Structures and light emission properties of nanocrystalline FeSi2/Si formed by ion beam synthesis with a metal vapor vacuum arc ion source

Thin layers of nanocrystalline FeSi₂ embedded in Si structures have been formed by Fe implantation using a metal vapor vacuum arc (MEVVA) ion source under various implantation and thermal annealing conditions. The microstructures were studied in details and correlated with the photoluminescence (PL) properties. It is found that higher lattice coherence between the FeSi₂ nanocrystals and the Si matrix is associated with better light emission efficiency. Multiple-cycle implantation schemes were introduced and it is shown that with appropriate process design the dose quenching effect can be suppressed to achieve light emission enhancement in higher dose samples. De-convolution of the PL spectra into two or three peaks was performed and their temperature and excitation power dependence were analyzed. The analysis results indicate that the 1.55-μm emission really originated from FeSi₂ and that the emission peaks are likely donor- or accepted-level-related. MOS structures with the incorporation of implanted nanocrystalline FeSi₂ were fabricated. Electroluminescence (EL) spectra from these devices showed two peak features of which one peak corresponds to FeSi₂ emission and the other corresponds to enhanced Si band-edge emission. Clear room-temperature EL signals from these device structures were observed. A model is proposed to qualitatively understand the temperature dependence of the EL spectra.     

Computational design of high efficiency FeSi2 thin-film solar cells

A novel PV cell structure in the form of a p-i-n construction is proposed by inserting a layer of FeSi₂ between two layers of crystalline silicon. The energy band diagram and PV properties are simulated. The structure parameters, such as the thickness, doping concentration and defect density of states of each layer, are taken into account. The optimized structure with a thickness less than 1 μm shows a large open circuit voltage (Voc) of 0.68 V and a high photoelectric conversion efficiency (Eff) of 24.7%, which is significantly larger than that of the FeSi₂/Si double-layer pn-heterojunction structure, and is comparable with the performance of crystalline Si solar cells with a thickness of 250 μm. The Voc and Eff increase further when replacing the crystalline Si at both sides of the FeSi₂ layer with amorphous Si layers. The present work is expected to open a new avenue in developing low-cost thin film solar cells on the basis of the well established Si technology.     

Low-temperature plasma-enhanced chemical vapor deposition of tungsten and tungsten nitride

Tungsten and tungsten nitride layers have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Tungsten layers deposited at low deposition temperatures T150 °C using this method showed good uniformity over dielectric and silicon substrate areas. As the deposition temperature decreased, the silicon consumed during the deposition reaction decreased, at T150 °C no silicon consumption was measurable. PECVD tungsten nitride layers were deposited directly on oxidized silicon substrates with no requirement for a nucleation layer. As the NH₃ flow rate was increased, whilst maintaining all other parameters constant, deposited layers were found to change from metal tungsten to tungsten-rich amorphous layer to W₂N. The resistivity of the layers was found to be high compared to published literature for higher-temperature deposited layers. The high resistivity is attributed to the incorporation of fluorine into the layer at low deposition temperatures. A deposition process was established for smooth amorphous tungsten-rich W _x N layers at 150 °C.     

Thermoelectric properties of p- and n-type FeSi2 prepared by spray drying, compaction and sintering technique

FeSi₂ alloys doped with Mn and Co (p-type Fe_0.926Mn_0.074Si₂, n-type Fe_0.980Co_0.020Si₂) were prepared by vacuum induction melting. The ingots were pulverized in a jet-mill, and the powders were granulated by spray drying method using aqueous polyvinyl alcohol (PVA) binder in order to investigate the possibility of production on a large scale. The powders granulated exhibited excellent characteristics of flowability, leading to the smooth feeding into a die cavity in compacting. The powders obtained were compacted at a pressure of 98 MPa. It was debindered at 723 K for 1 h and sintered at 1423 K for 5 h in a hydrogen atmosphere, so that the residual carbon and oxygen contents in a sintered body approached to those in the starting powder. Subsequently it was annealed at 1113 K for 100 h in an argon atmosphere, to produce the semiconducting -FeSi₂ phase. The thermoelectric figures of merit (Z) for optimum p- and n-type FeSi₂ are 1.75 × 10^–4 (K^–1) and 2.0 × 10^–4 (K^–1) at about 900 K, respectively, which agree roughly with those obtained by Tani and Kido for FeSi₂ materials prepared by the spark plasma sintering method. It indicates that the spray drying method leading to the production on a large scale is available for the fabrication of the thermoelectric FeSi₂ materials.     

