Mercury-sensitized Photochemical Vapor Deposition Research Articles

Low-temperature Deposition of Hydrogenated Microcrystalline Silicon Thin Films by Photochemical Vapor Deposition Technique and Their Application to Thin Film Solar Cells

Hydrogenated microcrystalline silicon (µc-Si:H) thin films were prepared by mercury-sensitized photochemical vapor deposition (photo-CVD) technique at various deposition temperatures. Crystalline volume fraction showed a monotonic decrease with increasing temperature from 125 to 200 °C caused by the enhanced desorption of surface hydrogen atoms. Since a similar decrease has been reported at higher temperatures than 400 °C in plasma-enhanced vapor deposition technique, we conclude that photon energy from ultraviolet radiation serves to activate surface species in photo-CVD process. In spite of the low deposition temperature, the obtained films had excellent material properties. Then, thin film solar cells were fabricated on a Type-U substrate with various haze ratios provided by Asahi Glass using such films as the i-layer. A substrate with a high haze ratio of about 50% was effective in realizing a better light-trapping structure. By decreasing the deposition temperature of the i-layer to 140 °C, conversion efficiency was improved markedly due to the improved film quality brought about by the contributions of photons from ultraviolet radiation to the enhancement of the surface migration of the radicals for reducing defects in the resulting film. Consequently, a maximum solar cell efficiency of 7.4% was obtained.

Dependence of hydrogen and oxygen incorporation on deposition parameters in photochemical vapor deposited mercury free silicon nitride films

Silicon nitride films have been deposited on p-type Si (100) by mercury-sensitized photo-chemical vapor deposition (photo-CVD) method varying deposition pressure and substrate temperature. Energy dispersive X-ray fluorescence spectra of the samples show that the incorporation of mercury in the films, if any, is below 20 ppm. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy studies show the incorporation of oxygen and hydrogen in all the films, which is a function of the deposition parameters. Higher substrate temperature favors the formation of SiH bonds and reverse is the case for the formation of SiNH bonds. The sample deposited at low temperature (170 °C) shows the presence of less unreacted silicon (4%) in comparison to the sample (12.5% unreacted silicon) deposited at higher deposition temperature (250 °C), but the variation of pressure shows no significant change in terms of the unreacted silicon. The incorporated hydrogen and oxygen passivate surface defects thereby influencing interface electronic state densities ( D it) and fixed insulating charges ( Q ss).

Electrical properties of photo-CVD boron-doped hydrogenated nanocrystalline silicon-carbide (p-nc-SiC:H) films for uncooled IR bolometer applications

Electrical properties of boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) films grown by mercury-sensitized photo-chemical vapor deposition method have been investigated for uncooled IR bolometer applications. The temperature coefficient of resistance (TCR), conductivity, and 1/ f noise characteristics have been measured and the detectivity of a bolometer has been estimated from them. It has been observed that as ethylene gas flow ratio (C 2H 4/SiH 4) increases, TCR of films increases due to larger optical bandgap. However, the 1/ f noise also increases due to the reduction of the number of total carriers in the films for higher activation energy ( E a). The maximum TCR of 2.3%/K has been measured for the films deposited with an ethylene gas flow ratio of 0.47, while the maximum detectivity has been estimated for those with an ethylene gas flow ratio of 0.07 because of the lowest 1/ f noise.

Microcrystalline Silicon Films and Solar Cells Prepared by Photochemical Vapor Deposition on Textured SnO2 with High Haze Factors

