Device-quality a-Si:H films have been deposited by hot-wire chemical vapor deposition (HWCVD). We have investigated the influence of deposition parameters on the film growth and properties. The most important deposition and growth processes that influence the optoelectronic material properties of a-Si:H deposited by HWCVD are clarified. During the deposition process attention must be paid to accurately control the substrate temperature, which is a key parameter to obtain device-quality films. A heat transport model is presented to be able to correct for the heating of the substrate by the filaments. It is found that films deposited at high deposition temperatures are under a high compressive stress. We show how the hydrogen incorporation in the layer is influenced by hydrogenation of subsurface layers by the atomic hydrogen flux that is inherent to the HWCVD process. We further identify the fundamental differences between plasma enhanced CVD and HWCVD material.

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Poly-silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) from hydrogen diluted silane gas at a low temperature (430 °C). The optical gap of the poly-silicon films is 1.1 eV, though with a higher optical absorption than c-Si. The grains have a preferential orientation (220) perpendicular to the substrate with an average crystallite size of 70 nm. The crystalline volume fraction is 95% with complete coalescence of grains. Large structures up to 0.5 μm could be observed in the AFM micrograph. The activation energy (0.54 eV) and the low carrier concentration (1011 cm−3) indicate a fully intrinsic nature of the films. The μτ product of carriers is 7.1×10−7 cm2V−1 whereas the ambipolar diffusion length (LD) is 334 nm. The excellent photo-conductive properties are attributed to the low (∼1017 cm−3) defect density. The HWCVD poly-silicon films showed a very small temperature dependence of mobility, indicating negligible trapping of carriers at the grain boundaries. Preliminary n-i-p cells incorporating poly-silicon i-layer yielded 3.15 % efficiency.

In this paper we present the results of the optimization of hydrogenated amorphous silicon films deposited by the hot-wire method in a larger area system. Using a two-wire design, we succeeded in depositing films that exhibit uniform electrical properties over the whole 4” x 4” Corning 7059 glass substrate. At a substrate temperature of 430 °C. and a pressure of 20 μbar we obtained a growth rate of ∼2 nm/s. The temperature of the tungsten filaments was kept at 1850 °C. The values for the photoconductivity and dark conductivity were 8.9×10−6 S/cm and 1.6×10−10 S/cm respectively, whereas the ambipolar diffusion length, as measured with the Steady-State Photocarrier Grating technique (SSPG), amounted to 145 nm. This value is higher than for our device quality glow-discharge (GD) films, which yield devices with efficiencies higher than 10%. The hydrogen content was 9.5%.We report on the density-of-states (DOS) distribution in the films, which was measured with the techniques of Thermally Stimulated Conductivity (TSC) and Constant Photocurrent Method (CPM). Furthermore, we describe the behavior of the electrical properties on light-induced degradation. Finally, we incorporated these films in solar cells, using conventional GD doped layers. Preliminary SS/n-i-p/ITO devices yielded efficiencies in excess of 3% under 100 mW/cm2 AM 1.5 illumination. Further work concerning the optimization of the interfaces is in progress.

We present the first thin film transistors (TFTs) incorporating a low hydrogen content (5 - 9 at.-%) amorphous silicon (a-Si:H) layer deposited by the Hot-Wire Chemical Vapor Deposition (HWCVD) technique. This demonstrates the possibility of utilizing this material in devices. The deposition rate by Hot-Wire CVD is an order of magnitude higher than by Plasma Enhanced CVD. The switching ratio for TFTs based on HWCVD a-Si:H is better than 5 orders of magnitude. The field-effect mobility as determined from the saturation regime of the transfer characteristics is still quite poor. The interface with the gate dielectric needs further optimization. Current crowding effects, however, could be completely eliminated by a H2 plasma treatment of the HW-deposited intrinsic layer. In contrast to the PECVD reference device, the HWCVD device appears to be almost unsensitive to bias voltage stressing. This shows that HW-deposited material might be an approach to much more stable devices.