Growth and properties of ferromagnetic In1−xMnxSb alloys

We discuss a new narrow-gap ferromagnetic (FM) semiconductor alloy, In_1−xMn_xSb, and its growth by low-temperature molecular-beam epitaxy. The magnetic properties were investigated by direct magnetization measurements, electrical transport, magnetic circular dichroism, and the magneto-optical Kerr effect. These data clearly indicate that In_1−xMn_xSb possesses all the attributes of a system with carrier-mediated FM interactions, including well-defined hysteresis loops, a cusp in the temperature dependence of the resistivity, strong negative magnetoresistance, and a large anomalous Hall effect. The Curie temperatures in samples investigated thus far range up to 8.5 K, which are consistent with a mean-field-theory simulation of the carrier-induced ferromagnetism based on the 8-band effective band-orbital method.     

Interface-related effects on confined excitons in GaAs/AlxGa1−xAs single quantum wells

Confined excitons in non-abrupt GaAs/Al_xGa_1−xAs single quantum wells are studied. The graded interfaces are described taking into account fluctuations in their thickness a and positioning with respect to the abrupt interface picture. Numerical results for confined (0,0),(1,1) and (0,2) excitons in GaAs/Al_0.3Ga_0.7As quantum wells show that while the interfacial fluctuations produce small changes (<0.5 meV) in the exciton binding energies, the confined exciton energies can be red- or blue-shifted as much as 25 meV for wells with mean width of 50 Å and 2 ML wide interfaces.     

Photoluminescence and X-ray characterisation of Si/Si1−xGex multiple quantum wells

We present an experimental approach to correlate optical and structural properties of Si/Si_1−xGe_x multiple quantum wells as determined by photoluminescence (PL) and X-ray diffraction, respectively. The optical properties of the quantum wells were characterised by studying the dependence of luminescence on temperature and excitation density. An enhanced PL yield and an increased quenching temperature were observed for a sample grown at 650°C as compared to one grown at 600°C. Pronounced interdiffusion across the multiple quantum well interfaces as well as significant lattice distortions within the SiGe layers have been observed.     

MBE growth and magnetotransport studies of ferromagnetic Ga1−xMnxSb semiconductor layers on hybrid ZnTe/GaAs substrates

Narrow-gap Ga_1−xMn_xSb layers grown on hybrid ZnTe/GaAs substrates are observed to be ferromagnetic by SQUID magnetization and anomalous Hall effect measurements. The layers display an easy axis of magnetization perpendicular to the layer plane, in contrast to in-plane easy axis orientation observed in Ga_1−xMn_xSb grown on GaSb substrates. Resistivity measured in the Ga_1−xMn_xSb/ZnTe/GaAs system shows a well-defined maximum at temperatures close to the ferromagnetic/paramagnetic transition. We determined the spontaneous resistivity anisotropy in Ga_0.98Mn_0.02Sb grown on hybrid ZnTe/GaAs substrates and compared it to that observed on Ga_0.98Mn_0.02Sb grown on a GaSb buffer. These results should provide a valuable test for future theories of transport in ferromagnetic semiconductors.     

Zn1−xMnxO : A typical member of the II-VI:Mn DMS family

ZnO:Mn thin films are grown by MOCVD technique. Mn(x) varies in 0<x<0.44 range. Vegrad’s law has been verified for the lattice parameters. EPR measurements prove the substitution incorporation of Mn²⁺ on zinc sites. The behavior of EPR line-width regarding temperature is discussed. All ZnO:Mn layers show antiferromagnetic interaction and a effective exchange constant. Observation of excitons, giant Zeeman effect, and trace of the Brillouin function is evidence for high quality of the crystal lattice and of substitutional incorporation of manganese ions in place of Zn, as Mn²⁺.     

Determination of the g-factor of n-type HgTe/Hg1−xCdxTe single quantum wells

The effective g-factor of modulation doped n-type HgTe single quantum wells, SQWs, has been determined by the coincidence method in tilted magnetic fields to lie between 15 and 35. For symmetrically doped samples the effective g-factor has been found to be constant for different filling factors; however, for asymmetric SQWs, a large increase with increasing filling factor has been observed. This can be ascribed to a combination of Zeeman spin splitting and Rashba spin–orbit splitting. Reasonable agreement has been achieved between theoretical calculations based on the 8×8 k · p method and experimental results.     

Exciton energy broadening due to interface fluctuations in ZnSe/ZnSxSe1−x strained quantum wells

The excitonic properties of a ZnSe/ZnS_xSe_1−x strained quantum well (QW) are calculated taking into account interface effects. Numerical results obtained with ZnS_0.18Se_0.82/ZnSe QWs show that graded interfaces can be responsible for a strong broadening of excitonic spectra.     

Negative luminescence from In1−xAlxSb and CdxHg1−xTe diodes

Indium aluminium antimonide (In_1−xAl_xSb) and cadmium mercury telluride (Cd_xHg_1−xTe) heterostructure diodes, which comprise a near intrinsic active region bounded by more highly doped contact regions, exhibit positive or negative luminescence at medium to long infrared wavelengths when forward or reverse biased respectively at room temperature. In reverse bias, the carrier densities in the near intrinsic region are reduced below their equilibrium values by the effects of exclusion and extraction. In consequence, the radiative recombination is reduced and the devices emit less infrared radiation than the thermal equilibrium value. The observed intensity of the negative luminescence is in general agreement with expected values.     

