Ab initio calculations of phonon spectra of (GaP)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>(AlP)<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript> superlattices

Using the method of linear response, vibrational spectra and densities of states of GaP and AlP crystals and monolayer GaP/AlP superlattices are calculated. Phonon modes of (GaP) _n (AlP) _m superlattices with various numbers of monolayers are calculated for the center of the Brillouin zone. The obtained results are compared with the Raman scattering data and the effect of nonideality of the interface on phonon frequencies is discussed.     

Transverse optical phonon splitting in GaAs/AlAs superlattices grown on the GaAs(311) surface studied by the method of Raman light scattering

GaAs_n/AlAs_m superlattices grown on the GaAs (311)A and (311)B surfaces by molecular-beam epitaxy were studied by Raman light scattering. The form of the Raman scattering tensor allowed the TO _y and TO _x modes to be separately observed using various scattering geometries (the y and x axes correspond to atomic displacements along and across facets formed on the (311)A surface, respectively). The TO1_y and TO1_x modes exhibited splitting in superlattices grown on a faceted GaAs(311)A surface. The degree of splitting increased for superlattices with an average GaAs layer thickness of 6 monoatomic layers and less. No splitting was observed for superlattices grown under the same conditions on the (311)B surface, which indicates that the splitting effect is probably due to the formation of GaAs quantum wires on the faceted (311)A surface.     

Electron mobility and electron scattering by polar optical phonons in heterostructure quantum wells

Basic features of confined electron scattering in quantum wells (QWs) by confined polar optical (PO) phonons are analyzed. The dependence of electron mobility in Al_0.25Ga_0.75As/GaAs/Al_0.25Ga_0.75As QWs on the well width is calculated. It is demonstrated that increases and decreases in electron mobility (relative to the bulk value) as a function of the QW width occur due to resonance intersubband scattering. The dependence of electron mobility limited by PO phonon scattering on the electron density n _s in the QW is calculated. It is shown that anomalous behavior of electrical conductivity, which in certain cases decreases with increasing electron density, can take place in Al_0.25Ga_0.75As/GaAs/Al_0.25Ga_0.75As QWs for n _s >10¹⁶ m^?2.     

Raman study of GaAs quantum wires grown with partial filling of corrugated (311)A AlAs surfaces

Raman spectroscopy powered by theoretical modeling of vibrational modes was shown to be an effective tool to examine interface structure of superlattices (SLs). In this work we studied GaAs_n/AlAs_m (n=1–10 monolayers) corrugated superlattices (CSLs) grown on (311)A GaAs substrates using Raman spectroscopy and high-resolution transmission electron microscopy. The strongest modification of the calculated Raman spectra is found for the finite length of the GaAs-domain in the CSL in the case of partial (<1 nm) GaAs filling of the AlAs surface. This theoretical result is in a very good agreement with the sharp transformation of the experimental Raman spectra observed for this thickness range. Thus, calculated and experimental Raman spectra demonstrated a good agreement for both complete (≥1 nm) and partial (<1 nm) GaAs filling of the AlAs surface.     

Nonohmic conductance and mechanisms of energy relaxation in 2D electron gas in GaAs/InGaAs/GaAs heterostructures

The effects of heating 2D electron gas in GaAs/InGaAs/GaAs structures at temperatures of 0.4–4 K and electron densities of (1.5–6) × 10¹⁵ m^?2 are studied. For the entire range of temperatures and densities studied, the rate of energy relaxation agrees well with theoretical predictions of the energy relaxation via electron scattering at the piezoelectric and deformation potential of acoustic phonons. It is shown that the widely used analytical expressions for energy relaxation rate, which were obtained in low-and high-temperature limits, are applicable only when the relations between the Fermi quasi-momentum and the momentum of thermal phonons are strictly observed. It is shown that the dependence of the energy relaxation rate on the electron density is a nonmonotonic function. For example, at the lattice temperature of 1.4 K, it initially increases as the electron density decreases to n = 2.5 × 10¹⁵ m^?2, but then starts to decrease as the density decreases further.     

