Thermoelectric Properties for a Suspended Microribbon of Quasi-One-Dimensional TiS<Subscript>3</Subscript>

Transition-metal trichalcogenides MX₃ (M = Ti, Zr, Nb, Ta; X = S, Se) are well-known inorganic quasi-one-dimensional conductors. Among them, we have investigated the thermoelectric properties of titanium trisulfide TiS₃ microribbon. The electrical resistivity ρ, thermal conductivity κ, and thermoelectric power S were measured using 3ω method. The weight mean values were found to be ρ = 5 mω m and κ = 10 W K^?1 m^?1 along the one-dimensional direction (b-axis) of the TiS₃ microribbon. Combined with the thermoelectric power S = ?530 μV K^?1, the figure of merit was calculated as ZT = 0.0023. This efficiency is the same as that of randomly oriented bulk TiS₃. We also estimated the anisotropy of σ and κ using the present results and those for randomly oriented bulk material. The obtained weak anisotropy for TiS₃ is attributable to strong coupling between triangular columns consisting of TiS₃ units. These experimental results are consistent with theoretical results obtained using density functional theory (DFT) calculations.     

Permittivity spectra and characteristic energy losses of electrons in ZnO at 100 K

For the first time, full sets of fundamental optical functions have been obtained for zinc oxide in the range 0–30 eV at 100 K for E ⊥ c and E ‖ c polarizations. Spectra of the transverse and longitudinal components of transitions and their basic parameters (peak energies E _i , half-widths H _i of transition bands, band areas S _i , and oscillator strengths f _i ) have also been determined for the first time. The calculations are performed using synchrotron experimental reflectance spectra. The main features of spectra of the optical functions and components of transitions are established. These features are compared to the results of known theoretical calculations of the bands and spectra of optical functions.     

Reflection spectra of two polymorphic modifications of cadmium arsenide

The optical properties of cadmium arsenide are studied. Reliable new data on the reflection spectra are obtained owing to the use of perfect crystals and high-precision spectral equipment. The reflection spectra of the polymorphic modifications α″-Cd₃As₂ (space group D _4h ¹⁵ ) and α′-Cd₃As₂ (space group ) are D _4h ¹¹ are recorded at room and liquid-nitrogen temperatures in polarized light (E ⊥ c, E ‖ c) at incident-light photon energies of 1–5 eV. For the α′ modification, anisotropy is observed in the reflection spectra for the first time. The obtained results are compared with the known experimental and theoretical data.     

Carrier Lifetimes in Lightly-Doped <Emphasis Type="Italic">p</Emphasis>-Type 4H-SiC Epitaxial Layers Enhanced by Post-growth Processes and Surface Passivation

We investigated limiting factors of carrier lifetimes and their enhancement by post-growth processes in lightly-doped p-type 4H-SiC epitaxial layers (N _A ～ 2 × 10¹⁴ cm^?3). We focused on bulk recombination, surface recombination, and interface recombination at the epilayer/substrate, respectively. The carrier lifetime of 2.8 μs in an as-grown epilayer was improved to 10 μs by the combination of V_C-elimination processes and hydrogen annealing. By employing surface passivation with deposited SiO₂ followed by POCl₃ annealing, a long carrier lifetime of 16 μs was obtained in an oxidized epilayer. By investigating carrier lifetimes in a self-standing p-type epilayer, it was revealed that the interface recombination at the epilayer/substrate was smaller than the surface recombination on a bare surface. We found that the V_C-elimination process, hydrogen annealing, and surface passivation are all important for improving carrier lifetimes in lightly-doped p-type epilayers.     

Progress of MCT Detector Technology at AIM Towards Smaller Pitch and Lower Dark Current

