CVD Diamond Thin Films for Alpha Particle Detector Application
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CVD Diamond Thin Films for Alpha Particle Detector Application S. G. Wang, Q. Zhang, S. F. Yoon, J. Ahn Microelectronics Centre, School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore. ABSTRACT In this paper, CVD diamond thin films for alpha particle detector have been prepared using hot-filament chemical vapor deposition (HFCVD) technique. Alpha particle detectors with coplanar and sandwich electrode geometry have been fabricated on the diamond films using liftoff technology. The detector performance has been tested using a 241 Am source with a characteristic energy of 5.5 MeV. An average charge collection efficiency of 32 % for the detector with coplanar and of 21 % for the detector with sandwich geometry is obtained.
INTRODUCTION Diamond has high radiation hardness, good thermal conductivity, wide band gap and fast charge collection. These unique properties make it an attractive material for radiation detection applications [1-4]. The main parameters assessing the quality of CVD diamond films to be used as particle detectors are charge collection distance (CCD) and efficiency [5,6]. The charge collection distance is an average drift distance and is given by δ=(µe+µh)τE
(1)
where µe, µh are electron and hole mobilities, respectively, τ mobility weighted lifetime, and E applied electric field. The detector efficiency η is defined as the ratio of the collected charge Qc to the total charge Q0 generated by the incident ionizing particles η= Qc/Q0
(2)
η is related to δ by Hecht theory [7]: η=δ/L [1-δ/(4G) (1-e-2G/δ) (1+e2(G-L)/δ)]
(3)
where G is the ionizing particle penetration depth. L is the detector thickness. From Equations (1) and (3), it can be seen that the lifetime τ critically determines the δ and η. High quality diamond films can be successfully deposited by chemical vapor deposition (CVD) techniques. However, CVD diamond films are typically polycrystalline with grain sizes from a few to tens of micron. In addition, there exist some impurities in the films. The presence of grain boundaries and impurities will strongly reduce the τ, and thus the δ and η, resulting in poor charge transport and low charge collection efficiency. This in turn limits its application in high energy particle detection [1,8-10]. H9.32.1
Some efforts have been done to improve the detector efficiency. They include using high quality diamond films to reduce the structural defects and impurities [5,8], using thick diamond films to reduce the grain boundaries [11, 12], and pre-irradiating the detector to partially passivate defect-related trapping levels in the gap [8]. For diamond-based alpha particle detectors, a simple metal-diamond-metal structure is adopted to construct a detector, instead of a p-n junction diode used in conventional siliconbased detectors [13-15]. Coplanar and sandwich structures are commonly used in diamond-based detectors. A comparative study on detectors with coplanar and sandwich electrode geometry, based on the identical diamond fil
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