G. Livescu

Quantum well carrier sweep out: relation to electroabsorption and exciton saturation

The authors studied the effects of changing the barrier design of GaAs-Al/sub x/Ga/sub 1-x/As quantum wells on the electroabsorption, exciton saturation, and carrier sweep-out times. Five samples with x values ranging from 0.2 to 0.4 and barrier thicknesses from 35 to 95 AA were studied. Within this range, the authors find that the electroabsorption is not very sensitive to the barrier thickness, but that the ionization field of the excitons approximately doubles for an increase of x from 0.2 to 0.4. The samples with high, thick barriers have lower internal quantum efficiencies than those with low, thin barriers. It was found that the exciton saturation intensity increases with increasing applied field, and decreasing barrier thickness or height. Time-resolved electroabsorption measurements confirm the variation in sweep-out rates between samples, and indicate that the escape mechanism at low field is probably a thermally-assisted tunneling process. >

Free carrier and many-body effects in absorption spectra of modulation-doped quantum wells

The temperature-dependent optical absorption and luminescence spectra of GaAs/AlGaAs and InGaAs/InAlAs n-doped modulation-doped quantum wells is discussed with emphasis on the peak seen at the edge of the absorption spectra of these samples. A many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton-the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band. This calculation accounts for the strong dependence of the absorption edge peak on both the temperature and carrier concentration, in good qualitative agreement with experimental data and with previously published results. The changes induced by the carriers on the subband structure through self-consistent calculations are also analyzed, and it is concluded that in these symmetric structures, the changes are small for achievable carrier densities. >

High-power cw vertical-cavity top surface-emitting GaAs quantum well lasers

We have devised a novel vertical‐cavity top surface‐emitting GaAs quantum well laser structure which operates at 0.84 μm. The laser combines peripheral current injection with efficient heat removal and uses only the epitaxially grown semiconductor layers for the output mirrors. The structure is obtained by a patterned deep H+ implantation and anneal cycle which maintains surface conductivity while burying a high resistance layer. Peripheral injection of current occurs from the metallized contact area into the nonimplanted nonmetallized emission window. For 10‐μm‐diam emitting windows, ∼4 mA thresholds with continuous‐wave (cw) room‐temperature output powers ≳1.5 mW are obtained. Larger diameter emitting windows have maximum cw output powers greater than 3 mW. These are the highest cw powers achieved to date in current injected vertical‐cavity surface‐emitting lasers.

Band offset determination in analog graded parabolic and triangular quantum wells of GaAs/AlGaAs and GaInAs/AlInAs

The band offset parameter Qc = ΔEc/ΔEg for both GaAs/AlGaAs (lattice matched to GaAs), and GaInAs/AlInAs (lattice matched to InP) was extracted from the optical interband transition energies obtained from both triangular and parabolic quantum well shapes of various widths. The wells were grown using continuous analog compositional grading as opposed to the discrete, superlattice (digital) grading used by previous researchers. Electron beam electroreflectance (EBER) was the primary technique used to measure the interband transition energies. By combining the theoretical energies from quantum mechanical potential well calculations with EBER measured energies, it was possible to extract band offset values in a self‐consistent manner. Qc values obtained were 0.658±0.009 and 0.650±0.001 for GaAs/AlGaAs and GaInAs/AlInAs, respectively. Measurements also revealed that Qc was both temperature and concentration independent within the range of composition studied.

EXCITON SATURATION IN ELECTRICALLY BIASED QUANTUM WELLS

We have measured the heavy hole excitation saturation intensity in GaAs/AlGaAs quantum wells as a function of applied electric field and AlGaAs barrier design. We find that the saturation intensity increased with increasing applied field, and decreasing barrier thickness or height, because of increased carrier sweep‐out rates. Time‐resolved sweep‐out time and temperature‐dependent saturation intensity measurement point out the roles of both thermionic emission and tunneling in the field and barrier‐dependent carrier escape time. By reducing the barrier Al composition from 30 to 20%, we achieved an increase in the saturation intensity by a factor of ∼6.

Modulation of absorption in field-effect quantum well structures

Experimental and theoretical investigations of the absorption in a single-modulation-doped quantum well (QW) used as conducting channel of a field-effect transistor are presented. By applying a voltage to the gate, the electron concentration can be varied between 0 and approximately 10/sup 12/ cm/sup -2/. The continuous transition can be optically followed from an undoped to a highly doped QW. Effects of band filling are observed, along with renormalized effects at the first subband edge and electrostatic effects at the higher ones. It is shown that optical techniques can give in situ information on the electron density and temperature as well as on the electrostatic fields inside field-effect structures. >

High‐speed absorption recovery in quantum well diodes by diffusive electrical conduction

We present here picosecond time‐resolved electroabsorption measurements in GaAs quantum well p‐i‐n diode structures. While the dynamics of the vertical transport is not completely understood at present, our data reveal the importance of the ‘‘lateral’’ propagation of the photoexcited voltage pulse over the area of the doped regions. We propose a two‐dimensional ‘‘diffusive conduction’’ mechanism, which predicts a fast relaxation of the electrical pulse, with time constants ranging from 50 fs to 500 ps, determined by the size of the exciting spot, the resistivity of the doped regions, and the capacitance of the intrinsic region.

Spatial light modulator and optical dynamic memory using a 6×6 array of self-electro-optic-effect devices

Using a 6 X 6 array of integrated quantum-well self-electro-optic-effect devices, we demonstrate an optically addressed spatial light modulator able to convert a visible, incoherent image into coherent infrared (IR) light. Depending on the IR wavelength used, the output is either a positive, binary-thresholded version of the input (bistable mode) or its linear, negative (self-linearized) mode. This device can also function as a dynamic bistable memory that can retain its internal state without power for times as long as 30 sec.

Electron effective mass and band‐gap dependence on alloy composition of AlyGaxIn1−y−xAs, lattice matched to InP

AlyGaxIn1−y−xAs structures were prepared by molecular beam epitaxy to determine both the electron effective mass and band‐gap dependence of the InP lattice‐matched alloy. Cyclotron resonance and photoluminescence measurements were used, respectively. The effective mass obtained after nonparabolicity correction is m*=0.0403+0.0817y. The band‐gap relationships obtained at 300 and 5.5 K are Eg (eV)=0.752+1.453y, and Eg (eV)=0.792+1.530y, respectively.

Low‐temperature‐grown GaAs quantum wells: Femtosecond nonlinear optical and parallel‐field transport studies

We find that GaAs‐AlGaAs quantum wells grown at low temperature (300 °C) by molecular beam epitaxy are nearly semi‐insulating and exhibit a broadened excitonic resonance. Studies of the femtosecond time‐resolved nonlinear optical saturation response and parallel‐field transport properties indicate that a subpicosecond nonradiative decay dominates the carrier recombination of this material. Annealing of low‐temperature‐grown quantum wells causes the arsenic point defects to form arsenic metallic precipitates within the quantum wells and up‐shifts the absorption edge.