Crystallite size dependent carrier recombination rate and thermal diffusivity in undoped and boron doped CVD diamond layers

Abstract

Carrier dynamics under interband carrier injection conditions (213nm) was studied in undoped and boron-doped microcrystalline diamond layers with different grain size. The grain size, determined by scanning electron microscopy and electron backscattered diffraction, varied from 130m on the growth side to approximate to 1-2m on the nucleation side of a 1.0mm-thick undoped layer. Carrier lifetimes measured by differential transmittivity (DT) technique varied from 1 to 4ns on the growth side to approximate to 220ps on the nucleation side. Also the carrier diffusivity was found higher on the growth side. The B-doped layer with 40m grain size at the growth side exhibited 380ps carrier lifetime, which decreased to 130ps on the nucleation side. Even shorter lifetimes (approximate to 100-200ps and approximate to 10ps, correspondingly) were revealed in this layer by differential reflectivity decay due to impact of subsurface defects. Therefore we conclude that the recombination rate in presence of large grains is dominated by bulk non-radiative traps, as diffusion time of carriers to reach grain boundaries is much longer (few s). An impact of grain boundaries to recombination is expected in case of smaller grains. Thermal grating decay provided values of thermal diffusivity D-th in the range of 12-6cm(2)s(-1), respectively, on the growth and nucleation side of the layers, and its decrease with reduction of grain size was attributed to phonon scattering on grain boundaries.