Microcrystalline silicon (µc-Si) films and solar cells were prepared by mercury-sensitized photochemical vapor deposition (photo-CVD). The changes in the structural properties of a series of µc-Si films grown under various H2 dilution and deposition pressure conditions were discussed. The results indicated that the properties of µc-Si films depend strongly on the atomic hydrogen. The microstructures of µc-Si films on textured SnO2 with different haze factors (from 13% to 65%) were observed with a scanning electron microscope and a transmission electron microscope. The observations revealed that the µc-Si layers grew from the initial stage of deposition with columnar grains and that they were conformal to the surface of textured SnO2. The grain boundary density of µc-Si film on SnO2 with higher haze factors was lower than that on SnO2 with lower haze factors. The effect of textured SnO2 with different haze ratios on p–i–n µc-Si cell characteristics was discussed and it was found that the higher haze factors showed a higher degree of light trapping in our µc-Si cells. We deposited µc-Si cells with an intrinsic µc-Si layer thickness of 430 nm on a textured SnO2 sample with a haze factor of 42% and achieved a conversion efficiency of 6.55%.

Role of incorporated hydrogen in non-stoichiometric photo-deposited silicon nitride films

Silicon nitride films were deposited by mercury-sensitized photochemical vapor deposition utilizing a gaseous mixture of Si 2H 6 (2% in Ar) and NH 3 under 253.7 nm ultraviolet light irradiation. Non-stoichiometric silicon-rich and nitrogen-rich samples were deposited by varying gas flow ratio. High partial pressure of disilane results in the formation of clusters, with the incorporation of hydrogen in the form of SiH and NH stretching modes below the detection level of Fourier transform infrared spectroscopy. In the nitrogen-rich sample the bonded hydrogen concentration is 0.56×10 18 cm −2. After annealing the samples at 350 °C for 3 h in a pressure of 50 mTorr, the interface electronic state density decreases for silicon-rich sample and increases for nitrogen-rich sample.

Phosphorous Doping of Strain-Induced Si1-yCy Epitaxial Films Grown by Low-Temperature Chemical Vapor Deposition

Pseudomorphic Si1-yCy alloy layers were grown on Si(001) substrates by plasma-enhanced chemical vapor deposition and mercury-sensitized photochemical vapor deposition. By depositing the films using SiH4, H2 and SiH2(CH3)2 at a very low substrate temperature of 200°C, we successfully obtained epitaxial Si1-yCy films with a high substitutional C composition of up to 3.5%. In situ P doping was carried out using PH3. Si1-yCy films required annealing to activate the P atoms. However, the measured Hall mobility of the Si1-yCy films decreased after annealing at 700°C. After thermal treatment at 900°C for 1 min, the mobility recovered to the same level as that of bulk Si and an electron concentration of about 1019 cm-3 was achieved.

Highly conductive boron-doped nanocrystalline silicon-carbide film prepared by low-hydrogen-dilution photo-CVD method using ethylene as a carbon source

Boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) films with a low-concentration of hydrogen-dilution were grown by a mercury-sensitized photo-chemical vapor deposition method using silane (SiH 4), hydrogen (H 2), diborane (B 2H 6), and ethylene (C 2H 4) as a carbon source. From the Raman and FTIR spectrum measurements, the p-nc-SiC:H film is composed of nanosize crystal silicon embedded in a hydrogenated amorphous silicon-carbide matrix. A dark conductivity as high as 1.7×10 −1 S/cm, with an optical bandgap is 2.0 eV, and a crystal volume fraction of 50%, were obtained. We tested these films as window material for amorphous silicon solar cells, obtaining an initial conversion efficiency of 10.4% without using any back reflectors.

Novel Carbon Source (1,3-Disilabutane) for the Deposition of P-Type a-SiC

AbstractWe proposed a new carbon source, 1,3-disilabutane (H3Si-CH2-SiH2-CH3:1,3-DSB), to grow hydrogenated amorphous silicon carbide (a-SiC:H) films by mercury-sensitized photochemical vapor deposition (photo-CVD). We described preliminary results of undoped and p-type a-SiC films deposited using 1,3-DSB. It was found that the optical energy gap of the films was changed even at very small 1,3-DSB/silane ratios of few percents. P-type doping was carried out by using diborane and we obtained the films with a darkconductivity of 1.3x10-4 S/cm at the optical bandgap of 2.1 eV. In addition, we applied this material for a p-layer of a p-i-n type a-Si based solar cell and we have achieved relatively high conversion efficiency of 9.55%.

