Preparation and properties of Bi2−x As x S3 thin films by solution-gas interface technique

The solution gas interface technique by which thin films of Bi_2−x As _x S₃ were deposited is described in this paper. The semiconducting properties of the interface grown Bi_2−x As _x S₃ thin films are studied. The optical absorption, dark resistivity and thermoelectric power of the films were studied and results are reported.     

Effect of complexing agent on growth process and properties of nanostructured Bi2S3 thin films deposited by chemical bath deposition method

Nanostructured Bi₂S₃ thin films have been prepared onto amorphous glass substrates by chemical bath deposition method at room temperature using bismuth nitrate and sodium thiosulphate as cationic and anionic precursors with EDTA as complexing agent in aqueous medium. The X-ray diffraction study reveals that the films deposited without the complexing agent are amorphous in nature and becomes nanocrystalline in the presence of EDTA. The resistivity for the films prepared from EDTA complexed bath is decreased due to the improvement in grain structure. The decrease in optical bandgap and activation energy is observed as the thickness of the film varies from 45 to 211 nm on account of the variation of the volume of complexing agent in reaction bath. Studies reveal that the growth mechanism of Bi₂S₃ gets affected in the presence of complexing agent EDTA and shows impact on structural, electrical and optical properties.     

Photo-induced optical changes in amorphous AsS films

Photo-induced optical changes were investigated in thin film structures of As_xS_100?x with x between 10 and 43. For As₄₀S₆₀ the magnitude of the change in optical path length at 6328 Å for the reversible photodarkening process is about one-third of that for the irreversible one. The relaxation time of the former is approximately twice that of the latter.In specimens with excess chalcogen, the influence of annealing becomes weaker with increase in sulphur content. The change in optical path length for the virgin films has a minimum at a composition of about As₂₅S₇₅. These characteristics suggest that sulphur plays an active role in the photo-induced transformation.     

The reaction mechanism of the spray Ion Layer Gas Reaction process to deposit In2S3 thin films

The spray Ion Layer Gas Reaction (ILGAR) is a well-established, patented and commercial process used primarily to deposit In₂S₃ as buffer layers in thin film solar cells. In this paper we investigate the growth mechanism of the spray In₂S₃ ILGAR process by characterising the intermediate growth stages of films, following the growth mechanism with a quartz crystal microbalance and tracking the gaseous side-and-intermediate products during film growth, using a mass spectrometer. A basic growth mechanism model is then proposed based on an aerosol assisted chemical vapour deposition of an In(O_x,Cl_y,(OH)_z) film, as the first stage process, followed by the conversion of the intermediate film using H₂S gas to In₂S₃.     

Micro-structures of BF2- and As-implanted polycrystalline silicon thin films of various thicknesses and heat treatments

Characterization of annealed BF₂⁺- and As⁺-doped polycrystalline silicon (polysilicon) films is presented. Effects of heat treatment, doping concentration, and thickness of film on the grain size and mobility of polysilicon films are investigated and discussed. By using transmission electron microscopy (TEM), it is found that the grain size, effective carrier concentration, and carrier mobility of a polysilicon thin film increases with increasing film thickness. Our results show that a high concentration of As dopant could enhance the recrystallization of the polysilicon films. Heavily As⁺-doped samples were seen to have a relatively larger grain size compared to the lightly doped film. The maximum grain size of about 278 nm can be realized in a polysilicon film with 150 nm in thickness. In contrast, the B dopant has a negligible effect on the recrystallization of polysilicon films. With increasing film thickness and thermal annealing temperature, a high performance polysilicon film with high mobility and grain size can be obtained.     

Effects of post-deposition annealing on the structure and optical properties of Y2O3 thin films

Y₂O₃ thin film waveguides were prepared by RF magnetron sputtering. The effects of post-deposition annealing on the structure and optical properties have been investigated. The structural evolution of Y₂O₃ films with annealing temperature was investigated by X-ray diffraction (XRD). Spectroscopic ellipsometry was employed to determine the optical properties of Y₂O₃ films annealed at various temperatures. It was found that with increasing annealing temperature, the refractive index (n) of Y₂O₃ films increases. The optical band gap of Y₂O₃ films shifts to higher energy after higher temperature annealing, which is likely due to the reduction of defects and the change of crystalline structure in Y₂O₃ films.     

Optical monitoring of etching in inorganic resists

An as-deposited As₂S₃ thin film is demonstrated as a negative working inorganic resist suitable for silicon microstructure fabrication. An in situ optical technique for monitoring the etching process in inorganic resists is described for pattern delineation. The etching control in dielectric films is treated as a special case.     

