Freestanding single crystal chemical vapor deposited diamond films produced using a lift-off method: Response to a-particles from 241 Am and crystallinity Nobuteru Tsubouchi a,⇑ , Y. Mokuno a , A. Kakimoto b , F. Fujita b , J.H. Kaneko b , H. Yamada a , A. Chayahara a , S. Shikata a a Diamond Research Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan article info Article history: Received 12 August 2011 Received in revised form 11 November 2011 Available online 22 November 2011 Keywords: CVD Single crystal diamond Freestanding Lift-off process Crystallinity Diamond particles detector a-Particles abstract Thick (100 lm) undoped diamond films were grown homoepitaxially on single crystal (SC) diamond substrates by microwave plasma chemical vapor deposition (CVD). To form a freestanding SC diamond film (plate), the substrate was pre-ion-implanted with high-energy ion beams before the film growth, and after the thick-film deposition, the substrate was eliminated using a lift-off method, resulting in fab- rication of a SC CVD diamond plate. Two samples were prepared; sample 1 was grown on a (0 0 1) ori- ented, nitrogen doped CVD SC diamond at 900 °C with the input microwave power of 1.7 kW, while sample 2 was grown on a (0 0 1) oriented, high-pressure high-temperature synthesized type-Ib SC dia- mond at 900 °C with the input microwave power of 1.25 kW. The formed SC plates have high optical transparencies, indicating no remarkable optical absorptions seen in the wavelength from ultraviolet to near infrared. The photoluminescence (PL) spectra of both samples show strong free exciton FE peaks, while in sample 2 relatively strong optical emissions corresponding to nitrogen related centers were observed in the visible region. After the metal electrodes were formed on both faces of the SC diamond plate to fabricate a sandwich-type diamond particle detector, the energy spectra of 5.486 MeV a-particles from 241 Am were measured. The charge collection efficiencies (CCEs) of sample 1 were CCE = 98% for a hole transport and CCE = 89% for an electron transport, respectively, while CCEs of sample 2 were CCE = 80% for a hole transport and CCE = 78% for an electron transport, respectively. These results indicate that both holes and electrons in sample 2 were trapped much more than those in sample 1. Possible can- didates of carrier capture centers are nitrogen and/or nitrogen-vacancy centers observed in PL, nonradi- ative defect (complex) centers, extended defects such as threading dislocations observed in micrographs taken with polarizers. The different growth conditions most likely affected crystallinity and responses to a-particles of the samples. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Diamond has long been expected as a potential material for a radiation detector in harsh environments where conventional semiconductors (e.g., Si) cannot be used, because diamond has ex- treme radiation hardness, chemical inertness, etc. Although high quality, single crystal (SC) diamond with a large effective volume is demanded for a radiation detector, high-pressure high-tempera- ture (HPHT) and plasma chemical vapor deposition (CVD) methods used typically for the growth of synthetic diamonds have not ever provided a thickness (growth-rate) and crystallinity enough to fab- ricate diamond particle (radiation) detectors. Recently, high-rate growth (100 lm/h) of SC CVD diamond has been reported [1], which suggested a possibility of fabrication of thick diamond wa- fers. To date, also in our group, high-rate CVD homoepitaxial thick film growth has been attempted [2] using a home-made special de- signed sample holder for facilitating production of high-density source plasmas. In addition, we have ever developed a ‘‘lift-off method’’ [3] using ion implantation based on several reports [4– 6] to separate a thick diamond film from a seed substrate, and fi- nally fabricated a SC diamond plate (freestanding film) [7]. The procedure of our lift-off method is as follows: prior to the film growth, a highly defective layer is produced in the subsurface of the diamond substrate using high-energy, high-dose ion implanta- tion. After this pretreatment, thick diamond film growth is per- formed, and then the graphitic defective layer is dissolved by electrochemical etching, finally resulting in removal of the sub- strate from the diamond film. The combination of the thick CVD diamond homoepitaxial film growth with our above lift-off method gives an effective way to make freestanding diamond SC films (plates), allowing us to fabricate of diamond particle detectors. 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.11.013 ⇑ Corresponding author. E-mail address: nobu-tsubouchi@aist.go.jp (N. Tsubouchi). Nuclear Instruments and Methods in Physics Research B 286 (2012) 313–317 Contents lists available at SciVerse ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb