Electronic properties of cleaved CdTe(110) surfaces

Photoemission yield spectroscopy measurements were performed on a set of n- and p-doped CdTe single crystals. The surfaces were obtained by cleavage in ultrahigh vacuum and characterized by low energy electron diffraction and Auger electron spectroscopy. On clean and properly cleaved surfaces, no band bending was found, neither on n- nor on p-type samples, showing the absence of intrinsic surface states in the gap. The ionization energy is found at 5.80±0.05 eV. Oxygen adsorption removes defect-induced surface states on the valence band side of the gap and develops a band bending on n-type samples which indicates the presence of acceptor surface states in the gap down to 0.70 eV below the conduction band edge. The ionization energy remains constant.     

First-principles study of the band gap tuning and doping control in CdSe_xTe_(1-x) alloy for high efficiency solar cell

CdTe is one of the leading materials for low cost, high efficiency thin-film solar cells with a nearly ideal band gap of 1.48 eV. However, its solar to electricity power conversion efficiency(PCE) is hindered by the relatively low open circuit voltage(VOC) due to intrinsic defect related issues. Here, we propose that alloying CdTe with CdSe could possibly improve the solar cell performance by reducing the "ideal" band gap of CdTe to gain more short-circuit current from long-wavelength absorption without sacrificing much VOC. Using the hybrid functional calculation, we find that the minimum band gap of the CdTe_(1-x)Se_x alloy can be reduced from 1.48 eV at x = 0 to 1.39 eV at x = 0.32, and most of the change come from the lowering of the conduction band minimum. We also show that the formation of the alloy can improve the p-type doping of CuCdimpurity based on the reduced effective formation energy and nearly constant effective transition energy level, thus possibly enhance VOC, thus PCE.     

The observation of a direct energy band gap for CuInTe2 single crystals using electroreflectance techniques

The band gap of CuInTe₂ has been determined with the aid of electroreflectance measurements. The single crystal samples were produced by a zone growth technique and exhibited p-type electrical characteristics. Both electrolyte and dry sandwich techniques have been used and we find that the direct energy gap has values of 1.06 eV at 290 K and 1.12 eV at 77 K.     

Surface photovoltage and internal photoemission at the anodized InSb surface

Surface photovoltage measurements have been made as a function of wavelength and temperature on a number of variously doped samples of n- and p-type InSb in the carrier concentration range of 8.9 × 10¹³ to 1.0 × 10¹⁸ cm^?3. The measurements were made using an MIS sandwich employing for the dielectric an anodically formed layer of In₂ O₃. Differential capacitance measurements have shown that, when cooled in the dark, the surface of the n-type material is near flat band whereas that of the p-type is depleted. Illumination with photons of energy in excess of ～ 1.5 eV leads to a shift of the surface potential to larger negative values presumably as a result of optical activation of electrons from fast interfacial surface states to slow states near the InSb surface. Internal photoemission measurements lend support to this model and suggest that, in the absence of any applied bias an internal field within the oxide causes the electrons excited from the semiconductor to move towards the metal. A theory for the surface photovoltage in the presence of a continuum of surface states is developed. It is concluded from theory, and supported by experiment, that surface trapping as well as recombination can exert a considerable influence on the photovoltaic response.     

Characterization of thermal, optical and carrier transport properties of porous silicon using the photoacoustic technique

In this work, the porous silicon layer was prepared by the electrochemical anodization etching process on n-type and p-type silicon wafers. The formation of the porous layer has been identified by photoluminescence and SEM measurements. The optical absorption, energy gap, carrier transport and thermal properties of n-type and p-type porous silicon layers were investigated by analyzing the experimental data from photoacoustic measurements. The values of thermal diffusivity, energy gap and carrier transport properties have been found to be porosity-dependent. The energy band gap of n-type and p-type porous silicon layers was higher than the energy band gap obtained for silicon substrate (1.11 eV). In the range of porosity (50-76%) of the studies, our results found that the optical band-gap energy of p-type porous silicon (1.80-2.00 eV) was higher than that of the n-type porous silicon layer (1.70-1.86 eV). The thermal diffusivity value of the n-type porous layer was found to be higher than that of the p-type and both were observed to increase linearly with increasing layer porosity.     

On the electronic structure of cleaved GaAs(110) surfaces exposed to formic acid

GaAs(110) surfaces cleaved in UHV and exposed to HCOOH have been studied by work function measurements (Kelvin method), electron energy loss spectroscopy (ELS) and by low energy electron diffraction (LEED). From the different changes of the work function on n- and p-type material information about intrinsic and extrinsic surface states is derived. In the loss spectra the adsorbed formate species causes a loss near 9 eV. The intensity of the loss near 20 eV generally ascribed to an excitonic transition from the Ga 3d core level into surface states is reduced only by a factor of two after saturation with HCOOH. This might be related to the c(2 × 2) superstructure observed in LEED, which suggests a saturation coverage of half a monolayer.     

Influence of the thickness of CdTe and ZnTe layers on cathodoluminescence spectra of strained CdTe/ZnTe superlattices with quantum-dot layers

The influence of the thickness of ZnTe barrier layers on the cathodoluminescence spectra of strained CdTe/ZnTe superlattices containing layers of quantum dots with an average lateral size of approximately 3 nm has been investigated. In samples with thick barrier layers (30, 15 nm), the cathodoluminescence spectra of quantum dots exhibit one band with a maximum at E = 2.03 eV. It has been revealed that, at a barrier layer thickness of ∼3 nm, the luminescence band is split. However, at a ZnTe layer thickness of 1.5 nm, the luminescence spectrum also contains one band. The experimental results have been interpreted with allowance made for the influence of elastic biaxial strains on the energy states of light and heavy holes in the CdTe and ZnTe layers. For the CdTe/ZnTe heterostructure with quantum dots in which the thickness of the deposited CdTe layer is 1.5 monolayers and the thickness of the barrier layer is 100 monolayers, the cathodoluminescence spectrum contains 2LO-phonon replicas. This effect has been explained by the resonance between two-phonon LO states and the difference between the energy states in the electronic spectrum of wetting layer fragments.     

Surface photovoltage studies of n-type and p-type InP

Surface photovoltage spectroscopy (SPV) was used to study the initial stages of oxidation of single crystal InP(110) in an attempt to understand the nature and origin of the surface states that develop. Distinct surface states were seen to develop on n-type as the surface was exposed to oxygen. A surface state, associated with cleavage damage, was also observed on p-type. A detailed fit to the experimental data was made by using a model of the dependence of surface charge on photon energy. This was used to unfold the position and intensity of the states. States trailing into the band gap from the bulk bands were seen on both n- and p-types. The analysis also indicated that pairs of isolated states, a donor and an acceptor state, were produced. On p-type, these were present on the clean, cleaved surface while they developed with oxygen exposure on n-type. These states are consistent with the point defect states proposed by the unified defect model. The time response of the SPV signal was also recorded for these surfaces. They were analysed by careful fitting to a model describing the charging and discharging characteristics. This revealed that the midgap state on n-type had a fast and a slow component.     

Modification of Al/Si interface and Schottky barrier height with chemical treatment

The influence of different chemical treatments on the electrical behaviour of n- and p-type Al/Si Schottky junctions was studied. A Schottky barrier height of 0.91 eV was achieved on p-type Si probably due to the unpinning of the Fermi-level at the Al/Si interface. This is one of the highest barrier height values reported so far for a solid-state Schottky junction prepared to p-Si. A doping level reduction was observed in the vicinity of the Si surface for wafers with native oxide and for those boiled in acetone or annealed in forming gas. It was observed unexpectedly that the reactive plasma etch used for the formation of mesa structures decreases the apparent Schottky barrier height. The relation between the sum of n- and p-type Schottky barrier heights and forbidden gap is discussed.     

Energy gap revealed by low-temperature scanning-tunnelling spectroscopy of the Si(111)-7×7 surface in illuminated slightly doped crystals

Physical properties of the Si(111)-7×7 surface of low-doped n- and p-type Si samples are studied in the liquid helium temperature region by scanning-tunnelling microscopy and spectroscopy. Conduction required for the study is provided by illumination of the surface. Application of illumination completely removes the band bending near the surface and restores the initial population of the surface states. Our results indicate the existence of the energy gap 2Δ?=?40?±?10?meV in the intrinsically populated Si(111)-7×7 surface.     

Surface states in Si(111)2×1 and Ge(111)2×1 by optical reflectivity

Surface states in Si(111)2×1 and Ge(111)2×1 are detected by the method of the change of external reflectivity, both at energies below and above the gap. Optical transitions at 2.6 eV in Si and at 1.8 eV and 3.1 eV in Ge, as well as the already known transitions below the gap are observed.     

The role of extended defects in the formation of the spectrum of electronic states in high-purity p-CdTe polycrystals with stoichiometric composition

The spectra of the low-temperature photoluminescence in the cases of band-to-band and subgap laser excitation and the luminescence-excitation spectra of high-purity [111]-textured p-CdTe polycrystals with stoichiometric composition were measured. The spectrum of electronic states in the 1.3–1.6 eV energy range and the changes in this spectrum as a result of annealing of the samples in cadmium vapor were studied. It is shown that the electron levels located in the CdTe band gap that are responsible for the Z and Y luminescence bands (?ω=1.36 and 1.47 eV) are formed as a result of interaction of extended defects with background impurities.     

Free excitons at room temperature in cadmium telluride : A photoelectrochemical evidence

Photoelectrochemical experiments with n-type cadmium telluride electrodes are reported. When the semiconductor samples are sufficiently pure and exhibit a low free carrier concentration, the photocurrent vs. photon energy characteristic, at fixed potential, presents an intense and narrow peak which may be correlated to the excitonic absorption of CdTe. A forbidden gap energy value is deduced E_g = 1.475 eV at room temperature.     

掺杂非晶氧化硅薄膜中三元化合态与电子结构的第一性原理计算

基于密度泛函理论和分子动力学方法,研究了ITO-SiO_x(In,Sn)/n-Si异质结光伏器件中非晶SiO_x层的氧化态和电子结构.计算结果表明:具有钝化隧穿功能的超薄(2 nm)非晶SiO_x层,是由In,Sn,O,Si四种元素相互扩散形成的,其中In,Sn元素在SiO_x网格中以In-O-Si和Sn-O-Si成键态存在,形成了三元化合物.In和Sn的掺杂不仅在SiO_x的带隙中分别引入了E_v+4.60 eV和E_v+4.0 eV两个电子能级,还产生了与In离子相关的浅掺杂受主能级(E_v+0.3 eV).这些量子态一方面使SiO_x的性能得到改善,在n-Si表面形成与反型层相衔接的p-型宽禁带"准半导体",减少了载流子的复合,促进了内建电场的建立.另一方面有效地降低了异质结势垒高度,增强了ITO-SiO_x(In,Sn)/n-Si光伏器件中光生非平衡载流子的传输概率,促进了填充因子的提升(72%).     

Mn掺杂6H-SiC的第一性原理研究

本文通过密度泛函理论第一性原理平面波超软赝势计算方法计算了Mn掺杂6H-SiC的电子结构与光学性质。计算结果显示掺杂Mn后的6H-SiC为间接带隙p型半导体,且带隙较本征体有所降低,带隙由2.022 eV降为0.602 eV,电子从价带跃迁所需能量减少。掺杂后的Mn的3d能级在能带结构中以杂质能级出现,提高了载流子浓度,导电性增强。光学性质研究中,掺杂Mn后的介电函数虚部在低能处增加,电子激发态数量增多,跃迁概率增大。掺杂后的光吸收谱能量初值也较未掺杂的3.1 eV扩展到0 eV,反射谱发生红移。由于禁带宽度的降低使得光电导率起始范围得到扩展。     

Identification of the double and single acceptor state of isolated NiGa in GaAs

The question of the exact energy positions of isolated Ni_Ga as well as their optical and electrical properties have not yet been fully clarified in GaAs. We present a systematic study by deep-level transient spectroscopy, deep-level optical spectroscopy and optical absorption performed on several Ni-doped GaAs materials: n- and p-type LEC (liquid-encapsulated Czochralski) grown and p-type layers grown by liquid-phase epitaxy (LPE). All the electrical and optical results are up to now relatively coherent with the following identifications: (i) the double-acceptor charge state (Ni⁺ /Ni²⁺) is at E_c - 0.4 eV, (ii) the single-acceptor charge state (Ni²⁺ / Ni³⁺) is at E_v + 0.2 eV. However, when Ni is introduced during LPE in p-type materials we do not detect the Ni²⁺ /Ni³⁺ level which suggest a very low solubility of Ni in the LPE growth conditions.     

