P.A.T.T. van Veenendaal

The influence of different catalyzers in hot-wire CVD for the deposition of polycrystalline silicon thin films

Polycrystalline silicon thin films grown by hot-wire chemical vapour deposition, using tungsten and tantalum as filament material, show different material properties. The different filament materials can cause this difference, due to different surface reactions and catalytic properties. X-Ray photoelectron spectroscopy on used-filaments shows a much lower silicon content in the near-surface region of the tantalum filaments, as compared to the tungsten ones. Furthermore, it is shown that the silicon content on the tungsten filament increases linearly with time, while the silicon content on the tantalum filament saturates quickly. A comparison of the silicon contents on the different filaments shows the possible presence of a liquid phase on the tungsten filaments surface during deposition. This liquid phase can cause the short lifetime of tungsten filaments compared to tantalum ones.

The influence of the filament temperature on the structure of hot-wire deposited silicon

Exposure to hydrogen significantly cools the filament from the set temperature. This can mainly be explained by the power dissipation due to dissociation of hydrogen. The effect of silane on the filament temperature is more complicated. Below a certain threshold temperature (1850 K for W, 1750 K for Ta) a silicon-rich silicide is deposited on the filament, partially shielding it for further dissociation reactions. A drop in deposition rate accompanies this. Above another but higher threshold temperature (2000 K for W and 1950 K for Ta) the silicon-rich silicide is evaporated from the filament and the dissociation reactions occurred and thus the deposition rate are restored. Below these threshold temperatures (2 2 0) oriented materials can be produced.

Deposition of HWCVD poly-Si films at a high growth rate

Abstract The process parameters for high growth rate poly-silicon films by hot-wire chemical vapour deposition have been explored. A four-wire hot wire assembly has been employed for this purpose. High silane to hydrogen flow ratios and high wire temperatures are the key process parameters to achieve high growth rate and growth rates higher than 5 nm/s can be achieved. The process conditions to incorporate high hydrogen content into the material for passivation of defects and donor states have been identified as high hydrogen dilution and lower wire temperature. With these procedures poly-Si films deposited at 1.3 nm/s showed high ambipolar diffusion length of 132 nm. Incorporating such poly-Si films as i-layer, n–i–p solar cell on stainless steel substrate without back reflector showed an efficiency of 4.4%.

a-Si:H/poly-Si tandem cells deposited by hot-wire CVD

Abstract Innovative multibandgap a-Si:H/poly-Si tandem solar cells have been developed, where the two absorbing layers have been deposited by hot-wire CVD. These n–i–p/n–i–p cells have been deposited on a flexible stainless steel substrate, where the microcrystalline doped layers are made by PECVD. No enhanced back reflector was applied. Although the bottom cell shows a shunting problem under low-light conditions, the best tandem cell has an efficiency of 8.1% under AM-1.5 illumination, a fill factor of 0.60, an open-circuit voltage of 1.18 V, and a short-circuit current density of 11.4 mA / cm 2 . The total thickness of the tandem structure is only 1.1 μm.

Thin Film a-Si/poly-Si Multibandgap Tandem Solar Cells With Both Absorber Layers Deposited by Hot Wire Cvd

The first competitive a-Si/poly-Si multibandgap tandem cells have been made in which the two intrinsic absorber layers are deposited by Hot Wire Chemical Vapor Deposition (HWCVD). These cells consist of two stacked n-i-p type solar cells on a plain stainless steel substrate using plasma deposited n- and p-type doped layers and Hot-Wire deposited intrinsic (i) layers, where the i-layer is either amorphous (band gap 1.8 eV) or polycrystalline (band gap 1.1 eV). In this tandem configuration, all doped layers are microcrystalline and the two intrinsic layers are made by decomposing mixtures of silane and hydrogen at hot filaments in the vicinity of the substrate. For the two layers we used individually optimized parameters, such as gas pressure, hydrogen dilution ratio, substrate temperature, filament temperature, and filament material. The solar cells do not comprise an enhanced back reflector, but feature a natural mechanism for light trapping, due to the texture of the (220) oriented poly-Si absorber layer and the fact that all subsequent layers are deposited conformally. The deposition rate for the throughput limiting step, the poly-Si i-layer, is ≍ 5-6 A/s. This layer also determines the highest substrate temperature required during the preparation of these tandem cells (500 °C). The initial efficiency obtained for these tandem cells is 8.1 %. The total thickness of the silicon nip/nip structure is only 1.1 µm.