We present our latest results on cooled p-on-n planar mercury cadmium telluride (MCT) photodiode technology. Along with a reduction in dark current for raising the operating temperature (T _op), AIM INFRAROT-MODULE GmbH (AIM) has devoted its development efforts to shrinking the pixel size. Both are essential requirements to meet the market demands for reduced size, weight and power and high-operating temperature applications. Detectors based on the p-on-n technology developed at AIM now span the spectrum from the mid-wavelength infrared (MWIR) to the very long wavelength infrared (VLWIR) with cut-off wavelengths from 5 μm to about 13.5 μm at 80 K. The development of the p-on-n technology for VLWIR as well as for MWIR is mainly implemented in a planar photodetector design with a 20-μm pixel pitch. For the VLWIR, dark currents significantly reduced as compared to ‘Tennant’s Rule 07’ are demonstrated for operating temperatures between 30 K and 100 K. This allows for the same dark current performance at a 20 K higher operating temperature than with previous AIM technology. For MWIR detectors with a 20-μm pitch, noise equivalent temperature differences of less than 30 mK are obtained up to 170 K. This technology has been transferred to our small pixel pitch high resolution (XGA) MWIR detector with 1024 × 768 pixels at a 10-μm pitch. Excellent performance at an operating temperature of 160 K is demonstrated.     

Improvement of RTS Noise in HgCdTe MWIR Detectors

Random telegraph signal (RTS) noise is present in all bands of the infrared spectrum from λ _c = 2.5 μm (short-wavelength infrared) to λ _c = 15.75 μm (very long-wavelength infrared) and decreases the performance of infrared photodetectors. The main features of RTS noise such as the jump amplitude and RTS frequency are defined, and their dependence as a function of focal-plane array (FPA) temperature was measured for all bands of the infrared spectrum. Both of these features comply with a Boltzmann activation law \( \left( { \propto {\hbox{e}}^{{\frac{{ - E_{\rm{a}} }}{k_{\rm B} T}}} } \right) \), and their activation energies scale with the bandgap. Comparison of three different HgCdTe mid-wavelength infrared photodetector technologies was also performed, showing that the optimized n-on-p improvement of operability (AOP) and p-on-n high-operating-temperature technologies show a reduced number of pixels exhibiting RTS noise (by about two decades) in comparison with standard n-on-p technology.     

Characterization of HgCdTe Films Grown on Large-Area CdZnTe Substrates by Molecular Beam Epitaxy

Recent advances in growth of Hg_1?x Cd _x Te films on large-area (7 cm × 7.5 cm) CdZnTe (CZT) substrates is presented. Growth of Hg_1?x Cd _x Te with good uniformity on large-area wafers is achieved using a Riber 412 molecular beam epitaxy (MBE) tool designed for growth of Hg_1?x Cd _x Te compounds. The reactor is equipped with conventional CdTe, Te, and Hg sources for achieving uniform exposure of the wafer during growth. The composition of the Hg_1?x Cd _x Te compound is controlled in situ by employing a closed-loop spectral ellipsometry technique to achieve a cutoff wavelength (λ _co) of 14 μm at 78 K. We present data on the thickness and composition uniformity of films grown for large-format focal-plane array applications. The composition and thickness nonuniformity are determined to be <1% over the area of a 7 cm × 7.5 cm wafer. The films are further characterized by Fourier-transform infrared spectroscopy, optical microscopy, and Hall measurements. Additionally, defect maps show the spatial distribution of defects generated during the epitaxial growth of the Hg_1?x Cd _x Te films. Microdefect densities are in the low 10³ cm^?2 range, and void defects are below 500 cm^?2. Dislocation densities less than 5 × 10⁵ cm^?2 are routinely achieved for Hg_1?x Cd _x Te films grown on CZT substrates. HgCdTe 4k × 4k focal-plane arrays with 15 μm pitch for astronomical wide-area infrared imagers have been produced using the recently developed MBE growth process at Teledyne Imaging Sensors.     

Leakage currents over the Surface of CdHgTe-based photodiodes

The leakage current I _p over the surface of Cd_xHg_1−x Te-based photodiodes that have a cutoff wavelength of the photosensitivity spectrum of λ=9.8–11.6 μm and are fabricated by implanting Zn⁺⁺ ions into the p-type solid solution is investigated. The surface character of the I _p current is indicated by a coordinate shift of the peak in the sensitivity profile of n ⁺-p junctions, which is measured in a scanning mode by the beam of a CO₂ laser with a wavelength of 10.6 μm, with an increase in voltage U across the photodiode and the shift of spectral characteristics to shorter wavelengths with increasing U. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 7, 2004, pp. 890–895. Original Russian Text Copyright ? 2004 by Biryulin, Turinov, Yakimov.     

