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

Surface processes in CdTe molecular-beam epitaxy were studied using in situ mass spectrometry. Modulated molecular Cd and Te₂ beams were used for measuring kinetic parameters. The experiments were performed at crystal temperatures of 600–730 K. The results were processed within a model in which condensation and evaporation occur through adsorption and desorption stages. The desorption rate was 2–10 s^?1 for Te₂ and more than 30 s^?1 for Cd. The CdTe evaporation activation energy and desorption energies were determined as E _ev = 1.1 eV, E _d (Cd) = 1.0 eV, and E _d (Te) = 0.6 eV. The adsorbate coverage was estimated as n(Cd) < 0.01 and n(Te) = 0.1–1 Te.     

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).     

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.     

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.     

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.     

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.     

Effect of interband scattering on kinetic coefficients and estimates of the parameters of the band spectrum of Sb2Te3

It has been shown that uncertainties in the interpretation of experimental data on transport phenomena in Sb₂Te₃ are resolved in the two-band model with the consistent inclusion of the interband hole scattering. The performed calculation is in quantitative agreement with the experimental data in the temperature range from 77 to 400 K for the following parameters of the band spectrum: the effective mass of the density of states of light holes m _d1 ≈ 0.6m ₀ (where m ₀ is the free electron mass), the effective mass of the density of states of heavy holes m _d2 ≈ 1.8m ₀, and the energy gap between nonequivalent extrema of the valence band ΔE _v(T) ≈ 0.15–2.5 × 10^?4 T eV.     

Optical properties and electronic band structure of CdGa2Te4

Optical properties of a defect-chalcopyrite-type semiconductor CdGa₂Te₄ have been studied by optical absorption, spectroscopic ellipsometry (SE), and electroreflectance (ER) measurements. Optical absorption measurements suggest that CdGa₂Te₄ is a direct-gap semiconductor having the band gap of ∼1.36 eV at 300 K. The complex dielectric-function spectra, ε(E)=ε₁(E)+iε₂(E), measured by SE reveal distinct structures at energies of the critical points (CP's) in the Brillouin zone. ER spectrum facilitates the precision determination of the CP parameters (energy position, strength, and broadening). By performing the band-structure calculation, these CP's are successfully assigned to specific points in the Brillouin zone.     

Effect of Cd impurity on the electrical properties of a-Se80Te20

Temperature dependence of dark and photoconductivity is studied in amorphous films of Se₈₀Te_20−xCd_x in the temperature range (300 K to 380 K) with a view to see the effect of Cd impurity on a-Se₈₀Te₂₀ binary alloy. It is observed that, at low concentration of Cd (x = 0.5), the dark and photoconductivity increase at all temperatures. However, at higher concentration of Cd (x = 10), an appreciable reduction in these parameters occurs in the same temperature range. The photosensitivity (σ_ph/σ_d) remains unchanged at x = 0.5 but decreases quite significantly at x = 10. The results are explained in terms of impurity doping at x = 0.5 and alloying effect at x = 10.     

Defect studies in small CdTe clusters

We study Cd vacancy formation in prototype stoichiometric and non-stoichiometric CdTe clusters with and without passivation. For certain clusters like Cd₁₃Te₁₆, vacancy leads to severe distortion of the geometry due to propagation of defect. Annealing of the vacancy out of the cluster is observed in all unpassivated clusters. Passivated clusters retain their initial geometry and vacancy induced structural distortions are not seen in these clusters since the defect gets localized. Vacancy also induces intragap states. However, it was observed that the passivation of the dangling bonds created by the vacancy removes the intragap states. In an attempt to have CdTe clusters with extrinsic carriers, we substituted a Cd atom by its adjacent atoms Pd/Ag/In/Sn in these CdTe clusters. Substitutional doping of Cd by metal atoms increases the stability of unpassivated clusters. For certain clusters, metal atom doping leads to a half-metallic character. Pd/Ag-doped clusters are p-type semiconductors whereas In-doped clusters are n-type semiconductors. Sn doping in these clusters does not result in n-type semiconductors.     

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.     

Thermal kinetics and short range order parameters of Se80X20 (X = Te,Sb) binary glasses

Bulk Se₈₀Te₂₀ and Se₈₀Sb₂₀ glasses were prepared using the melt–quench technique. Differential scanning calorimetry (DSC) curves measured at different heating rates (5 K/min≤α≤50 K/min) and X-ray diffraction (XRD) are used to characterize the as-quenched specimens. Based on the obtained results, the activation energy of glass transition and the activation energy of crystallization (E _g, E _c) of the Se₈₀Te₂₀ glass are (137.5, 105.1 kJ/mol) higher than the corresponding values of the Se₈₀Sb₂₀ glass (106.8, 71.2 kJ/mol). An integer n value (n=2) of the Se₈₀Te₂₀ glass indicates that only one crystallization mechanism is occurring while a non-integer exponent (n=1.79) in the Se₈₀Sb₂₀ glass means that two mechanisms are working simultaneously during the amorphous–crystalline transformations. The total structure factor, S(K), indicates the presence of the short-range order (SRO) and the absence of the medium-range order (MRO) inside the as-quenched alloys. In an opposite way to the activation energies, the values of the first peak position and the total coordination number (r ₁, η ₁), obtained from a Gaussian fit of the radial distribution function, of the Se₈₀Te₂₀ glass are (2.42 nm, 1.99 atom) lower than the corresponding values (2.55 nm, 2.36 atom) of the Se₈₀Sb₂₀ specimens.     

