Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure

CdTe/CdS heterojunction solar cell structure has been fabricated using simple, easy and low-cost methods. To fabricate this structure, CdS and CdTe thin films are deposited onto FTO-coated conducting glass substrates by chemical bath deposition (CBD) and electrodeposition method, respectively. The optimized growth conditions are chosen for both CdS and CdTe films by investigating the optical, structural and morphological properties of both the as-deposited and annealed films. Optical measurement showed that CdS films have higher transmittance and lower absorbance, and CdTe films have lower transmittance and higher absorbance in the near infrared region. The band gap of CdS films is estimated to lie in the range 2.29–2.41 eV and that of CdTe films is in the range 1.53–1.55 eV. X-ray diffraction (XRD) study reveals that CdS and CdTe films are polycrystalline with preferential orientation of (1 1 1) plane. Scanning electron microscopy (SEM) study reveals that both films are smooth, void-free and uniformly distributed over the surface of the substrate. Fabricated CdTe/CdS structure showed the anticipated rectifying behaviour, and the rectifying behaviour is observed to improve due to CdCl₂ treatment.     

Photoemission studies of the interactions of CdTe and Te with Si(100)

The interactions between CdTe, and in particular Te, and the (100) surface of Si have been probed using photoemission and low energy electron diffraction with a view to investigating the mechanisms responsible for (100) and (111) growth orientations for CdTe on Si(100). The interfacial reactions have been studied both on room temperature deposition followed by annealing and on depositions at typical epitaxial growth temperatures. In both cases the same precursor stage of an ordered submonolayer of Te on the Si(100) surface has been identified. Line shape analysis of the Si 2p core level has suggested a structural model in which Te adatoms make up an incomplete monolayer bound in bridge sites. This model is in excellent agreement both with the (1 × 1) LEED pattern and recent SEXAFS studies of this surface. The implications of the cubic symmetry of this surface in terms of the subsequent growth orientation of CdTe are discussed. Termination of the surface by Te was also seen to induce band bending suggestive of Fermi level pinning at around midgap, in contrast to the passivating behaviour of other group VI elements on this surface. The Si 2p core level line shape analysis on termination by Te has also provided evidence to support the “covalent dimer” interpretation of the clean dimerised Si(100) surface.     

水溶性CdTe/CdS纳米晶表面SiO2壳层的反相胶束法包覆

"提出了一种水相中制备CdTe/CdS核壳结构纳米粒子的方法.用Te粉作为碲源,用Na2S作为硫源,在50 ℃下制备了CdTe/CdS核壳结构纳米粒子. 用紫外可见吸收光谱和荧光光谱分析了CdS壳层对CdTe核的影响. 随CdS壳层厚度的增加,紫外可见吸收光谱和荧光光谱均发生了红移. CdS壳层厚度较薄时,CdTe/CdS纳米晶的荧光强度较CdTe纳米粒子有显著提高;而CdS壳层厚度较厚时,CdTe/CdS纳米晶的荧光强度会逐渐降低. 用反相胶束法在CdTe/CdS核壳结构纳米粒子的表面包被一层SiO2,     

Silver: a novel growth catalyst for Ge nanoislands on Si(113)

The impact of silver pre‐adsorption on germanium growth on Si(113) was investigated using in‐situ low‐energy electron microscopy (LEEM) as well as low‐energy electron diffraction (LEED). The adsorption of silver leads to the formation of a regular pattern of nanofacets along the [1 0] direction. The periodicity of this pattern in [33 ] direction was determined to (44 ± 4) nm. From LEED series at different energies the facets were identified to be of (111) and (115) orientation. While the (111) facets show a (√3 × √3)‐R30° reconstruction, the (115) facets exhibit a (2 × n) superstructure. The subsequent growth of Ge results in the formation of nanoislands that are aligned along the facets. These Ge islands have an anisotropic shape with typical sizes of about 100 nm in [33 ] direction and 400 nm in [1 0] direction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoreflectance characterization of CdTe thin films grown by Molecular Beam Epitaxy on GaAs(100)

CdTe films have been grown on top of GaAs(100) by means of Molecular Beam Epitaxy (MBE) at 300 °C substrate temperature. Different procedures for the CdTe growth and for the preparation of the GaAs substrates resulted in diverse crystalline qualities of the CdTe films. We present the results obtained from PhotoReflectance (PR) measurements of these films employing HeNe and Ar-ion lasers as modulating excitation. For Ar excitation, the ratio of CdTe to GaAs signal strength for the E ₀ transition is enhanced, allowing a differentiation of the contributions from film and substrate. Both the PR line shape and intensity are correlated to the structural quality of the CdTe films. One of the samples presented a below-band-gap transition of the GaAs substrate around 30±5 meV from E ₀ which is attributed to donor states produced by Te atoms diffused in the interface; this result demonstrates the high sensitivity of the photoreflectance technique to the structural properties of interfaces.     

