Hot Wire Cell Method Research Articles

Fabrication and characterization of strained Si 1− yC y n-MOSFETs grown by Hot Wire Cell method

By applying low-temperature process below 800 °C, we have fabricated n-MOSFET with a strained Si 1− y C y channel. The strained Si 1− y C y films were grown on Si substrate with a substrate temperature of 200 °C by using a Hot Wire Cell method. In the previous work, the degradation of the device performance was found due to the poor interface between the directly grown Si 1− y C y channel layer and the source/drain regions. In this work, we applied the new device fabrication process and improved the electrical characteristics of the Si 1− y C y MOSFET. Furthermore, it was succeeded to enhance the effective electron mobility of n-MOSFET by introducing stress into the Si 1− y C y channels.

Intrinsic Microcrystalline Silicon Thin Films Prepared by Hot-Wire Cell Method and Their Application to Solar Cells

Microcrystalline silicon (µc-Si:H) thin films and solar cells were prepared by a hot-wire cell (HW-cell) method. A maximum conversion efficiency of 6.0% (Voc: 0.50 V, Jsc: 19.69 mA/cm2, F.F.: 0.61, active area: 0.086 cm2, AM1.5) was obtained for µc-Si:H solar cells prepared at a low filament temperature of 1800°C. The influence of filament temperature on µc-Si:H film properties was investigated. As a result, it was found from secondary ion mass spectroscopy (SIMS) results that tungsten (W) and aluminum (Al) concentrations largely increase with increasing filament temperature from 1800°C to 2100°C. Next, we found that a considerable degradation of solar cell performances occurs with aging. The degradation of intrinsic µc-Si:H absorber layers with aging was investigated, and it was found that the peak at 1000–1100 cm-1 originating from Si–O–Si bonds and spin density markedly increase. SIMS results also showed that O and C atoms in our µc-Si:H films are as low as 2×1018 cm-3 in the initial state and the films can easily undergo post oxidation. However, it was also found that a deposition at a high substrate temperature of more than 165°C is very effective in realizing high-stability µc-Si:H films and that a 3-µm-thick a-Si:H cap layer can prevent post oxidation.

2-Step Growth Method and Microcrystalline Silicon Thin Film Solar Cells Prepared by Hot Wire Cell Method

Hot Wire Cell (HW-Cell) method has been developed in order to grow microcrystalline silicon (µc-Si:H) thin films. The influence of various deposition parameters on the structural and electrical properties of the films was investigated to improve film quality. It was found that the concentrations of O and C atoms in µc-Si:H films could be reduced from the order of 1021 cm-3 to the order of 1020 cm-3 by decreasing the partial pressure of SiH4 from 100 mTorr to 3 mTorr. Then, a novel 2-step growth method was proposed in order to reduce the incubation layer in the initial growth of µc-Si:H i-layer. By using this method, Jsc largely increased (10.11 → 18.32 mA/cm2), and as a result, the conversion efficiency of 3.9% could be achieved. The influence of the incubation layer on solar cell performances was also investigated by a numerical analysis. To date, a conversion efficiency of 5.3% (Voc: 0.48 V, Jsc: 20.56 mA/cm2, F.F.: 0.54, active area: 0.086 cm2, AM1.5) was obtained for µc-Si:H solar cells with an i-layer thickness of 1.0 µm. Furthermore, high-rate depositions were investigated and a maximum deposition rate of 11.5 nm/s could be achieved. µc-Si:H solar cells fabricated at a high deposition rate of 1.5 nm/s showed a conversion efficiency of 2.8% (Voc: 0.42 V, Jsc: 12.31 mA/cm2, F.F.: 0.54, active area: 0.086 cm2, AM1.5).

