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Characterisation of Defects in Semiconductor Materials

Dr Siobhan Daly

Dopants and impurities are readily incorporated into semiconductor and other materials and give rise to defects centres in the material which may induce energy levels in the forbidden band gap of the material. The characterisation of both the optical and electrical properties of such defects and their influence on the host material is a competitive and active research area world wide.

Photothermal Ionisation Spectroscopy (PTIS)

Dr Siobhan Daly

PTIS is a hybrid optical and electrical technique which is complimentary to existing techniques and has the advantage of probing directly the interaction between the excited state structure of defect related levels in the forbidden band gap of a semiconductor and the band edges.

It is a thermally enhanced photoconductivity method. It is a two step process consisting of an optical excitation of charge carriers from the ground state into an excited state of a defect followed by a phonon related thermal ionisation of the excited state into the corresponding band, as indicated in the figure. As in the case of optical absorption spectroscopy electrons (holes) are excited from the ground state to a bound excited state, however, instead of observing the reduction of photon flux at photon energies corresponding precisely to such transitions, one measures the flow of free carriers, in the form of a current, produced by the excitation process into the relevant band. The detection of the phonon induced excitation of the charge carriers from the excited states of a defect related energy level system into the band will reveal information on the interaction of the defect with the surrounding host crystal.

In situations where low concentrations, or small volumes of defect related shallow and deep levels are present in the band gap, extremely sensitive and where possible highly resolving analytical measurement techniques are required, PTIS meets such requirements.