Aluminum-seeded growth of microcrystalline Si thin film onto SnO2 substrate

To improve the crystallinity of thin boron-doped microcrystalline Si (p-μc-Si:H) films at an initial growth stage, we tried an Al-seeding method, which is to deposit an ultrathin (∼50 Å) Al layer onto a transparent conducting oxide before preparing the p-μc-Si:H film by using a mercury-sensitized photochemical vapor deposition method. It was found that the Al layer serves as seeds to facilitate the p-μc-Si:H with high crystallinity at an initial growth stage and the Al seeds stimulate the nucleation of Si crystallites. It was also found that the absorption loss in the ultrathin (∼50 Å) Al-seed layer is negligible in the visible wavelength region due to its natural oxidation. When used in superstrate-type p-i-n amorphous silicon solar cells, even the Al-seeded p-μc-Si:H film having a several hundred angstrom thickness acts as an excellent p layer compared to the conventional p-μc-Si:H.

Improvement of Stabilized Efficiency of Amorphous Silicon Solar Cell by SiH2Cl2 Addition

The characteristics of mercury-sensitized photo-chemical vapor deposition (CVD) grown a-Si:H films and solar cells in the case of dichrolosilane (SiH2Cl2) addition were investigated. The films with a small amount of SiH2Cl2 addition exhibited high photosensitivity, low hydrogen content and low defect density. However, the cells with an i-layer grown by SiH2Cl2 addition showed a low fill factor. It was also observed by secondary ion mass spectroscopy (SIMS) measurements that about 1019 atom/cm3 Cl was incorporated into a-Si:H films, even at the flow rate ratio of 0.01 (SiH2Cl2 to SiH4). Furthermore, it was found that the excess Cl in a-Si:H films was removed by H2 dilution. In the case of H2 dilution together with SiH2Cl2 addition to the i-layer of a-Si solar cells, a high fill factor and high quantum efficiencies, particularly in the medium wavelength region was observed. From numerical analysis, it was determined that the improvement was due to the increase of the electric field in the medium of the i-layer after light soaking. As a result, we achieved a high stabilized efficiency of 9.0% for a 1 cm2 single-junction a-Si:H solar cell.

Boron-doped amorphous diamondlike carbon as a new p-type window material in amorphous silicon p-i-n solar cells

A boron-doped hydrogenated amorphous diamondlike carbon (a-DLC:H) was prepared using a mercury-sensitized photochemical vapor deposition (photo-CVD) method. The source gases were B2H6 and C2H4. By increasing the boron doping ratio (B2H6/C2H4) from 0 to 12 000 ppm, the dark conductivity increased from ∼10−9 to ∼10−7 S/cm. A boron-doped a-DLC:H with an energy band gap of 3.8 eV and a dark conductivity of 1.3×10−8 S/cm was obtained at a doping ratio of 3600 ppm. By using this film, amorphous silicon (a-Si) solar cells with a novel p-a-DLC:H/p-a-SiC double p-layer structure were fabricated using the photo-CVD method and the cell photovoltaic characteristics were investigated as a function of a-DLC:H layer thickness. The open circuit voltage increased from 0.766 V for the conventional cell with a 40-Å-thick p-a-SiC to 0.865 V for the cell with a p-a-DLC:H (15 Å)/p-a-SiC (40 Å) double p-layer structure. The thin (<15 Å) p-a-DLC:H layer proved to be an excellent hole emitter as a wide band gap window layer.

