Detection of background boron impurity in multisectoral type IIa HPHT single-crystal diamond plate by means of optical and admittance spectroscopy techniques

The object of the present study is a single-crystal multisectorial diamond plate synthesized by HPHT technique in Fe-Co environment with boron and nitrogen getters. These undoped type IIa diamonds find extensive use in X-ray optics, UV photodetectors, and ionizing radiation detectors, all of which necessitate precise control over the samples’ impurity content. Complementary non-destructive optical and electrical techniques were employed in a comprehensive study to assess the background concentration of boron in sectors <100>, <113>, <115> of the plate. The discussion addresses the features of the application of these methods for detecting extremely low concentrations and the spatial heterogeneity of the measured values across the sectors of type IIa diamond plates. UV transmission spectra near the fundamental edge were obtained and mathematical modeling was performed to study exciton and phonon effects affecting the diamond absorption in this spectral range. The room temperature FTIR spectra was shown to be insufficiently sensitive for controlling impurities in the plate; the spectra measured at the liquid nitrogen temperature allowed us to determine the boron concentration in the <115> sector as 3.5 ppb (6.3×1014 cm–3). The differences in current-voltage characteristics measured in various points of the plate are discussed. The performed capacitance-voltage measurements showed a strong dependence on the test signal frequency and enabled the assessment of low free charge concentrations (at the level of 1015 cm-3 and below, with the value varying by plate sector and frequency). The activation energy of boron was assessed by electrical measurements in DC (370 meV) and AC (345 meV) modes, the differences in the obtained values are discussed. Overall, this study shows that electrical measurements provide a non-destructive approach capable of determination background impurity content in specially undoped single-crystal diamond below the limits of conventional optical methods.