Far-infrared radiation from n-InGaAs/GaAs quantum-well heterostructures in high lateral electric fields under injection conditions

The infrared radiation emitted by hot electrons in n-InGaAs/GaAs quantum-well heterostructures subjected to a lateral electric field is investigated under conditions of carrier injection from the current contacts. In structures with double tunneling-coupled wells one of which is δ-doped, a pronounced increase in the intensity of far-infrared radiation upon the onset of carrier injection is observed. At the same time, this effect is lacking in single-quantum-well structures with doped wells or barriers. The observed increase in the radiation intensity is associated with the direct intersubband transitions of electrons which contribute to emission upon the real-space transfer of charge carriers between wells. The intensity of these transitions increases due to compensation of the space charge existing between the wells by the injected holes.     

On the temperature delocalization of carriers in GaAs/AlGaAs/InGaAs quantum-well heterostructures

The effect of temperature delocalization in semiconductor lasers (emission wavelength λ = 1060 nm) based on symmetric and asymmetric separate-confinement heterostructures fabricated by metal-organic vapor-phase epitaxy (MOVPE) is studied. Experimental and calculated estimates show that the carrier concentration in the waveguide increases by an order of magnitude when the temperature of a semiconductor laser is raised by ～100°C. It is found that an increase in the temperature of the active zone leads to enhancement of the temperature delocalization of both electrons and holes. It is shown that the delocalization of holes begins at higher temperatures, compared with that of electrons. It is demonstrated experimentally that the onset of temperature delocalization depends on the threshold carrier concentration in the active region of a laser at room temperature. It is found that raising the energy depth of the active region by choosing the waveguide material makes it possible to fully suppress the temperature-delocalization process up to 175°C.     

Negative differential resistance in InAs/GaSb single-barrier heterostructures

The authors report the first observation of negative differential resistance (NDR) at room temperature in InAs/GaSb barrier heterostructures, where the conduction band in InAs lies below the GaSb valence band.<>     

Electroreflectance spectra of InGaN/AlGaN/GaN quantum-well heterostructures

p?n InGaN/AlGaN/GaN heterostructures with InGaN/AlGaN multiple quantum wells are studied by electroreflectance spectroscopy. The structures are grown by metal—organic epitaxy and arranged with the p region in contact with the heat sink. Light is incident on and reflected from the structures through the sapphire substrate. To modulate the reflectivity, rectangular voltage pulses and a dc reverse bias are applied to the p?n junction. A line corresponding to interband transitions in the region of InGaN/AlGaN multiple quantum wells is observed in the electroreflectance spectra. The peak of this line is shifted to shorter wavelengths from the peak of injection luminescence of the light-emitting diode structures. The low-field model developed by Aspnes is used to describe the electroreflectance spectra. By choosing the parameters of the model to fit the experimental data, the effective band gap of the active region of the structure, E _g ^* , is determined at 2.76–2.78 eV. The experimental dependence of E _g ^* on the applied voltage is attributed to the effect of piezoelectric fields in the InGaN quantum wells. In the electroreflectance spectra, an interference pattern is observed in the wide spectral range from 1.4 to 3.2 eV. The interference is due to the dependence of the effective refractive index on the electric field.     

Spectra of persistent photoconductivity in InAs/AlSb quantum-well heterostructures

Persistent photoconductivity at T = 4.2 K in AlSb/InAs/AlSb heterostructures with two-dimensional (2D) electron gas in InAs quantum wells is studied. Under illumination by IR radiation (?ω = 0.6–1.2 eV), positive persistent photoconductivity related to the photoionization of deep-level donors is observed. At shorter wavelengths, negative persistent photoconductivity is observed that originates from band-to-band generation of electron-hole pairs with subsequent separation of electrons and holes by the built-in electric field, capture of electrons by ionized donors, and recombination of holes with 2D electrons in InAs. It is found that a sharp drop in the negative photoconductivity takes place at ?ω > 3.1 eV, which can be attributed to the appearance of a new channel for photoionization of deep-level donors in AlSb via electron transitions to the next energy band above the conduction band.     

Imperfections and resonant tunneling in quantum-well heterostructures

We discuss effects of structural imperfections on the I-V characteristics and local density-of-states of semiconductor double-barrier heterostructures. Using a linear-chain model for GaAs/AlGaAs structures, we simulate fluctuations in layer thicknesses, defects at the heterointerfaces, and disorder.     

