Modeling novel effects in transient current measurements of single-crystal CVD diamond with carrier excitation by MeVα-particles

Abstract Temperature- and electric field-dependent transient current measurements with 4.6 MeV high-energy α-particle excitation were reported recently in commercially available CVD diamonds and were used to extract drift velocities and drift mobilities of electrons and holes (Jansen et al., 2012, 2013 [1–3]). These data showed unexpected characteristic temperature-dependent pulse current profiles which remained unexplained as well as other novel observations. Here we interpret all experimental findings by a detailed physical model. It demonstrates that the measured electron or hole currents consist of two components one of which is constant in intensity whereas the other is temperature-dependent due to thermal activation of carriers out of a low-temperature storage state shown to consist of electron-hole droplets (EHDs); this is a condensed phase of electrons and holes. EHDs form in regions with electrons and holes embedded between adjacent regions with electrons or holes alone. This local separation is caused by the electric field during carrier relaxation from the highly excited states. The model describes all experimental findings very satisfactorily, some of them quantitatively. Maxima in the transit time as a function of temperature were experimentally reported (Jansen et al., 2013 [2, 3]) which successively become more pronounced at decreasing electric fields while shifting to lower temperatures. This observation is modeled by temperature-controlled re-population of the “slow” and “fast” conduction band valleys split apart by the applied electric field upon lifting the sixfold valley orientational degeneracy. The model suggests that the novel effects are characteristic of the very high excitation energy of the free excess carriers and would not be observable for near-band gap excitation.