Effect of hydrogen on the properties of Pd/GaAs/InGaAs diode structures with quantum wells

The effect of hydrogen on photoelectric properties and photoluminescence of Pd/GaAs/InGaAs diode structures with quantum wells (QWs) was investigated. The dependence of the structure characteristics on the thickness of the GaAs anodic oxide layer is revealed, and the optimum oxide thickness for the fabrication of hydrogen sensors is determined. It is established that the existence of metal bridges in a thin oxide layer has a significant influence on the I-V curves of the structures. It is shown that the presence of QWs leads to an increase in the structure’s sensitivity to hydrogen. Using the QWs as local defect probes, formation of the defects resulting from the deposition of a Pd electrode both on natural and on anodized GaAs surface is studied. It is found that defects in the QWs of the diode structures can be passivated by introduction of atomic hydrogen through the Pd electrode upon exposure of the structures to an atmosphere of molecular hydrogen.     

(Zn, Mg)O/ZnO-based heterostructures grown by molecular beam epitaxy on sapphire: Polar vs. non-polar

Zinc oxide (ZnO) has recently attracted considerable attention because of its unique physical properties and its potential applications in the blue and UV spectral range. Up to now, ZnO-based heterostructures have mostly been grown in a c-orientation. The growth of non-polar layers along the a-direction [1 1 2¯ 0] has been proposed to avoid any built-in electric fields in the c-direction. Polar and non-polar quantum wells (QWs) embedded in (Zn, Mg)O barriers were grown on an optimized buffer. We compare the photoluminescence (PL) emission of a- and c-oriented QWs. From this comparison, we demonstrate that the QWs exhibit confinement but no indication of quantum confined Stark effect, contrary to what is observed in c-oriented structures. In the non-polar orientation, it is shown that the thermal quenching is not related to the thermal escape of excitons from the ZnO area, since the calculated activation energies are much lower.     

Threshold characteristics of an IR laser based on deep InAsSb/AlSb quantum well

The basic threshold characteristics of a semiconductor IR laser based on a heterostructure with deep InAs_0.84Sb_0.16/AlSb quantum wells (QWs) have been studied. The threshold carrier densities and threshold current densities of radiative and Auger recombination (AR) were found. It is shown that at certain QW parameters the AR rate is strongly (by several orders of magnitude) suppressed. In this case, the emission wavelength falls within the interval 2–3.5 μm, which corresponds to the mid-IR spectral range. The internal quantum efficiency of emission at the lasing threshold was calculated and its dependence on the QW width within the AR suppression range was demonstrated. The laser structure was optimized with respect to the number of QWs.     

Photoconductivity Amplification in a Type-II <Emphasis Type="Italic">n</Emphasis>-GaSb/InAs/<Emphasis Type="Italic">p</Emphasis>-GaSb Heterostructure with a Single QW

Significant photocurrent/photoconductivity amplification is observed at low reverse biases in a type-II n-GaSb/InAs/p-GaSb heterostructure with a single quantum well (QW), grown by metal-organic vapor phase epitaxy. A sharp increase in the photocurrent by more than two orders of magnitude occurs under exposure of the heterostructure to monochromatic light with a wavelength of 1.2–1.6 μm (at 77 K) and the application of a reverse bias in the range 5–200 mV. The optical gain depends on the applied voltage and increases to 2.5 × 10² at a reverse bias of 800 mV. Theoretical analysis demonstrated that the main role in the phenomenon is played by the screening of the external electric field by electrons accumulated in the deep InAs QW and by the mechanism of the tunneling transport of carriers with a small effective mass. It is shown that the effect under study is common to both isotype and anisotype type-II heterojunctions, including structures with QWs and superlattices.     

Characteristic temperature of a tunneling-injection quantum dot laser: Effect of out-tunneling from quantum dots

A laser structure is studied, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). An extended theoretical model is developed allowing for out-tunneling leakage of carriers from QDs into the opposite-to-injection-side QWs (electrons into the p-side QW and holes into the n-side QW). Due to out-tunneling leakage, parasitic recombination of electron-hole pairs occurs outside QDs – in the QWs and optical confinement layer. The threshold current density j_th and the characteristic temperature T₀ are shown to be mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, a tunneling-injection laser shows potential for significant improvement of temperature stability of j_th – the characteristic temperature T₀ remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations.     