Photoelectrical memory in GaAs/AlGaAs multilayer quantum-well structures

An increase in the dark current (by 2–3 orders of magnitude) in GaAs/Al_xGa_1−x As multilayer quantum-well structures with x⋍0.4 is observed after illumination of the structures with optical light (λ<1.3 μm). This increase is sustained for an extended time (more than 10³ s) at low temperatures. It then decreases to its initial value upon heating of the sample. A model of the barrier with local sag of the conduction band facilitating tunneling is proposed. The conduction band sag and the magnitude of the current grow due to optical ionization of uncontrolled deep level clusters present in the barrier and decrease due to subsequent capture of electrons from the conduction band by the deep levels upon heating. Fiz. Tekh. Poluprovodn. 32, 209–214 (February 1998)     

Band-edge line-up in GaAs/GaAsN/InGaAs heterostructures

Triple GaAs/GaAsN/InGaAs heterostructures were grown by MBE on GaAs substrates, and their optical properties were studied. The band-edge line-up in GaAs/GaAsN and InGaAs/GaAsN heterostructures was analyzed by correlating the experimental photoluminescence spectra with the known parameters of the band diagram in (In,Ga)As compounds. It is shown that a GaAs/GaAsN heterojunction is type I, while an In_xGa_1?x As/GaAsN heterojunction can be type I or type II, depending on the In content x.     

Multichannel carrier scattering at quantum-well heterostructures

An efficient combined numerical-analytical technique is developed for calculating states of the continuum spectrum in systems with quantum wells (QWs) with an arbitrary potential shape, described by a system of coupled Schrödinger equations, e.g., hole states in semiconductor QWs. Continuum-spectrum states are found exactly using the approach similar to the scattering theory. Scattering states (the in/out-solutions) and the S-matrix for the case of multichannel scattering in one-dimensional systems with QWs are constructed, and their symmetry is determined and analyzed. The method is applied to studying the hole scattering by GaInAs-InGaAsP QWs with strained layers. The hole transmission and reflection coefficients and the delay-time energy dependence are calculated in relation to parameters of the structures and values of the transversal momentum components. In the energy range in which the channel with heavy hole conversion into a propagating light hole is closed, scattering of the heavy hole on a QW has a resonant nature.     

Induced disorder of AlAs-AlGaAs-GaAs quantum-well heterostructures

The effects of induced disorder on the layered structure and luminescence properties of Al_xGa_1-xAs quantum-well heterostructures (QWH’s) grown at Ts = 750°C by metalorganic chemical vapor deposition (MO-CVD) are investigated. High-temperature thermal anneals in the range 1000–800°C > T_s result in the interdiffusion of Al and Ga and disordering of the QWH active region. Similar results are obtained at the much lower temperature 500–600°C < T₈ by impurity (Zn) diffusion, which greatly enhances the Al-Ga interdiffusion process. It is also possible, by the use of a Si₃N₄ surface mask, to disorder selectively only certain portions of a QWH, thus leaving unchanged the as-grown quantum-well layered structure in the remaining areas. Lower-bandgap QWH’s can be dis ordered and changed into higher-bandgap material, including from direct gap to indirect gap     

Thermal relaxation in strained InGaAs/GaAs heterostructures

The kinetics of strain relaxation during annealing of epitaxial InGaAs films grown on GaAs by MOCVD at atmospheric pressure was studied by optical microscopy and transmission electron microscopy. The density of misfit dislocations and crosshatching was measured as a function of annealing temperature and annealing time. These experiments show that the generation of misfit dislocations during the annealing process is thermally activated. The kinetic rate constant increases as thickness increases. After long annealing times, the sample reaches its steady-state condition in which a residual strain apparently still exists. Apparently this residual strain is not accommodated by misfit dislocations and does not change with annealing temperature nor sample thickness.     

Spin relaxation in asymmetrical heterostructures

Electron spin relaxation by the D’yakonov-Perel’ mechanism is investigated theoretically in asymmetrical III–V heterostructures. Spin relaxation anisotropy for all three dimensions is demonstrated for a wide range of structural parameters and temperatures. Dependences of spin relaxation rates are obtained both for a GaAs-based heterojunctions and triangular quantum wells. The calculations show a several-orders-of-magnitude difference between spin relaxation times for heterostructure parameters realized in experiments.     

