Time–Temperature–Transformation (TTT) Diagrams for Crystallization of Metal Oxide Thin Films

Time–temperature–transformation (TTT) diagrams are proposed for the crystallization of amorphous metal oxide thin films and their specific characteristics are discussed in comparison to glass‐based materials, such as glass‐ceramics and metallic glasses. The films crystallize from amorphous to full crystallinity in the solid state. As an example the crystallization kinetics for a single‐phase metal oxide, ceria, and its gadolinia solid solutions are reported made by the precipitation thin‐film method spray pyrolysis. The crystallization of an amorphous metal oxide thin film generally follows the Lijschitz–Sletow–Wagner (LSW) Ostwald ripening theory: Below the percolation threshold of 20 vol% single grains crystallize in the amorphous phase and low crystallization rates are measured. In this state no impact of solute on crystallization is measurable. Once the grains form primary clusters above the threshold the solute slows down crystallization (and grain growth) thus shifting the TTT curves of the doped ceria films to longer times and higher temperatures in comparison to undoped ceria. Current views on crystallization of metal oxide thin films, the impact of solute dragging, and primary TTT diagrams are discussed. Finally, examples on how to use these TTT diagrams for better thermokinetic engineering of metal oxide thin films for MEMS are given, for example, for micro‐Solid Oxide Fuel Cells and resistive sensors. In these examples the electrical properties depend on the degree of crystallinity and, thereby, on the TTT conditions.     

Revisiting Metal Sulfide Semiconductors: A Solution‐Based General Protocol for Thin Film Formation,Hall Effect Measurement,and Application Prospects

Nanostructured thin films of metal sulfides (MS) are highly desirable materials for various optoelectronic device applications. However, a general low‐temperature protocol that describes deposition of varieties of MS structures, especially in their film form is still not available in literatures. Here, a simple and highly effective general solution‐based deposition protocol for highly crystalline and well‐defined nanostructured MS thin films from ethanol on variety of conducting and non‐conducting substrates is presented. The films display remarkable electronic properties such as high carrier mobility and high conductivity. When NiS thin film deposited on a flexible polyethylene terephthalate (PET) substrate is used as a fluorine doped tin oxide (FTO)‐free counter electrode in dye‐sensitized solar cells, it exhibits a solar‐to‐electric power conversion efficiency of 9.27 ± 0.26% with the highest conversion efficiency as high as 9.50% (vs 8.97 ± 0.07% exhibited by Pt‐electrode). In addition, the NiS film deposited on a Ti‐foil has demonstrated an outstanding catalytic activity for the hydrogen and oxygen evolution reactions from water.     

Crystallization and Microstructure of Yttria‐Stabilized‐Zirconia Thin Films Deposited by Spray Pyrolysis

The crystallization and microstuctural evolution upon thermal treatment of yttria‐stabilized zirconia (YSZ, Zr_0.85Y_0.15O_1‐δ) thin films deposited by spray pyrolysis at 370 °C are investigated. The as‐deposited YSZ films are mainly amorphous with a few crystallites of 3 nm in diameter and crystallize in the temperature range from 400 °C to 900 °C. Fully crystalline YSZ thin films are obtained after heating to 900 °C or by isothermal dwells for at least 17 h at a temperature as low as 600 °C. Three exothermic heat releasing processes with activation energies are assigned to the crystallization and the oxidation of residuals from the precursor. Microporosity develops during crystallization and mass loss. During crystallization the microstrain decreases from 4% to less than 1%. Simultaneously, the average grain size increases from 3 nm to 10 nm. The tetragonal phase content of the YSZ thin film increases with increasing temperature and isothermal dwell time. Based on these data, gentle processing conditions can be designed for zirconia based thin films, which meet the requirements for Si‐based microfabrication of miniaturized electrochemical devices such as micro‐solid oxide fuel cells or sensors.     

