Electron-confined LO-phonon scattering in GaAs-Al0.45Ga0.55As superlattice

We develop a theoretical model to the scattering time due to the electron-confined LO-phonon in GaAs-Al_xGa_1-xAs superlattice taking into account the sub-band parabolicity. Using the new analytic wave function of electron miniband conduction of superlattice and a reformulation slab model for the confined LO-phonon modes, an expression for the electron-confined LO-phonon scattering time is obtained. In solving numerically a partial differential equation for the phonon generation rate, our results show that forx = 0.45, the LO-phonon in superlattice changes from a bulk-like propagating mode to a confined mode. The dispersion of the relaxation time due to the emission of confined LO-phonons depends strongly on the total energy.     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

Raman spectroscopy—A versatile tool for characterization of thin films and heterostructures of GaAs and AlxGa1−xAs

It is shown that Raman spectroscopy can provide useful information on characteristic properties of thin crystalline films of compound semiconductors. Crystal orientation, carrier concentration, scattering times of charge carriers, composition of mixed crystals and depth profiles can be studied in thin layers and heterostructures of GaAs and Al_xGa_1−xAs. The advantages and disadvantages of Raman scattering compared to conventional characterization methods are discussed.     

Raman scattering determination of folded acoustical phonon dispersion curves in large period GaAs/GaAlAs superlattices

We report the first experimental determination of the dispersion curves of folded acoustical modes in large period GaAs/Ga_1?xAl_xAs superlattices, obtained by scanning the incident wavelength in a Raman scattering experiment. Moreover, the resonant scattering evidences disorder activated Raman lines similar to those already reported in Ga_1?xAl_xAs single layers. These results imply the coexistence in Ga_1?xAl_xAs of dispersive virtual crystal acoustical modes with non dispersive disorder induced contributions. These features are explained on the basis of the Coherent Potential Approximation.     

Hot electron photoluminescence in GaAs crystals

The spectrum and the linear polarization of photoluminescence of hot electrons in GaAs crystals were investigated. Oscillations in the hot photoluminescence (HPL) spectrum due to the subsequent emission of LO-phonons were observed. The study of HPL depolarization in an external magnetic field yielded the scattering time due to the emission of a LO-phonon by a hot electron in the Γ-valley (τ_?0 = 1 × 10^?13 sec) as well as the Γ?L intervalley scattering time. The radiative recombination of hot electrons created in the central Γ-valley via the subsidiary L-valley was observed. The distribution function of hot electrons in a wide energy range was evaluated from the spectra.     

Evidence by Raman scattering on In1−xGaxAsyP1−y of the two-mode behaviour of In1−xGaxP

We report the Raman scattering study of optical vibrations in indium rich In_1?xGa_xAs_yP_1?y epitaxial layers grown on InP. We evidence the splitting of the LO line of InP even for very small x and demonstrate it is due to the presence of gallium in the samples. These results lead us to ascribe to In_1?xGa_xP a somewhat modified two-mode behaviour and to In_1?xGa_xAs_yP_1?y a four-mode behaviour.     

Vibrational spectroscopy of the Ga1−x AlxP epitaxial layers grown on GaP(111) substrate by the liquid-phase epitaxial technique

The paper reports the results of measurements of the lattice IR reflection and Raman scattering spectra for the Ga_1?x Al_xP (x=0–0.8) films grown on the GaP(111) substrate by the liquid-phase epitaxy technique. The dispersion analysis of the experimental spectra has demonstrated that, for the studied system of the Ga_1?x Al_xP alloy, the vibrational spectra of the alloys with different compositions exhibit three modes of the Ga-P vibrations and one mode of the Al-P vibrations. The frequencies of modes only slightly depend on the composition x of the Ga_1?x Al_xP alloy, but the composition considerably affects the oscillator strengths of these modes.     

Ferromagnetism in heterostructures based on a dilute magnetic semiconductor

Ferromagnetism of magnetic impurity atoms located in the barrier regions of various heterostructures (solitary heterojunction, single quantum well, double quantum well, or superlattice) is considered theoretically. The indirect magnetic interaction of impurities occurs via charge carriers localized in quasi-two-dimensional conducting channels of these structures due to “penetration” of the wavefunction of charge carriers into the barrier regions. The wavefunctions defined analytically in the triangular potential model are virtually the same as in “exact” numerical calculations (joint solution of the Poisson and Schrödinger equations). The corresponding Curie temperatures are determined, which may attain approximately 500 K in Ga_{1 ? x} Mn _x As-based structures according to calculations.     

