Effect of porous silicon layer on the performance of Si/oxide photovoltaic and photoelectrochemical cells

Photovoltaic and photoelectrochemical systems were prepared by the formation of a thin porous film on silicon. The porous silicon layer was formed on the top of a clean oxide free silicon wafer surface by anodic etching in HF/H₂O/C₂H₅OH mixture (2:1:1). The silicon was then covered by an oxide film (tin oxide, ITO or titanium oxide). The oxide films were prepared by the spray/pyrolysis technique which enables doping of the oxide film by different atoms like In, Ru or Sb during the spray process. Doping of SnO₂ or TiO₂ films with Ru atoms improves the surface characteristics of the oxide film which improves the solar conversion efficiency.The prepared solar cells are stable against environmental attack due to the presence of the stable oxide film. It gives relatively high short circuit currents (I_sc), due to the presence of the porous silicon layer, which leads to the recorded high conversion efficiency. Although the open-circuit potential (V_oc) and fill factor (FF) were not affected by the thickness of the porous silicon film, the short circuit current was found to be sensitive to this thickness. An optimum thickness of the porous film and also the oxide layer is required to optimize the solar cell efficiency. The results represent a promising system for the application of porous silicon layers in solar energy converters. The use of porous silicon instead of silicon single crystals in solar cell fabrication and the optimization of the solar conversion efficiency will lead to the reduction of the cost as an important factor and also the increase of the solar cell efficiency making use of the large area of the porous structures.     

ITO靶刻蚀时间对沉积ITO薄膜光电性能的影响

在磁控溅射过程中,通过观察在2～50 h的刻蚀时间下制备的ITO薄膜的表面形貌,研究了在磁控溅射过程中ITO靶表面的黑色凸起的形成过程,并探讨了刻蚀时间对沉积ITO薄膜的光电性能的影响。结果表明,随着刻蚀时间的增长,ITO靶材表面的In和Sn分布发生变化,导致ITO膜不均匀。薄膜的电学和光学性能因结节的形成而显著退化。而当刻蚀时间在40 h以下时,ITO薄膜的光电性能变化不大。然而,当蚀刻时间继续延长,薄膜的光电性能会迅速恶化。     

刻蚀工艺对四面体非晶碳膜生长及其性能的影响

目的研究不同等离子体刻蚀工艺对基体和四面体非晶碳膜(ta-C)的影响,并进一步考察不同电弧等离子体刻蚀时间对ta-C薄膜结构的影响。方法采用自主设计研制的45°单弯曲磁过滤阴极真空电弧镀膜设备,进行不同等离子体刻蚀以及ta-C薄膜的沉积。使用等离子体发射光谱仪表征离子种类及其密度,使用椭偏仪表征薄膜厚度,原子力显微镜表征刻蚀后的基体粗糙度,拉曼光谱仪和XPS表征薄膜结构,TEM分析薄膜的膜基界面结构。结果辉光刻蚀工艺中,作用的等离子体离子以低密度的Ar离子为主;而电弧刻蚀时,作用的等离子体离子为高密度的Ar离子和少量的C离子,并且能够在基体表面形成约15 nm的界面层,并实现非晶碳膜(a-C)的预沉积。随电弧等离子体刻蚀时间增加,ta-C薄膜的sp3含量有所降低。结论相比于辉光刻蚀,电弧刻蚀利于制备较厚的ta-C薄膜。这主要是因为电弧刻蚀时,基体表面形成良好的界面混合层,并预沉积了非晶碳膜,形成a-C/ta-C的梯度结构,有助于增强膜基结合力。     

