Field effect surface passivation of SiO2/Si interfaces by heat treatment with high-pressure H2O vapor

We investigated a simple field effect passivation of the silicon surfaces using the high-pressure H₂O vapor heating. Heat treatment with 2.1×10⁶ Pa H₂O vapor at 260°C for 3 h reduced the surface recombination velocity from 405 cm/s (before the heat treatment) to 38 cm/s for the thermally evaporated SiO_x film/Si. Additional deposition of 140 nm-SiO_x films (x<2) with a high density of fixed positive charges on the SiO₂/Si samples further decreased the surface recombination velocity to 22 cm/s. We also demonstrated the field effect passivation for n-type silicon wafer coated with thermally grown SiO₂. Additional deposition of 210 nm SiO_x films on both the front and rear surfaces increased the effective lifetime from 1.4 to 4.6 ms. Combination of thermal evaporation of SiO_x film and the heat treatment with high-pressure H₂O vapor is effective for low-temperature passivation of the silicon surface.     

Characterization of pulsed laser crystallization of silicon thin film

Increases in the melt duration of silicon films were achieved by electrical current heating during and after pulsed excimer laser heating. When 50 nm thick amorphous silicon films formed on glass substrate were irradiated by 28-ns-pulsed excimer by applying 1.8 μs long pulsed-voltage at 100 V to the films, the silicon films were melted for the duration of the voltage pulse. The power threshold for heat energy for this long melting by the self-heating effect of the silicon films was 3.0×10⁵ W/cm². The high electrical conductivity of the silicon film (2.9×10⁻² S/cm) was found after regrowth of the silicon using a laser energy density of 360 mJ/cm² and a pulsed voltage of 150 V. The advantages of the long melt duration for large crystalline growth are discussed.     

Stability of thin film solar cells having less-hydrogenated amorphous silicon i-layers

In this study, highly stabilized hydrogenated amorphous silicon films and their solar cells were developed. The films were fabricated using the triode deposition system, where a mesh was installed between the cathode and the anode (substrate) in a plasma-enhanced chemical vapor deposition system. At a substrate temperature of 250 °C, the hydrogen concentration of the resulting film (Si–H=4.0 at%, Si–H₂<1×10²⁰ cm⁻³) was significantly less than that of conventionally prepared films. The films were used to develop the i-layers of solar cells that exhibited a significantly low degradation ratio of 7.96%.     

Effects of water in phthalocyanine layers

The molecular water concentration inside zinc phthalocyanine (ZnPc) thin films was measured. After exposure to air, gas effusion experiments show that the ZnPc layers contain (1.7±0.4)×10²⁰ water molecules per cm³, which corresponds to 1 H₂O per 10 ZnPc units. We can distinguish a mobile and an immobilized population of H₂O in ZnPc films. The mobile part effuses out at room temperature when exposing the films to a low pressure of 10⁻² mbar, whereas temperature activation is needed to reach a complete out-diffusion of water. The effusion process was observed to proceed with a diffusion coefficient D_H2O of (1.3±0.3)×10⁻¹⁰ cm² s⁻¹ at 296 K. The rate of water effusion directly correlates with the timescale of the decrease of surface conductivity when exposing the layers to an equally low pressure. This indicates the existence of an electrically active surface layer of water molecules, which is refilled from the bulk of water molecules during the effusion process.     

Photo atomic layer deposition of transparent conductive ZnO films

Low-resistivity ZnO films were grown by photo atomic layer deposition (photo-ALD) technique using diethylzinc (DEZ) and H₂O as reactant gases. Self-limiting growth was achieved for the temperature range from 105°C to 235°C. It was found that UV light irradiation was very effective to increase the electron concentration of the films and the electron concentration of 5 × 10²⁰ cm⁻³ was achieved even in undoped ZnO. Thus, the resistivity of the films grown with UV irradiation was one order of magnitude less than that grown without UV irradiation. The minimum resistivity of 6.9 × 10⁻⁴Ω cm was obtained by photo- ALD method without any intentional doping.     

