Type II heterojunctions in an InGaAsSb/GaSb system: Magnetotransport properties

Magnetotransport properties of the narrow-gap In_xGa_1?x As_ySb_1?y /GaSb heterojunctions grown by liquid-phase epitaxy with various In content in the solid solution (x=0.85–0.95 and E _g ≤0.4 eV) were studied. It is shown that, depending on the In content in these heterostructures, type II staggered-lineup (x=0.85) or broken-gap heterojunctions (x=0.95) with high mobility in the electron channel at the interface (μ?20000 cm²/(V s)) can be realized. For x=0.92, depending on temperature, both types of heterojunctions were observed. Obtained results are in good agreement with the band energy diagram of the type II InGaAsSb/GaSb heterostructures under study.     

Magnetotransport properties of type II heterojunctions based on GaInAsSb/InAs and GaInAsSb/GaSb

The results of detailed study of the magnetotransport properties of broken-gap type II heterojunctions in a GaInAsSb/InAs(GaSb) system are reported. An electron channel with a high charge-carrier mobility (as high as 50000–60000 cm²/(V s)) is observed and studied for the first time in an isotype broken-gap p-GaInAsSb/p-InAs heterostructure. The effects of electron-channel depletion and semimetal-semiconductor transition in the case of heavy doping of the quaternary alloy with acceptors are studied. Magnetotransport properties at temperatures of 4.2–200 K are studied in detail. Data on the energy spectrum and parameters of two-dimensional charge carriers at the heteroboundary are obtained. It is ascertained experimentally that, depending on the composition, either staggered (at x = 0.85) or broken-gap (at x = 0.95) heterojunctions can be formed in the Ga_1?x In_xAs_ySb_1?y /GaSb, which is confirmed by theoretical calculations. The anomalous Hall effect and negative magnetoresistance were observed in GaInAsSb/InAs:Mn grown on substrates doped heavily with Mn magnetic acceptor impurity so that the hole concentration was as high as p > 5 × 10¹⁸ cm^?3; these phenomena are caused by exchange interaction of Mn ions in InAs with high-mobility charge carriers in the electron channel at the heterointerface.     

Experimental and theoretical investigation of defects at (1 0 0) Si1−xGex/oxide interfaces

The identification of a nontrigonal Ge dangling bond at SiO₂/Si_1−xGe_x/SiO₂ heterostructures and its electrical activity are discussed, both from experimental and theoretical points of view. This dangling bond is observed from multifrequency electron-spin resonance experiments performed at 4.2 K, for typical Ge concentrations in the range 0.4 ≤ x ≤ 0.85. The electrical activity of this defect is revealed from capacitance-voltage characteristics measured at 300 and 77 K, and is found to behave like an acceptor defect. First-principles calculations of the electronic properties of this Ge dangling bond indicate that its energy level approaches the valence band edge of the Si_1−xGe_x layer as the Ge content increases, confirming its acceptor-like nature.     

Effect of Zr4+ Content on the Grain Growth,Dielectric Relaxation Behavior,and Ferroelectric Properties of Ba0.4Sr0.6Ti1−x Zr x O3 Nano-Ceramics Prepared by Different Methods Assisted by Fast Microwave Sintering

The effect of Zr⁴⁺ content on the grain growth, dielectric relaxation, and piezoelectric properties of Ba_0.4Sr_0.6Ti_1?x Zr _x O₃ (BSTZ; x = 0, 0.02, 0.04, 0.06) ceramics prepared by solid-state (SS) and sol–gel modified hydrothermal (SH) methods assisted by fast microwave sintering was investigated in this study. A combination of x-ray diffraction (XRD), scanning electron microscopy (SEM), impedance analysis, and ferroelectric analysis was used. All the ceramics had pure perovskite structures at room temperature, as seen from XRD patterns, indicating that Zr⁴⁺ was incorporated into Ba_0.4Sr_0.6TiO₃ lattices to form a solid solution. In the SEM micrographs, SH samples had higher densities and smaller and more homogeneous grain size than SS samples, which was in agreement with density measurements. Nano-ceramics were obtained by this method. When the temperature dependence of dielectric constant and dielectric loss was studied, SH samples had higher permittivity, better thermally activated relaxation, and lower dielectric loss at high temperature. Ferroelectric characteristics can still be detected in Ba_0.4Sr_0.6Ti_1?x Zr _x O₃ ceramics and residual polarization (P _r) decreased with increasing Zr⁴⁺ content.     

