Thermoelectric properties of symmetric and asymmetric double quantum well structures

The electronic states and carrier transport in (100)PbTe/Pb _{1 ? x} Eu _x Te double quantum wells are theoretically analyzed. The dependences of the mobility and Seebeck coefficient on the thickness of the internal barrier in symmetric and asymmetric structures are investigated. It was found that at great distance between the wells even small violation of the structure symmetry and essential reconstruction of electron wave functions results in suppression of intersubband scattering with carriers transfer between the wells and provides the correct limit to isolated quantum well in kinetic coefficients. Some possibilities of increasing the thermoelectric power factor are found, and a suitable set of structure parameters is calculated within the proposed model.     

Thermally activated resonance tunneling in asymmetric systems of CdSe/ZnSe double quantum wells with self-assembled quantum dots

In the luminescence study of double quantum wells formed by depositing two CdSe layers with different nominal thicknesses into a ZnSe matrix, a heavy dependence of the photoluminescence spectrum on the thickness of the ZnSe barrier separating the quantum wells, the excitation photon energy, and temperature is observed. The photoluminescence spectra are studied at barrier widths of 34, 50, and 63 monolayers, excitation photon energies of 3.06, 2.71, and 2.54 eV, and temperatures T in the range of 5–200 K. Upon above- (3.06 eV) and below-barrier (2.71 eV) excitation, the photoluminescence spectrum exhibits two bands, I ₁(T) and I ₂(T), corresponding to the annihilation of excitons localized in the quantum dots of the shallow and deep quantum wells. An increase in temperature to ～50 K yields only a slight decrease in the total integrated emission intensity of both bands I _PL(T) and the intensities of each of the two bands, I ₁(T) and I ₂(T). A further increase in temperature results in substantial redistribution of the photoluminescence intensity between the two wells, which is attributed to the tunneling of excitons from the QD (quantum-dot) states of the shallow well to states of the deep well. This process is of the activation character and manifests itself as a sharp decrease in the integrated emission intensity related to the shallow quantum well, I ₁(T), and a simultaneous increase in the integrated emission intensity of quantum dots of the deep quantum well, I ₂(T). The experimentally detected effect is most profound in the range of temperatures T = 110–130 K and in the samples with a barrier thickness of 50 monolayers. It is most likely that the tunneling is of a resonance nature. This inference follows from the fact that the barrier width is much larger than the well widths for both wells, which predetermines only slight penetration of the wave functions into the neighboring well, and the effect of tunneling itself is only slightly supressed, as the barrier thickness is increased. At the same time, the activation energy is at least three time higher that the optical phonon energy, which cannot be explained on the basis of existing theory.     

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.     

Optical transitions in Cd x Hg1 ? x Te-based quantum wells and their analysis with account for the actual band structure of the material

Quantum-confinement levels in a Cd _x Hg_{1 ? x} Te-based rectangular quantum well are calculated in the framework of the four-band Kane model taking into account mixing between the states of electrons and three types of holes (heavy, light, and spin-split holes). Comparison of the calculation results with experimental data on the photoluminescence of Cd _x Hg_{1 ? x} Te-based quantum wells suggests that optical transitions involving the conduction and light-hole bands are possibly observed in the spectra.     

Shallow acceptors in strained Ge/Ge1−x Six heterostructures with quantum wells

The energies of localized acceptor states in quantum wells (strained Ge layers in Ge/Ge_1?x Si_x heterostructures) were analyzed theoretically in relation to the quantum well width and the impurity position in the well. The impurity absorption spectrum in the far IR range is calculated. Comparison of the results of the calculation with experimental photoconductivity spectra allows an estimation of the acceptor distribution in the quantum well to be made. In particular, it was concluded that acceptors may largely concentrate near the heterointerfaces. The absorption spectrum is calculated taking into account the resonance impurity states. This allows the features observed in the short-wavelength region of the spectrum to be interpreted as being due to transitions into the resonance energy levels “linked” to the upper size-quantization subbands.     

Influence of temperature on the mechanism of carrier injection in light-emitting diodes based on InGaN/GaN multiple quantum wells

The experimental current-voltage characteristics and dependences of the external quantum yield on the current density of light-emitting diodes based on InGaN/GaN multiple quantum wells for the wide temperature range T = 10–400 K are presented. It is shown that, at low-temperatures T < 100 K, the injection of holes into the quantum wells occurs from localized acceptor states. The low-temperature injection of electrons into p-GaN occurs due to quasi-ballistic transport in the region of multiple quantum wells. An increase in temperature leads to an increase in the current which is governed by thermally activated hole and electron injection from the allowed bands of GaN.     