CdTe aggregates in KBr crystalline matrix

In this work, we report the experimental results on the fabrication and optical characterization of Czochralski (Cz) grown KBr single crystals doped with CdTe crystallites. The results of the optical absorption have shown two bands, the first one located at 250 nm demonstrates the incorporation of cadmium atoms in the KBr host followed by a partial chemical decomposition of CdTe, the second band located at 585 nm shows an optical response of CdTe aggregates. Photoluminescence spectra at room temperature before annealing showed a band located at 520 nm (2.38 eV), with a blue shift from the bulk gap of 0.82 eV (E_g (CdTe)=1.56 eV). While the photoluminescence spectra after annealing at 600 °C showed a band situated at 640 nm (1.93 eV), these bands are due to band-to-band transitions of CdTe nanocrystals with a blue shift from the bulk gap at 0.38 eV. Blue shift in optical absorption and photoluminescence spectra confirm nanometric size of dopant. X-ray diffraction (XRD) spectra have shown the incorporation of CdTe aggregates in KBr.     

Effect of annealing on properties of electrochemically deposited CdTe thin films

Thin films of CdTe have been deposited onto stainless steel and fluorine-doped tin oxide (FTO)-coated glass substrates from aqueous acidic bath using electrodeposition technique. The different preparative parameters, such as deposition time, bath temperature and pH of the bath have been optimized by photoelectrochemical (PEC) technique to get good quality photosensitive material. The deposited films are annealed at different temperature in presence of air. Annealing temperature is also optimized by PEC technique. The film annealed at 200 °C showed maximum photosensitivity. Different techniques have been used to characterize as deposited and also as annealed (at 200 °C) CdTe thin film. The X-ray diffraction (XRD) analysis showed the polycrystalline nature, and a significant increase in the XRD peak intensities is observed for the CdTe films after annealing. Optical absorption shows the presence of direct transition with band gap energy 1.64 eV and after annealing it decreases to 1.50 eV. Energy dispersive analysis by X-ray (EDAX) study for the as-deposited and annealed films showed nearly stoichiometric compound formation. Scanning electron microscopy (SEM) reveals that spherically shaped grains are more uniformly distributed over the surface of the substrate for the CdTe film.     

分子束外延生长[111]晶向CdTe的研究

本文采用分子束外延(MBE)方法在BaF₂衬底上直接外延生长了CdTe(111)薄膜. 反射高能电子衍射(RHEED)实时监控生长表面, 衍射图样揭示了CdTe(111)在BaF₂表面由二维生长向三维生长的变化过程．XRD表征验证了外延生长的CdTe薄膜的单晶性质．由红外透射光谱测量和理论拟合相结合, 得到了CdTe外延薄膜室温带隙宽度E_g=1.511 eV.     

光谱分析气体状态对近空间升华沉积CdTe多晶薄膜的影响

在CdTe多晶薄膜太阳电池制备中用近空间升华法生长了CdTe多晶薄膜,沉积工作气体的状态决定了薄膜的结构、性质。文章首先分析了近空间沉积的物理机制,测量了近空间沉积装置内的温度分布,使用氩氧混合气体为工作气体,其中重点讨论了该气体状态(包括气氛和气压)与薄膜的初期成核的关系,即择优取向程度和光能隙与气氛和气压的关系。结果表明,(1)不同气氛下沉积的CdTe薄膜均为立方相结构。随氧浓度的增加,σ增加,氧浓度为6%时,σ最大,之后随氧浓度增加,σ降低,在12%达到最小,然后随氧浓度的增加而增加, 在氧浓度为9%时沉积的结晶更完整。CdTe薄膜的光能隙为1.50～1.51 eV;(2)在氩氧气氛下氧浓度为9%,不同气压下制备的样品,均有立方相CdTe, 此外, 还有CdS和SnO₂:F衍射峰。CdTe晶粒随气压增加有减小趋势,随气压的增加,透过率呈下降趋势,相应的CdTe吸收边向短波方向移动;(3)在氩氧气氛下氧浓度为9%,采用衬底温度550 ℃,源温度620 ℃,沉积时间4 min时制备的CdTe多晶薄膜获得了转换效率优良的结构为SnO₂:F/CdS/CdTe/Au的集成电池。