化学刻蚀法调控铝合金阳极氧化膜的表面结构及防腐性能

为了提高铝合金的耐蚀性能,采用化学刻蚀与阳极氧化相结合的方法在铝合金表面构造了微纳结构,经进一步化学修饰后得到耐蚀性能良好的表面防护膜层。利用扫描电子显微镜、红外光谱仪表征所制备膜层的表面形貌和化学成分,采用激光共聚焦显微镜测定样品的表面粗糙度,通过接触角测量仪和电化学工作站对膜层的润湿性和防腐性能进行表征,考察刻蚀时间对于膜层表面结构和耐蚀性能的影响规律。结果表明:当刻蚀时间为3min时,膜层的耐蚀性能最佳:相对于未经刻蚀的样品腐蚀电位正移了0.15V,腐蚀电流下降了两个数量级。且接触角最大(152°),这是由于此条件下制备的薄膜表面微/纳结构最完整、比例最合理。     

预处理对金刚石薄膜质量及结合力的影响

目的改善硬质合金表面金刚石薄膜的结合力。方法采用热丝CVD法在硬质合金基体上制备金刚石薄膜,研究对比喷砂+一步法、喷砂+两步法、Al Cr N过渡层和传统两步法这四种预处理对金刚石薄膜质量及其结合性能的影响。对预处理后硬质合金基体表面的形貌和粗糙度进行分析,并通过扫描电子显微镜、X射线衍射、拉曼光谱及洛氏硬度计表征金刚石薄膜的形貌、结构及性能。结果喷砂有利于在基体表面获得均匀分布的凹坑,提高金刚石的形核密度及均匀性,尤其改善了金刚石颗粒的团聚现象。Al Cr N过渡层虽然表面粗糙度不高,但有大量的凸起颗粒,提供了极佳的形核点,也在一定程度上优于传统两步法的表面金刚石形核密度。在金刚石薄膜沉积参数不变的前提下,传统两步法预处理获得的涂层结合力为HF4级,喷砂结合一步法和两步法获得的结合力分别达到了HF3级和HF1级,但Al Cr N过渡层的结合力表现较差。结论 Al Cr N过渡层能阻挡Co的扩散,提高了金刚石的纯度,但金刚石膜的内应力较大,结合力差。喷砂和刻蚀复合预处理不仅能提高金刚石的结晶质量和纯度,金刚石薄膜的结合力也得到改善。     

Deposition and etching of fluorocarbon thin films in atmospheric pressure DBDs fed with Ar-CF4-H2 and Ar-CF4-O2 mixtures

The deposition and etching of plasma-polymerized fluorocarbon thin films were studied in filamentary dielectric barrier discharges (FDBDs) fed with Ar-CF₄-H₂ and Ar-CF₄-O₂ mixtures, respectively. The etching/polymerization competition was investigated as a function of the feed composition.Hydrogen addition to CF₄ promotes thin films deposition, with a maximum deposition rate at 20% H₂, and reduces the F/C ratio of the deposit, while the oxygen addition promotes the etching of the plasma-deposited film. It is demonstrated that fluorine atoms can perform the etching of the fluoropolymer also without ion bombardment. The correlation between the trend of the etch rate and the trend of the surface chemical composition of fluoropolymers etched in Ar-CF₄-O₂ mixtures allows to enhance hypotheses on the reaction mechanism and on the role of the different active species involved in plasma-surface interactions.     

ESCA studies of the composition profile of low temperature oxide formed on chromium steels—II. Corrosion in oxygenated water

ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the chemical composition of passive films formed on chromium steels with 4–30% Cr at 25 and 70°C in oxygenated water for different times. The composition-depth profile of the films has been investigated by successive stripping of the film by ion etching.The composition of the film varies with exposure time: short time yields an Fe---Cr ratio corresponding to that of the alloy; prolonged exposure produces increasing enrichment of chromium through the whole layer, with the highest concentration at the outer surface. The passive film generally seems to consist of two layers: outermost one of (Cr(OH)₃) and underneath one of iron-chromium oxide (Fe_1+xCr_2−xO₄. The low alloy steels (3·9 and 7·8%Cr) were passive for a limited time during which the Cr enrichment was high (8 times); after breakdown of the chromium-rich passive layer, it was replaced by FeOOH.When a dry-formed oxide film, which consists mainly of iron oxide, is exposed to water, its composition slowly changes until that of a water-formed film is reached.     

