Microcrystalline germanium thin films prepared by reactive RF sputtering

We study microcrystalline germanium (μc-Ge) film as narrow gap semiconductor materials for infrared absorbers by reactive sputtering with inert gas/mixtures. H₂ mixed with Ne, Ar and Xe was used as sputtering gases, in order to research effects of the ion damage. A higher deposition rate is obtained by using inert gases with a larger mass. But the crystallinity becomes lower by the damage due to larger mass ions. In the Ar/H₂ mixtures, the structure changes from crystalline to amorphous with increase in the Ar/H₂ flow rate ratio. The damage of Xe ion is too large to crystallize the films, but the influence of Ne on the crystallinity is not significant. The photo-sensitivity is obtained in the mixed structures between crystalline and amorphous given by proper ion damages. The amorphous parts probably contribute suppression of the grain-boundary defects. The observation of photo-sensitivity indicates the possibility of μc-Ge as a narrow gap material for PV cells.     

Low-temperature deposition of polycrystalline silicon thin films by hot-wire CVD

Polycrystalline silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430°C. The material properties have been optimized for photovoltaic applications by varying the hydrogen dilution of the silane feedstock gas, the gas pressure and the wire temperature. The optimized material has 95% crystalline volume fraction and an average grain size of 70 nm. The grains have a preferential orientation along the (2 2 0) direction. The optical band gap calculated from optical absorption by photothermal deflection spectroscopy (PDS) showed a value of 1.1 eV, equal to crystalline silicon. An activation energy of 0.54 eV for the electrical transport confirmed the intrinsic nature of the films. The material has a low dangling bond-defect density of 10¹⁷ cm³. A photo conductivity of 1.9 × 10⁻⁵ Ω⁻¹cm⁻¹ and a photoresponse (σ_ph/σ_d) of 1.4 × 10² were achieved. A high minority-carrier diffusion length of 334 nm as measured by the steady-state photocarrier grating technique (SSPG) and a large majority-carrier mobility-lifetime (μτ) product of 7.1 × 10⁻⁷cm²V⁻¹ from steady-state photoconductivity measurement ensure that the poly-Si : H films possess device quality. A single junction n---i---p cell made in the configuration n⁺-c-Si/i-poly-Si: H/p-μc-Si : H/ITO yielded 3.15% efficiency under 100 mW/cm² AM 1.5 illumination.     

Optical characterization of a-C:N thin films deposited by RF sputtering

In this work we present the main results of the optical properties study of amorphous carbon nitride (a-C:N) thin films prepared by reactive radio frequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto glass substrates, from a graphite target, in a pure nitrogen plasma. During the deposition, the pressure of nitrogen and the power density were maintained at 10⁻² mbar and 0.79 W cm⁻², respectively. Optical properties of these films were deduced from optical transmission spectra in the ultraviolet–visible–near infrared (UV–Vis–NIR) range. The refractive index follows well the Cauchy law with an extrapolated value of 1.68 in the far IR region. The optical band gap of the a-C:N films is about 1.2 eV. This value is relatively high in comparison with that of amorphous carbon films (0.8 eV) obtained in similar conditions. The incorporation of nitrogen in the amorphous carbon network leads to an increase of the optical band gap.     

Nanocrystalline silicon thin films deposited by high-frequency sputtering at low temperature

Intrinsic and n-type hydrogenated nanocrystalline silicon thin films (nc-Si:H) were deposited at a temperature as low as 95 °C by high-frequency (HF) sputtering, with hydrogen dilution percentage varying from 31% to 73%. In order to study the properties of the films prepared by this method, the samples were examined by infrared absorption spectroscopy (IR), X-ray diffraction (XRD), SEM, spectroscopic ellipsometry (SE), laser Raman spectrometry and atomic force microscopy (AFM). XRD measurements showed that this film has a new microstructure, which is different from the films deposited by other methods. In addition, an n-type nc-Si:H/p-type c-Si heterojunction solar cell, which has an open circuit voltage (V_oc) of 370 MV and a short-circuit current intensity (J_sc) of 6.5 mA/cm², was produced on the nanocrystalline silicon thin film. After 10 h light exposure under AM1.5 (100 MW/cm²) light intensity at room temperature, radiation degradation has not been found for the device.     

Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD

Polycrystalline silicon (poly-Si) films ( 10 μm) were grown from dichlorosilane by a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 Å/s at the substrate temperature (T_s) of 1030°C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn⁺ junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm², 1 cm²).     

The light stability of microcrystalline silicon thin films deposited by VHF-PECVD method

Microcrystalline silicon thin film is deposited under different conditions by plasma enhanced chemical vapor deposition. The light stability with different crystallinity and grain size is studied, and the growth mechanism is analyzed using the scaling behavior of roughening surface evolution. Degradation of photoconductivity mainly depends on crystallinity and grain size, but fundamentally, on the growth mechanism. Materials with high crystallinity and large grain size are more stable under light soaking. With the increasing of deposition pressure and input power, growth process transfers to zero diffusion limit growth mechanism, and films deposited present less grain size and poor light stability.     

Raman and XPS characterization of vanadium oxide thin films deposited by reactive RF sputtering

In this paper we report on Raman and XPS characterization of vanadium oxide thin films deposited by RF-sputtering. The samples were deposited by using a vanadium target in different oxygen fluxes, so that the stoichiometry (O/V ratio) of the oxide was varied. Several physical parameters of the films indicate a strong structural difference between the sample deposited at lower oxygen flux (1 scc m) and those obtained with higher flux (from 1.25 to 9 scc m). The increase of O/V ratio corresponds to a lower crystallinity of the thin films as indicated by the initial lowering and the final disappearance of the characteristic Raman mode of V₂O₅ (crystal) at about 140 cm⁻¹. For the highest flux samples new broad bands develop, typical of amorphous materials, both in polarized as well as in depolarized Raman spectra.     

XPS study of amorphous carbon nitride (a-C:N) thin films deposited by reactive RF sputtering

Amorphous carbon nitride (a-C:N) thin films were deposited by reactive radiofrequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto silicon substrates, from a graphite target of very high purity, in an atmosphere of pure nitrogen. The chemical properties of these films were studied by X-ray photoelectron spectroscopy (XPS). The XPS spectra of the a-C:N films reveal that nitrogen is well incorporated in the amorphous carbon network. The atomic percentage of nitrogen in the a-C:N films, calculated from the XPS spectrum, is about 32%. In addition to C–C and CC bonds, the analysis of the chemical shifts of C 1 s and N 1 s core level peaks show that nitrogen is bonded to carbon in CN double bonding and CN triple bonding configurations. The content of the CN triple bonds is found to be more important than the CN double bonds.     

Characterization of hydrogenated amorphous silicon-carbon films deposited by hybrid-plasma CVD

We propose a new deposition process, hybrid-plasma CVD, to overcome the difference in decomposition of source gases. In this new process, SiH₄ are CH₄are decomposed individually in a RF plasma and a microwave plasma, respectively, for the deposition of CH₄ hydrogenated amorphous silicon-carbon. Effectent decomposition of CH₄ by a microwave plasma reduces the excess hydrogenation of carbon atoms. Reactions in the hybrid-plasma CVD are investigated by quadruple mass spectroscopy. Film structures are investigated by Fourier transform infrared absopption and X-ray photoelectron spectroscopy, and electrical properties are also examined. The films show low hydrogenation of the C atom compared to the films of glow discharge method. Moreover, the films with optical bandgap less than 2.2 eV show photoconductivesies in the range of 10⁻⁵ S/cm.     

