Strained-layer Quantum Wells Research Articles

Double AC photoreflectance spectroscopy of semiconductors

We report a new way of making photoreflectance (PR) measurements to overcome the problem of photoluminescence (PL) that is often encountered in low temperature PR measurements. In conventional PR, the probe beam is dc while the pump beam is chopped and in phase detection is done at that chopping frequency. At low temperatures a large PL signal arises at the chopping frequency of the pump beam (a laser) and swamps the PR signal. We overcome this problem by chopping both the pump beam as well as the probe beam at two different frequencies and detecting the PR signal at the sum frequency. This way we avoid the PL signal which now comes at a frequency which is not the frequency of detection. In this paper we discuss the details of this technique and present some low temperature PR data on epitaxial GaAs and GaAs-In/sub x/Ga/sub 1-x/As-GaAs strained layer quantum wells along with fitted lineshapes to show the feasibility of this technique.

High Power Semiconductor Lasers for Optical Fiber Amplifiers.

We review the state-of-the-art high power semiconductor lasers emitting at 0.98 and 1.48 μm, which are used for the pumping sources of the Er-doped fiber amplifiers. The introduction of strained-layer quantum wells to the active layer, together with graded-indexseparate-confinement-heterostructure enables us to improve the output power of semiconductor lasers. As a result, Er-doped fiber amplifiers with high signal output power are commercially available.

Characterization of GaP / InGaP and GaP / GaAsP strained-layer quantum wells grown by metalorganic chemical vapor deposition

GaP/InGaP and GaP/GaAsP strained-layer quantum well (QW) structures with 3.0–5.0 nm thick well layers in the direct gap region were grown on GaP substrates by metalorganic chemical vapor deposition (MOCVD) using triethylgallium (TEGa) and ethyldimethylindium (EDMIn) for the column III sources and 100% AsH 3 and PH 3 for the column V sources. The strained QW structures were characterized using TEM and 4.2 K photoluminescence (PL). Transmission electron microscope (TEM) images indicate that the structures are uniform with sharp, defect-free interfaces. The QW layers are clearly resolved and appear to be free of misfit dislocations or other defects. The InGaP QW structures exhibit improved 4.2 K PL linewidths and a shift of the peak energies to shorter wavelengths compared to InGaP bulk layers of the same compositions. The GaAsP QW structures also exhibit sharp PL emission linewidths. The band alignments for the GaP/InGaP and GaP/GaAsP strained heterojunctions were calculated using local-density-functional pseudopotential formalism and the model-solid approach. Optical transition energies between the conduction band and valence subbands at the Γ point were calculated for the InGaP and GaAsP strained QWs, taking into account both the strain-modified band structures and quantum confinement effects. By employing the results derived from the theoretical predictions, we show that the 4.2 K PL spectra of the InGaP strained wells exhibit both the n = 1 heavy-hole and light-hole related transitions. We also show that the degree of removal of valence band degeneracy can be determined from the PL spectra.

The effects of strain on intrasubband scattering rates in InP-based strained-layer quantum-well lasers

We analyze the strain effects (both compressive and tensile) on the intrasubband scattering rates for electrons and holes in InP-based strained-layer quantum-well (SL-QW) lasers. Carrier-carrier and carrier-LO-phonon interactions are taken into account on an equal basis within the fully dynamic random phase approximation for multi-subband structures at finite temperatures. The principal influences of the strain-induced changes in the valence band structures on the scattering rates are discussed in terms of the changes in the valence-band density of states and the phonon coupling with holes. We show that the hole-hole interaction plays a dominant role in determining the hole scattering rates regardless of the carrier energy. On the other hand, for electron scattering rates, the dominant scattering mechanism switches from electron-hole interaction to electron-electron interaction as the initial energy of electron increases. We also demonstrate that it is essential to take into account the mutual interactions among the individual scattering processes, such as carrier-carrier scattering and carrier-LO-phonon scattering, when evaluating the intrasubband scattering rates in SL-QW lasers.

Observation of “parity-forbidden” optical transitions in GaAs/GaAsP strained-layer quantum well

The photoluminescence (PL) of the GaAs/GaAsP strained-layer single quantum well structures have been investigated in detail. It was observed in the PL spectra that besides the normal “parity-allowed” n=1 electron to heavy hole excitonic transition, le-1hh, there are several relatively weak but unambiguous emission peaks which appear at the high energy side of the dominant le-1hh. The good agreement of the experiment with the calculation suggests that the “parity-forbidden” combinations should be responsible for these anomalous optical transitions.

