Probing the phase composition of silicon films in situ by etch product detection

Abstract

Exploiting the higher etch probability for amorphous silicon relative to crystalline silicon, the transiently evolving phase composition of silicon films in the microcrystalline growth regime was probed in situ by monitoring the etch product (SiH4) gas density during a short H2 plasma treatment step. Etch product detection took place by the easy-to-implement techniques of optical emission spectroscopy and infrared absorption spectroscopy. The phase composition of the films was probed as a function of the SiH4 concentration during deposition and as a function of the film thickness. The in situ results were corroborated by Raman spectroscopy and solar cell analysis.

At the limit of total silane gas utilization for preparation of high-quality microcrystalline silicon solar cells at high-rate plasma deposition

“…It is observed that for each deposition condition, U sg is more or less constant over the covered range of SCs. Note that even for the regime of S1, where U sg ϳ 25% only ͑in other words, there is plenty of Si atoms available in the plasma͒, r d still strongly depends on the source gas flow of SiH 4 . At higher SC, r d is expected to saturate at the point where the power P will be rate limiting for the dissociation of SiH 4 .…”

“…Note that even for the regime of S1, where U sg ϳ 25% only ͑in other words, there is plenty of Si atoms available in the plasma͒, r d still strongly depends on the source gas flow of SiH 4 . At higher SC, r d is expected to saturate at the point where the power P will be rate limiting for the dissociation of SiH 4 . The dependency of r d on P in this regime is studied by the effect of the increase in P from 0.13 to 0.40 mW/ cm 2 for S1 and S2, respectively.…”

“…3. A high SC opt is an essential precondition for high-rate deposition because of the high availability of SiH 4 .…”

“…As the c-Si: H properties strongly depend on the underlying layers, the most relevant and direct optimization procedure is to evaluate series of complete c-Si: H-based p-i-n solar cells with varying i-layer properties. 3,4 An important precondition for the growth of c-Si: H is the presence of atomic hydrogen. 9 High concentrations of atomic hydrogen occur at low silane concentrations ͑SC͒ of the source gas mixture from silane and hydrogen.…”

“…The optoelectronic properties strongly depend on its crystallinity; c-Si: H solar cells optimally perform at the "optimal phase mixture" of the c-Si: H intrinsic absorber layer for which it is deposited in a regime that is close to the transition to the amorphous phase. [2][3][4][5] Generally, the material quality decreases with increasing deposition rate, 6 which is partly due to the high plasma power densities that involve a highenergy ion bombardment of the film during deposition. The application of high excitation frequencies 7 or high deposition pressures 8 or a combination of both 3 in PECVD is known to suppress the ion energy.…”