Effects of spatial reproduction as a result of the interference of electron waves in two-dimensional semiconductor nanostructures with parabolic quantum wells

Effects of spatial nonuniformity for the probability flux density j_x(x, z) or for the density of the quantum-mechanical current ej_x(x, z), where e is the elementary charge) are studied. These effects arise in two-dimensional semiconductor nanostructures that consist of thin rectangular and wide parabolic quantum wells that are consecutively arranged in the direction of the electron-wave propagation (the x axis) and are oriented along the dimensional-quantization axis z. A nonuniform distribution of j_x(x, z) arises as a result of interference of electron waves that propagate simultaneously in a wide quantum well over different quantum-dimensional subbands. Particular attention is paid to the effects of the spatial reproduction of electron waves in the nanostructures under consideration. It is shown that the transverse distribution j_x(0, z) existing at the entry to the wide quantum well is reproduced to a certain accuracy at a distance of X₁ from the entry. In addition, the initial distribution j_x(0, z) is reproduced periodically in the sections X_q = qX₁ (coefficients q are integers). The results of numerical calculations of magnitudes of these effects in the structures that are symmetric with respect to the z axis are reported; a modification of the effects under consideration in asymmetric nanostructures is considered.     

Band offsets in Zn 1−x Cd x Te/ZnTe single-quantum-well structures grown by molecular-beam epitaxy on GaAs(001) substrates

ZnTe heteroepitaxial layers and ZnTe/Zn _1?x Cd _x Te/ZnTe strained quantum-confinement structures grown by molecular-beam epitaxy on GaAs(001) were studied by low-temperature cathodoluminescence spectroscopy and current-relaxation deep-level transient spectroscopy (DLTS). A peak related to electron emission from the ground size-quantization level in the conduction-band was observed in the DLTS spectra of quantum-confinement structures. The conduction-band offset parameter Q _C was determined from the DLTS and cathodoluminescence data. For Zn _1?x Cd _x Te/ZnTe single-quantum-well structures with x=0.2–0.22, Q _C equals 0.82 ± 0.05. The effect of internal elastic strain on the band offsets and Q _C at the Cd _x Zn _1?x Te quantum well interfaces was calculated; the results of calculations agree well with experimental data.     

Parameter-dependent third-order nonlinear susceptibility of parabolic InGaN/GaN quantum dots

The electron states confined in wurtzite In_xGa_1−xN/GaN-strained quantum dots (QDs) have been investigated in the effective-mass approximation by solving the Schrödinger equation, in which parabolic confined potential and strong built-in electric field effect due to the piezoelectricity and spontaneous polarization have been taken into account. The third-order nonlinear susceptibility of the QDs in various directions (both parallel to z direction and vertical to z direction) have been calculated, and the magnitude reaches 10⁻¹⁴ m²/V². It has been shown from the results that the order of the built-in electric field in the strained QD is of MV/cm. Furthermore, the results of how the third-order nonlinear susceptibility depend on the radius R of QDs, the height L of QDs, the In content x of QDs and the relaxation rate Γ₁₀ have been given.     

Characterization of In x Ga1−x As/GaAs quantum-well heterostructures by C-V measurements: Band offsets, quantum-confinement levels, and wave functions

The method of C-V profiling combined with self-consistent solution of Schrödinger’s and Poisson’s equations was used to determine with high precision the absolute values of the conduction-band offsets, energies of quantum-confinement levels, and charge-carrier concentrations in quantum-confinement subbands of In _x Ga_1?x As/GaAs quantum-well heterostructures with In content corresponding to the pseudomorphic growth mode (0 < x < 0.3). A characterization technique based on capacitance measurements is developed, enabling one to determine the main electronic parameters of quantum-confinement heterostructures.     

Electrical properties and structure of chalcogenide glasses containing bivalent tin

Two valence states of tin atoms are identified by Mössbauer and X-ray photoelectron spectroscopy in (As₂Se₃)_1?z (GeSe) _z?x (SnSe) _x glasses; it is shown that the presence of bivalent tin in the structural network of a glass does not give rise to impurity conductivity and impurity optical absorption. It is suggested to regard (As₂Se₃)_1?z (GeSe) _z?x (SnSe) _x and (As₂Se₃)_1?z (GeSe₂) _z?x (SnSe₂) _x glasses as semiconductor solid solutions whose electrical properties depend both on the electrical properties of the starting components and on the composition of the solid solutions.     

