Mass spectrometry study of the kinetics of CdTe molecular-beam epitaxy: Part II. Cd, Te2, Te, and CdTe

The results of an in situ mass-spectrometric study of surface processes occurring during CdTe molecular beam epitaxy are presented. The measurements of kinetic parameters are performed with modulated Cd and Te₂ molecular beams with an intensity of 0.1–5.0 ML/s at a crystal temperature of 550–730 K. The experimental results are treated using a model in which condensation and evaporation proceed through adsorption and desorption steps. The desorption rates are 2–15 and 150 s^?1 for Te₂ and Cd, respectively. The activation energy of CdTe “evaporation” is found to be 1.2 eV; the desorption energies are E _d(Cd) = 1.0 eV and E _d(Te₂) = 0.3 eV. The adsobate coverage with cadmium atoms and tellurium molecules is estimated to be n(Cd) < 0.01, n(Te₂) = 0.02–0.20, and n(Te) = 0.2–1.0.     

Influence of deformation on the conductivity and photoconductivity of p-Type CdTe

Samples of p-type CdTe were subjected to deformation. Studies were performed of the influence of 60°-dislocations on the electrical conductivity, on the lifetime of minority carriers and on the relative change of the conductivity due to illumination. Dislocations were produced by bending the sample that was cut at suitable orientation. In this way, one obtained the 60°-dislocations, either of α- or β-type, according to the kind of bending involved. From the measurement of the temperature dependence of the conductivity it was ascertained that an increase in acceptor concentration occurs on the level with activation energy of 0.3 eV (identical to that of V^cd or a complex with V^cd), regardless of the type of dislocations. The concentration of acceptors on the level with activation energy 0.3 eV increases on increasing the dislocation density. The most probable explanation is that the deformation and, especially, annealinginduced migration of the dislocations produced give rise also to V^cd and/or V^Te (both types simultaneously). V^cd acts as an acceptor and V^Te as a donor. It follows that the increase in V^Te can not be established from this measurement. The lifetime of the minority carriers is almost unaffected by introducing the dislocations into the samples. From the temperature dependence of the relative change in conductivity due to illumination one can determine the activation energy of a dislocation level provided that one assumes the dislocations acting as recombination centers. Under this assumption one obtainsE _d =0.21–0.24 eV (for type α) andE _d =0.55–0.59 eV (for type β), measured from the top of the valence band.     

Wet chemical etched CdTe thin film solar cells

Wet chemical etching process on as-deposited CdTe surface using nitric-phosphoric (NP) acid improved the efficiency of CdS/CdTe solar cells from 10.1% to 13.8%. Nitric-phosphoric (NP) acid solution etched native oxide (TeO₂) layer and resolved excess cadmium on CdTe surface. After the heat treatment process activated the CdCl₂, CdO layer which was believed to be a diffusion barrier of chlorine did not grow on the etched CdTe surface and new (V_Cd^2?–2Cl_Te¹⁺)⁰ complexes was located at E_V + 0.045 eV. New (V_Cd^2?–2Cl_Te¹⁺)⁰ complexes acted as a shallow acceptors and induced to improve V_oc and J_sc. The surface of CdTe thin film has been studied using Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and low temperature Photoluminescence (PL).     

Specific features of the optical and electrical properties of polycrystalline CdTe films grown by the thermal evaporation method

The results of studying the physical properties of thin CdTe films obtained by the thermal evaporation method have been presented. The optical constants and the band gap of the films under study have been determined (E _g = 1.46 eV). It has been established based on the investigation of optical properties and the Raman spectrum of the films that they possess high structural quality. The activation energy of the electrical conductivity of CdTe films has been determined: E _a = 0.039 eV. The measured spectral dependences of the impedance of CdTe thin films are characteristic of the inhomogeneous medium with two time constants: τ_gb = R _gb C _gb = 1/ω_gb = 1.62 × 10^?3 s and τ_g = R _g C _g = 1/ω_g = 9.1 × 10^?7 s for grain boundaries and grains, respectively.     

Identification of Cu and Ag acceptors in CdTe

We report here on the identification of the two dominant acceptor levels in high purity p type CdTe, with Cu and Ag on Cd site. This identification is based on back doping experiments coupled with electrical measurements and photoluminescence studies. Cu and Ag can form easily complex centers when a supersaturation is achieved. The way of obtained good doping without complexation, is explained. The principal bound exciton lines are at 1,5896 eV (Cu) and at 1,5885 eV (Ag). The precise hole binding energies obtained from optical data are E_A (Cu) = 146 meV and E_A (Ag) = 108 meV.     

