Widegap a-Si:H films prepared at low substrate temperature

Wide bandgap hydrogenated amorphous silicon (a-Si:H) films have been prepared by the PECVD method at a low substrate temperature (80°C) controlling the incorporation of hydrogen (bonded with silicon) into the film. Optimizing the deposition parameters viz. hydrogen dilution, rf power, a-Si:H film with E_g ∼ 1.90 eV and σ_ph ≥ 10⁻⁴ Scm⁻¹ has been developed. This film exhibited better optoelectronic properties compared to a-SiC:H of similar optical gap. The quantum efficiency measurement on the Schottky barrier solar cell structure showed a definite enhancement of blue response. Surface reaction as well as structural relaxation under suitable deposition condition have been claimed to be responsible for the development of such material.     

Inhomogeneities in PECVD deposited a-Si:H films induced by a spacing between substrate and substrate holder

Homogeneous deposition of a-Si:H films on glass substrates is routinely done by plasma deposition. A major cause of non-uniformity is the presence of a spacing between substrate and (grounded) electrode to which it is mounted. We have observed a reduced deposition rate if a gap exists between the glass substrate and the metal electrode. The amount of deposition-rate reduction scales with the size of this gap. A reduction of 50% of the initial deposition rate is measured in the case of a gap of 2-mm thickness, for a 65 MHz SiH₄/H₂ plasma at 0.35 mbar. In addition, the material quality is affected. Filling the gap with a dielectric (Corning 7059 glass) leads to a smaller reduction, i.e., of 20%. These effects are explained by using the equivalent electrical circuit of the plasma reactor system: an extra capacitor representing the gap is added. For a conductive substrate no deposition-rate reduction is observed, which supports the electrical origin of the effect.     

Structural, optical and electrical characterizations of μc-Si:H films deposited by PECVD

The effects of the silane concentration f on the structural, optical and electrical properties of undoped hydrogenated silicon films prepared in a plasma-enhanced chemical vapour deposition system have been studied. The electrical conductivity and Hall mobility appear to be controlled by microstructures induced by silane concentration and a clear electrical transport transition from crystalline to amorphous phase has been found when 3%<f<4%. A two-phase model has been used to discuss the electrical properties.     

A-Si:H buffer in a-SiGe:H solar cells

Profiled a-SiGe:H-buffer layers between the doped and the absorption layers of amorphous silicon germanium (a-SiGe:H) solar cells are routinely used to avoid bandgap discontinuities and high-defect densities at the p/i- and i/n interface. Here, we present a much simpler approach replacing the profiled a-SiGe:H-buffer layers at both interfaces by a-Si:H-buffer layers. It is demonstrated that for a-SiGe:H solar cells (thickness of the E_G=1.5 eV part is 54 nm) these structures yield similar open circuit voltage V_OC and fill factor (FF) compared to the bandgap profiled layer at the same short circuit current density j_SC. The influence of thickness, optical bandgap and position of the buffer layers on the solar cell performance is investigated.     

Comments on: “Gallium-doped ZnO thin films deposited by chemical spray”: [Sol. Energy Mater. Sol. Cells 87 (2005) 107–116]

In this communication, I will discuss some aspects concerning a paper recently published by H. Gomez et al. on the properties of gallium-doped ZnO thin films deposited by chemical spray methods [H. Gomez, A. Maldonado, M. de la L. Olvera, D.R. Acosta, Sol. Energy Mater. Sol. Cells 87 (2005) 107.]. I believe that there are some wrong considerations of basic principles, some incorrect experimental procedures and inconsistencies on the analysis and throughout the paper. The “Introduction” reflects some knowledge on the subject, but it seems that the authors of the aforementioned paper use outdated background sources in some cases, and inconsistently apply prior research in others.The “Experimental procedure” shows some serious lack of knowledge concerning thin films that are potentially applicable as transparent electrodes in photovoltaic devices. In the “Results and discussion”, I found some of the arguments and statements contradictory, mainly when the effects of their vacuum annealing were being discussed. Considerations on the “Structural properties” are necessary, as they are not appropriate for comparative purposes. Finally, a last point, but not less important than the others, is the “Conclusions” of the paper. Some suggestions are made below for the improvement of this, and several others, aspects of the paper.     

