Strained-layer Quantum Wells Research Articles

Einstein's Photoemission from Quantum Confined Superlattices.

This paper is dedicated to the 83th Birthday of Late Professor B. R. Nag, D.Sc., formerly Head of the Departments of Radio Physics and Electronics and Electronic Science of the University of Calcutta, a firm believer of the concept of theoretical minimum of Landau and an internationally well known semiconductor physicist, to whom the second author remains ever grateful as a student and research worker from 1974-2004. In this paper, an attempt is made to study, the Einstein's photoemission (EP) from III-V, II-VI, IV-VI, HgTe/CdTe and strained layer quantum well heavily doped superlattices (QWHDSLs) with graded interfaces in the presence of quantizing magnetic field on the basis of newly formulated electron dispersion relations within the frame work of k · p formalism. The EP from III-V, II-VI, IV-VI, HgTe/CdTe and strained layer quantum wells of heavily doped effective mass superlattices respectively has been presented under magnetic quantization. Besides the said emissions, from the quantum dots of the aforementioned heavily doped SLs have further investigated for the purpose of comparison and complete investigation in the context of EP from quantum confined superlattices. Using appropriate SLs, it appears that the EP increases with increasing surface electron concentration and decreasing film thickness in spiky manners, which are the characteristic features of such quantized hetero structures. Under magnetic quantization, the EP oscillates with inverse quantizing magnetic field due to Shuvnikov-de Haas effect. The EP increases with increasing photo energy in a step-like manner and the numerical values of EP with all the physical variables are totally band structure dependent for all the cases. The most striking features are that the presence of poles in the dispersion relation of the materials in the absence of band tails create the complex energy spectra in the corresponding HD constituent materials of such quantum confined superlattices and effective electron mass exists within the band gap which is impossible without the concept of band tails. The well-known result of EP for bulk semiconductors having parabolic energy bands can be obtained as a special case of our generalized formulation and thus confirming the compatibility test. The content of this paper finds four important applications and we have suggested the methods of experimental determinations of important transport quantities in the field of quantum effect devices of nanoscience and nanotechnology.

Many-body effects on the ground-state energy in semiconductor quantum wells

The ground-state energy due to exchange interaction and screening of the Coulomb correlation for the electron-hole plasma in strained-layer quantum wells is examined as functions of sheet carrier density and biaxial strain. We calculate the leading-order self-energy within the full random-phase approximation for consideration of many-body effects, taking into account the valence-band non-parabolicity. We solve the Luttinger–Kohn Hamiltonian in the k·p method considering valence-band mixing to obtain the valence-band structure for the holes. It is shown that the ground-state energy strongly depends on the sheet carrier density and strain. We also see that the screening of Coulomb correlation plays an important role in determination of the ground-state energy of the strained-layer quantum wells.

In x Ga 1–x As Strained-Layer Quantum Well in a Pseudomorphic Heterostructure: High-Resolution XRD Characterization for Different Quantum-Well Thicknesses

Strained-layer quantum wells of different thicknesses are realized in the form of an n-Al0.25Ga0.75As/InxGa1 – xAs/GaAs pseudomorphic heterostructure. The structural properties of the quantum wells are determined by high-resolution double-crystal XRD and by photoluminescence spectroscopy. The depth profile of In mole fraction is derived from the rocking curves. It indicates that the quantum-well interfaces are strongly smeared and hence their In mole fractions are less than the target value, 0.19. The XRD data are compared with the photoluminescence ones.

Optical properties of ZnS/ZnMgS strained-layer quantum wells

We report optical properties of ZnS/ZnMgS strained-layer single quantum wells. The main photoluminescence (PL) peak from ZnS is attributed to light-hole (LH) free excitons. The emission due to heavy-hole (HH) excitons was also observed. With decreasing well width, these emission lines shift to higher energy owing to quantum confinement. Time-resolved PL spectra suggest rapid relaxation from HH band to LH band in the ZnS well. Localization of excitons in the ZnMgS barrier, which is due to fluctuation of alloy composition, is also suggested. On the other hand, localization of excitons in the ZnS well, caused by the fluctuation of the well width, was hardly observed for the sample with good quality. The LH exciton emission showed fast initial decay time of 16–36 ps, which seems to also depend on the sample quality.