Deep level structure and compensation mechanism in In-doped CdTe crystals

Deep Level Transient Spectroscopy (DLTS) and Optical Deep Level Transient Spectroscopy (ODLTS) experiments have been conducted on a series of In-doped CdTe crystals grown by the Bridgman or the travelling heater (THM) methods using Te as the solvent. The THM samples are n-type but strongly compensated. Annealing at 700°C under high Cd vapour pressure leads to a decompensation of the crystals. The electron concentration is then a measure of the donor (In) concentration, which was in the range 3 × 10¹⁶−1.5 × 10¹⁸cm⁻³. Six and eight electron traps are, respectively detected in the non-annealed and annealed samples at a concentration level 10²–10³ times below the net electron concentration. They cover the energy range 0.2–0.8 eV. Similar traps are found in both types of crystals, the concentration of which increases with In content and after annealing. The presence of In interstitial-type defects is suggested. A main hole trap at 0.12eV is detected by ODLTS in compensated samples with a concentration close to the donor concentration. Low temperature electrical measurements show that this trap is ionized under equilibrium conditions. It appears to be the main compensating acceptor centre. A plausible microscopic structure is In_cdV_cd. This study shows that In-doped CdTe grown by THM is electrically compensated and that In-containing neutral associates or precipitates play a minor role in the compensation mechanism.     

Direct and indirect dipole-dipole coupling between111Cd,113Cd and125Te in solid CdTe

In nuclear magnetic resonance spectra of¹¹³Cd and¹²⁵Te in solid CdTe a splitting has been found. The intensity of the satellites is proportional to the natural abundance of¹¹¹Cd,¹¹³Cd and¹²⁵Te. From a NMR rotation pattern of¹²⁵Te in a CdTe single crystal in addition to a direct dipole-dipole coupling between¹²⁵Te and¹¹¹Cd,¹¹³Cd an indirect anisotropic dipole-dipole interaction has been observed. If the direct dipoledipole coupling is calculated from crystal geometry and from the gyromagnetic ratios of¹¹³Cd and¹²⁵Te, the indirect dipole-dipole interaction isJ (¹¹³Cd–¹²⁵Te)=(765±90)Hz perpendicular andJ (¹¹³Cd–¹²⁵Te)=(435±120)Hz parallel to the internuclear axis.     

Interface photoluminescence of the SnO2:F/CdS:In/CdTe thin film solar cells prepared partially by the spray pyrolysis technique

CdS/CdTe solar cells were built by depositing a 200 nm layer of SnO₂:F on glass substrates by the spray pyrolysis (SP) technique, a 500 nm CdS:In layer by the same technique and a 1–1.5 μm CdTe layer by vacuum evaporation. The cells were CdCl₂ heat-treated in nitrogen atmosphere for 30 min at 350 °C. The photoluminescence (PL) spectra were measured at the CdS/CdTe interface for two cells with different values of the CdTe layer's thickness at the temperature T=60 K. A deconvolution peak fit was performed from which it is found that the peaks are characteristic of the solid solution CdS_xTe_1?x. The parabolic relation that relates the bandgap energy with the composition was used to estimate x, where x is [S]/([Te]+[S]) and [Te], [S] are the concentrations of Te and S atoms, respectively. The results show that the interface is smooth and the change of the bandgap occurs gradually. The solar cell of the thicker CdTe layer showed more interdiffusion at the CdS/CdTe interface and better photovoltaic characteristics.     

Desorption of halogens from some Nb,Mo, Ta and W single crystal surfaces

A systematic investigation of the thermal desorption of halogens from well characterized (111), (100) and (110) 4d (Nb, Mo) and 5d (Ta, W) transition metal surfaces has been carried out under low coverage conditions (θ < 10^?2 of a monolayer). Characterization of the surfaces was achieved by LEED, AES and work function determinations while the desorption kinetics were recorded in a large temperature range (1700–2300 K) using a pulsed ionic beam method. The new data concerning some Ta and W surfaces are presented and the results of this systematic study are discussed. It is shown that the halogen desorption parameters, e.g., desorption energies and preexponential factors, are independent of both surface structure and d bond filling of the substrate; E(F) ～4.75 eV, E(Cl) ～4.1 eV, E(Br) ～3.7 eV and τ₀ ～10^?13 ?10^?14 S. The halogen behaviour is compared with that of other adsorbates and with the predictions of a general chemisorption model.     

