X-ray induced photoconductivity and its correlation with structural and chemical defects in heteroepitaxial diamond

Abstract

Three sets of heteroepitaxial diamond crystals grown under nominally identical process conditions on Ir/YSZ/Si(001) substrates have been studied with respect to structural defects, chemical purity, and x-ray induced photoconductivity (PC). The dislocation density that varied systematically between 107 and 109 cm−2 had a minor influence on dark conductivity and photoconductive gain G. In contrast, the substitutional nitrogen (NN) and substitutional boron (NB) defects, which are both present at very low concentrations (≤1 ppb), turned out to be the crucial factors controlling the electrical behavior. Small differences between both resulted in variations of the photocurrents by up to 5 orders of magnitude. The maximum in G of 0.75×104 was measured in the sample with highest dark conductivity. It could be explained conclusively within our model calculations on gain formation by NN≤NB. For low gain samples, we found NN>NB. However, the measured G values were far from theoretical predictions. This indicates a dominating role of additional traps. After x-ray switch-off, persistent photoconductivity (PPC) was observed in high gain samples. It was attributed to an energetic barrier hampering the recharging of nitrogen atoms by hole capture. As a possible source, strain fields generated by dislocations are suggested.