During selective epitaxial growth of diamond through SiO2 masks, silicon terminations were formed on a diamond surface by replacing oxygen terminations under the masks. The high temperature of selective growth and its reductive atmosphere possibly allowed Si atoms in SiO2 to interact with the diamond surface, resulting in silicon terminated diamond (C–Si diamond) composed of a monolayer or thin multi-layers of carbon and silicon bonds on diamond. Diamond metal oxide semiconductor field effect transistors (MOSFETs), with a C–Si diamond channel and selectively grown undoped or heavily boron-doped (p+) source/drain (S/D) layers, have been fabricated. Both the MOSFETs with undoped and p+ S/D exhibited enhancement mode (normally off) FET characteristics. The drain current (IDS) of the undoped device reached −17 mA/mm with threshold voltage (VT) −19 V; the p+ device attained a high IDS −165 mA/mm with a VT of −6 V being one of the best normally off diamond FETs. Transmission electron microscopy and energy dispersive x-ray spectroscopy confirmed the presence of C–Si diamond under the SiO2 masking area. The field effect mobility and interface state density at the C–Si/SiO2 (220 nm)/Al2O3 (100 nm) MOS capacitor are 102 cm2 V−1 s−1 and 4.6 × 1012 cm−2 eV−1, respectively. The MOSFET operation of C–Si diamond provides an alternative approach for diamond.
We report angle-resolved photoemission spectroscopy (ARPES) results of A-site ordered perovskite CaCu3Ti4O12. We have observed the clear band dispersions, which are shifted to the higher energy by 1.7 eV and show the band narrowing around 2 eV in comparison with the local density approximation calculations. In addition, the high energy multiplet structures of Cu 3d 8 final-states have been found around 8 -13 eV. These results reveal that CaCu3Ti4O12 is a Mott-type insulator caused by the strong correlation effects of the Cu 3d electrons well hybridized with O 2p states. Unexpectedly, there exist a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states.A-site ordered perovskite CaCu 3 Ti 4 O 12 (CCTO) has generated considerable interest due to the extremely high dielectric constant ( ) as high as 10 4 -10 5 over a wide range of temperature from 100 to 600 K, which holds a promise for high performance capacitor [1]. Prior to applications, there have been many studies to identify the intrinsic mechanism of the high . Although the consistent conclusion of the origin has not been established yet, it has been widely accepted that the high would come from defects and/or disorder structures, e.g. a relaxor like dipole fluctuation in nanosize domain [2], an internal barrier layer capacitance [3], and the nanoscale disorder of Ca and Cu-site [4]. Generally, the origin of the high of CCTO has been considered to be different from that of conventional ferroelectric materials, because of the absence of structural transition accompanying with the abrupt change of around 100 K [5,6]. Therefore, in order to understand these intrigue physical properties, the electronic structures should be clarified experimentally. In particular, relations between the electronic structure and the strong correlation effects are central issues [7][8][9]. For example, the high and an insulator phase of CCTO can not be explained by theoretical calculations within the local density approximation (LDA), which are not considered to properly treat strong correlation between electrons. The strong correlation effects can be also expected from the crystal structure of CCTO, which contains the CuO 4 plane units in similar to the CuO 2 plane of the high-T c cuprates as shown in Fig. 1 (b) [10]. Recently, it has been reported that a family compound, CaCu 3 Ru 4 O 12 , shows the heavy fermion behavior and the non-Fermi liquid, supporting the importance of the strongly correlation effects [11]. Hence, it has been believed that CCTO would be a Mott-type insulator, even though the experimental band dispersions have never been observed.Here, we first report the clear observation of band dispersions of CCTO by the angle-resolved photoemission spectroscopy (ARPES) measurements. ARPES experiments were performed at the beamline BL5U of UVSOR, using photon energies (hν) from 30 to 93 eV. Measurements were carried out at room temperature (T = 300 K) in a vacuum better than 2 × 10 −8 Pa. Total energy res...
We have performed the photoemission and inverse photoemission experiments to elucidate the origin of Mott insulating states in A-site ordered perovskite CaCu 3 Ti 4 O 12 (CCTO). Experimental results have revealed that Cu 3d-O 2p hybridized bands, which are located around the Fermi level in the prediction of the local-density approximation (LDA) band calculations, are actually separated into the upper Hubbard band at ∼ 1.5 eV and the lower Hubbard band at ∼ −1.7 eV with a band gap of ∼ 1.5-1.8 eV. We also observed that Cu 3d peak at ∼ −3.8 eV and Ti 3d peak at ∼ 3.8 eV are further away from each other than as indicated in the LDA calculations. In addition, it is found that the multiplet strucutre around −9 eV includes a considerable number of O 2p states. These observations indicate that the Cu 3d and Ti 3d electrons hybridized with the O 2p states are strongly correlated, which originates in the Mott-insulating states of CCTO.
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