Effect of target compositions on the crystallinity of β-FeSi2 prepared by ion beam sputter deposition method

Targets with the elemental composition of Fe, Fe₂Si and FeSi₂ were employed in the present study to grow β-FeSi₂ film on Si (100) substrate by means of ion beam sputter deposition (IBSD) method. The results revealed that when FeSi₂ target was employed, a Si-rich phase, α-FeSi₂ (Fe₂Si₅), was predominant at temperatures above 973 K, while β-FeSi₂ phase was observed only in the limited temperature range at around 873 K. In this case, Si was originated both from the sputtered target and the substrate, thus, the supply of Si was considered to be excessive to sustain β structure. On the other hand, the films prepared with Fe target became polycrystalline as they grow thicker than 100 nm. In order to optimize the supply of Fe and Si for epitaxial growth, Fe₂Si target was employed, where highly (100)-oriented β-FeSi₂ layer of 120 nm in thickness was obtained at 973 K.     

Deposition and microhardness of SiC from the Si2Cl6-C3H8-H2-Ar system

We obtained SiC coating layers on a graphite substrate using hexachlorodisilane (Si₂Cl₆, boiling point 144° C) as a silicon source and propane as a carbon source. We examined the deposition conditions, contents of carbon, silicon and chlorine in the deposits, and the microhardness. Mirror-like amorphous silicon layers were deposited in the reaction temperature range 500 to 630° C. well-formed silicon carbide layers with good adherency to the substrate were obtained above 850° C. The lowest deposition temperature of SiC was estimated to be 750 to 800° C. The Vickers microhardness of the SiC layer was about 3800 kg mm^–2 at room temperature and 2150 kg mm^–2 at 1000° C.     

Chemical vapour deposition of Si3N4 from a gas mixture of Si2Cl6, NH3 and H2

Si₃N₄ layers were obtained on a quartz substrate from a gas mixture of Si₂Cl₆, NH₃ and H₂ under a reduced pressure in a temperature range of 800 to 1300‡ C. Amorphous Si₃N₄ layers that were dense and adherent to the substrate were obtained in a temperature range of 800 to 1100‡ C. On the other hand,α-Si₃N₄ layers were obtained at 1200‡ C and a source-gas ratio (N/Si) of 1.33 to 1.77. The lowest deposition temperature of amorphous Si₃N₄ was considered to be about 700‡ C. The microhardness of amorphous Si₃N₄ obtained in a temperature range of 800 to 1100‡ C was 2400 to 2600 kg mm⁻² (load: 50 g), and that ofα-Si₃N₄ obtained at 1200‡ C was 3400 kg mm⁻². Chlorine contents in the Si₃N₄ layer decreased with increasing deposition temperature and source-gas ratio (N/Si), and with decreasing total pressure.     

Crystallographic characteristics and fine structures of semiconducting transition metal silicides

Silicide-based photonic materials have attracted a great deal of research interest due to their compatibility with the well-developed silicon technology. Extensive efforts have been made for the synthesis and characterisation of these materials. This paper covers some aspects of the microstructural and crystallographic characteristics of ion beam synthesised silicides such as the semiconducting iron and ruthenium silicides, using transmission electron microscopy. A previously predicted new orientation relationship has been found to exist between the Si substrate and ion beam synthesised βFeSi₂ nanocrystals, which are free of 90° rotational order domain boundaries.     

Single crystalline β-FeSi2 grown using high-purity FeSi2 source

We have investigated the effect of FeSi₂ source purity on the electrical property of β-FeSi₂ grown from solution. A high-purity FeSi₂ source avoided a contamination of Cu and W metals was synthesized by melting a high-purity Fe (5N) and Si (5N-up) in a quartz ampoule. Glow discharge mass spectrometry revealed that the purity of the FeSi₂ source synthesized using 5N-Fe and a quartz-ampoule-melting process is one order of magnitude higher than that of the conventional arc-melted FeSi₂ source using 4N-Fe. The β-FeSi₂ crystals grown using the high-purity FeSi₂ and Zn solvent showed n-type conduction, whereas those grown using the arc-melted FeSi₂ showed p-type. The carrier concentration of the n-type crystals was (4.9-6.3) × 10¹⁸ cm^− 3, which was more than 10 times higher than that of the p-type crystals (5.2 × 10¹⁷ cm^− 3). From the ICP-MS and SIMS analysis of the grown crystals, we found that dominant impurity concentrations (Cr, Mn, Co, Ni, Cu, Zn and W) in the p-type crystals were higher than those in the n-type ones. Therefore, the p-type conductivity of undoped crystals grown using Zn solvent results from unintentional doping by the high impurity level of the used FeSi₂ source.     