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.     

Effect of film thickness on electrical property of microcrystalline silicon

We report dependences of electrical properties on SiH₄/H₂ dilution rate and film thickness for microcrystalline silicon films formed by a hydrogen radical-induced chemical vapor deposition (HRCVD) method. The electrical conductivity of the films at SiH₄ 18 sccm /H₂ 120 sccm was markedly increased to 10⁻³ S/cm as film thickness increased above 100 nm. Crystalline grains with (2 2 0) orientation were formed. Theoretical analysis revealed that grain boundaries among (2 2 0) grains had a low defect density of 1×10¹² cm⁻² so that the high conductivity was achieved.     

In situ hydrogen plasma treatment for improved transport of (4 0 0) oriented polycrystalline silicon films

Polycrystalline silicon (poly-Si) films were deposited on glass by very high-frequency (100 MHz) plasma enhanced chemical vapor deposition from a gaseous mixture of SiF₄ and H₂ with small amounts of SiH₄. (2 2 0) oriented films prepared at small SiF₄/H₂ ratios (<30/40 sccm) showed intrinsic transport properties of poly-Si. However, the room temperature dark conductivity (σ_d) of the (4 0 0) oriented film was very high for intrinsic poly-Si, 7.2×10⁻⁴S/cm. This conductivity exhibited a T^−1/4 behavior, suggesting a high defect density at the grain boundaries. It was found that in situ hydrogen plasma treatment successfully produced (4 0 0) oriented poly-Si with a reasonably low σ_d of 4.5×10⁻⁷S/cm and a good photoconductivity of 1.3×10⁻⁴S/cm.     

Fabrication of stable hydrogenated amorphous silicon from SiH2Cl2 by ECR-hydrogen-plasma

Relatively stable high bandgap hydrogenated amorphous silicon (a-Si:H) films were prepared by the microwave electron cyclotron resonance (ECR) hydrogen plasma CVD method using SiH₂Cl₂ source gas. The substrate position relative to the position of the generation of reactive species affects the structure of these films. The films prepared under optimized condition showed rather high bandgap, 1.83 eV. However, the defect density was low, 3 × 10¹⁵ cm⁻³, and the photosensitivity was greater than 7 orders of magnitudes. The defect density was found to saturate at relatively low values ( 3 × 10¹⁶ cm⁻³) independent of the illumination intensity.     

High rate growth of device-grade microcrystalline silicon films at 8 nm/s

We have developed a novel technique for large-area high rate growth of microcrystalline silicon films by plasma-enhanced chemical vapor deposition, designing a novel cathode with interconnected multi-holes, which leads to produce uniformly flat-distributed stable high-density plasma spots near cathode surface. The spatial distribution of plasma at holes on cathode surface was analyzed using optical emission spectroscopy for SiH₄/H₂ plasma with various pressures with a view to optimizing deposition conditions. Improvement of properties of high-rate-grown films was discussed with regard to silane depletion as well as the temperature of film-growing surface. Microcrystalline silicon films with a low defect density of 5×10¹⁵ cm⁻³ obtained at a high rate approaching 8 nm/s demonstrate the effectiveness of the novel cathode.     

Pyrite (FeS2) thin films prepared by spray method using FeSO4 and (NH4)2Sx

A simple spray method for the preparation of pyrite (FeS₂) thin films has been studied using FeSO₄ and (NH₄)₂S_x as precursors for Fe and S, respectively. Aqueous solutions of these precursors are sprayed alternately onto a substrate heated up to 120°C. Although Fe–S compounds including pyrite are formed on the substrate by the spraying, sulfurization of deposited films is needed to convert other phases such as FeS or marcasite into pyrite. A single-phase pyrite film is obtained after the sulfurization in a H₂S atmosphere at around 500°C for 30 min. All pyrite films prepared show p-type conduction. They have a carrier concentration (p) in the range 10¹⁶–10²⁰ cm⁻³ and a Hall mobility (μ_H) in the range 200–1 cm²/V s. The best electrical properties (p=7×10¹⁶ cm⁻³, μ_H=210 cm²/V s) for a pyrite film prepared here show the excellence of this method. The use of a lower concentration FeSO₄ solution is found to enhance grain growth of pyrite crystals and also to improve electrical properties of pyrite films.     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

