I. Tanaka

Γ-X mixing effects on photoluminescence intensity in GaAs/AlAs type-II superlattices

We have investigated Γ-X mixing effects on the no-phonon assisted (pseudodirect) transition between the n=1 Xz-electron and the n=1 Γ-heavy-hole states in [001]-(GaAs)m/(AlAs)m type-II superlattices (m=8−13 monolayers) by using photoluminescence (PL) and PL-excitation (PLE) spectroscopies. We have estimated the relative oscillator strength of the pseudodirect transition to the direct transition between the n=1 Γ-electron and Γ-heavy-hole states from the relative PLE intensity and the extrapolated relative PL intensity at infinite temperature. It is found that the relative oscillator strength is almost constant value of ∼3×10−4 for each sample. According to a first-order perturbation theory, we evaluate a Γ-X mixing factor as a function of the layer thickness from the relative oscillator strength, and show that the mixing factor, which gradually increases from ∼0.8 to ∼1.9 meV with decreasing the layer thickness, is mainly determined by the envelope-function overlap of the Γ and Xz electrons.

High sensitivity of electroreflectance to stark-ladder transitions in a GaAs/AlAs superlattice

Abstract Using electroreflectance spectroscopy, we have succeeded for the first time to observe the Stark-ladder formation of the second ( n = 2) quantization state in addition to that of the n = 1 state in a GaAs (6.4 nm)AlAs (0.9 nm) superlattice with a p-i-n structure at 77 K. From the comparison between electroreflectance and photocurrent spectra, it is demonstrated that electroreflectance spectroscopy is much more sensitive for probing the Stark-ladder transition than photocurrent spectroscopy. We discuss the high sensitivity of electroreflectance from the modulation mechanism.

Resonant coupling between buried single-quantum-well and Wannier-Stark-localization states in a GaAs/AlAs superlattice

Abstract We have investigated the resonant coupling between the quantized state in a GaAs single quantum well (6.4 nm) buried in the center of a GaAs(3.2 nm)/AlAs(0.9 nm) superlattice and the Wannier-Stark-localization state in the superlattice by using electroreflectance spectroscopy. It is found that the electroreflectance-line shapes of the heavy-hole and light-hole exciton transitions associated with the first (n = 1) subbands in the buried single quantum well drastically change under the resonant-coupling condition: the splitting feature of the line shapes due to the formation of the bonding and antibonding states and the intensity reduction due to the wave-function delocalization over the coupling space. We have detected the various resonant couplings of electrons and holes. The experimental results are discussed from a transfer-matrix analysis with Airy functions.

Optical Gain of Stimulated Emission Due to Exciton-Exciton Scattering Processes In Cui Thin Films

We report the stimulated-emission characteristics of CuI thin films with a thickness of 400 v nm grown on a (001) NaCl substrate by vacuum deposition. The energies of the heavy-hole and light-hole excitons in the CuI thin films are split at 10 v K by a thermal strain effect. Under intense excitation conditions, we have observed two types of photoluminescence bands originating from inelastic scattering of two n =1 heavy-hole excitons. One type is due to scattering into the n =2 heavy-hole-exciton state, and the other is due to scattering into the n =1 light-hole-exciton state. Optical gain spectra around the energies of these photoluminescence bands have been obtained by pump-probe measurements of transmittance. We discuss the excitation-power dependence of the optical gain based on a model for stimulated emission due to exciton-exciton scattering processes.

Electroreflectance and transfer-matrix analysis of Stark-ladder transitions in a GaAs/AlAs superlattice

We have investigated Stark-ladder transitions in a GaAs(6.4 nm)/AlAs(0.9 nm) superlattice (SL) by using electroreflectance (ER) and photocurrent (PC) spectroscopy for the detection and a simple transfer-matrix (TM) method for the analysis. It is demonstrated that ER spectroscopies is much more sensitive to detect the Stark-ladder transitions than PC spectroscopy and that the TM method is powerful for the analysis. In addition, we discuss the resonant coupling between the Wannier-Stark localization states of the electron subbands from the ER results and the TM analysis.

Hole-subband-order reversal in GaAsInxAl1−xAs strained-layer superlattices investigated by photoreflectance spectroscopy

Abstract We have measured photoreflectance (PR) spectra of the exciton transitions associated with the n =1 electron and hole subbands in GaAs (d G ) In 0.2 Al 0.8 As (d I ) strained-layer superlattices with d G = d I of 10, 40, and 100 A and a GaAs (100 A)/AlAs (100 A) superlattice to investigate the hole-subband order. The PR-intensity profiles of the heavy-hole and light-hole excitons demonstrate that the order of the n =1 heavy-hole and light-hole subbands is changed by the In concentration (strain effects) and the layer thickness (quantum-size effects). We have analyzed the experimental results according to an effective-mass approximation including strain effects.

Anisotropic properties of photoluminescence in a GaAs/AlAs type-II superlattice

Abstract We report the first study of photoluminescence (PL) emitted from a (1 1 0) cleaved edge plane of a (GaAs)12/(AlAs)12 type-II superlattice. We observed the direct (type-I) and indirect (type-II) PL bands at 77 K in surface and edge geometries. The type-II edge PL from the recombination between X electrons of AlAs and Γ heavy holes of GaAs is hardly affected by reabsorption effects through propagation along interfaces, which is in contrast with the strong reabsorption of the type-I edge PL. It has been clearly demonstrated that the type-II edge PL is polarized along interfaces. We discuss the polarization anisotropy on the basis of a first-order perturbation theory for a Γ-X mixing.