Optical properties of Cd1−xZnxTe thin films fabricated through sputtering of compound semiconductors

Cd_1−xZn_xTe thin film fabrication is necessary for its photovoltaic and imaging applications in large scale. Thermally annealed and thereby interdiffused r.f. sputtered multilayers comprising of CdTe and ZnTe have been utilized here for the fabrication of Cd_1−xZn_xTe thin films. Photoluminescence and change of resistance of the multilayer under illumination were studied using different annealing temperatures and varying number of repetitions. It was found that three number of repetitions annealed at 300 °C exhibited the best results.     

Growth and studies of Hg1−xCdxTe based low dimensional structures

The band structure of HgTe quantum wells (QWs) has been determined from absorption experiments on superlattices in conjunction with calculations based on an 8×8 k·p model. The band structure combined with self-consistent Hartree calculations has enabled transport results to be quantitatively explained.Rashba spin–orbit, (SO) splitting has been investigated in n-type modulation doped HgTe QWs by means of Shubnikov–de Haas oscillations (SdH) in gated Hall bars. The heavy hole nature of the H1 conduction subband in QWs with an inverted band structure greatly enhances the Rashba SO splitting, with values up to 17 meV.By analyzing the SdH oscillations of a magnetic two-dimensional electron gas (2DEG) in modulation-doped n-type Hg_1−xMn_xTe QWs, we have been able to separate the gate voltage-dependent Rashba SO splitting from the temperature-dependent giant Zeeman splitting, which are of comparable magnitudes. In addition, hot electrons and Mn ions in a magnetic 2DEG have been investigated as a function of current.Nano-scale structures of lower dimensions are planned and experiments on sub-micrometer magneto-transport structures have resulted in the first evidence for ballistic transport in quasi-1D HgTe QW structures.     

Fractional-dimensional polaron corrections in asymmetric GaAs-Ga1−xAlxAs quantum wells

Polaron effects in asymmetric GaAs-Ga_1−xAl_xAs quantum wells (QWs) are investigated within the framework of the fractional-dimensional space approach and by using second-order perturbation theory. A well-width dependence of the polaron corrections with a dip and a peak is obtained for both symmetric and asymmetric QWs. The dip and the peak occur in the case of asymmetric QWs for larger well widths than in the case of symmetric QWs. An enhancement of the contrast between the dip and the peak of the polaron energy shift is found for the case of asymmetric QWs. These results show the convenience of using asymmetric QWs instead of symmetric ones in any experimental attempt of detecting the dip and the peak of the polaron energy shift.     

Resonant Rayleigh scattering from excitons in CdxZn1−xTe:ZnTe quantum wells: measurement of homogeneous linewidths

The technique of resonant Rayleigh scattering is used to determine the homogeneous linewidth across the inhomogeneously broadened exciton resonance in a Cd_0.25Zn_0.75Te/ZnTe multiple quantum well structure. An order of magnitude increase of the Rayleigh scattering signal over background is observed on tuning a narrow-band laser through the exciton resonance at low temperatures. Spectral and temporal measurements show the effect to be a true scattering process rather than luminescence. The interface and alloy fluctuations in the quantum well give rise to spatial fluctuations in the dielectric response of the system while the large exciton resonance causes strong enhancement of scattering. The homogeneous linewidth was calculated across the exciton resonance. The technique is compared with the dephasing and hole-burning techniques more commonly used in homogeneous linewidth measurements.     

Calculated effects of interface grading in GaAs----Ga1−xAlxAs quantum wells

Effects of interface grading on energy levels of electrons in GaAs---Ga_1−xAl_xAs quantum wells have been estimated using both a tight-binding formalism and an effective-mass Hamiltonian of the BenDaniel-Duke form. Graded interfaces a few atomic layers thich have only a small effect on energy levels in both schemes. Self-consistent calculations for electrons in a relatively wide (40 nm) quantum well show how the lowest levels change from those characteristic of the empty well to those characteristic of two weakly coupled heterojunctions as the electron density is increased.     

Ferromagnetic Zr1−xMnxCo2 laves phase compounds

Structure and magnetic properties of the Zr_1−xMn_xCo_2+δ alloys were studied for 0 x <0.7, δ=0, 0.45. The cubic C15 Laves phase structure shows Mn solubility up to x≈0.4. The other Laves phase with the hexagonal C36 structure found for x0.5 apparently has a small region of Mn solubility in the vicinity of Zr_0.4Mn_0.6Co₂. Though the parent Mn-free compounds are known to be paramagnetic, the Mn-substituted alloys show ferromagnetic behavior with the Curie temperatures up to 625 K and the room-temperature saturation magnetization of about 100 emu/g. The onset of ferromagnetism with the Mn substitution for Zr may be caused by polarization of itinerant 3d electrons, like it was earlier supposed for the off-stoichiometric ZrCo_2+δ. The universal composition dependencies of the intrinsic magnetic properties for different δ can be obtained, if plotted against the amount of zirconium atoms missing in its sublattice. The room-temperature anisotropy with the noticeable anisotropy field of 24 kOe and the 1 1 0 easy magnetization direction laying in a basal plane was found in the hexagonal Zr_0.5Mn_0.5Co₂.     

