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).     

Vibrational spectra of Matrix isolated binary and mixed cadmium dihalides

The Raman spectra CdX₂ and CdXY (X, Y = Cl, Br, I) molecules isolated in solid krypton at 20°K are reported together with the matrix isolation infrared spectrum of CdX₂ and CdXY (X, Y = Br, I) molecules. Isotopic effects and force constants are discussed and several irregularities in them are interpreted as possible indications of slightly bent structures. The bending vibration frequency of CdClI as well as several bands assigned to dimers are also reported.     

Luminescence centers in phosphorus-doped Cd1−xMgxTe mixed crystals

The emission and excitation spectra of P-doped Cd_1?xMg_xTe (x ? 0.45) mixed crystals have been measured over the temperature range of 300 ～ 4.2 K to reinvestigate the photoluminescence process of two broad emission bands; the 690 nm band of the as- grown crystals and the 610 nm band of the annealed crystal. The excitation spectrum of each band has a characteristic peak at 550 nm near the absorption edge of this crystal. Together with the data of the decay characteristic and the halfwidth of each band, we discuss the difference in the recombination process when the crystal is excited under two different radiation at 365 and 570 nm (close to the 550 nm radiation). It is concluded that both emission bands arise from donor-acceptor pairs and a localized center or self- activated center; the associated pair is V_Cd-interstitial P in the as-grown and V_Te ? P_Te in the annealed crystal.     

First principles study of O defects in CdSe

Recently, the vibrational signatures related to oxygen defects in oxygen-doped CdSe were measured using ultrahigh resolution Fourier transform infrared (FTIR) spectroscopy by Chen et al.(2008) [1]. They observed two absorption bands centered at ∼1991.77 and 2001.3 cm⁻¹, which they attributed to the LVMs of O_Cd, in the samples grown with the addition of CdO and excess Se. For the samples claimed to be grown with even more excess Se, three high-frequency modes (1094.11, 1107.45, and 1126.33) were observed and assigned to the LVMs of O_Se–V_Cd complex. In this work, we explicitly calculated the vibrational signatures of O_Cd and O_Se–V_Cd complex defects based on first principles approach. The calculated vibrational frequencies of O_Cd and O_Se–V_Cd complex are inconsistent with the frequencies observed by Chen et al., indicating that their observed frequencies are from other defects. Potential defects that could explain the experimentally observed modes are suggested.     

Far-infrared detection with Hg1−xCdxTe

We report detection in liquid He cooled n−Hg_1−xCd_xCd_xTe at wavelengths between 140 and 1200 μm. With the mole fraction, x, of Cd <0.4m the alloy behaves as an electron bolometer with similar detectivity, but much improved bandwidth, compared with n-InSb. With x > 0.48 the detection process is extrinsic photoconductivity with an energy gap of a few meV. Lattice absorption severely limits detection in the wavelength range 220–340 μm.     

Thermoelectric power of quasi-one-dimensional Nb3X4 with X=S, Se andTe

The thermoelectric power of linear chain synthetic metals Nb₃X₄ (with X=S, Se and Te) was measured from 5 to 300 K. The thermopower is negative indicating a dominant transport by electrons. Common to three compounds, in lower temperature regions the thermopower rises linearly with temperature but soon saturates. With respect to Nb₃S₄ and Nb₃Se₄ we have found no special anomaly of the thermopower except for a little higher magnitude.With respect to Nb₃Te₄ anomalies in the thermopower vs temperature appear at about 80 and 20 K which are explained in terms of the charge- density-wave phase transition from the simultaneous measurement of the resistivity and the observation of the electron diffraction patterns.     

Impurity–defect emission from undoped Cd<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Zn<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Te single crystals near the fundamental absorption edge

Shallow impurity–defect states in undoped Cd_1–xZn_xTe (x ～ 3–6%) single crystals have been studied using low-temperature photoluminescence measurements. It has been found that the effect exerted by zinc is mainly reduced to a rigid shift of all the specific features associated with the exciton radiation, which made it possible, with a high (～0.3 meV) accuracy, to measure the band gap and the zinc concentration in solid solutions. Hydrogen-like donors with the ground-state energy of ～14 meV and four types of acceptors with average activation energies of 59.3 ± 0.6 meV, 69.6 ± 1.5 meV, 155.8 ± 2.0 meV, and 52.3 ± 0.6 meV have been identified in all the crystals studied. Based on a comparison with the results of the analysis of the impurity background and the data available in the literature on impurity–defect emission in undoped CdTe, the first three acceptors can be assigned to the substitutional impurities Na_Cd, P_Te, and Cu_Cd, respectively. The most shallow acceptor (52.3 ± 0.6 meV) is a complex defect in which there is a nonstandard excited level separated by only 7 meV from the ground level. This level is formed apparently due to the removal of degeneracy, which is characteristic of T_D acceptors, by the low-symmetry potential of the complex defect.     