Resonant electron tunneling in GaN/Ga1−x AlxN(0001) strained structures with spontaneous polarization and piezoeffect

Electron tunneling through the GaN/Ga_1?x Al_xN(0001) wurtzite strained structures is investigated by the pseudopotential and scattering matrix methods. It is shown that the results of multiband calculations at low aluminum concentrations (x<0.3) are adequately described within the single-valley model in the envelope wave function method accounting for the dependences of the effective mass on the energy and strain. Upon electron tunneling through two-barrier structures, sharp resonance peaks are observed at a barrier thickness of several monolayers and the characteristic collision time in the resonance region is equal to ～1 ps. The internal electric fields associated with spontaneous and piezoelectric polarizations lead to a “red” or “blue” shift in the resonance energy according to the thickness and location of barriers with respect to the polar axis. In the (GaN)_n(Ga_1?x Al_xN)_m superlattices, the internal fields can form the Stark ladder of electronic states at a small number of ultrathin layers even in the absence of external fields.     

Raman scattering from GaAs-InxGa1−xAs strained-layer superllatices

Measurements of Raman scattering were performed on GaAs-In_xGa_1?xAs strained-layer superlattices, grown by molecular beam epitaxy, with lattice periods ranging from 30 ～ 250 Å and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-Al_xGa_1?xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk In_xGa_1?xAs alloys as the ratio of the one-layer thickness of In_xGa_1?xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer superlattice and the phonon frequency is determined by the averaged In concentration.     

Resonant Raman scattering in nanostructures with InGaAs/AlAs quantum dots

Raman scattering by optical phonons in In_xGa_{1 ? x} As/AlAs nanostructures with quantum dots has been studied experimentally for compositions corresponding to x = 0.3?1 under out-resonance conditions. Features due to scattering by GaAs-and InAs-like optical phonons in quantum dots have been detected, and the phonon frequencies have been determined as a function of the dot composition. With increasing excitation energy, a red shift is observed in the frequency of the GaAs-like phonon in quantum dots, which testifies to Raman scattering selective by the size of quantum dots. Under resonant conditions, multiphonon light scattering by optical and interface phonons is observed up to the third order, including overtones of the first-order phonons of InGaAs and AlAs materials and their combinations.     

Disorder-induced Raman scattering of In1−xGaxP in the transverse acoustic phonon region

Raman scattering measurements have been performed on In_1?xGa_xP (0.62?x?1) over the entire frequency range of first and second order scattering. Besides the already known disorder activated band between the LO(Γ) and TO(Γ) modes a new disorder activated band is found in the region of transverse acoustic phonons around 87 cm^-1. The position of the new band shifts only slightly with composition while its strength and line-shape change.     

Resonant Raman scattering by plasmons in n-type Ge

We have measured the Raman efficiency for plasmon scattering in n-type Ge as a function of laser excitation frequency in the range of the E₁ and E₁+Δ₁ optical gaps (2.1–2.5 eV). Our results can be explained by a mechanism involving the macroscopic electric field that accompanies the plasma oscillation in a manner similar to that responsible for Fröhlich-interaction-induced forbidden LO-phonon resonances in polar semiconductors plus some contribution of the standard cdf mechanism for scattering by plasmons.     

Electronic Raman scattering from inter-valence band transition in photo-excited GaP and GaAs1−xPx crystals

Raman scattering from photo-created free carriers in undoped GaP and GaAs_1?xP_x (x = 0.85, 0.73 and 0.66) under high excitation intensity has been studied. Two new Raman bands have been observed and assigned to electronic transitions from the split-off hole band to the heavy hole band and from the light hole band to the heavy hole band. The spin-orbit splitting energies in these crystals have been determined from the analysis of observed Raman bands, and compared with other experimental values.     

Solid immiscibility and liquid structure in the Ga2(SexTe1−x)3 alloy system

Differential thermal analysis and X-ray diffraction studies of the system Ga₂(Se_xTe_1?x)₃ are reported and correlated with measurements of the electrical conductivity in the liquid state. The experiments reveal that Ga₂Te₃ and Ga₂Se₃ do not form a continuous series of pseudo-binary solid solutions but exhibit solid state immiscibility in the range 0.50 ? x ? 0.90. The composition dependence of the conductivity of the liquid alloys exhibits structure in the range of the solid state phase separation. Such “memory” in the liquid of a solid phase separation has not been reported previously and suggests the importance of concentration fluctuations in determining the electronic properties of the liquid alloys.     

