Valence Band Mixing Effects Research Articles

Confinement-induced valence-band mixing in CdS quantum dots observed by two-photon spectroscopy.

One- and two-photon absorption spectra of the quantum-confined transitions of CdS quantum dots are reported. The results show significant deviations from calculations based on the parabolic-valence-band approximation. The near-degenerate one- and two-photon transitions observed experimentally, and confirmed theoretically using Luttinger theory and numerical matrix diagonalization, clearly demonstrate the effects of valence-band mixing and Coulomb interaction.

Theoretical analysis of strained-layer InGaAs/GaAs quantum-well lasers with gain suppression and valence-band mixing

Linear gain, gain suppression, and L-I characteristics of strained-layer InGaAs/GaAs quantum-well laser are studied theoretically, taking into account valence-band mixing effects with biaxial compressive strain. It is found that the biaxial compressive strain substantially alters subband structure by pushing the light-hold subband bands into higher-energy states. It also alters the optical gain of a quantum-well laser. In particular, the biaxially compressed strained-layer InGaAs/GaAs quantum well shows a pronounced preference for TE polarization over TM polarization and the higher optical gain than does a typical GaAs/AlGaAs quantum well. The L-I characteristics are obtained self-consistently from the rate equations for the carrier and the photon densities and the calculated L-I curve shows reasonable agreement with the experimental data.

Interband optical transitions in doping superlattices

The absorption coefficient associated with interband transitions in doping superlattices is studied theoretically. Our calculations are based on a multiband effective-mass theory which simultaneously determines the electron and hole subband states self-consistency. Valence band mixing effects are calculated using a Luttinger-Kohn Hamiltonian. The absorption coefficients of both hetero- and homo-doping superlattices are evaluated from the complex dielectric function. The calculated absorption coefficients show that there are remarkable differences between our model and the conventional, uncoupled parabolic band model. Our results show that the valence band mixing can essentially eliminate the strong step-like structure in the absorption spectrum predicted by the conventional model. This suggests that the lack of an experimental observation of the step-like structure in the absorption spectrum may be due to valence band mixing and need not be attributed to potential fluctuations, spatial variations in the design parameters, or inhomogeneous excitation.

Theoretical gain in strained InGaAs/AlGaAs quantum wells including valence-band mixing effects

In this letter, we present the first detailed theoretical study of gain in strained InGaAs/AlGaAs quantum wells, taking into account the complex nature of the valence-subband structure, which must be included in any realistic model. We first compare the material gain as a function of carrier and radiative current density for a strained and unstrained quantum well. We then present calculations of theoretical differential gain, carrier density, and radiative current density at transparency as a function of indium mole fraction in the well.<lz> <lz> <lz>

Electric field dependence of exciton oscillator strengths in GaAs-AlxGa1-xAs quantum wells studied by photocurrent spectroscopy.

Photocurrent spectroscopy of GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As multiple-quantum-well structures with an electric field applied perpendicular to the heterointerface is used to study exciton oscillator strengths as a function of electric field up to \ensuremath{\sim}${10}^{5}$ V/cm. The electric field dependence of exciton oscillator strengths is attributed to a complicated interaction between local variation of zone-center electron- and hole-wave-function overlap and strong valence-band mixing. The results of experiment are compared with a theoretical calculation based on a multiband effective-mass approach which incorporates valence-band mixing effects, and in which good agreement is found. Strong mixing between the 2s and 2p states of the n=1 light-hole exciton and the 1s state of the n=2 heavy-hole to n=1 electron exciton is identified.

Valence-band mixing effects on the gain and the refractive index change of quantum-well lasers

The effects of valence-band mixing on the gain and on the refractive index change of the quantum-well laser and the effect of an applied electric field perpendicular to the quantum wells for gain switching are studied theoretically. Our calculations are based on the multiband effective-mass theory (k⋅p method) and the density-matrix formalism with the intraband relaxation taken into account. First, we calculate the nonparabolic valence-band structure by the finite difference method after making a unitary transformation of the Luttinger-Kohn Hamiltonian. The calculated gain for our model shows remarkable differences in both spectral shape and peak amplitude as compared with those for the conventional model of the parabolic valence band. The peak gain is reduced considerably and the gain spectrum is more symmetric in our model compared with that for the conventional model. The refractive index change shows a negative increment in the active region for both the TE and TM polarizations resulting in the antiguidance. However, the TM polarization shows more negative change which would result in the suppression of the TM modes as compared with the TE modes. As another example, the perpendicular electric field effects on the quantum-well laser with lateral current injection are also calculated. A red shift and a reduction of the gain spectrum are shown when an electric field is applied. This may have applications as a tunable and high-speed switching quantum-well laser.