Effect of Carrier Localization on Optical Gain Formation in CdxZn1-xS/ZnS Quantum Wells

We present a theoretical study on optical gain formation due to the localized state in CdxZn1-xS/ZnS strained-layer quantum wells. We have calculated the optical gain coefficient, which is produced when carriers are localized in a quantum disk due to potential fluctuation of the alloy in a CdxZn1-xS well. It is assumed in our model that the density of the localized state in this quantum disk can be followed by Gaussian distribution function. It has been revealed that the formation of optical gain due to the localization of carriers takes place at a carrier concentration of about 1×1018 cm-3, which is one order of magnitude lower than that of band-to-band transition.

Comment on “Two-dimensional spin confinement in strained-layer quantum wells”

For quasi-two-dimensional p channels, second-order corrections of the perturbation theory to the Zeeman splitting of heavy-hole subbands, which originate from the in-plane magnetic-field component, were found to have magnetic-field dependence and an order of magnitude different from those calculated in a previous paper [R.W. Martin et al., Phys. Rev. B 42, 9237 (1990)]. Because of the zero first-order corrections, second-order corrections determine the behavior of energy separations between magnetic levels of low-lying heavy-hole subbands in a tilted magnetic field which is quite different from that of electrons.

InGaAsP Lasers on GaAs Fabricated by the Surface Activated Wafer Direct Bonding Method at Room Temperature

1.3 µm InGaAsP/InP strained-layer quantum wells ridge waveguide lasers have been successfully fabricated on a GaAs substrate by the surface activated wafer direct bonding method at room temperature. In this method, the surfaces of two wafers are activated by Ar fast atom beam irradiation and mated in a high vacuum. A high bonding strength was attained. No microcracks and voids were found under transmission electron microscopy(TEM). A threshold current density of 500 A/cm2 was achieved. This technique is very promising to realize a monolithic of optoelectronic integrated circuits for optical communications and interconnections.

1.55 μm High Speed Partly Gain-Coupled DFB Lasers with Periodically Etched Strained-Layer Quantum Wells

The high speed performance of partly gain-coupled (GC) DFB lasers consisting of periodically etched strained-layer quantum wells (QW's) is reviewed with comparisons to the equivalent index-coupled (IC) DFB lasers with the same active layers. It is shown that the GC DFB laser has a –3 dB modulation bandwidth of 22 GHz at 10 mW with a stable single mode oscillation at the longer side of the Bragg Stop-band due to in-phase gain coupling. A theoretical analysis is also presented based on the local-normal-mode transfer-matrix laser model which takes into account both the longitudinal distribution of laser parameters and carrier transport effects. The mechanism for high modulation bandwidth of the GC DFB laser is attributed to a higher differential gain due to a reduced carrier transport time which is provided by an effecient carrier injection from the longitudinal etched interface of the QW's.

A Multistate Switch with Double Delta-Doped Strained-Layer Quantum Wells

A GaAs–InGaAs switching device, prepared by molecular beam epitaxy (MBE), has been investigated. A double delta-doped (δ-doped) strained-layer quantum-well structure is employed to produce the potential wells for the carrier confinement and the potential barriers for carriers thermionically emitted over them. Equipped with the different doping levels of the δ-doped sheets, the barriers are successively collapsed to form a double negative-differential-resistance (NDR) phenomenon in the current–voltage ( I–V ) characteristics when a sufficient external bias is applied to this device. Moreover, because the temperature variation changes the carrier confinement efficiency and carrier thermal energy, the NDR characteristics are sensitive to environmental temperature. The influence of temperature on the device performance is also discussed.

GSMBE growth and characterization of strained-layer MQWs in a P-i-N Configuration

InxGa1-xAs/InP (0.39 less than or equal to x less than or equal to 0.68) strained-layer quantum wells having 20 wells with thickness of 50 Angstrom in a P-i-N configuration were grown by gas source molecular beam epitaxy (GSMBE). High-resolution X-ray diffraction rocking curves show the presence of up to seven orders of sharp and intense satellite reflection, indicative of the structural perfection of the samples. Low-temperature photoluminescence and low-temperature absorption spectra were used to determine the exciton transition energies as a function of strain. Good agreement is achieved between exciton transition energies obtained experimentally at low temperature with those calculated using the deformation potential theory.