AC conductivity of nanoparticles CoxFe(1−x)Fe2O4 (x=0, 0.25 and 1) ferrites

Nanoparticles of Co_xFe_(1−x)Fe₂O₄ (x=0, 0.25 and 1) were prepared by the chemical co-precipitation method. X-ray diffraction and scanning electron microscopy were used to determine the average particle size and morphology of the prepared samples. AC conductivity is found to vary as ω^s in the frequency range 42–5×10⁶ Hz. The impedance analysis reveals that low conductivity and high impedance values are observed at low temperatures. The Nyquist impedance plots of the present investigation clearly depict the inherent phenomenon involved in the conduction mechanism of Co doped Fe₃O₄ ferrites. Regarding frequency dependence of Co_xFe_(1−x)Fe₂O₄ AC conductivity the observed behavior clearly indicates that the present ferrites are semiconductor-like.     

The interface between Hg1−xCdxTe and its native oxide

The results of a study of the electrical properties of the interface between Hg_1?xCd_xTe with x = 0.21 and its native oxide at 77°K are presented. The native oxide is formed by anodic oxidation and results in an interface with reproducible properties. The surface charge, the surface mobility and the effective lifetime are obtained from galvanomagnetic measurements and are related to the semiconductor bulk parameters, the oxide thickness and the annealing conditions. The surface state charge and the metal-semiconductors work function difference are obtained from the shift of the flat band voltage of metal-oxide-semiconductor (MOS) capacitor characteristics. The interface between Hg_1?xCd_xTe and its native anodic oxide is characterized by a density of fast surface states of the order of 5 × 10¹¹cm^?2 (eV^?1) near the middle of the bandgap. The density of states increases towards the band edges to the order of 10¹³cm^?2 (eV^?1). The measured flat band voltage is approximately ?0.5 V for an oxide thickness of 500 Å and for an n-type semiconductor with an electron carrier concentration in the range 1–3 × 10¹⁵cm^?3 at 77°K. The fixed oxide surface state charge is positive for both p-type and n-type semiconductors and is of the order of 6 × 10¹¹ charges per cm^?2. The surface properties, the significance and the reproducibility of the results are evaluated.     

Enhanced barrier height of AuIn1−xGaxAsyP1−y Schottky diodes

Barrier heights of Au Schottky diodes made on In_1?xGa_xAs_yP_1?y epitaxial layers with interfacial films have been measured. The interfacial layer was either a thin (?25 Å) natural oxide film or an oxide film grown by HNO₃ oxidation for which the thickness was much greater (300–500 Å). Room temperature forward-biased current-voltage and capacitance-voltage measurements show that enhancement of the barrier heights ranging from 0.15 to 0.32 eV, depending on composition, is obtained for the natural oxide films. The enhancement is much less for the HNO₃ oxide layers. The best value of the ideality factor n = 1.4 and saturation current densities J_s ranging from 10^?4 to 10^?8 A cm^?2 are obtained. For the thin natural oxide interfacial films, the results can be explained reasonably with a thermionic emission model taking into account the interfacial oxide layer.     

Interfacial reactions and electrical properties of Hf/p-Si0.85Ge0.15

Interfacial reactions and electrical properties of Hf/p-Si_0.85Ge_0.15 as a function of the annealing temperature were studied. Hf₃(Si_1−xGe)₂ and Hf(Si_1−xGe)₂ were initially formed at 500°C and 600°C, respectively. At temperatures above 400°C, Ge segregation out of the reacted layers associated with strain relaxation of the unreacted Si_0.85Ge_0.15 films appeared. At 780°C, agglomeration occurred in the Hf(Si_1−xGe_x)₂ films. All the as-deposited and annealed Hf/p-Si_0.85Ge_0.15 samples showed the formation of an ohmic contact. The lowest specific contact resistance around 10⁻⁵ ω cm² could be obtained for the Hf₃ (Si_1−xGe_x)₂ contacts to p-Si_0.85Ge_0.15 formed at 500°C. Below 500°C, the decrease of specific contact resistance with the annealing temperature is mainly caused by the formation of Hf₃(Si_1−xGe_x)₂ and an interfacial Ge-rich layer between the Hf₃(Si_1−xGe_x)₂ and unreacted Si_0.85Ge_0.15 films, while above 600°C, the increase of specific contact resistance may be due to the formation of Hf(Si_1−xGe_x)₂ and SiC as well as the roughness of the Hf(Si_1−xGe_x)₂ films.     