Structural and optical properties of hot wall deposited CdSe thin films

Cadmium selenide (CdSe) films were prepared by hot wall deposition technique using optimized tube length under a vacuum of 6 mPa on to well-cleaned glass and ITO substrates. The X-ray diffraction analysis revealed that the films are polycrystalline in nature for lower thickness and at lower substrate temperatures, but with increasing thickness and increasing substrate temperature a more preferred orientation along (0 0 2) direction was observed. The crystallite size (D), dislocation density (δ) and strain () were calculated. An analysis of optical measurements revealed a sharp absorption around 700 nm and a direct allowed transition. The band gap was found to be around 1.7 eV. The effect of thickness and substrate temperature on the fundamental optical parameters like band gap, refractive index and extinction coefficient are studied.     

MgH2 thin films deposited by one-step reactive plasma sputtering

The morphology, crystal structure, hydrogen content, and sorption properties of magnesium hydride thin films prepared by reactive plasma-assisted sputter deposition were investigated. Few micrometers-thick films were deposited on Si and SiO₂/Si substrates, at low pressure (0.4 Pa) and close to room temperature using (Ar + H₂) plasma with H₂ fraction in the range 15–70%. The microstructure and hydrogen content of the films are closely related to the surface temperature and hydrogen partial pressure during the deposition process. Operating in pulsed-plasma mode allows the hydrogenation rate of the MgH₂ thin film to top up to 98%, thereby producing a nearly fully hydrogenated film in a single-step process. The positive effect of the pulsed process is explained by the significant decrease in the whole energy flux incident on the surface and the favourable impact of the transient process for the rearrangement/relaxation of the materials. As for the hydrogen storage properties, desorption experiments and cycling of the films show the destabilizing effect of Mg₂Si formation at the interface between the film and the Si substrate resulting in a drastically increased desorption kinetics compared to less reactive SiO₂ substrate. However, the reaction is regrettably not reversible upon hydrogenation and the hydrogen storage capacity is consequently reduced upon cycling. Nevertheless, the deposition process carried out on inert substrates would offer true potential for reversible storage. Finally, our experimental results, which show the possibility to preferentially grow the metastable medium pressure γ-MgH₂ phase, open possibilities for the synthesis of more complex metastable phases such as magnesium-based ternary compounds.     

Structural and electrochemical studies of WO3 films deposited by pulsed spray pyrolysis

Transparent conductive and WO₃ electrochromic thin films were deposited by spray pyrolysis technique. The films were deposited using solutions of WCl₆ in dimethylformamide on SnO₂:F (FTO) substrates with different sheet resistances. Noticeable effects of substrate on structural, morphological and optical properties of the WO₃ films and on its electrochromic behavior are presented and discussed. Hexagonal and monoclinic WO₃ structures were obtained on amorphous glass substrates; also the monoclinic structure on polycrystalline FTO substrates was obtained. Cyclic structural changes during the colored and blanched states were found from XRD and electron diffraction result analysis: The hydrogen tungsten bronze in the tetragonal phase after the hydrogen extraction change to the original WO₃ monoclinic phase.     

Gallium-doped ZnO thin films deposited by chemical spray

Gallium-doped zinc oxide (ZnO:Ga) thin films were deposited on glass substrates by the spray pyrolysis technique. The effect of the variation of the [Ga]/[Zn] rate in the starting solution, the substrate temperature as well as the post-annealing treatments on the physical properties was examined. The electrical properties of the films show an improvement with the Ga incorporation and the annealing treatment. All the films were found to be polycrystalline and show a (0 0 2) preferential growth, irrespective of the deposition conditions. The films were of n-type conductivity with an electrical resistivity in the order of 8×10⁻³ Ω cm and optical transmittance higher than 80% in the visible region. These results makes chemically sprayed ZnO:Ga potentially applicable as transparent electrode in photovoltaic devices.     

