Defekte in epitaktisch gewachsenen Silizium-Schichten (Niedertemperaturepitaxie)

Gegenstand der Arbeit ist die Analyse von Defekten in Silizium Schichten, die mit Elektronenzyklotronresonanz unterstützter Gasphasenabscheidung ("electron cyclotron resonance chemical vapour deposition", ECRCVD) bei Substrattemperaturen unterhalb 600°C deponiert wurden. Untersucht wurde e...

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Bibliographic Details
Main Author: Petter, Kai
Contributors: Fuhs, Walther (Prof.) (Thesis advisor)
Format: Dissertation
Language:German
Published: Philipps-Universität Marburg 2006
Physik
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Table of Contents: Subject of this work is the analysis of defects in silicon layers, which were deposited with electron cyclotron resonance chemical vapour deposition (ECRCVD) at substrate temperatures below 600°C. A model system was examined, where epitaxially grown silicon (epi-Si) layers with a thickness of approximately 1 micrometer were deposited onto the ideal substrate, a (100) oriented crystalline silicon (c-Si) wafer. The epi-Si layers are nominally undoped, however, all exhibit n-type conductivity with charge carrier concentrations (n_Hall) in the range of 10^15 cm^-3 to 10^18 cm^-3. A comparison of n_Hall with impurity concentrations in the layers showed that oxygen is involved in the formation of the donors and that presumably two oxygen atoms are necessary for the formation of one donor. In low-temperature electron spin resonance (ESR) investigations a resonance (k1) was observed, which has a g-value typical for shallow donors in silicon and whose spin density is nearly identical to n_Hall. This behavior can be explained in a model assuming that k1 acts as the main donor in the epi-Si layers. The observed slight deviations from this behaviour, as well as the changes in the ESR spectrum by light irradiation and a rise in temperature, can be explained assuming that also acceptor states near the conduction band are present, which may be introduced due to structural defects in the epi-Si layers. Transmission electron microscopy (TEM) investigations revealed that there are structural defects present in the epi-Si layers. Apart from stacking faults and pyramid-shaped microcrystalline regions, different kinds of line defects were observed. One type of line defect could be identified to consist of partial edge dislocations. Additional analysis using photoluminescence revealed evidence that another type of defect are so called "line interstitial defects", which are agglomerations of silicon self interstitials. In order to determine the density of the different types of defects, defect etching experiments were carried out. A detailed analysis of the shapes and crystallographic orientations of the resulting etch pits made it possible to assign the observed etch pits to the different types of defects observed in TEM investigations, making it possible to determine the density of the different defects easily. The overall density of structural defects is in the range of 10^8 cm^-2. Investigation on boron doped epi-Si layers showed that the same types of structural defects develop in these layers. A solar cell prepared from a 2 micrometer thick boron doped epi-Si layer had an efficiency of 4.2%. This is below the value which is expected for a defect-free solar cell of this thickness. A comparison with simulations showed that this can be explained with the high defect density of 4x10^8 cm^-2. In order to reduce the influence of the defects so that these do not affect the solar cell significantly, a reduction of the defect density by a factor of 100 would be necessary.