用光电子能谱研究CdTe太阳电池的背接触特性

CdTe太阳电池的背电极须采用高功函数金属。通过采用光电子能谱(XPS)分析了高功函数金属Au和Ni分别作为背电极的CdTe太阳电池背接触特性,发现在背电极剥离后,Au在ZnTe/ZnTe∶Cu背接触层表面以Au单质形式存在,扩散深度较浅;Ni扩散到ZnTe/ZnTe∶Cu复合层的深度比Au大,且大多呈离子态,与ZnTe/ZnTe∶Cu复合层中的富Te离子形成Ni_xTe,提高了掺杂浓度,使电池性能获得改善。在两样品中还发现,不论是Te的峰还是Zn的峰,其峰的位移变化都很小,说明两样品中Te和Zn的存在形式没有发生变化。     

CdTe太阳电池的不同背电极和背接触层的特性研究

用Ni替代Au来作为CdTe太阳电池的背电极，比较了Ni,Ni/Au,Au/Ni及Au背电极对电池性能的影响.发现Ni作为背电极和ZnTe/ZnTe:Cu复合层接触，电池的开路电压V_oc略有降低，填充因子FF有增有减，变化幅度不大，但因短路电流I_sc有较大的提高，转换效率η平均增长4％.测试了不同背电极的CdTe太阳电池的暗I-V和C-V特性，对背电极剥离后的样品进行了XPS测试分析.结果表明，Ni扩散到ZnTe/ZnTe:Cu复合层的深度比Au多，且大多呈离子态，与ZnTe/ZnTe:Cu复合层中的富Te离子形成Ni_xTe，提高了掺杂浓度，使电池性能获得改善. 关键词： 金属背电极 复合背接触层 转换效率 CdTe太阳电池     

197Au Mössbauer study of Au/Ni artificial multilayers

¹⁹⁷Au Mössbauer measurements have been performed at 16 K on the Au/Ni artificial multilayers having three different thickness of the layers those are 10Å Au/10Å Ni, 30Å Au/30Å Ni and 53Å Au/53Å Ni on a 250Å pure Au buffer layer. Mössbauer spectra obtained can be decomposed into mainly two components. One is an unperturbed component having an identical isomer shift value to the bulk Au metal. The other is the component perturbed strongly by the Ni layer indicating a broad contribution at positive velocity side and its intensity depends on the thickness of the Au layer. The spectrum from 10Å Au/10Å Ni multilayer is an entirely perturbed one and its area ratio to the component rising from pure Au buffer layer indicates the large Debye-Waller-factor suggesting the supermodulus effect in this multilayer.     

Core hole satellites in Ni and Au Ni

The 2p core hole satellite structure in Ni and 5% Ni in Au are compared and discussed in terms of recent theoretical models. The large differences found for these two materials are explained in terms of the large differences in the Ni 3d band, and virtual bound state widths. The satellite structure for Au Ni is found to agree with the multiplet splitting of the free atom in a 2p⁵3d⁹ configuration.     

Ab initio study of structural, electronic and optical properties of Be-doped CdS, CdSe and CdTe compounds

Full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method within density functional theory (DFT) has been utilized to calculate structural, electronic and optical properties of Be-doped CdS, CdSe and CdTe compounds with the dopant concentration x in the range 0≤x≤1. For the contribution of exchange-correlation potential, we used Wu-Cohen generalized gradient approximation (GGA) to calculate structural parameters, whereas both Wu-Cohen and Engel-Vosko GGA have been applied to calculate electronic structure of the materials. Only a slight deviation from Vegard's law has been observed for the calculated lattice constants and bulk moduli of the alloys. Structural and chemical factors that affect the band-gap bowing of these semiconductor alloys have been estimated and discussed. Density of states curves and charge density maps have been calculated and analyzed. Lastly, optical properties of both binary and their related ternary alloys have been discussed in terms of the calculated dielectric function. The resultant optical parameters are compared with the available experimental data and other calculations.     