Towards an understanding of surfactant action in the epitaxial growth of metals: The case of Sb on Ag (111)

We address the role of surfactant adsorbates in determining changes in the homoepitaxial growth mode of metals, discussing the case of Sb on Ag (111). From ab initio calculations, we extract evidence that the mechanism operative in this system is that Sb induces an irregular shape and an increase in density of the growing Ag islands, and an ensuing increase of the number of attempts for an adatom to descend to a lower terrace. This results from a combination of peculiar properties of this system: Sb is adsorbed insubstitutional surface sites, leading to the formation of a Sb–Ag surface alloy; deposited Ag has reduced mobility on Sb-covered Ag (111), from which follows a higher nucleation probability. The island shape is irregular since the surface alloy is disordered. Surface seggregation of Sb once the growing layer is completed furthers the phenomenon for many deposited Ag layers. Our explanation of the surfactant action of Sb on Ag (111) does not require a reduction of the downstep diffusion barrier, which may, however, be a concurrent factor helpful to interlayer mass transport and layerby-layer growth.     

Performance improvement in CdTe solar cells by modifying the CdS/CdTe interface with a Cd treatment

The performance of n-CdS/p-CdTe solar cells is often degraded under light soaking or thermal stress, even though the technology of CdTe solar cells is close to a commercial level. The Cu diffusion from a Cu back contact to a CdS window layer might degrade the cell's performance. To prevent the Cu diffusion, a very-thin intrinsic CdTe layer was introduced at the n-CdS/p-CdTe interface by depositing a very-thin Cd metal layer on the CdS film and converting the Cd metal into intrinsic CdTe during p-CdTe deposition at high temperature. By the Cd treatment on CdS surface, pinholes or voids were eliminated at the CdS/CdTe and the intermixing of Te and S at the interface was much suppressed. The depletion width was much increased and the intensity of LTPL peak was increased. The analysis suggested that an intrinsic CdTe interlayer was formed and the surface recombination rate was suppressed by the intrinsic interlayer. As a result, the short circuit current of the CdTe solar cell was significantly increased due the increased current gain in the blue wavelength region. The thermal stability of the CdTe solar cell was also greatly improved and the Cu diffusion was retarded by the intrinsic CdTe interlayer at the n-CdS/p-CdTe.     

Structure of CdTe thin films grown in quasiclosed volume

A study has been made of how the condensation conditions and the state of the substrate surface affect the structure of CdTe films grown on mica in a quasiclosed volume. Monocrystalline CdTe films on mica grow under conditions close to equilibrium when the coefficient of vapor supersaturation =1.06. The existence of silver islets on the mica surface stimulates the growth of perfect films with a smooth surface under conditions not as close to equilibrium with =1.5.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 101–104, June, 1978.     

The adsorption of In on the surface of (0 0 1) CdTe

To understand CdTe doping with In, first-principle calculations are performed to obtain the various kinds of surface-structure for In on CdTe (0 0 1) surface. Of all the structures examined, the structure of CdTe (0 0 1) as caused by In adsorption atoms at the fourfold hollow sites with 0.25 monolayer coverage is the most energetically favorable. In atoms are adsorbed on the Cd-terminated surface, whereas below the Te-terminated surface. For the Cd-terminated surface, cadmium vacancy can form spontaneously and is energetically favorable. In atoms are likely to be adsorbed/incorporated at an interstitial site on Te-terminated CdTe (0 0 1) surfaces for most of the range of the chemical potential.     