Application of surface charge lithography to nanostructuring of GaN epilayers

It is shown that treatment of GaN epilayers by a low energy low dose focused ion beam with subsequent photoelectrochemical etching represent an efficient tool for GaN nanostructuring. Direct “writing” of a surface negative charge trapped by radiation defects allows one to fabricate thin GaN walls with a thickness as low as 100 nm using focused ion beam treatment. The obtained results show that the undercut etching inherent to GaN etching through windows defined by surface charge lithography depends on the depletion length in the doped GaN material and does not occur in the structures below a critical size of 200 nm in our case. The text was submitted by the authors in English.     

酸蚀刻剂对钛基体及其IrO2-PbO2涂层表面形貌及电化学性能的影响研究

采用热分解法制备Ti/IrO₂-PbO₂阳极,深入研究硫酸、硝酸、盐酸、草酸、盐酸/草酸蚀刻顺序对Ti/IrO₂-PbO₂阳极性能的影响规律,借助场发射扫描电子显微镜、X射线衍射、循环伏安法、线性扫描伏安法、电化学交流阻抗谱、加速寿命试验等考察钛基体及氧化物涂层的形貌、结构及电化学行为。结果表明：钛基体在双酸中的腐蚀效果优于单酸,获得更致密更均匀的表面结构。双酸腐蚀使钛表面拥有完整的TiH_x晶型,有助于提高涂层负载量,增强活性层与基体的结合力。改变双酸蚀刻顺序对阳极电化学性能有一定的影响。草酸/盐酸蚀刻剂处理Ti/IrO₂-PbO₂阳极具有最佳的电催化活性与最长的加速寿命。     

Evolution of the structural and magnetotransport properties of magnetite films depending on the temperature of their synthesis on the SiO<Subscript>2</Subscript>/Si(001) surface

The methods of transmission and reflection electron diffraction have been used to investigate the structure of Fe₃O₄ films depending on the temperature of their synthesis on an Si substrate coated with an ultrathin layer of SiO₂. The thus-grown polycrystalline films of magnetite had a texture, the axis of which was perpendicular to the surface of the SiO₂ film. It has been revealed that, with an increase in the growth temperature, a structural rearrangement occurs which is characterized by an increase in the volume fraction of grains with the preferred (311) orientation. A study of the magnetotransport properties of the films has shown that the magnitude of their magnetoresistance increases with an increase in the temperature of their synthesis. It has been established that in the Fe₃O₄/SiO₂/Si system with a tunneling-thin layer of SiO₂ the magnetoresistance decreases as a result of the flow of an electric current through the silicon substrate.     

Esca-studies of the composition profile of low temperature oxide formed on chromium steels—I. Oxydation in dry oxygen

ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the composition of oxide films formed on chromium steels with 6–20%Cr at room temperature in pure oxygen at 0·2 atm. during different times from 2 min to 46 h. Ion etching was used to clean the surface before oxidation and to investigate the composition profile by successive stripping of the oxide layer.It is demonstrated that the total chromium content in the oxide is proportional to the concentration in the metal, but that it varies with depth inside the film. At the oxide-gas boundary the chromium concentration is less than in the metal, while chromium is enriched at the metal phase boundary. It is suggested that the oxide consists of (Fe,Cr)₂O₃ in the outermost zone and FeCr₂O₄ at the inside, with the reservation that the method is not sensitive to deviations from stoichiometry.The thickness of the layer has been determined as 25 Å from the intensity ratio of the deconvoluted ESCA peaks of the metallic and oxidized states. The thickness and the composition profile of the film do not vary noticeably with the oxidation time.     

Prevention of hillock formation during micro-machining of silicon by using OTS-SAM and SiO2 coatings

The feasibility of using a dual coating system consisting of SiO₂ and OTS-SAM thin films on the micro-machining characteristics of silicon wafer were investigated with the aim to eliminate the formation of undesirable hillocks. The outermost OTS-SAM coating was used as a sacrificial layer to pattern the SiO₂ film, which in turn served to pattern the silicon substrate. After selectively removing the OTS-SAM coating by micro-machining, HF and KOH chemical etching processes followed to remove the SiO₂ layer and create patterns on the silicon substrate. By this process, groove patterns of about 1 μm width could be successfully fabricated on a silicon wafer without the formation of undesirable hillocks.     

