High field limitation of Poole-Frenkel emisson caused by tunneling

Abstract

Capture and emission of carriers by deep impurities is an important channel of non-radiative recombination limiting luminescence efficiency. The electric field dependence of the ionization probability of charged deep impurities in semiconductors is usually attributed to the Poole-Frenkel effect. Here we show that the characteristic electric field dependence of the Poole-Frenkel effect can only be observed at sufficiently small electric field strengths. At higher field strengths the electric field stimulated thermal emission is outweighted by phonon assisted tunneling. This frequently ignored fact is important for the analysis of deep impurity charge states. The experimental investigations have been carried out on charged impurities (Si:Au, Cu-pairs in Si, Ge:Hg, Ge:Cu) and on DX-centers in AlGaAs and AlGaSb which are proved to be neutral. High voltage DC sources and high power pulsed FIR lasers have been used to apply static as well as terahertz electric fields. Short FIR laser pulses allow contactless and homogeneous application of strong electric field strengths, avoiding problems associated with avalange breakdown, current pinching etc. [E. Ziemann et al., MRS Symp.Proc. 510, 595 (1998)]. The present terahertz frequency measurements were performed in the temperature and frequency range where alternating fields act as static fields and both the Poole-Frenkel effect and phonon assisted tunneling are independent of frequency.