High collection efficiency in chemical vapor deposited diamond particle detectors

Diamond is a very attractive material to realize nuclear detectors due do its outstanding properties. In order to realize the detectors described in this work, diamond films were grown on silicon by microwave chemical vapor deposition (CVD) using an H2-CH4 gas mixture. The α particle spectra were measured by using a 5.5 MeV 241Am source. A maximum collection efficiency η as high as 70%, with an average value of 50%, was obtained in a 115 μm thick sample after β particle irradiation (`priming effect'). These values correspond to collection distances, as calculated by the Hecht formula, of 250 μm and 120 μm respectively (higher than the film thickness) thus demonstrating the very high quality of the samples. Unprimed efficiency η = 50% maximum, 30% average was also obtained on other samples. The efficiency and the resolving power of the detectors were studied as a function of the external electric field. A Monte Carlo simulation of the α particle detection process was developed, and the results compared to the experimental spectra. It is concluded that the priming process saturates in-grain defects limiting the as-grown detector performance, so that charge collection distance is only limited by grain boundaries located close to the substrate side. Thus, further improvement can be reasonably obtained by increasing the film thickness and/or by removing the defective layer at the substrate interface.