Metal-assisted etching of p-silicon—Pore formation and characterization

Etching of silicon and formation of definite porous surfaces can be carried out by different methods. Metal-assisted etching represents a convenient method for the application of induced etching for beneficial applications. Porous silicon layers (PSL) on Si are useful and important in solar energy conversion and optoelectronics. Porous silicon on silicon increases the effective area and thus higher optical absorption as well as solar conversion efficiency can be achieved. The effective optical properties of PSL have found great interest in optoelectronics. In the last few years PSL of definite pore structures have been prepared by metal-assisted etching of p-Si in aqueous hydrofluoric acid solutions containing different oxidizing agents. Potassium dichromate, at definite concentration and after optimum etching time of p-Si on which Pt nuclei were electroless deposited, has shown promising effects. The effect of etching time, K₂Cr₂O₇ concentration and HF concentration on the main characteristics of the porous structure was investigated and discussed. In this respect electrochemical impedance spectroscopy (EIS) was used. The experimental data were fitted to theoretical data according to a proposed electrical equivalent circuit model. The morphology of the formed layers and surface contaminations were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques.The results have shown that PSL with nano and micro pores were formed on p-Si when etched in HF-K₂Cr₂O₇ aqueous solutions. At 22.0 mol L⁻¹ HF and relatively high concentration of K₂Cr₂O₇ [>0.05 mol L⁻¹] a passive K₂SiF₆ layer was formed on the Si surface with a thickness that is affected by the concentrations of both HF and K₂Cr₂O₇. The passive K₂SiF₆ layer reduces the effectiveness of the PSL in both the solar conversion process and also its electrical and optical characteristics.     

Deposition and characterization of CuInSe2 films for solar cells using an optimized chemical route

CuInSe₂ (CISe) thin films have been deposited on glass using successive ionic layer adsorption and reaction (SILAR). The as-deposited films are treated at 400 °C in argon atmosphere and etched in KCN solution to remove detrimental secondary phases. The preparation and temperature of the precursor solutions, the duration of the reaction cycles and the duration of the annealing stage have been optimized. The films have been characterized employing grazing incident X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive scanning spectroscopy. Relevant semiconductor parameters have been calculated. Photoelectrochemical tests confirm p-type conduction. The films are crystalline and the stoichiometry can be improved by renewing the precursor solution after completing half of the cycles, annealing for 90 min and later etching in KCN. The quality of the material seems to be promising for application in solar cell devices.     

Glass surface modification by lithography-free reactive ion etching in an Ar/CF4-plasma for controlled diffuse optical scattering

Micro- and nanostructuring of glass surfaces is normally realized by pattern transfer via a lithographical process including wet chemical or reactive ion (i.e. dry) etching. In this contribution we introduce a technology for roughening glass surfaces without lithography. We make use of self-masking that appears under certain process parameters during reactive ion etching in an Ar/CF₄-plasma. This way a multitude of different surface roughness morphologies can be realized. In a first step an about 10 nm thin unstructured metallic layer (preferably of Cu) is deposited onto the cleaned glass substrates. This layer is not laterally structured by lithography, but rather by nucleation or coalescence. These metal spots impose an initial etch pattern on the glass surface, which is getting increasingly coarse during further etching. Choice of the metal layer and variation of the etch parameters like etch time, total pressure, gas fluxes of Ar and CF₄, ion energy, and plasma density affect the resulting morphology. Each morphology is characterized by its root-mean-square surface roughness and the correlation length of its height profile. We mainly used borosilicate thin glass D263?T from Schott AG, Mainz, Germany as substrates. The resulting individual surface structures like cones, hemispheres, pits, and trenches possess typical dimensions of 0.1 to 0.8 μm. The diverse surface roughness morphologies show individual scattering characteristics so that nearly any proportion of diffuse to total transmitted light power is realized. Thus user-defined light diffusers are feasible. The process can be scaled up to large substrate sizes.     

