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.     

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.     

Material properties of microcrystalline silicon for solar cell application

The paper reviews the material requirements of microcrystalline silicon (μc-Si) in terms of the device operation and configuration for thin film solar cells and thin film transistors (TFTs). We investigated the material properties of μc-Si films deposited by using 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD) from a conventional H₂ dilution in SiH₄. Two types of intrinsic μc-Si films deposited at the high pressure narrow electrode gap and the low pressure wide electrode gap were studied for the solar cell absorption layers. The material properties were characterized using dark conductivity, Raman spectroscopy, and transmission electron microscope (TEM) measurements. The μc-Si quality and solar cell performance were mainly determined by microstructure characteristics. Solar cells adopting the optimized μc-Si film demonstrated high stability with no significant changes in solar cell performance after air exposure for six months and subsequent illumination for over 300 h. The results can be explained that low ion bombardment and high atomic hydrogen density under the PECVD condition of the high pressure narrow electrode gap produce high-quality μc-Si films for solar cell application.     

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.     

Microcrystalline silicon films and solar cells deposited by PECVD and HWCVD

The application of microcrystalline silicon (μc-Si:H) in thin-film solar cells is addressed in the present paper. Results of different technologies for the preparation of μc-Si:H are presented, including plasma enhanced chemical vapour deposition (PECVD) using 13.56 MHz (radio frequency, rf) and 94.7 MHz (very high frequency, vhf) and hot-wire chemical vapour deposition (HWCVD). The influence of the silane concentration (SC) on the material and solar cell parameters is studied for the different techniques as the variation of SC allows to optimise the solar cell performance in each deposition regime. The best performance of μc-Si:H solar cells is always observed near the transition to amorphous growth. The highest efficiency obtained so far at a deposition rate of 5 Å/s is 9.4%, achieved with rf-PECVD in a deposition regime of using high pressure and high discharge power. High deposition rates and solar cell efficiencies could be also achieved by vhf-PECVD. An alternative approach represents the HWCVD which also demonstrated high deposition rates for μc-Si:H. However, good material quality and solar cell performance could only be achieved at low substrate temperatures and, consequently, low deposition rates. The μc-Si:H solar cells prepared by HWCVD exhibit comparably high efficiencies up to 9.4% and exceptionally high open circuit voltages up to 600 mV but at lower deposition rates (≈1 Å/s). The properties of PECVD and HWCVD solar cells are carefully compared.     

Substrate temperature and hydrogen dilution: parameters for amorphous to microcrystalline phase transition in silicon thin films

Amorphous to microcrystalline phase transition in hydrogenated silicon (Si:H) is realized separately with the variations of substrate temperature and hydrogen dilution. The Raman spectroscopy reveals structural transformations and marks the transition. It occurs at 450°C with 10% silane concentration, whereas that is noted at 250°C with a silane concentration of 4.5%. The material evolved in the transition region is a well-developed amorphous matrix containing a small fraction (12%) of crystallites. A uniform distribution of small (100 Å) crystallites in the films is observed by transmission electron microscopy. The transition material is photosensitive.     

Formation mechanism and characteristics of nonequilibrium plasma by pulsed silent discharge

A nonequilibrium condition is often used in plasma processing. A silent discharge is one of the simplest methods for realizing this condition. In the case that a square pulsed voltage has been applied, this method enables high-energy input into a reaction field. However, the mechanism for this has never been clarified. In this study, an experiment on pulsed silent discharge has been carried out using a discharge tube with a corona electrode system and oxygen gas. The characteristics of this discharge were then investigated, and its mechanism estimated. As a result, it was clarified that the pulsed silent discharge, especially in the case of a developing positive streamer, has highly nonequilibrium characteristics. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(4): 207–218, 1997     

Structural,optical and electrochemical transient photoresponse properties of ZnO/Ga2O3 nanocomposites prepared by two-step CVD method

The influence of ZnO layer thickness on the structural, optical and electrochemical transient photoresponse properties of Ga₂O₃/ZnO heterostructured nanocomposites is evaluated. The increasing thickness of ZnO layer from 0 to 7.868 μm on the nanocomposites show an increase in the concentration of photogenerated electron-hole pairs, improve carrier separation and transportation to the electrolyte. This results to an enhanced photocurrent density from 164 to 833 μA/cm² at 1 V vs Ag/AgCl electrode. The unhybridized β-Ga₂O₃ film and the ZnO/Ga₂O₃ nanocomposites exhibit nanoblock-like and nanoplate-like morphologies, respectively. Structural studies reveal hexagonal ZnO and β-Ga₂O₃ crystalline phases for the nanocomposites. Optical bandgap of the reference β-Ga₂O₃ film is found to be 4.79 eV whereas, the nanocomposites exhibit two bandgaps: 3.30–3.25 eV belonging to the crystalline hexagonal ZnO phase, and 4.87–4.83 eV traced to the β-Ga₂O₃ phase. Photoluminescence spectroscopy exhibits blue-green emissions for the unhybridized film and ultraviolet-green emissions for the nanocomposites.     