Solubility and lattice location of Au in CdTe by backscattering techniques

The solubility of Au in CdTe, at 800°C and 900°C, has been determined as a function of P_cd (stoichiometry), and the lattice location of the Au has been investigated by ion channeling (2 MeV, ⁴He⁺) techniques. The measured Au solubilities varied from ～10¹⁸cm^?3 to ～10²⁰cm^?3 and were found to increase with increasing temperature and with decreasing cadmium partial pressure roughly as $\text{P}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}{}_{\mathrm{cd}}$. Lattice location studies showed that from ～50% to ～80% of the Au remained in a substitutional, dissolved state after quenching to room temperature. In addition, angular yield scans indicate that the substitutional fraction of the incorporated Au is displaced off lattice sites by ～0·2 Å. No flux peaking was seen in the angular yield scans and the non-substitutiolial fraction of the incorporated Au, manifested at room temperature, may consist either of precipitated or (displaced) interstitial Au. Diffusion coefficients of Au in CdTe have been estimated at 900°C, at the extreme boundaries of the phase field, using the ion channeling technique. The results indicate that the Au diffusivity decreases markedly with increasing P_cd.These observations are found to be consistent with a dissociative model for Au in CdTe in which the major native defects are doubly ionized cadmium Frenkel defects and approximately singly ionized substitutional Au.     

Surface Diffusion Induced Growth of Solid Solution over a Free Crystal Surface

It has been shown that interdiffusion along a free vicinal crystal surface can lead, at comparably low temperatures, to layered growth of solid solution over the surface without the involvement of bulk diffusion. The alloy concentration distribution along the surface, as well as the normal rate of solution growth has been calculated. The formation of a layer of the solid solution has been experimentally observed on a vicinal (111) surface of Ni single crystal as a result of surface interdiffusion between Ni and a thin film of Au deposited on a part of the Ni surface. The surface diffusion coefficients, as well as other parameters responsible for the exchange rate between the adatoms and kinks of elementary steps have been measured in the temperature range 550–700°C.     

Magnetic surfaces

The magnetic ground state, magnetic anisotropies and spin excitations of surfaces, interfaces and ultra-thin films of ferromagnetic 3d-metals are discussed. Enhanced magnetic ground state moments and altered hyperfine fields as predicted by ab initio band calculations have not been conclusively verified by experiments up to now. Future calculations should take into account dipolar fields and the role of interface roughness. Very large magnetic anisotropies are observed at magnetic surfaces and interfaces. In Ni/Cu multilayered films, the superposition of surface and stress-induced anisotropies was used to switch the easy axis of magnetization from the film plane to a perpendicular orientation by a proper choice of the Ni layer thickness. This could be an attractive possibility to develop new magnetic materials for technical applications. The temperature dependence of the spontaneous magnetization at surfaces and in ultra-thin films deviates from the behaviour of bulk material. Size effects as well as surface effects of spin wave excitations are discussed, comparing theoretical and experimental results. The need for more complete theories including surface exchange, surface anisotropy and realistic surface structures is emphasized.     

197Au Mössbauer study of Au/Ni and Au/Fe metallic superlattices

¹⁹⁷Au Mössbauer measurements have been performed for Au/Ni and Au/Fe metallic superlattices at below 75 K. For Au/Ni superlattices, the area ratio in a spectrum between a superlattice component and that of the pure Au buffer layer has been determined at 25, 50 and 75 K. From the area ratios, it is found that the recoil-free fraction of Au in Au(10 Å)/Ni(10 Å) is larger than that of the bulk Au, suggesting the existence of the supermodulus effect in this superlattice. The¹⁹⁷Au Mössbauer spectrum obtained from Au(5 Å)/Fe(8 Å) is entirely magnetic at 16 K, suggesting the existence of a magnetic hyperfine interaction at¹⁹⁷Au nuclei through the transferred electron spin polarization.     

A Simple Theoretical Method to Predict the Hardness of Pure Metal Crystals

Design of superhard bulk materials requires predicting their hardness, challenging current theories for material design. By introducing a concept of condensing force （CF）, it is shown via ab initio calculations for fcc （Ni, Cu, Al, Ir, Rh, Au, Ag, Pd） and hcp Re crystals that materials with larger CF can have greater hardness. Since the calculation of CF is easy, this method might prove a convenient way to evaluate the hardness of newly designed materials.     

Studies of the surface structures formed by the alternated electrodeposition of Cd and Te on the low-index planes of Au: II. STM studies

Scanning tunneling microscopy (STM) studies of the alternated deposition of Te and Cd atomic layers on the low-index planes of Au are presented here. These investigations were designed to gain an understanding of the surface chemistry involved in the deposition of CdTe by electrochemical atomic layer epitaxy (ECALE). ECALE is the electrochemical analog of atomic layer epitaxy (ALE). In the present study, the structures formed by the alternated electrodeposition of Te and Cd atomic layers, at underpotential, were examined by STM on the low-index planes of Au.

