Valence Subbands Research Articles

InAs1−xSbx/In1−yGayAs multiple-quantum-well heterostructure design for improved 4–5 μm lasers

A new strained-layer multiple-quantum-well (MQW) heterostructure comprising InAs1−xSbx wells and In1−yGayAs barrier layers is proposed for improved 4–5 μm lasers. Both well and barrier compositions are close to InAs and are potentially well suited for high-quality crystal growth of MQWs on either InAs or GaSb substrates. Simple modeling shows favorable valence-band alignment suitable for carrier confinement, free of the staggered band alignment problem generally found in the conventional designs. Perturbation calculation of the effect of the compressive strain in the wells also shows sufficient valence subband separation. This and the reduced hole mass are expected to considerably lower the threshold carrier density and Auger recombination.

Polarized luminescence study of ordered nonlinear optical material ZnGeP2

Polarized photoluminescence (PL) and cathodoluminescence studies have been made as a function of excitation laser wavelength or electron beam energy for the ordered nonlinear optical material ZnGeP2 grown by the Bridgman method. The luminescence signal is seen to be strongly polarized. Furthermore, several PL peaks appear on top of one large broad band, and the dominant peak shifts to a higher energy position as the excitation energy increases. This observation can be explained by the transitions from three conduction bands (Γ6, Γ7, Γ7) to acceptor level one triplet ground state levels which parody the top three valence subbands (Γ6, Γ7, Γ6). This explanation agrees well with the theoretical energy band diagram of this pseudodirect band-gap material.

Photoinduced absorption within the valence Γ and the conduction L-subband manifolds in undoped GaSb/AlSb superlattices

We report the observation of L-valley conduction intersubband absorption together with Γ- valence intersubband in optically excited undopeal, indirect binary-binary short period GaSb/AlSb superlattices. The transitions were identified by comparing their position and polarization with calculations which take into account both valence and conduction subbands. The dependence of the intersubband absorption on the temperature, excitation intensity and modulation frequency, indicates that the photoexcited carriers are trapped with an activation energy of ≃50 meV and are long lived (≃5μ sec).

Analysis of GaInP/AlGaInP compressive strained multiple-quantum-well laser

We have analyzed GaInP/AlGaInP compressive strained MQW lasers, with theoretical calculation and experimental results. Our calculations of TE polarized gain, where the valence subband mixing and the heterobarrier leakage current are taken into account, are in good agreement with the experimental results. When a compressive strain of up to 0.5% is induced in the quantum wells, the density of states near the valence band edge is decreased, due to the reduction of heavy-hole and light-hole subband mixing. At the threshold condition, the compressive strain reduces not only the radiative recombination current, but also the hetero-barrier leakage current. Therefore, the threshold current is reduced, and its temperature dependence is found to be small, In the analysis, we also show that when larger compressive strain of more than 0.5% is induced in the 40-/spl Aring/-thick quantum wells, the threshold characteristics are degraded.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dispersion measurements of hot holes in quantum wells

An optical method for obtaining the valence subband dispersion of GaAs/AlGaAs quantum wells, including warping, is reviewed. Utilizing the recombination of hot electrons at neutral acceptors, the method gives meV accuracy. The dispersion has been measured for quantum wells with widths ranging from 3.3 nm to 9.8 nm. Measurements have been made for wavevectors from zero to greater than 10% of the Brillouin zone, corresponding to holes with as much as 50 meV of kinetic energy. The measurements are in excellent agreement with k.p calculations.

Strain Effect on 630 nm GaInP/AlGaInP Multi-Quantum Well Lasers

We theoretically analyzed the GaInP/AlGaInP strain effect of compressive- and tensile-strained multi-quantum well (MQW) lasers with the lasing wavelength of 630 nm and under a fixed optical confinement condition. Since the compressive-strained MQW has the weaker valence subband mixing, it can greatly reduce the hole band density of states, and results in the reduction of the threshold current. On the contrary, the tensile strain increases the density of states, so the strain of up to 0.5% has no effect in reducing the threshold current despite the large matrix element. In the comparison with the experimental results, however, the threshold current density of the compressive-strained MQW obtained in our calculation is much lower than that in the experimental results. It seems that the nonradiative recombination current, which is not included in our calculation, increases in the compressive-strained MQW because of its narrow well width.

Electroluminescence spectroscopy of resonant tunnelling in GaAs-AlAs superlattices

Resonant tunnelling between different subbands of adjacent wells is investigated with electroluminescence spectroscopy as a function of forward bias voltage. The observed near-bandgap electroluminescence lines indicate a strong inhomogeneity in the electric field as well as in the carrier distribution. This interpretation is supported by calculations. Electroluminescence lines originating from excited conduction and valence subband states demonstrate an efficient occupation of these subbands by resonant tunnelling.

Tensile-strain effects in quantum-well and superlattice band structures.

