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A study of amorphous silicon thin films by optical methods

Sidopekso, Satwiko (1992) A study of amorphous silicon thin films by optical methods. PhD thesis, Murdoch University.

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Abstract

The technological importance of thin films of such materials as amorphous silicon and amorphous carbon has created a demand for convenient and accurate methods of determining the optical properties of such materials when deposited on a variety of substrates.

The techniques for the determination of the optical constants of amorphous thin films on transparent, semitransparent and strongly absorbing substrates have been reviewed. Whether the films are transparent or semitransparent and whether the substrates are transparent or strongly absorbing depends not only on the nature of the films and substrates but on the wavelength employed for the measurement. The various methods used can be divided into two groups, those using interference effects and those using change of state of polarisation. We can further divide the methods into those using reflection, those using transmission and those using both transmission & reflection. Another criterion is that of wavelength used, some techniques use a range of wavelengths, others a fixed single wavelength. A further distinguishing parameter is the angle of incidence, which may be normal, variable or fixed at some angle other than normal.

Thin film amorphous silicon was deposited on metal, glass and silicon substrates by three techniques, RF sputtering of silicon in a hydrogen atmosphere, atmospheric pressure chemical vapour deposition from a mixture of silanes and plasma enhanced chemical vapour deposition from silane. A variety of optical methods were used for analysing the optical properties of the films, including refractive index, absorption coefficient, thickness and optical gap.

The instruments used for the measurements of the amorphous silicon film were; simple interferometer, UV-visible spectro-photometer, fourier transform infrared spectrometer and ellipsometer. The UV-visible spectrophotometer and the FTIR both produce interference fringes as a function of wavelength and although useful and popular with other workers proved to be limited in the information they could give. The simple interferometer has low precision but produces rapid and reliable results for the thickness of the films independently of their refractive index.

The ellipsometer has been used to determine the thickness and refractive index of the amorphous silicon films at a fixed wavelength (HeNe 632.8 nm) and angle of incidence. An analysis based on a three layer model was developed to take account of an interface layer and a surface oxide layer. A typical film, 0.8 microns thick, has a refractive index of 3.56 with an absorption coefficient of 0.56 . The interface layer has a refractive index of 1.43 and a thickness up to 4 nanometers while the surface oxide has a similar refractive index and a thickness of up to 3 nanometers. The UV-visible spectrophotometer measurements were also used to find the optical gap, which for the films considered was in the range of 1.3 ev to 1.7 ev. This was confirmed by parallel measurement of the spectral photoresponse of photovoltaic cells produced from the films.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Mathematical and Physical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Cornish, John
URI: http://researchrepository.murdoch.edu.au/id/eprint/51554
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