Optical properties of hybrid quantum-well–dots nanostructures grown by MOCVD

The deposition of In _x Ga_1–x As with an indium content of 0.3–0.5 and an average thickness of 3–27 single layers on a GaAs wafer by metalorganic chemical vapor deposition (MOCVD) at low temperatures results in the appearance of thickness and composition modulations in the layers being formed. Such structures can be considered to be intermediate nanostructures between ideal quantum wells and quantum dots. Depending on the average thickness and composition of the layers, the wavelength of the photoluminescence peak for the hybrid InGaAs quantum well–dots nanostructures varies from 950 to 1100 nm. The optimal average In _x Ga_1–x As thicknesses and compositions at which the emission wavelength is the longest with a high quantum efficiency retained are determined.     

Synchrotron investigations of an electron energy spectrum in III–V-based nanostructures

Using synchrotron radiation, the spectra of an X-ray absorption near-edge structure in a region of P-L _{2, 3} edges of a band spectrum are obtained for the first time. The spectra give insight into the local density of states in the conduction band for MOCVD-grown nanostructures with InP quantum dots on GaAs(100) substrates and for porous InP layers obtained by anodic pulse electrochemical etching of single-crystal InP(100) wafers. For all of the nanostructures, quantum-confinement effects are observed, which manifest themselves in the emergence of an additional level 3.3 eV above the conduction band bottom, as well as in variation in the band gap of the materials under investigation upon the dimensional quantization of the electron spectrum. A band-to-band origin of the luminescence spectra for the nanostructures investigated is assumed.     

Resonance Coulomb scattering at shallow donors in AlGaAs/n-GaAs/AlGaAs quantum wells

The characteristics of the Coulomb scattering of conduction electrons at shallow donor centers in Al _x Ga_{1 ? x} As/n-GaAs/Al _x Ga_{1 ? x} As quantum well heterostructures are studied theoretically with consideration for the influence of resonance states. The resonance states emerge below the excited quantum-confinement subbands because of the presence of donors. It is found that the spectra of total and transport Coulomb-scattering cross sections exhibit asymmetric resonance features in the vicinity of resonance energies. It is shown that these features do not exhibit a small effect: in the vicinity of resonance energies, the cross sections can differ from the corresponding quantities in the nonresonance case several times.     

Population inversion between Γ subbands in quantum wells under the conditions of Γ-<Emphasis Type="Italic">L</Emphasis> intervalley transfer

Monte Carlo simulations of electron transport in Al_xGa_1−x As/GaAs/In_yGa_1−y As double-quantum-well heterostructures in high lateral electric fields are carried out. It is shown that, under the conditions of intervalley Γ-L electron transfer, there exists a population inversion between the first and the second quantum-confinement subbands in the Γ valley. The population inversion appears in the fields exceeding 4 and 5.5 kV/cm at 77 and 300 K, respectively. The gain in a superlattice composed of such quantum wells is estimated to be on the order of 100 cm⁻¹ for radiation with a wavelength of 12.6 μm. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 224–229. Original Russian Text Copyright ? 2003 by Aleshkin, Andronov, Dubinov.     

Tunneling effects in tilted magnetic fields in n-InGaAs/GaAs structures with strongly coupled double quantum wells

The effects of tunneling between two parallel two-dimensional electron gases in n-InGaAs/GaAs nanostructures with strongly coupled double quantum wells with a change in the in-plane component of a tilted magnetic field (up to B _‖ = 9.0 T) in the temperature range T = 1.8–70.0 K are investigated. A nonmonotonic temperature dependence of the inverse quantum lifetime τ _q ^? (T) is obtained from analysis of the dependence of the longitudinal resistance on the parallel component of the tilted magnetic field at fixed temperatures, ρ _xx (B _‖, T). The quadratic portion of this dependence is found to be due to the contribution of inelastic electron-electron scattering. The decrease in the inverse quantum lifetime τ _q ^? (T) at T > 0.1T _F cannot be described within known theories; it seems, it is not related to the processes of electron momentum relaxation.     

Point quantum contacts in disordered Si MOS structures with an inversion p-type channel: Nonlinear behavior of the system in the longitudinal and transverse electric fields

The behavior of the lateral conductance G of mesoscopic Si-MOS structures with an inversion p-type channel and a high concentration of built-in charges of ions (N _t≤3×10¹²cm^?2) in the conditions of the insulator-metal percolation transition is discussed. The following features are observed in the dependences of G on the transverse (V _g) and longitudinal (V _d) voltages at temperatures ≥77 K: a quasi-plateau G(V _g) at G ≈ 2e ²/h and a minimum G(V _d) at . It is shown that the data obtained from the field effect are consistent with the results of calculations of the characteristics of the point quantum contact (the curvature parameters for the potential in the longitudinal and transverse directions are ?ω _x ≈ ?ω _y ≈ 10 meV) but are completely inconsistent with the value of ?ω _x ≈ 300 meV determined from the dependence G(V _d). This discrepancy is related to nonlinearity of a system of quantum contacts with respect to the longitudinal and transverse electric fields. It is shown that the number of quantum contacts along the percolation path varies in the range 1 ≤ N ≤ 30 under the effect of an electric field.     