Growth and Characterization of Vertical-Cavity Surface-Emitting Lasers Grown on (311)A-Oriented GaAs Substrates by Molecular Beam Epitaxy

We have fabricated InGaAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) grown on GaAs(311)A substrates by molecular beam epitaxy. High-quality crystals for VCSEL structures on (311)A having flat and smooth heterointerfaces were obtained by optimizing the growth conditions. It was experimentally demonstrated that the optical anisotropic gain of (311)-oriented strained-layer quantum wells is effective for polarization control. A [*BAR*2*BAR*33] polarization mode is stable against the injection currents of 3 times the threshold. In addition, the threshold current density of 80 A/(cm2·well) is achieved for a device with a laser aperture as large as 25 µ m square, which is comparable to or even lower than that of (100)-oriented VCSELs.

A1.3 μm strained quantum well laser on a graded InGaAs buffer with a GaAs substrate

We grew a 1.3 μm strained-layer quantum well (SL-QW) laser with InGaP cladding layers on a lattice-relaxation buffer layer by metalorganic vapor phase epitaxy. For the lattice-relaxation buffer, we used a compositionally graded InGaAs/GaAs structure. The significantly reduced surface roughness of the InGaP cladding layers achieved by supplying a large amount of H2Se enables CW operation of our 1.3 μm SL-QW laser. We achieved a low threshold current of less than 10 mA and a high characteristic temperature of 100K around room temperature.

Temperature dependence of the threshold current in gain-coupled distributed feedback lasers with periodically etched quantum wells: Mechanism for an appearance of infinite T

A phenomenological model describing the temperature dependence of the threshold current of 1.3 μm gain-coupled distributed feedback (DFB) lasers with periodically etched strained-layer quantum wells is presented and compared to experimental results. The model can predict the minimum threshold current as a function of temperature (infinite T0), which has been observed experimentally. The mechanism for the appearance of this minimum is explained by the decrease of the detuning of the DFB-mode wavelength from the material gain-peak wavelength as the temperature increases. The condition for single-mode operation is also discussed with use of the effective gain of DFB laser structures.

Spectroscopy of biexcitons in [formula omitted] strained-layer quantum wells

We investigate the excitation-intensity-dependent photoluminescence spectroscopy at 10 K of the biexciton recombination in Zn1 − xCdxSeZnSe strained-layer quantum-wells with Cd-contents of 12 and 14% and QW thicknesses of 30 and 60 Å. A simple analysis of the line shape of both exciton and biexciton features and photoluminescence excitation spectroscopy allows us to give an accurate value of the biexciton binding energy.

Optical Determination of the Heavy-hole Effective Mass of (In, Ga)As/ GaAs Quantum Wells

We determine the reduced mass of heavy-hole exciton and the heavy-hole in-plane mass for a series of (In, Ga)As/GaAs strained layer quantum wells using the magnetoluminescence measurements of the exciton ground state and the modified perturbation approach. In the theoretical calculation of the magnetoexciton ground state, the exciton reduced mass is considered as an adjustable parameter, and two variation parameters are used in the unperturbed wave function which is expressed in terms of subband wave functions in the growth axis and the product of two-dimensional hydrogen and oscillatorlike wave functions for the in-plane component. We take into account the energy dependence of transverse and in-plane electron masses in the two-band effective mass approximation. The electron effective mass decreases as either quantum-well width or indium composition increases, and so does the heavy-hole in-plane mass down to the value at the decoupling limit (mhh, p = 0.11m0).

Temperature dependence of the photoluminescence of Zn1−xCdxSe/ZnSe strained-layer quantum wells

We investigate by temperature-dependent photoluminescence (PL) spectroscopy between 9 K and 300 K Zn1−xCdxSe/ZnSe strained-layer quantum wells (QWs) with Cd contents ranging between 12% and 19% and QW thicknesses between 9 and 175 Å, i.e., with confinement energies up to 220 meV. In the whole temperature range the PL spectra are dominated by E1-HH1 free-exciton recombinations. Between 9 and 300 K the intensity of this line is reduced by three to four orders of magnitude while transitions involving excited states progressively emerge. An analysis of the thermal quenching of the PL intensity reveals that for all confinement energies the escape of excitons out of the QWs is the mechanism responsible for this quenching near 300 K.

Very high characteristic temperature and constant differential quantum efficiency 1.3-μm GaInAsP-InP strained-layer quantum-well lasers by use of temperature dependent reflectivity (TDR) mirror

A very high characteristic temperature T/sub 0/ of 150 K (25-70/spl deg/C) or 450 K (25-50/spl deg/C) and an almost constant differential quantum efficiency operation in the temperature range of 25-70/spl deg/C were achieved in 1.3-/spl mu/m GaInAsP-InP strained-layer quantum-well (SL-QW) lasers by use of a novel temperature dependent reflectivity (TDR) mirror composed of multiple quarter-lambda thickness /spl alpha/-Si-SiOx dielectric films with quarter-lambda shift in the vicinity of center portion, The mechanism of high T/sub 0/ and constant differential quantum efficiency were explained using the structural parameters, transparent current density and gain coefficient of a SL-QW laser that are derived experimentally. The effect of TDR mirror was confirmed by measuring the temperature dependence of net gain of a SL-QW laser with TDR mirror. It was found that less temperature dependent net gain due to the decrease of mirror loss with temperature played an important role for improving the temperature characteristics of threshold current. Almost constant differential quantum efficiency over a wide temperature range is attributed to the increase of the facet reflectivity with temperature.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

SOME INVESTIGATIONS OF THE PHYSICAL PROPERTIES OF (Ga, In)As-GaAs HETEROSTRUCTURES GROWN WITH BUILT-IN PIEZOELECTRIC FIELD

The properties of single and double (Ga,ln)As-GaAs strained-layer quantum wells embedded in (pin) diodes are studied. These properties are found to be orientation-dependent, mainly due to the existence of a strong internal piezoelectric field in the (Ga,ln)As layers when the growth axis is polar. We first calculate how large the influences of the (pin) and piezoelectric field are to produce carrier tunnelling out of the active part of the heterostructure. This enables us to compute the carrier’s lifetime in the heterostructures and the corresponding resonance widths. Next, we compare the binding energies of interacting electron and hole pairs in double quantum wells with or without internal piezo electric fields. We show that the exciton binding energy is less sensitive to the piezoelectric field than the oscillator strength. Under photo excitation, many body-effects and bandgap renormalization can be easily produced in strained-layer quantum wells with internal built-in piezo-electric fields. We illuminated at low temperature single and double Ga 0.92 ln 0.08 As-GaAs strained layer quantum wells grown either along the (001) or (111) direction, and tuned over several decades the densities of photo-injected carriers. The comparison between experimental data and the results of a Hartree calculation including the space charge effects reveals that many body interactions are efficiently photo-induced in the (111)-grown samples. Moreover, we show that the tunnelling of the two lowest-lying heavy-hole levels can be stimulated for moderate carrier densities making such structures promissive in order to realise self electrooptic effect device (SEED) modulators.

Zeeman splitting and g factor of heavy-hole excitons in InxGa1-xAs/GaAs quantum wells.

Zeeman splittings have been investigated for n=1 heavy-hole excitons ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs strained-layer quantum wells at 1.8 K and at magnetic fields up to 6 T applied parallel to the growth axis (001). The sign and magnitude of the splitting to precision of \ifmmode\pm\else\textpm\fi{}2 \ensuremath{\mu}eV were obtained from the shift in the exciton photoluminescence line upon switching between right and left circularly polarized detection. Measurements have been made for well widths between 3 and 12 nm and for x values of 0.075 and 0.11. Linear field dependence is observed below 1.5 T giving exciton g factors. For higher fields in narrow wells the dependence becomes nonlinear. We have calculated the g factor using an eight-band k\ensuremath{\cdot}p model and obtain satisfactory agreement with experiment by using the Luttinger parameter \ensuremath{\kappa} for ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As as a variable parameter. From this we obtain an empirical concentration dependence \ensuremath{\kappa}\ensuremath{\approxeq}7.68x+1.1(1-x)-4.0x(1-x).

Theoretical analysis of temperature sensitivity of differential gain in 1.55-μm InGaAsP-InP quantum-well lasers

We study the temperature sensitivity of the differential gain in InGaAsP-InP strained-layer (SL) quantum-well (QW) lasers operating at a wavelength of 1.55 μm. Electrostatic deformation in conduction-band and valence-band profiles is taken into account by solving Poisson's equation and the effective-mass equations for conduction and valence bands in a self-consistent manner. We demonstrate that electrostatic deformation in both band profiles plays a significant role in determining the temperature sensitivity of the differential gain in 1.55-μm InGaAsP-InP SL QW lasers. The physical mechanism for limiting the differential gain at elevated temperatures is also discussed.