Deposition of atomic layers of each element were achieved by underpotential deposition (UPD). Studies of the UPD of Cd by STM were not performed due to Cd's instability in aqueous solutions after withdrawal from solution and loss of potential control. Studies of Te UPD were performed, however, since Te is stable in aqueous media without potential control. Ordered Te structures were observed using STM on all three Au low-index planes. Atomic resolution imaging of Cd UPD on Te atomic layers proved more difficult than imaging simple Te deposits, however, ordered CdTe structures were observed on each Au plane. Higher bias voltages were generally required to image the CdTe deposits. On Au(100) the CdTe deposit appears to adopt a structure equivalent to a (100) slice of bulk CdTe (zinc blende), but contracted 10% due to the lattice missmatch on the Au surfaces. Similar structures were observed on the other two low index planes, as well; although Cd-Cd distances were closer to those found in bulk CdTe.     

The theoretical and experimental study of the ionization processes during the low energy ion sputtering

The process by which atoms are ionized as they are sputtered from a metal surface has been analyzed both theoretically and experimentally. In the theoretical part the expressions for ionization coefficient R⁺ of atoms having the ionization energy much larger than the metal work function have been derived using a molecular orbital method. The effect of the level crossing was estimated in an approximate way. In the experimental part the SIMS experiments on clean Ni and Al surfaces and on Ni surface covered with a submonolayer of adsorbed K, Na and Al are reported. It has been found and it is for the first time reported that the energy distribution of ions sputtered from a submonolayer of adatoms is independent of energy (200–2500 eV) and mass (Ar⁺ Xe⁺ of incident ions and depends only upon the adsorption energy of the adatom. The energy distribution of ions sputtered from bulk samples has been found dependent on the primary ion energy. The measurement of the absolute value of R⁺ has shown that there is a strong correlation between the number of the adatom valence d-electrons and the value of R⁺, the value of R⁺ being smaller for atoms with more d-electrons. These experimental data have been compared with the theoretical expressions and the important role of the mechanism which takes into account the bending of the adatom energy level has been assessed.     

Surface contributions to the XPS spectra of nanostructured NiO deposited on HOPG

In this work we present an in situ X-ray photoelectron spectroscopy (XPS) study of the growth of NiO on highly ordered pyrolitic graphite (HOPG). The XPS spectra were measured as a function of the equivalent NiO coverage. Also, ex-situ atomic force microscopy (AFM) images were taken for some of these stages in order to follow the morphology of the NiO deposits. For low coverages the lineshapes of the Ni 2p spectra differ strongly from those of bulk NiO. This has been related to the large surface contribution. The O 1s XPS spectra also show a surface related structure which follows the same trend observed in the Ni 2p spectra.     

Properties of graded group IIa fluoride buffer layers for semiconductor heteroepitaxy

Composition graded epitaxial CaF₂-SrF₂-BaF₂ stacks with up to 14% lattice mismatch have been grown onto Si(111) by molecular beam epitaxy (MBE). Despite large lattice- and thermal expansion mismatch, the fluoride layers exhibit specular surfaces, are untwinned, and the minimum ion channeling yield is nearly as low as in bulk material. A unique feature is their ability to relax mechanical stress due to lattice- and thermal expansion mismatch even near room temperature in a nonthermal manner. They were applied as buffer layers to grow device-quality epitaxial lead chalcogenides and CdTe on Si, and may find further applications as epitaxial buffers for still other combinations of semiconductor materials with lattice mismatches of up to 20%.     