Influence of the variband-layer thickness on the energy and frequency characteristics of In x(z)Ga1 − x(z)As Gunn diodes

The operation of variband-In _x(z)Ga_{1 − x(z)}As Gunn diodes with an active-region length of 2.5 μm and an n ⁺-n cathode contact is studied by using a two-temperature model of electron intervalley transfer in a varib-and semiconductor. It is established that, in diodes, dipole domains or accumulation layers may be formed depending on the variband-layer thickness. The use of variband In _x(z)Ga_{1 − x(z)}As in the active region with an appropriate variband-layer thickness allows one to enhance the output power and the generation efficiency by a factor of approximately 1.5 and to increase the width of the frequency range of the diode operation approximately twofold as compared to that of an In_0.2Ga_0.8As-based diode. Original Russian Text ? Yu.V. Arkusha, E.D. Prokhorov, I.P. Storozhenko, 2006, published in Radiotekhnika i Elektronika, 2006, Vol. 51, No. 3, pp. 371–378.     

Spin-orbit splitting dependent resonant third-order nonlinear optical susceptibility in InGaN/GaN multiple quantum wells

For the transition between valence band and conduction band, the third-order nonlinear optical susceptibility χ⁽³⁾ for degenerated four-wave mixing in In_xGa_1−xN/GaN multiple quantum wells (MQWs) has been calculated. The contributions of spin-orbit split-off energy to the resonant third-order nonlinear optical susceptibility of the modes, whose polarization is vertical to the [0 0 1] direction of the MQWs, are discussed in detail. The correlations between the peaks of χ⁽³⁾, which are due to the transitions from the spin-orbit split-off energy level to first conduction subband, and the width of the quantum well and the constituents of the semiconductor material are obtained.     

A detailed analysis of the metal-semiconductor contact

A general quantum and electronic theory able to explain the electric and photoelectric experimental properties of the metal-semiconductor contacts is proposed. The theory consists firstly in calculating the electric space charge due to the quantum mechanical tunneling of the electrons from the metal into the semiconductor, and vice-versa, and to the metal and semiconductor bands bending. Then the electric charge so obtained is utilised to solve in an appropriate and complete way the Poisson equation so as to determine the electric field and potential as functions of the abscissa x. The electric field F(x) is employed to obtain a new expression for the junction capacitance C, holding in the general case of a non-uniform charge, whereas the electric potential ν_i(x) is used to calculate general expressions for the thermionic and photoelectric currents i and i_ph, respectively, taking into account in this both the tunneling probability through the energy barrier and the many-valley structure of the semiconductor energy bands. Finally, from ν_i(x), C, i and i_ph four new expressions of the energy barrier height of the contact are deduced. The theoretical results relative to the barrier height so determined (which hold for both n-and p-type semiconductors) are compared with published experimental values obtained, by means of capacitance and photocurrent measurements: (a) on contacts between n-type CdS and Au, Cu, Ag and Pt; (b) on contacts between n-type GaAs and Au, Ag, Cu, Sn, Al and Pt and; (c) on contacts between p-type GaAs and Au and Al. The agreement between the theoretical and experimental values is very good.     

The effect of built-in electric field in GaN/AlGaN quantum wells with high AIN mole fraction

Spontaneous and piezoelectric polarization in hexagonal GaN/AlGaN heterostructures give rise to large built-in electric fields. The effect of the builtin electric field in GaN/Al_xGa_1−xN quantum wells was investigated for x=0.2 to 0.8 by photoluminescence studies. The quantum well structures were grown by molecular beam epitaxy on (0001) sapphire substrates. Cross-sectional transmission electron microscopy performed on the samples revealed abrupt interfaces and uniform layer thicknesses. The low temperature (4 K) photoluminescence peaks were progressively red-shifted due to the quantum confined Stark effect depending on the AlN mole fraction in the barriers and the thickness of the GaN quantum well. Our results verify the existence of very large built-in electric fields of up to 5 MV/cm in GaN/Al_0.8Ga_0.2N quantum wells.     

Growth and optical characterization of strained CdZnTe/ CdTe quantum wells

We have grown strained Cd_1-xZn_xTe(x ≈ 0.2)/CdTe single and multiple quantum wells by molecular beam epitaxy. GaAs was used as a substrate. The well widths were systematically increased until the critical thickness was exceeded. Low-temperature (liquid helium) photoluminescence (PL) spectroscopy was used to characterize the films. Two prominent PL peaks were observed: one arising from the quantum well and the other from the barrier material. The energy of the quantum well luminescence is consistent with theory when strain is included. The critical layer thickness for the CdTe quantum wells was found to be between 150 and 175 å, in agreement with the model of Matthews and Blakeslee.     

