Electrical and Optical Properties of (n)ZnO/(p)CdTe Heterojunction and Its Performance as a Photovoltaic Converter

Thin film heterojuctions of the type (n)ZnO/(p)CdTe with different doping concentration were prepared by vacuum evaporation, and their electrical and optical properties, both in dark and under illumination at room temperature as well as elevated temperatures, were studied. Different junction parameters such as ideality factors, barrier heights, Richardson constant, short-circuit current, etc. were determined from I–V characteristics. These parameters were found to change significantly on hydrogenation and annealing of the junctions and also on variation of temperature. The structures showed the change of the photovoltaic (PV) effect, giving a fill factor of 0.57 for hydrogen (H)-treated with an open-circuit voltage of 345 mV and a short-circuit current density of 75.72× 10⁻⁴ mA·cm⁻² and 0.42 for untreated with an open-circuit voltage of 244 mV and a short-circuit current density of 52.00× 10⁻⁴ mA·cm⁻² for doping concentrations of N _a=2.43× 10¹⁶/cm³(2.53% Sb doped CdTe) and N _d=3.88× 10¹⁵/cm³(2.74% Al doped ZnO). The junctions were found to possess a series resistance as high as 437 Ω for an untreated sample and 95 Ω for H-treated samples even under illumination. Proper doping, annealing, and hydrogenation are necessary to reduce the series resistance so as to achieve an ideal solar cell.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Scanning Kerr Microscopy of the Spin Hall Effect in <Emphasis Type="Italic">n</Emphasis>-Doped GaAs with Various Doping Concentration

We investigated the doping concentration (N _D) dependence of the extrinsic spin Hall effect (SHE) in n-doped GaAs with N _D raging from 3×10¹⁶ cm⁻³ to 5×10¹⁷ cm⁻³. By using scanning Kerr microscopy (SKM) measurements, we observed the Kerr rotation signal due to the spin accumulation near the channel edges in all the samples with different N _D. Moreover, the position and in-plane magnetic field dependence of the Kerr rotation signal are found to vary with N _D. We analyzed the N _D dependence of the spin Hall conductivity by taking account of the N _D-dependent spin lifetime based on the typical drift-diffusion model.     

An approach to enhanced acceptor concentration in ZnO:N films

Owing to the low doping concentration of nitrogen and strong compensation of intrinsic donors, the attainment of highly conductive p-type ZnO films remains one of the largest challenges for the application of ZnO. An approach has been proposed to increase the doping concentration of nitrogen in ZnO by exposing the ZnO:N films in the ambient of nitrogen plasma periodically in this paper. Hall measurements and photoluminescence spectroscopy indicate that this approach is effective in improving the hole concentration in ZnO films. Under the optimized conditions, a p-type ZnO film with a hole concentration of 1.68 × 10¹⁸ cm⁻³ has been achieved.     

Electrical characterization of low temperature deposited oxide films on ZnO/<Emphasis Type="Italic">n</Emphasis>-Si substrate

Thin films of silicon dioxide are deposited on ZnO/n-Si substrate at a low temperature using tetra-ethylorthosilicate (TEOS). The ZnO/n-Si films have been characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The border trap density (Q_bt) and fixed oxide charge density (Q_f/q) of the SiO₂/ZnO/n-Si films are found to be 3.9 × 10¹⁰ cm⁻² and 1.048 × 10¹¹ cm⁻², respectively. The trapping characteristics and stress induced leakage current (SILC) have also been studied under Fowler-Nordheim (F-N) constant current stressing.     