Magnetic properties of single crystalline Zn1−xCoxO thin films

We report on the magnetic properties of single crystalline thin films of Zn_1−xCo_xO (x=0.003–0.14) grown by plasma-assisted molecular beam epitaxy. In order to understand the role of intermediate charge carriers in the magnetic properties of this material two types of films were fabricated, with and without Ga-codoping. Magnetic measurements were made between 2 and 300 K in fields up to 5 T with a Quantum Design SQUID magnetometer. We found that all the tested films exhibit paramagnetic behavior following the Curie–Weiss law, χ=C/(T−θ), with negative Curie–Weiss temperatures and that this behavior holds even under strong n-doping. We show that the magnetization data, M(H), in function of the Co content provide additional evidence in favor of the antiferromagnetic Co–Co interaction in this material. We also observe that these data exhibit an ‘easy plane’ magnetic anisotropy for all the studied Co concentrations. Finally, we develop a simple cluster model, in order to describe the magnetic properties of ZnCoO, which is found to be in good agreement with our experiments.     

Transport, optical and magnetotransport properties of hetero-epitaxial InAsxSb1−x/GaAs(x0.06) and bulk InAsxSb1−x (x0.05) crystals: experiment and theoretical analysis

We briefly review the growth and structural properties of InAs_xSb_1−x (x0.05) bulk single crystals and InAs_xSb_1−x (x0.06) epitaxial films grown on semi-insulating GaAs substrates. Temperature-dependent transport measurements on these samples are then correlated with the information obtained from structural (XRD, TEM, SEM) and optical (FTIR absorption) investigations. The temperature dependence of mobility and the Hall coefficient are theoretically modelled by exactly solving the linearized Boltzmann transport equation by inversion of the collision matrix and the relative role of various scattering mechanisms in limiting the low temperature and 300 K mobility is estimated. Finally, the first observation of Shubnikov oscillations in InAsSb is discussed.     

A spectroscopic ellipsometric investigation of new critical points of Zn1−xMnxS epilayers

Zn_1−xMn_xS epilayers were grown on GaAs (1 0 0) substrates by hot-wall epitaxy. X-ray diffraction (XRD) patterns revealed that all the epilayers have a zincblende structure. The optical properties were investigated using spectroscopic ellipsometry at 300 K from 3.0 to 8.5 eV. The obtained data were analyzed for determining the critical points of pseudodielectric function spectra, (E) = ₁(E) + i₂(E), such as E₀, E₀ + Δ₀, and E₁, and three E₂ (Σ, Δ, Γ) structures at a lower Mn composition range. These critical points were determined by analytical line-shapes fitted to numerically calculated derivatives of their pseudodielectric functions. The observation of new peaks, as well as the shifting and broadening of the critical points of Zn_1−xMn_xS epilayers, were investigated as a function of Mn composition by ellipsometric measurements for the first time. The characteristics of the peaks changed with increasing Mn composition. In particular, four new peaks were observed between 4.0 and 8.0 eV for Zn_1−xMn_xS epilayers, and their characteristics were investigated in this study.     

Oscillator strength of type-II light-hole exciton in InxGa1−xAs/GaAs strained single quantum wells

We have investigated the excitonic properties of In_0.15Ga_0.85As/GaAs strained single quantum wells by using photoreflectance spectroscopy and a variational calculation method. We clearly detected the photoreflectance signal of the type-II light-hole exciton, which consists of an electron confined in the InGaAs layer and a light hole located in the thick GaAs layer, in addition to the type-I heavy-hole exciton confined in the InGaAs layer. The calculated results of the overlap integral of the envelope function in the type-II light-hole exciton predict that the oscillator strength is remarkably enhanced with decreasing the InGaAs-layer thickness. This is demonstrated by the layer-thickness dependence of the photoreflectance intensity of the type-II light-hole exciton.     

Surface photovoltage investigations of Cd1−xMnxTe for x = 0.01 and 0.10

Surface photovoltage investigations of Cd_1−xMn_xTe monocrystals for x = 0.01 and 0.10 were performed in the temperature range between 100 and 300 K with a modified Kelvin method at a pressure of 10⁻⁵ Pa. The surfaces with orientation (110) were ground, polished with “Gamal”, and rinsed in acetone and alcohol. Three types of effects were observed on the surface spectroscopy curves: A sharp increase in photovoltage, connected with the electron band-to-band transitions for a photon energy equal to the energy gap. Photovoltage quenching attributed to the existence of surface states with energy just above the edge of the valence band. Increase in photovoltage in the range between 0.9 and 1.0 eV resulting from electron transitions between the valence band and energy states connected with manganese ions.