Growth Related Material Properties of Hydrogenated Amorphous Silicon
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Growth related material properties of hydrogenated amorphous silicon Citation for published version (APA): Smets, A. H. M. (2002). Growth related material properties of hydrogenated amorphous silicon. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR556557 DOI: 10.6100/IR556557 Document status and date: Published: 01/01/2002 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 04. Oct. 2021 Growth Related Material Properties of Hydrogenated Amorphous Silicon PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit van Eindhoven, op gezag van de Rector Magnificus, prof.dr. R.A. van Santen, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op donderdag 30 mei 2002 om 16.00 uur door Arno Hendrikus Marie Smets geboren te Maasbree Dit proefschrift is goedgekeurd door de promotoren: prof. dr.ir. M.C.M. van de Sanden en prof. dr.ir. D.C. Schram The work presented in this thesis was carried out at the Department of Applied Physics of the Eindhoven University of Technology. It is part of research program of the "Nederlandse Onderneming voor Energie en Milieu" (NOVEM). This research is also carried out within the framework of a TDO project (Center of Technology for durable Development) entitled: "Thin amorphous silicon film solar cells". Printed by Universiteitsdrukkerij Technische Universiteit Eindhoven Cover design by Jan-Willem Luiten CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Growth Related Material Properties of Hydrogenated Amorphous Silicon / by Arno Smets. – Eindhoven : Eindhoven University of Technology, 2002. - Proefschrift ISBN 90-386-1969-3 NUGI 812 Trefwoorden: gehydrogeneerd amorf silicium / materiaalonderzoek / plasmadepositie / hoge depositie snelheid / ellipsometrie / cavity ring down absorptie spectroscopie / groei mechanisme Subject headings: hydrogenated amorphous silicon / material properties / plasma deposition / ellipsometry / cavity ring down absorption spectroscopy / growth mechanism Aan mijn ouders Contents 1. Introduction 1 2. Hydrogen in the hydrogenated amorphous silicon network: its relation to the microstructure and growth mechanism 19 3. Hydrogenated amorphous silicon deposited by means of the expanding thermal plasma technique: material properties related to growth rate and substrate temperature 57 4. In situ single wavelength ellipsometry studies of high rate hydrogenated amorphous silicon growth using a remote expanding thermal plasma 83 5. Scaling behavior of the hydrogenated amorphous silicon roughness evolution during growth as a tool to study the radical-surface kinetics 99 6. Direct optical measurements on the sub gap of hydrogenated amorphous silicon thin films by means of the cavity ring down absorption technique 131 7. Conclusions 165 Summary 167 Samenvatting 169 List of publications related to the work 171 Dankwoord 173 Curriculum vitae 175 Chapter 1 Introduction 1.1 Worldwide Energy Consumption The last century has been unique with respect to the drastic increase in the demand for energy. The world wide energy consumption increased by a factor of 14 between 1890 up to 1990 [Hol90]. In 1890, 52 % of the total energy used was produced using non-commercial sources of energy, such as fuelwood, agriculture residues and dung [Tol92]. In 1990 the worldwide energy consumption was dominated by the fossil fuels: oil, gas and coal each having a share of 36%, 19 % and 32 % respectively [Tol92]. Non-fossil energy sources, such as hydropower and nuclear power, had a small share of 9% and 4% in the world's energy consumption in 1990. The dominance of the use of fossil fuels has drawbacks. The consumption of fossil fuels is responsible for the increase in the greenhouse gas carbon dioxide by 25 % (1990) compared to the preindustrial period (i.e. before 1785) [IPC90] and is believed to be responsible for the increase in the global mean temperature of 0.3-0.6 oC [IPC90,Tol92]. This development causes many concerns. One of these concerns is that the increase in global mean temperature could induce drastic climate changes on earth. Another concern is the fact that the fossil fuel resources are not inexhaustible. Consequently the interest in and the need for sustainable energy sources has increased, among which wind energy, biomass, geothermal and solar energy are the important alternative resources. The research presented in this thesis is related to solar energy in particular as it is focused on hydrogenated amorphous silicon (a-Si:H), the basis material used in a thin film solar cell. For example, the importance of the solar cell for future energy consumption has been demonstrated in a recent study performed by Shell International [Evo96], in which a photovoltaic (PV) energy share of 15-20% in the world energy production in 2060 is predicted. 1.2 Thin film solar cells At present, more than 80 % of the produced solar cells are crystalline silicon. These commercial modules have efficiencies of 12-15 % but also high production costs. The c-Si solar cell has a price-performance ratio of 5.4 €/Wp [Ach00] (Wp stands for the power produced by a solar cell under standard test conditions 1000 W/m2 AM1.5 light radiation, normal incidence and temperature of 25 oC). To become competitive with energy produced from fossil fuels like oil, coal and gas the price of solar electricity has to become at least 2.3 €/Wp [Ach00]. A price reduction on relatively short term for c-Si solar cell technology can be obtained by large scale production and innovations in Chapter 1 Introduction 25 Si CIGS CdTe 20 a-Si Cu2S 15 10 efficiency (%) efficiency 5 0 1950 1960 1970 1980 1990 2000 year Fig. 1.1 The evolution of solar cell's efficiency (on lab scale) for various types of solar cells: crystalline silicon (solid squares), CIGS (solid circles), CdTe (open triangles), a-Si (solid triangles), Cu2S (open diamonds). This figure is obtained from ref. [Sch01]. cell design, materials and production technolgoies. It is expected that the c-Si performance ratio can be reduced to 3.2 €/Wp for c-Si solar cells in 2007 [Ach00]. The current dominance of the c-Si solar cell has its origin in the fact that c-Si is the most commonly used and investigated semiconductor material in the world. Consequently the crystalline silicon solar cell is the most highly developed solar cell. This is nicely illustrated in Fig. 1.1, obtained from Ref. [Sch01], in which the history of the efficiency of crystalline silicon solar cells (obtained in laboratories) and thin film solar cells is depicted. The history of the c-Si solar cell can be divided into two time periods: the period before 1980, in which the c-Si efficiency increased up to ~15 % and the period after 1980, in which due to the rapidly developing semiconductor industry the efficiency increased up to ~25 % on lab scale. There is also great interest in thin film solar cells, since it is expected that these solar cells can be produced at lower costs, resulting in lower price-performance ratios in the near future. Furthermore, an additional advantage of thin film solar cells is that in principle they can be deposited on flexible, cheap substrates. Typical thin film materials used are copper-sulphide, cadmium-sulphide, cadmium-telluride, copper indium diselinide and hydrogenated amorphous silicon. However, as shown in figure 1.1 the development of the efficiency status of the thin film solar cells is roughly 20 years behind that of the crystalline silicon solar cell. The main reason is that the complex properties of the amorphous materials, which are not as well understood as the crystalline material properties, require a lot of research [Sch01]. Consequently, thin film solar cells had in 1999, with a worldwide module production of 25 MWp/year, a minor contribution to the worldwide total module production of 200 MWp/year [Sho01]. However, the worldwide plans and the construction of thin film solar cell production lines, promises a larger market share in the future. In this thesis we will focus on the basis material in the hydrogenated amorphous silicon (a-Si:H) thin film solar cell. Hydrogenated amorphous silicon is the most widely 2 Introduction Chapter 1 L I G H T glass transparent conductive oxide p-layer intrinsic a-Si:H - - + n-layer + metal contact Fig.