Thermal neutron damage in CdS and CdTe

Photoluminescence and electrical resistivity changes in CdS and CdTe produced by thermal neutrons are discussed. The damage is produced principally by the neutron capture reaction ¹¹³Cd (n,y) ¹¹⁴Cd. Since the reaction product ¹¹⁴Cd is stable, complications arising from impurity introduction is minimal. The cumulative recoil nuclear recoil energy is about 143 eV, but is not the recoil energy at the time atomic displacement occurs. Thermal and fast neutrons enhance the CdS luminescence band at 7200A in the ratio of 28:1, but the resistivity changes are in the ratio of 40:1 Cd interstitial is suggested as the luminescence center. Hall measurements on n-type CdTe suggest that only Cd defects are produced for low thermal neutron doses. The acceptor introduction rate is about 1.0 to 0.6, compared to 0.098 for CdS. These are in good agreement with the values reported by R. O. Chester. The fast neutron effects in high resistivity CdS reported by Johnson indicate the need for further measurement.     

Study of acceptor states in CdTe by donor-acceptor pair excitation luminescence

Excitation of donor-acceptor pair luminescence has been studied in CdTe doped with lithium or chlorine. The excitation spectrum of the lithium acceptor is determined and fitted with the effective mass theory of Baldereschi and Lipari. Revised values of the valence band parameters are deduced: μ = 0.8, δ = 0.054, Ry = 24 meV. The analysis of the 1.45 eV luminescence band in compensated Cl-doped crystals shows the existence of donor-acceptor pair transitions. Three acceptor centers are identified: E_A = 89, 111 and 119 meV, and the contribution of a deep donor (E_D > 40 meV) is demonstrated. Besides intracentre type excitation transitions of the 1.45 eV band have been observed in non-compensated chlorine-doped crystals. Thus several recombination channels and distinct acceptor states contribute to the composite 1.45 eV luminescence band.     

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.     

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).     

The influence of annealing temperature and tellurium (Te) on electrical and dielectrical properties of Al/p-CIGSeTe/Mo Schottky diodes

p-CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂ type thin films were synthesized by thermal evaporation method on Mo coated glass substrates. To obtain Al/CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂/Mo Schottky diode structure for two compositions of x = 0.0 and 0.6, Al metal was evaporated on upper surface of CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂ as a front contact. Al/p-CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂/Mo structures were annealed temperature range from 150 °C to 300 °C for 10 min under vacuum. The electrical and dielectrical properties of Al/p-CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂ (CIGSeTe) Schottky barrier diodes (SBD) have been investigated. Capacitance–Voltage (C–V) characteristics, Conductance–Voltage (G/w–V) characteristics and interface state density were studied in order to obtain electrical and dielectrical parameters. The effects of interface state density (N_ss), series resistance (R_s), the dielectric constant (?′), dielectric loss (?″), dielectric loss tangent (tan δ), ac electrical conductivity (σ_ac) and carrier doping densities were calculated from the C–V and G/w–V measurements and plotted as a function of annealing temperature. It was observed that the values of carrier doping density N_A for annealing temperature at 150 °C decreased from 2.83 × 10⁺¹⁵ cm^?3 to 2.87 × 10⁺¹⁴ cm^?3 with increasing Te content from x = 0.0 to 0.6. The series resistance for x = 0.0 found to be between 10 and 75 Ω and between 50 and 230 Ω for x = 0.6 in the range of annealing temperature at 150–300 °C.     

Binding energy of an electron to a three-defect-complex in CdTe

Recent measurements on halogen doped CdTe shows an unidentified electron trapping level at 0.05 eV. Using a mass action approach to self compensation involving a substitutional halogen, X_Te, and a cadmium vacancy, V_Cd, it can be shown that in addition to the two-defect complex, V_CdX_Te, the three-defect complex (V_Cd2X_Te) is present in substantial concentrations. A calculation of the binding energy of an electron to this three-defect-complex using the configurational interaction and one electron wave function, leads to a bound state with a depth about right for the unidentified level.     

Zenith-angle dependences of ρ s,600 and ρ μ,600, in giant air showers

Results of a global analysis of data from the Yakutsk array and AGASA are presented. The zenith-angle dependences ρ _s,600(θ) and ρ _μ,600(θ) of the densities of all charged particles and muons (the threshold energy is E _μ ≈ 1.0·secθ GeV) at a distance of 600 m from the axis of giant air showers (GAS) of energies E ₀≥10¹⁹ eV are considered. These dependences are compared with the results of the calculations based on the QGSJET model and performed for the case of primary protons. The results of the calculations within this model agree well with data from both arrays at E ₀≤2×10¹⁸ eV, but they are in a glaring contradiction with GAS data. The experiments indicate that the lateral structure of showers changes at E ₀≥(3–5)×10¹⁸ eV. In all probability this is due to some new processes accompanying their evolution. The neglect of this fact can lead to considerably overestimating the GAS energy (by a factor of 1.5–2.5).