Theoretical study of auger recombination processes in deep quantum wells

The basic processes and mechanisms of Auger recombination of nonequilibrium carriers in a semiconductor heterostructure with deep InAs_0.84Sb_0.16/AlSb quantum wells (QWs) are analyzed. It is shown that a zero-threshold Auger recombination process involving two heavy holes predominates in sufficiently narrow QWs, and a resonant process involving two electrons is dominant in wide QWs. The range of QW widths at which the Auger recombination is suppressed in a given structure to the greatest extent (suppression region) is determined. In this case, the threshold process involving two electrons remains the basic nonradiative recombination process, with its probability being several orders of magnitude lower than those for the zero-threshold and resonant mechanisms. In turn, the zero-threshold mechanism involving two electrons is totally impossible in the heterostructure under study because of the large conduction-band offset (which markedly exceeds the energy gap). Also, the range of emission wavelengths that corresponds to the suppression region is estimated. It is shown that the interval calculated belongs to the mid-IR range.     

Impact of strain on the band offsets of important III–V quantum wells: InxGa1−xN/GaN,GaAs/InxGa1−xP and InxGa1−xAs/AlGaAs

Lattice strain has immense effect on the optoelectronic properties of III–V semiconductor quantum wells (QWs), since it introduces a pronounced change on the band properties of QWs and it is often purposefully introduced to improve device performance. In this paper we report the results of our experimental and theoretical studies on, how the important parameter, the band offset, changes with strain for In_xGa_1−xN/GaN, GaAs/In_xGa_1−xP and In_xGa_1−xAs/AlGaAs QWs. Experimentally the band offsets have been studied through capacitance transient measurements in the form of deep level transient spectroscopy (DLTS) on suitable QWs within a Schottky diode. The energy levels in a QW are considered to be analogous to a deep trap in the forbidden energy gap. From detailed balance between the emission and capture, Arrhenius type expressions were derived to analyze transient emission data, from which the band offsets were computed. Theoretically the band positions at the heterointerfaces have been calculated from the equations developed, which directly correlate the position of the bands with the strain at the interface. The strain is calculated from the In mole fractions and lattice constants. The parameters implicitly involved are the elastic stiffness constants (C₁₁ and C₁₂), the hydrostatic deformation potential of the conduction band (a′), the hydrostatic deformation potential (a) and the shear deformation potential (b) for the valance band. The results should be useful to research workers in the field of optoelectronics.     

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.     

Super-flat (411)A interfaces and uniformly corrugated (775)B interfaces in GaAs/AlGaAs and InGaAs/InAlAs heterostructures grown by molecular beam epitaxy

Extremely flat interfaces, i.e. effectively atomically flat interfaces over a wafer-size area were realized in GaAs/AlGaAs quantum wells (QWs) grown on (411)A GaAs substrates by molecular beam epitaxy (MBE). These flat interfaces are called as “(411)A super-flat interfaces”. Besides in GaAs/AlGaAs QWs, the (411)A super-flat interfaces were formed in pseudomorphic InGaAs/AlGaAs QWs on GaAs substrates and in pseudomorphic and lattice-matched InGaAs/InAlAs QWs on InP substrates. GaAs/AlGaAs resonant tunneling diodes and InGaAs/InAlAs HEMT structures with the (411)A super-flat interfaces were confirmed to exhibit improved characteristics, indicating high potential of applications of the (411)A super-flat interfaces. High density, high uniformity and good optical quality were achieved in (775)B GaAs/(GaAs)_m(AlAs)_n quantum wires (QWRs) self-organized in a GaAs/(GaAs)_m(AlAs)_n QW grown on (775)B GaAs substrates by MBE. The QWRs were successfully applied to QWR lasers, which oscillated at room temperature for the first time as QWR lasers with a self-organized QWR structure in its active region. These results suggest that MBE growth on high index crystal plane such as (411)A or (775)B is very promising for developing novel semiconductor materials for future electron devices.     

