Vibrational and electronic properties of impurities in semiconductors

Abstract

Properties of impurities in semiconductors have been investigated by means of absorption spectroscopy. The studies have concentrated not on one topic but on a range of types of impurity behaviour: they may be summarized under three main headings. (1) Fine Structure in Absorption Bands. The shapes and temperature-dependent widths of absorption bands of impurities may be understood qualitatively in terms of vibrational coupling with the lattice. The spectra studied show evidence of Jahn-Teller coupling, one particular form of such an interaction, and it has been found possible in most cases to relate the fine structure observed to features in the phonon spectra of the host lattices. (2) Antiresonances. Since the materials studied are such that in some cases a clear and complete ascription of spectral fine structure can be made in terms of Jahn-Teller coupling, it is possible unambiguously to identify certain other features not previously observed in any systems. These are vibronic anti-resonances, resulting from interference between vibrational levels of the lattice and impurity states of mixed electronic-vibrational nature. In the absence of adequate models, these features are discussed phenomeno- logically. An electronic analogue of these effects, involving spontaneous ionization of excited impurities, has been looked for, but no convincing instances have been found, (3) Photoionization. The charge state and electronic configuration of an impurity depend not only on bonding requirements at the lattice site, but on other factors such as the position of the Fermi level in the bulk medium, and may be changed by photo-excitation. Electrons or holes being removed from ar. impurity and released into crystal bancs results in intense absorption. The form of the absorption cross-section for these processes follows a simple power law near the threshold of ionization, so the value of the ionization energy may be determined quite readily. In this way the positions of a deep level of nickel in zinc selenide and in zinc sulphide have been determined with respect to the conduction and valence bands of these materials. This important information has not been obtained directly for any other transition metal impurity. In addition, fine structure has been seen near the ionization thresholds in these materials. This is due to the coulomb potential of the ionized centre, charged with respect to the lattice, being able to hind holes in hydrogenic orbits. The phonon coupling to this shallow level is found to be much stronger than is observed in absorption bands due to photoionization.