Conduction band offset of strained InGaP by quantum well capacitance-voltage profiling

The conduction band alignment of compressively strained In_1−xGa_xP relative to lattice matched InGaP/GaAs has been determined by capacitance-voltage profil-ing. A modified version of Kroemer’s capacitance-voltage profiling method is developed wherein a quantum well is profiled instead of a single heterojunction. A one-dimensional Poisson-Schrodinger solver was used to fit the reconstructed carrier profiles corresponding to a value of ΔE_c at varying temperatures. Schottky barrier diode structures containing a single strained InGaP quantum well were grown by low pressure metalorganic chemical vapor deposition. The two strained compositions studied contained 35 and 31% gallium. Conduction band offsets of 101 and 131 meV were found for the 35 and 31% samples, respectively, with an estimated accuracy of ±5 meV. These results agreed closely with values predicted by empirical calculations.     

Materials characteristics of pseudomorphic high electron mobility transistor structures with InxGa1−xas single quantum well and GaAs-InxGa1−xas (0.25 < x < 0.4) thin strained superlattice active layers

Growth and characterization results are presented for pseudomorphic high electron mobility transistor structures with In_xGa_1-xAs single quantum well and GaAs(h ₁)In _x Ga_1−x As(h ₂) thin strained superlattice active layers where 0.25≤x ≤ 0.4. All of the samples were grown by molecular beam epitaxy. Hall effect at 77 K, photoluminescence at 2 K, in-situ reflection high energy electron diffraction, and transmission electron microscopy measurements are discussed. Critical layer thickness measurements are compared with the Matthews-Blakeslee theory. Photoluminescence transition energies are compared with a self-consistent solution to Schrodinger’s and Poisson’s equations.     

Growth of GaAs1−xPx/GaAs and InAsxP1−x/InP strained quantum wells for optoelectronic devices by gas-source molecular beam epitaxy

In this paper we show that pseudomorphically strained heterostructures of InAs _x P_1−x /InP may be an alternative to lattice-matched heterostructures of In_1−x Ga _x As _y P_1−y /InP for optoelectronic applications. We first studied the group-V composition control in the gas-source molecular beam epitaxy (GSMBE) of the GaAs_1-x P _x /GaAs system. Then we studied GSMBE of strained InAs _x P_1−x /InP multiple quantum wells with the ternary well layer in the composition range 0.15 <x < 0.75. Structural and optical properties were characterized by high-resolution x-ray rocking curves, transmission electron microscopy, absorption and low-temperature photoluminescence measurements. High-quality multiple-quantum-well structures were obtained even for highly strained (up to 2.5%) samples. The achievement of sharp excitonic absorptions at 1.06, 1.3 and 1.55μm at room temperature from InAs _x P_1−x /InP quantum wells suggests the possibility of long-wavelength optoelectronic applications.     

Temperature-dependent excitonic absorption in long-period multiple In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs quantum well structures

Temperature variations in the fundamental absorption edge of long-period In _x Ga_{1 − x} As/GaAs structures are studied for samples with different numbers of quantum wells and similar periods. The quantum wells were close in composition and width. Experimental data are interpreted in the model of exciton-polariton light transfer involving localized excitons in confined structures with a finite number of quantum wells. The experimentally observed low-temperature anomaly of the integrated absorption coefficient is attributed to reemission of resonance localized excitons along a finite chain of quantum wells, with no excitonic transfer. The radiative decay time of an exciton in a single quantum well is estimated from the experimental data. It is demonstrated that, at low temperatures, the major contribution to the width of the experimentally observed absorption line corresponding to the ground heavy-hole exciton state is made by inhomogeneous broadening of the line by the field of potential fluctuations associated with the compositional disorder of the alloy. At low temperatures, the inhomogeneous broadening is much more pronounced than the broadening governed by the true radiative and nonradiative dissipative decay.     

Simulation of capacitance-voltage characteristics of heterostructures with quantum wells using a self-consistent solution of the Schrödinger and Poisson equations

Methods are developed for calculating capacitance-voltage characteristics and finding the concentration profile of free charge carriers in semiconductor doped heterostructures containing a quantum well. The capacitance-voltage characteristic of a heterostructure with a quantum well was calculated using a numerical self-consistent solution of the Poisson and Schrödinger equations in the context of a unified quantum-mechanical approach. The suggested method was applied to the simulation and analysis of the experimental capacitance-voltage characteristics of heterostructures with strained InGaAs/GaAs quantum wells.     

