under these conditions show superior structural and optical quality. Strong excitonic features are observed in the room-temperature absorption spectra of a number of multiple-quantum-well structures with well widths ranging from 30 A(ring) to 170 A(ring) . Calculations of the exciton transition energies using a simple empirical two-band model are in excellent agreement with experiment, even for a structure containing quantum wells in tensile strain in which the ordering of ground-state light- and heavy-hole excitons is reversed. The optical absorption spectrum of a 50-A(ring) -period (In,Ga)As/(In,Al)As superlattice shows room-temperature excitons involving electronic states at both the bottom and top of the minibands. Exciton line widths for these quantum-well structures, measured using low-temperature photoluminescence, are consistent with the limits imposed by random alloy fluctuations. We tentatively explain the lower optimum V/III pressure ratio for growth of (In,Ga)As in terms of the increase in kinetic energy of As4 molecules (compared with the kinetic energy of molecules from a conventional As4 source) and the consequent enhancement in the efficiency of dissociation of As4 molecules into As2 molecules at the growing surface.

Notes: We present a simple model for photocurrent spectroscopy of quantum-well p-i-n diodes that provides a quantitatively accurate desciption of the dependence of photocurrent on absorption coefficient and applied bias. The model incorporates the transit-time effect described previously [R. P. Leavitt and J. L. Bradshaw, Appl. Phys. Lett. 59, 2433 (1991)] as a limiting case. It also includes the two major effects of residual background doping in the intrinsic region of the diode: nonuniform electric fields, which affect the transport of carriers, and incomplete depletion at low electric fields, which reduces the amount of photocurrent collected. We show that the background-doping effect alone can mimic the transit-time effects: reduction in the overall carrier collection efficiency, saturation of photocurrent spectral features, and the presence of minima in photocurrent where absorption spectra show maxima. We obtain a closed-form expression for the photocurrent in the general case where both transit-time and background-doping effects are significant. Excellent agreement is obtained between model calculations and experimental room-temperature photocurrent spectra for an 89-period 100-A(ring) GaAs/100-A(ring) Al0.3Ga0.7As multiple-quantum-well diode, where the background doping density, the effective electron mobility, and the built-in potential are treated as adjustable parameters. The background doping density and the built-in potential obtained from the fit are in excellent agreement with independent measurements. We apply the model to predict the dependence of photocurrent on the intrinsic-region thickness of the diode. We also show a dramatic asymmetry between photocurrent spectra measured with light incident from the front and from the back of the diode, and we discuss the impact of this asymmetry on the performance of self-electro-optic-effect devices. We also find good agreement between the model predictions and the photocurrent results of Whitehead et al. [Appl. Phys. Lett. 52, 345 (1988)]. Further, our model qualitatively describes the dependence of the photoluminescence intensity on electric fields in multiple-quantum-well p-i-n diodes.

Notes: Previously reported chemical vapor deposition of 3C-SiC on 6H-SiC has resulted in films with a high density of double positioning boundaries (DPBs). We have found that growth on as-grown faces of 6H-SiC crystals can yield films that are largely free of DPBs. The (111) 3C-SiC films, up to 12 μm thick, were evaluated by optical and electron microscopy and low-temperature photoluminescence (LTPL). The LTPL spectra of the films were similar to those of high quality Lely-grown 3C-SiC.

Notes: Previously reported growth of SiC films on SiC by chemical vapor deposition (CVD) used Acheson and Lely α-SiC crystal substrates. We report the CVD growth and evaluation of high quality 6H-SiC films on 6H-SiC wafers cut from large boules grown by the modified-sublimation process. The single-crystal 6H-SiC films were grown on wafers oriented 3° to 4° off the (0001) plane toward the 〈112¯0〉 direction. The films, up to 12 μm thick, had surfaces that were smooth and featureless. The high quality of the films was demonstrated by optical and electron microscopy, and low-temperature photoluminescence.

Notes: We consider the effects on photocurrent spectra of the finite transit times of carriers through the 2-μm-thick intrinsic region of a p-i-n diode containing multiple quantum wells. If the transit time is comparable to or greater than the carrier recombination time, the photocurrent spectra show severe distortion. We have observed an anomalously large decrease in the diode's quantum efficiency, accompanied by a compression of the spectral features and eventually an apparent splitting of each photocurrent peak into two separate peaks, as the electric field is lowered. A carrier-transport model is presented that accurately describes the essential experimental features.

Notes: Coupled triple quantum-well GaAs/AlGaAs heterostructures have been designed to exhibit simultaneous electron and hole tunneling from the central well to opposite side wells at a fixed applied bias. Band offset is the critical parameter for the design of structures with simultaneous resonances. Photocurrent measurements reveal which triple quantum-well structures exhibit simultaneous resonances. A band offset ratio near 62:38 is required to correctly engineer structures with simultaneous resonances.

Notes: A mid-IR (3.3–3.5 μm) type-II interband cascade laser has been demonstrated at temperatures up to 300 K in pulsed mode and 150 K in cw mode. Threshold current densities as low as 13.2 A/cm2 and power efficiencies as large as 17% have been achieved under cw conditions at 80 K. © 2002 American Institute of Physics.

Notes: Midinfrared (3.6–3.8 μm) interband cascade lasers based on InAs/GaInSb type-II quantum wells have been demonstrated in the continuous-wave (cw) mode with low-threshold current densities (e.g., ∼56 A/cm2 at 80 K) and power efficiencies exceeding 9%. At a relatively low current of 0.4 A, we observed ∼100 mW/facet of optical power out at 80 K (124 mW at 60 K) from lasers mounted epilayer-side up with uncoated facets. These lasers were able to operate in the cw mode at temperatures up to 127 K. Also, in the pulsed mode, devices lased at temperatures up to 250 K and displayed, at 80 K, a peak power efficiency exceeding 11%. © 2000 American Institute of Physics.

Notes: Several novel features are observed in the low-temperature photoluminescence spectra of AlxGa1−xAs layers grown on GaAs by metalorganic chemical vapor deposition. GaAs substrate luminescence spectra are observed through AlxGa1−xAs layers as thick as 22 μm. Both carrier diffusion to the substrate and AlxGa1−xAs luminescence are identified as the excitation mechanisms of the GaAs luminescence, and a minority-carrier diffusion length of 10 μm is estimated. The presence of photon recycling in the AlxGa1−xAs layers is demonstrated by comparison of photoluminescence (PL) spectra for layers with and without GaAs substrates, as well as PL examination of two AlxGa1−xAs heterostructures.