Post Hydrogen Treatment Effects of Boron-doped a-SiC:H p-Layer of a-Si:H Solar Cell Using a Mercury-Sensitized Photo-Chemical Vapor Deposition Method

Post hydrogen treatment effects of boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film used as a p-layer of p/i/n type amorphous silicon based solar cells using a mercury-sensitized photochemical vapor deposition method were investigated by measuring the thickness, electrical, and optical properties of the film before and after hydrogen treatment. It was found that the boron-doped a-SiC:H film was simultaneously etched and passivated by the treatment. The performance of the cell with a hydrogen treated p-layer was improved by ∼7% due to an increase in open circuit voltage (V oc) and fill factor (F.F.), compared to that of the untreated cell, although the p-layer thickness was nearly identical in both cases. The increase in V oc and F.F. could be explained by an increase in the built-in potential due to a decrease in the film activation energy. This electrical property improvement was well explained by the passivation effect of a SiH2/SiH ratio decrease of the film calculated by FTIR spectra. These film changes by post hydrogen treatment are considered to occur in the bulk of the boron-doped a-SiC:H film.

Etching and passivation effects on boron-doped amorphous silicon carbide p layer of amorphous silicon solar cell by hydrogen treatment using a mercury-sensitized photochemical vapor deposition method

Etching and passivation effects of hydrogen treatment of boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film used as a p layer of p-i-n type amorphous silicon based solar cells using a mercury-sensitized photochemical vapor deposition method were investigated. For the hydrogen treatment of the p-layer film, longer p-layer deposition time was needed to obtain the same thickness as for no hydrogen treatment because of hydrogen etching effect. However, the cell performance was improved by ∼7% due to an increase in the open circuit voltage (Voc) and fill factor (FF) although the p-layer thickness was nearly identical in both cases. The increase in the Voc and FF could be explained by an increase in the built-in potential due to a decrease in the film activation energy. Moreover, the electrical property improvement of the film was well explained by the passivation effect of a SiH2/SiH ratio decrease and a hydrogen content increase calculated from Fourier transformed infrared absorption measurements.

Preparation of High-Quality Undoped Microcrystalline Silicon with High Deposition Rate Using Mercury-Sensitized Photochemical Vapor Deposition Method

We prepared undoped microcrystalline silicon films (µ c- Si:H) using a mercury-sensitized photochemical vapor deposition system under various film deposition conditions of SiH4/(SiH4+H2) gas ratio and total gas flow rate, and investigated the dependence of the film quality on the deposition conditions in order to obtain high-quality undoped microcrystalline films at a high deposition rate. Up to a SiH4/(SiH4+H2) gas ratio of ∼0.1, the crystallite size of the µ c- Si:H films decreased linearly as the ratio increased, but the volume fraction of crystallites remained constant at about 80%. The dark conductivity of the films was very low, in the range of 10-7–10-6 Scm-1 under nearly all experimental conditions, and so a high photosensitivity of ∼550 was obtained without the addition of a dopant gas such as diborane. In addition, a high deposition rate of ∼30 Å/min was obtained at a low total flow rate of 21 sccm. No degradation of the microcrystallinity was observed for the low total flow rate.

Properties and Time Dependences of Silicon Oxynitride Films Deposited by Low-Temperature Photochemical Vapor Deposition

Silicon oxynitride films ( SiNx Oy ) were deposited using mercury-sensitized photochemical vapor deposition. The photochemical vapor deposition (photo-CVD) SiNx Oy film was used as an antireflective coating (ARC) for a single-crystal Si solar cell. It was found that the refractive index of SiNx Oy film deposited at low temperatures showed maked time dependence.

Improvement of a-Si Solar Cell Fabricated by Mercury-Sensitized Photochemical Vapor Deposition Using H2 Dilution Technique

The fabrication of hydrogen-diluted amorphous silicon (a-Si) film by the mercury-sensitized photochemical vapor deposition (photo-CVD) method has been performed. The hydrogen content in the film along with the optical bandgap was found to increase with the hydrogen/silane dilution ratio, while the dihydride/monohydride bond ratio decreased at the substrate temperature of 150° C. As a result, a decrease in the defect density was observed. Moreover, the photo- and dark conductivities were maintained in the range of 3×10-5 and 1×10-10 S/cm, respectively, in spite of the fact that the optical bandgap widened. The solar cells with hydrogen diluted i-layers were fabricated and showed improvement in the open-circuit voltage and fill factor (FF), resulting in the increase of both initial and stabilized efficiencies. The forward- and reverse-bias dark I-V characteristics measurement of solar cells revealed improvement in quality and stability of the hydrogen-diluted i-layers.

