B. A. Weinstein

Energy Level Alignments in Strained‐Layer GaInP/AlGaInP Laser Diodes: Model Solid Theory Analysis of Pressure‐Photoluminescence Experiments

We report high-pressure photoluminescence (PL) experiments on GaInP/AlGaInP laser structures and a comprehensive interpretation of the results via calculations based on the Model Solid Theory (MST) of Van de Walle et al. [1, 2]. This methodology allows an accurate description of the band structure and band offsets in complex heterostructures over a wide range of sample compositions and applied pressures. Measurements are performed on Ga 0.4 In 0.6 P/(Al 0.6 Ga 0.4 ) 0.5 In 0.5 P (nominal compositions) double heterojunction samples having active layers of either 125 or 30 A. The pressure data determine a reliable set of deformation potentials for the three constituent binary compounds in this system, InP, AlP and GaP. Using these deformation potentials and a realistic equation of state, the MST calculation gives a reasonable representation of the observed PL spectra over the full presure range (0 to 5 GPa) studied. The conduction band offset ΔE c , valence band offset ΔE v , and the bulk bandgap energies are computed in this materials system for a wide variety of conditions. At ambient pressure we find ΔE c : ΔE v 70: 30, in accord with the consensus value of the band-offset ratio in similar heterostructures. However, our calculations predict band-offset variations with both composition and applied pressure that are non-negligible.

Pressure Tuning of Many-Electron Impurity Interactions in Confined Semiconductor Structures

We report studies of the free carrier and donor-bound FIR excitations of a confined electron gas in modulation doped GaAs/AlGaAs quantum wells (QW) as a function of the QW electron density. Applied pressure is used to tune the electron density via the Γ–X well–barrier crossover. As electrons are removed from the QWs, we observe successively the quenching of cyclotron resonance, the evolution of the D— singlet-like magnetoplasmon resonance into the D— singlet transition of isolated donors, and the emergence of the neutral donor D0 1s–2p+ line. Calculations predict a sharp drop in the QW electron density for 2.3 to 3.1 GPa, in accordance with experiment. A rapid decrease with B-field in the blue shift of the magnetoplasmon resonance at 2.2 GPa in one sample shows that pressure has shifted the ν < 1 filling-factor regime to a factor-of-two lower field.

Pitfalls of Using Pressure to Assign the Luminescence of Large‐Lattice‐Relaxation Defects

Deep defects are often assumed to be insensitive to applied pressure because of their localized character. However, in recent photoluminescence (PL) experiments, several deep acceptor bands in ZnSe were found to shift with pressure substantially faster than the ZnSe bandgap. This shows that the optical (viz., PL) levels of these acceptors become more shallow under compression, a result that, if also true for the thermal defect levels, is important for p-type doping problems in II±VI semiconductors. We report investigations of the C3v-relaxed isolated Zn-vacancy (VZn) in ZnSe that help to resolve these issues. High-pressure PL and PL-excitation (PLE) experiments and calculations are performed on this system. We find that the VZn-related PL and PLE bands have pressure coefficients that are, respectively, larger and smaller than that of the ZnSe bandgap. Hence, the Stokes-shift decreases with pressure. These results can not be understood without taking explicit account of lattice relaxation. We employ a defect-molecule model with atomic wavefunctions to calculate semi-empirical configuration-coordinate diagrams for the VZn defect as a function of pressure. We find that compression increases the Jahn-Teller coupling, but not sufficiently to overcome lattice stiffening. Overall, the VZn thermal level deepens, inhibiting p-type doping.

Heterostructure interface effects on the far-infrared magneto-optical spectra of InAs/Gasb quantum wells

Abstract The electronic and optical properties of InAs/GaSb heterostructures depend on the type of bonding at the interfaces, InSb bonds or GaAs bonds. We have studied cyclotron resonance (CR) in the far-infrared on two samples, each consisting of a single 30 nm InAs quantum well surrounded by thick GaSb barriers. The only intended difference between the samples is the interface bonding type, Ga–As bonds and In–Sb bonds. The CR for the sample with Ga–As interface bonds shows two lines whose positions are determined by nonparabolicity effects, whereas the sample with In–Sb bonds shows multiple lines due to strong cross-interface coupling between the InAs conduction band Landau levels (LLs) and the valence band LLs in GaSb. We find that the CR is a sensitive probe of interface bonding type for such structures.

