Silane Depletion Research Articles

A global model study of silane/hydrogen discharges

An algorithm to automatically build a general global chemical model on the basis of a set of chemical reactions is developed for capacitively coupled discharges. The methodology is applied to silane/hydrogen discharge regimes relevant for the deposition of microcrystalline silicon thin films for solar cell fabrication. The input parameters of the model are merely the process conditions such as absorbed power, pressure, gas flow, gas mixture and gap distance as well as the electron energy distribution function. Computational time is less than 30 s for an analytical description of the electron energy distribution and less than 40 s in the case of a look-up table for one set of process parameters for a silane/hydrogen gas mixture. The electron Boltzmann equation solver BOLSIG+ is used to determine the most appropriate electron energy distribution depending on different process conditions of this application. The numerical results of the global model are compared with measurements of silane depletion from the literature and show good agreement. A wide range of process conditions relevant for the deposition of thin-film silicon is covered. An analysis of the effect of different process conditions on the resulting plasma composition is performed. This shows the potential of a global model for silane/hydrogen discharges.

Processed optimization for excellent interface passivation quality of amorphous/crystalline silicon solar cells

This study evaluates the interface passivation quality of the amorphous/crystalline heterointerface and the performance of the heterojunction with intrinsic thin layer solar cells via values of a plasma parameter characterized by the deposition pressure (p)×electrode distance (d). Increasing the product of p×d leads to a lower crystalline fraction and higher hydrogen content, and enhances the c-Si surface passivation. This p×d evaluation factor is also compared with other evaluation factors, such as the silane depletion fraction and film-crystallinity. The tendencies of minority carrier lifetimes with respect to these evaluation factors were similar. Using the highest p×d value of 48, the photovoltaic parameter of the device yielded an open-circuit voltage of up to 710mV, in turned giving an efficiency of 19.12%.

Optimization of intrinsic hydrogenated amorphous silicon deposited by very high-frequency plasma-enhanced chemical vapor deposition using the relationship between Urbach energy and silane depletion fraction for solar cell application

Intrinsic hydrogenated amorphous silicon (i-a-Si:H) films were deposited by very high frequency (VHF) plasma enhanced chemical vapor deposition (PECVD) technique. It was found that there were three distinct deposition rate regions, when the deposition pressure and power were varied according to Paschen's law. The silane depletion fraction (SDF) is related to the reaction rates and the sticking probability of the radicals, which is smaller in the 1st region, where the SiH3 radical is the dominant deposition precursor giving rise to higher film density and a low Urbach energy (~68meV). The third region has higher SDF, where more SiH2 radicals are generated resulting in a reduction in film density and increased structural disorder due to polyhydride formation. The good quality films obtained with the condition of the 1st region, showed low SDF at lower pressure and power, following Paschen's law. Some of these i-a-Si:H films were used to fabricate p–i–n type solar cells. The measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (Voc)=800mV, short circuit current density (Jsc) of 16.3mA/cm2, fill-factor (FF) of 72%, and photovoltaic conversion efficiency (η) of 9.4%, which may be due to improved intrinsic layer. Jsc, FF and Voc of the cell can be improved further with optimized cell structure and with i-a-Si:H having a lower number of defects.

Gas Source Depletion Study of High-Order Silanes of Silicon-Based Epitaxial Layers Grown with RPCVD and Low Temperatures

In this paper, silicon gas-source depletion of silane, disilane, and a high-order silane, TF, is studied on oxide wafers. Two different growth mechanisms were discovered for TF. The faster mechanism depletes readily at temperatures as low as 525 oC. The "slower" mechanism does not deplete even at temperatures as high as 600 oC. This second growth mechanism has a growth rate as high as 13 nm/min at 550 oC and 43 nm/min at 600 oC under a chamber pressure 100 Torr. Two techniques, of reducing growth temperature and reducing growth pressure, are shown to suppress gas-source depletion. After the suppression of gas-source depletion, the faster mechanism exhibited a growth rate of 35 nm/min at 550 oC and a chamber pressure of 10 torr. Due to the two different growth mechanisms of TF, uniform growth deposition can be achieved for both low and high pressures for temperatures up to 600oC.