Optical absorption and photoconductivity in gamma-irradiated zinc selenide crystals

The possibility of radiation-induced enhancement of photoconductivity (σ_ph) in crystalline zinc selenide (ZnSe) has been studied. The electron concentration in the initial ZnSe:O crystal was N _Eg = 1.26 × 10¹⁶ cm⁻³ as determined from the optical density of the sample at E _g = 2.58 eV. The irradiation of the ZnSe:O crystal by gamma-photons from ⁶⁰Co source to a dose of 10⁶ Gy leads to the creation of five new energy levels (Γ_6v, 5.76 eV; L _1.3v, 4.85 eV; Zn_i, 3.34 eV; O_Se, 3.13 eV; X, 2.72 eV), a decrease in the electron concentration to N _Eg = 0.63 × 10¹⁶ cm⁻³, and a twofold increase in σ_ph. The doping with Te to 0.2 wt % also creates the L _1.3v level and increases N _Eg to 2.02 × 10¹⁶ cm⁻³ and σ_ph to 2.01 × 10⁻¹⁰ Ω⁻¹. The irradiation of Te-doped crystals leads to the creation of an additional resonant level (Zn_i, 3.34 eV) and a two-fold increase in σ_ph. An increase in the content of Te to 0.5 wt % results in a shift of the Zn_i level to 3.39 eV and a growth in the conductivity to σ_ph = 7.64 × 10⁻¹⁰ Ω⁻¹. However, the gamma-irradiation of these crystals leads to decomposition of this center and to a decrease in the photoconductivity.     

Microstructural effects on the phase transitions and the thermal evolution of elastic and piezoelectric properties in highly dense,submicron-structured NaNbO<Subscript>3</Subscript> ceramics

The dielectric, piezoelectric and elastic coefficients, as well as the electromechanical coupling factors, of NaNbO₃ submicron-structured ceramics have been obtained by an automatic iterative method from impedance measurements at resonance. Poled thin discs were measured from room temperature up to the depoling one, close to 300 °C. Dielectric thermal behaviour was determined also for unpoled ceramics up to the highest phase transition temperature. Ceramics were processed by hot-pressing from mechanically activated precursors. Microstructural effects on the properties are discussed. The suppression of the classical maximum in dielectric permittivity in unpoled ceramics at the phase transition at 370 °C was found when a bimodal distribution of grain sizes, with a population of average grain size of 110 nm in between much coarser grains, is observed. The appearance of a phase transition at 150 °C took place when Na vacancies are minimised. The occurrence of a non-centrosymmetric, ferroelectric phase, in the unpoled ceramic from room temperature to ~300 °C, highly polarisable resulting in high ferro–piezoelectric properties was also observed in the ceramic which presents grain size below 160 nm. Maximum values of k _p = 14%, d ₃₁ = −8.7 × 10⁻¹² C N⁻¹ and N _p = 3772 Hz m at room temperature, and k _p = 18%, d ₃₁ = −25.4 × 10⁻¹² C N⁻¹ and N _p = 3722 Hz m at 295 °C were achieved in the best processing conditions of the ceramics.     

Highly Transparent and Conductive Zn<Subscript>0.86</Subscript>Cd<Subscript>0.11</Subscript>In<Subscript>0.03</Subscript>O Thin Film Prepared by Pulsed Laser Deposition

Zn_0.86Cd_0.11In_0.03O alloy semiconductor film was deposited on quartz substrate by pulsed laser deposition technique. Cd is used to change the optical band gap and In is used to increase the carrier concentration of the ZnO film. XRD studies confirm that the structure of Zn_0.86Cd_0.11In_0.03O is hexagonal wurtzite structure without CdO phase appeared. FE-SEM shows that the grain size of Zn_0.86Cd_0.11In_0.03O film is smaller than that of ZnO. These films are highly transparent (∼85%) in visible region. Most importantly, the electrical properties of Zn_0.86Cd_0.11In_0.03O film highly improved with In doped. It has low resistivity (4.42×10⁻³ Ω cm) and high carrier concentration (5.50×10¹⁹ cm⁻³) that enable this film a promising candidate for window layer in solar cells and other possible optoelectronic applications.     