Exciton binding energies and photo-induced tunnelling of carriers in (Ga,In)As-GaAs heterostructures grown with built-in piezoelectric field

We compare the binding energy of interacting electron and hole pairs in double quantum wells with and without internal piezo-electric fields. We show that the exciton binding is less sensitive to the piezo electric field than the oscillator strength. This allows many body-effects and bandgap renormalization to be easily produced in strained-layer quantum wells with internal built-in piezo-electric fields, under photo excitation. Our observation was made at low temperature by comparing the behaviour of Ga 0.92 In 0.08 gAs-GaAs strained layer single and double quantum wells grown along the (001) and (111) directions when the densities of photoinjected carriers are tuned over several decades. Comparison between experimental data and the results of a Hartree calculation including the space charge effects reveals that manybody interactions are efficiently photo-induced in the (111)-grown samples. Moreover, tunnelling of the two first excited heavy-hole levels can be stimulated for moderate carrier densities making such structure promissive for realising self electrooptic effect device (SEED) modulators

Band lineup and in-plane effective mass of InGaAsP or InGaAlAs on InP strained-layer quantum well

We describe the band lineups of InGaAlAs on (001) InP as well as InGaAsP on (001) InP system with strain effects, based on the Harrison model. We show that the compressive strain does not affect the band position so much, and tensile strain raises the band position in the InGaAsP system. It is also shown that both compressive and tensile strains raise the band positions in the InGaAlAs system. The conduction and valence band positions of InGaAs, InGaAsP, and InGaAlAs relative to InP valence band are given in approximate formulas as a function of the strain. We calculate the energy versus in-plane wave vector relationship of the InGaAsP/InGaAs(P) InGaAlAs/InGa(Al)As on InP strained quantum-well systems. We obtain the in-plane effective mass of the strained quantum-well system by fitting the dispersion relationship to a parabolic curve. The in-plane effective masses of several kinds of strained quantum-well systems are listed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-performance uncooled 1.3-μm Al/sub x/Ga/sub y/In/sub 1-x-y/As/InP strained-layer quantum-well lasers for subscriber loop applications

Design considerations for fabricating highly efficient uncooled semiconductor lasers are discussed. The parameters investigated include the temperature characteristics of threshold current, quantum efficiency, and modulation speed. To prevent carrier overflow under high-temperature operation, the electron confinement energy is increased by using the Al/sub x/Ga/sub y/In/sub 1-x-y/As/InP material system instead of the conventional Ga/sub x/In/sub 1-x/As/sub y/P/sub 1-y//InP material system. To reduce the transparency current and the carrier-density-dependent loss due to the intervalence-band absorption, strained-layer quantum wells are chosen as the active layer. Experimentally, 1.3-/spl mu/m compressive-strained five-quantum-well lasers and tensile-strained three-quantum-well lasers were fabricated using a 3-/spl mu/m wide ridge-waveguide laser structure. For both types of lasers, the intrinsic material parameters are found to be similar in magnitude and in temperature dependence if they are normalized to each well. Specifically, the compressive-strained five-quantum-well lasers show excellent extrinsic temperature characteristics, such as small drop of 0.3 dB in differential quantum efficiency when the heat sink temperature changes from 25 to 100/spl deg/C, and a large small-signal modulation bandwidth of 8.6 GHz at 85/spl deg/C. The maximum 3 dB modulation bandwidth was measured to be 19.6 GHz for compressive-strained lasers and 17 GHz for tensile-strained-lasers by an optical modulation technique. The strong carrier confinement also results in a small k-factor (0.25 ns) which indicates the potential for high-speed modulation up to 35 GHz. In spite of the aluminum-containing active layer, no catastrophic optical damage was observed at room temperature up to 218 mW for compressive-strained five-quantum-well lasers and 103 mW for tensile-strained three-quantum-well lasers. For operating the compressive-strained five-quantum-well lasers at 85/spl deg/C with more than 5 mW output power, a mean-time-to-failure (MTTF) of 9.4 years is projected from a preliminary life test. These lasers are highly attractive for uncooled, potentially low-cost applications in the subscriber loop.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of well number in 1.3-μm GaInAsP/InP GRIN-SCH strained-layer quantum-well lasers

Effects of well number in 1.3-/spl mu/m GaInAsP/InP GRIN-SCH compressively strained-layer quantum-well lasers were investigated from the viewpoint of the crystal quality of the strained-layer quantum wells and low threshold current operation at high temperature. As a result, we find that the optimum well number that gives the minimum threshold current increases with operating temperature and it was limited to the degradation of crystal quality of the quantum wells due to the critical thickness. A very high CW operating temperature of 170/spl deg/C was obtained. A very high output power of over 300 mW was also achieved. >

Observation of many-body effects and band-gap renormalization in low-dimensional systems with built-in piezoelectric fields.

