High Frequency Plasma Enhanced Chemical Vapor Deposition Research Articles

Investigation of structural disorder using electron temperature in VHF-PECVD on hydrogenated amorphous silicon films for thin film solar cell applications.

Electrode distances and gas flow ratios are important parameters for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma-enhanced chemical-vapor deposition (VHF-PECVD) system. In this work, we investigated the relationship between the electrode distances and gas flow ratios on the properties of i-type a-Si:H films. The electrical, chemical and structural properties are improved with decreasing electrode distances (20-40 mm) at a hydrogen ratio [R (H2/SiH4) = 4], due to the low electron temperature and heating effect. A low electron temperature generates silane-related-reactive species (SiH3) and decreases structural disorder resulting in high quality i-type a-Si:H films. The electrical, chemical and structural properties of the a-Si:H films are confirmed using Al coplanar electrodes, FTIR, Raman spectroscopy, and spectroscopy ellipsometry (SE). When a solar cell is fabricated using the a-Si:H film, J(sc) of 13.2-14.8 mA/cm2, photoconductivity of 1.5 x 10(-5)-8.6 x 10(-6) S/cm, Si--H2 content of 0-1.24 at.%, and hydrogen content of about 10 at.% are obtained. These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process.

Growth characteristics of amorphous-layer-free nanocrystalline silicon films fabricated by very high frequency PECVD at 250°C

Amorphous-layer-free nanocrystalline silicon films were prepared by a very high frequency plasma enhanced chemical vapor deposition (PECVD) technique using hydrogen-diluted SiH4 at 250°C. The dependence of the crystallinity of the film on the hydrogen dilution ratio and the film thickness was investigated. Raman spectra show that the thickness of the initial amorphous incubation layer on silicon oxide gradually decreases with increasing hydrogen dilution ratio. High-resolution transmission electron microscopy reveals that the initial amorphous incubation layer can be completely eliminated at a hydrogen dilution ratio of 98%, which is lower than that needed for the growth of amorphous-layer-free nanocrystalline silicon using an excitation frequency of 13.56 MHz. More studies on the microstructure evolution of the initial amorphous incubation layer with hydrogen dilution ratios were performed using Fourier-transform infrared spectroscopy. It is suggested that the high hydrogen dilution, as well as the higher plasma excitation frequency, plays an important role in the formation of amorphous-layer-free nanocrystalline silicon films.

Optimization of Recombination Layer in the Tunnel Junction of Amorphous Silicon Thin-Film Tandem Solar Cells

The amorphous silicon/amorphous silicon (a-Si/a-Si) tandem solar cells have attracted much attention in recent years, due to the high efficiency and low manufacturing cost compared to the single-junction a-Si solar cells. In this paper, the tandem cells are fabricated by high-frequency plasma-enhanced chemical vapor deposition (HF-PECVD) at 27.1 MHz. The effects of the recombination layer and the i-layer thickness matching on the cell performance have been investigated. The results show that the tandem cell with a p+recombination layer and i2/i1thickness ratio of 6 exhibits a maximum efficiency of 9.0% with the open-circuit voltage (Voc) of 1.59 V, short-circuit current density (Jsc) of 7.96 mA/cm2, and a fill factor (FF) of 0.70. After light-soaking test, our a-Si/a-Si tandem cell with p+recombination layer shows the excellent stability and the stabilized efficiency of 8.7%.

Effects of RF power and pressure on performance of HF-PECVD silicon thin-film solar cells

High-frequency plasma-enhanced chemical vapor deposition (HF-PECVD) is a widely applicable method of deposition over a large area at a high rate for fabricating silicon thin-film solar cells. This investigation presents the properties of hydrogenated amorphous silicon (a-Si:H) films and the preparation of highly-efficient p–i–n solar cells using an RF (27.1 MHz) excitation frequency. The influence of the power (10–40 W) and pressure (20–50 Pa) used during the deposition of absorber layers in p–i–n solar cells on the properties and mechanism of growth of the a-Si:H thin films and the solar cells is studied. The a-Si:H thin films prepared under various deposition conditions have widely varying deposition rates, optical-electronic properties and microstructures. When the deposition parameters were optimized, amorphous silicon-based thin-film silicon solar cells with efficiency of 7.6% were fabricated by HF-PECVD. These results are very encouraging for the future fabrication of highly-efficient thin-film solar cells by HF-PECVD.

High-Rate Deposition of High-Quality Hydrogenated Amorphous Silicon Germanium Using Very High-Frequency Plasma-Enhanced Chemical Vapor Deposition with a High Hydrogen Dilution

Ways of improving the deposition rate of hydrogenated amorphous silicon germanium (a-SiGe:H) while maintaining its film quality, using the very high-frequency plasma-enhanced chemical vapor deposition (VHF-CVD) technique, were investigated. It was concluded that the VHF-CVD technique with a high hydrogen dilution of silane and germane is suitable for high-quality a-SiGe:H fabrication at a relatively high deposition rate. The reason why the high deposition rate is compatible with high quality is related to the high density of hydrogen radicals, which are proper to the VHF-CVD technique. The low plasma potential of VHF plasma contributes to high-quality a-SiGe:H film deposition by suppressing ion bombardment and excessive penetration of hydrogen atoms into the film. In device fabrication, the VHF-CVD technique is also useful because a high deposition rate with low ion bombardment may suppress negative effects to the underlying layers during deposition of the a-SiGe:H photovoltaic layer. This investigation showed that almost the same initial and stabilized efficiency for an a-Si/a-SiGe tandem solar cell was obtained at a five times higher deposition rate for the a-SiGe:H photovoltaic layer than those in conventional RF-CVD methods using the same reaction chamber.

Optical characteristics of intrinsic microcrystalline silicon

We studied the optical properties of intrinsic microcrystalline silicon $(\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ deposited by very high frequency plasma-enhanced chemical-vapor deposition system at different silane concentrations (SC) by spectroscopy ellipsometry and photothermal deflection spectroscopy. The bulk property of the samples was probed because the impact of the surface layer was significantly reduced by mechanical polishing. At high SC, we extracted the optical characteristics of the disordered part by assuming a superposition of ideal $c\ensuremath{-}\mathrm{Si}$ and mathematically generated amorphous function. At low SC, we studied the reason for a large deviation of absorption coefficient in the energy range between 1.6 and 3.2 eV from the value predicted by effective medium theory. We considered the scattering loss by the inhomogeneity of $\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ and introduce the dense medium radiative transfer formalism to an optical scattering simulation. We simulated the Stokes vector of p- and s-polarized light at oblique incidence. From this formalism, we could predict depolarization of p and s wave by scattered incoherent light. Further, we also suggested strain effect as the second possible reason for the enhanced absorption near the onset of the indirect transition in highly crystalline $\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$