The effect of argon pressure on the structural and photocatalytic characteristics of TiO2 thin films deposited by d.c. magnetron sputtering

TiO₂ thin films of 200-300 nm thickness were deposited by d.c. magnetron sputtering onto glass substrates from a semiconducting TiO_2−x target in pure Ar using pressures between 0.1 and 1.0 Pa. The obtained TiO₂ coatings are transparent and have refractive indices between 2.5 and 1.9. Post deposition heat treatment at different temperatures was performed to achieve crystallization of anatase TiO₂. The as-deposited and heat treated films were examined with UV-VIS (transmission), SEM and XRD to investigate the influence of the argon pressure during deposition on the structural development during heat treatment. Additionally, the photocatalytic activity of the films was tested by measuring the decomposition rate of ethanol in a controlled gas atmosphere simulating air, and was related to their respective microstructures.     

Multilayer amorphous hydrogenated carbon (a-C:H) and SiOx doped a-C:H films for optical applications

In this study SiO_x doped amorphous hydrogenated carbon (a-C:H) films were formed from hexamethyldisiloxane (with hydrogen transport gas) by closed drift ion beam deposition applying variable ion beam energy (300-800 eV). The band gap dependence on the deposition energy was determined and used in production of SiO_x doped a-C:H and a-C:H (formed from acetylene gas) multilayer (two and four layers) stack. Optical properties of the multilayer structures as well as individual layers were analysed in the UV-VIS-NIR range (200-1000 nm). It was shown that employing double or four layer systems, the reflectivity of the multilayer structure-crystalline silicon can be tuned to almost 0% at specific wavelength range (550-950 nm), important in solar cell applications.     

High performance metal-insulator-metal capacitors with atomic vapor deposited HfO2 dielectrics

Thin HfO₂ films were grown as high-k dielectrics for Metal-Insulator-Metal applications by Atomic Vapor Deposition on 8 inch TiN/Si substrates using pure tetrakis(ethylmethylamido)hafnium precursor. Influence of deposition temperature (320-400 °C) and process pressure (2-10 mbar) on the structural and electrical properties of HfO₂ was investigated. X-ray diffraction analysis showed that HfO₂ layers, grown at 320 °C were amorphous, while at 400 °C the films crystallized in cubic phase. Electrical properties, such as capacitance density, capacitance-voltage linearity, dielectric constant, leakage current density and breakdown voltage are also affected by the deposition temperature. Finally, TiN/HfO₂/TiN stacks, integrated in the Back-End-of-Line process, possess 3 times higher capacitance density compared to standard TiN/Si₃N₄/TiN capacitors. Good step coverage (> 90%) is achieved on structured wafers with aspect ratio of 2 when HfO₂ layers are deposited at 320 °C and 4 mbar.     

Influence of high temperature processing of sol-gel derived barium titanate thin films deposited on platinum and strontium ruthenate coated silicon wafers

Thin films of barium titanate (BTO) of 200 nm thickness, derived from an alkoxide-carboxylate sol-gel process, were deposited on Pt/Ti and SrRuO₃/ZrO₂-8%Y₂O₃ coated Si wafers. Films with a dense columnar microstructure were obtained by repeated deposition of thin amorphous layers from low-concentrated sols, and crystallization at 800 °C. This method added 10 nm thickness to the crystalline BTO film in each deposition step. The harsh processing conditions had a negative impact on the platinized silicon wafers, where Pt-Si silicides were formed. This led to diffusion of Si into BTO and interfacial silicate formation. The interfacial silicate layer was the cause of deteriorated dielectric and ferroelectric properties of the BTO layer. Use of SrRuO₃/ZrO₂-8%Y₂O₃/Si substrates solved the problem. No diffusion of Si was observed, and BTO films with good dielectric and ferroelectric properties were obtained.     

Rapid Deposition of Buffer Layers for YBCO Coated Conductors on Biaxially-Textured Ni Tapes

Epitaxial growth of CeO₂ buffer layers on biaxially textured (001)Ni tapes was studied using pulsed laser deposition. Relationships between microstructure and deposition parameters were systematically studied in order to develop reliable long tape coating processes. It was found that orientation and texture of CeO₂ buffer layers were sensitive to deposition parameters. X-ray diffraction analyses showed that CeO₂ buffer layers had pure (001) orientation at 860 °C. Under optimized deposition conditions, highly (001) oriented CeO₂ buffer layers have been achieved at a high deposition rate of 1.5 nm/s in-plane texture and out-plane texture of CeO₂ buffer layers were 4.25 degrees and 5.85 degrees, respectively.     