Optical and electrochemical properties of polyether derivatives of perylenediimides adsorbed on nanocrystalline metal oxide films

We report optical and electrochemical properties of polyether derivatives of perylenediimides (PDIs) thin films formed in various materials (semiconductor, insulator, amorphous and self-assembly). Perylenediimides adsorbed on nanocrystalline TiO₂ (NT) nanocrystalline alumina (NA), amorphous silicon (PS) and neat self-assemblied (SA) films were prepared and characterized based on spectroscopic, electrochemical, spectro-electrochemical techniques. The absorption and fluorescence spectra of PDIs in chloroform exhibit vibronic features. The fluorescence quantum yields (Φ_f) of PDIs with end amino substituents in chloroform solutions are over 0.95, while the quantum yield of triethoxyphenyl substituted PDI Φ_f value is 0.024 in solution. Optical spectroscopy proves that PDIs in metal oxide thin films form aggregated type complexes. An electrochromism, a color change from red to blue/violet, is observed on metal oxide films, that indicates existence of mono and dianion forms of PDIs. Reversibility of electrochemical reductions in NT film depends on the scanning rate. However, electrochromism in NA films is stable and reversibility is independent from scanning rate. Stable mono and diaionic species are formed on NA films. SA films show broad absorption peaks during the voltammetric scan. On the other hand, the first reduction onset potentials of PDIs are almost equal to the onset potential of capacitive current of TiO₂ which lead to low efficiency in dye-sensitized solar cells.     

Properties of Flame Sprayed Ce0.8Gd0.2O1.9‐δ Electrolyte Thin Films

Thin films of Ce_0.8Gd_0.2O_1.9‐δ (CGO) are deposited by flame spray deposition with a deposition rate of about 30 nm min^?1. The films (deposited at 200 °C) are dense, smooth, and particle‐free and show a biphasic amorphous/nanocrystalline microstructure. Isothermal grain growth and microstrain are determined as a function of dwell time and temperature and correlated to the electrical conductivity. CGO films annealed for 10 h at 600 °C present the best electrical conductivity of 0.46 S m^?1 measured at 550 °C. Reasons for the superior performance of films annealed at low temperature over higher‐temperature‐treated samples are discussed and include grain‐size evolution, microstrain relaxation, and chemical decomposition. Nanoindentation measurements are conducted on the CGO thin films as a function of annealing temperature to determine the hardness and elastic modulus of the films for potential application as free‐standing electrolyte membranes in low‐temperature micro‐SOFCs (solid oxide fuel cells).     

Rigorous optical modeling and optimization of thin‐film photovoltaic cells with textured transparent conductive oxides

Typical thin‐film photovoltaic (PV) cells incorporate a textured transparent conductive oxide to enhance light trapping and efficiently harvest solar energy. Rigorous coherent optical simulations of these devices and a complete characterization of these textured films are a challenging problem because of the several orders of magnitude difference between the wavelengths of interest and the spatial dimension of the sample that needs to be evaluated. In this paper, a practical approach for rigorous and predictive modeling of optical properties of thin‐film PV cells incorporating a vast variety of light‐trapping structures including semi‐coherent textured films and patterned coherent structures is presented. In contrast to the existing semi‐empirical device models, it is demonstrated that the presented methodology can accurately predict the scattering properties of textured fluorine‐doped tin oxide and aluminum‐doped zinc oxide conductive transparent films. It is further shown that the optical response of single‐junction and tandem‐junction PV devices incorporating such films can also be predicted with good accuracy as compared with the measured results. Next, a methodology to identify the sufficient statistical fingerprints of semi‐coherent textured films that are needed to unambiguously predict the light propagation in thin‐film cells is presented. This comprehensive approach then lends itself to identifying the optimal surface morphology needed for strong light trapping. This rigorous approach automatically includes the effects of important loss mechanisms such as the surface plasmon‐enhanced absorption in textured metal surfaces that are otherwise very difficult to account for semi‐coherent approaches based on scalar scattering theory. Copyright © 2011 John Wiley & Sons, Ltd.     

Films surface temperature calculation during growth by sputtering technique

A calculation of film surface temperature during thin films growth by sputtering technique is proposed. The calculation procedure is based on the conversion into heat of the total flux energy of species impinging the film surface during growth. The results indicate that the film''s surface temperature depends drastically on material substrate thermal conductivity and thickness on one hand, and the plasma conditions namely the discharge power on the other. The predicted film surface temperatures were used to explain the microstructure evolution of hydrogenated amorphous silicon (a-Si:H) thin films deposited by reactive radio frequency (RF) sputtering on different substrates.     

PECVD法制备纳米晶硅薄膜的研究进展

简要介绍了纳米晶硅薄膜的微结构表征方法,重点讨论了PECVD制备方法中工艺参数对薄膜结构的影响,并探讨了氢在薄膜形成和生长中的作用。通过优化氢稀释率、衬底温度、反应气压、激励功率和激发频率等工艺参数可提高纳米晶硅薄膜的晶化率并改善薄膜质量。结合喇曼光谱、X射线衍射谱、傅里叶红外光谱和高分辨透射电镜等表征方法可深入研究薄膜形成机理,对进一步探索薄膜光电特性有重要意义。分析了等离子体化学气相沉积(PECVD)制备方法中各工艺参数对薄膜质量和沉积速率的影响,指出其存在的问题,并探寻了今后的研究方向。     

Influence of Dielectric Surface Chemistry on the Microstructure and Carrier Mobility of an n‐Type Organic Semiconductor

This paper examines the microstructure evolution of 3,4,9,10‐perylene‐tetracarboxylic bis‐benzimidazole (PTCBI) thin films resulting from conditions imposed during film deposition. Modification of the silicon dioxide interface with a hydrophobic monolayer (octadecyltrichlorosilane (OTS‐18)) alters the PTCBI growth habit by changing the unit cell contact plane. PTCBI films deposited on oxide surface have an orientation of (011), while films atop OTS‐treated oxide surface have a preferred orientation of (001). The quality of the self assembled monolayer does not appear to influence the PTCBI growth preference significantly yet it enhances the carrier mobility, suggesting that charge traps are adequately passivated due to uniform monolayer coverage. High‐quality monolayers result in n‐type carrier mobility values of 0.05 cm²V^–1s^–1 Increasing the substrate temperature during PTCBI film deposition correlates with an increase in mobility that is most significant for films deposited on OTS‐treated surface.     

Efficient and Flexible Thin Film Amorphous Silicon Solar Cells on Nanotextured Polymer Substrate Using Sol–gel Based Nanoimprinting Method

The mechanical flexibility of substrates and controllable nanostructures are two major considerations in designing high‐performance, flexible thin‐film solar cells. In this work, we proposed an approach to realize highly ordered metal oxide nanopatterns on polyimide (PI) substrate based on the sol‐gel chemistry and soft thermal nanoimprinting lithography. Thin‐film amorphous silicon (a‐Si:H) solar cells were subsequently constructed on the patterned PI flexible substrates. The periodic nanopatterns delivered broadband‐enhanced light absorption and quantum efficiency, as well as the eventual power conversion efficiency (PCE). The nanotextures also benefit for the device yield and mechanical flexibility, which experienced little efficiency drop even after 100,000 bending cycles. In addition, flexible, transparent nanocone films, obtained by a template process, were attached onto the patterned PI solar cells, serving as top anti‐reflection layers. The PCE performance with these dual‐interfacial patterns rose up to 8.17%, that is, it improved by 48.5% over the planar device. Although the work was conducted on a‐Si:H material, our proposed scheme can be extended to a variety of active materials for different optoelectronic applications.     

HfO2薄膜折射率非均质性生长特性研究

在加热的BK7基板上,采用电子束蒸发(EB)工艺制备了一系列不同厚度的HfO2单层膜,对HfO2薄膜生长过程中的折射率非均质性进行了研究。光谱分析表明薄膜非均质性与其厚度息息相关。X射线衍射(XRD)测试表明不同非均质性薄膜对应不同的微观结构;薄膜的微观结构主要由薄膜的生长机制决定。当膜厚较薄时,薄膜不易结晶,呈无定形态,此时薄膜呈正非均质性。如果沉积温度足够高,则薄膜达到一定厚度后开始结晶,此后薄膜折射率就会逐渐下降。随着薄膜继续生长,薄膜晶态结构保持恒定不再变化,非均质性也会因此保持不变达到极值。     

Recrystallization of Electrodeposited Copper Thin Films During Annealing

The microstructure evolution of electrodeposited copper thin films was studied at room and elevated temperatures. The effects of isothermal and nonisothermal treatments were investigated. The heating rate in nonisothermal treatment was found to significantly control the recrystallization temperature and time. The Johnson–Mehl–Avrami–Kolmogorov model was applied to describe the fraction recrystallized under isothermal conditions. It was proven that the recrystallization of copper films during nonisothermal annealing can be modeled using the additivity rule. Based on the isothermal results, a model was applied to predict the recrystallization kinetics during nonisothermal heat treatments.     

Transparent Highly Ordered TiO2 Nanotube Arrays via Anodization of Titanium Thin Films

Titanium thin films, 400 nm to 1000 nm thick, fabricated by radio frequency (rf) sputter deposition are anodized in an electrolyte containing acetic acid and hydrofluoric acid to form optically transparent films of highly ordered titania nanotube arrays. Real‐time monitoring of the anodization current, at a fixed potential, is used to controllably eliminate the Ti layer underneath the titanium oxide nanotube array without disturbing the architecture. Fabrication variables critical to achieving the transparent nanotube‐array film include annealing temperature of the anodized, initially amorphous nanotube array and Ti‐film sputter deposition variables, including rate, film thickness, and substrate temperature. Structural investigations on the transparent nanotube arrays reveal only the presence of the anatase phase even after annealing at 500 °C. In contrast, both rutile and anatase phases were observed in films with a metal layer underneath the nanotubes and annealed in an oxygen ambient above 430 °C. Rutile growth occurs at the nanotube–metal interface as metal oxidation takes place during annealing. The average refractive index of the transparent nanotube‐array film is found to be 1.66 in the UV‐vis range, with a calculated porosity of 67 %; the bandgap is determined as 3.34 eV, with a bandgap tail extending to 2.4 eV.     

Single‐Crystal Germanium Growth on Amorphous Silicon

Growing single‐crystal semiconductors directly on an amorphous substrate without epitaxy or wafer bonding has long been a significant fundamental challenge in materials science. Such technology is especially important for semiconductor devices that require cost‐effective, high‐throughput fabrication, including thin‐film solar cells and transistors on glass substrates as well as large‐scale active photonic circuits on Si using back‐end‐of‐line CMOS technology. This work demonstrates a CMOS‐compatible method of fabricating high‐quality germanium single crystals on amorphous silicon at low temperatures of <450 °C. Grain orientation selection by geometric confinement of polycrystalline germanium films selectively grown on amorphous silicon by chemical vapor deposition is presented, where the confinement selects the fast‐growing grains for extended growth and eventually leads to single crystalline material. Germanium crystals grown using this method exhibit (110) texture and twin‐mediated growth. A model of confined growth is developed to predict the optimal confining channel dimensions for consistent, single‐crystal growth. Germanium films grown from one‐dimensional confinement exhibit a 200% grain size increase at 1 μm film thickness compared to unconfined films, while 2D confinement growth achieved single crystal Ge. The area of single crystalline Ge on amorphous layers is only limited by the growth time. Significant enhancement in room temperature photoluminescence and reduction in residual carrier density have been achieved using confined growth, demonstrating excellent optoelectronic properties. This growth method is readily extensible to any materials system capable of selective non‐epitaxial deposition, thus allowing for the fabrication of devices from high‐quality single crystal material when only an amorphous substrate is available.     

基于不同单光子能量拉曼谱的氢化硅薄膜微观特性研究

用等离子体增强化学汽相沉积法（PECVD）在玻璃和单晶硅（c-Si）衬底上分别制备了氢化纳米硅（nc-Si：H）和非晶硅（a-Si：H）薄膜,用紫外、可见和近红外3种不同波长的激光线对不同形态的Si薄膜进行拉曼散射实验研究。研究发现,这些Si薄膜在不同的单光子能量的激光线激发下的拉曼谱线形也不同。进而通过对Si薄膜材料...     

Aqueous Deposition of Ultraviolet Luminescent Columnar Tin‐Doped Indium Hydroxide Films

Indium tin oxide (ITO) has attracted intense interest as the most important transparent conducting oxide (TCO) that sees wide use in many opto‐electronic and photo‐chemical devices. The goal of this study is to explore the possibility of depositing ITO thin films using a bioinspired aqueous deposition route as an alternative. On the surface of sulfonated‐self assembled monolayers, Sn‐doped indium hydroxide films are obtained via a hydrogen peroxide‐assisted method. As a result, the as‐deposited indium tin hydroxide films possess a single hexagonal phase of In(OH)₃· xH₂O (0 ≤ x ≤ 1) with Sn doping percentage of (1.7 ± 0.2) at % and a column‐like hierachical microstructure. Structural, compositional and property studies, including electron microscopy, X‐ray diffraction, photoelectron spectroscopy, optical transmittance, photoluminescence and four‐probe conductivity measurements, are conducted. The possible mechanism based on oriented attachment is discussed for the film growth. Strong room temperature photoluminescence within the near UV range is observed in the case of Sn‐doped, but not in the one of the pure In(OH)₃· xH₂O films. Annealing of the indium tin hydroxide films above 200 °C gives nanocrystalline Sn:In₂O₃ films with higher UV and visible transparency and electrical conductivity compared with those of pure In₂O₃ films. The influence of annealing atmosphere is investigated.     