Synthesis and optical characterization of nanocrystalline CdTe thin films

From several years the study of binary compounds has been intensified in order to find new materials for solar photocells. The development of thin film solar cells is an active area of research at this time. Much attention has been paid to the development of low cost, high efficiency thin film solar cells. CdTe is one of the suitable candidates for the production of thin film solar cells due to its ideal band gap, high absorption coefficient. The present work deals with thickness dependent study of CdTe thin films. Nanocrystalline CdTe bulk powder was synthesized by wet chemical route at pH≈11.2 using cadmium chloride and potassium telluride as starting materials. The product sample was characterized by transmission electron microscope, X-ray diffraction and scanning electron microscope. The structural characteristics studied by X-ray diffraction showed that the films are polycrystalline in nature. CdTe thin films with thickness 40, 60, 80 and 100 nm were prepared on glass substrates by using thermal evaporation onto glass substrate under a vacuum of 10⁻⁶ Torr. The optical constants (absorption coefficient, optical band gap, refractive index, extinction coefficient, real and imaginary part of dielectric constant) of CdTe thin films was studied as a function of photon energy in the wavelength region 400–2000 nm. Analysis of the optical absorption data shows that the rule of direct transitions predominates. It has been found that the absorption coefficient, refractive index (n) and extinction coefficient (k) decreases while the values of optical band gap increase with an increase in thickness from 40 to 100 nm, which can be explained qualitatively by a thickness dependence of the grain size through decrease in grain boundary barrier height with grain size.     

Investigation of non-acoustical parameters of compressed melamine foam materials

A series of careful non-acoustical parameters measurements using 5 ‘Illtec’ melamine foam and 10 ‘Basotect TG’ melamine foam samples have been made. Flow resistivity, tortuosity, porosity, viscous characteristic length and thermal characteristic length of two types of compressed melamine foam materials with different foam magnifications have been investigated. It has been found that a relationship between the flow resistivity, fibre equivalent diameter and bulk density exists for each of the compressed melamine foam materials.This paper also discusses relationships between the non-acoustical parameters and compression rates in the compressed melamine foam media.     

Core level binding energy shifts in Ni on Au and Au on Ni overlayers

Core level binding energy shifts of the Ni-2p and Au-4? lines have been measured for Ni on Au and Au on Ni overlayers down to mean coverages of less than 0.1 monolayers. When normalized to the maximum shift at submonolayer coverage the Ni on Au and Au on Ni shifts show the same dependence as function of the monolayer coverage. Using the thermodynamical approach for the calculation of binding energy shifts in metals, recently developed by Mårtensson and Johansson, the submonolayer shifts together with experimental results of core level binding energy shifts in dilute NiAu and AuNi alloys are used to calculate surface segregation energies in these alloys. They are compared with semiempirical determinations of these energies by Seah.     

Kaon production in hadronic matter

Subthreshold kaon production has been studied in symmetric nucleus-nucleus collisions as a function of the nucleus mass, beam energy and centrality. In Au+Au collsions at 1 AGeV theK ⁺ multiplicity increases more than linearly with increasing number of participating nucleons. Transport calculations have to assume a soft equation of state in order to reproduce the data. The in-mediumK ^? cross section measured in Ni+Ni collisions is enhanced by about a factor of 7 as compared to the free cross section when using theK ⁺ cross section at equivalent beam energies as a normalization.     

Study on optical and electrical properties of gold-doped silicon fabricated by femtosecond laser

Microstructured silicon (Si) materials have been fabricated by femtosecond (fs) laser ablation and have been hyperdoped with gold (Au) impurities. The ablated Si materials showed large and thermostable infrared absorption at 1.1–2.5 μm wavelengths, which was contributed by sub-band absorption and laser-induced defects absorption. The Au–Si alloy was formed after laser irradiation onto the Au-coated Si surface, which was determined by XRD characterization. Using N-type Si substrate, the fabricated Au-doped Si performed lower sheet carrier density due to the self-compensation effect between deep donor and acceptor energy lever of Au in bulk Si material. From Hall measurement, both the p- and n- types of Au-doped Si samples can be obtained by controlling the type of Si substrate.     

Formation of DX-centers in indium doped CdTe

In CdTe, the achievable n-type doping is limited by the formation of DX-centers. A characteristic feature of DX-centers is the ‘persistent photoconductivity (PPC)’ which is created by illumination at low temperatures and caused by a metastable state of the DX-center. The DX-center and the PPC effect in n-type CdTe are theoretically explained by the ‘large lattice relaxation model’. PAC measurements on In doped CdTe using ¹¹¹In/¹¹¹Cd and, in addition, resistivity measurements on the same samples have been performed. Below 150 K, the samples showed a PPC effect that was accompanied by an increase of about 20% of the carrier concentration. This effect is not accompanied by any changes of the observed EFG. Possible explanations of the EFG observed, originally assigned to the DX-center, will be discussed. Finally, first reports on the investigation of DX-centers in CdTe using the radioactive isotope ¹¹⁷Cd decaying to ¹¹⁷In are presented.