Preparation and optical properties of Co-doped ZnSe single crystals

ZnSe single crystals doped via Co diffusion are investigated. The diffusion was carried out from metal Co in He or Ar atmosphere. The spectra of optical density in the wavelength range 0.4–2 μm are investigated. It is found that the absorption edge shifts as the concentration of doping impurities increases. This shift is caused by the formation of the Zn_{1 ? x} Co_xSe alloy. The diffusion profile of the Co dopant is determined via measurement of the relative optical density of the crystals in the visible spectral region. The Co diffusivities (D) in the ZnSe crystals at T = 1103–1273 K are calculated. The analysis of the temperature dependence D(T) made it possible to determine the coefficients in the Arrhenius equation, namely, D ₀ = 3.4 × 10⁶ cm²/s and E ₀ = 3.8 eV.     

Measurement of space charge generation-recombination current in Hg1−xCdxTe photodiodes by deep level transient spectroscopy

Deep Level Transient Spectroscopy (DLTS) measurements have been used to characterize n⁺ ? pHg_1?xCd_xTe junction photodiode performance. Deep level results obtained on a x = 0.320 liquid phase epitaxial grown photodiode and a x = 0.219 bulk quench anneal-grown photodiode have identified deep Shockley-Read recombination centers. Detailed characterization of trap energy, trap density, and capture cross sections for these traps located within the diode depletion region have been used to predict a space charge generation-recombination current and dynamic resistance-area product at zero bias voltage. This paper presents for the first time a direct correlation of DLTS parameters with photodiode device performance.     

Effects of high doping on the bandgap bowing for AlxGa1−xN

This paper gives the composition dependence of the bandgap energy for highly doped n-type Al_xGa_1−xN. We report results of the bowing parameter obtained using a random simulation. Three groups of Al_xGa_1−xN semiconductors were considered and which are distinguishable by their non degenerate or degenerate character in the doping density (10¹⁷?N_D?10²⁰ cm⁻³). A striking feature is the large discrepancy of the bandgap bowing (−2.02?b?2.94 eV), as was demonstrated from our calculations. This suggests that high doping may be a possible cause able to induce the large range of bowing parameters reported for Al_xGa_1−xN alloys.     

Layered Growth of Lattice-Mismatched Ga x In1−x P on GaP Substrates by Liquid Phase Epitaxy

We report layered growth of Ga _x In_1?x P on GaP substrates using single-step liquid phase epitaxy (LPE) with a Sn-based melt when the lattice mismatch is greater than 0.4 % (x < 0.95). Compositional control was observed by (1) varying the cooling rate and (2) changing the melt-back temperature at the beginning of the growth. Possible growth mechanisms are proposed to explain the principles of both approaches of compositional control. Smooth epilayers have been observed. High resolution x-ray diffraction was used to characterize the composition of the epilayers, and room temperature photoluminescence was reported for one of the samples with the composition of x = 0.11. Plan-view TEM measurements revealed threading dislocation densities on the order of 10⁸ cm^?2 in the upper regions of the Ga _x In_1?x P epilayers. In contrast, when using In-based melts, LPE of Ga _x In_1?x P on GaP (100) substrates exhibited island growth at large misfits, whereas edge growth dominated when using GaP (111B) substrates under equivalent growth conditions.     

Ga1-xinxas - inp abrupt heterostructures grown by MOVPE at atmospheric pressure

High quality InP and Ga_1-x In_xAs layers have been grown on InP substrates using MOVPE growth at atmospheric pressure. Excellent material quality has been obtained using triethylindium and trimethylgallium sources(n = 1.7 10¹⁴ cm^-3, μ = 106 000 cm²V^-1s^-1 at 77 K for InP andn = 1 ? 3 10¹⁵ cm^-3, μ= 75 000 cm²V^-1 s^-1 at 77 K for Ga_1-xIn_xAs). The InP/Ga_1-xIn_xAs interface width obtained is very small (10 Å). The first PIN diodes grown by the process exhibit excellent characteristics.     