Effects of substrate temperature on the properties of facing-target sputtered Al-doped ZnO films

ZnO:Al films (Al 2.5 wt%) were deposited using a DC facing targets magnetron sputtering via two ZnO targets mixed with Al₂O₃. The structural, electrical and optical properties of the deposited films were strongly influenced by substrate temperature. Films with better texture, higher transmission, lower resistivity and larger carrier concentration were obtained for the samples fabricated at higher substrate temperature. The optimal condition for deposition of ZnO:Al film with the lowest resistivity of 3.18×10⁻⁴ Ω cm, the highest carrier concentration of 4.58×10²⁰ cm⁻³, and a transmission toward 85% in the visible range was obtained at 200 °C. This film proposes a promising future for the application of the practical window and contact layers for solar cells.     

Dielectric and conduction studies on hot-wall deposited CdSe films

CdSe films are deposited using hot-wall deposition technique on glass and ITO substrates. From the XRD analysis, the structural parameters like crystallite size, dislocation density and strain were calculated. Films had preferential orientation along (0 0 2) and the structure of the film corresponded to wurtzite nature. From the EDAX analysis a slight increase in the cadmium content is observed as thickness increases. The dielectric study has been carried out on the stoichiometric films at different frequencies and temperatures to study their effect on capacitance, dielectric constant and dielectric loss. To explore the effect of illumination on these fundamental dielectric parameters, measurements are taken in dark as well as under an illumination of 1000 lx. and observed slight variation in these parameters. The temperature coefficient of capacitance, relative permittivity and linear expansion coefficient are evaluated. From the AC conduction studies the conduction was found to be due to hopping. Variation of conductivity with temperature reveals the presence of two activation energies. The Mott–Schottky plot for the films yields the value for carrier concentration in the range 10¹⁷–10¹⁸ cm⁻³ and the conduction was found to be n-type.     

Control of the properties of wide bandgap a-SiC : H films prepared by RF PECVD method by varying methane flow rate

The influence of increase in flow rate of CH₄ in a source gas mixture of SiH₄+CH₄+H₂ for the preparation of high-quality wide band gap a-SiC : H film by the RF PECVD method has been studied. We have been able to increase the optical gap of the film by increasing CH₄ flow rate under appropriate deposition conditions. These films are structurally better which also shows good opto-electronic properties. This has been achieved mainly by using CH_n (where n=3, 2 or 1) precursors in the plasma as the etchant for weak bonds on the growing surface of a-SiC : H films.     

Influence of silane flow on structural, optical and electrical properties of a-Si:H thin films deposited by hot wire chemical vapor deposition (HW-CVD) technique

The electrical, structural and optical properties of hydrogenated amorphous silicon (a-Si:H) films deposited from pure silane (SiH₄) using hot wire chemical vapor deposition (HW-CVD) technique are systematically studied as a function of silane flow rate between 5 and 30 sccm. We found that the properties are greatly affected by the silane flow rate over the range we studied. The device quality a-Si:H films with a photosensitivity >10⁵ were deposited by HW-CVD at a deposition rate >10 Å s⁻¹ using low silane flow rate. However, a-Si:H films deposited at higher silane flow rate and/or higher deposition rates show degradation in their structural and electrical properties. The FTIR studies indicate that the hydrogen bonding in a-Si:H films shifts from mono-hydrogen (Si–H) to di-hydrogen (Si–H₂) and (Si–H₂)_n complexes when films were deposited at higher silane flow rate. The hydrogen content in the a-Si:H films increases with increase in silane flow rate and was found to be less than 10 at.%. The Raman spectra show increase in disorder and the Rayleigh scattering with increase in silane flow rate. The optical band gap also shows an increasing trend with silane flow rate. Therefore, only the hydrogen content cannot be accounted for the increase in the optical band gap. We think that the increase in the optical band gap may be due to the increase in the voids. These voids reduce the effective density of material and increase the average Si–Si distance, which is responsible for the increase in the band gap. Silane flow rate of 5 sccm, appears to be an optimum flow rate for the growth of mono-hydrogen (Si–H) bonded species having low hydrogen content (4.25 at%) in a-Si:H films at high deposition rate (12.5 Å s⁻¹), high photosensitivity (10⁵) and small structural disorder.     

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.     

Photoelectrochemical investigations on electrochemically deposited CdSe and Fe-doped CdSe thin films

Thin films of CdSe and Fe-doped CdSe (Fe:CdSe) were deposited onto stainless steel substrates by electrodeposition technique. The photoelectrochemical investigations have been carried out using the cell configurations CdSe/1 M (Na₂S–S–NaOH)/C and Fe:CdSe/1 M (Na₂S–S–NaOH)/C for studying the current–voltage (I–V) characteristics in dark and under illumination, photovoltaic output, spectral response, photovoltaic rise and decay characteristics. The studies reveal that films are n-type conductivity. The junction quality factor in light (n_l), series and shunt resistance (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. After Fe doping, efficiency and FF of PEC solar cell is found to be improved from 0.34% and 31.12 to 1.80% and 35.78, respectively.     