Initial transient status during silicon thin film deposition under high pressure

Quadruple mass spectrometer and voltage–current (V–I) probe were used to in-situ process diagnostics during deposition of silicon thin films, especially the phase of initial transient status (ITS). The silane concentration variation in plasma region (Cp) was measured in real time, the transient depletion of silane at the beginning of discharge was observed in traditional gas mixing method and pre-hydrogen method. Meantime the electrical parameters were monitored with V–I probe, the fluctuation of discharge voltage and the current over time in the stage of the ITS were given. The relationship between electrical parameters and Cp was analyzed.

The silane depletion fraction as an indicator for the amorphous/crystalline silicon interface passivation quality

In silicon heterojunction solar cells, thin amorphous silicon layers passivate the crystalline silicon wafer surfaces. By using in situ diagnostics during plasma-enhanced chemical vapor deposition (PECVD), the authors report how the passivation quality of such layers directly relate to the plasma conditions. Good interface passivation is obtained from highly depleted silane plasmas. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells, yielding aperture efficiencies up to 20.3% on 4 cm2 cells.

Hydrogen-dominated plasma, due to silane depletion, for microcrystalline silicon deposition

Plasma conditions for microcrystalline silicon deposition generally require a high flux of atomic hydrogen, relative to SiHα=0→3 radicals, on the growing film. The necessary dominant partial pressure of hydrogen in the plasma is conventionally obtained by hydrogen dilution of silane in the inlet flow. However, a hydrogen-dominated plasma environment can also be obtained due to plasma depletion of the silane in the gas mixture, even up to the limit of pure silane inlet flow, provided that the silane depletion is strong enough. At first sight, it may seem surprising that the composition of a strongly depleted pure silane plasma consists principally of molecular hydrogen, without significant contribution from the partial pressure of silane radicals. The aim here is to bring some physical insight by means of a zero-dimensional, analytical plasma chemistry model. The model is appropriate for uniform large-area showerhead reactors, as shown by comparison with a three-dimensional numerical simulations. The SiHα densities remain very low because of their rapid diffusion and surface reactivity, contributing to film growth which is the desired scenario for efficient silane utilization. Significant SiHα densities due to poor design of reactor and gas flow, on the other hand, would result in powder formation wasting silane. Conversely, hydrogen atoms are not deposited, but recombine on the film surface and reappear as molecular hydrogen in the plasma. Therefore, in the limit of extremely high silane depletion fraction (>99.9%), the silane density falls below the low SiHα densities, but only the H radical can eventually reach significant concentrations in the hydrogen-dominated plasma.

Physics and chemistry of hot-wire chemical vapor deposition from silane: Measuring and modeling the silicon epitaxy deposition rate

We measure and successfully model the deposition rate (R) of epitaxial Si by hot-wire chemical vapor deposition (HWCVD) onto (100) silicon over a wide range of growth conditions. A deposition rate model based on the fundamentals of gas-filament and gas-substrate interactions is presented; the results are consistent with the observed dependences of R on gas pressure, flow, and filament area. Gas-phase measurements of silane depletion allow calculation of the average radical sticking coefficient from the film deposition rate. Our findings indicate that the epitaxial deposition rate can be increased sufficiently to enable an economical epitaxial film-silicon photovoltaic tech-nology on low-cost foreign substrates. The model can be simply adapted to apply to the HWCVD of amorphous, nanocrystalline, and polycrystalline Si.

Input silane concentration effect on the a-Si:H to [formula omitted] transition width

In this work the microstructure transition width from amorphous to microcrystalline silicon is discussed. It is shown that the width of the transition depends on the input silane concentration level and indirectly on the silane depletion level. The higher the input silane concentration and depletion, the wider the transition. Experimental results are then compared to an analytical model and good agreement is obtained with a semi-empirical approach that takes into account the effect of the silane density in the plasma on the electron density.

