Electric Field Effect on Multiphonon Transitions at Deep Centres

Abstract

It has been demonstrated both experimentally and theoretically that multiphonon transitions from deep levels into band states, under certain conditions, are extremely sensitive to an applied electric field. The effect has been clearly seen in the case of the EL2 centre in GaAs /1/ and of the Au-related neutral acceptor in Si /2,3/. In the case of GaAs a good level of theoretical understanding has already been reached by the work of Makram-Ebeid et. al. /1/ although there remain unresolved problems also in this case. In comparison to GaAs the situation for Si is more complex due to the indirect gap, the multi-valley structure and effective mass anisotropy of the conduction band. It is the purpose of this paper to develop a theory of the electric field effect on multiphonon emission which applies also to the more complex situation of Si. Further we will present experimental results on field dependent emission rates from the neutral Au-acceptor and the A-centre in Si and give an adequate theoretical interpretation for them. The authors of /1/ constructed their theory of multiphonon transitions in an electric field by extending the theory of tunneling between gap and band states to the case where multiphonon transitionscan take place in addition, i.e. where tunneling processes become phonon-assisted. In this paper we start from the standard theory of multiphonon transitions and extend it to the case where an electric field is present. In this way the various field effects considered so far, like phonon assisted tunneling and Poole-Frenkel effect, are automatically taken into account, but additional effects are also regarded like tunneling enhanced multiphonon transitions. Stark effect and field induced line shape broadening of the deep level. In part 2 of the paper we present the theory. In part 3 we describe the experimental procedure. A comparison between theory and experiment is made in part 4. Some unresolved problems in the case of the EL2 centre in GaAs are also adressed in this part.