Microsensor based on low temperature cofired ceramics and gas-sensitive thin film

A novel design of gas sensor using low temperature cofired ceramics (LTCC) and thin film technologies is presented. The LTCC structure is composed essentially of two ceramic layers with interlayer thick film Pt heater, interdigitated electrodes on top, contact pads and metallic connections realised by vias. The thin films of both SnO₂ and In₂O₃, intentionally doped and activated, were deposited on top of the structure. With some modifications of the lamination process and heat treatment parameters, the authors obtained the upper ceramic layer with the roughness not exceeding 250 nm, what was suitable for thin film technology. The films deposited onto such LTCC structure revealed the sensing properties very similar to the reference films deposited onto glass. The gas-sensitive films were tested with changing concentrations of reducing and oxidising gases in air. The necessary sensor working temperature was obtained and stabilised using a custom-built digital controller. The low heat capacity of the sensor structure enabled also a sinusoidal temperature control. The satisfactory results obtained by the authors indicate that the connection of LTCC and thin film technologies can lead to the fabrication of good quality gas sensors.     

Transfer of CuInS2 thin film by lift-off process and application to superstrate-type thin-film solar cells

Transfer of a CuInS₂ thin film grown on a Mo/soda-lime glass substrate was investigated using a lift-off process. The CuInS₂ thin film was flatly exfoliated, with preferential peeling occurring in the CuInS₂/MoS₂ interface vicinity. This suggests that the interfacial MoS₂ layer behaves as a sacrificial layer. The lift-off process was also applied to solar cell fabrication. A superstrate-type CuInS₂ thin-film solar cell was fabricated and exhibited no significant degradation of conversion efficiency compared with a substrate-type CuInS₂ thin-film solar cell. The lift-off process could therefore also be applied to fabricate the upper part of a tandem solar cell structure.     

Spray pyrolyzed β-In2S3 thin films: Effect of postdeposition annealing

Indium sulfide thin films prepared using spray pyrolysis, with In/S ratio 2/3 in the solution, were annealed in vacuum at 300 and 400 °C. The effect of this treatment on properties of the films was studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, optical absorption, transmission and electrical measurements. Optical constants of the films were calculated using the envelope method. Annealing did not affect the optical properties of the film much, but the resistivity of the films showed a drastic decrease and the grain size increased. In₂S₃ thin films have potential use as buffer layer in photovoltaic heterojunction devices.     

An ellipsometric study of relaxation-induced changes in the optical characteristics and structural inhomogeneity of As2S3 glassy thin films

The optical characteristics of thin (～2-μm-thick) films of As₂S₃ chalcogenide glass prepared by thermal deposition in vacuum have been determined from the results of multiangle ellipsometric measurements using He-Ne laser radiation (λ = 0.6328 μm), with allowance for weak absorption in this spectral range. Relaxation-induced changes in the optical properties of both freshly prepared (unannealed) and annealed As₂S₃ films are determined. The possibility of using ellipsometry for the qualitative estimation of the degree of inhomogeneity of the film and its variation in the course of structural transformations during relaxation or under the action of other factors is considered.     

Electrodeposition of As2Se3 thin films

Semiconducting As₂Se₃ thin films have been prepared from an aqueous bath at room temperature onto stainless steel and fluorine-doped tin oxide (F.T.O.)-coated glass substrates using an electrodeposition technique. It has been found that As₂O₃ and SeO₂ in the volumetric proportion as 4:6 and their equimolar solutions of 0.075 M concentration forms good quality films of As₂Se₃. The films are annealed in a nitrogen atmosphere at temperature of 200 °C for 2 h. The films are characterised by scanning electron microscopy, X-ray diffraction and optical absorption techniques. Studies reveal that asdeposited and annealed thin films are polycrystalline in nature. The optical band gap has been found to be 2.15 eV for the above-mentioned composition and concentration of the film.     

In situ fabrication of Bi2S3 nanocrystal film for photovoltaic devices

A facile wet-chemical method to prepare Bi₂S₃ thin films with flake nanostructures directly on ITO glass substrate is presented in this paper for the first time. The product was characterized by X-ray powder diffractometer (XRD), Raman spectrometer, scanning electron microscope (SEM), and atomic force microscope (AFM). The one-step solvothermal elements treatment on the ITO substrate spare time to form film by spin-coating process and the film could be tightly attached to the ITO electrode. A conjugated polymer, poly 3-hexylthiophene (P3HT), was then spin-coated on the as-prepared Bi₂S₃ film to form an inorganic-organic hybrid thin film. The photovoltaic performance of the resulting solar cell device was also investigated.     

