Liquid-phase synthesis of silicon and silicon carbide thin-films via atmospheric pressure chemical vapor deposition

RWTH Publications (RWTH Aachen)

Abstract

Hydrogenated amorphous silicon (a-Si:H) and silicon carbide (a-SiC:H) thin films are essential materials for optoelectronic and photovoltaic devices. High-quality layers of these materials are typically produced using vacuum-based deposition techniques, such as plasma enhanced chemical vapor deposition (PECVD). However, these methods require expensive equipment and complex infrastructure, which limits their economic viability. Atmospheric pressure chemical vapor deposition (APCVD) offers a promising alternative, as it avoids the need for ultra-high vacuum infrastructure. Successfully addressing challenges in precursor handling, film quality, and process reproducibility could make this approach a cost-effective method for producing a-Si:H and a-SiC:H thin films. This thesis seeks to overcome the limitations of existing APCVD systems by developing a custom-built system that can reproducibly deposit high-quality a-Si:H and a-SiC:H films using liquid precursors. The research systematically investigates how key process parameters – such as deposition temperature, deposition time, precursor volume, injection frequency, and precursor dilution with cyclooctane – affect film growth and composition, as well as hydrogen incorporation, purity, defect density, and the resulting electrical and structural properties. The goal is to use the insights gained from these investigations, together with targeted post-deposition treatments, to optimize film properties for a range of applications and thereby demonstrate that liquid precursors, particularly trisilane and 1,4-disilabutane, can enable low-temperature, scalable deposition of high-quality functional semiconductor materials when precisely controlled. Extensive material characterization was carried out using Fourier transform infrared spectroscopy, Raman spectroscopy, spectroscopic ellipsometry, electron spin resonance spectroscopy, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and electrical conductivity measurements. For a-Si:H, films with hydrogen contents up to 10 at%, defect densities as low as 3 × 10^18 cm^(−3) and photosensitivities up to 10^5 were achieved. While the defect density is higher than for standard PECVD films, such hydrogen content and photosensitivity are typical for functional a-Si:H layers for semiconductor applications. The effective passivation of crystalline silicon (c-Si) substrates resulted in lifetimes of up to 1.9 ms, comparable to those achieved with standard c-Si substrates passivated using PECVD a-Si:H layers. Post-deposition hydrogenation and catalytic doping significantly enhanced electronic properties, and films crystallized without blistering into polycrystalline silicon via rapid thermal annealing, enabling high-throughput annealing processes. APCVD using 1,4-disilabutane enabled the formation of amorphous SiC films at intermediate temperatures below 600 °C, with carbon concentrations and conductivities comparable to or exceeding PECVD analogs. Post-deposition annealing and laser crystallization facilitated the formation of a nanocrystalline SiC and Si mixture. In-situ nitrogen doping by 1,1,3,3-tetramethyldisilazane – a nitrogen, silicon, carbon and hydrogen containing molecule – during a-SiC:H deposition enabled conductivities up to 0.03 S cm^(−1), typical for n-type nanocrystalline SiC. The findings demonstrate that APCVD, when combined with well-designed precursor chemistry and deposition control, is a cost-effective alternative for producing a-Si:H, a-SiC:H, and their crystalline derivatives. This enables new application routes in photovoltaic technologies, such as passivated contact solar cells, and in other advanced electronic devices like micro-electro-mechanical systems.

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