High Rate Growth of MOCVD-Derived GdYBCO Films Based on a Simple Self-Heating Method

The home-designed metal organic chemical vapor deposition (MOCVD) system was applied to prepare the Gd_0.5Y_0.5Ba₂Cu₃O_7-δ (GdYBCO) films at a high deposition rate in order to improve the production efficiency and reduce the preparation cost of high temperature superconducting tapes. Based on a simple self-heating method, the distance between the shower head and the substrate surface can be reduced effectively to increase the concentration of metal organic sources on the substrate surface, which can commendably improve the deposition rate of GdYBCO films and the utilization ratio of metal organic sources. At last, the GdYBCO films were successfully prepared on the LaMnO₃ template at the high deposition rate of 1 μm/min by the MOCVD process based on the simple self-heating method and the critical current (I_c) was more than 220 A/cm-width (77 K, 0 T), corresponding to the critical current density (J_c) more than 4.4 MA/cm² (77 K, 0 T).     

Improvement in Thermoelectric Performance of SnS Due to Electronic Structure Modification Under Biaxial Strain

Ab-initio calculations using the full potential linearized augmented plane-wave technique and the semi-classical Boltzmann theory are used to study thermoelectric properties of unstrained SnS and at 1%, 2% and 3% applied biaxial tensile (BT) strain. The studies are carried out at 800 K for p-type and n-type carriers. For an increase in BT strain, lattice constants of SnS change causing changes in the band structure and increase in the band gap which in turn modifies thermoelectric coefficients. In the case of unstrained SnS, the maximum thermopower (S) obtained is 426 μV/K at carrier concentration 5.40?×?10¹⁸ cm^?3 for p-type carriers and 435 μV/K at carrier concentration 1.68?×?10¹⁸ cm^?3 for n-type carriers. At 3% applied BT strain, S is increased to 696 μV/K at carrier concentration 4.61?×?10¹⁷ cm^?3 for p-type carriers and 624 μV/K at carrier concentration 3.21?×?10¹⁷ cm^?3 for n-type carriers. The power factor (PF) increases?～?2 times at 3% BT strain as compared to unstrained SnS, and it is 6.20 mW K^?2 m^?1 for p-type carriers. For n-type carriers, PF at 3% applied BT is slightly less than the PF for unstrained SnS, which is 6.81 mW K^?2 m^?1. For both types of carriers, the figure of merit (ZT) is found to be?～?1.5 for unstrained SnS. For p-type carriers ZT is enhanced 1.4 times at 3% applied BT strain as compared to that of unstrained SnS. However, for n-type carriers, ZT does not change drastically with increase in BT strain.     

Structural and Electrical Properties of Ag/<Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO₂/p-Si/Al heterostructure. Thin films of pure TiO₂ were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO₂ film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO₂, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO₂/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 10⁵ Am^?2 K^?2, which is of the order of the theoretical value 3.2 × 10⁵ Am^?2 K^?2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO₂ film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO₂ films was found to be 3.60 eV.     

MOCVD growth and Mg-doping of InAs layers

Epitaxial layers of Mg-doped InAs were grown by MOCVD, and electrical properties of these layers were studied. The doping with magnesium in the course of MOCVD growth allows one to obtain strongly compensated p-InAs with a high hole density (p≈2×10¹⁸ cm^?3) and a low carrier mobility (μ≈50 cm²/(V s)) at T=300 K. When the samples are lightly doped with Mg, neutral impurities are bound with Mg, and n-type InAs layers with a carrier mobility exceeding that in undoped samples are formed.     

Functionalized Multiwalled Carbon Nanotube Electrochemical Sensor for Determination of Anticancer Drug Flutamide

