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Title: Growth and crystallisation of silicon for solar cells
Author: Quinn, Thomas Edward
ISNI:       0000 0004 2733 7666
Awarding Body: London South Bank University
Current Institution: London South Bank University
Date of Award: 2011
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Polycrystalline silicon seed layer formation by aluminium-induced crystallisation (AIC) for solar cell applications was investigated. Precursor amorphous and microcrystalline silicon layers were deposited on SiO2 substrates using electron-cyclotron resonance plasma-enhanced chemical vapour deposition (ECR PECVD) and RF sputter deposition followed by thermal evaporation or RF sputter deposition of aluminium layers. Samples were then thermally annealed leading to layer exchange and crystallisation of the silicon layer. Selected samples were used as seed layers for ECR PECVD epitaxial thickening. Processing was usually done at temperatures less than 600°C compatible with glass substrates. Deposition parameters (substrate temperature, deposition time, gas flow rates, chamber pressure and magnetic currents) for ECR PECVD Si growth using SiH4 and H2 on bare and oxidised silicon wafers were varied to examine the effect on the crystallinity of deposited layers. This was related to properties of the plasma using diagnostic measurements. A method of adding argon to the chamber, found to be beneficial to material properties of deposited films, without causing the often observed reduction in growth rates was found. A study was then undertaken to optimise growth conditions of precursor silicon and aluminium layers for suitable seed layers by AIC. Using ECR PECVD deposited silicon and evaporated aluminium was found to lead to continuous layers after short annealing time, sometimes less than 15min, than using sputtered aluminium or silicon precursor layers which did not form continuous layers after several hours of annealing at 500°C. Using thin microcrystalline silicon was also found to lead to better film quality after annealing than thicker amorphous silicon films. Increasing the temperature during AIC from 250°C to 475°C was found to reduce the annealing time to typically 30 minutes at the highest temperature while allowing relatively large grains of several microns to form. Growth of a thick Si-Al interfacial oxide also lead to larger grain sizes. Under optimal preparation and annealing conditions, i.e. ECR PECVD μc-Si growth conditions and an annealing temperature of around 400°C, AIC produced continuous polycrystalline silicon seed layers with large grains up to 100μm, a preferential (001) orientation and a relatively smooth surface morphology with Ra values typically in the range of 20-30nm. Improvements in surface morphology and crystallinity were seen after subjecting AIC polycrystalline silicon to excimer laser irradiation at optimal fluence. Based on the above results and previous published work the process of AIC is discussed. ECR PECVD overgrowth on AIC seed layers was found to be epitaxial but the crystalline quality diminishes with thickness. Adding argon during deposition was found to be beneficial in producing thick, epitaxially grown layers with 2.5μm thick epitaxial layers achieved.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available