Structural and electronic properties of zincblende phase of Tl x Ga1−x As y P1−y quaternary alloys: First-principles study

Using the first-principles band-structure method, we have calculated the structural and electronic properties of zincblende TlAs, TlP, GaAs and GaP compounds and their new semiconductor Tl _x Ga_1?x As _y P_1?y quaternary alloys. Structural properties of these semiconductors are obtained with the Perdew and Wang local-density approximation. The lattice constants of Tl _x Ga_1?x As, Tl _x Ga_1?x P ternary and Tl _x Ga_1?x As _y P_1?y quaternary alloys were composed by Vegard’s law. Our investigation on the effect of the doping (Thallium and Arsenic) on lattice constants and band gap shows a non-linear dependence for Tl _x Ga_1?x As _y P_1?y quaternary alloys. The band gap of Tl _x Ga_1?x As _y P_1?y , E _g (x, y) concerned by the compositions x and y. To our awareness, there is no theoretical survey on Tl _x Ga_1?x As _y P_1?y quaternary alloys and needs experimental verification.     

A new semiconductor superlattice with tunable electronic properties and simultaneously with mobility enhancement of electrons and holes

A new artificial semiconductor superlattice with tunable electronic properties and simultaneously with significant mobility enhancement of both 2-dimensional electrons and 2-dimensional holes has been prepared by molecular beam epitaxy. The structure consists of a periodic sequence ofn-Al _x Ga_1?x As/i-GaAs/n-Al _x Ga_1?x As/p-Al _x Ga_1?x As/ i-Ga.As/p-Al _x Ga_1?x As stacks with undoped Al _x Ga_1?x As spacers between the intentionally doped Al _x Ga_1?x As and the nominally undopedi-GaAs layers. In this newheterojunction doping-superlattice we have for the first time achieved a spatial separation of electrons and holes by half a superlattice period as well as simultaneously a spatial separation of both types of free carriers from their parent ionized impurities. These unique properties are demonstrated by the strongly increased tunability of bipolar conductivity with bias. In addition, the observed temperature dependence of Hall mobilities provides direct evidence for a strong mobility enhancement of both electrons and holes in the spatially separated 2-dimensional accumulation channels formed in the lower band gap material.     

Localized exciton-phonon complex as an intermediate state in light scattering

A $\text{sharp}$ LO-phonon replica of the first order Raman spectrum of (Ga_xIn_1-xP and Ga(As_1-xP_x) has been observed for the first time. The experiment shows strong resonance with varying incoming photon energy which only appears in the presence of alloying and disorder. The results can only be understood if the relevant resonant intermediate state is a localized sharp exciton strongly interacting with the LO-phonon system, i.e. a localized exciton-phonon complex. Under those conditions, a simple model yields a ratio of the two-phonon to one-phonon intensities which is given by straightforward Franck-Condon overlaps.     

Calculation of I–V curves in narrow-gap semiconductors with symmetric electron and hole energy dispersion laws

The hot electron phenomena are considered in the narrow-gap semiconductors with Lax band structure. Methods for calculation of I–V curves are developed in the case when only the quasielastic scattering processes occur. The following factors are taken into consideration: Bloch structure of wave functions; electron-electron interaction; change in concentration of carriers due to the warming of electrons in the electric field; anisotropy and many-valley character of band spectrum. Results obtained here may be useful for the interpretation of experimental data for alloys Bi_1-xSb_x, lead and tin chalcogenides.     

Electron scattering and transport phenomena in small-gap zinc-blende semiconductors

We give expressions for electrical conductivity, thermoelectric power and thermal conductivity of conduction band electrons in small-gap zinc-blende semiconductors, obtained by solving the Boltzmann equation by a variational procedure. The term resulting from the phonon-drag is included in the Boltzmann equation. The following electron scattering mechanisms are investigated: inter and intraband scattering by optical phonons via polar and nonpolar interactions, scattering by charged centers (ionized defects and heavy holes) and by neutral centers, as well as scattering by acoustic phonons. Particular attention is paid to the screening of the electron-optical phonon polar interaction by free carriers, which is particularly important in the case of a linear energy band. The formula for the intraband RPA dielectric function for the case of the linear band is given.The general formulation of all the problems investigated permits direct application of the results given in this paper to both intrinsic or n-type HgTe-type and InSb-type semiconductors, including mixed crystals, e.g. Cd_xHg_1?xSe near the cross point.