Methanol conversion from methane and water vapor by electric discharge (Effect of electric discharge process on methane conversion)

The possibility of methanol conversion from a methane and water-vapor gas mixture was investigated for a new and highly efficient energy conversion system. Reforming process of methanol to hydrogen can be used for low-temperature thermal energy utilization. Direct methanol production from a methane and water-vapor mixture by spark or glowlike discharges has been achieved experimentally. A high methanol mole fraction of 0.5% has been obtained by both discharges. The effects of applied high voltage time, total pressure, and ratio of gas mixture on the conversion efficiency have been clarified experimentally. The electric energy consumption for methanol production by the spark discharge method is 1/100 that by the glow discharge method. The methanol conversion process has also been analyzed theoretically by considering the dissociation of the initial mixture gas by electrons and 104 elementary reactions. The results suggest that a very short period energy input such as a spark discharge can effectively produce methanol compared with a steady-state discharge such as a glowlike discharge. © 1999 Scripta Technica, Heat Trans Asian Res, 28(5): 404–417, 1999     

Study on hydrogenated silicon nitride for application of high efficiency crystalline silicon solar cells

The hydrogenated silicon nitride films (SiN_x:H) deposited by plasma enhanced chemical vapor deposition (PECVD) technique is commonly used as an antireflection coating as well as surface passivating layer of crystalline silicon solar cells. The refractive indices of SiN_x:H films could be changed by varying the growth gas ratio R(=NH₃/SiH₄+NH₃) and annealing temperature. For optimum SiN_x:H film, the optical and chemical characterization tools by varying the film deposition and annealing condition were employed in this study. Metal-insulator-semiconductor (MIS) devices were fabricated using SiN_x:H as an insulator layer and they were subjected to capacitance-voltage (C-V) and current-voltage (I-V) measurements for electrical characterization. The effect of rapid thermal annealing (RTA) on the surface passivation as well as antireflection properties of the SiN_x:H films deposited at various process conditions were also investigated for the fabrication of low cost and high efficiency silicon solar cells.     

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.     

Effect of alkaline-doping on photoelectrochemical activity of electrodeposited cuprous oxide films

P-type Cu₂O films with alkaline ions (Li⁺, Na⁺ and K⁺) of unintentional dopants on indium tin oxide coated glass substrate are successfully fabricated via a simple electrodeposition method for photoelectrochemical (PEC) hydrogen generation. The SEM and XRD analysis show the as-grown films with the pyramid-like morphology and cubic structure, and the composition of alkaline-doped Cu₂O films are examined using XPS spectroscopy to demonstrate the substitution of alkaline ions in the Cu₂O lattice. The optical analyses, including the absorbance and low-temperature photoluminescence spectra, confirm a bandgap of 2.3 eV and the presence of structural defects in alkaline-doped Cu₂O films. The Mott-Schottky plot shows the flat band potentials of the alkaline-doped Cu₂O films to be approximately ?0.1 V and the hole concentrations in the order of 10¹⁷ cm^?3. Significantly, the Cu₂O:Li film photocathode exhibits a higher photocurrent of ?2.2 mA cm^?2 at a potential of ?0.6 V vs Ag/AgCl which are greater than those of Cu₂O:K and Cu₂O:Na films due to greater preferred orientation degrees along (111) and less structural defects, because the ionic radii of Cu and Li is similar. These results demonstrate the great potential of alkaline doped Cu₂O films in solar-related applications.     

Linear discharge model,power losses and overall efficiency of the solid oxide fuel cell with thin film samarium doped ceria electrolyte. Part I: Linear discharge model

In this work, a solid oxide fuel cell with 60 μm samarium doped ceria film as the electrolyte is fabricated with a co-pressing technique. The performance of the cell is measured at 600, 650 and 700 °C. The corresponding maximum power outputs are 236, 331 and 401 mW cm^?2, respectively. The measured current–voltage (I–V) curves are straight lines. A linear discharge model is derived based on the Gorelov and Liu modified electromotive force (EMF) equations. The model fits the measured I–V curves with the maximum errors less than 1.5%. The overall activation overpotential of the cell is therefore postulated to be proportional to the polarization current.     

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.     