Electrical properties of <Emphasis Type="Italic">n</Emphasis>-ZnO/<Emphasis Type="Italic">p</Emphasis>-CuO heterostructures

The n-ZnO/p-CuO heterostructure is prepared, and its I-V characteristic is measured. It is shown that the heterostructure conductivity is primarily determined by the CuO layer and the n-ZnO/p-CuO heterojunction itself.     

Effect of the electric field on the intensity and spectrum of emission from InGaN/GaN quantum wells

Comparative study of the photoluminescence (PL) from quantum wells (QWs) in forward-biased p-GaN/InGaN/n-GaN structures and electroluminescence from these structures has been carried out. It is shown that, upon application of a forward bias, a characteristic red shift of the spectral peak is observed, together with a broadening of the PL line and simultaneous burning-up of the PL. This results from a decrease in the field strength in the space charge region of the p-n junction and suppression of the tunneling leakage of the carrier from band-tail states in the active InGaN layer. An analysis of the results obtained demonstrated that the tunneling strongly affects the quantum efficiency and enabled evaluation of the internal quantum efficiency of the structures. It is shown that nonequilibrium population of band-tail states in InGaN/GaN QWs depends on the injection type and is controlled by the capture of carriers injected into a QW, in the case of optical injection, and by carrier tunneling “below” the QW under electrical injection.     

Tight-binding study of the optical properties of GaN/AlN polar and nonpolar quantum wells

The electronic structure of wurtzite semiconductor superlattices (SLs) and quantum wells (QWs) is calculated by using the empirical tight-binding method. The basis used consists of four orbitals per atom (sp³ model), and the calculations include the spin-orbit coupling as well as the strain and electric polarization effects. We focus our study on GaN/AlN QWs wells grown both in polar (C) and nonpolar (A) directions. The band structure, wave functions and optical absorption spectrum are obtained and compared for both cases.     

Low-temperature photoluminescence and X-ray diffractometry study of InxGa1−x As quantum wells

The structures grown by molecular-beam epitaxy with In_xGa_1?x As quantum wells (QWs) in GaAs were studied by X-ray diffractometry and low-temperature photoluminescence techniques. The inhomogeneity of the QW composition along the growth direction was established. Energy positions of the exciton recombination lines in the QWs with step-graded In distribution were calculated, and good agreement with the experimental data was obtained.     

Coupling of electron states in the InAs/GaAs quantum dot molecule

Deep level transient spectroscopy (DLTS) is used to study electron emission from the states in the system of vertically correlated InAs quantum dots in the p-n InAs/GaAs heterostructures, in relation to the thickness of the GaAs spacer between the two layers of InAs quantum dots and to the reverse-bias voltage. It is established that, with the 100 Å GaAs spacer, the InAs/GaAs heterostructure manifests itself as a system of uncoupled quantum dots. The DLTS spectra of such structures exhibit two peaks that are defined by the ground state and the excited state of an individual quantum dot, with energy levels slightly shifted (by 1–2 eV), due to the Stark effect. For the InAs/GaAs heterostructure with two layers of InAs quantum dots separated by the 40 Å GaAs spacer, it is found that the quantum dots are in the molecule-type phase. Hybridization of the electron states of two closely located quantum dots results in the splitting of the levels into bonding and antibonding levels corresponding to the electron ground states and excited states of the 1s ⁺, 1s ^?, 2p ⁺, 2p ^?, and 3d ⁺ types. These states manifest themselves as five peaks in the DLTS spectra. For these quantum states, a large Stark shift of energy levels (10–40 meV) and crossing of the dependences of the energy on the electric field are observed. The structures with vertically correlated quantum dots are grown by molecular beam epitaxy, with self-assembling effects.     

