Temperature dependent excitonic effects in silicon
The exciton binding energy and phonon energies are the two key parameters in defining the bandgap energy of a semiconductor. A more accurate measurement of bandgap energy can be obtained by adding the excitonic binding energy and the corresponding phonon energy to the optically measured threshold. Observing excitonic effects in the absorption process is an effective means of understanding the phonon and exciton behaviour and the material’s bandgap. When the temperature rises, the excitonic effect is not completely dissipated but rather immersed in a thermally broadened spectrum. Thus, a highly accurate and sensitive characterisation tool needs to be developed to investigate the material’s excitonic and temperature-depended behaviour. In this project, a wavelength modulation spectroscopy (WMS) instrument is developed for the observation of the exciton-assisted absorption in silicon at various temperatures. The technique enhances weak spectral features that are immersed in a signal background that is generally three orders of magnitude larger than the signal itself. The construction of a wide range wavelength modulation spectrometer is described in detail. Experimental improvements in resolving fine details are also summarised. With this setup, the transmission spectra of silicon near its bandgap edge are measured under various temperatures. The interpretation of the result is carefully revised and applied according to the updated excitonic perturbation theory. The temperature-dependent excitonic binding energy and phonon energies are extracted. The revised temperature dependent silicon bandgap energy is then given.
Spectroscopy, Silicon, Characterization, WMS
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