Interaction of electrons with optical phonons localized in a quantum well

The scattering rate of electrons in a quantum well by localized polar optical and interface phonons is considered. The dependence of the force of the electron-phonon interaction on the frequency of optical phonons in materials of the heterostructure forming the electron and phonon quantum wells is determined. It is shown that, by varying the composition of semiconductors forming the quantum well and its barriers, it is possible to vary the scattering rates of electrons by a factor of several times. The scattering rates of electrons by polar optical phonons are calculated depending on the fractions In _x and In _y in the composition of semiconductors forming the In _x Al_{1 ? x} As/In _y Ga_{1 ? y} As quantum wells. Dependences of the mobility and saturated drift velocity of electrons in high electric fields and quantum wells In _y Ga_{1 ? y} As on the composition of the In _x Al_{1 ? x} As barriers introduced into quantum wells are determined experimentally. The electron mobility increases, while the saturated drift velocity decreases as the fraction of In _x in the composition of barriers is increased.     

Light-emitting tunneling nanostructures based on quantum dots in a Si and GaAs matrix

InGaAs/GaAs and Ge/Si light-emitting heterostructures with active regions consisting of a system of different-size nanoobjects, i.e., quantum dot layers, quantum wells, and a tunneling barrier are studied. The exchange of carriers preceding their radiative recombination is considered in the context of the tunneling interaction of nanoobjects. For the quantum well-InGaAs quantum dot layer system, an exciton tunneling mechanism is established. In such structures with a barrier thinner than 6 nm, anomalously fast carrier (exciton) transfer from the quantum well is observed. The role of the above-barrier resonance of states, which provides “instantaneous” injection into quantum dots, is considered. In Ge/Si structures, Ge quantum dots with heights comparable to the Ge/Si interface broadening are fabricated. The strong luminescence at a wavelength of 1.55 μm in such structures is explained not only by the high island-array density. The model is based on (i) an increase in the exciton oscillator strength due to the tunnel penetration of electrons into the quantum dot core at low temperatures (T < 60 K) and (ii) a redistribution of electronic states in the Δ₂-Δ₄ subbands as the temperature is increased to room temperature. Light-emitting diodes are fabricated based on both types of studied structures. Configuration versions of the active region are tested. It is shown that selective pumping of the injector and the tunnel transfer of “cold” carriers (excitons) are more efficient than their direct trapping by the nanoemitter.     

Quantized conductance in silicon quantum wires

The results of studying the quantum-mechanical staircase for the electron and hole conductance of one-dimensional channels obtained by the split-gate method inside self-assembled silicon quantum wells are reported. The characteristics of quantum wells formed spontaneously between the heavily doped δ-shaped barriers at the Si(100) surface as a result of nonequilibrium boron diffusion are analyzed first. To this end, secondary-ion mass spectrometry, and also the detection of angular dependences of the cyclotron resonance and ESR, is used; these methods make it possible to identify both the crystallographic orientation of the self-assembled quantum wells and the ferroelectric properties of heavily doped δ-shaped barriers. Since the obtained silicon quantum wells are ultrathin (～2 nm) and the confining δ-shaped barriers feature ferroelectric properties, the quantized conductance of one-dimensional channels is first observed at relatively high temperatures (T≥77 K). Further, the current-voltage characteristic of the quantum-mechanical conductance staircase is studied in relation to the kinetic energy of electrons and holes, their concentration in the quantum wells, and the crystallographic orientation and modulation depth of electrostatically induced quantum wires. The results show that the magnitude of quantum steps in electron conductance of crystallographically oriented n-type wires is governed by anisotropy of the Si conduction band and is completely consistent with the valence-valley factor for the [001] (G ₀=4e ²/h and g _v=2) and [011] (G ₀=8e ²/h and g _v=4) axes in the Si(100) plane. In turn, the quantum staircase of the hole conductance of p-Si quantum wires is caused by independent contributions of the one-dimensional (1D) subbands of the heavy and light holes; these contributions manifest themselves in the study of square-section quantum wires in the doubling of the quantum-step height (G ₀=4e ²/h), except for the first step (G ₀=2e ²/h) due to the absence of degeneracy of the lower 1D subband. An analysis of the heights of the first and second quantum steps indicates that there is a spontaneous spin polarization of the heavy and light holes, which emphasizes the very important role of exchange interaction in the processes of 1D transport of individual charge carriers. In addition, the temperature-and field-related inhibition of the quantum conductance staircase is demonstrated in the situation when kT and the energy of the field-induced heating of the carriers become comparable to the energy gap between the 1D subbands. The use of the split-gate method made it possible to detect the effect of a drastic increase in the height of the quantum conductance steps when the kinetic energy of electrons is increased; this effect is most profound for quantum wires of finite length, which are not described under conditions of a quantum point contact. It is shown in the concluding section of this paper that detection of the quantum-mechanical conductance under the conditions of sweeping the kinetic energy of the charge carriers can act as an experimental test aiding in separating the effects of quantum interference in modulated quantum wires against the background of Coulomb oscillations as a result of the formation of QDs between the delta-shaped barriers.     