Contact resistivities of sputtered TiN and Ti---TiN metallizations on solar-cell-type-silicon

Contact resistivities of TiN and Ti---TiN contacts on a shallow junction solar-cell-type silicon substrate have been investigated. The contact materials were sputter-deposited. The method of the transmission line model was applied for contact resistivity measurements. The contact resistivity of the n⁺Si---TiN contact system was 2 × 10⁻³ Ωcm² ± 50 per cent and remained constant after annealing up to 700°C for 30 min. For the n⁺Si---Ti---TiN system, the contact resistivity of 9 × 10⁻⁴ Ωcm² ± 50 per cent was measured. A heat treatment of 700°C. 30 min decreases this value by one order of magnitude and the interposed Ti fully reacts with Si and forms a TiSi₂ layer. The voltage drop caused by the n⁺Si---TiN contact system in a standard non-concentrator solar cell is negligible. The n⁺Si---TiSi₂---TiN contact system should be acceptable for Si solar cells used at up to 100 times solar concentration.     

Effect of proton irradiation on electrical properties of CuInSe2 thin films

High-energy proton irradiation (380 keV and 1 MeV) on the electrical properties of CuInSe₂ (CIS) thin films has been investigated. The samples were epitaxially grown on GaAs (0 0 1) substrates by Radio Frequency sputtering. As the proton fluence exceeded 1×10¹³ cm⁻², the carrier concentration and mobility of the CIS thin films were decreased. The carrier removal rate with proton fluence was estimated to be about 1000 cm⁻¹. The electrical properties of CIS thin films before and after irradiation were studied between 80 and 300 K. From the temperature dependence of the carrier concentration in CIS thin films, we found N_D=9.5×10¹⁶ cm⁻³, N_A=3.7×10¹⁶ cm⁻³ and E_D=21 meV from the fitting to the experimental data on the basis of the charge balance equation. After irradiation, a defect level was created, and N_T=1×10¹⁷ cm⁻³ for a fluence of 3×10¹³ cm⁻², N_T=5.7×10¹⁷ cm⁻³ for a fluence of 1×10¹⁴ cm⁻² and E_T=95 meV were also obtained from the same fitting. The new defect, which acted as an electron trap, was due to proton irradiation, and the defect density was increased with proton fluence.     

Determination of emittance of selective absorbing surfaces

The hemispherical emittance of the selective absorbing coating on the outside of the inner glass tube of an all-glass evacuated collector tube has been determined, using calorimetry at steady state in the temperature range 50–300°C and gas pressure range 1.0×10⁻³–1.6 Pa at the jacket between cover and inner glass tubes. Calculated gas heat flux q_c and equivalent emittance _g based on the theory of gas conduction at medium and low pressures have been determined. The calculations agree well with experiments. The experimental results indicate that heat losses of all-glass evacuated collector tubes due to gas convection and conduction are negligible when the gas pressure in the tube is less than 5×10⁻² Pa.     

Transformation of amorphous iron oxide thin films predeposited by spray pyrolysis into a single FeSe2-phase by selenisation

The present paper investigates a simple and non-toxic method to transform amorphous iron oxide pre-deposited by spray pyrolysis of FeCl₃·6H₂O (0.03 M)-based aqueous solution onto glass substrates heated at 350 °C into FeSe₂ thin films. The amorphous iron oxide films were heat treated under a selenium atmosphere (10⁻⁴ Pa) at different temperatures for 6 h. X-ray diffraction (XRD) was used to investigate the structure of the obtained films. Single FeSe₂-phase films having good crystallinity were obtained at a selenisation temperature of 550 °C. Optical analyses of the FeSe₂ films obtained at 550 °C enabled us to deduce a large absorption coefficient (, ). Surface scanning electron microscopy (SEM) observations show inhomogeneous films. Electrical conductivity of the as-prepared films was measured at high and low temperatures.     