We show that many-body effects and band-gap renormalization can be easily produced in strained-layer quantum wells with internal built-in piezoelectric fields, under photoexcitation. Our observation was made at low temperature by comparing the behavior of ${\mathrm{Ga}}_{0.92}$${\mathrm{In}}_{0.08}$As-GaAs strained-layer quantum wells grown along the (111) and (001) directions and tuning the densities of photoinjected carriers over several decades. Comparison between experimental data and the results of Hartree calculations including the charge space effects reveals that many-body interactions are photoinduced.

Determining the electric field in [111] strained-layer quantum wells

The electric field in a [111] growth-axis strained-layer quantum well embedded in a p-i-n diode is determined by measuring the polarization vector in the quantum well. The polarization is determined from the critical reverse bias necessary to produce zero electric field in the quantum well. The critical reverse bias is obtained from electroreflectance spectra, which have quantum well optical features that exhibit a 180° phase change at flat band. A depletion model of the p-i-n diode with an embedded quantum well is used to relate the electric field in the quantum well to the applied bias and the polarization vector in the well.

Optical properties of single and double (111)-grown (Ga, In)As-GaAs strained-layer quantum wells under strong photo-injection

We show that manybody-effects and bandgap renormalization can be easily produced in strained-layer quantum wells with internal built-in piezo-electric fields, under photo excitation. Our observation was made at low temperature by comparing the behaviour of Ga 0.92 In 0.08 As-GaAs strained layer single and double quantum wells grown along the (001) and (111) directions when the densities of photoinjected carriers is tuned over several decades. Comparison between experimental data and the results of a Hartree calculation including the space charge effects reveals that manybody interactions are efficiently photo-induced in the (111)-grown samples. Moreover, in the case of double quantum wells, additional transitions appear in the photoluminescence spectra, due to the tunnelling of the two first excited heavy-hole levels for moderate densities

Influence of piezoelectric fields on Rydberg energies in (Ga,In)As-GaAs single quantum wells embedded inp-i-nstructures

We have performed a variational calculation of the heavy-hole exciton for (Ga,In)As-GaAs strained-layer quantum wells embedded in p-i-n structures. The calculation has been made for sample growth along both the (001) and (111) directions. We show that the influence of the piezoelectric field may sometimes lead to strong orientation-dependent properties. In particular, when piezoelectric fields are present, the radiative lifetimes of heavy-hole excitons are strongly dependent on the thickness of the (Ga,In)As layer

Formation of optical gain due to exciton localization in Cd xZn 1− xS-ZnS strained-layer quantum wells

An exciton-related mechanism of optical gain formation has been studied in Cd x Zn 1− x S-ZnS strained-layer quantum wells which can cover ultraviolet spectral regions. Energy states of excitons in the quantum wells are broadened inhomogeneously by alloy composition fluctuations in ternary alloy well layers in addition to interface fluctuations between well and barrier layers. Excitons created initially are therefore localized at the lower energy states of inhomogeneously broadened exciton resonance. Optical gain spectra were analyzed on the basis of a phase-space-filling effect of these localized exciton states in which the energy shift of stimulated emission from the exciton resonance were well explained. It is proposed that the localization of excitons contributes to the formation of optical gain in wide-bandgap II–VI semiconductor quantum wells.

Quantitative characterization of modulation-doped strained quantum wells through line-shape analysis of room-temperature photoluminescence spectra

Room-temperature photoluminescence (PL) has been used to characterize modulation-doped AlGaAs/InGaAs/superlattice strained layer quantum wells. A phenomenological line-shape model has been developed which can be fitted to PL spectra in order to obtain key parameters such as the subband energies, Fermi energy, and transition amplitudes. Quantum well sheet densities calculated from fits to the PL spectra (taken at both room temperature and 77 K) have been compared to sheet densities obtained from low-temperature Hall measurements. It has also been shown how variations in quantum well composition, width, and symmetry can be characterized by shifts in values of the relevant fitting parameters.