Rectification in AlxGa1-xAs-GaAs N-n heterojunction devices

The previously unsolved problem of rectification at Al_xGa_1-xAs-GaAs N-n heterojunction is found to originate from a vague concept regarding the maximum junction grading width which can sustain rectification. The theoretical current density vs voltage characteristics of this heterojunction system are derived from thermionic emission theory. It is found that, unless the impurity concentration of the AlGaAs layer (prepared by LPE techniques) is less than 10¹⁶ cm^?3, typical 90–200 Å metallurgical grading widths at the N-n heterojunction interface produce either ohmic or poorly rectifying characteristics. These results explain (1) the lack of rectification in most N-n Al_xGa_1-xAs-GaAs heterojunctions reported in the literature and (2) the recent observation of significant rectification in high purity (N)Al_0.3Ga_0.7As-(n)GaAs heterojunctions reported by Chandra and Eastman.     

Highly (1 0 0)-oriented Pb(Zr0.20Ti0.80)O3/(Pb1−xLax)Ti1−x/4O3 multilayered thin films prepared by RF magnetron sputtering

The Pb(Zr_0.20Ti_0.80)O₃/(Pb_1−xLa_x)Ti_1−x/4O₃ (x = 0, 0.10, 0.15, 0.20) (PZT/PLT_x) multilayered thin films were in situ deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering technique with a PbO_x buffer layer. With this method, all PZT/PLT_x multilayered thin films possess highly (1 0 0) orientation. The PbO_x buffer layer leads to the (1 0 0) orientation of the multilayered thin films. The effect of the La content in PLT_x layers on the dielectric and ferroelectric properties of the PZT multilayered thin films was systematically investigated. The enhanced dielectric and ferroelectric properties are observed in the PZT/PLT_x (x = 0.15) multilayered thin films. The dielectric constant reaches maximum value of 365 at 1 KHz for x = 0.15 with a low loss tangent of 0.0301. Along with enhanced dielectric properties, the multilayered thin films also exhibit large remnant polarization value of 2P_r = 76.5 μC/cm², and low coercive field of 2E_c = 238 KV/cm.     

Electron states at electrolyte/n-GaN and electrolyte/n-InGaN interfaces

The differential capacitance and differential active conductance of rectifying contacts of n-GaN and n-In _x Ga_{1 ? x} N (x ?? 0.15) with an electrolyte (0.2 M aqueous solutions of NaOH, NaCl, or HCl) have been studied. It was found that electron states with energies corresponding to the upper half of the energy gap of a semiconductor exist at the interface between these semiconductors and a NaOH solution. The density and characteristic recharging time of states noticeably contributing to the differential capacitance and differential active conductance at probe-voltage frequencies of 0.3?C1 kHz grow with their binding energy and, for states lying at 0.15?C0.3 eV below the conduction-band bottom of n-GaN, fall within the ranges 10¹²?C2 × 10¹³ cm^?2 eV^?1 and 10^?4?C10^?2 s, respectively. For contacts with NaCl and HCl solutions, there are no states of this kind. It is assumed that the observed states are related to the adsorption of hydroxyl groups.     

Properties of Zn1 − x Co x O films produced by pulsed laser deposition with fast particle separation

The study is concerned with the structural, optical, magnetic, and transport properties of Zn_{1 ? x} Co _x O (x = 0.05–0.45) films produced on Al₂O₃ (0001) substrates at a temperature of T _s = 500°C by pulsed laser deposition with fast particle separation. The film thickness is d = 60–300 nm. It is found that the Zn_{1 ? x} Co _x O ternary alloy retains its wurtzite-type crystal structure up to x = 0.35, if the films are produced at low buffer-oxygen pressures (～10^?6 Torr). It is established that, in these conditions, the electron concentration is higher than 10²⁰ cm^?3 because of the high density of oxygen donor vacancies. In this case, the films start to exhibit ferromagnetism in the magnetization and the anomalous Hall effect at temperatures above 100 K. The sign of the anomalous Hall effect is found to be positive and opposite to the sign of the normal Hall effect, as occurs in Co metal layers. This is indicative of the cluster nature of ferromagnetism of the Zn_{1 ? x} Co _x O films. For thin Zn_{1 ? x} Co _x O layers (d = 60 nm, x = 0.2) in a transverse magnetic field, profound hysteresis of the magnetoresistance is observed, which is indicative of the out-of-plain easy axis of magnetization of the films. The magnetic anisotropy is attributed to the structuring of the layers (elongation of magnetic clusters along the growth axis of the films). The structuring can lead to noticeable strengthening of the layer ferromagnetism.     