Effect of 8 MeV electron irradiation on electrical properties of CuInSe2 thin films

Radiation damages due to 8 MeV electron irradiation in electrical properties of CuInSe₂ thin films have been investigated. The n-type CuInSe₂ films in which the carrier concentration was about 3×10¹⁶ cm⁻³, were epitaxially grown on a GaAs(0 0 1) substrate by RF diode sputtering. No significant change in the electrical properties was observed under the electron fluence <3×10¹⁶ e cm⁻². As the electron fluence exceeded 10¹⁷ e cm⁻², both the carrier concentration and Hall mobility slightly decreased. The carrier removal rate was estimated to be about 0.8 cm⁻¹, which is slightly lower than that of III–V compound materials.     

Preparation of dense and uniform La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) films for fundamental studies of SOFC cathodes

Thin films of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) were deposited on (1 0 0) silicon and on GDC electrolyte substrates by rf-magnetron sputtering using a single-phase oxide target of LSCF. The conditions for sputtering were systematically studied to get dense and uniform films, including substrate temperature (23–600 °C) background pressure (1.2 × 10⁻² to 3.0 × 10⁻² mbar), power, and deposition time. Results indicate that to produce a dense, uniform, and crack-free LSCF film, the best substrate temperature is 23 °C and the argon pressure is 2.5 × 10⁻² mbar. Further, the electrochemical properties of a dense LSCF film were also determined in a cell consisting of a dense LSCF film (as working electrode), a GDC electrolyte membrane, and a porous LSCF counter electrode. Successful fabrication of high quality (dense and uniform) LSCF films with control of thickness, morphology, and crystallinity is vital to fundamental studies of cathode materials for solid oxide fuel cells.     

Properties of fluorine-doped SnO2 films prepared by the pyrosol deposition method

Fluorine-doped SnO₂ (SnO₂:F) films were prepared in ordinary atmosphere on heated Corning 7059 glass substrates by the pyrosol deposition method with solutions consisting of SnCl₄·5H₂O, NH₄F, CH₃OH, H₂O and HCl. It was found that the substrate temperature and the chemical composition of the solutions largely affect the deposition rate and the properties of the SnO₂:F films. Under the optimized deposition condition, a resistivity as low as ≈4.3 × 10⁻⁴ Ω cm and a specular transmittance of ≈79% could be attained for a ≈0.6 μm thick film. X-ray diffraction measurements showed that these films were polycrystalline with the tetragonal cassiterite structure and grew with a (200) preferred orientation. The surface morphology observed by scanning electron microscopy changed from round-shape to pyramidal-shape above a substrate temperature of 450°C. A similar change in the surface morphology also took place when the CH₃OH/H₂O mol ratio in solution was less than 0.1. X-ray photoelectron spectroscopy indicated that the fluorine concentrations in the films, being significantly diminished, also increase with increasing the fluorine concentrations in the starting solutions.     

Physical properties of Bi doped CdTe thin films grown by the CSVT method

A study of the physical properties of CdTe thin films doped with Bi is presented. CdTe:Bi thin films were deposited by the close space vapor transport (CSVT) technique using powdered CdTe:Bi crystals grown by the vertical Bridgman method. CdTe:Bi crystals were obtained with nominal Bi doping concentrations varying in the 1×10¹⁷–8×10¹⁸ cm⁻³ range. The physical properties of CdTe:Bi thin films were studied performing photoluminescence, X-ray, SEM, photoacoustic spectroscopy and resistivity measurements. We observed a decrease of the resistivity values of CdTe:Bi films with the Bi content as low as 6×10⁵ Ω-cm for Bi concentrations of 8×10¹⁸ cm⁻³. These are meaningful results for CdTe-based solar cells.