Photo-CVD silicon nitride thin layers as pH-ISFET sensitive membrane

Silicon nitride films have been deposited by the process of mercury-sensitized photochemical vapor deposition (photo-CVD), using a gaseous mixture of SiH 4 and NH 3 under 253.7 nm UV light irradiation. The main advantages of this technique are low deposition temperature (200–400 ° C) and no radition damage. The optimal conditions of the deposit are defined in order to obtain a stable sensor with a surface sensitive to pH: optimal temperature of 200 °C, gas flow rate (SiH 4/NH 3) of 1/80 and deposition time of 8 min. An annealing is necessary at 400 °C in N 2 atmosphere for 20 s. Under these conditions, a deposition thickness f 560 Å and refractive index of 1.97 were obtained. The silicon nitride ISFET obtained has a quasi-nernstian pH response with a sensitivity equal to 53.5±0.5 mV pH −1, due tothe NH 2 sites at the surface of the silicon nitride deposit. The stability of the response when the silicon nitride surface is kept in contact with an aqueous solution is about five months.

Low-temperature epitaxial growth of undoped and n-doped silicon by photochemical vapor deposition using SiH 4/SiH 2Cl 2/H 2/PH 3 mixtures

Undoped and doped silicon films were grown epitaxially on (100) substrates of the same material from a gas mixture of dichlorosilane, silane, hydrogen and phosphine, employing mercury-sensitized photochemical vapor deposition (photo-CVD). The films were characterized using reflection high energy electron diffraction (RHEED), Raman spectroscopy, infrared transmittance and electrical measurements. RHEED patterns and Raman measurements revealed the formation of single-crystal silicon films grown at temperatures as low as 150 °C. The best crystallinity was observed on films with a low hydrogen content (calculated from infrared measurements) and was equivalent to that reported previously for photo-CVD films grown from Si 2H 6/SiH 2F 2/H 2 mixtures at lower growth rates. At the phosphine molar fraction 1 × 10 −5 used in this work, the electrical properties of n-doped films are affected in an appreciable manner by hydrogen dilution and temperature.

Reaction Mechanism of Mercury-Sensitized Photochemical Vapor Deposited Silicon Oxide

The deposition mechanism for silicon oxide (SiOx) films grown by the mercury-sensitized photochemical vapor deposition (photo-CVD) method using silane (SiH4) and nitrous oxide (N2O) under irradiation of a low-pressure mercury lamp has been studied. An increasing-naximum-decrease pattern of the deposition rate with increasing silane partial pressure ratio (Ps=PSiH4/Ptotal) has been observed. Deposition rates for the SiO-like and SiO2-like films have been analyzed based on the Langmuir-Hinshelwood surface reaction theory. Rate equations were also derived. By means of the Arrhenius plots, activation energy of the surface reaction and the heat of adsorption can be determined.

Effect of Annealing on Undoped a-Si:H Films Prepared by Mercury-Sensitized Photochemical Vapor Deposition

ABSTRACTThe effects of annealing on undoped a-Si:H films were studied. Films of a-Si:H were prepared by mercury-sensitized photochemical vapor deposition, and were annealed immediately. The hydrogen configuration was altered in the annealing process below 200°C, although films were prepared at a substrate temperature of 230°C. At the same time, electron drift mobility greatly increased. These observations indicate a reduction in the number of tail states in the a-Si:H band-gap. Thus the annealing below the substrate temperature clearly influences not only the bonded-hydrogen atoms but also the Si-Si network in the films to some extent.