Characterization and Comparison of Strained Si1-yCy Metal Oxide Semiconductor Field-Effect Transistor Grown by Gas-Source Molecular Beam Epitaxy and Hot Wire Cell Method

Strained Si1-yCy metal oxide semiconductor field-effect transistors (MOSFETs) were fabricated by gas-source molecular beam epitaxy (GS-MBE) and the Hot Wire (HW) Cell method, and their electrical characteristics were compared. The strained Si1-yCy films were grown by GS-MBE at 600°C and by the HW-Cell method at 200°C. The electron mobility of the MOSFET fabricated by GS-MBE showed a large decrease while that fabricated by the HW-Cell method showed a slight decrease. It was considered that this difference was due to the difference in non-substitutional carbon content. We found that the increase in growth temperature caused the decrease in substitutional carbon content and increase in non-substitutional carbon content. These results indicated that lowering the growth temperature decreases the non-substitutional carbon content and improves the electrical characteristics of Si1-yCy films.

Highly Conductive Boron Doped Microcrystalline Si Films Deposited by Hot Wire Cell Method and its Application to Solar Cells

Boron doped microcrystalline silicon (p-µc-Si) films were successfully deposited by the Hot Wire Cell method using a gas mixture of pure silane (SiH4) and diborane (0.5% B2H6 in H2). The influence of various deposition parameters on the structural and electrical properties of the films was investigated to obtain highly conductive p-µc-Si films. A high dark conductivity (σd) of 84.6 S/cm and a low activation energy of 0.02 eV were achieved for 550-nm-thick films. For thin films with a thickness of 15 nm, a σd of 4×10-2 S/cm was obtained. The thin p-µc-Si was incorporated into p–i–n amorphous silicon (a-Si) and microcrystalline silicon (µc-Si) solar cells, in which intrinsic layers were deposited by photo chemical vapor deposition (photo-CVD). The initial conversion efficiencies of 9.04% and 6.17% were obtained for a-Si and µc-Si solar cells, respectively.

Amorphous Silicon Thin Films Prepared by Hot Wire Cell Method and Its Application to Solar Cells

Amorphous Si (a-Si:H) and microcrystalline Si (µc-Si:H) materials were successfully deposited by the Hot Wire Cell method. We have studied these materials from the viewpoint of improving their qualities and the solar cell performances. As a result, it was found that the peak at 2100 cm-1 which originates from Si–H2 bonding in the infrared absorption spectra could be dramatically reduced at a low deposition pressure of 10 mTorr. Furthermore, it was shown by secondary ion mass spectroscopy (SIMS) analysis that the concentrations of O and C atoms in a-Si:H films could be reduced to the order of 1018 cm-3 at low deposition pressures of 1–10 mTorr. The conversion efficiency of 7.5% was achieved for superstrate-type pin a-Si solar cells with a deposition rate of 0.4 nm/s under AM1.5 insolation.

Highly conductive microcrystalline silicon carbide films deposited by the hot wire cell method and its application to amorphous silicon solar cells

Microcrystalline silicon carbide (μc-Si 1− x C x ) films were successfully deposited by the hot wire cell method using a gas mixture of SiH 4, H 2 and C 2H 2. It was confirmed by Fourier transform infrared and X-ray diffraction analyses that the films consisted of μc-Si grains embedded in a-Si 1− x C x tissue. The p-type μc-Si 1− x C x films were deposited using B 2H 6 as a doping gas. A dark conductivity of 0.2 S/cm and an activation energy of 0.067 eV were obtained. The p-type μc-Si 1− x C x was used as a window layer of a-Si solar cells, in which the intrinsic layer was deposited by photo-chemical vapor deposition, and an initial conversion efficiency of 10.2% was obtained.

Epitaxial growth of strained Si 1− yC y films by the hot-wire cell method and its application to metal oxide semiconductor devices

We succeeded in obtaining strained Si 1− y C y films at a substrate temperature of 200 °C by the hot-wire cell method. The substitutional carbon concentration in films annealed at 700 °C was 0.9%, while it was limited to 0.13% for a sample grown by gas-source molecular beam epitaxy (MBE) at a substrate temperature of 700 °C. We investigated the thermal stability of strained Si 1− y C y films for device application. Annealing at over 900 °C caused the formation of 3C-SiC and relaxation of the strain occurred. From this result, we found that the process temperature should be lower than 800 °C. A low-temperature MOSFET process, in which all process temperatures after deposition of Si 1− y C y were lower than 800 °C, was developed and a strained Si 1− y C y MOSFET was fabricated.