AlGaAs/InGaAs/GaAs quantum-well power MISFET at millimeter-wavefrequencies

An AlGaAs/InGaAs/GaAs quantum-well MISFET developed for power operation at millimeter-wave frequencies is described. The InGaAs channel is heavily doped to increase the sheet carrier density, resulting in a maximum current density of 700 mA/mm with a transconductance of 480 mS/mm. The 0.25-μm×50-μm device delivers a power density of 0.76 W/mm with 3.6-dB gain and 19% power-added efficiency at 60 GHz. At 5.2 dB gain, the power density is 0.55 W/mm. A similar device built on an undoped InGaAs channel had much poorer power performance and no speed advantage     

Vertical integration of a GaAs/AlGaAs quantum-well laser and along-wavelength quantum-well infra-red photodetector

A short-wavelength (~0.8 μm) GaAs/AlGaAs graded-index separate-confinement heterostructure quantum-well laser has been monolithically integrated with a long-wavelength (~8 μm) GaAs/AlGaAs multiple-quantum-well infra-red photodetector on a semi-insulating GaAs substrate by molecular beam epitaxy. The vertical integration method is used and the combined structure is a pinin structure. Both the laser and detector exhibit excellent characteristics. At room temperature, the ridge waveguide laser has an extremely low threshold current of 25 mA and a differential quantum efficiency above 65% with a stripe width of 20 μm. The quantum-well detector has a peak response at 8 μm and a responsivity of 0.7 A/W     

Enhanced bandgap resonant nonlinear susceptibility in quantum-well heterostructures

Calculation shows that the bandgap resonant nonlinear susceptibility in a multi-quantum-well structure can be a factor of approximately 2 greater than in the bulk. The calculation uses the mechanism of blocking of band-to-band transitions, likely to be present in materials of interest for advanced optical-fibre communication systems.     

Optical characteristics of quantum-well heterostructures in saturation conditions

The resalts of theoretical investugation for spectral characterstics of gain, refraction index of QW heterostructures depending on quantum layer thickness the value of saturation field, as well as their field dependence are presented.     

GaAs基GaSb体材料及InAs/GaSb超晶格材料的MBE生长

采用分子束外延方法在GaAs(100)衬底上生长GaSb体材料,以此GaSb为缓冲层生长了不同InAs厚度的InAs/GaSb超晶格,其10K光致发光谱峰值波长在2.0～2.6 μm.高分辨透射电子显微镜观察证实超晶格界面清晰,周期完整.     

Picosecond photoluminescence dynamics in an InGaAs/GaAs quantum-well heterostructure

The results of experimental studies of the subpicosecond relaxation dynamics of photoexcited charge carriers in an In_0.22Ga_0.78As/GaAs quantum-well heterostructure are reported. From photoluminescence studies of the structure by the upconversion technique, the cooling rate of charge carriers in the quantum well and the time of charge-carrier trapping into the well are estimated to be ??1 ps at 300 K and at ??6.5 ps at 10 K.     

Epitaxial growth and characterization of DyP/GaAs, DyAs/GaAs, and GaAs/DyP/GaAs heterostructures

There is a significant interest in the area of improving high temperature stable contacts to III-V semiconductors. Two attractive material systems that offer promise in this area are dysprosium phosphide/gallium arsenide (DyP/GaAs) and dysprosium arsenide/gallium arsenide (DyAs/GaAs). Details of epitaxial growth of DyP/GaAs and DyAs/GaAs by molecular beam epitaxy (MBE), and their characterization by x-ray diffraction, transmission electron microscopy, atomic force microscopy, Auger electron spectroscopy, Hall measurements, and high temperature current-voltage measurements is reported. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01% and is stable in air with no sign of oxidation, even after months of ambient exposure. Both DyP and DyAs have been grown by solid source MBE using custom designed group V thermal cracker cells and group III high temperature effusion cells. High quality DyP and DyAs epilayer were consistently obtained for growth temperatures ranging from 500 to 600°C with growth rates between 0.5 and 0.7 μm/h. DyP epilayers are n-type with electron concentrations of 3 × 10²⁰ to 4 × 10²⁰ cm⁻³, room temperature mobilities of 250 to 300 cm²/V·s, and a barrier height of 0.81 eV to GaAs. DyAs epilayers are also n-type with carrier concentrations of 1 × 10²¹ to 2 × 10²¹ cm⁻³, and mobilities between 25 and 40 cm²/V·s.