Influence of pressure and silane depletion on microcrystalline silicon material quality and solar cell performance

Hydrogenated microcrystalline silicon growth by very high frequency plasma-enhanced chemical vapor deposition is investigated in an industrial-type parallel plate R&D KAI™ reactor to study the influence of pressure and silane depletion on material quality. Single junction solar cells with intrinsic layers prepared at high pressures and in high silane depletion conditions exhibit remarkable improvements, reaching 8.2% efficiency. Further analyses show that better cell performances are linked to a significant reduction of the bulk defect density in intrinsic layers. These results can be partly attributed to lower ion bombardment energies due to higher pressures and silane depletion conditions, improving the microcrystalline material quality. Layer amorphization with increasing power density is observed at low pressure and in low silane depletion conditions. A simple model for the average ion energy shows that ion energy estimates are consistent with the amorphization process observed experimentally. Finally, the material quality of a novel regime for high rate deposition is reviewed on the basis of these findings.

Non-intrusive plasma diagnostics for the deposition of large area thin film silicon

Plasma diagnostics for large area, industrial RF parallel-plate reactors can be useful for process optimization and monitoring, provided that their implementation is practical and non-intrusive. For instance, Fourier transform infrared (FTIR) absorption spectroscopy and/or time-resolved optical emission spectroscopy (OES) can easily be retro-fitted into the pumping line of a reactor. Both techniques were used to measure the fractional depletion of silane in silane/hydrogen plasmas. By means of a simple analytical plasma chemistry model, it is shown that the silane depletion is related to the silicon thin film properties such as microcrystallinity. Uses of the diagnostics are demonstrated by two examples: (i) the optimal plasma parameters for high deposition rate of microcrystalline silicon, along with efficient gas utilization, are shown to be high input concentration and strong depletion of silane; and (ii) the optimal reactor design, in terms of fast equilibration of the plasma chemistry, is shown to be a closed, directly-pumped showerhead reactor containing a uniform plasma.

Time-dependent gas phase kinetics in a hydrogen diluted silane plasma

The gas phase kinetics in a high-pressure hydrogen diluted silane plasma has been studied at time scales of 10−2–6×102 s. The time-resolved gas phase composition shows the following kinetics at different time scales: silane decomposition and polysilane generation in ≲2×10−1 s, nanoparticle formation and plasma density reduction in 10−1–100 s, polysilane accumulation in 100–102 s, and silane depletion and electrode heating in ≳101 s. Disilane radicals are implied to be the dominant film precursors in addition to silyl radicals.

Solar cell of 6.3% efficiency employing high deposition rate (8 nm/s) microcrystalline silicon photovoltaic layer

Microcrystalline silicon (μc-Si) films deposited at high growth rates up to 8.1 nm/s prepared by very-high-frequency-plasma-enhanced chemical vapor deposition (VHF-PECVD) at 18–24 Torr have been investigated. The relation between the deposition rates and input power revealed the depletion of silane. Under high-pressure deposition (HPD) conditions, the structural properties were improved. Furthermore, applying μc-Si to n–i–p solar cells, short-circuit current density ( J SC) was increased in accordance with the improvement of microstructure of i-layer. As a result, a conversion efficiency of 6.30% has been achieved employing the i-layer deposited at 8.1 nm/s under the HPD conditions.

Gas phase considerations for the growth of device quality nanocrystalline silicon at high rate

In order to increase industrial viability and to find niche markets, high deposition rate and low temperature depositions compared to standard deposition conditions are two recent trends in research areas concerning thin film silicon. In situ diagnostic tools to monitor gas phase conditions are useful in quick optimization processes of deposition parameters without going into time consuming material characterizations. Optical emission spectroscopy is an efficient technique to monitor/predict growth rate and phase of the material (amorphous or nanocrystalline). However, at high growth rate conditions which are generally achieved at high chamber pressures ( p), the simple correlation breaks down. We see that at high pressure condition a higher H α/Si * is needed for the onset of crystallinity than that is found at lower pressure conditions. Additional methods such as estimating the silane depletion from the experiment and the flux ratio of atomic hydrogen to deposited silicon atoms from simulations can be used for fine-tuning the amorphous to nanocrystalline transition regime. On the other hand, intensity of Si * line loses its character as a monitoring tool for deposition rate. Moreover, the plasma changes its character when the pressure is varied, even when the pd product is kept constant. In situ diagnosis of the ion energy distribution function by a retarding field ion energy analyzer has thrown new lights on the role of hydrogen dilution for depositions at low substrate temperature conditions, namely to compensate the loss in ion energy due to lower gas temperature.

