ALBERT

Notes: Changes in the optical phase and transmission with applied electric fields were measured in GaAs/AlGaAs waveguides containing asymmetric coupled quantum wells. At sufficiently high applied bias, the lowest energy optical transition supported by this system is that between the lowest energy hole subband of the wide well and the lowest energy electron subband of the narrow well. We show that this interwell transition produces significant contributions to both the electroabsorption and the electrorefraction in the coupled quantum well system.

Notes: Low-temperature photocurrent and photoluminescence spectroscopies have been used to quantify the effects of electric field nonuniformities (on the order of 100 kV/cm) within the ∼1-μm-thick intrinsic regions of GaAs/AlGaAs multiple quantum well p-i-n photodiodes. The smearing out of excitonic spectral features due to nonuniform fields agrees well with the results of calculations performed using a single-band envelope-function approximation together with a standard depletion model for a p−/n+ junction.

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: 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 have demonstrated an infrared photodetector based on intersubband transitions in doped (In, Ga)As quantum wells in which the excited state is confined by the minigap between two minibands in a surrounding superlattice. Sharp peaks were observed at 78 K in the infrared photocurrent spectra in the 2.7–3.6 μm wavelength region corresponding to transitions in which the excited state energy is substantially above the (In, Al)As conduction-band edge. Broader, longer-wavelength photocurrent features corresponding to transitions into the lower-lying miniband states were also observed. The spectral features in the photocurrent spectra are well described by calculations of the optical absorption cross section.© 2001 American Institute of Physics.

Notes: We have measured room-temperature photocurrent spectra of p-i-n diode structures containing multiple superlattice-equivalent quantum wells wherein each quantum well consists of a few periods of a fine-period (In,Ga)As/(In,Al)As superlattice. Both the absolute exciton absorption energies and the Stark shifts are in excellent agreement with calculations, which predict a larger Stark shift for this structure than for a quaternary quantum well with the same width and absorption edge. Calculated electric-field-induced changes in the real and imaginary parts of the refractive index show that these structures are promising candidates for semiconductor intensity and phase modulators for wavelengths around 1.3 μm.

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: 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: We have designed and demonstrated a narrow bandpass filter, which is composed of a double-barrier structure, for an infrared hot-electron transistor with a cutoff wavelength at 14 μm. The filter is capable of suppressing the low energy thermally assisted tunneling current as well as the high energy thermionic emission current that are not degenerate with the photoelectrons. As a result, the detector can be operated at 77 K with a 45% BLIP level and an estimated NEΔT of 38 mK. © 1995 American Institute of Physics.