Specific features of the spectra and relaxation kinetics of long-wavelength photoconductivity in narrow-gap HgCdTe epitaxial films and heterostructures with quantum wells

The spectra and relaxation kinetics of interband photoconductivity are investigated in narrow-gap Hg_{1 ? x} Cd _x Te epitaxial films with x = 0.19–0.23 and in structures with HgCdTe-based quantum wells (QWs), having an interband-transition energy in the range of 30–90 meV, grown by molecular-beam epitaxy on GaAs (013) substrates. A long-wavelength sensitivity band caused by impurities or defects is found in the spectra of the structures with quantum wells in addition to the interband photoconductivity. It is shown that the lifetimes of nonequilibrium carriers in the structures with QWs is less than in bulk samples at the same optical-transition energy. From the measured carrier lifetimes, the ampere-watt responsivity and the equivalent noise power for a film with x = 0.19 at a wavelength of 19 μm are estimated. When investigating the relaxation kinetics of the photoconductivity at 4.2 K in high excitation regime, it is revealed that radiative recombination is dominant over other mechanisms of nonequilibrium-carrier recombination.     

Influence of hydrogen sulfide on electrical and photoelectric properties of Al-<Emphasis Type="Italic">p</Emphasis>-Si-SnO<Subscript>2</Subscript>:Cu-Ag heterostructures

I-V characteristics, spectral photosensitivity, and the dependence of the photocurrent on bias, as well as the effect of a 1%-H₂S/N₂ gas mixture, were studied for an Al-p-Si-SnO₂:Cu-Ag heterostructure. The charge transport for carriers in the dark and under illumination was found to obey the law J ∝ U². Exposure to hydrogen sulfide results in a spectral shift of the photovoltage curve to shorter wavelengths. The kinetics of the decrease in photocurrent in a hydrogen sulfide medium is characterized by long relaxation times.     

Specific features of electroluminescence in heterostructures with InSb quantum dots in an InAs matrix

The electrical and electroluminescence properties of a single narrow-gap heterostructure based on a p-n junction in indium arsenide, containing a single layer of InSb quantum dots in the InAs matrix, are studied. The presence of quantum dots has a significant effect on the shape of the reverse branch of the current-voltage characteristic of the heterostructure. Under reverse bias, the room-temperature electroluminescence spectra of the heterostructure with quantum dots, in addition to a negative-luminescence band with a maximum at the wavelength λ = 3.5 μm, contained a positive-luminescence emission band at 3.8 μm, caused by radiative transitions involving localized states of quantum dots at the type-II InSb/InAs heterointerface.     

Reverse <Emphasis Type="Italic">I-V</Emphasis> characteristics of a diode-connected heterostructure MESFET

The reverse gate current of heterostructure MESFETs is studied experimentally. The behavior of the reverse I-V characteristic is shown to be consistent with the tunneling mechanism of the current. The curve is discussed with regard to two special features of a planar device configuration: the full depletion of the doped layer and the δ-shaped character of the doping profile.     

Self-consistent analysis of the band structure of doped lattice-matched GaNAsBi-based QWs operating at 1.55 μm

Band structures of n–i doped lattice-matched GaNAsBi/GaAs quantum wells are studied theoretically using a self-consistent calculation (based on the envelop function formalism) combined with the 16-band anti-crossing model. Operating at 1.55 μm, these QWs can represent active zones of temperature-insensitive optoelectronic device applications intended for optical fiber communications. We have calculated physical parameters of the structures such as the confining potential profiles, the Fermi level, the subband energies and their corresponding wavefunctions as well as the oscillator strength of inter-band transitions, the subband occupations, and the confined electrons density distributions. Finally, the absorption coefficient spectra of GaNAsBi-based QWs are also computed.     

Quantum Confinement and Carrier Localization Effects in ZnO/Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O Wells Synthesized by Pulsed Laser Deposition

The optical properties of ZnO/Mg _x Zn_1−x O (x = 0.17) quantum wells (QWs) grown on c-plane sapphire substrates by pulsed laser deposition are presented. A blueshift in the low-temperature photoluminescence (PL) of the QWs illustrates quantum confinement effects as a function of ZnO well widths in the range from 3 nm to 10 nm. Enhanced luminescence properties are observed with increasing quantum confinement. PL data indicate weak polarization effects associated with the heterojunctions. Temperature-dependent PL measurements indicate carrier/exciton localization with activation energy of approximately 4−5 meV, which are attributed to potential fluctuations at the well-barrier interface.