A tunneling spectroscopy study of the temperature dependence of the forbidden band in Bi2Te3 and Sb2Te3

Tunneling measurements of dI/dV, d ² I/dV ², and d ³ I/dV ³ were formed along the C ₃ axis (normally to layers) for Bi₂Te₃ and Sb₂Te₃ layered semiconductors in the temperature range 4.2<T>29 5 K. Temperature dependences of the forbidden band energy E _g were obtained. The forbidden band energy in Bi₂Te₃ was 0.20 eV at room temperature and increased to 0.24 eV at T=4.2 K. The E _g value for Sb₂Te₃ was 0.25 eV at 295 K and 0.26 eV at 4.2 K. The distance between the top of the higher valence band of light holes and the top of the valence band of heavy holes situated lower was found to be ΔE _V≈19 meV in Bi₂Te₃; this distance was independent of temperature. The conduction bands of Bi₂Te₃ and Sb₂Te₃ each contain two extrema with distances between them of ΔE _c≈25 and 30 meV, respectively.     

Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M_4,5 (5p → 3d_3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M_4,5 to the Te M_4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).     

Determination of the activation energy for OH desorption in the H2 + O2 reaction on Polycrystalline Platinum

We have investigated the temperature dependence of the desorption rate of OH produced in the oxidation of hydrogen on polycrystalline Pt by using laser induced fluorescence (LIF). Arrhenius type plots of OH desorption rates give quite straight lines, with an “apparent” desorption energy, E^a_OH, in the rang 1.4–2.3 eV depending on the relative hydrogen concentration. E^a_OH, differs from the “true” desorption energy, E^d_OH, due to the temperature dependence of the surface coverage of θ_OH,. Knowing θ_OH, from kinetic modelling calculation of this reaction we deduce E^d_OH, to be 2.0 ± 0.15 eV.     

Photoluminescence and electric spectroscopy of dislocation-induced electronic levels in semi-insulated CdTe and CdZnTe

We studied deformation-induced defects in semi-insulating CdTe and CdZnTe by infrared photoluminescence (PL), contact less photoconductivity and resistivity. Plastic deformation increased the concentrations of grown-in defects, namely, those of an important midgap level E_C−0.74 eV in CdTe and Cd_1−xZn_xTe (x<0.1), the materials of choice in today’s X-ray and gamma ray detector technology. We confirmed the direct correlation between Y-emission and the dislocation density in both compounds. The Y-band intensified near an indenter deformation or near a scribing line, but was barely visible in low-dislocation areas (etch pit density <2×10⁵ cm⁻²). Our results correlate with recent findings that dislocation-induced defects and their clusters degrade charge collection in radiation detectors. Photoluminescence of midgap levels can serve as a tool to identify areas of degraded performance in semi insulated CdTe and CdZnTe crystals for radiation detectors.     

Modification of the surface state and doping of CdTe and CdZnTe crystals by pulsed laser irradiation

The photoelectric and electrical properties of high-resistivity p-like CdTe and Cd_0.96Zn_0.04Te single crystals and barrier structures on their base before and after laser irradiation in different conditions are studied. Irradiation of samples with nanosecond ruby laser pulses was carried out in two different ways. In the first case, the Cd(Zn)Te crystals were subjected to laser action directly from the surface and irradiation within a certain range of intensities resulted in a decrease in the surface recombination rate and increase in the photoconductivity signal. The surface region with a wider bandgap in CdZnTe crystals was formed. In the second case, the samples were irradiated from the side pre-coated with a relatively thick In dopant film and it caused rectification in the I-V characteristics as a result of laser-induced doping of the thin Cd(Zn)Te surface region and formation of a built-in p-n junction. The application of the fabricated M-p-n structured In/Cd(Zn)Te/Au diodes for X-ray and γ-ray detectors is discussed.     

Temperature dependance of the fundamental absorption edge in CdTe

Optical measurements made on CdTe put in light an extrinsic transition previously used to determine the interband edge of this compound. The donor level involved has a depth of 30 meV inside the bandgap at 77°K and of 60meV at 300°K. The value obtained for its temperature coefficient, suggests an association of this level with the L minimum of the conduction band.Also, determined is the true optical bandgap of CdTe between many conflicting results. One obtains: E_g = 1.529 meV with a temperature coefficient of ?3.10^?4eV/°K.     