Effects of Ta2O5/Y2O3 Codoping on the Microstructure and Microwave Dielectric Properties of Ba(Co0.56Zn0.40)1/3Nb2/3O3 Ceramics

The effects of Ta₂O₅/Y₂O₃ codoping on the microstructure and microwave dielectric properties of Ba(Co_0.56Zn_0.40)_1/3Nb_2/3O₃-xA-xB (A = 0.045 wt.% Ta₂O₅; B = 0.113 wt.% Y₂O₃) ceramics (x = 0, 1, 2, 4, 8, 16, 32) prepared according to the conventional solid-state reaction technique were investigated. The x-ray diffraction (XRD) results showed that the main crystal phase in the sintered ceramics was BaZn_0.33Nb_0.67O₃-Ba₃CoNb₂O₉. The additional surface phase of Ba₈CoNb₆O₂₄ and trace amounts of Ba₅Nb₄O₁₅ second phase were present when Ta₂O₅/Y₂O₃ was added to the ceramics. The 1:2 B-site cation ordering was affected by the substitution of Ta⁵⁺ and Y³⁺ in the crystal lattice, especially for x = 4. Scanning electron microscopy (SEM) images of the optimally doped ceramics sintered at 1340°C for 20 h showed a compact microstructure with crystal grains in dense contact. Though the dielectric constant increased with the x value, appropriate addition would result in a tremendous modification of the Q × f and τ _f values. Excellent microwave dielectric properties (ε _r = 35.4, Q × f = 62,993 GHz, and τ _f  = 2.6 ppm/°C) were obtained for the ceramic with x = 0.4 sintered in air at 1340°C for 20 h.     

High-temperature ferromagnetism of Si1 ? x Mn x films fabricated by laser deposition using the droplet velocity separation technique

The transport and magnetic properties of Si_{1 ? x} Mn _x films of thickness 55?C70 nm with various Mn content (x = 0.44?C0.6) are studied in the temperature range of 5?C400 K and in magnetic fields up to 2 T. The films are grown by pulsed laser deposition on Al₂O₃ (0001) substrates at a temperature of 340°C using velocity separation of deposited particles. The films exhibit metal conductivity and the resistivity ?? = (2?8) × 10^?4 ?? cm, typical of highly degenerate semiconductors. It is found that the anomalous component of the Hall effect dominates over the normal component at T = 300 K for the Si_{1 ? x} Mn _x alloy with x ?? 0.5, and that the Curie temperature significantly exceeds room temperature and is estimated as ??500 K from magnetization measurements (for MnSi silicide the Curie temperature is T _C = 30 K). It is shown that the anomalous component of the Hall conductivity at low temperatures is controlled by ??side-jump?? and (or) ??intrinsic?? mechanisms independent on the carrier scattering time. The results are explained by features of the formation of defects with localized magnetic moments in the case of Si_{1 ? x} Mn _x films with x ?? 0.5 and by the significant role of matrix spin fluctuations in the exchange between these defects.     

Effect of Se Substitution on Structural and Electrical Transport Properties of Bi0.4Sb1.6Se3x Te3(1−x) Hexagonal Rods

In the current study, novel hexagonal rods based on Bi_0.4Sb_1.6Te₃ ingots dispersed with x amount of Se (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_0.4Sb_1.6Se_3x Te_3(1?x) were synthesized via a standard solid-state microwave route. The morphologies of these rods were explored using field-emission scanning electron microscopy (FESEM). The crystal structure of the powders was examined by x-ray diffraction (XRD) analysis, which showed that powders of the 0.0 ≤ x ≤ 0.8 samples could be indexed to the rhombohedral phase, whereas the sample with x = 1.0 had an orthorhombic phase structure. The influence of variations in the Se content on the thermoelectric properties was studied in the temperature range from 300 K to 523 K. Alloying of Se into Bi_0.4Sb_1.6Te₃ effectively caused a decrease in the hole concentration and, thus, a decrease in the electrical conductivity and an increase in the Seebeck coefficient. The maximal power factor measured in the present work was 7.47 mW/mK² at 373 K for the x = 0.8 sample.