Magneto-optical studies of strain effects on the excitons in InxGa1-xAs/AlyGa1-yAs strained quantum wells.

Low-temperature (4.2 K) magnetoreflectance measurements have been performed on a series of In x Ga 1-x As/Al y Ga 1-y As strained-layer quantum wells (QW's) with the InAs mole fraction x varying from 0 to 0.2, for three different well widths, and for the magnetic-field range of 0-9 T. The diamagnetic shift data of 1s, 2s, 3s,... excitons are fit to a theoretical model to determine the exciton binding energies and the in-plane reduced effective masses, which are presented as a function of x, for excitons in 80, 120, and 340 A In x Ga 1-x As/Al y Ga 1-y As QW's. In the case of high-barrier QW's, the binding energies and effective masses appear to decrease linearly with increasing x over the range studied

Electronic structure of (In,Ga) As(Ga, Al) As strained-layer quantum wells

We discuss the growth of epitaxial structures of (In, Ga) As on GaAs substrates, with elastic and plastic strain, and study the effect of the corresponding built-in strain on the photoluminescence of quantum wells inserted into such buffer layers. In addition, we examine the effect of built-in strain on the full electronic structure of strained-layer quantum wells. Reflectivity data taken at 2 K, where we could measure ground state type I, as well as excited type I and type II transitions, have revealed that a correct description of these heterostructures requires including the effect of the spin-orbit split-off states in the valence band physics as soon as the configuration of the electron-to-light-hole potential profiles switches from type II to type I. We anticipate that this effect should be included for the calculation of threshold currents in quantum well lasers made from such heterostructures.

Valence band structure calculations of GaAs/Ga xIn 1-xP strained-layer quantum wells

Valence band structure of GaAs/Ga xIn 1-xP strained-layer quantum wells are investigated within an envelope function formalism which takes into account the valence subbands mixing through the Luttinger-Kohn Hamiltonian and the strain effects due to the lattice-mismatch through the Pikus-Bir Hamiltonian. For a Ga mole fraction x greater than 0.516 (compression), the top valence band is heavy-hole-like. For x less than 0.516 (tension strain), the top valence subband can be either heavy-hole-like or light-hole-like, depending on the well width and on the shear deformation potential. Interesting features are also obtained for the effective masses of the valence subbands from ϵ( K → ) energy dispersion calculations along the in-plane K → direction.

Structural characterization of highly strained InAs N monolayer lasers and quantum well structures by X-ray diffraction and transmission electron microscopy

X-ray interference effect and transmission electron microscopy are used to study the relaxation process in a series of laser structures as a function of InAs content in the quantum well. It is shown that the X-ray interference effect is a powerful, fast and non-destructive method to assess the strain status in samples of this kind. A set of strained layer laser structures containing N monolayers of InAs ( Nx(InAs) 1(GaAs) 3 with ( N = 1, 3, 5, 7) in an 8 nm quantum well active region and a set of strained layer quantum wells consisting of P monolayers of InAs ( Px(InAs) 1(GaAs) Q with ( P = 2, 4 and ( Q = 2, 4) were grown [Dotor et al., J. Crystal Growth 127 (1993) 46] by atomic layer molecular beam epitaxy. X-ray interference effect and cross-section transmission electron microscopy analysis of the samples show that in the series of lasers with N monolayers of InAs the whole laser structure is coherent with the substrate (and consequently dislocation free) for 1 and 3 monolayers of InAs, while a sample with 5 monolayers of InAs is in a certain stage of relaxation (dislocation density n d ≌10 7 cm -2) and a sample with 7 monolayers of InAs is almost completely relaxed ( n d≌10 8 cm -2). In strained layer quantum well samples, the influence of the InAs/GaAs thickness ratio ( P/ Q) on the critical thickness has also been studied. These results are compared with those predicted by theoretical critical thickness models. Optical characterization as well as threshold current measurements of the lasers are correlated with X-ray diffraction and transmission electron microscopy relaxation status results.

InGaAs/InP strained layer quantum wells grown by molecular beam epitaxy

Different heterostructures containing InGaAs/InP quantum wells, separate confinement heterostructures and superlattices are grown by molecular beam epitaxy on InP substrates. These layers are grown in tensile and compressive strain and their luminescence emission wavelength is kept around 1550 nm. The growth parameters (temperature, substrate quality) are found to be of prime importance to obtain high quality materials.