Charge carrier mobility in n-CdxHg1−x Te crystals subjected to dynamic ultrasonic stressing

The Hall mobility was studied in the n-Cd_xHg_1?x Te crystals subjected to dynamic ultrasonic stressing (W _US≤10⁴ W/m², f=5–7 MHz). It was found that, in field of the ultrasonic deformation, an increase in the carrier mobility in the impurity conduction region (T<120 K) and a decrease in the intrinsic conduction region (T>120 K) occurred in all tested samples. In this case, the magnitude of the sonic-stimulated variation in μ_H increases with decreasing structural perfection of a crystal. Different mechanisms of ultrasonic influence on μ_H with regard to scattering by optical phonons, ionized impurities, and alloy potential are analyzed, with the current flow conditions in the crystal taken into account. It is shown that, in the impurity conduction region, the main cause of the sonic-stimulated increase of the Hall mobility is the smoothing of the macroscopic intracrystalline potential that results from the inhomogeneity of the crystals. In the intrinsic conduction region, a decrease in mobility is caused by an increase in the intensity of scattering by the optical phonons.     

Heat‐Transport Mechanisms in Superlattices

The heat transport mechanisms in superlattices are identified from the cross‐plane thermal conductivity Λ of (AlN)_x–(GaN)_y superlattices measured by time‐domain thermoreflectance. For (AlN)_{4.1 nm}–(GaN)_{55 nm} superlattices grown under different conditions, Λ varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest Λ, Λ of (AlN)_{4 nm}–(GaN)_y superlattices decreases monotonically as y decreases, Λ = 6.35 W m⁻¹ K⁻¹ at y = 2.2 nm, 35 times smaller than Λ of bulk GaN. For long‐period superlattices (y > 40 nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G ≈ 620 MW m⁻² K⁻¹. For y < 40 nm, the apparent value of G increases with decreasing y, reaching G ≈ 2 GW m⁻² K⁻¹ at y < 3 nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)_{4.1 nm}–(GaN)_{4.9 nm} superlattice irradiated by 2.3 MeV Ar ions to a dose of 2 × 10¹⁴ ions cm⁻² is reduced by <35%, suggesting that heat transport in these short‐period superlattices is dominated by long‐wavelength acoustic phonons. Calculations using a Debye‐Callaway model and the assumption of a boundary scattering rate that varies with phonon‐wavelength successfully capture the temperature, period, and ion‐dose dependence of Λ.     

Modeling of Thermoelectric Properties of Magnesium Silicide (Mg2Si)

Thermoelectric (TE) materials based on alloys of magnesium (Mg) and silicon (Si) possess favorable properties such as high electrical conductivity and low thermal conductivity. Additionally, their abundance in nature and lack of toxicity make them even more attractive. To better understand the electronic transport and thermal characteristics of bulk magnesium silicide (Mg₂Si), we solve the multiband Boltzmann transport equation within the relaxation-time approximation to calculate the TE properties of n-type and p-type Mg₂Si. The dominant scattering mechanisms due to acoustic phonons and ionized impurities were accounted for in the calculations. The Debye model was used to calculate the lattice thermal conductivity. A unique set of semiempirical material parameters was obtained for both n-type and p-type materials through simulation testing. The model was optimized to fit different sets of experimental data from recently reported literature. The model shows consistent agreement with experimental characteristics for both n-type and p-type Mg₂Si versus temperature and doping concentration. A systematic study of the effect of dopant concentration on the electrical and thermal conductivity of Mg₂Si was also performed. The model predicts a maximum dimensionless figure of merit of about 0.8 when the doping concentration is increased to approximately 10²⁰?cm^?C3 for both n-type and p-type devices.     

Kapitsa effect in crystals with superlattices

It is shown that the longitudinal Kapitsa effect in crystals with superlattices can be explained taking into account Landau quantization and its influence on carrier scattering by the deformation potential of acoustic phonons. Formulas for the Kapitsa coefficient are derived for various degrees of electron gas degeneracy.     

Strain induced lateral ordering in Ga0.22In0.78As/Ga0.22In0.78P short period superlattices on (001) InP

The application of the strain induced lateral ordering process to the strain-compensated (Ga_0.22In_0.78As)_m (Ga_0.22In_0.78P)_m short period superlattices is investigated. The superlattices have been grown at low temperatures by solid source molecular beam epitaxy (MBE) on (001) InP. These superlattices have been used in multiquantum well heterostructures using InP as barriers. The anisotropic polarization of photoluminescence shows the existence of lateral modulation. Dark-field images using the 220 reflection gives modulated contrast in the superlattice layers. High-resolution transmission electron microscopy shows local variations of the interplanar spacing of the (200) planes as well as the angles they form with the (002) planes.     