Effect of substrate temperature on structural, optical and electrical properties of ZnO thin films deposited by pulsed laser deposition

ZnO thin films were grown by the pulse laser deposition (PLD) method using Si (100) substrates at various substrate temperatures. The influence of the substrate temperature on the structural, optical, and electrical properties of the ZnO thin films was investigated. All of the thin films showed c-axis growth perpendicular to the substrate surface. At a substrate temperature of 500 °C, the ZnO thin film showed the highest (002) peak with a full width at half maximum (FWHM) of 0.39°. The X-ray Photoelectron Spectroscopy (XPS) study showed that Zn was in excess irrespective of the substrate temperature and that the thin film had a nearly stoichiometrical composition at a substrate temperature of 500 °C. The photoluminescence (PL) investigation showed that the narrowest UV FWHM of 15.8 nm and the largest ratio of the UV peak to the deep-level peak of 32.9 were observed at 500 °C. Hall effect measurement systems provided information about the carrier concentration, mobility and resistivity. At a substrate temperature of 500 °C, the Hall mobility was the value of 37.4 cm²/Vs with carrier concentration of 1.36 × 10¹⁸ cm⁻³ and resistivity of 2.08 × 10⁻¹ Ω cm.     

Current–voltage characteristics and impedance spectroscopy of LiNbO<Subscript>3</Subscript> films grown by RF magnetron sputtering

Polycrystalline LiNbO₃ films with random orientation of grains on (001)Si substrates have been grown by RF magnetron sputtering method. Electrical conductance of the formed (001)Si–LiNbO₃ heterostructures is defined through hopping mechanism by charge localization centers (CLC) in the band gap of LiNbO₃ with concentration N _t  = 2.3 × 10²⁴ m⁻³. Analysis of the impedance frequency spectrum has disclosed two relaxation processes of Maxwell–Wagner type with relaxation times τ₁ = 0.1 s and τ₂ = 1 × 10⁻⁴ s. Thermal annealing at T = 650 °C leads to an increase in the average grain size from 50 to 95 nm; it also leads to a decrease in the CLC concentration down to N _t  = 2.8 × 10²⁰ m⁻³. Electrical conductance of (001)Si–LiNbO₃ heterostructures after thermal annealing is determined by space charge limited conduction mechanism. There have been defined parameters of dielectric hysteresis loops. It has been demonstrated that thermal annealing leads to a decrease in values of remanent polarization and coercive field.     

Photoresponse of n -ZnO/ p -Si heterojunction towards ultraviolet/visible lights: thickness dependent behavior

A series of n-ZnO/p-Si thin film heterojunctions have been fabricated by a low cost sol–gel technique for different ZnO film thicknesses and the dark as well as photo current–voltage (I–V) characteristics have been investigated in details. The heterojunction with ZnO thickness of 0.46 μm shows the best diode characteristics in terms of rectification ratio, I _F/I _R = 5.7 × 10³ at 5 V and reverse leakage current density, J _R = 7.6 × 10⁻⁵ A cm⁻² at −5 V. From the photo I–V curves and wavelength dependent photocurrent of the heterojunctions, it is found that the junction with 0.46 μm ZnO thickness shows the highest sensitivity towards both UV and visible lights.     

Synthesis and chemical properties of photoluminescent self-doped polyanilines

Fluorescent self-doped polyanilines (PAS-AntPy-a and PAS-AntPy-b) were obtained by the pyridinium sulfonation of poly(2-methoxyaniline-5-sulfonic acid) (PAS) with 4-(2-anthracene-9-yl-vinyl)pyridine (AntPy). The degrees of pyridinium sulfonation in PAS-AntPy-a and PAS-AntPy-b were 0.70 and 0.97, respectively. A neutral polyaniline with an AntPy side unit (PANI-AntPy) was synthesized by the oxidative polymerization of N-(2-aminophenyl)-4-(2-anthracene-9-yl-vinyl)pyridinium chloride. The UV-vis spectra of PAS-AntPy-a and PAS-AntPy-b exhibited an absorption due to the polaron band that was derived from the protonation of amine groups in the polyaniline backbone with the remaining sulfonic acid proton. In contrast, PANI-AntPy did not exhibit absorption due to the polaron band. PAS-AntPy-a, PAS-AntPy-b, and PANI-AntPy were photoluminescent in a solution. The electric conductivity of PAS-AntPy-a was σ = 1.5 × 10⁻⁷ S cm⁻¹, which was higher than that of H₂SO₄ doped PANI (4.2 × 10⁻⁹ S cm⁻¹) reported previously.     

