Plasma characterization and modelling for c-Si solar cells thin film deposition

Abstract: In this work, plasmas produced during thin film deposition are characterized. These thin films are used as optical and passivation layers for crystalline silicon solar cells. The proposed investigations focused on plasmas constituted by silane (SiH4) and ammonia (NH3) gas mixtures due to their extensive use for the deposition of dielectric layers in particular silicon nitride (SiNx). Silicon nitride is the standard anti-reflection coating layer for solar cells applications. Moreover, plasma characterization using optical emission spectroscopy (OES) is the focus of this work, due to its fast and non-intrusive nature making it ideal for industrial applications. The novelty of this research lies in the quantitative use of plasma emission to extract important plasma parameters, followed by the the utilization of the results for modelling purposes. The investigated plasma parameters are the electron density and the shape of the electron energy distribution function at different values of the set pressure and the silane to ammonia gas ratio. Using OES, the extraction of some important plasma parameters requires a small addition of noble gas (argon in this work) into the SiH4-NH3 mixture. Furthermore, an adequate emission model for the noble gas excited states is needed. As a first step, different emission models were investigated to describe the excited states of a pure argon gas. It has been found that 2p levels dynamics can be described by a so-called extended corona model considering excitation from ground and metastable states but also cascade and radiation trapping contribution from 1s states. Argon emission calculated from the model was fitted to OES data to extract the electron density and the shape of the electron energy distribution (or the electron temperature considering Maxwellian electron energy distribution function) at different pressure values. In pure argon plasma, the obtained parameters are in the range of 1.6 eV for the electron temperature and 5×1016 m-3 for the electron density. However, in a SiH4-NH3-Ar gas mixture where argon represents a small proportion of the admixture, the described model is no more valid. The quenching of 1s metastable states by background gases (silane SiH4 and ammonia NH3) was found to be the most important depopulating reaction and must be added to the emission model. By adding the effect of quenching, the emission model is extended to this gas mixture and plasma parameters are calculated for different values of ammonia-to-silane gas ratios. As a direct application, the obtained plasma parameters are used for plasma simulation. The obtained plasma parameters are used as input parameters for a SiH4-NH3 plasma chemistry model. The SiH4-NH3 plasma model is a global model based on particle density balance equations, including contributions from volumetric and surface reactions, flow and diffusion. OES-based plasma parameters are used to calculate electron impact dissociation rates for silane and ammonia. Aminosilane species were found to be the most important precursors for silicon nitride (SiNx) deposition in the investigated process condition range. A surface chemistry model was developed, involving the diffusion, the adsorption, the incorporation and the reflection of radicals. The surface chemistry model is used to calculate the deposition rate at different values of ammonia-to-silane gas ratio. A good agreement between the experimental and the calculated deposition rate was found. The calculated radical densities were used to interpret the experimental SiNx layer compositions at different gas ratio values. When varying the ammonia-to-silane gas flow, three different plasma regimes were found depending on the main precursor for the layer growth. Moreover, SiNx layer characteristics highly depend on the regime in which it was deposited. The passivation quality of SiNx layers deposited at different growth regimes was analyzed using QSSPC and FTIR measurements. A decrease of the as-deposited effective lifetime τeff is observed with the increase of ammonia-to-silane gas ratio from 160 to 10 μs. In the final part of the thesis, hydrogen loss after firing (due to bulk and out-diffusion) is correlated to the layer properties

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2021

Classification and Topics

• Keyword: Solar cells ; Plasma ; Solarzelle ; Optische Spektroskopie ; Passivierung

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2022
• Creator: Rachdi, Lazhar
• Contributor: Glunz, Stefan ; Wöllenstein, Jürgen ; Zacharias, Margit ; Albert-Ludwigs-Universität Freiburg. Fakultät für Angewandte Wissenschaften

Further information

• DOI: 10.6094/UNIFR/223718
• URN: urn:nbn:de:bsz:25-freidok-2237188
• Rights: Kein Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 15.08.2025, 7:28 AM CEST