An electrochemical sensor based on functionalized multiwalled carbon nanotubes (MWCNT_f) has been developed and applied for determination of anticancer drug flutamide in pharmaceutical and artificial urine samples. The electrode was prepared by modifying a glassy carbon electrode with MWCNT_f, denoted herein as MWCNT_f/GCE. The MWCNT_f/GCE electrode exhibited high catalytic activity, high sensitivity, and high stability and was applicable over a wide concentration range for flutamide. The effects of the scan rate, pH, and nature of the electrolyte on the electrochemical behavior of flutamide on the MWCNT_f/GCE were investigated. The results showed that this electrode presented the best square-wave voltammetric response to flutamide in Britton–Robinson buffer solution at pH 5.0 at frequency of 50 Hz and amplitude of 0.06 V. The proposed sensor presents a wide linear response range from concentration of 0.1 μmol L^?1 up to 1000 μmol L^?1 (or 27.6 μg L^?1 up to 0.27 g L^?1), with limit of detection, limit of quantification, and sensitivity of 0.03 μmol L^?1, 0.1 μmol L^?1, and 0.30 μA μmol^?1 L, respectively. The MWCNT_f/GCE electrode was successfully applied for determination of flutamide in pharmaceutical formulations and artificial urine samples, giving results in agreement with those obtained by a comparative method described in literature. A paired Student’s t-test revealed no statistical difference between the reference and proposed method at 95% confidence level. The average recovery for fortified samples was 101 ± 1%.     

Internal quantum efficiency of stimulated emission of (λ=1.55 µm) InGaAsP/InP laser diodes

The stimulated emission (η _i ^st ) of InGaAsP/InP separate-confinement double heterostructure lasers operating at λ=1.5–1.6 μm has been studied experimentally and theoretically. Laser heterostructures with a varied design of the waveguide layer were grown by MOCVD. The maximum internal quantum efficiency η _i ^st ≈97% was obtained in a structure with a double-step waveguide characterized by minimum leakage into the p-emitter above the generation threshold. The high value of η _i ^st is provided by low threshold and nonequilibrium carrier concentrations at the interface between the waveguide and p-emitter. The calculation yields η _i ^st values correlating well with the experimental data. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 243–248. Original Russian Text Copyright ? 2003 by Skrynnikov, Zegrya, Pikhtin, Slipchenko, Shamakhov, Tarasov.     

Fabrication of a <Emphasis Type="Italic">p</Emphasis>–<Emphasis Type="Italic">n</Emphasis> Heterojunction Using Topological Insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>–Si and Its Annealing Response

A junction device has been fabricated by growing p-type Bi₂Te₃ topological insulator (TI) film on an n-type silicon (Si) substrate using a thermal evaporation technique. Annealing using different temperatures and durations was employed to improve the quality of the film, as confirmed by microstructural study using x-ray diffraction (XRD) analysis and atomic force microscopy (AFM). The p–n diode characteristics of the junction devices were studied, and the effect of annealing investigated. An improved diode characteristic with good rectification ratio (RR) was observed for devices annealed for longer duration. Reduction in the leakage or reverse saturation current (\( I_{\rm{R}} \)) was observed with increase in the annealing temperature. The forward-bias current (\( I_{\rm{F}} \)) dropped in devices annealed above 400°C. The best results were observed for the sample device annealed at 450°C for 3 h, showing figure of merit (FOM) of 0.621 with RR ≈ 504 and \( I_{\rm{R}} \) = 0.25 μA. In terms of ideality factor, the sample device annealed at 550°C for 2 h was found to be the best with \( n \) = 6.5, RR ≈ 52.4, \( I_{\rm{R}} \) = 0.61 μA, and FOM = 0.358. The majority-carrier density \( \left( {N_{\rm{A}} } \right) \) in the p-Bi₂Te₃ film of the heterojunction was found to be on the order of 10⁹/cm³ to 10¹¹/cm³, quite close to its intrinsic carrier concentration. These results are significant for fundamental understanding of device applications of TI materials as well as future applications in solar cells.     

<Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-Si-<Emphasis Type="Italic">n</Emphasis>-CdF<Subscript>2</Subscript> heterojunctions

Boron diffusion and the vapor-phase deposition of silicon layers are used to prepare ultrashallow p⁺-n junctions and p⁺-Si-n-CdF₂ heterostructures on an n-CdF₂ crystal surface. Forward portions of the I–V characteristics of the p⁺-n junctions and p⁺-Si-n-CdF₂ heterojunctions reveal the CdF₂ band gap (7.8 eV), as well as allow the identification of the valence-band structure of cadmium fluoride crystals. Under conditions in which forward bias is applied to the p⁺-Si-n-CdF₂ heterojunctions, electroluminescence spectra are measured for the first time in the visible spectral region.     