High density 2DEG in III-V semiconductor heterostructures and high-electron-mobility transistors based on them

Situation in high-electron-mobility transistor (HEMT) technology is discussed. The N-AlGaAs/InGaAs/GaAs pseudomorphic HEMT’s are now considered as most advanced for mmwave monolithic circuits, but metamorphic N-In_xAl_1−x As/In_yGa_1−y As/In_xAl_1−x As HEMT’s grown on GaAs substrates are very promising for the future high-frequency devices. High density 2DEG in HEMT’s is analyzed by means of the Hall effect and photoluminescence measurements. Processing technology of the sub-0.25-μm pseudomorphic HEMT’s, metamorphic HEMT’s and their characteristics are also described. Fiz. Tekh. Poluprovodn. 33, 1064–1065 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Analysis of mechanisms for electron scattering in GaAs/AlxGa1−x As superlattices with doped quantum wells for longitudinal resonant current flow in high electric fields and at low temperatures

Formulas are derived for, and a numerical analysis made of, the dependence of the transverse phase relaxation time on electron energy for resonant current flow through GaAs/Al_xGa_1−x As superlattices with doped quantum wells. The parameters are chosen to be close to those of superlattices used for creating photodiodes for operation at λ⋍10 μm. The analysis is limited to the interactions of electrons with neutral atoms and impurity ions at low temperatures. Resonant current flow is ensured by an electric field that brings the ground state and the first excited state of the “Stark ladder” into resonance with neighboring, weakly interacting quantum wells. Fiz. Tekh. Poluprovodn. 33, 438–444 (April 1999)     

Low-field anomaly of the hall effect in disordered two-dimensional systems

This study is devoted to investigation of the nonlinear behavior of the Hall resistance in low magnetic fields. When investigating two-dimensional electron gas in single GaAs/In _x Ga_{1 − x} As/GaAs quantum wells, it is shown that the anomaly of the Hall effect in disordered systems can be described taking into account the second-order quantum corrections to conductivity.     

Study of defects in heterostructures with GaPAsN and GaPN quantum wells in the GaP matrix

Heterostructures with GaP/GaP_{1 − x} N _x and GaP/GaP_{1 − x − y} As _x N _y quantum wells grown by the MOCVD method are studied by methods of capacitance-voltage profiling and capacitive and current deep level transient spectroscopy. In heterostructures with GaP/GaP_{1 − x} N _x quantum wells, intrinsic defects with deep levels of 0.17 and 0.08 eV are revealed. It is shown that a considerable decrease in the concentration of these defects occurs with the substitution of a ternary GaP_{1 − x} N _x alloy forming the region of the quantum well by a GaP_{1 − x − y} As _x N _y quaternary alloy. The nature of emerging defects and mechanisms of decreasing their concentration are discussed.     

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.     

InGaAs/InP heterostructures with strained quantum wells and quantum dots (λ=1.5–1.9 µm)

Strongly strained In_xGa_1−x As/In_0.53Ga_0.47As/InP heterostructures with indium content x=0.69−1.0 in the active region were investigated experimentally and theoretically. Two types of structures were obtained by vapor-phase epitaxy from metalorganic compounds: 1) with isolated compression-strained quantum wells and 2) with self-organized nanosize InAs clusters (quantum dots). The temperature dependence of the quantum radiation efficiency of samples with quantum wells in the temperature range 77–265 K is characterized by T ₀=43 K. One reason for the low value of T ₀ is electron delocalization in the active region. The maximum radiation wavelength obtained in structures with quantum dots is 1.9 μm at 77 K. Fiz. Tekh. Poluprovodn. 33, 1105–1107 (September 1999)     

Determination of the density of states in quantum wells and quantum dot arrays by the capacitance-voltage method

The possibility of determining the electronic density of states in quantum wells and quantum dot arrays in heterostructures from the capacitance-voltage curve is investigated. In heterostructures fluctuations of the composition and geometrical dimensions play an important role. It is shown that to reconstruct the exact density of states from the measured capacitance-voltage curve is impossible, because this problem is ill-posed from the mathematical point of view. An approximate method is proposed for solving the problem, involving the determination of a “reduced” density of states. It is shown that the reduced density of states is close to the true density if the characteristic energy scale governing the variation of the latter is much greater than the thermal energy kT. The proposed method is used to find the density of states in the conduction band of a quantum well in an In_0.22Ga_0.78As/GaAs heterostructure. Fiz. Tekh. Poluprovodn. 33, 1246–1252 (October 1999)     

Optical intersubband transitions in strained quantum wells utilizing In1−x GaxAs/InP solid solutions

Infrared absorption in strained p-type In_1−x Ga_xAs/InP quantum wells is investigated for both possible types of strain (tensile and compressive). It is observed that the normalincidence absorption increases considerably under compressive strain (when the ground state is a heavy-hole state) and decreases under tensile strain (when the ground state is a light-hole state). The peak absorption in the compressed quantum well can attain very large values, on the order of 5000 cm⁻¹ at a hole density ∼ 10¹² cm⁻²; this attribute makes “compressed” p-type quantum wells attractive for IR detection applications. Fiz. Tekh. Poluprovodn. 33, 83–90 (January 1999)     

Microstructural Aspects of Nucleation and Growth of (In,Ga)As-GaAs(001) Islands with Low Indium Content

Molecular beam epitaxy growth of multilayer In _x Ga_1-x As/GaAs(001) structures with low indium content (x = 0.20–0.35) was studied by X-ray diffraction and photoluminescence in order to understand the initial stage of strain-driven island formation. The structural properties of these superlattices were investigated using reciprocal space maps, which were obtained around the symmetric 004 and asymmetric 113 and 224 Bragg diffraction, and ω/2θ scans with a high-resolution diffractometer in the triple axis configuration. Using the information obtained from the reciprocal space maps, the 004 ω/2θ scans were simulated by dynamical diffraction theory and the in-plane strain in the dot lattice was determined. We determined the degree of vertical correlation for the dot position (“stacking”) and lateral composition modulation period (LCM) (lateral ordering of the dots). It is shown that initial stage formation of nanoislands is accompanied by LCM only for [110] direction in the plane with␣a period of about 50 to 60 nm, which is responsible for the formation of a quantum wire like structure. The role of In _x Ga_1-x As thickness and lateral composition modulation in the formation of quantum dots in strained In _x Ga_1-x As/GaAs structures is discussed.     