Preparation and Transport Properties of Bi2O2Se Single Crystals

Bi₂O₂Se single crystals were grown by a gas-phase transport reaction with a temperature gradient. X-ray diffraction revealed that the products crystallized in a tetragonal-type lattice with lattice parameters a?=?0.38866?nm and c?=?1.22001?nm. The samples were characterized by measuring the electrical conductivity, Hall coefficient, and Seebeck coefficient as functions of temperature between 80?K and 470?K. The obtained experimental data allowed us to calculate the reduced Fermi level, provided that the single-valley parabolic model applied. The corresponding value of the electron effective mass proved to be m _ef????0.29. Free electron mobility is governed by the scattering of carriers by acoustic phonons.     

Studies of interdiffusion in GemSin strained layer superlattices

We present the results on the characterization and interdiffusion behavior of Ge _m Si _n strained layer superlattices (SLS’s) composed of alternating monolayers of pure Ge and pure Si. Such Ge _m Si _n SLS’s were grown on top of thick relaxed Ge _y Si_1-y buffer layers so as to symmetrize the strain distribution and to maintain the pseudomorphic growth of the superlattices. Samples with different superlattice periodicities (i.e. d = d_Ge + d_Si and different layer thickness ratios (i.e. d_Ge:d_Si were prepared for comparison. Raman scattering spectroscopy and x-ray diffraction were used to characterize these samples. Initial results on thermal stability of these Ge _m Si _n SLS’s are also reported     

(InSb)m/(HgTe)n短周期超晶格的电子结构

Theelectronicstructureof（InSb）m/（HgTe）nshortperiodsuperlatticesgrownalongthe（001）directionis studiedtheoreticallyusingnorm-conservingpseudo-potentialstogetherwiththelocal-densityapproximationforthe exchange-correlationpotential.Thebandstructuredependsonthevalueofmandn,thenumberofmono-layersand on the ordering of atoms at the InSb/HgTe interface in one unit cell. Our calculation indicates that the superlattice can be a semiconductor having a band gap between the occupied and unoccupied bands, or a metal with no band gap at the Fermi energy. According to the further calculation of total charge density between（InSb）m/（HgTe）nwith different structures, a clearly different behavior happens when the structure changes from a system with a gap and a system without a gap.     

Optical-absorption spectra of PbS/C-based Fibonacci superlattices with phonon-assisted transitions

The method of laser-induced evaporation was used to form the Fibonacci superlattices in an n-PbS/C system (C stands for diamond-like carbon). The optical-absorption spectra in the range of 1.12–3.25 eV were compared with the electron energy spectrum calculated on the basis of the Kronig-Penney model. The electron-affinity energy in the diamond-like carbon was found to be 0.25±0.05 eV. Fine structure in the absorption spectrum in the range of 1.7865–1.7914 eV is attributed to band-to-band transitions that involve phonons with energies of (0.92–2.35)×10^?3 eV.     

静压下（CdSe）_m／（ZnSe）_n－ZnSe超短周期超晶格量子阱的共振喇曼散射研究

在静压和液氮温度下观察到（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中重空穴激子的复合发光和多达４阶的类ＺｎＳｅＬＯ多声子喇曼散射，并观察到厚ＺｎＳｅ势垒层的带边发光和限制在厚势垒层中的类ＺｎＳｅＬＯ声子散射．结果表明，加压后（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中的类ＺｎＳｅ的１ＬＯ和２ＬＯ声子模频率分别以３．７６和７．１１ｃｍ－１／ＧＰａ的速率向高频方向移动，超晶格阱层光致发光峰的压力系数为５９．８ｍｅＶ／ＧＰａ．与（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格共振时的类ＺｎＳｅ１ＬＯ声子模频率比与ＺｎＳｅ势垒层共振时的类ＺｎＳｅ１ＬＯ声子模频率低２．０ｃｍ－１，反映了（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中ＬＯ声子的限制效应     

Growth and optical properties of strained (AIP)n (GaP)n short-period superlattices

Biaxial tensile strain is introduced into (AlP)_n(GaP)_n superlattices (SLs) by growing the SLs on slightly lattice-mismatched InGaP intermediate layers on GaP(00l) substrates. A significant enhancement of photoluminescence intensity is observed for the strained (AlP)_n (GaP)_n short-period SLs, especially for those with n≤ 3.     