Investigation of the ZnS-CdHgTe interface

The ZnS-Cd_xHg_1?xTe interface was investigated using the capacitance-voltage characteristics of MIS structures in experimental samples. During fabrication of the n⁺-p junctions based on p-Cd_xHg_1?xTe, the density of states within the range N _ss =(1–6)×10¹¹ cm^?2 eV^?1 at T=78 K was obtained. The experiments showed that the conditions in which n⁺-p junctions are fabricated only slightly affect the state of the ZnS-CdHgTe interface. The negative voltages of the at bands V _FB , even if immediately after deposition of the ZnS films V _FB >0, point to the enrichment of the ZnS-p-CdHgTe near-surface layer with majority carriers, specifically, holes. This led to a decrease in the leakage current over the surface. During long-term storage (as long as ～15 years) in air at room temperature, no degradation of differential resistance R _d , current sensitivity S _i , and detectivity D* of such n⁺-p junctions with a ZnS protection film was observed.     

Fine Structure of the long-wavelength edge of exciton-phonon absorption and hyperbolic excitons in silicon carbide of 6<Emphasis Type="Italic">H</Emphasis> polytype

The fine structure of the long-wavelength edge of the polarization spectra of exciton-phonon absorption in moderate-purity n-type 6H-SiC crystals with a concentration of uncompensated donors N_D?N_A=(1.7–2.0)×10¹⁶ cm^?3 at T=1.7 K was studied. The analysis of new special features found at the absorption edge and the reliable detection of the onset of exciton-phonon steps related to the emission of phonons from acoustical and optical branches allowed highly accurate determination of a number of important parameters such as the band gap, the exciton band gap, the exciton binding energy, and the energies of spin-orbit and crystal-field splitting of an exciton. For the first time, transitions with the emission of LA phonons to the 1S exciton state with an M₁-type dispersion law were detected in E ∥ Z(C) polarization (the electric-field vector is parallel to the optical axis of the crystal). This observation supports the previously predicted “two-well” structure of the conduction band minimum in 6H-SiC.     

Valence-band offsets in strained SiGeSn/Si layers with different tin contents

Admittance spectroscopy is used to study hole states in Si_0.7–y Ge_0.3Sn _y /Si quantum wells in the tin content range y = 0.04–0.1. It is found that the hole binding energy increases with tin content. The hole size-quantization energies in structures containing a pseudomorphic Si_0.7–y Ge_0.3Sn _y layer in the Si matrix are determined using the 6-band kp method. The valence-band offset at the Si_0.7–y Ge_0.3Sn _y heterointerface is determined by combining the numerical calculation results and experimental data. It is found that the dependence of the experimental values of the valence-band offsets between pseudomorphic Si_0.7–y Ge_0.3Sn _y layers and Si on the tin content is described by the expression ΔE _V ^exp = (0.21 ± 0.01) + (3.35 ± 7.8 × 10^–4)y eV.     

Simulation of Electronic Center Formation by Irradiation in Silicon Crystals

We present the results of a study on localized electronic centers formed in crystals by external influences (impurity introduction and irradiation). The main aim is to determine the nature of these centers in the forbidden gap of the energy states of the crystal lattice. For the case of semiconductors, silicon (Si) was applied as model material to determine the energy levels and concentration of radiation defects for application to both doped and other materials. This method relies on solving the appropriate equation describing the variation of the charge carrier concentration as a function of temperature n(T) for silicon crystals with two different energy levels and for a large set of N ₁, N ₂ (concentrations of electronic centers at each level), and n values. A total of almost 500 such combinations were found. For silicon, energy level values of ε ₁ = 0.22 eV and ε ₂ = 0.34 eV were used for the forbidden gap (with corresponding slopes determined from experimental temperature-dependent Hall-effect measurements) and compared with photoconductivity spectra. Additionally, it was shown that, for particular correlations among N ₁, N ₂, and n, curve slopes of ε ₁/2 = 0.11 eV, ε ₂/2 = 0.17 eV, and α = 1/2(ε ₁ + ε ₂) = 0.28 eV also apply. Comparison between experimental results for irradiation of silicon crystals by 3.5-MeV energy electrons and Co⁶⁰ γ-quanta revealed that the n(T) curve slopes do not always coincide with the actual energy levels (electronic centers).