Anion and cation vacancies in CdTe

The cadmium vacancy (V_Cd) and the tellurium vacancy (V_Te) in CdTe are identified by Electron Paramagnetic Resonance (EPR). The EPR spectrum of the singly ionised V_Te reveals cubic (unpertubed) symmetry and the hyperfine structure shows that the unpaired electron is equally spread over the four Cd neighbors. Further figand hyperfine interactions with the more distant neighbors are resolved by Electron Nuclear Double Resonance (ENDOR). The V_Cd is a double acceptor and the EPR spectrum is observed in its singly negative charge state. The symmerty is found to be trigonal, which can be explained in a model in which the hole occupies a dangling bondt ₂ orbital and the orbital degeneracy is removed by a static Jahn-Teller distortion. The hyperfine interaction shows that the hole is localised on one of the four Te neighbors.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

An appraisal of the molecular model for the vk centre

The V_k centre in halide crystals is often described in terms of an X₂^? molecule ion. Recent molecular calculations for the halogen molecule negative ions have enabled a detailed comparison to be made between the theoretical anion and the results from optical and spin resonance studies on the crystals. The optical absorption line widths are naturally dependant on the host lattice, but the excitation energies and spin resonance constants may be readily interpreted in terms of the molecule. Certain anomalies in the spin resonance data for the heteronuclear defects, in particular ClI^? and BrI^?, show that the simple wavefunction constructed from “s” and “p” atomic orbitals does not adequately describe the polarisation of the large halogen atoms, and a more flexible wavefunction will be needed to calculate the spin resonance constants for these ions.     

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.     

Spectral bundles and the DRY-Conjecture

Supersymmetric heterotic string models, built from a Calabi-Yau threefold X endowed with a stable vector bundle V, usually start from a phenomenologically motivated choice of a bundle V_v in the visible sector, the spectral cover construction on an elliptically fibered X being a prominent example. The ensuing anomaly mismatch between c₂(V_v) and c₂(X), or rather the corresponding differential forms, is often ‘solved’, on the cohomological level, by including a fivebrane. This leads to the question whether the difference can be alternatively realized by a further stable bundle. The ‘DRY’-conjecture of Douglas, Reinbacher and Yau in math.AG/0604597 gives a sufficient condition on cohomology classes on X to be realized as the Chern classes of a stable sheaf. In 1010.1644 [hep-th], we showed that infinitely many classes on X exist for which the conjecture is true. In this note, we give the sufficient condition for the mentioned fivebrane classes to be realized by a further stable bundle in the hidden sector. Using a result obtained in 1011.6246 [hep-th], we show that corresponding bundles exist, thereby confirming this version of the DRY-Conjecture.     

129Te Mössbauer emission spectroscopic study of the V-Te system: V3Te4, V5Te8 and VTe2 phases

The hyperfine interaction in the VTe system (V₃Te₄, V₅Te₈ and VTe₂ phases) was studied by means of the ¹²⁹Te Mössbauer emission spectroscopy. The transferred magnetic hyperfine field at the ¹²⁹I site was observed at 16K for the V₅S₈-type V_2.60Te₄ which has T_N at 52K. It was found that the ionicity estimated from the isomer shift decreases with decreasing the vanadium content, which is in better agreement with the earlier results on similar chalcogenide systems such as CrTe.     

Si中S0/Se0,S0/Te0和Se0/Te0对的电子结构

利用紧束缚近似下的格林函数方法,讨论了Si中硫属元素混对杂质(即S⁰/Se⁰,S⁰/Te⁰和Se⁰/Te⁰)基态的电子结构。混对杂质在Si禁带中引入两个A₁能级,其中成键性的A₁能级位置在反键性的A₁能级之上。数值计算得到的混对杂质能级与实验符合得相当好。理论分析表明,在Si中测到的那些未定的比最近邻混对杂质能级更浅的能级(S⁰/Se⁰(X₁),S⁰/Te⁰(X₁),Se⁰/Te⁰(X₁)…)不是由非最近邻位型的混对杂质引入的。本文还指出了一个极性分子放入Si晶体中,两个不同原子间s波函数的转移方向与通常极性分子相反,并讨论其物理原因。 关键词：     

Effect of donor-acceptor defect pairs on the crystal structure of In and Ga rich ternary compounds of Cu-In(Ga)-Se(Te) systems

A comparative study of the effect of donor-acceptor defect pairs on the unit cell parameters a, c and V of the ordered defect compounds that are intermediate phases of the pseudo-binary [Cu₂(Se,Te)]_1−X[(In₂,Ga₂)(Se₃,Te₃)]_X system has been carried out. It is found that a, c and V decrease linearly with the increase in the fraction of cation vacancies to the total number of cation positions, m, or the fraction of the interacting donor-acceptor defect l per unit, respectively, in the chemical formula. The reduction in the unit cell dimensions is explained as due to the decrease in the effective cation radius r_eff caused by the increase in m or l or decrease in n. The linear dependence of r_eff on a, c, and V has important consequences. This behavior can be used to predict the unit cell parameters of other ODCs that may have chalcopyrite-related structure and have not been reported so far.     

