Protocrystalline Silicon at High Rate from Undiluted Silane

Abstract

Hot Wire Chemical Vapor Deposition (HWCVD) is shown to be a fast method for the deposition of protocrystalline silicon films from undiluted silane. Intrinsic silicon-hydrogen films (2 µm thick) have been deposited by HWCVD on plain stainless steel as well as on stainless steel precoated with a n-type doped microcrystalline silicon layer. In X-ray diffraction experiments, the linewidths of the first sharp peak (FSP) were 5.59 ± 0.09 degrees and 5.29 ± 0.11 degrees, respectively, indicating improved medium-range order and a template effect due to the µc-Si:H n-layer. For thinner layers (0.7 µm thick), the linewidths of the FSP were 5.29 ± 0.09 degrees and 5.10 ± 0.09 degrees. These FSPs are as narrow as for optimized i -layers made by H 2 -diluted plasma deposition, however, at a much higher deposition rate (1 nm/s), at moderate temperature (250 °C), and without the use of H 2 dilution. In accompanying transmission electron micrographs, the layers show a significant concentration of elongated small voids in the growth direction that are not interconnected. Small Angle X-ray Scattering (SAXS) results are consistent with these observations. We suspect that the void nature allows the bulk of the film to be more ordered. The utilization of such layers in n-i-p solar cells on plain stainless steel leads to cells with a remarkably good stability, showing a decrease of the fill factor of less than 10 % during 1500 h of light soaking.

Thin film silicon devices deposited at 100°C: A study on the structural order of the photoactive layer

“…The relatively void-rich nature of the HWCVD films goes along with the behavior of the A parameters. The elongated, not interconnected, void structure has earlier been observed in protocrystalline silicon type of HWCVD materials made at 250°C [9]. The HREM of the HWCVD sample also shows dispersed tiny highly ordered regions (lattice fringes) of around $1-1.5 nm (Fig.…”

“…The origin of elongated voids (after an incubation phase) in the optimized HWCVD film suggests the release of internal stress in the film though the creation of voids when a critical stress level is reached and this leads to a relaxed network, manifested in a small C value. This mechanism was suspected earlier with reference to the optimized protocrystalline silicon HWCVD films made at 250°C [9].…”

Hot Wire CVD for thin film triple junction cells and for ultrafast deposition of the SiN passivation layer on polycrystalline Si solar cells

“…We use Ta filaments at a temperature of 1850°C, pure SiH 4 feed gas and a process pressure of 0.02 mbar, resulting in a deposition rate of 1 nm/s [1]. A fingerprint of the protocrystalline nature of this material is the narrow width of the first sharp peak in X-ray diffraction (XRD) [2].…”

“…Special precautions were taken to protect the Asahi SnO 2 :F coated substrates onto which the cells were deposited [1]. While the cells deposited at 1 nm/s reached an initial efficiency of 8.9% (0.88 V, 14.2 mA/cm 2 , FF = 0.71), we achieved 8.5% at an r d of 1.6 nm/s, 8.1% efficiency at an r d of 2.1 nm/s, and at an r d of 3.2 nm/s still a high efficiency of 7.5% was obtained.…”

On the development of single and multijunction solar cells with hot-wire CVD deposited active layers

“…Hot-wire chemical deposition (HWCVD) has been proved to be able to prepare high quality intrinsic hydrogenated amorphous silicon (a-Si:H) [1][2][3], proto-crystalline silicon (proto-Si:H) [4], nanocrystalline silicon (nc-Si:H) (also called microcrystalline silicon lc-Si:H) [5,6], polycrystalline silicon (poly-Si:H) [7] (also called high-crystallinity lc-Si:H) and amorphous silicon germanium (a-SiGe:H) [8] materials that are suitable for use as the active layers in silicon based thin film solar cells. Compared to conventional plasma enhanced chemical vapor deposition (PECVD), HWCVD has the property that the process is independent of electromagnetic properties of the substrate.…”