Surface Acoustic Wave Induced Birefringence in In0.21Ga0.79As-GaAs Multiple Quantum Wells for Optical Modulation ≈1000 nm under Normal Incidence

This paper presents observation of quantum Confined Stark effect induced by the surface acoustic waves (SAW) propagating in multiple quantum well (MQW) layers. It also presents the evidence for the first time of SAW induced birefringence under surface normal mode operation in MQWs. MQW layers usually do not exhibit significant birefringence when the light is normally incident. The launching of surface acoustic waves in In_0.21Ga_0.79As-GaAs MQW layers produces significantly different strain along the direction of propagation (y) as compared to the lateral direction (x), and this in turn results in differential absorption coefficient Δα (= α_x - α_y) and birefringence (Δn_bir). The enhanced birefringence due to excitonic effect depends on the magnitude of RF power exciting the SAW transducer. An interdigitated transducer operating at 119 MHz with an aperture (w) to wavelength (Λ) ratio of 25 was used to launch the surface acoustic waves. Typical values of Δn are in the range of 0.01-0.02 for polarized light at ≈1.01 μm wavelength when the RF power is varied between 0.5-1.2 Watt. Computations of Δn are in agreement with experimental data. The SAW propagation also induces a perpendicular component E_z of electric field which results in conventional Stark effect producing absorption and index changes in MQWs that vary as a function of SAW power.     

Effect of transverse constant electric field on the electron interference effects related to the under-barrier penetration of quantum-mechanical current density in 2D semiconductor nanostructures

The effect of transverse constant electric field F on the under-barrier penetration of the local quantum-mechanical current density in a 2D semiconductor structure is theoretically studied. The structure represents two quantum wells with identical widths that are sequentially located along the propagation direction of electron wave: the first well has the rectangular cross section, and the second well exhibits a semi-infinite rectangular potential barrier with height V ₀ that is modified by the transverse electric field. When an electron wave whose energy is less than resulting height of the potential barrier V _eff is incident from the first well on the barrier under certain conditions, the coordinate-dependent exponentially decaying penetration of the local quantum-mechanical current density may take place under the barrier due to the interference of electron waves in the nanostructure. It is demonstrated that the penetration parameters depend on field strength F.     

AlGaN-based quantum-well heterostructures for deep ultraviolet light-emitting diodes grown by submonolayer discrete plasma-assisted molecular-beam epitaxy

Features of plasma-assisted molecular-beam epitaxy of AlGaN compounds at relatively low temperatures of the substrate (no higher than 740°C) and various stoichiometric conditions for growth of the nitrogen- and metal-enriched layers are studied. Discrete submonolayer epitaxy for formation of quantum wells and n-type blocking layers without varying the fluxes of components was used for the first time in the case of molecular- beam epitaxy with plasma activation of nitrogen for the nanostructures with the Al _x Ga_{1 ? x} N/Al _y Ga_{1 ? y} N quantum wells. Structural and optical properties of the Al _x Ga_{1 ? x} N layers in the entire range of compositions (x = 0–1) and nanostructures based on these layers are studied; these studies indicate that there is photoluminescence at room temperature with minimum wavelength of 230 nm. Based on the analysis of the photoluminescence spectra for bulk layers and nanoheterostructures and their temperature dependences, it is concluded that there are localized states in quantum wells. Using the metal-enriched layers grown on the c-Al₂O₃ substrates, heterostructures for light-emitting diodes with Al _x Ga_{1 ? x} N/Al _y Ga_{1 ? y} N quantum wells (x = 0.4–0.5, y = x + 0.15) were obtained and demonstrated electroluminescence in the ultraviolet region of the spectrum at the wavelength of 320 nm.     

An experimental investigation of the off-diagonal Seebeck effect on textured YBCO high-Tc superconductor

The off-diagonal Seebeck effect was investigated on a melt-textured YBa₂Cu₃O_7−δ (YBCO) high-T_c superconductor. It was found that the transverse voltage V_x was proportional to the longitudinal temperature difference Δ T_z, measured directly with a pair of differential thermocouples, for textured samples with their c-axes tilted with respect to the surface’s normal. This supported the idea that the off-diagonal thermoelectric effect accounts for the anomalously high laser-induced transverse voltage on the oriented YBCO superconducting thin films. The variation of the V_x against the sample’s thickness d, at a given Δ T_z, deviated from the inversely proportional relationship when the sample was too thin. The deviation was discussed qualitatively in terms of deteriorated surface layers. The data of V_x(d) was fitted to obtain a reasonably reliable ∣ S_c−S_ab∣ value of 12 μ V K⁻¹.