The effects of tensile strain on the energy-band structures of semiconductor quantum wells and superlattices (SL's) are studied theoretically, with emphasis on structures with unique valence-subband configurations achievable only through the use of tensile strain. Quantum wells are treated using finite-element envelope-function calculations which fully treat interactions between the light-hole, heavy-hole, and split-off valence bands, whereas strained SL's are modeled using a superlattice K\ensuremath{\cdot}p approach modified to treat strain effects. The two models are described in detail, tested for appropriate cases where both models should be applicable, and applied to prototype structures based on the III-V ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$/${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$As and ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/InP heterostructure systems. Single quantum wells are considered first. Transition energies are calculated and conveniently plotted as functions of composition (or strain) and layer thickness for both systems, and valence-subband band structures and k-dependent optical matrix elements are examined in detail for both systems in regimes where crossing of the uppermost light- and heavy-hole bands is induced by composition or well-width changes. Superlattices in both material systems are then considered, with emphasis on structures in which crossing of the uppermost valence subbands is induced by variation of barrier width. Band structures and optical matrix elements are calculated for wave vectors along directions parallel and perpendicular to the layer planes both in free-standing ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$/${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$As SL's with strain shared between the well and barrier layers and ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/InP SL's strained to lattice match to InP substrates. Finally, general features of band structures and optical matrix elements in tensile-strained structures inferred from these studies are summarized. Implications for tensile-strained quantum-well lasers are discussed briefly.

Optical properties of symmetrically strained (GaIn)As/Ga(PAs) superlattices grown by metalorganic vapour phase epitaxy

Symmetrically strained (GaIn)As/Ga(PAs) superlattice structures (symm. SLS) have been realized by metalorganic vapour phase epitaxy (MOVPE). The high structural as well as optical quality of these symm. SLS has been established by detailed high-resolution X-ray diffraction (XRD) and in particular photoluminescence (PL) and excitation spectroscopy (PLE) investigations. The optical recombination at low temperatures is due to excitons localized in statistical potential fluctuations in the (GaIn)As well layer. The exciton binding energy is determined in these symm. SLS to (5.5 ± 1) meV according to a theoretical description of the PLE spectra using a two-dimensional (2D) absorption model. Therefore, the exciton binding energy is significantly reduced as compared to unstrained GaAs quantum wells, caused by a considerably smaller in-plane hole mass for the top most valence subband. An estimate of this mass based on the determined exciton binding energy yields a value of about 0.05 m 0.

Wave function engineering in compressive- and tensile-strained laser structures

The engineering of the valence subbands for device applications has concentrated on the energy separation between heavy- and light-hole states. We show that the degree of overlap between the envelope functions of heavy- and light-hole states can affect the in-plane dispersion of the highest hole subband. We consider ways to reduce this overlap by spatially separating the heavy- and light-hole states to different layers, while maximizing their energy separation. Strain-compensated superlattices where opposite strains are introduced in the well and barrier regions offer such possibilities and lead to a significant increase of the optical gain in semiconductor lasers. We consider the In xGa 1- x As yP 1- y / In x'Ga 1- x' As y'P 1- y' system grown on an InP substrate where the wells are under biaxial compression while the barriers are under tension. In this type of structures, the electron and heavy-hole states are confined to the compressive layers whereas the light-holes are confined to the tensile layers. We also discuss the possibility of confining light-hole and electron states to wells under tension, of potential benefit for lasers operating in the transverse magnetic (TM) mode.

Valence-subband level crossing in GaAs/GaAsP strained-barrier quantum well structures observed by circularly polarized photoluminescence excitation spectroscopy

We have investigated the level configuration of the valence subbands in GaAs/GaAs 1− x P x ( x = 0.23) strained-barrier quantum well structures as a function of the well width. Circularly polarized photoluminescence excitation (CPPLE) spectroscopy was used to identify the heavy- or light-hole character of the optical transitions. Distinct polarization properties were observed in the strain-split heavy- and light-hole bands in the GaAsP barriers, which indicates the photoexcited electrons in the barriers retain spin orientation well even after the capture into the well and the relaxation process. Utilizing this polarization tendency of the transitions in the barriers, we could distinguish unequivocally the heavy- and light-hole character of the quantum well-related transitions. By varying the well width systematically, we observed the level crossing between n = 1 heavy- and light-hole transitions around the well width of 4 nm. This is in good agreement with the calculation based on the effective mass approximation.

Optimum strain for the suppression of Auger recombination effects in compressively strained InGaAs/InGaAsP quantum well lasers

The primary mechanism that leads to the suppression of Auger recombination effects in compressively strained quantum wells is found due to the existence and the appropriate positioning of negative-curvature zones in the first valence subbands (zones in k space where the radial curvature of the first valence subband is negative). By studying previous Monte Carlo calculation results, strong correlations between the negative-curvature zones and its adverse effects on Auger recombinations are observed, thereby confirming this finding. Through this understanding, it is inferred that compressive strain is most effective for reduction of Auger recombination effects when the amount of strain is no more than 1.0%. Beyond 1.0%, the gain in device performance with respect to more compressive strain is expected to be greatly diminished.

