Time Dependence of Charge Transfer Processes in Diamond Studied with Positrons

We have developed a method called optical transient positron spectroscopy and apply it to study the optically induced carrier trapping and charge transfer processes in natural brown type IIa diamond. By measuring the positron lifetime with continuous and pulsed illumination, we present an estimate of the optical absorption cross section of the vacancy clusters causing the brown color. The vacancy clusters accept electrons from the valence band in the absorption process, giving rise to photoconductivity.

Figure 1

${\tau }_{\mathrm{ave}}$ (upper panel) and the fraction of negative clusters estimated from Eq. (3) (lower panel) as a function of illumination flux. The excitation wavelength and the measurement temperature are indicated. The ratio $g_s/\sigma$ is fitted according to Eq. (2) and is given in units ${10}^{15}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$.

Figure 2

The positron lifetime spectra obtained after 3.1 eV illumination at room temperature. The labels correspond to time in the dark after switching off the illumination. A lifetime spectrum of a colorless diamond is shown for comparison.

Figure 3

The fraction of negative clusters at 200 and 300 K calculated from the trapping rate. The photoconductivity decay curves shown in the upper part were measured at RT with 2.7 and 3.1 eV illumination using the van der Pauw configuration. The solid lines are fitted based on the model presented in the text.

Figure 4

The optical absorption coefficient $\alpha$ compared to the optical absorption cross section $\sigma$ at different photon energies. The slope of the fitted line is the vacancy cluster concentration $4\times {10}^{15}{\mathrm{cm}}^{-3}$. The error bars are estimated from the scatter of the estimated cross section values.