Photo-induced changes in optical properties of As2S3 and As2Se3 films deposited at normal and oblique incidence

Photostructural transformations in amorphous chalcogenide films have been a subject of intensive research so far. In this paper we discuss the changes in the optical properties of typical As-based chalcogenide glasses (As₂S₃ and As₂Se₃) on exposure to ultraviolet (UV) light. An attempt has been made to systematically investigate the optical parameters like extinction coefficient, refractive index and optical bandgap of the films by measuring the same for as-grown and UV-exposed amorphous films of As₂S₃ and As₂Se₃ prepared by vacuum evaporation technique.     

Structural,optical and electrical properties of spray pyrolytically deposited thin films of CuInS2

CuInS₂ is a promising chalcopyrite semiconducting material for solar cell fabrication. Using aqueous solutions of cupric chloride, indium trichloride and thiourea, we deposited thin CuInS₂ films on glass at 350°C and studied their structural, optical and electrical properties. From the XRD pattern the chalcopyrite structure of these films was confirmed. The films were polycrystalline. The grain size estimated from scanning electron micrographs was found to be of the order of 1μm. Resistivity of the film was measured for temperatures ranging from 77 to 473 K. Band gap values were determined from optical transmission data. Hall mobility and carrier concentration at room temperature were calculated using Van der Pauw-Hall method.     

Non-isothermal crystallization of As2Se3 glass

The results of non-isothermal crystallization studies performed at different heating rates on batches of As₂Se₃ glasses prepared from melts at 400°C, 600°C and 800°C are reported. The peak temperature of crystallizationT _p, the enthalpy of crystallization ΔH _c and the activation energy for crystallizationE _c are independent of the melt temperature used in the preparation. Bulk nucleation with three-dimensional growth of crystals is indicated for As₂Se₃. The values of ΔH _c andE _c are found to be respectively 23·3 ± 0·9 cal/g and 36·5 ± 0·9 kcal/mol for As₂Se₃.     

Effect of Temperature on Diffraction Efficiency of Holograms Recorded in Arsenic Trisulphide Thin Films

Hologram recording in arsenic trisulphide thin films of thickness 300 nm, prepared by electron beam evaporation on substrates between room temperature and 150°C, have been investigated experimentally. These holograms were evaluated for the diffraction efficiency (DE) and resolution. The results were compared with published results for As₂ S₃ films prepared in gelatin. The DE was investigated for beam ratios of 1, 5 and 10, and was found to decrease for increasing temperature and beam ratio. Thus decrease in DE at an elevated substrate temperature (150°C) is attributed to microstructural changes taking place in the As₂ S₃ films.     

Optical properties change in amorphous (As2S3)0.87Sb0.13 thin films by photo and thermal induced process

Exposure with above band gap light and thermal annealing at a temperature near to glass transition temperature, of thermally evaporated amorphous (As₂S₃)_0.87Sb_0.13 thin films of 1 μm thickness, were found to be accompanied by structural effects, which in turn, lead to changes in the optical properties. The optical properties of thin films induced by illumination and annealing were studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Photo darkening or photo bleaching was observed in the film depending upon the conditions of the light exposure or annealing. These changes of the optical properties are assigned to the change of homopolar bond densities.     

Pyrite (FeS2) thin films deposited by sol-gel method

Nanocrystalline pyrite (FeS₂) films were achieved by the sol-gel dip-coating process and sulfurization treatment. The microstructural, optical and electrical characteristics were investigated and the effect of sulfurization time on film properties was discussed. The XRD spectra show that FeS₂ film can be obtained for 1 h sulfurization and no other phase appears. The morphology of the precursor Fe₂O₃ films shows a porous and loose structure. However, with the sulfurization time increasing, the precursor films completely transformed into the pyrite films which have a compact and smooth structure. The pyrite films with a different sulfurization time have the optical absorption edges changed in the range of 0.90-0.99 eV. With the increase of sulfurization time, the carrier concentration increases and the carrier mobility decreases. It is speculated that crystallographic defects in the films could play an important role in film properties.     

In2O3-ZnO transparent conductive oxide film deposition on polycarbonate substrates

Amorphous transparent conductive oxide films in the In-Zn-O system were deposited on polycarbonate (PC) substrates by simultaneous DC sputtering of an In₂O₃ target and a ZnO target with either 4 wt% Al₂O₃ or 7.5 wt% Ga₂O₃ impurities. Although the resistivity of the amorphous, non-doped In-Zn-O film on PC was about one order of magnitude higher than that on the glass substrate, the resistivity of the In-Zn-O films with Ga₂O₃ impurities on PC substrates was reduced to the level of the non-doped In-Zn-O films on glass substrates. The addition of Al₂O₃ or Ga₂O₃ to the In-Zn-O films also induced the widening of the optical band gap, which would improve transparency at blue wavelengths.