Charge Transport and Radiation Induced Defects in Single-Crystal CVD Diamond

Field: Physics and Astronomy (Physical Sciences)

Due to a number of unique intrinsic properties, diamond has been considered and studied as a solid state ionisation chamber radiation detector for many years. Although its indirect band gap (5. 48 eV) should list it as an insulator, diamond is electrically considered and studied as a semiconductor. Despite its numerous advantages, both poly-crystalline (PC) and single crystal (SC) chemical vapour deposition (CVD) diamonds suffer from crystal defects and impurities. More particularly, PC-CVD diamonds have shown a high concentration of grain boundaries and dislocations. SC-CVD diamonds on the other hand offer the opportunity to study grain boundary free crystals. In order to improve diamond’s detection performance, the various parameters affecting the charge transport properties need to be understood, especially defects leading to degradation from the expected behaviour of an “ideal” crystal. The first part of this work focuses on high quality, electronic grade SC-CVD diamond samples. Various optical and electrical experimental techniques are used to study and understand the limitting factors of charge transport and their properties. The samples show excellent optical and electrical characteristics with a very low dislocation density, good Raman signal uniformity, low leakage currents, near 100% charge collection efficiency (CCE) levels reached, high mobility and saturation velocity values and acoustic phonon being the dominant charge carrier scattering mechanism for low temperatures. Ion beam induced charge (IBIC) on a different sample of similar quality illustrates excellent CCE uniformity. Polarisation effects are observed in all devices but prove to only influence the spectroscopic results at low applied electric fields. The second section of this work focuses on the effects of radiation damage on SC-CVD diamond detector operation. One SC-CVD diamond sample received rectangular spatially localised proton irradiation in various doses. IBIC mapping was employed to study how this damage affects CCE values. CCE showed a decrease with increased proton dose. Three SC-CVD samples were damaged with neutrons and various characterisation experimental results are discussed, to determine how neutron radiation damage affects SC-CVD diamond detectors. Significant spectroscopic deterioration appears only for the sample with the highest received neutron dose. Finally, a high temperature annealing step was performed to assess any recovery in detector performance. Charge carrier current pulses indicate electric field non-uniformity after annealing. The recent commercial availability of high quality, electronic grade SC-CVD diamond offers the opportunity to study this material as a planar radiation detector in a detailed manner. Furthermore, due to its radiation hardness and excellent detection properties, SC-CVD diamond is a good detector candidate for use in radiation harsh environments. The function of radiation damaged PC-CVD diamond detectors has been extensively studied, but only limited work has been performed on SC-CVD diamond detectors function after radiation damage. The study of near intrinsic, radiation damaged and subsequently annealed SC-CVD diamond detectors presented in this work aims to understand how the bulk charge transport properties are affected by the inherent growth, radiation damage introduced and thermally evolved crystal defects.