Growth of AlN Films and Its Process Development for the Fabrication of Acoustic Devices and Micromachined Structures

AlN films were grown on silicon substrates by RF reactive magnetron sputtering. At high sputtering powers, (002) preferred orientation as well as Al-N absorption band becomes prominent. The surface roughness and grain size of sputtered films were found to increase with RF power. Surface acoustic wave (SAW) device has been made on the grown (002) oriented piezoelectric AlN film with interdigital transducer (IDT) electrodes spacing corresponding to a wavelength of 60 μm. The centre frequency of the SAW filter was found to be 84.304 MHz, which gives a phase velocity of 5058 m/s with an electromechanical coupling coefficient (K ²) of 0.34%. Low etch rate of AlN films were observed in doped TMAH solution. Three-dimensional suspended Cr/AlN/Cr/SiO₂ microstructures were also fabricated by wet chemical etching.     

Electrochemical and morphological characterization of electrodeposited poly(2,2′:5′,2″-terthiophene) for photovoltaic applications

The growth, structure and morphology of polyterthiophene films has been characterised using a range of electrochemical and structural techniques. The kinetics of the growth process, including the diffusion coefficient of the terthiophene monomer and the rate constant of the polymerization process has been measured. The electrochemical analysis is consistent with a Stranski–Krastanov growth mode involving the completion of a dense precursor layer prior to the growth of a more porous bulk overlayer. Fabricating polyterthiophene films under diffusion-controlled conditions appears to maximise the thickness of the dense precursor layer. The implication of these results on the photovoltaic energy conversion efficiency of photoelectrochemical cells fabricated from electropolymerized terthiophene is discussed.     

Improved electromigration reliability of surface acoustic wave devices using Ti/Al-Mo/Ti/Al-Mo electrodes

In order to obtain both high electromigration (EM) reliability and fine-dimensional control in high-frequency surface acoustic wave (SAW) devices, 4-layered Ti/Al-Mo/Ti/Al-Mo electrode films were investigated on 128° Y-X LiNbO₃ substrates by sputtering deposition. The results indicated that the 4-layered films had an improved EM reliability compared to conventional Al-0.5wt.%Cu films. Their lifetime is approximately three times longer than that of the Al-0.5wt.%Cu films tested at a current density of 5 × 10⁷ A/cm² and a temperature of 200°C. Moreover, the 4-layered films were easily etched in reactive ion etching and fine-dimensional control was realized during the pattern replication for high-frequency SAW devices. For the 4-layered films, an optimum Mo quantity and sputtering parameters were very significant for high EM reliability.     

Improvement of epitaxy and crystallinity in YBa2Cu3Oy thin films grown on silicon with double buffer of ECO/YSZ

A novel double buffer of Eu2CuO4 （ECO）/YSZ （yttrium-stabilized zirconia） was developed for growing YBa2Cu3Oy （YBCO） thin films on Si substrates. In these films, the severe reaction between Si and YBCO is blocked by the first YSZ layer, whereas, the degradation of crystallinity and superconductivity in the grown YBCO is greatly improved by the second ECO layer. Such an ECO material possesses a very stable 214-T＇ structure and excellent compatibilities with YBCO and YSZ. The result shows that the epitaxy and crystallinity of YBCO deposited on Si could be considerably enhanced by using the ECO/YSZ double buffer. The grown films are characterized by high-resolution X-ray diffraction, grazing incidence X-ray reflection, and transmission electron microscopy （TEM）, respectively. It is found that well defined interfaces are formed at YBCO/ECO/YSZ boundaries. No immediate layer could be seen. The defect density in all grown layers is kept at a lower level. The YBCO film surface turns out to be very smooth. These films have full superconducting transitions above 88 K and high current carrying capacity at 77 K. The successful growth of highly epitaxial YBCO thin films on silicon with ECO/YSZ buffer, demonstrate the advantages of such a double buffer structure.