Unusually High Optical Transmission in Ca:Ag Blend Films: High‐Performance Top Electrodes for Efficient Organic Solar Cells

Highly transparent electrodes are demonstrated based on thermally evaporated calcium:silver blend thin–films, which show unusually high transmission well above the expectations from bulk material properties and thin film optics. These electrodes exhibit a low sheet resistance of 27.3 Ω/, combined with an extraordinarily high mean transmittance of 93.0% in the visible spectral range (σ_dc/σ_opt = 186.7), superior to the commonly used inorganic electrodes made from indium tin oxide (ITO). Additionally, the metal blend electrode is flexible, showing a constant sheet resistance down to a bending radius of 10 mm and can be employed on top of organic devices without causing damage to the organic material. The spontaneously formed unique microstructure of a polycrystalline Ag network with randomly distributed nanoapertures, surrounded by a calcium shell, enables broadband transmittance enhancement due to amplified plasmonic coupling. Consequently, top‐illuminated organic solar cells using such metal blend electrodes achieve a power conversion efficiency of 7.2% (which defines a new record for top illuminated organic solar cells) and even exceed the efficiency of similar bottom‐illuminated reference solar cells (6.9%) employing common ITO electrodes.     

Improved micromorph solar cells by means of mixed‐phase n‐doped silicon oxide layers

A good light trapping scheme is necessary to improve the performance of amorphous/microcrystalline silicon tandem cells. This is generally achieved by using a highly reflective transparent conducting oxide/metal back contact plus an intermediate reflector between the component cells. In this work, the use of doped silicon oxide as alternative n‐layer in micromorph solar cells is proposed as a means to obtain high current values using a simple Ag back contact and no extra reflector between the component cells n‐doped silicon oxide layers with a wide range of optical and electrical properties have been prepared. The influence of different deposition regimes on the material properties has been studied. The main findings are the following: (i) when carbon dioxide is added to the gas mixture, sufficiently high hydrogen dilution is necessary to widen the transition region from highly conductive microcrystalline‐like films to amorphous material characterized by low electrical conductivity; (ii) lower refractive index values are found with lower deposition pressure. Optimal n‐doped silicon oxide layers have been used in both component cells of micromorph devices, adopting a simple Ag back contact. Higher current values for both cells are obtained in comparison with the values obtained using standard n‐doped microcrystalline silicon, whereas similar values of fill factor and open circuit voltage are measured. The current enhancement is particularly evident for the bottom cell, as revealed by the increased spectral response in the red/infrared region. The results prove the high potential of n‐doped silicon oxide as ideal reflector for thin‐film silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Gate Capacitance‐Dependent Field‐Effect Mobility in Solution‐Processed Oxide Semiconductor Thin‐Film Transistors

Solution‐processed oxide semiconductors (OSs) used as channel layer have been presented as a solution to the demand for flexible, cheap, and transparent thin‐film transistors (TFTs). In order to produce high‐performance and long‐sustainable portable devices with the solution‐processed OS TFTs, the low‐operational voltage driving current is a key issue. Experimentally, increasing the gate‐insulator capacitances by high‐k dielectrics in the OS TFTs has significantly improved the field‐effect mobility of the OS TFTs. But, methodical examinations of how the field‐effect mobility depends on gate capacitance have not been presented yet. Here, a systematic analysis of the field‐effect mobility on the gate capacitances in the solution‐processed OS TFTs is presented, where the multiple‐trapping‐and‐release and hopping percolation mechanism are used to describe the electrical conductivity of the nanocrystalline and amorphous OSs, respectively. An intuitive single‐piece expression showing how the field‐effect mobility depends on gate capacitance is developed based on the aforementioned mechanisms. The field‐effect mobility, depending on the gate capacitances, of the fabricated ZnO and ZnSnO TFTs clearly follows the theoretical prediction. In addition, the way in which the gate insulator properties (e.g., gate capacitance or dielectric constant) affect the field‐effect mobility maximum in the nanocrystalline ZnO and amorphous ZnSnO TFTs are investigated.