Low-temperature STM/STS studies on boron-doped (111) diamond films Terukazu Nishizaki a,Ã , Yoshihiko Takano b , Masanori Nagao b , Tomohiro Takenouchi c , Hiroshi Kawarada c , Norio Kobayashi a a Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan c School of Science and Engineering, Waseda University, Tokyo 169-8555, Japan article info Keywords: A. Superconductors C. Scanning tunnelling microscopy (STM) D. Electronic structure abstract We have performed scanning tunneling microscopy/spectroscopy (STM/STS) experiments on (111)- oriented epitaxial films of heavily boron-doped diamond (T c 5.4 K). We present that tunneling conductance spectra show temperature-dependent spatial variations. In the low-temperature region (T ¼ 0.47 K), the tunneling spectra do not show strong spatial dependence and a superconducting energy gap is observed independent of the surface morphology. In the high-temperature region (T ¼ 4.2 K), on the other hand, the tunneling conductance spectra show significant spatial dependence, indicating the inhomogeneous distribution of the superconducting property due to the distribution of boron atoms. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction Lightly boron-doped diamond is a p-type semiconductor with an activation energy of 0.37 eV. With increasing boron concen- tration n beyond the critical value n c 3  10 20 cm 3 for the metal–insulator (MI) transition, the boron-doped diamond shows metallic behavior. Recently, superconductivity was discovered in the heavily boron-doped diamond polycrystalline samples (T c 2.3–4 K for n4.9  10 21 cm 3 ) synthesized by high-pressure high-temperature (HPHT) method [1] and boron-doped diamond thin films (T c ¼ 4.2–7.4 K) prepared by microwave plasma-assisted chemical vapor deposition (MPCVD) method [2,3]. The super- conducting transition temperature T c depends on the concentra- tion of boron, the synthesized process, and the orientation of the film growth [2–4], and the highest T c reached so far is 7.411.4 K for (111)-oriented epitaxial films with n8.4  10 21 cm 3 [5]. Various theoretical proposals [6–11] and experiments [12–20] have been performed to understand the electronic structure and the mechanism of the superconductivity in boron-doped dia- mond. Scanning tunneling microscopy/spectroscopy (STM/STS) is a powerful tool to detect high-energy-resolution information about the electronic state with sub-nanometer spatial resolution. In the low-temperature region (T5T c ), the differential conduc- tance (dI/dV) spectra provide direct information on the local density of states (LDOS) of quasiparticles N(E, r) and consequently on the superconductor gap structure, the low-energy quasiparticle excitations at the Fermi level, and the electron–phonon coupling strength. Here, E is the energy of quasiparticles and r the position of the STM tip. In this paper, we report low-temperature STM/STS experiments on (111)-oriented epitaxial films of heavily boron-doped dia- mond. We present the spatial variation of LDOS and super- conducting energy gap structures at T ¼ 0.47 and 4.2 K. 2. Experimental Heavily boron-doped epitaxial diamond films were grown on the (111)-oriented type Ib diamond substrates by using the MPCVD method [3]. The superconducting transition temperature was T c ¼ 5.4 K and the boron concentration was estimated to be n6  10 21 cm 3 [5]. STM/STS measurements were performed by using 3 He-refrigerator-based STM under the ultra-high vacuum (UHV) condition [16,17]. The spatial variation of the LDOS was measured at T ¼ 0.47 and 4.2K. In order to reduce the oxygen contamination, the film was annealed at 450 1C in the UHV chamber just before STM/STS measurements [16]. 3. Results and discussion It is known that the film surface of the heavily boron-doped diamond shows rough structures with a typical size of 1 mm and fine microstructures with a typical size of 5–20 nm covering over the entire surface [15,16]; our STM topography on the (111)- oriented epitaxial film is consistent with the previous studies ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jpcs Journal of Physics and Chemistry of Solids 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.06.011 Ã Corresponding author. Tel.: +8122 215 2029; fax: +8122 215 2026. E-mail address: terukazu@imr.tohoku.ac.jp (T. Nishizaki). Journal of Physics and Chemistry of Solids 69 (2008) 3027– 3030