Hydrostatic pressure dependence of negative-donor-ion singlet and singlet-like bound magnetoplasmon transitions in doped GaAs/AlGaAs quantum wells

Abstract Applied hydrostatic pressure modifies the Coulomb bound states of a quasi-two-dimensional electron gas in quantum wells by increasing the effective mass and by tuning the free electron density. Here, we explore these mechanisms by measuring the effects of pressure on the cyclotron resonance, the D 0 1 s → 2 p + transition, and the D − -singlet and singlet-like transitions in low-and high-density, modulation-doped GaAs quantum wells. For low doping density, detailed calculations employing a pressure-dependent electron mass agree well with the observed magnetic field and pressure dependencies. For high doping density and low fields, the blue-shift of the D − -singlet-like transition at fields below 8 T decreases with applied pressure as anticipated, due to loss of free electrons via the Γ–X crossover. However, near ∼7.5 T , this singlet-like transition exhibits an anomalous branching for pressures above 4 kbar , which indicates the presence of a resonant level and obscures the blue-shift at high fields.

Full-spectrum optically detected resonance (ODR) spectroscopy of GaAs/AlGaAs quantum wells

Abstract Resonant magneto-absorption of far-infrared (FIR) laser radiation by neutral and negatively charged donors and free electrons, and the mechanisms of coupling of the power absorbed to various photoluminescence recombination paths in a Si-doped GaAs/AlGaAs multiple-quantum-well structure was studied by optically detected resonance (ODR) spectroscopy. A sensitive charge-coupled-device detection scheme was used to record complete PL/ODR spectra at high resolution with good signal-to-noise. The rich and complex ODR spectra were analyzed by a line-fitting procedure. Results on the neutral donor 1s–2p + transition show that at low FIR laster intensities, the recombination is modified by processes that do not involve carrier heating. At high FIR laser intensities, carrier heating effects dominate.

Acceptor state instabilities in ZnSe under hydrostatic pressure

Abstract Photoluminescence experiments on ZnSe doped with P and As have been conducted at high pressure and low temperature. We find evidence that deep states arising from these impurities become unstable between 15 and 25 kbar. Although our results are not yet definitive, some interesting implications for current models are discussed.

Pressure Tuning of Competing Charged and Neutral Exciton States in Quasi‐2D Semiconductor Structures

Photoluminescence studies of neutral and charged excitons in modulation doped GaAs/Al0.3Ga0.7As quantum wells are performed as functions of applied pressure, temperature, and excitation power and frequency. Varying both pressure and incident power allows sensitive selection of the different exciton transitions. The G±X crossover in the barriers at 7 to 9 kbar accelerates photo-pumping of electrons to the barriers. A semi-empirical kinetic model is able to account for the power and pressure dependencies of this process.

Phonons and Phonon‐Mixing in ZnSe Isotopic Crystals, Pressure‐cycled Domains, and Nanorods

Pressure‐Raman spectra are compared for bulk crystalline 68Zn76Se and ZnSe nanorods. The broadening of TO(Γ) and LO(Γ) is explained by multiphonon mixing and the position of a gap in the 2‐phonon DOS. Nucleation of the phase transition in 68Zn76Se leads to striking similarities in the Raman spectra of retrieved bulk and nanorod samples.

Far-Infrared Spectroscopy of Quasi-2D Impurity States in Semiconductor Nanostructures Under High Hydrostatic Pressure

We review recent experimental advances by the SUNY at Buffalo group in performing far-infrared spectroscopy under fine-tuning of applied high hydrostatic pressure and high magnetic fields. Experiments are reported for the effects of pressure on cyclotron resonance (CR) and Si donors in GaAs epilayers, a semi-metallic InAs/AlSh/GaSb structure, and modulation doped GaAs/AlGaAs quantum wells (QWs). Our most extensive results are for the latter system: We clearly observe pressuremediated competition between free (i.e., Landau level) and bound electron states arising from both neutral and charged donor species. With increasing pressure, there is a progression of the observed spectra from being dominated by CR and a transition due to negatively charged donors, to showing only the main 1s-2p+ transition of neutral donors. The reason for this evolution is the decrease in electrons due to the crossover of Si impurity levels associated with different well and barrier conduction-band extrema. However, we find strong evidence that electrons are separately lost to a trap, which becomes active several kbar below this crossover. Reasonable agreement is found between experiment and theory for the magnetic field dependencies of the CR and the donor transitions. A new, apparently anomalous, transition is observed, and its origins are discussed.