Research on silane depletion status during the deposition of silicon thin films by high-pressure PECVD

In this paper a series of hydrogenated silicon thin films were prepared by high-pressure radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) using various plasma powers. The influence of plasma power on Raman crystallinity and deposition rate was investigated to study the silane depletion level during the deposition of silicon thin films. Based on these results the status of silane depletion were classified as un-depleted, depleted and over-depleted status. Additionally, the structural and opto-electrical properties were also investigated for those materials deposited under different silane depletion status. The results demonstrated that the μc-Si:H films, which are deposited at depleted status, have good opto-electrical properties and are suitable for application as intrinsic layers in solar cells.

Plasma silane concentration as a determining factor for the transition from amorphous to microcrystalline silicon in SiH4/H2 discharges

In this work, the microstructure transition from amorphous to microcrystalline silicon is defined in terms of the silane concentration in the plasma as opposed to the silane concentration in the input gas flow. In situ Fourier transform infrared absorption spectroscopy combined with ex situ Raman spectroscopy has been used to calibrate and validate this approach. Results show that a relevant parameter to obtain μc-Si : H from SiH4/H2 mixtures is the plasma composition, which is determined not only by the gas dilution ratio but also by the silane depletion fraction. It is also shown that μc-Si : H can only be deposited efficiently, in terms of gas utilization, at a high rate by using high input concentration and depletion of silane.

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

Abstract 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 SiH 4 /H 2 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 5×10 15 cm −3 obtained at a high rate approaching 8 nm/s demonstrate the effectiveness of the novel cathode.

High-efficiency µc-Si solar cells made by very high-frequency plasma-enhanced chemical vapor deposition

Microcrystalline silicon-based single-junction p–i–n solar cells have been fabricated by very high-frequency plasma enhanced chemical vapor deposition using a showerhead cathode at high pressures and under silane depletion conditions. The i-layers are made near the transition from amorphous to crystalline. It was found that, especially at high crystalline fractions, the open-circuit voltage and fill factor are very sensitive to the morphology of the substrate. At an i-layer deposition rate 0·45 nm/s, we have measured a stabilised efficiency of 10% (Voc = 0·52 V, FF = 0·74) for a cell made on texture-etched ZnO:Al. The performance is stable under light soaking. The defect density of the absorber layer is in the 1015 cm−3 range. In spite of the presence of oxygen contamination, good electrical properties and good infrared cell response are obtained. Copyright © 2006 John Wiley & Sons, Ltd.

Effect of power density on the structure properties of microcrystalline silicon film prepared by high-density low-ion-energy microwave plasma

The effect of microwave power density on the structure properties of microcrystalline silicon films (μc-Si films) deposited by high-density (>1012 cm−3) low-ion-energy microwave plasma at 400 °C substrate temperature has been investigated with X-ray diffraction, Raman spectra and atomic force microscopy. The μc-Si films are deposited under conditions of depletion of silane in order to avoid the combined effect of deposition rate and plasma bombardment. The experimental results indicate that the reduction of the power density causes a shoulder to form in the Raman spectra and the microstructure of the films changes from dominant (110) to random. Based on the experimental results, it is deduced that a large quantity of ions impinging on the growth surface can prevent a polymerization reaction among the silane radicals and promote the formation of the (110) preferred orientation μc-Si film. The present results are successfully explained in terms of the mechanism of collision of ions with the precursor adsorbed on the growth surface.

A new method of measuring the spatial distribution of depletion fraction of silane plasma by mass spectrometer

A newly established movable sampling apparatus of mass spectrometer is used to measure the spatial distribution of depletion fraction of silane plasma. A straight-line fit method of deducing the depletion fraction of silane is proposed. Theoretical analysis and test results demonstrate that the proposed new method is universal and more accurate than the existing one. There exist a largest peak near the middle of two electrodes and two peaks near the electrodes in the spatial distribution of silane depletion fraction, which are related to the distribution of electric field and the silane plasma sheaths.