Characterization of low temperature epitaxial Si and Si 1− yC y films grown by hot wire cell method

The hot wire (HW) cell method was applied to grow epitaxial Si and strained Si 1− y C y films at very low substrate temperature. The epitaxial Si films were successfully obtained on Si(001) at a substrate temperature ( T sub) of 150–250 °C and a SiH 4 pressure of 0.015–0.03 Torr by using only SiH 4. The structure of the Si films became polycrystalline or microcrystalline either at T sub of over 300 °C or the pressure of over 0.06 Torr. Carbon was introduced to the films by using C 2H 2 gas and the epitaxial growth of Si 1− y C y at low T sub was also possible by using the hydrogen dilution. The concentration of substitutional C was 0.9%, when the ratio [C 2H 2/SiH 4] was 0.04. Furthermore, X-ray reciprocal lattice space mapping, indicated the pseudomorphic growth of Si 1− y C y alloy on Si. The thermostability of strained Si 1− y C y films grown by HW cell method was also investigated and the concentration of the substitutional C in the films decreased by annealing at over 800 °C.

Low temperature epitaxial growth of Si and Si1−yCy films by hot wire cell method

The Hot Wire (HW) Cell method was applied to grow epitaxial Si films at a substrate temperature of 200°C. C2H2 gas was added to this system and C containing epitaxial Si films were also obtained. After annealing the films at 600–700°C, the vibration mode at 607 cm−1, which indicated the presence of the C atoms located at the Si substitutional sites, was observed in Fourier transform infrared absorption and Raman scattering spectroscopy. The X-ray diffraction peak of the annealed films shifted to a greater extent compared to the substrate, which indicated the decrease in the lattice constant of the epitaxial layer. The carbon composition was controlled by changing the partial pressure of the C2H2 gas and we successfully obtained 0.8 at.% substitutional carbon composition.

High-rate deposition of silicon thin-film solar cells by the hot-wire cell method

The hot-wire cell method has been developed to grow polycrystalline and amorphous Si thin films with relatively high growth rates of 0.4–3.0 nm s−1. It was found that polycrystalline Si films can be obtained at substrate temperatures of 175–400°C without hydrogen dilution when the filament temperature is 2000–2100°C. Valency control has been carried out using PH3 and B2H6. Up to now, high conductivities of 13 and 4 S cm−1 have been achieved for n- and p-type polycrystalline Si thin films, respectively. Superstrate-type polycrystalline Si and amorphous Si solar cells prepared with deposition rates of 0.4–1.0 nm s−1 showed efficiencies of 1.6 and 4.3% under AM1.5 illumination, respectively. We found by SIMS analysis that a high concentration of O and C atoms, of the order of 1020–1021 cm−3, is incorporated into the film, which limits the performance of the present cell.

High-rate deposition of polycrystalline silicon thin films by hot wire cell method using disilane

A new process, hot wire cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low-temperature and high deposition rate. In the hot wire cell method, reactant gases are decomposed by a heated tungsten filament. Polycrystalline silicon films can be deposited at deposition rates of 1.2 nm/s for mono-silane (SiH 4) and 2.8 nm/s for disilane (Si 2H 6). By using disilane as a reactant gas, it is possible to achieve a high deposition rate without any change in the quality of the films.