Electric field dependence of exciton transition energies in GaAs-AlxGa

Photocurrent spectroscopy of GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As multiple-quantum-well structures in an electric field perpendicular to the heterointerface is used to study exciton transition energies as a function of electric field up to \ensuremath{\sim}${10}^{5}$ V/cm. A large number of excitonic transitions, as many as 16, are identified. The \ensuremath{\Delta}n\ensuremath{\ne}0 forbidden excitonic transitions grow in intensity with increasing electric field, whereas the \ensuremath{\Delta}n=0 allowed excitonic transitions show the opposite. Both negative and positive shifts of exciton transition energies are observed. The results of these observations are compared with those of a theoretical calculation based on a multiband effective-mass approach which incorporates valence-band mixing effects, and a good agreement is found.

Excitonic transitions in GaAs/GaxAl1−xAs quantum wells observed by photoreflectance spectroscopy: Comparison with a first-principles theory

Excitonic transitions in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As multiple quantum wells are observed using photoreflectance spectra obtained with a novel double-monochrometer spectrometer. The first pass of the monochrometer produces the infrared probe beam while the second pass is used as a synchronous band-pass filter, resulting in a better signal-to-noise ratio than the low-pass filter conventionally used, and allowing simultaneous collection of the photoluminescence spectrum. The photoreflectance data are analyzed using a multiband effective-mass theory which includes valence-band mixing and assumes a first-derivative modulation of the built-in field as the dominant modulation mechanism. The entire photoreflectance spectrum is simultaneously fit using a first-principles theory in which only the exciton linewidth and value of the built-in electric field are not determined by the theory. Full advantage is thus taken of the signal intensity information, in contrast to comparisons currently encountered in the literature where positions of excitons are usually compared to values predicted by a finite-square-well calculation, without valence-band mixing effects. The theory is shown to have much success in explaining the observed photoreflectance signal from a 200-A\r{} multiple quantum well at 77 K, including the qualitative line shape for any given exciton.

Index of refraction of GaAsAl xGa 1−xAs superlattices and multiple quantum wells

Recent research of superlattices and multiple quantum wells has generated considerable interest in the optical waveguiding properties of these structures for optoelectronic applications. As a result we present a theoretical study of the index of refraction of superlattices and determine its variation as a function of frequency and the superlattice parameters, i.e., layer width and AlAs composition. Γ-region exciton and valence-band mixing effects are included in the model. It is found that these two effects have an important influence on the value of the index of refraction and that superstructure effects rapidly decrease for energies greater than the superlattice potential barriers. Because of the quasi-two-dimensional character of the Γ-region excitons, our results indicate that the superlattice index of refraction can vary by ∼ 2% at the quantized, bound-exciton, transition energies. Overall, the theoretical results are in good agreement with the experimental data.

Direct Observation of Valence-Band Mixing in GaAs/GaPAs Strained-Layer Superlattices

ABSTRACTLow-temperature magneto-luminescence studies are used to examine the in-plane valence-band dispersion in GaAs/GaPAs strained-layer-superlattice structures. The results provide a direct observation of valence-band mixing effects in strained-layer systems.Theoretical and experimental studies of the effect of valence-band mixing upon the magneto-transport properties and magneto-luminescence data for GaAs/AlGaAs latticed-matched superlattices have been recently reported.[1–] Due to the presence of the 2D-quantum well, the valence-band degeneracy between the light-hole and heavy-hole bands at is removed. These studies show, however, that the lowest energy valence-band mass is still strongly dependent upon due to valence-band mixing. On the other hand, because of the large biaxial compression in strained-layer-super lattices (SLS), valence-band mixing effects should be minimal.[8–9] Recent magneto-transport measurements[10] on p-type InGaAs/GaAs SLS structures have provided information regarding nonparabolic valence-bands in these structures.