Deep level transient spectroscopy investigation of deep levels in CdS/CdTe thin film solar cells with Te:Cu back contact

Deep levels in Cds/CdTe thin film solar cells have a potent influence on the electrical property of these devices. As an essential layer in the solar cell device structure, back contact is believed to induce some deep defects in the CdTe thin film. With the help of deep level transient spectroscopy (DLTS), we study the deep levels in CdS/CdTe thin film solar cells with Te:Cu back contact. One hole trap and one electron trap are observed. The hole trap H1, localized at E_v+0.128~eV, originates from the vacancy of Cd (V_Cd. The electron trap E1, found at E_c-0.178~eV, is considered to be correlated with the interstitial Cu_i⁼ in CdTe.     

A molecular beam study on the interaction of NO with a Pt(111) surface

NO adsorbs on Pt(111) with a (temperature independent) initial sticking coefficient S₀=0.88. The fraction of molecules not being chemisorbed is directly inelastically scattered back due to failure of translational energy accommodation. The nonlinear variation of s with coverage can well be described by a precursor-state model, the precursor state being formed by NO molecules translationally and rotationally accommodated in a physisorbed second layer. Dissociation is essentially restricted to defect sites and is negligible on perfect (111) planes. These defect sites (present in small concentration) are first populated and are also sampled by the modulated beam technique yielding an activation energy for desorption E_d = 33.1 kcal/mole and preexponential factor v_d = 10^15.5s^?1. Isothermal desorption measurements yielded E_d and v_d as a function of coverage: E_d rapidly drops from its initial value (at defect sites) to about 27 kcal/mole — which value is considered as representing the adsorption energy on a perfect (111) plane — and then decreases continuously due to effective repulsive interactions. Simultaneously v_d is decreasing to about 10¹² s^?1 at θ = 0.25 which marks the equilibrium coverage to be reached at 300 K. If the surface is precovered with oxygen atoms the NO sticking coefficient is reduced to 0.6, and the desorption parameters are lowered to E_d = 17.1 kcal/mole and v_d= 10^12.6s^?1 (at zero NO coverage).     

First principles study of half-metallic ferromagnetism in Cr-doped CdTe

We report a systematic study of the structural, electronic and magnetic properties of Cr-doped CdTe for various Cr concentrations x (=0.25, 0.5, 0.75 and 1.0) using first principles calculations based on the density functional theory (DFT). The electronic band structure of the alloy has been calculated using the Wu-Cohen (WC) as well as the Angel-Vosko (EV) generalized gradient approximation (GGA) for the exchange-correlation potential. The analysis of the density of states (DOS) curves shows the half-metallic ferromagnetic character with half-metallic gap more than 0.52 eV. While the origin of half-metallic ferromagnetism is explained, the band structure calculations are used to determine s (p)-d exchange constants N₀α (conduction band) and N₀β (valence band) that agree with typical magneto-optical experiment. It is found that the p-d hybridization reduces the magnetic moment of Cr from its free space charge value and produces small magnetic moments on the Cd and Te sites. Lastly, we discuss the robustness of half-metallicity with respect to the variation of lattice constants of the Cr_xCd_1−xTe alloys.     

Hybridization and electron interaction in nickel compounds

The bare Coulomb interaction, defined by the reaction 2dⁿ = d^n?1 + d^{n + 1} ? U is determined for some Ni compounds. It is found that U is larger than the 3d bandwidth even for metallic compounds. More relevant to the metal nonmetal transition is the optical gap energy E_G = U ? (E(3d^n?1) ? E(L)) where E(3d^n?1) and E(L) are the d-electron and ligand electron ionization potentials respectively.     

An optical study of vacuum evaporated amorphous Ge<Subscript>20</Subscript>Te<Subscript>80−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films using transmission and reflection spectra

The present paper reports the effect of Bi addition on the optical behavior (optical band gap and refractive index) of Ge₂₀Te_80?x Bi _x (where x=0, 1.5, 2.5, 5.0) glassy alloys by analyzing the transmission and reflection spectra of their thin films in the 900–2400 nm range. Films are deposited on glass substrate using a thermal evaporation technique under vacuum. Various optical parameters viz. refractive index, extinction coefficient, absorption coefficient, optical band gap, etc. are determined and the effect of Bi incorporation on these parameters is studied. The refractive index has been found to increase with increasing Bi content over the entire spectral range and this behavior is due to the increased polarizability of the larger Bi atomic radius (1.46 Å) compared to Te atomic radius (1.36 Å). Dispersion energy, E _d , average energy gap, E ₀ and static refractive index, n ₀ is calculated using Wemple–DiDomenico model. Optical band gap is estimated using Tauc’s extrapolation and is found to decrease from 0.86 to 0.73 eV with the Bi addition. This behavior of the optical band gap is interpreted in terms of the electronegativity difference of the atoms involved and the cohesive energy of the system.