Amorphous-to-Polycrystalline Silicon Transition in Hot Wire Cell Method

Hot Wire Cell method has been newly developed and successfully applied to grow polycrystalline silicon films at a low temperature with a relatively high growth rate. In the Hot Wire Cell method, reactant gases are decomposed as a result of reaction with a heated tungsten filament placed near a substrate and polycrystalline silicon films can be deposited at a growth rate of 0.9 nm/s without hydrogen dilution. The film crystallinity is changed from polycrystalline to amorphous by decreasing the total pressure. The model calculation of the Hot Wire Cell method is carried out and it is assumed that transition of crystallinity may be due to the shift in the preferential impinged radicals. X-ray analysis clearly showed that the films grown at the filament temperature of 1700°C have a very strong (220) preferential orientation. The films consist of large grains as well as small grains. These results suggest that the Hot Wire Cell method is a promising candidate to grow device-grade polycrystalline silicon films for photovoltaic application.

Comparison of gas-phase reactions in low-temperature growth of Si films by photochemical vapor deposition and the hot wire cell method

Mercury sensitized photochemical vapor deposition and hot wire cell method were studied as growth techniques of Si films at low temperature (<400°C). In both techniques, the boundaries indicating the structural transition of the Si films were observed to depend on the substrate temperature and the total pressure. The results of the theoretical analysis indicated that the film structure is correlated with the ratio of atomic hydrogen to SiH 3 radical on the growing surface. By the hot wire method, polycrystalline Si films were successfully obtained on glass substrate without H 2 dilution. These films had a growth rate >1 nm/s, which was about 10 times larger than that of the films grown by photochemical vapor deposition. This method can generate a larger flux of SiH 3 and the flux of H is also large enough to induce crystallization on glass substrate.

New approach to low-temperature Si epitaxy by using hot wire cell method

Hot wire (HW) cell method was applied to Si epitaxy and high-quality epitaxial Si films were obtained at a pressure of approximately 0.05 Torr. The growth rate increased linearly with increasing SiH 4 flow rate. It was found that Si epitaxy was possible for substrate temperatures ranging from 200 to 400°C. However, there was a critical thickness for epitaxy below 600°C.

Hydrogen Dilution Effect on the Crystallinity of Silicon Films Grow by Hot Wire Cell Method

AbstractA new process, the Hot Wire Cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low temperature and high growth rate. In the Hot Wire Cell method, reactant gases are decomposed as a result of reacting with a heated tungsten filament placed near to a substrate and polycrystalline silicon films can be deposited at a growth rate of 1.2nm/s without hydrogen dilution and 0.9nm/s with the use hydrogen dilution. The film crystallinity changed from amorphous to polycrystalline due to the addition of hydrogen, thus hydrogen dilution was effective for improving film crystallinity. Furthermore, we obtained (220) oriented polycrystalline silicon thin films with a 90% crystal fraction by the use of hydrogen dilution. These results showed that the Hot Wire Cell method is promising for the deposition of device-grade polycrystalline silicon films for photovoltaic applications.

Low Temperature Deposition of Polycrystalline Silicon Thin Films Prepared by Hot Wire Cell Method

AbstractHot wire (HW) cell method has been newly developed and successfully applied to grow polycrystalline silicon films at a low temperature with a relatively high growth rate. In the HWcell method, silane is decomposed by reaction with a heated tungsten wire placed near the substrate. It is found that polycrystalline silicon films can be obtained at substrate temperatures of 175-400°C without hydrogen dilution. The film crystallinity is changed from polycrystalline to amorphous with decreasing the total pressure. The X-ray analysis clearly showed that the films grown at the filament temperature of 1700°C have a very strong (220) preferential orientation. The films consist of large grains as well as small grains, and it was found from cross-sectional SEM that the films have columnar structure. These results suggested that the HW-cell method would be a promising candidate to grow device-grade polycrystalline silicon films for photovoltaic application.

Thermal Conductivity and Rotational Relaxation in Some Polar Gases

Measurements of thermal conductivities of hydrogen sulfide, hydrogen chloride, and ammonia have been carried out over the temperature ranges −78.5°–200°, 0°–200°, and −26.2°–20.1°C, respectively, by the hot-wire cell method. The data have been interpreted by taking into consideration the effects of resonant exchange of rotational energy. The temperature dependence of rotational collision numbers Zrot derived from thermal conductivity values has also been investigated.