AlGaInAs/InP strained-layer quantum well lasers at 1.3 µm grown by solid source molecular beam epitaxy

Al_xGa_yIn_1−x−yAs/InP strained-layer multiple-quantum-well lasers emitting at 1.3 μm have been grown by solid source molecular beam epitaxy, and the performance characteristics have been studied. The lasers contain 4, 5, or 6 compressively strained quantum wells in the active region. They exhibit low transparency current densities, high gain coefficients, and high characteristic temperatures compared to conventional GaInAsP/InP quantum well lasers. The results show that desired lasing features can be achieved with relatively simple layer structures if the doping profiles and waveguide structures are properly designed and the material is grown to high structural perfection.     

Valence band offsets in strained GaAs1−xPx/GaAs heterojunctions

Valence band offsets at [100]-oriented heterojunctions between tensile-strained GaASj_1−xP_x and unstrained GaAs are studied experimentally and theoretically. Light-hole (LH) and heavy-hole (HH) offsets are first extracted from the well-width dependence of valence subband splittings observed in luminescence spectra of tensile-strained GaAs_1−xP_x/GaAs quantum wells of various compositions (x = 0.06,0.09, and 0.19). This data is then combined with results from two other laboratories, yielding a set of 30 independent experimental offset values for junctions with compositions throughout the range 0.06≤x ≤0.32. The data are found to be highly consistent, with linear fits δE_LH = −140x (meV) and δE_HH= −401x (meV) describing the measured offsets to within less than 5 meV on average. Experimental results are then compared with theoretical predictions for the GaAs_1−x P_x/GaAs system obtained from a tight-binding model for strained heterojunctions. Predictions from the tight-binding calculations are found to lie within experimental scatter for the LH offsets, which define the valence band edge in these heterostructures, while magnitudes of the tight-binding HH offsets exceed measured values by ~20% on average.     

Surface wave excitation auger electron spectroscopy of lnGaAs/GaAs(001) grown by alternate molecular-beam epitaxy

An angle-resolved electron-beam-excitation Auger-electron-spectroscopy, called as “surface wave excitation Auger electron spectroscopy (SWEAES),” was developed to characterize the semiconductor surface on an atomic scale. SWEAES enables us to get information about (1) surface valence electron states, (2) surface inner potentials concerned with high energy electron diffraction at the surface wave resonance (SWR) condition, (3) surface composition of ultra-thin heterostructures, and (4) confinement effects of diffracted surface electron waves at the SWR condition. These effects were demonstrated for In_1-x Ga _x As/GaAs(001)-c(8 × 2)-In/Ga and (GaAs)₂/(InAs)₁/GaAs(001) strained single quantum well surfaces. The current address: Dept. Elect. Eng., Faculty of Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-Ku, Kobe, 657, JAPAN     

Influence of the cap layer thickness on photoluminescence properties in strained InGaAs/GaAs single quantum wells

The influence of the GaAs cap layer thickness on the luminescence properties in strained In_0.20Ga_0.80As/GaAs single quantum well (SQW) structures has been investigated using temperature-dependent photoluminescence (PL) spectroscopy. The luminescence peak is shifted to lower energy as the GaAs cap layer thickness decreases, which demonstrates the effect of the GaAs cap layer thickness on the strain of InGaAs/GaAs single quantum wells (SQW). We find the PL quenching mechanism is the thermal activation of electron hole pairs from the wells into the GaAs cap layer for the samples with thicker GaAs cap layer, while in sample with thinner GaAs cap layer exciton trapping on misfit dislocations is dominated.     

Investigation of the band structure of the strained systems InGaAs/GaAs and InGaAs/AIGaAs by high-pressure photoluminescence

In_xGa_1-xAs quantum wells grown pseudomorphically in GaAs and AlGaAs with values ofx up to 0.25 have been studied by photoluminescence under high hydrostatic pressure. We concentrate here on the pressure range where the emissions quench and take on the characteristics of theX-minima. In the InGaAs/GaAs structures, these transitions display an unexpected pressure coefficient, -2.6 meV/kbar, twice that of theX minima in GaAs. We assign these transitions to theX minima in the wells, and therefore make a direct measurement of the strainedX positions as a function of composition. In the InGaAs/AIGaAs structures the crossovers occur against theX-minima in the barriers and these crossovers yield an accurate value for the band offset ratio for InGaAs/GaAs heterojunctions which is found to be 60:40 (CB:VB).     

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-Al_0.25Ga_0.75As/In _x Ga_{1 – x} As/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.