Study of the photon down‐conversion effect produced by thin silicon‐rich oxide films on silicon solar cells

In the present work, we studied the photon down‐conversion effect produced by thin films of silicon oxide with embedded silicon nanocrystals also called silicon‐rich oxide (SRO). These films have been used to absorb high energy light and the re‐emission of two or more low energy photons (~1.1 eV) with the goal of improving the external quantum efficiency and consequently the conversion efficiency of silicon solar cells. According to our results, the incorporation of a thin SRO film on the solar cell surface increases the short circuit current and the FF of the silicon solar cells; the enhancement of spectral response is due to the high photoluminescence intensity of the SRO in the visible region when irradiated with UV light. An improvement of 38% in the solar cell efficiency has been observed in our particular solar cell fabrication process by the use of an SRO film with high photoluminescence intensity, which replaces the conventional silicon dioxide film. Copyright © 2016 John Wiley & Sons, Ltd.     

Electrochromic properties of Ni oxide thin films in diluted acidic electrolytes and their stability

Electrochromic (EC) properties of sputtered Ni oxide films have been examined in 1 M KCl+H₂SO₄ acidic aqueous solutions with H₂SO₄ concentrations of 0–50 mM. EC coloration efficiency comparable to that in alkaline electrolytes was obtained in all the solutions and no remarkable degradation in charge capacity was observed up to 100 cycles. These results offer support for the practical construction of efficient complementary EC devices using dilute acidic aqueous electrolytes.     

The effect of annealing on the properties of diamond-like carbon protective antireflection coatings

In addition to its similarity to genuine diamond film, diamond-like carbon (DLC) film has many advantages, including its wide band gap and variable refractive index. Therefore, as one of the diverse applications, DLC film can be utilized as a protective coating for IR windows and an anti-reflective coating for solar cells. For this study, DLC films were prepared by the radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) method on silicon substrates using methane (CH₄) and hydrogen (H₂) gas. We examined the effects of the post-annealing temperature and the annealing ambient on structural, electrical and optical properties of DLC films. The films were annealed at temperatures ranging from 300 to 900 °C in steps of 200 °C using rapid thermal annealing equipment in nitrogen ambients. The thickness of the film was observed by scanning electron microscopy (SEM) and surface profile analysis. The variation of structure according to the annealing treatment was examined using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The reflectance of DLC thin film was investigated by UV–vis spectrometry and its electrical properties were investigated using a four point probe and I–V meter. The carrier lifetime of the film was also checked.     

Wet-etch texturing of ZnO:Ga back layer on superstrate-type microcrystalline silicon solar cells

Surface wet etching is applied to the ZnO:Ga (GZO) back contact in μc-Si thin film solar cells. GZO transparency increases with increasing deposition substrate temperature. Texturing enhances reflective scattering, with etching around 5-6 s producing the best scattering, whereas etching around 5 s produces the best fabricated solar cells. Etching beyond these times produces suboptimal performance related to excessive erosion of the GZO. The best μc-Si solar cell achieves FF=68%, V_OC=471 mV and J_SC=21.48 mA/cm² (η=6.88%). Improvement is attributed to enhanced texture-induced scattering of light reflected back into the solar cell, increasing the efficiency of our lab-made single μc-Si solar cells from 6.54% to 6.88%. Improved external quantum efficiency is seen primarily in the longer wavelengths, i.e. 600-1100 nm. However, variation of the fabrication conditions offers opportunity for significant tuning of the optical absorption spectrum.     

Effect of power and pressure on the properties of hydrogenated amorphous silicon films prepared by DC glow discharge

Hydrogenated amorphous silicon films prepared at different rates of deposition up to 18 Å s^-1, on the anode of a DC glow discharge reactor have been studied for their optoelectronic properties. In this system, use of earthed shield to confine the plasma has been utilised and one does not need to use any “grid”, as has previously been found necessary to deposit device quality films on insulating substrates. Films are grown at three pressure regimes, i.e. 0.3, 0.5 and 1.0 Torr without any dilution of silane and, it is found, that the best quality films at high rates can only be obtained at ≈ 0.5 Torr silane pressure in this system (δ_ph/δ_D ≈ 10⁵ and E₀ values 54.8 meV at 13 Å s^-1).     

Low temperature polycrystalline silicon: a review on deposition, physical properties and solar cell applications

This review article gives a comprehensive compilation of recent developments in low temperature deposited poly Si films, also known as microcrystalline silicon. Important aspects such as the effect of ions and the frequency of the plasma ignition are discussed in relation to a high deposition rate and the desired crystallinity and structure. The development of various ion energy suppression techniques for plasma enhanced chemical vapour deposition and ion-less depositions such as HWCVD and expanding thermal plasma, and their effect on the material and solar cell efficiencies are described. The recent understanding of several important physical properties, such as the type of electronic defects, structural effects on enhanced optical absorption, electronic transport and impurity incorporation are discussed. For optimum solar cell efficiency, structural considerations and predictions using computer modelling are analysed. A correlation between efficiency and the two most important process parameters, i.e., growth rate and process temperature is carried out. Finally, the application of these poly Si cells in multijunction cell structures and the best efficiencies worldwide by various deposition techniques are discussed.