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)     

Electroreflectance spectra from multiple InGaN/GaN quantum wells in the nonuniform electric field of a <Emphasis Type="Italic">p–n</Emphasis> junction

A line at E = 2.77 eV (with a width of Γ = 88 meV) related to interband transitions in the region of multiple quantum wells in the active region is detected in the electroreflectance spectra of the GaN/InGaN/AlGaN heterostructure. As the modulation bias is reduced from 2.9 to 0.4 V, the above line is split into two lines with energies of E ₁ = 2.55 eV and E ₂ = 2.75 eV and widths of Γ₁ = 66 meV and Γ₂ = 74 meV, respectively. The smaller widths of separate lines indicate that these lines are caused by interband transitions in particular quantum wells within the active region. The difference between the interband transition energies E ₁ and E ₂ in identical quantum wells in the active region is related to the fact that the quantum wells are in an inhomogeneous electric field of the p–n junction. The magnitudes of the electric-field strengths in particular quantum wells in the active region of the heterostructure are estimated to be 1.6 and 2.2 MV/cm.     

Terahertz luminescence of GaAs-based heterostructures with quantum wells under the optical excitation of donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ～3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO₂ laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm^?1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers N _D ≈ 10¹¹ cm^?2 can be attained under the excitation-flux density 10²³ photons/(cm² s).     

Luminescence in ZnMnTe/ZnMgTe and CdMnTe/CdMgTe structures with different parameters of quantum wells

The low-temperature luminescence of ZnMnTe/ZnMgTe and CdMnTe/CdMgTe quantum well structures with different quantum well widths and different Mn proportions is studied at optical-excitation power densities ranging from 10⁴ to 10⁶ W cm^?2. Because of saturation of the lowest excited state ⁴ T ₁ of the 3d shell of Mn²⁺ ions, transitions to higher states start to play an important role. As a result, the intracenter luminescence of Mn²⁺ ions deteriorates at high excitation levels. Simultaneously, the temperature-dependent saturation of the main exciton-emission band e1hh1 of the quantum wells occurs, and the band e2hh2 emerges. As the optical excitation is increased, the intracenter luminescence band of Mn²⁺ ions changes its shape. This effect is attributed to the faster saturation of the excited states of interface ions. For CdMnTe/CdMgTe structures, the effect of the quantum well width and Mn content on the relation between the emission intensities corresponding to excitons in quantum wells, excitons in barriers, and the 3d shell of Mn²⁺ ions is established.     

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.     

Size Effects in Transport Properties of PbSe Thin Films

This paper presents an overview and analysis of our earlier obtained experimental results on the dependences of kinetic properties of single PbSe quantum wells and PbSe-based superlattices on the PbSe layer thickness d. The observed oscillatory character of these dependences is attributed to quantum size effects due to electron or hole confinement in quantum wells. Some general regularities and factors that determine the character of these quantum size effects are established. The influence of the oxidation processes and doping on the d-dependences of the transport properties is revealed. A periodic change in the conductivity type related to quantum size oscillations is detected. It is shown that the experimentally determined values of the oscillation period Δd are in good agreement with the results of theoretical calculations based on the model of a rectangular quantum well with infinitely high walls, taking into account the dependence of the Fermi energy ε _F on d and the availability of subbands below ε _F. It is established that the Δd value for the superlattices is practically equal to the Δd value observed for the single PbSe thin film.     

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.     

