Search

Plasma enhanced chemical vapor deposition of silicon on glass substrates leads to formation of silicon amorphous films with partial crystallization of nano-grains in the amorphous matrix. We studied the transition of amorphous to microcrystalline silicon during such deposition. Formation of silicon nano-grains was detected by means of photoluminescence and Raman spectroscopy. The crystalline fraction and the mean size of the nano-grains were estimated by the position and the intensity of the peaks in the Raman spectrum. We showed that the fraction of crystalline silicon in the layers and the size of the nano-grains are strongly dependent on the growth conditions. The photoluminescence spectra exhibit distinct features related to recombination in the amorphous and in the crystalline phases. A significant narrowing of the photoluminescence peak related to the amorphous phase with increasing crystalline fraction indicates a structural modification in the amorphous silicon. It suggests an ordering process occurring before the start of the actual crystallization. A peak at about 1.4 eV was associated with isolated nano-crystalline grains within the amorphous matrix. A correlation between the peak energy and grain size was found, indicating effects of carrier quantum confinement. The experimental results confirm the established theoretical models for growth of microcrystalline silicon films. [ABSTRACT FROM AUTHOR]

Hot-wire chemical vapor deposition (HWCVD) is a promising technique for the deposition of silicon nitride layers (a-SiN[sub x]:H) at low temperatures. In contrast to the commonly used plasma-enhanced chemical vapor deposition, no ion bombardment is present in HWCVD, which makes it particularly attractive for the deposition of passivation layers on structures that are sensitive to the impact of energetic ions. We deposit hot-wire a-SiN[sub x]:H from a mixture of silane and ammonia at substrate temperatures in the range of 300-500 °C. Layers deposited with an ammonia/ silane gas-flow ratio of R=30 are close to stoichiometry (N/Si=1.33) with a hydrogen content around 10 at. %. Such films have been implemented in hot-wire a-Si:H thin-film transistors. Deposition with R>30 did not result in an increase of the N content, but led to more porous films. Infrared spectroscopy revealed that moisture penetrates these layers and that oxygen is incorporated in the network under air exposure. Cross-sectional transmission electron microscopy images show that these layers contain spherical voids with diameters of several nanometers. In contrast, films deposited with a lower gas-flow ratio (R<30) are inert and do not contain these voids. Both types of films show columnar growth. To better understand the deposition process, we used deuterated silane (SiD[sub 4]) as a source gas. We found that no deuterium was incorporated in the films. This gives rise to the assumption that SiH[sub 4](SiD[sub 4]) is cracked effectively at the filaments. We infer that the ammonia species are scarcely dissociated at the filaments but rather in the gas phase by the atomic hydrogen (deuterium) originating from the dissociated silane. The abundance of atomic hydrogen in the gas phase is crucial for the breakup of ammonia and the incorporation of N in the film. [ABSTRACT FROM AUTHOR]

In the past few years hot-wire chemical vapor deposition (HWCVD) has become a popular technique for the deposition of silicon-based thin-film transistors (TFTs). Several groups have been using hot-wire deposited amorphous and microcrystalline silicon as the active layers in TFTs. In such devices either thermal SiO2 or plasma-deposited silicon nitride was the gate insulator. Recently ‘All-Hot-Wire TFTs’ have been realized, with also the silicon nitride deposited by HWCVD. This paper reviews the characteristics of hot-wire TFTs with amorphous and microcrystalline silicon using plasma- or hot-wire deposited silicon nitride as the gate insulator. It has been shown that hot-wire TFTs have a higher stability upon gate-bias stress as compared to their plasma-deposited counterparts. We present an overview of the stability of hot-wire TFTs deposited at a range of substrate temperatures. The higher stability of hot-wire TFTs that have been deposited at temperatures of 400–500 °C is ascribed to an enhanced structural order, i.e. a higher degree of medium-range order of the silicon network. [Copyright &y& Elsevier]

Abstract: A new regime of high rate deposition of near-stoichiometric silicon nitride by Hot Wire Chemical Vapor Deposition was investigated. The layers were tested on multicrystalline silicon solar cells in order to assess their antireflection and passivation properties. The present design of the filament arrangement and the showerhead gas supply system allows for virtually unlimited scale-up and deposition rates of around 3 nm/s were obtained. By varying the SiH4 flow, the refractive index at 630 nm (n) could be controlled from 1.90±0.05 to 2.12±0.05, and the extinction coefficient at 400 nm (k) was <0.007 for all films of interest. The cells had state-of-the-art values for all photovoltaic parameters, similar to cells with a conventional SiN x antireflection coating. An efficiency of 14.3% was reached using HWCVD SiN x for multicrystalline Si solar cells with an industrial emitter. [Copyright &y& Elsevier]