Al-doped vanadium dioxide thin films deposited by PLD

Thin films of vanadium dioxide (VO₂) and Al³⁺-doped VO₂ were deposited on silicon and glass substrates using pulsed laser deposition (PLD). Optimized processing conditions were determined for depositing pure VO₂ with monoclinic phase by laser ablation of a V₂O₅ target. Al³⁺-doping levels in the VO₂ films were varied by altering the relative laser ablation time on the Al₂O₃ and V₂O₅ targets. The change in electrical conductivity with temperature in the semiconductor to metallic phase transition was measured for pure VO₂ and Al³⁺-doped VO₂ films. Doping the VO₂ films with Al³⁺ lowered the transition temperature directly on increasing the Al³⁺ content from 67 °C for the pure VO₂ films to 40 °C at 10% Al³⁺. The magnitude of the resistance change from semiconductor to metallic states also decreased with increase in Al³⁺ doping. The results imply that Al³⁺-doped VO₂ films could be a good candidate for energy-efficient “smart window” coatings used for architecture applications.     

Photoelectrical properties of ZnS thin films deposited from aqueous solution using pulsed electrochemical deposition

ZnS is an n-type semiconductor with a wide direct band gap (3.7 eV at room temperature), and it is very suitable as a window layer in heterojunction photovoltaic solar cells. We deposited ZnS thin films on Sn-doped In₂O₃-coated glass substrate using pulsed electrochemical deposition (ECD) from aqueous solutions containing Na₂S₂O₃ and ZnSO₄ with two different compositions, the first group grown from ZnSO₄-rich solution, and the second grown from Na₂S₂O₃-rich solution. We investigated electrical properties of the ZnS thin films and properties of contacts with different metals evaporated on the surfaces. We found that Au and In contacts have Ohmic-like characteristics to ZnS. Furthermore, we observed photoconductivity of the ZnS thin films by means of photoelectrochemical (PEC) measurements. We found that for both the groups of ZnS thin films, the as-deposited film shows weak photosensitivity and after annealing at 300 °C the photosensitivity improved.     

Analysis of semiconductor thin films deposited using a hollow cathode plasma torch

The hollow cathode plasma torch has been used for several years. One of the major applications has been the deposition of dielectric thin films. However, this technique has also been used to deposit metals where high-speed deposition is needed. It has proven to be useful in deposition of coatings onto the inside of substrates of complex shape, high-speed etching, and deposition of thin films at atmospheric pressure. In recent years, we have adapted the technique to deposit high-quality amorphous and polycrystalline semiconducting films. A large variety of measurement techniques have been employed to determine the film properties and the results are reported here.     

Crystallographic analysis of high quality poly-Si thin films deposited by atmospheric pressure chemical vapor deposition

Crystallinity of thin film polycrystalline silicon (poly-Si) grown by atmospheric pressure chemical vapor deposition has been investigated by X-ray diffraction measurement and Raman spectroscopy. Poly-Si films deposited at high temperatures of 850–1050°C preferred to 2 2 0 direction. By Raman spectroscopy, the broad peak of around 480–500 cm⁻¹ belonged to microcrystalline Si (μc-Si) phase was observed even for the poly-Si deposited at 950°C. After high-temperature annealing (1050°C) 3 3 1 direction of poly-Si increased. This result indicates that the μc-Si phase at grain boundary became poly-Si phase preferred to 3 3 1 direction by high-temperature annealing. Effective diffusion length of poly-Si films deposited at 1000°C was estimated to be 11.9–13.5 μm and 10.2–12.9 μm before and after annealing, respectively.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.     

Microcrystalline silicon solar cells fabricated by VHF plasma CVD method

A series of systematic investigations on microcrystalline silicon (μc-Si:H) solar cells at high deposition rates has been studied. The effect of high deposition pressure and narrow cathode-substrate (CS) distance on the deposition rate and quality of microcrystalline silicon is discussed. The microcrystalline silicon solar cell is adopted as middle cell and bottom cell in a three-stacked junction solar cell. The characteristics of large area three-stacked junction solar cells, whose area is 801.6 cm² including grid electrode areas, are studied in various deposition rates from 1 to 3 nm/s of microcrystalline silicon. An initial efficiency of 13.1% is demonstrated in the three-stacked junction solar cell with microcrystalline silicon deposited at 3 nm/s.