Schottky-barrier photodiode using p-diamond epilayer grown on p + -diamond substrates Masataka Imura a, ⁎, Meiyong Liao b , Jose Alvarez c , Yasuo Koide b a World Premier International (WPI) Research Center, International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b Sensor Materials Center, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan c Laboratoire de Génie Electrique de Paris (UMR 8507 CNRS), Ecole Supérieure d'Electricité, Universités Paris VI et Paris XI, 11 rue Joliot-Curie Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France abstract article info Available online 9 November 2008 Keywords: Diamond Schottky-barrier photodiode Ib-type diamond substrate p + -diamond substrate In order to understand a substrate effect on photoresponse properties of diamond-based Schottky-barrier photodiode (SPD), we utilize a heavily boron-doped p + -diamond substrate for a thin epilayer growth and the SPD fabrication. Vertical- and planar-types SPD's show excellent rectifying property with ideality factor close to unity and on-resistance as small as 1 Ω·cm 2 . Both the SPD's are operable in photovoltaic and reverse bias modes with a quantum efficiency of 2.2 ± 0.5% and does not provide a photoconductivity gain and a persistent photocurrent before and after annealing at 500 °C for 120 min. By comparing the photoresponse properties of SPD's on the p + -diamond substrate with on a Ib-diamond substrate, we conclude that a homojunction between p-diamond epilayer and Ib-diamond substrate governs a photo-carriers generation and a transport process, which produces an excellent spectral response property of SPD on the Ib-diamond substrate. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor diamond is a promising candidate for deep-ultravio- let (DUV) photodetectors which can be operated in high-temperature, high-radiation flux, and chemically-harsh environments because of its advantages of the highest thermal conductivity of 22 W/cm·K, wide bandgap of 5.5 eV, and chemical, radiation, and thermal stabilities. The highly-sensitive DUV photodetector with high-speed response is expected to be used as flame detection, and the robust DUV photo- detector without degradation under long-term strong DUV irradiation is essential for using as monitor of high-power excimer lamp and/or laser. The performance of single-crystalline diamond-based DUV photo- detector has been remarkably improved [1–4]. We have also developed solar-blind and thermally-stable DUV photodiodes using semi-transparent WC Schottky and annealed Ti/WC ohmic contacts (where a slash “/” shows the deposition sequence), which were fabricated as the planar-type device on boron (B)-doped single-crystalline p- diamond homoepitaxial layers on Ib-type nitrogen (N)-doped insulating diamond (100) substrates by a microwave plasma chemical vapor deposition (MPCVD) technique. A characteristic feature of the DUV photodiode by using the Ib-type diamond substrate is that the N of the substrate will affect and involve into the photoresponse properties, which leads to a huge photoconductivity gain [5–8]. For the reason, it is necessary to select a suitable substrate for the development of high-performance diamond photodetector. In this study, we fabricate Schottky barrier photodiode (SPD) with the vertical- and planar-types by using heavily B-doped p + -diamond (100) substrates and compare the photoresponse properties of the both the SPD's with planar-type SPD fabricated on Ib-type diamond substrate. The purpose of this paper is to investigate the substrate influence on the photoresponse properties of the SPD. 2. Experimental The p-diamond epilayers were homoepitaxially grown by MPCVD on the heavily B-doped p + -diamond (100) substrates commercialized by Technological Institute for Super-hard and Novel Carbon Materials (TISNCM). The substrate was 2.5×2.5×0.5 mm 3 in size, and the B con- centration ([B]) was measured to be around 1 × 10 20 cm - 3 by secondary- ion mass spectroscopy (SIMS). The diamond epilayers were grown by CH 4 and H 2 gases. Residual B inside the MPCVD chamber is believed to be unintentionally incorporated into the diamond epilayer with a con- centration ranging from 10 15 to 10 16 cm - 3 analyzed by SIMS. The ratio of CH 4 to H 2 was 0.08%, and the corresponding flow rates of CH 4 and H 2 were 0.4 and 500 sccm, respectively. The reactor pressure was fixed at 106 hPa during growth. The growth was performed at ~800 °C. The thickness of epilayer was about 0.5 μm, which was a same thickness of p- diamond epilayer of SPD by using N-doped Ib-type diamond substrate. Prior to fabricating the SPD, the diamond epilayer was oxidized for 1 h in a boiling acid solution of H 2 SO 4 /HNO 3 (1:1) to remove the surface conductive hydrogenated layer. The standard photolithographic and lift-off techniques were used for the fabrication process. The metal contacts were deposited using a sputter deposition apparatus at a base Diamond & Related Materials 18 (2009) 296–298 ⁎ Corresponding author. Tel.: +81 29 8513354x8715; fax: +81 29 860 4706. E-mail address: imura.masataka@nims.go.jp (M. Imura). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.065 Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond