Technology and electronic properties of PHEMT AlGaAs/In y(z)Ga1 − y(z)As/GaAs compositionally graded quantum wells

Graded In _y Ga_{1 ? y} As quantum well epitaxial technology is developed for engineering the band potential profile. The crystal structure of the samples is clarified by high-resolution X-ray diffraction. The influence of quantum-well bending on the crystal and electron transport properties is studied on one- and two-side δ-doped Al_0.23Ga_0.77As/In _y Ga_{1 ? y} As/Al_0.23Ga_0.77As PHEMT heterostructures. The highest InAs content gradient reached is 1.2%/nm for the mean InAs content y = 0. 2. Optimization of the InAs content grading leads to an increase in the electron mobility and concentration. This effect is related to the straightening and deepening of the quantum-well potential profile. In addition, the electron wavefunction shifts toward the quantum-well center, thus reducing electron scattering.     

MOVPE grown Ga1−xInxAs solar cells for GaInP/GaInAs tandem applications

Lattice-mismatched Ga_1−xIn_xAs solar cells with an absorption edge between 900 and 1150 nm have been grown on GaAs substrates. Different graded Ga_1−xIn_xAs buffer layers and solar cell structures were evaluated to achieve a good electrical performance of the device. External quantum efficiencies comparable to our best GaAs solar cells were measured. The best 1 cm² cell with a bandgap energy of 1.18 eV has an efficiency of 22.6% at AM1.5g and a short circuit current density of 36.4 mA/cm². To our knowledge, this is the highest reported efficiency for a Ga_0.83In_0.17As solar cell.     

Features of defect formation during the growth of double heterostructures for injection lasers based on Al x Ga1 − x As y Sb1 − y /GaSb materials

A lack of lattice defects and, in particular, a lack of dislocations in the active layer in complex multilayer heteroepitaxial systems is the basic condition for the efficient and reliable operation of optoelectronic microdevices. Minimum elastic stresses in multilayer heteroepitaxial systems and their lack in the active layer at that elevated temperature that occurs in an efficiently operating electronic device is the second necessary condition for its long-term operation.     

Temperature dependence of the photoluminescence properties and band gap energy of InxGa1−xAs/GaAs quantum wells

The effective band gap energy of In_xGa_1−xAs/GaAs strained quantum wells (QWs) is investigated by photoluminescence spectroscopy (PL) in the range 12–295 K. The temperature dependence of the band gap energy of strained QWs correlates well with that of bulk In_xGa_1−xAs of similar composition. Deviations from the band gap variation of bulk material at low temperatures (12–90 K) are interpreted in terms of exciton localization. The differences ΔE(12 K) between the measured PL peak energies and the expected transition energies at 12 K (obtained by simulating the measured temperature dependence of the PL peak positions by the well-known Varshni relation) are suggested to be closely related to the Stokes shifts that often exist between PL and PL excitation spectra of QWs. A linear relation is found between the PL full-width at half-maximum measured at 12 K and ΔE for a range of QWs prepared under different growth conditions. Excitonic recombination is inferred to be dominant in the PL transitions at the highest temperatures investigated—even at room temperature.     

Growth and Characterization of In x Ga1−x As/GaAs1−y P y Strained-Layer Superlattices with High Values of y (~80%)

Strained-layer superlattice (SLS) structures, such as InGaAs/GaAsP lattice matched to GaAs, have shown great potential in absorption devices such as photodetectors and triple-junction photovoltaic cells. However, until recently they have been somewhat hindered by their usage of low-phosphorus GaAsP barriers. High-P-composition GaAsP was developed as the barrier for InGaAs/GaAsP strained-layer superlattice (SLS) structures, and the merits of using such a high composition of phosphorus are discussed. It is believed that these barriers represent the highest phosphorus content to date in such a structure. By using high-composition GaAsP the carriers are collected via tunneling (for barriers ≤30 Å) as opposed to thermionic emission. Thus, by utilizing thin, high-content GaAsP barriers one can increase the percentage of the intrinsic in a p-i-n structure that is composed of InGaAs wells in addition to increasing the number of periods that can be grown for given depletion width. However, standard SLSs of this type inherently possess undesirable compressive strain and quantum size effects (QSEs) that cause the optical absorption of the thin InGaAs SLS wells to shift to higher energies relative to that of bulk InGaAs of the same composition. To circumvent these deleterious QSEs, stress-balanced, pseudomorphic InGaAs/GaAsP staggered SLSs were grown. Staggering was achieved by removing a portion of one well and adding it to an adjacent well. The spectral response obtained from device characterization indicated that staggering resulted in thicker InGaAs films with reduced cutoff energy. Additionally, these data confirm that tunneling is a very effective means for carrier transport in the SLS.     

Design of Dislocation-Compensated ZnS y Se1−y /GaAs (001) Heterostructures

The understanding of lattice relaxation and dislocation dynamics in lattice-mismatched semiconductors makes it possible to design metamorphic device structures utilizing the dislocation compensation mechanism for reduced defects, improved performance, and enhanced reliability. We have developed a dislocation dynamics model accounting for misfit–threading interactions and have applied it to ZnS _y Se_1?y /GaAs (001) heterostructures.1 Dislocation compensation involves the removal of threading dislocations associated with one sense of misfit dislocations by bending them over to create misfit dislocations of the opposite sense at an intentionally mismatched interface. Here we investigated the design of dislocation-compensated ZnS _y Se_1?y /GaAs (001) heterostructures and considered the sulfur mole fraction tolerances applicable to such structures. We considered two types of structures: type A involved a uniform-composition (ungraded) layer on top of a uniform-composition buffer, while type B involved a uniform-composition layer on a linearly graded buffer. For each structure type we studied the requirements on the thickness and compositional profile of the buffer layer to optimize the removal of mobile threading dislocations from the top uniform (device) layer as well as the allowed tolerance in compositional overshoot to achieve structures with low threading dislocation density. We show for both types of structure that (i) for given compositional overshoot at the buffer–device layer interface, there is an optimum buffer thickness which minimizes the dislocation density; and (ii) for given buffer thickness there is an optimum overshoot which minimizes the dislocation density.     

Metamorphic GayIn1−yP/Ga1−xInxAs tandem solar cells for space and for terrestrial concentrator applications at C > 1000 suns

The use of Ga_1−xIn_xAs instead of GaAs as a bottom solar cell in a Ga_yIn_1−yP/Ga_1−xIn_xAs tandem structure increases the flexibility of choosing the optimum bandgap combination of materials for a multijunction solar cell. Higher theoretical efficiencies are calculated and different cell concepts are suggested for space and terrestrial concentrator applications. Various Ga_yIn_1−yP/Ga_1−xIn_xAs material combinations have been investigated for the first time and efficiencies up to 24·1% (AM0) and 27·0% (AM1·5 direct) have been reached under one-sun conditions. An efficiency of 30·0–31·3% was measured for a Ga_0·35In_0·65P/Ga_0·83In_0·17As tandem concentrator cell with prismatic cover at 300 suns. The top and bottom cell layers of this structure are grown lattice-matched to each other, but a large mismatch is introduced at the interface to the GaAs substrate. This cell structure is well suited for the use in next-generation terrestrial concentrators working at high concentration ratios. For the first time a cell efficiency up to 29–30% has been measured at concentration levels up to 1300 suns. A small prototype concentrator with Fresnel lenses and four tandem solar cells working at C = 120 has been constructed, with an outdoor efficiency of 23%. Copyright © 2001 John Wiley & Sons, Ltd.     

Study of heterostructures with a combined In(Ga)As/GaAs quantum dot/quantum well layer and a Mn δ layer

Using high-resolution transmission electron microscopy and photoelectric spectroscopy methods, the effect of Mn δ layer embedding and GaAs coating layer growth techniques in structures with In(Ga)As/GaAs quantum dots and wells on their structural and optoelectronic characteristics is studied. It is shown that the low-temperature GaAs coating layer in a structure with a Mn δ layer is structurally inhomogeneous and can cause a decrease in the quantum-dot photosensitivity.     

1.06 μm high-power InGaAs/GaAsP quantum well lasers

The high power and low internal loss 1.06 μm InGaAs/GaAsP quantum well lasers with asymmetric waveguide structure were designed and fabricated. For a 4000 μm cavity length and 100 μm stripe width device, the maximum output power and conversion efficiency of the device are 7.13 W and 56.4%, respectively. The cavity length dependence of the threshold current density and conversion efficiency have been investigated theoretically and experimentally; the laser diode with 4000 μm cavity length shows better characteristics than that with 3000 and 4500 μm cavity length:the threshold current density is 132.5 A/cm², the slope efficiency of 1.00 W/A and the junction temperature of 15.62 K were achieved.     

Design of S-Graded Buffer Layers for Metamorphic ZnS y Se1−y /GaAs (001) Semiconductor Devices

We present design equations for error function (or “S-graded”) graded buffers for use in accommodating lattice mismatch of heteroepitaxial semiconductor devices. In an S-graded metamorphic buffer layer the composition and lattice mismatch profiles follow a normal cumulative distribution function. Minimum-energy calculations suggest that the S-graded profile may be beneficial for control of defect densities in lattice-mismatched devices because they have several characteristics which enhance the mobility and glide velocities of dislocations, thereby promoting long misfit segments with relatively few threading arms. First, there is a misfit-dislocation-free zone (MDFZ) adjacent to the interface, which avoids dislocation pinning defects associated with substrate defects. Second, there is another MDFZ near the surface, which reduces pinning interactions near the device layer which will be grown on top. Third, there is a large built-in strain in the top MDFZ, which enhances the glide of dislocations to sweep out threading arms. In this paper we present approximate design equations for the widths of the MDFZs, the built-in strain, and the peak misfit dislocation density for a general S-graded semiconductor with diamond or zincblende crystal structure and (001) orientation, and show that these design equations are in fair agreement with detailed numerical energy-minimization calculations for ZnS _y Se_1?y /GaAs (001) heterostructures.     

Dilute nitrides and 1.3 μm GaInNAs quantum well lasers on GaAs

We present epitaxial growth of GaInNAs on GaAs by molecular beam epitaxy (MBE) using analog, digital and N irradiation methods. It is possible to realize GaInNAs quantum wells (QWs) with a maximum substitutional N concentration up to 6% and a strong light emission up to 1.71 μm at 300 K. High quality 1.3 μm GaInNAs multiple QW edge emitting laser diodes have been demonstrated. The threshold current density (for a cavity of 100×1000 μm²) is 300, 300, 400 and 940 A/cm² for single, double, triple and quadruple QW lasers, respectively. The maximum 3 dB bandwidth reaches 17 GHz and high-speed transmission at 10 Gb/s up to 110 °C under a constant voltage has been demonstrated.     

Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.     

Transport properties of InGaAs/GaAs Heterostructures with δ-doped quantum wells

The lateral transport of electrons in single- and double-well pseudomorphic GaAs/n-InGaAs/GaAs heterostructures with quantum wells 50–100 meV deep and impurity δ-layers in the wells, with concentrations in the range 10¹¹ < N _s < 10¹² cm⁻², has been investigated. Single-well structures with a doped well at the center exhibit a nonmonotonic temperature dependence of the Hall coefficient and an increase in low-temperature electron mobility with an increase in the impurity concentration. The results obtained indicate that the impurity-band electron states play an important role in the conductivity of these structures. Involvement of the impurity band also allows to explain adequately the characteristics of the conductivity of double-well structures; in contrast to single-well structures, band bending caused by asymmetric doping is of great importance. The numerical calculations of conductivity within the model under consideration confirm these suggestions.     

Comparison of different concepts of InAs quantum dot growth on GaAs for 1.3-μm-range lasers

The DWELL (dots-in-a-well), DUWELL (dots-under-the-well), and GC-SRL (gradient-composition-strain-reducing-layer) concepts of InAs quantum dot epitaxial growth under identical conditions have been compared. The laser structures are designed for operation near 1.3 ??m. An improved procedure is proposed for estimating the InAs growth rate. The dependence of the photoluminescence peak??s spectral position and intensity on the structure type and epitaxy conditions has been studied. A 10-stack DUWELL ridge waveguide laser diode with ground-state lasing at room temperature has been demonstrated.     

Optoelectronic properties of cubic BxInyGa1−x−yN alloys matched to GaN for designing quantum well Lasers: First-principles study within mBJ exchange potential

A Full-Potential Linearized Augmented Plane Wave calculation within density functional theory is performed to investigate the electronic and optical properties of cubic B_xIn_yGa_1−x−yN alloys matched to GaN with low-Boron content (x≤0.187). The exchange-correlation potential is treated by the local density approximation (LDA) to calculate the structural properties. The band structure and density of states of these compounds are well predicted by modified Becke–Johnson (mBJ) exchange potential compared to LDA and generalized gradient approximation (GGA). Also, the optical properties are calculated by the mBJ exchange potential. The computed structural parameters are found to be in good agreement with experimental and theoretical data. The B_xIn_yGa_1−x−yN alloy is expected to be lattice matched to GaN substrate for (x=0.125, y=0.187). The incorporation of B and In into GaN substrate allows the reduction of the band gap energy. The real and imaginary parts of the dielectric function, refractive index, reflectivity and absorption coefficient are discussed on the basis on the energy band structure and the calculated density of states. The optical properties of B_xIn_yGa_1−x−yN depend on the incorporated Boron content (with y=0.187). This means that B_xIn_yGa_1−x−yN could constitute an active layer in single quantum well for the design of high-efficiency solar cells and optoelectronic devices as Laser Diodes operating in the UV spectral region.     

