Pulse Shape Study of Chemical Vapor Deposited Diamond Alpha Particle Detectors
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Pulse Shape Study of Chemical Vapor Deposited Diamond Alpha Particle Detectors S.G. Wang, P.J. Sellin, A. Lohstroh, M.E. Özsan Department of Physics, School of Electronics and Physical Sciences, University of Surrey, Guildford GU2 7XH, United Kingdom
ABSTRACT We report a study of pulse shapes of a radiation detector with a sandwich structure fabricated from chemical vapor deposited (CVD) polycrystalline diamond. The pulse shapes were recorded at room temperature using 5.486 MeV alpha particles from 241Am source. Only “fast” component was observed in the electron predominated pulses, whereas both “fast” and “slow” components were obtained in the hole predominated pulses, suggesting that electron charge drift is prompt and no detrapping occurred. In contrast, hole charge drift is slower than expected and trapping-detrapping took place during hole travel process.
INTRODUCTION Diamond has many unique properties, such as a wide band gap (~5.5 eV), high electron and hole mobilities (1800 and 1200 cm2 V-1 s-1 respectively), high resistivity (>1011 Ω cm) and very low dark currents. This makes it an attractive material for use as a particle detector [1-3]. Besides natural single diamond, chemical vapor deposited (CVD) diamond grown with different techniques has been available since 1980s [4] and it is currently extensively researched in applications for radiation detectors [3,5-7]. The charge collection distance and efficiency are generally used to assess the performance of diamond detectors. The overall mean drift distance δ, which is also referred to as the charge collection distance, is given by δ=δe+δh =(µeτ e +µhτ e)E
(1)
where δe, δh are the electron and hole mean drift distances, µe, µh are the electron and hole mobilities, τ e and τ h are the lifetime for the free electrons and holes, respectively, and E is the applied electric field. The charge collection efficiency η is defined as the ratio of the collected charge Qc to the total charge Q0 generated by the incident alpha particles η= Qc/Q0
(2)
where η is related to δ with Hecht’s equation [8], which is given generally by η=δh/L [1-exp-(χ/δh)] + δe/L [1-exp-((L-χ)/δe)]
(3)
where L is the detector thickness, and χ is the depth of the radiation interaction measured from the cathode. Long carrier drift distances are required for particle detection, but the presence of either “shallow” or “deep” impurities and defects in CVD diamond can deteriorate the charge
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transport properties by decreasing the effective mobility and lifetime of the charge carriers. Consequently in order to understand the role of defects in particle detection, it is important to study the dynamic behaviour of the charge transport in CVD diamond detectors. The time of flight (TOF) technique provides a direct measurement of charge carrier drift times, and hence mobility, through analysis of the preamplifier pulse shapes. Recent studies by Marinelli et al. [9] reported a significant “slow” component in the preamplifier pulse shapes produced by hole transport, which was only obs
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