Electric-field-controlled effects of spatial recurrence and multiplication of electron waves in two-dimensional semiconductor nanostructures

Effects of recurrence and multiplication in the spatial distribution of the probability-flux density j _x(x, z) (or the quantum-mechanical current density ej _x(x, z), where e is the elementary charge), which arise from electron-wave interference in two-dimensional semiconductor nanostructures, are analyzed, and the possibility of controlling these effects by the application of a dc transverse electric field is examined. A type of nanostructure represented by two rectangular quantum wells (a wide one and a narrow one) whose widths are measured in the direction of the z axis (the quantum-confinement axis) with the wells arranged sequentially in the direction of propagation of the electron wave (the x axis) is considered. It is shown that, for an electron wave entering the wide well from the narrow well, the initial transverse distribution peak j _x(0, z) is reproduced with some accuracy at distances X _p = pX ₁ (recurrence) and, in nanostructures symmetric along the z axis, splits at distances X ₁/q into q identical peaks of magnitude reduced by a factor of q (multiplication) (here, p and q are integers). It is demonstrated that these effects can be controlled by a dc electric field applied in the transverse direction (along the z axis) in the region of the wide quantum well. A reduction in the effective well width and appearance of asymmetry in the transverse potential profile upon application of the electric field cause a radical change in the j _x(x, z) distribution in this quantum well and make possible inverse population of the quantum-confinement subbands.     

Inversion of the electron population in subbands of dimensional quantization with longitudinal transport in tunnel-coupled quantum wells

The scheme of a laser which can operate in the far-infrared range (λ ∼ 150 μm) is suggested. In order to attain the inversion of the subband population it was suggested that electron transport in three tunnel-coupled quantum wells in a strong electric field, which lies in the plane of quantum wells, be used. An important specific feature of the structure suggested is the presence of a single rough heterointerface. The electron trans-port was simulated by the Monte Carlo method for the Al_xGa_1−x As/GaAs (x=0.2–0.3) heterostructure. The simulation demonstrated that the population inversion in the first and second subbands of dimensional quantization is realized in the field above 1.2 kV/cm at T=4.2 and 77 K. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 6, 2002, pp. 724–729. Original Russian Text Copyright ? 2002 by Aleshkin, Dubinov.     

Shallow acceptors in strained multiquantum-well Ge/Ge1−x Six heterostructures

Far infrared photoconductivity spectra due to excitation of shallow acceptors in strained multiquantum well Ge/Ge_1?x Si_x (x≈0.1) heterostuctures are investigated. It is shown that these spectra are shifted toward longer wavelengths in the far infrared region compared with those of bulk p-Ge, owing to “built-in” strain and size quantization, which lead to splitting of the light-and heavy-hole subbands in the Ge layers. Shallow acceptor spectra are calculated variationally for bulk germanium under uniaxial tension, which is “equivalent” to the strained Ge layers in the heterostructures. Although this method is only appropriate for wide quantum wells (d _Ge≈800 Å), the calculations are shown to qualitatively account for photoconductivity spectra obtained from narrower wells (d _Ge≈200 Å) as well.     

Luminescence spectra and efficiency of GaN-based quantum-well heterostructure light emitting diodes: Current and voltage dependence

Luminescence spectra and quantum yield in light emitting diodes (LEDs) based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells (MQWs) were studied in the range of currents J=10⁻⁶–10⁻¹ A. Minor spread in the quantum yield at operating currents (±15% at J≈10 mA) was observed in these LEDs, which were fabricated by Hewlett-Packard. The spread is due to differences in the current and voltage dependences of the diode emission intensity, caused by differences in the charged center distribution across the space-charge region (SCR) of the structures and in the role of the tunnel current component at low voltages. In the diodes with a thin (≲120 nm) SCR, a tunnel emission band was observed for J≲100 μA; the peak energy of this band ℏω_max=1.92–2.05 eV corresponds to the voltage applied. At low currents (J=0.05–0.5 mA), the spectral position of the main peak ℏω_max=2.35–2.36 eV is independent of the voltage and is determined by the radiative transitions between the localized states. At J>1 mA, this band shifts with the current (ℏω_max=2.36–2.52 eV). Its shape corresponds to the model for the occupation of states in the two-dimensional energy band tails, which are caused by the microscopic potential fluctuations. The four parameters in this model are related to the calculated energy band diagram of the MQW structure. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 7, 2001, pp. 861–868. Original Russian Text Copyright ? 2001 by Kudryashov, Mamakin, Turkin, Yunovich, Kovalev, Manyakhin. Part of this study was reported at the 3rd All-Russia Workshop on Structures and Devices Based on Gallium, Indium, and Aluminum Nitrides (Moscow State University, 1999); the 3rd International Conference on Nitride Semiconductors (Montpellier, 1999); and the 4th European Gallium Nitride Workshop (Nottingham, 2000).