The effect of growth conditions and N<Subscript>2</Subscript>/O<Subscript>2</Subscript> ambient on LO-phonon replicas during epitaxial growth of ZnO on c-sapphire

High quality heteroepitaxial thin films of ZnO:N were grown by pulsed laser deposition using a two-step growth method and annealed in situ at different temperatures and ambient conditions. Films were analyzed by X-ray diffraction (XRD), electrical measurements, and photoluminescence experiments at low temperatures to investigate the effect of nitrogen doping. The XRD results demonstrate epitaxial growth on the c-sapphire substrates, with average grain size of 57 nm. Photoluminescence spectra reveals a peak at 3.061 eV (405.1 nm) which is part of the longitudinal-optical-phonon replicas of excitons bound to neutral acceptors \textA₁⁰  \textX_\textA {\text{A}}_{1}^{0} \,{\text{X}}_{\text{A}} at 3.348 eV (370.4 nm), attributed in recent investigations to a newly reported donor–acceptor pair. Electrical resistivity and Hall effect measurements were performed using standard four point van der Pauw geometry at room temperature. Fresh films exhibited a resistivity of 3.1 × 10⁻³ Ω cm, a carrier density of 1.3 × 10¹⁹ cm⁻³, and a mobility of 53 cm²/V s. During approximately 2 weeks the as-deposited films presented a p-type behavior, as shown by the positive sign of the Hall constant measured. Thereafter, films reverted to n-type. From electrical measurements and photoluminescence spectra, the acceptor energy was determined to be 150 meV, in close agreement with reported values. These results are consistent with those presented in the literature for high purity crystals or homoepitaxial thin films, even though samples for the present study were processed at lower annealing temperature.     

Thermophysical Property Measurements of Liquid Gadolinium by Containerless Methods

Thermophysical properties of liquid gadolinium were measured using non-contact diagnostic techniques with an electrostatic levitator. Over the 1585 K to 1920 K temperature range, the density can be expressed as ρ(T) = 7.41 × 10³ − 0.46 (T − T _m) (kg · m⁻³) where T _m = 1585 K, yielding a volume expansion coefficient of 6.2 × 10⁻⁵ K⁻¹. In addition, the surface tension data can be fitted as γ(T) = 8.22 × 10² − 0.097(T − T _m)(10⁻³ N · m⁻¹) over the 1613 K to 1803 K span and the viscosity as η(T) = 1.7exp[1.4 × 10⁴/(RT)](10⁻³ Pa · s) over the same temperature range.     

Effect of complexing anions on sorption of U(VI), <Superscript>90</Superscript>Sr,and <Superscript>90</Superscript>Y from aqueous solutions on layered double hydroxides of Mg,Al, and Nd