GaAsSb/GaAs strained structures with quantum wells for lasers with emission wavelength near 1.3 μm

Optimum conditions for the growth of the GaAs_{1 − x} Sb _x /GaAs heterostructures by the method of molecular-bean epitaxy are determined; it is shown that effective long-wavelength photoluminescence at T = 300 K can be obtained at wavelengths as long as λ = 1.3 μm by increasing the antimony incorporation. As the excitation power is increased, the appearance of a short-wavelength line (in addition to a shift of a photoluminescence maximum to shorter wavelengths characteristic of the type II heterojunctions) related to direct optical transitions in the real space takes place; this relation is confirmed by the results of studying the photoluminescence spectra with subpicosecond and nanosecond time resolution in the case of pulsed excitation.     

Structural and optical properties of heavily doped Al x Ga1 − x As1 − y P y :Mg alloys produced by metal-organic chemical vapor deposition

The high-resolution X-ray diffraction technique, Raman spectroscopy, and photoluminescence spectroscopy are used to study the structural, optical, and electron energy properties of epitaxial Al _x Ga_{1 ? x} As_{1 ? y} P _y :Mg alloy films grown by metal-organic chemical vapor deposition (MOCVD). It is shown that the introduction of a Mg impurity into the quaternary alloy provides high charge-carrier concentrations. A decrease in the growth temperature yields a decrease in the charge-carrier concentration in films doped with magnesium at a small gas-carrier flux of the acceptor impurity, whereas an increase in the flux results in an increase in the acceptor-impurity concentration, which is reflected in the character of the photoluminescence spectra.     

Comparison of Continuously- and Step-Graded ZnS y Se1−y /GaAs (001) Metamorphic Buffer Layers

The design of metamorphic buffer layers for semiconductor devices with reduced defect densities requires control of lattice relaxation and dislocation dynamics. Graded layers are beneficial for the design of these buffers because they reduce the threading dislocation density by (1) allowing the distribution of the misfit dislocations throughout the buffer layer therefore reducing pinning interactions, and (2) enhancing mobility from the high built-in surface strain which helps to sweep out threading arms. In this work, we considered heterostructures involving a linearly-graded (type A) or step-graded (type B) buffer grown on a GaAs (001) substrate. For each structure type, we studied the equilibrium configuration and the kinetically-limited lattice relaxation and non-equilibrium threading dislocations by utilizing a dislocation dynamics model. In this work, we have also considered heterostructures involving a constant composition ZnS _y Se_1?y device layer grown on top of a GaAs (001) substrate with an intermediate buffer layer of linearly-graded (type C) or step-graded (type D) ZnS _y Se_1?y . For each structure type, we studied the requirements on the thickness and compositional profile in the buffer layer for the elimination of all mobile threading dislocations from the device layer by the dislocation compensation mechanism.     

Properties of epitaxial (Al x Ga1 − x As)1 − y C y alloys grown by MOCVD autoepitaxy

The growth of epitaxial Al _x Ga_{1 ? x} As:C alloys by metal-organic chemical vapor deposition (MOCVD) at low temperatures results in the formation of quaternary (Al _x Ga_{1 ? x} As)_{1 ? y} C _y alloys, in which carbon atoms can be concentrated at lattice defects in the epitaxial alloy with the formation of impurity nanoclusters.     

Modelling of the electronic structure for an AlGaAs/GaAs/AlGaAs HFET with a δ-doping layer inside the quantum well

The effect of a δ-doping layer inside the quantum well of an AlGaAs/GaAs/AlGaAs HFET is studied by a self-consistent modelling of the band structure across the device in dependence on the geometric dimensions, the δ-doping density, and the gate voltage. The energy position of the δ-like donors inside the quantum well is attached to the lowest subband energy. Especially the results show the possibility to increase the channel charge and to control its spatial distribution which could make possible an increase of the mobility of the device. It is shown that the potential obtained by a Modified Thomas-Fermi Approximation is very suitable as a starting potential for the self-consistent calculation or even as a final result in special cases.