Physical properties of chemical bath deposited CdS thin films

Cadmium sulfide films of different thicknesses were deposited by chemical bath deposition (CBD) from a bath containing cadmium chloride, ammonium chloride, ammonium hydroxide and thiourea. The XRD patterns show that the films have a hexagonal phase with a preferential (0 0 2) orientation. The photoluminescence spectra show a defect structure, characteristics of the CdS films obtained by CBD. The electrical behavior in dark and under illumination, the optical properties and the band gap value reported in this work is in agreement with that reported in the literature.     

Computer-aided band gap engineering and experimental verification of amorphous silicon–germanium solar cells

A new band gap profile (exponential profile) for the active layer of the a-SiGe:H single junction cell has been designed and experimentally demonstrated. By computer simulations we show how bending the grading of the band gap in the i-layer contributes to the enhancement of the carrier collection, improving the fill factor and efficiency. The differences observed between experiments and simulations are studied using Rutherford Backscattering Spectrometry (RBS). The results highlight weak points during the deposition process, whose control enables us to bring together experimental and computational results.     

More stable low gap a-Si:H layers deposited by PE-CVD at moderately high temperature with hydrogen dilution

In the present work, several series with variation of deposition parameters such as hydrogen dilution ratio, VHF-power and plasma excitation frequency f_exc have been extensively analyzed. Compared with “conventional” more-stable layers obtained at 200–250°C and high H₂ dilution ratios of about 10, it was observed that electrical transport properties after light-induced degradation of layers deposited at “moderately high” temperatures (300–350°C) are equivalent but required lower H₂ dilution ratios (between 2 and 4). As a consequence, the deposition rate of more stable layers obtained at moderately high temperatures is increased by a factor of 2. Moreover, optical gaps of a-Si:H deposited at 300–350°C are significantly lower (by approx. 10 meV); furthermore, they decrease with f_exc.     

Effect of annealing on the electrical, optical and structural properties of hydrogenated amorphous silicon films deposited in an asymmetric R.F. plasma CVD system at room temperature

This paper reports the effect of annealing on hydrogenated amorphous silicon films (a-Si : H) deposited by r.f. self-bias technique on cathode in an asymmetric r.f. plasma CVD system at room temperature. Detailed study of the variation of the dark and photoconductivity (σ_D and σ_ph) as a function of temperature and light intensity, surface morphology, hydrogen evolution, optical absorption, subgap absorption and related parameters, thermal and structural disorder on the optical-absorption edge, IR vibrational modes and bonded hydrogen content have been carried out on unannealed and annealed samples at different temperatures (T_a) from 100°C to 550°C. It is found that the values of σ_ph increase and that of Urbach energy (E_o), subgap defect density (N_d) and the polyhydride to monohydride ratio decrease upto T_a=250°C and beyond 250°C the values of σ_ph decrease and that of E_o, N_d and the polyhydride to monohydride ratio increase. The best opto-electronic properties with much improved σ_ph and σ_ph/σ_D and dominant monohydride bonding are obtained after annealing the room temperature deposited film at 250°C for 1 h. The σ_D data obeys a Meyer Neldel rule in annealed a-Si : H films. The value of optical band gap is found to be related to the E_o and the hydrogen content. The Urbach energy (E_o) which is a measure of the disorder is the sum of structural and thermal disorder. The structural disorder part decreases with the annealing temperature upto 300°C and thereafter it increases. The curves of optical absorption coefficient versus photon energy at different T_a converge to a common point.     

The effect of heteropolyacids and isopolyacids on the properties of chemically bath deposited CdS thin films

The deposition of CdS films on ITO/glass substrates from a chemical bath containing cadmium acetate, ammonia, ammonium acetate and thiourea has been carried out with and without small amounts of heteropolyacids (HPA) (phosphotungstic acid (PTA): H₃[PW₁₂O₄₀], silicotungstic acid (STA): H₄[SiW₁₂O₄₀], phosphomolybdic acid (PMA): H₃[PMo₁₂O₄₀]) and isopolyacids (IPA) (tungstic acid (TA): H₂WO₄ and molybdic acid (MA): H₂MoO₄) for different deposition times. The chemical, morphological, structural and optical properties of the films have been determined. The composition in sulphur and in cadmium of the films’ surface and volume was determined for various HPA and IPA used in the deposition bath. The HPA and IPA which give the thickest film with the biggest grain size were deduced. The optical transmission at 400 nm of CdS films deposited with STA at short time (20 min) (50%) is higher than those of CdS deposited at longer time (6 h) (7%). The optical transmission of CdS deposited with STA at short time is higher (50%) than that of CdS deposited without STA (20%). The performances of heterojunctions CdS/CdTe solar cells fabricated from CdS films deposited with and without STA and CdTe films deposited without STA have been determined. It was shown that the CdS/CdTe heterojunction solar cells fabricated from CdS films deposited with STA exhibited better photon collection efficiency and solar cell efficiency (η=6%) than CdS/CdTe heterojunction solar cells fabricated from CdS films deposited without STA (η=3.3%).     