Calculation of the low-field mobility of quasi-two-dimensional electrons in a GaAs/Al0.36Ga0.64As superlattice at temperatures in the region of 77 K

The longitudinal and transverse mobilities of quasi-two-dimensional electrons in a GaAs/Al_0.36Ga_0.64As superlattice was calculated in the region of the liquid-nitrogen temperature and in the electron-concentration range from 10¹³ to 10¹⁵ cm^?3. Scattering of electrons by polar optical phonons, acoustic phonons, and impurity ions was taken into account. The Boltzmann equation in the linear approximation in the electric field with allowance made for inelasticity of scattering by polar optical phonons was solved by the sweep method. Analytical formulas for low-temperature longitudinal and transverse relaxation times with involvement of optical phonons were derived. The issue concerning the applicability of the Boltzmann equation to the analysis of longitudinal conductivity in the superlattices under consideration was studied.     

Thermal conductivity of doped PbTe-based solid solutions with off-center impurities

The coefficients of thermopower and electrical and thermal conductivity in the PbTe_0.8Se_0.1 S _0.1 solid solution with electron concentration (4.6–54) × 10¹⁸ cm^?3 are studied in the range of 85–300 K (and in some cases up to 700 K). The temperature dependences of electrical and thermal conductivity indicate that the low-temperature electron and phonon scattering initiated by the off-center impurity of sulfur exists. The temperature dependences of the electronic and lattice components of thermal conductivity are calculated in the approximation of a parabolic spectrum and electron scattering by acoustic phonons and neutral substitutional impurities. The lattice thermal conductivity is found to have a feature in the form of a shallow minimum in the range of 85–250 K. A similar feature, while not so clearly pronounced, is found to exist also in Pb_1?x Sn_xTe_1?x Se_x alloys (x≥0.15) with an off-center tin impurity. An analysis of the possible origins of this effect suggests that, at low temperatures, the Lorentz numbers L of the materials under study are smaller than the L₀ numbers employed which correspond to the above scattering mechanisms. The cause of the decrease in L is related to electron scattering at two-level systems, a mechanism whose effect grows with increasing electron energy. An analysis of experimental data obtained at high temperatures, as well as on undoped samples with the lowest possible carrier concentrations, yields the values of L for samples with different electron densities. The minimum value L/L₀ = 0.75 is obtained for a lightly doped sample at ～130 K.     

Electron mobility in n-doped zinc sulphide

A study of the mobility of n-doped wurtzite and zincblende ZnS is reported. We have determined nonequilibrium thermodynamic state of the ZnS—driven far away from equilibrium by a strong electric field—in the steady state. The dependence of the mobility (which depends on the nonequilibrium thermodynamic state of the sample) on the electric field strength is derived, which decreases with the strength of electric field. We analyzed the contributions to the mobility arising out of the different channels of electron scattering, namely, the polar optic, deformation, interactions with the phonons, and with ionized impurities. The case of n-ZnS WZ and ZB have been analyzed: as expected the main contribution comes from the polar-optic interactions in this strong-polar semiconductor. The other interactions are in decreasing order, the deformation acoustic and the one due to impurities.     

Resonant inelastic light scattering and photoluminescence in isolated nc-Si/SiO2 quantum dots

Observation at the room temperature the spectra of the resonant inelastic light scattering by the spatially confined optical phonons as well as the excitonic luminescence caused by confinement effects in the ensemble of isolated quantum dots (QDs) nc-Si/SiO₂ is reported. It is shown that the samples investigated are high purity and high crystalline perfection quality nc-Si/SiO₂ QDs without amorphous phase α-Si and contaminants. Comparison between the experimental data obtained and phenomenological model of the strong space confinement of optical phonons revealed the need of the more accurate form of the weighted function for the confinement of optical phonons. It is shown that simultaneous detection of the inelastic light scattering by the confinement of phonons and the excitonic luminescence spectra by the confined electron-hole pairs in the nc-Si/SiO₂ QDs allows selfconsistently to determine more accurate values of the diameter of the nc-Si/SiO₂ QDs.