Fabrication and Characterization of Electrically Functional Lanthanum Hexaboride Thin Films on Ultrasmooth Sapphire Subs
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1148-PP12-02
Fabrication and Characterization of Electrically Functional Lanthanum Hexaboride Thin Films on Ultrasmooth Sapphire Substrates
Yushi Kato1, Yusaburo Ono1, Yasuyuki Akita1, Makoto Hosaka1, Naoki Shiraishi1, Nobuo Tsuchimine2, Susumu Kobayashi2, and Mamoru Yoshimoto1 1 Department of Innovative & Engineered Materials, Tokyo Institute of Technology, Yokohama 226-8503, Japan. 2 TOSHIMA Manufacturing Company Limited, Saitama 355-0036, Japan.
ABSTRACT The crystal growth of lanthanum hexaboride (LaB6) thin films was examined by applying the laser molecular beam epitaxy (laser MBE) process. C-axis (100) highly-oriented LaB6 thin films could be fabricated on ultrasmooth sapphire (α-Al2O3 single crystal) (0001) substrates with atomic steps of 0.2 nm in height and atomically flat terraces. The obtained film exhibited a smooth surface with root mean square roughness of 0.15 nm. The lattice parameter of the LaB6 thin film was close to the bulk value reported previously. In the case of deposition on commercial mirror-polished sapphire substrates, the grown film was amorphous. The resistivity of the prepared crystalline LaB6 thin films was as low as 2.2 × 10−4 Ω cm and almost constant in the temperature range of 10–300 K. INTRODUCTION Lanthanum hexaboride (LaB6) is characterized by a high melting temperature, excellent thermal stability, high hardness, and excellent chemical stability [1–9]. It has already been applied to practical use in thermionic electron sources for TEM, SEM, and flat panel displays, which can offer high brightness and long service life because of the compound’s extremely small work function (~2.4 eV), high current and voltage capability, and low vapor pressure at high temperature. The crystal structure of LaB6 is a simple cubic CsCl-type arrangement of B6octahedra and metal ions [1–5], and the electric conduction of LaB6 shows metallic properties [7]. In addition, the electrical properties of hexaborides including various metal ions show a wide variation from semiconductor to superconductor [10], by doping divalent or trivalent ions into the crystal structure [11, 12]. The investigations providing the results outlined above have mostly been conducted on bulk samples of LaB6 [1–9]; in contrast, the properties of LaB6 thin films have not been well explored. There have been reported a few attempts to prepare LaB6 nanowire by chemical vapor deposition (CVD) [13], and LaB6 thin films by sputtering [14, 15], e-beam evaporation [16], and pulsed laser deposition (PLD) [17–19]. However, there are no reports on fabrication of epitaxial LaB6 thin films. We have previously investigated low temperature epitaxial growth of functional ceramic thin films of oxides [20–22] and nitrides [23, 24] by laser MBE (i.e., pulsed laser deposition in ultra-high-vacuum) using ultrasmooth sapphire (α-Al2O3 single crystal) (0001) substrates. These smooth substrates have atomic steps of 0.2 nm in height and atomically flat terraces of 60–80 nm in width [25]. Laser MBE has been intensively applied to the growth of
epitaxial
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