Sorption of ⁹⁰Sr and ⁹⁰Y from aqueous solutions on Mg-Al and Mg-Nd layered double hydroxides (LDHs) in various forms was studied. The distribution coefficients K _d of U(VI) and ⁹⁰Sr on LDH-Mg-Al-EDTA are 100–120 ml g⁻¹ in 15 min of contact of the solid and liquid phases at V/m = 50 ml g⁻¹. At the same time, under similar conditions, U(VI) and ⁹⁰Sr are not sorbed from aqueous solutions on LDH-Mg-Al-C₂O₄. The sorption of U(VI) from aqueous solutions containing H₂EDTA²⁻, C₂O₄²⁻, and CO₃²⁻ on LDH-Mg-Nd-CO₃ and LDH-Mg-Al-CO₃ strongly depends on the concentration of the complexing anions in the solution. In particular, for 10⁻³ M aqueous UO₂²⁺ solutions, with an increase in [C₂O₄²⁻] from 10⁻³ to 5 × 10⁻² M, K _d of U(VI) decreases from >5 × 10³ to 70 ml g⁻¹ for LDH-Mg-Al-CO₃ and from 170 to ∼0 ml g⁻¹ for LDH-Mg-Nd-CO₃. In the presence of 10⁻³ to 5 × 10⁻² M CO₃²⁻ in aqueous solution, U(VI) is not noticeably sorbed on LDH-Mg-Nd-CO₃ (K _d does not exceed 16 ml g⁻¹ at V/m = 50 ml g⁻¹), and on LDH-Mg-Al-CO₃ the sorption sharply decreases (K _d decreases from >5 × 10³ to ∼0 ml g⁻¹ at V/m = 50 ml g⁻¹). The presence of complexing anions in the solution does not appreciably affect the ⁹⁰Sr sorption, but noticeably affects the 90Y sorption. With an increase in their concentration, K _d of ⁹⁰Y appreciably decreases. The effect exerted by Sr²⁺ ions on the sorption of microamounts of U(VI) and by UO₂²⁺ ions on the sorption of microamounts of ⁹⁰Sr and ⁹⁰Y from aqueous solutions on LDH-Mg-Nd-CO₃ was also examined.     

Effect of TiO2 addition on the microstructure and electrical properties of ZnO-based linear resistance ceramics

ZnO-based linear resistance ceramics doped with TiO₂ were fabricated by the conventional ceramic method. The effect of TiO₂ on the microstructure and electrical properties of the ceramics was investigated in detail. The results show that the electrical properties, such as resistivity, nonlinear coefficient, resistance temperature coefficient and energy density, have been obviously influenced by TiO₂ doping. The optimal sample with the TiO₂ concentration of 7 wt% possesses a resistivity of 695 Ω · cm and an energy density of 780 J/cm³, which are improved by 171.5 and 68.8%, respectively. Spontaneously, the nonlinear coefficient of voltage decreases to 1.2 and the resistance temperature coefficient reaches −2.92 × 10⁻³/°C, decreased by 20 and 39.2%, respectively.     

Effect of Ga Doping Concentration on Electrical and Optical Properties of Nano-ZnO:Ga Transparent Conductive Films

Transparent conductive nano ZnO thin films with different Ga doping concentrations (1, 3, 5, 7 at.%) were prepared on glass substrate by RF magnetron sputtering. The influence of Ga doping concentration on the structural, electrical and optical properties of ZnO:Ga films was investigated by XRD, SEM, Hall measurement and optical-transmission spectroscopy. It shows that the nano ZnO:Ga films are dense and flat, and have polycrystalline structure with preferential (002) and weak (101) orientation. The grain sizes, carrier concentration and Hall mobility changes non-linearly with the increase of Ga-content. The lowest resistivity of 1.44×10⁻³ Ωcm appears at 3 at.% Ga doping concentration. The average transmittance of the films is about 80∼90% in the visible range. The optical band gap obtained for these films is larger than for pure ZnO (∼3.37 eV).     

Silicon metal-dielectric-semiconductor varicaps with an yttrium oxide dielectric

This paper reports an investigation of the electrophysical properties of metal-dielectric-semiconductor varicaps with an yttrium oxide dielectric, prepared by resistive vacuum evaporation of the rare-earth metal with subsequent thermal oxidation of the film in air at 500–550 °C. It is found that the electrical conductivity of the samples follows the Poole-Frenkel law. High-frequency capacitance-voltage characteristics are used to determine the specific capacitance of the dielectric, C ₀=0.027–0.03 μF/cm², the slope of the capacitance-voltage characteristic, dC/dV=35–40 pF/V, the fixed charge in the dielectric, Q _f=(1.7−2.7)×10⁻⁸ C/cm², and the density of surface states at the flat-band potential, N _ss=(1−2)×10¹¹ cm⁻²·eV⁻¹. The capacitance tuning range factor for the metal-dielectric-semiconductor varicaps is 2.5–3. These structures are shown to be applicable as metal-dielectric-semiconductor varicaps with a low control voltage and a high quality factor. Pis’ma Zh. Tekh. Fiz. 23, 50–55 (June 26, 1997)     

