In-situ Raman spectroscopy : a method to study and control the growth of microcrystalline silicon for thin-film solar cells

Optimum performance of a microcrystalline silicon (µc-Si:H) thin-film solar cell is achieved if the absorber layer material is deposited close to the phase transition towards amorphous Silicon. Hence exact knowledge about and control of the deposition process is of great importance. This work deals with the design and application of a novel experiment that enables in-situ Raman measurements during the parallel plate plasma enhanced chemical vapor deposition (PECVD) of µc-Si:H. Measurements of the crystalline volume fraction and the temperature of a growing film are carried out. To enable in-situ Raman measurement of central regions of the coated substrate in a PECVD system, optical access under normal incidence is necessary. An experimental setup in which an optical feed-through was integrated into a PECVD electrode was developed. This setup introduces a disturbance to the electrical field that sustains the plasma. By designing metallic shields the impact of the feed through was reduced considerably at low optical losses. The homogeneity of films deposited with the novel setup in different growth regimes – that are characterized by their deposition pressure ranging from 6 Torr to 18 Torr – was studied. A correlation between the magnitude of the inhomogeneity caused by the feed-through and the characteristics of the deposition regimes is found. Raman spectroscopy demands the illumination of a sample with a laser and the collection of the scattered radiation. Due to absorption of the laser light the temperature of the illuminated film is increased. Since the temperature determines the properties of a growing film the laser-induced temperature increase was studied. By pulsing the laser radiation a minimal temperature increase at maximal signal intensity was obtained. The crystalline volume fraction of a growing µc-Si:H layer was determined in-situ with the novel setup. A minimal temporal resolution of less than 17.5 s at sufficient signal-to-noise-ratio was achieved which corresponds to less than 9 nm of deposited material during one measurement interval at the industrial standard growth rate of 0.5 nm/s. The obtained results were compared to depth resolved measurements that were carried out after the deposition. An excellent agreement between both methods validates the reliability of the in-situ method. The initial phase of deposition is of great importance for the performance of a µc-Si:H thin-film solar cell. Hence the dependence of the evolution of the crystalline volume fraction during initial layer growth on the properties of the underlying seed layer was studied in-situ. A seed layer dependent increase and subsequent stabilization of the crystalline Volume fraction was observed. By actively controlling the deposition parameters based on these results it was possible to reduce the observed inhomogeneity of the Raman crystallinity in growth direction. A possible application of in-situ Raman spectroscopy as basis of an active process control was studied by testing the ability of in-situ Raman spectroscopy to detect fluctuations of the deposition parameters on the example of a disturbance of the process gas flow. It was possible to detect the reaction of the layer growth on a change of deposition conditions in-situ. By correlating the in-situ measurements to results obtained on solar-cells it was found that – unless the process fluctuation happens during the initial phase of deposition – it is possible to maintain state-of-the art solar cell performance by an active process control. Raman spectroscopy can be used to measure the temperature of a film. Up to now, it was only possible to estimate this important process parameter by measurements of the temperature of the substrate by a pyrometer. Raman Spectroscopy enables the direct determination of the temperature of a growing film. By modulating the plasma emission synchronized to the Raman measurements it was possible minimize the Signal-to-noise level. Hence the sensitivity of the measurements was increased to a level that enables the in-situ determination of the film temperature. Two deposition regimes were distinguished by their characteristic plasma induced temperature increase. In situ measurements show that an active control of the substrate heater results in a stabilized temperature of the growing layer throughout the deposition of a µc-Si:H film.

Grantor dc:publisher

Forschungszentrum Jülich, Zentralbibliothek

Year dc:date

2012

Author dc:creator

• Muthmann, Stefan

Contributors dc:contributor

• Rau, Uwe

• info:eu-repo/classification/ddc/530
• Photovoltaik
• Photovoltaikindustrie
• Silicium
• Raman-Spektroskopie
• Raman-Effekt
• PECVD-Verfahren
• Physik
• silicon
• Raman
• in-situ
• PECVD

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-122311%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:60609