Charge Collection and Propagation in Diamond X-Ray Detectors

Diamond is a unique material for x-ray energy conversion. Its high thermal conductivity and low coefficient of thermal expansion make it ideal for high heat load environments. High material strength and x-ray transmission also are potentially useful features for certain applications in x-ray science. However, its large bandgap, while offering insensitivity to visible light, makes charge trapping more likely (thermal detrapping less likely) than in silicon; energy conversion efficiency is also 3-4 times less even under the best conditions. Limitations to charge collection efficiency such as recombination and charge trapping have been investigated quantitatively using quasi-continuous tunable synchrotron radiation under flexible biasing schemes as well using detailed Monte Carlo simulations. In the case of charge collection efficiency, the magnitude of the applied field, initial particle energy, and probe depth are adjusted. The diffusion and drift of photo-generated charge clouds are explicitly considered for the specific scattering behavior of diamond. While recombination loss at the entrance window of diamond diodes is qualitatively similar to a treatment for an additional "dead" carbon window layer, the observed field and photon energy dependence implies that the more sophisticated model is more correct quantitatively. In addition, charge propagation in diamond is unique in that photoconductive gain is possible. Effectively, charge trapping of one carrier leads to screening of the applied field. In order to avoid photoconductive gain, either blocking contacts or explicit detrapping is required. Initial investigations of photoconductive gain as a function of applied field, waveform and photon energy have provided insight into the performance of state of the art single crystal diamond. Simple models are proposed to assist in extrapolating the observed behavior towards useful detector devices.