Deposition of hydrogenated amorphous silicon (a-Si:H) films by hot-wire chemical vapor deposition (HW-CVD) method: Role of substrate temperature

Hydrogenated amorphous silicon (a-Si:H) thin films were deposited from pure silane (SiH₄) using hot-wire chemical vapor deposition (HW-CVD) method. We have investigated the effect of substrate temperature on the structural, optical and electrical properties of these films. Deposition rates up to 15 Å s⁻¹ and photosensitivity 10⁶ were achieved for device quality material. Raman spectroscopic analysis showed the increase of Rayleigh scattering in the films with increase in substrate temperature. The full width at half maximum of TO peak (Γ_TO) and deviation in bond angle (Δθ) are found smaller than those obtained for P-CVD deposited a-Si:H films. The hydrogen content in the films was found <1 at% over the range of substrate temperature studied. However, the Tauc's optical band gap remains as high as 1.70 eV or much higher. The presence of microvoids in the films may be responsible for high value of band gap at low hydrogen content. A correlation between electrical and structural properties has been found. Finally, the photoconductivity degradation of optimized a-Si:H film under intense sunlight was also studied.     

A comparative study on the hydrogen absorption of thin films at room temperature deposited on non-porous glass substrate and nano-porous anodic aluminum oxide (AAO) template

The influence of different morphology of the thin films deposited on the non-porous glass and nano-porous anodic aluminum oxide (AAO) substrates on the hydrogen absorption at room temperature was studied. A well-known sandwich-like Pd/Mg/Pd film was investigated. It is observed that the film deposited on the porous AAO template demonstrates a better hydrogen absorption in the H₂ pressure range 10-600 mbar with respect to the same film supported by the non-porous glass substrate. Moreover, the layer grown on the AAO, owing to its specific morphology inherited from the nano-porous substrate, has revealed its resistance toward stress accumulation caused by the lattice expansion, showing no buckle-to-crack network formation upon the hydrogen uptake. This interesting feature is expected to improve the cycling properties and structural stability of the system, and may help to investigate better the interaction of the H₂ with metal.     

Properties of spray deposited titanium dioxide thin films and their application in photoelectrocatalysis

Thin films of titanium dioxide were deposited onto optically transparent, electrically conducting substrates (fluorine doped tin oxide on glass). The two oxide layers, SnO₂ and TiO₂, were deposited sequentially by spray pyrolysis. TiO₂ films of up to 800 nm thickness were prepared by varying the quantity of sprayed solution (titanyl acetylacetonate in methanol), at a growth rate of 0.15 nm/s.The effect of film thickness on the structural, optical and photoelectrochemical properties of TiO₂ films was studied. Scanning electron microscopy showed that the polycrystalline anatase films were compact. The grain size increased up to 1100 nm with increase in film thickness, whereas the crystallite size remained constant (40 nm) as shown by X-ray diffraction. The films had a transmittance of more than 70% in the visible region.Junctions of the semiconducting films with aqueous electrolytes were rectifying and photoactive. Films of 330 or 600 nm were thick enough to exhibit maximum photoelectrochemical response for light of a wavelength of 313 or 365 nm, respectively. Under depletion conditions, an IPCE (incident photon to current conversion efficiency) of 0.8 for a 330 nm thick film at 313 nm was obtained.Oxalic acid degradation under UVA light and under sunlight, applying electrical bias, was demonstrated using these electrodes.     

Optical properties of chemical vapor deposited thin films of molybdenum and tungsten based metal oxides

Thin films of tungsten oxide, molybdenum oxide and mixed MoO₃–WO₃ oxides were obtained by atmospheric pressure chemical vapor deposition (CVD). All the films were prepared using identical technological parameters and through investigation of the optical properties of as deposited and annealed at 400°C a comparative study is reported. Raman, IR and VIS spectrophotometry and spectral ellipsometry methods were used. The mixed MoO₃–WO₃ films have higher optical absorption with maxima at a closer position with respect to the human eye sensitivity peak at 2.5 eV. The observed electrochromic effect is better expressed in the mixed films; the electrical charge inserted is higher.