CdSyTe1-y多晶薄膜的制备及光谱表征

采用真空共蒸发法制备了CdSyTe1-y(0≤y≤1)多晶薄膜,并用X射线衍射谱(XRD)、能量色散谱(EDS)研究了CdSyTe1-y多晶薄膜的结构、组分。实验结果表明:石英振荡法监控的组分与EDS谱结果较为一致;当y<0·3时,CdSyTe1-y多晶薄膜为立方结构,当y≥0·3时,CdSyTe1-y多晶薄膜为六方结构。采用XRD线形分析法可计算出CdSyTe1-y多晶薄膜晶粒大小约20～50nm。最后,用紫外-可见-近红外谱(UV-Vis-NIR),测得300～2500nmCdSyTe1-y多晶薄膜的透过率曲线,并结合一阶Sellmeier模型的折射率色散关系,表征了CdSyTe1-y多晶薄膜的光学性质,获得了CdS0·22Te0·78多晶薄膜的光学厚度d～535nm,光能隙Eg～1·41eV,以及吸收系数α(λ)、折射率n(λ)等光学量。结果也表明,采用真空共蒸发法可以制备需要组分的CdSyTe1-y多晶薄膜,对CdSyTe1-y多晶薄膜光学性质的表征方法可推广到其他的半导体薄膜材料。     

Analysis of Bulk and Interface Phenomena in CdTe/CdS Thin-Film Solar Cells

CdTe layers have been grown on CdS layers to produce thin-film photovoltaic devices. Because of the large lattice mismatch of roughly 10%, CdTe and CdS can only be joined at the expense of a high density of misfit dislocations. Additionally, after deposition the CdTe layer contains submicrometer sized, [111] oriented, columnar grains with a high density of stacking faults and microtwins resulting in a poor electrical performance of the p-n junction. The performance of these cells can be improved by depositing a CdCl₂ layer on the CdTe absorber layer and subsequent annealing of the stack in air. This treatment induces interdiffusion of S and Te across the interface, which results in a better lattice match. During this anneal, CdTe is subject to grain growth, recovery and recrystallization. In samples annealed for different durations after different amounts of CdCl₂ were applied, grain growth is completed during the first minutes of annealing. Subsequent diffusion of Cl is detected along the CdTe grain boundaries. The presence of Cl enhances the recrystallization of the CdTe layer, starting from the CdTe surface, while recovery of the CdTe layer, mostly by the reduction of microtwins, takes place at the interface. The simultaneous occurrence of recrystallization and recovery leads to a preferred alignment of grain boundaries in CdTe parallel to the interface. Electron beam induced current measurements show the detrimental effect of these grain boundaries on the charge carrier collection efficiency of the cell. Based on these results, a modified growth procedure is proposed.     

InSb/CdTe heterostructures grown by MBE

InSb/CdTe heterostructures were grown by MBE, including a 10 layer superlattice. The structures were characterized by X-ray diffraction, van der Pauw measurements, and SNMS depth profiling. The interfaces widen because of interdiffusion and through the formation of an interface compound from the reaction of Te with the InSb surface. The interface compound is identified as strained InTe(II). The interfaces are still too wide for practical devices. Thermochemical analysis indicates that the reaction can be suppressed by applying a Cd overpressure. The diffusion of In and Sb in CdTe is very fast and will necessitate a MEE growth scheme at lower temperatures.     

CdTe,核 壳型CdTe/CdS及CdTe/ZnS量子点的合成及表征

在水相中制备了半导体CdTe纳米晶,核 壳型CdTe/CdS和CdTe/ZnS纳米晶（即量子点;QDs）.利用扫描隧道显微镜（STM）和荧光光谱(FS)对合成的纳米晶量子点进行了研究,并且根据FS的数据进行了量子效率的计算.STM的结果表明合成的量子点直径约为3 nm并且分布良好.为了提高量子效率,对Cd2+浓度和Cd2+∶S2－比例等反应条件进行了研究,结果表明随着回流时间的增加,核 壳型量子点CdTe/CdS的量子效率总体上呈下降趋势.CdTe/CdS在pH8.5,Cd2+∶S2－=10∶1（摩尔比）时可获得80.0%的最大量子效率.同时制备了核 壳型量子点CdTe/ZnS,其最大发射波长由551 nm（CdTe）红移到635 nm（CdTe/ZnS）表明量子点的尺寸在增长,但是量子效率下降到14.4%. 当前研究的量子点可适用于生物标记,生物成像,以及基于共振能量转移的生物传感研究.     

Buffer effects of titanium in the case of silver / a - Al 2 O 3(0001)

The influence of thin titanium layers on the growth of silver clusters on - Al ₂ O ₃ (0001) is investigated. We demonstrate through in situ RHEED measurements that titanium can relax stress in a growth mode where two lattice parameters show up simultaneously. Above a certain thickness, the lattice parameter closest to the bulk value of titanium dominates. Depending on the amount of stress in the titanium layer, silver films can either develop 3D textures or grow in epitaxy and form 2D like films.     