Boron doping of Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Si heterostructures grown by silicon sublimation in germane medium

Controlled boron doping of Si_{1 − x} Ge _x epilayers has been achieved during low-temperature growth of SiGe/Si(100) heterostructures by sublimation of boron-doped silicon in a germane medium. Boron-doped single-crystalline silicon plate was sublimed by resistive heating to ∼1300°C. Using this source, heterostructures with selectively doped layers, sharp dopant concentration profiles, and a maximum boron concentration of ∼1 × 10¹⁹ cm⁻³ were obtained.     

Luminescence properties of monodispersed spherical BaWO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> microphosphors for white light-emitting diodes

Monodispersed spheres (1–4 μm in diameter) of BaWO₄:Eu³⁺ (hereafter BWO:Eu) red-phosphor exhibiting intense emission at 615 nm were synthesized via a mild hydrothermal method. X-ray diffraction, scanning electron microscope, photoluminescence excitation and emission spectra, and decay curve were used to characterize the properties of BWO:Eu phosphors. An intense red emission was obtained by exciting either into the ⁵L₆ state with 394 nm or the ⁵D₂ state with 465 nm, that correspond to two popular emission lines from near-UV and blue LED chips, respectively. The values of Ω _2,4 experimental intensity parameters (13.8 × 10⁻²⁰ and 8.2 × 10⁻²⁰ cm²) are determined. The high-emission quantum efficiency of the BWO:Eu phosphor suggests this material could be promising red phosphors for generating white light in phosphor-converted white light-emitting diodes.     

Study of the structural and dielectric properties of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> and PbO addition on BiNbO<Subscript>4</Subscript> ceramic matrix for RF applications

In this paper, the structural and dielectric properties of BNO (BiNbO₄) was investigated as a function of the external RF frequency and temperature. The BNO Ceramics, prepared by the conventional mixed oxide method and doped with 3, 5 and 10 wt. % Bi₂O₃–PbO were sintered at 1,025 °C for 3 h. The X-ray diffraction patterns of the samples sintered, shown the presence of the triclinic phase (β-BNO). In the measurements obtained at room temperature (25 °C) was observed that the largest values of dielectric permittivity (ε′ _r ) at frequency 100 kHz, were for the samples: BNO5Bi (5 wt. % Bi₂O₃) and BNO5Pb (5 wt. % PbO) with values ε′ _r ~ 59.54 and ε′ _r ~ 78.44, respectively. The smaller values of loss tangent (tan δ) were for the samples: BNO5Bi and BNO3Pb (3 wt. % PbO) with values tan δ ~ 5.71 × 10⁻⁴ and tan δ ~ 2.19 × 10⁻⁴, respectively at frequency 33.69 MHz. The analysis as a function of temperature of the dielectric properties of the samples, obtained at frequency 100 kHz, showed that the larger value of the relative dielectric permittivity was about ε′ _r ~ 76.4 at temperature 200 °C for BNO5Pb sample, and the value smaller observed of dielectric loss was for BNO3Bi sample at temperature 80 °C, with about tan δ ~ 5.4 × 10⁻³. The Temperature Coefficient of Capacitance (TCC) values at 1 MHz frequency, present a change of the signal from BNO (−55.06 ppm/°C) to the sample doped of Bi: BNO3Bi (+86.74 ppm/°C) and to the sample doped of Pb: BNO3Pb (+208.87 ppm/°C). One can conclude that starting from the BNO one can increase the doping level of Bi or Pb and find a concentration where one have TCC = 0 ppm/°C, which is important for temperature stable materials applications like high frequency capacitors. The activation energy (H) obtained in the process is approximately 0.55 eV for BNO sample and increase with the doping level. These samples will be studied seeking the development ceramic capacitors for applications in radio frequency devices.