Comment on "Anisotropy of the confined hole states in a (311)A AlAs/GaAs/AlAs quantum-well system: Evidence for a camel's-back band structure"

In a recent paper [Phys. Rev. B 46, 15 586 (1992)], Hayden et al. found a weak in-plane dispersion of the lowest valence subbands in (311)A quantum-well systems by magnetotunneling spectroscopy. There is only qualitative agreement between the experimental and calculated dispersions. We point out that this actually implies that the observed interface corrugations govern the unusual electronic properties of non-(100)-oriented heterostructures and hence have to be taken into account in the interpretation of the experimental data and especially in the calculations.

The infrared response of lead phthalocyanine thin films to chlorine and iodine doping

Abstract Monoclinic thin films of PbPc were deposited on KBr disc substrates by a high-vacuum thermal-evaporation technique. The infrared response of the films was studied with respect to chlorine and iodine exposure in the range 400–4000 cm −1 . Doping with chlorine and iodine led to a generation of acceptor levels within the band gap and led to a splitting of the valence band. The data was analysed to estimate the intraband transition energy between the valence subbands. The ionization energy of the acceptor levels was estimated.

Optical measurements of warped valence bands in quantum wells

The valence subband dispersion of GaAs/AlGaAs quantum wells, including warping, is measured with meV accuracy using the radiative recombination of hot electrons at neutral acceptors. The measurements are shown to be in excellent agreement with k · p calculations. Differences in calculated dispersions from using various sets of Luttinger parameters proposed in the literature are discussed.

Fano resonances in the excitation spectra of semiconductor quantum wells.

Fano resonances are observed in the photoluminescence excitation spectra of GaAs-AlAs quantum wells. They originate from the Coulomb interaction that couples a discrete state and a continuum of excitonic states belonging to different pairs of conduction and valence subbands. This interpretation is supported by our calculations and by the behavior of the excitonic line shape in the presence of a magnetic field applied perpendicular to the layers.

Theoretical analysis of differential gain of 1.55 μm InGaAsP/InP compressive-strained multiple-quantum-well lasers

Basic design principles are formulated for increasing the differential gain of 1.55 μm InGaAsP/InP compressive-strained multiple-quantum-well (MQW) lasers. An InGaAsP quaternary active layer can provide more freedom of design compared to an InGaAs ternary active layer since the amount of strain and the quantum-well thickness can be independently determined in an InGaAsP material system for a given emission wavelength. Compressive strain ranging from 1% to 1.5% is large enough to reduce the density of states below one-third of that of unstrained wells. No further reduction is expected even if more compressive strain is applied. When the well thickness is, in turn, determined, it is essential to incorporate a trade-off between the conduction and valence subband energy spacings and the squared optical matrix elements. The extra enhancement of differential gain in MQW structures with a modulation p doping is also studied. By designing MQW structures according to these principles, differential gain can be increased to over 2×10−15 cm2, which corresponds to a relaxation oscillation frequency of more than 30 GHz at an output power of 20 mW.

Linear k term valence subband mixing and its effect on electroabsorption of tensile-strained GaAs/AlGaAs quantum wells

We have examined the contribution of a linear k term in the Luttinger Hamiltonian to electroabsorption of strained quantum wells. We have solved the 4/spl times/4 effective mass equation for the two lowest valence subbands and an appropriate set of differential equations for excitons. For bound excitons, our results indicate that the hh1/spl rarr/c1 transition is not forbidden for z-polarized electromagnetic waves, that the absorption spectrum can be independent of polarization, and that the hh1 and lh1 absorption edges, contrary to recent suggestion, cannot merge.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A suppression mechanism of Auger recombination effects in strained quantum wells induced by local negative curvature of the energy band structure

An analytical method has been developed to calculate distribution of carriers that undergo CHHS Auger recombinations in semiconductors. From this approach, it is further discovered that holes with a local negative effective mass are, statistically, not favored in the CHHS Auger recombination process. As extended regions in valence subbands of compressively strained quantum well structures possess a negative curvature-and thus a local negative hole effective mass-this mechanism is identified to be a significant factor that suppresses Auger recombination effects in compressively strained quantum well laser diodes. This suppression mechanism is also observed and confirmed by recent Monte Carlo calculation results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Second harmonic generation via excitations between valence sub-bands in p-type GaAsAlAs and SiSiGe quantum well structures

We present full-scale calculations of the frequency dependence of second harmonic generation due to intervalence sub-band transitions in p-type GaAsAlAs and SiSiGe asymmetric quantum well structures (QWSs). We demonstrate that the response is dominated by excitations lying throughout the Brillouin zone and that the states involved are not predicted by semiclassical models. Significant light and heavy hole momentum mixing away from the zone center is responsible for the second harmonic generation.