Surface micromorphology of CdTe(310) layers grown by molecular beam epitaxy

Surface morphology of CdTe(310) buffer layers grown by molecular beam epitaxy has been investigated by the method of reflection of high-energy electron diffraction. It was established that a clean CdTe(310) surface is atomically flat. Its reconstruction can be described by a unit cell coinciding with the unit cell of the unreconstructed (310) surface. It is determined that Te₂ adsorption in amounts of less than 0.2 monolayers results in the surface reconstruction with the formation of terraces parallel to the (100) plane and are 3/2a long. A system of (100) + (210) facets develops on the CdTe(310) surface with the increase in the Te adsorption layer’s thickness up to 0.3 monolayers and more.     

Pyroelectric coupling in thin film photovoltaics

We propose a theory of thin film photovoltaics in which one of the polycrystalline films is made of a pyroelectric material grains such as CdS. That film is shown to generate strong polarization improving the device open circuit voltage. Implications and supporting facts for the major photovoltaic types based on CdTe and CuIn(Ga)Se₂ absorber layers are discussed.

     

Single domain nonpolar (13$ \bar 4 $0) ZnO on (114) LaAlO3

An unconventional nonpolar plane (13$ \bar 4 $ 0) ZnO epitaxial film was grown on a 2‐inch (114) LaAlO₃ (LAO) substrate by pulsed laser deposition. Reflection high energy electron diffraction (RHEED) patterns of the grown ZnO surface demonstrate single crystalline characteristics with the orientation inclined with the a‐axis. Atomic force microscopy (AFM) shows that the grown ZnO film exhibits a stripe‐like surface morphology with the longitudinal direction parallel to the c‐axis. Cross‐sectional transmission electron microscopy (TEM) with selected area electron diffraction (SAED) was used to characterize the microstructure and to determine the growth plane of ZnO grown film as (13$ \bar 4 $ 0). In addition, XRD pole‐figure measurements confirm the single domain growth of (13$ \bar 4 $ 0) ZnO on (114) LAO. Room temperature photoluminescence spectra of the ZnO film measured across the substrate show the same near band edge emission peak at 3.29 eV, indicating that the nonpolar (13$ \bar 4 $ 0) ZnO film has excellent uniform optical properties. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Persistent layer‐by‐layer growth for pulsed‐laser homoepitaxy of $(000\bar 1)$ ZnO

Persistent layer‐by‐layer growth is demonstrated for pulsed‐laser homoepitaxy of ZnO thin films on $(000\bar 1)$ ZnO single crystals. Employing interval pulsed‐laser deposition (PLD), RHEED oscillations are stabilized over a film thickness of about 90 nm. For interval pulsed laser deposited films a considerably decreased root‐mean‐square surface roughness of 0.26 nm was found, in comparison to 0.74 nm for conventional PLD. A small asymmetry in the X‐ray diffraction (XRD) 2θ –ω scan reveals compressive strain in the thin film being slightly larger for interval PLD as compared to conventional PLD. The FWHM of the photoluminescence (PL) I₆ line is higher with about 500 µeV as compared to 350 µeV for the conventional PLD. Consequently, both XRD as well as PL indicate a slightly higher amount of charged defects for the interval PLD.

     

Initial stage of CdTe on Si(1 0 0) grown by MBE

The initial stage of CdTe growth on silicon has been investigated using angle-resolved photoemission and scanning tunneling microscopy (STM). In order to study initial stage of CdTe on Si, we have desorbed CdTe by annealing at 600 °C so that only one monolayer of Te remains on the Si(1 0 0) substrate. Te/Si(1 0 0)2×1 superstructure has been observed by LEED. Photoemission spectra indicate that Te atoms bond with the Si dangling bond. Atomically resolved STM images reveal that the Te atoms form dimers. It is observed that buckling direction of Te-dimer changes and the dimmers are broken in the site of some dimmer rows. It can be explained that the large lattice mismatch cause the switching of the buckling direction and the breaking of Te-dimer resulted surface relaxation.     

Atomic structure for Mg on InN(0001) surface

Structures of Mg adsorbed on InN(0001) surfaces are theoretically investigated by first-principle calculations. Of all the structures examined, the structure of R30° as caused by Mg adsorption at the Top sites with 1/3 monolayers coverage is most energetically favorable. Mg atoms may also substitute indium atoms, or accumulate at the voids inside InN film. The interstitial Mg defects may act as a potential source of compensation for the p-type behavior of Mg-doped InN at the surface.