X-ray photoemission spectra of valence electrons in V3X and Nb3X compounds

The X-ray photoemission spectra (XPS) of the A15 type compounds V₃Au, Nb₃Os, Nb₃Ir, Nb₃Pt and Nb₃Au have been studied. The inner level binding energies of the different components and the valence electron distribution were measured. The Nb4d and the X5d energy bands of the Nb₃X compounds appear to be more and more separate with increasing atomic number of the X component. The comparison between the results from X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) of corresponding V₃X and Nb₃X compounds points out the similarity of their electronic structures.     

ESR and photoacoustic spectra of V2O5−MO2 (M = Se, Te, Ge) glasses

ESR spectra of V₂O₅?MO₂ (M = Ge, Se, Te) glasses are investigated in the range 298–498 K. The spectra at 298 K are characteristic of V⁴⁺ with the 3d¹ electron localized on a single ${}^{51}\text{V}\text{(I =}\text{7}\text{2}$) in the glass network. At higher temperature, the hyperfine structure progressively broadens, leading eventually to a broad, single ESR peak. These results are consistent with thermally-induced electron hopping from V⁴⁺ to V⁵⁺. Photoacoustic spectra of the glass at 298 K are characteristic of V⁴⁺ in a distorted octa environment. A correlation of ESR and PAS data suggests that covalency increased as M is charged from Ge through Te to Se.     

Chemical composition of matrix-embedded ternary II–VI nanocrystals derived from first- and second-order Raman spectra

A one- and multiphonon Raman scattering study is performed for an extensive set of CdS_1–xSe_x, Cd_1–yZn_yS, Cd_1–yZn_ySe, and CdSe_1–xTe_x nanocrystals to investigate the applicability of first- and second-order Raman spectra for the determination of the matrix-embedded ternary nanocrystal composition. For one-mode ternary systems both the LO and 2LO phonon frequencies in the Raman spectra are shown to be a good measure of the nanocrystal composition. For two-mode systems, the approaches based on the difference of the LO phonon frequencies (first-order Raman spectra) or double LO overtone and combination tone frequencies (second-order Raman spectra) as well as on the LO phonon band intensity ratios are analysed. The weak electron–phonon coupling in the II–VI nanocrystals and the polaron constant values for the nanocrystal sublattices are discussed.     

Luminescent properties of nanoparticles YPXV1−XO4:Dy phosphors

Phosphors of nanoparticles YP_XV_1−XO₄:Dy³⁺ (0?X?1) have been prepared by a citrate sol-gel method. X-ray diffraction, transmission electron microscope (TEM), scanning electron microscope (SEM) and photoluminescence excitation and emission spectra were utilized to characterize the phosphors. The results of XRD showed that a solid solution formed in YP_XV_1−XO₄:Dy³⁺ phosphor series from X=0 to X=1 with zircon structure. TEM and SEM studies revealed that the obtained YP_XV_1−XO₄:Dy³⁺ nanocrystals appeared to be spherical with some agglomeration and their sizes ranged from 30 to 80 nm. Upon short ultraviolet excitation, the optical properties of all the powder presented that the characteristic transitions of Dy³⁺ due to ⁴F_9/2-⁶H_15/2 (blue) and ⁴F_9/2-⁶H_13/2 (yellow) were detected. Besides this, in the system of YP_XV_1−XO₄:Dy³⁺, the yellow-to-blue intensity ratio (Y/B) depended on the value of P/V greatly, with the increasing of X value and the decreased Y/B value. The phosphor found to yield white light when the value of X in the range of X=0.775-0.85, the optimum concentration for Dy³⁺ is 1 mol% of Y in the host, and the emission intensity increased with the annealing temperature.     

Vibrational spectroscopy at high pressures: Part 32 pressure-dependencies of the Raman active modes of CdX2 and PbFX type layer materials

The variation of Raman-active phonons of CdX₂, (X = Cl, Br and I), and of PbFX, (X = Cl and Br), has been investigated to about 40 kbar. Rigid layer shear modes were found for CdBr₂ and CdI₂ and shown to have much greater Gruneisen constants than intralayer modes.