A study of amorphous and crystalline phases in In2O3-10 wt.% ZnO thin films deposited by DC magnetron sputtering

We report on the processing, phase stability, and electronic transport properties of indium oxide (In₂O₃) doped with 10 wt.% zinc oxide (ZnO) deposited to a thickness of 100 nm using DC magnetron sputter deposition at room temperature and 350 °C. We compare the optimum oxygen content in the sputter gas for pure In₂O₃ and doped with (i) 10 wt.% ZnO and (ii) 9.8 wt.% SnO₂. Amorphous IZO films were annealed at 200 °C in air and N₂/H₂ and resistivity, Hall mobility, and carrier density along with molar volume change were monitored simultaneously as a function of time at temperature. We report that annealing the amorphous oxide in air at 200 °C does not lead to crystallization but does result in a 0.5% decrease in the amorphous phase molar volume and an associated drop in carrier density. Annealing in forming gas leads to an increase in carrier density and a small decrease in molar volume. We also report that when annealed in air at 500 °C, the amorphous IZO phase may crystallize either in the cubic bixbyite or in a recently observed rhombohedral phase.     

Properties of indium tin oxide films prepared by ion-assisted deposition

Conducting transparent films of indium tin oxide were deposited by 100 eV oxygen-ion-assisted deposition. A refractive index of 2.13 at 550 nm was obtained for films deposited onto ambient temperature substrates. The refractive index decreased with increasing substrate temperature to a value of 2.0 at 400°C. The sheet resistance of films 135 nm thick decreased from 800 Ω/□ for layers deposited onto room temperature substrates to around 25 Ω/□ at 400°C. Structural studies revealed that ion-assisted deposition onto ambient temperature substrates produced amorphous films, and that at temperatures above 100°C the films exhibit In₂O₃ crystallinity. In addition, it was found that the number of voids in the ion-bombarded films was reduced relative to that in films produced by conventional reactive evaporation.     

Subsurface structures in initial stage of FeSi2 growth studied by high-resolution Rutherford backscattering spectroscopy

The initial stage of iron silicide formation is investigated by high-resolution Rutherford backscattering spectroscopy. During the Fe deposition on Si(001) at 470 °C, the formation of FeSi₂ is confirmed by the surface peak analysis. Initially, FeSi₂ grows epitaxially so that one of the major crystallographic axes is parallel to the <111> axis of the Si substrate. With increasing Fe deposition, the deviation between the major crystallographic axis of the silicide region and Si<111> increases although the electron diffraction pattern is independent of the amount of Fe deposition. Therefore, the subsurface crystallographic structure of iron silicide is transformed from a cubic-like to a low-symmetry structure.     

Characterizations of SnO2 and SnO2:Sb thin films prepared by PECVD

Structural characterizations of tin oxide (SnO₂) thin films, deposited by plasma-enhanced chemical vapor deposition (PECVD), were investigated with scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results show that the films are porous, the crystalline structure transforms from crystalline to amorphous phase as deposition temperature changes from 500°C to 200°C, and the chemical component is non-stoichiometric (Sn:O is 1.0716 prepared at 450°C with a value of O₂ flow 3.5 l/min). Sheet resistance of the thin films decreases with increasing of deposition temperature. Whereas, sheet resistance increases with increasing of oxygen flow. Tin oxide doped with antimony (SnO₂:Sb) thin films prepared by same method have a better selectivity to alcohol than to carbon monoxide; the maximum sensitivity is about 220%. The gas-sensing mechanism of SnO₂ thin films is commentated.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

Growth of thick chemical solution derived pyrochlore La2Zr2O7 buffer layers for YBa2Cu3O7−x coated conductors

The preparation of several 100 nm thick La₂Zr₂O₇ (LZO) buffer layers on biaxially textured Ni-5 at.%W substrates using chemical solution deposition is studied. This oxide material is currently of great interest for the fabrication of YBa₂Cu₃O_7−x (YBCO) coated conductors. Buffer layers for these coated conductors are required to have thicknesses greater than 100 nm in order to guarantee a sufficient barrier function against metal diffusion from the substrate. In this work, single LZO buffer layers with thicknesses exceeding 200 nm have been prepared. Detailed investigations were carried out in order to study the texture development with increasing thickness as well as the microstructure of these layers. Independent of the thickness, high quality buffer layers showing a distinct biaxial texture up to the surface, smooth surfaces, and a sufficient barrier function against Ni diffusion from the substrate have been reproducibly obtained. The high performance of these chemical solution derived LZO buffer layers was confirmed by a YBCO critical current density J_c of 1.0 MA/cm² (77 K, 0 T) achieved for a coated conductor sample with a layer sequence YBCO/CeO₂/LZO(CSD)/Ni-5 at.%W where CeO₂ and YBCO were deposited by pulsed laser deposition.