Influence of grain environment on open circuit voltage of hot-wire chemical vapour deposited Si:H solar cells

Abstract Polycrystalline silicon films have been deposited by hot-wire chemical vapour deposition (HWCVD) at lower substrate temperatures. N–i–p cells, deposited on a flexible stainless steel substrate, yielded an efficiency of 4.15%, with increased open circuit voltage and fill factor (0.452 V and 0.65, respectively). The moderate efficiency is due to a thin i-layer of 600 nm, which delivers a current density of 14 mA/cm2. An increase of more than 20% in the Voc is achieved compared to our earlier n–i–p cell at high temperature (490 °C). Furthermore, for the first time, thin silicon films have been deposited by HWCVD using rhenium (Re) as filament material.

Processes in silicon deposition by hot-wire chemical vapor deposition

Hot-wire chemical vapor deposition is a rapidly developing CVD technique for the deposition of silicon thin films and silicon alloys and may become a competitor of the plasma-enhanced (PE) CVD method due to significant advantages such as high deposition rate, efficient source gas utilization, lack of ion bombardment, and low equipment cost. Little is known, however, about the mechanisms for catalytic decomposition of the source gases, gas phase reactions at commonly used pressures, and the growth reactions. In this article, the differences in the reactions at various filament materials are discussed and it is shown that the subsequent reactions in the gas phase and reactions contributing to film growth can be substantially different from those in PE-CVD, due to the lack of energetic electrons and ions. Further work is necessary to identify the role of each precursor for the deposition of amorphous and microcrystalline films.

Investigation of scaling-up issues in hot-wire CVD of polycrystalline silicon

In order to apply device quality polycrystalline silicon obtained by the hot-wire chemical vapour deposition technique in industrial processes a high deposition rate is desirable. Meanwhile it is important to maintain good thickness uniformity over large areas. To achieve this the catalytic area needs to be increased. The influence of increasing the catalytic area on the deposition rate and material properties was studied. Proportionally increasing the feedstock gas flows leads to an unexpected drop in deposition rate. The dissociation of the gases causes the filament temperature to decrease. Below 1850 K the dissociation process is limited by the silicon deposition on the filament. This leads to a drastic drop in deposition rate. By adjusting the process parameters to the increased catalytic area, device quality polycrystalline silicon is again obtained. The deposition rate of polycrystalline silicon has been increased from 6 to 21 A/s while maintaining the material properties.

Microwave mobility in profiled poly-Si thin films deposited on glass by hot-wire CVD

To study the mobility and lifetime of charge carriers in thin film polycrystalline silicon deposited by hot-wire chemical vapor deposition (HWCVD), time-resolved microwave conductivity (TRMC) measurements have been performed. With this technique it is possible to monitor the change in conductivity on pulsed laser excitation on a nanosecond time-scale, without contacting the layer with electrodes. Pulsed laser excitation has been performed at a laser wavelength of 320 nm. The samples studied are Polyl (highly defective), Poly2 (device quality) and profiled layers of Polyl and Poly2. The results for the Polyl film show that lifetimes of the charge carriers generated on front and back side illumination are similar and below a nanosecond. For Poly2, the mobility in the substrate region (μ = 0.17 cm 2 /Vs) is more than one order of magnitude lower than in the top region (μ = 3.8 cm 2 /Vs). The introduction of a thin Polyl layer, acting as a seed layer for the Poly2, results in a relatively small increase of the mobility in the substrate region compared to standard Poly2, while TRMC signals upon front illumination remain approximately constant.

Spectroscopic and kinetic ellipsometry studies of hot-wire deposited polycrystalline silicon on glass

In order to get more insight into the growth mechanism of polycrystalline silicon deposited by hot-wire chemical vapor deposition on Corning glass, spectroscopic and kinetic ellipsometry studies have been performed. Spectroscopic ellipsometry measurements have been performed on layers, deposited using different levels of hydrogen dilution. From this study, it followed that the crystallinity of the films becomes higher with higher hydrogen dilution. Kinetic ellipsometry measurements have been performed during the deposition of profiled layers, starting with a seed layer deposited at high hydrogen dilution. This study showed that by using a seed layer, highly crystalline layers could be deposited using a lower hydrogen dilution. Without the use of the seed layer, this lower hydrogen dilution yields amorphous silicon layers.