Kenji Nose

Electric conductivity of boron nitride thin films enhanced by in situ doping of zinc

The authors demonstrate that the electric conductivities of cubic and hexagonal boron nitride (c-BN and h-BN) thin films increased markedly by the in situ doping of zinc. The doped films were electrically semiconducting, and conductivities at room temperature increased from 10−8to10−2Ω−1cm−1 with increasing zinc concentration from 400to20000ppm. Activation energies for electric conduction (Ec) of c-BN decreased from 0.3to0.1eV with increasing zinc concentration, suggesting feasible shallow-level doping. On the other hand, h-BN thin films showed approximately 0.1eV higher Ec than those of c-BN’s, due to the formation of defective B–B bonds.

Effects of magnesium doping on growth and electric conductivity of nanocrystalline cubic boron nitride thin films

We have achieved in situ magnesium doping into nanocrystalline cubic boron nitride thin films during sputter deposition. It was clarified, through x-ray diffraction, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, that less than 1 at% magnesium doping does not significantly affect the growth of the cubic phase. Magnesium-doped films deposited in pure argon showed electric conductivity up to 104 times higher than the undoped film. Although the Hall voltage was not reproduced reliably for most of the cases, the films containing 2.1 at% magnesium clearly revealed p-type conduction with carrier concentration and mobility of 4 × 1014 cm−3 and 6 cm2 V−1 s−1, respectively, at 380 K. It was also found that the increase in electric conductivity with magnesium concentration was accompanied by the decrease in the relative amount of nitrogen in the film.

Semiconducting properties of zinc-doped cubic boron nitride thin films

We have examined the electronic properties of zinc-doped cubic boron nitride (cBN) thin films prepared by sputter deposition. The electric conductivity of films deposited in pure Ar increased as the concentration of zinc dopant increased, and hole conduction was identified by the measurement of thermoelectric currents. It was also found that the conductivity increment in such films was accompanied by a linear increase in the B∕(B+N) ratio. At the same time, no modification of the composition and the conductivity by incorporated zinc was observed when film growth took place in presence of nitrogen gas. The effect of the excess boron on the conductivity emerged only when films show semi-insulating behavior. These results suggest that Zn substitution for nitrogen causes high electric conductivity of cBN. The electric contact between Ti electrode and semiconducting cBN was examined by the transfer length method, and Ohmic conduction was observed in the Ti∕cBN contact. The specific contact resistance was affec...

Effect of Ti seed layers on structure of self-organized epitaxial face-centered-cubic-Ag(001) oriented nanodots

The influence of Ti seed layers on the structure of self-organized Ag nanodots, obtained with a Ti seed-layer-assisted thermal agglomeration method, has been investigated. The samples were grown on MgO(001) single crystal substrates by RF magnetron sputter deposition. The samples were deposited at room temperature and post-annealed at 350 °C for 4 h while maintaining the chamber vacuum conditions. The results of atomic force microscopy (AFM) observations indicated that the insertion of the Ti seed layer (0.6–5.0 nm) between the MgO substrate and Ag layer promotes the agglomeration process, forming the nanodot array. Comparisons between the AFM images revealed that the size of the Ag nanodots was increased with an increase in the Ti seed layer thickness. The atomic concentration of the film surface was confirmed by X-ray photoelectron spectroscopy (XPS). The XPS result suggested that the nanodot surface mainly consisted of Ag. Moreover, X-ray diffraction results proved that the initial deposition of the Ti seed layer (0.6–5.0 nm) onto MgO(001) prior to the Ag deposition yielded high-quality fcc-Ag(001) oriented epitaxial nanodots. The optical absorbance spectra of the fabricated Ag nanodots with various Ti seed layer thicknesses were obtained in the visible light range.

Electron field emission from undoped polycrystalline diamond particles synthesized by microwave-plasma chemical vapor deposition

Electron emission from polycrystalline diamond particles (PDPs) was obtained at low electric fields in the absence of intentional doping. The PDPs were synthesized on a silicon substrate using microwave-plasma chemical vapor deposition accompanied by bias-enhanced nucleation. Polycrystalline diamond particles of two different sizes, i.e., ∼500 nm and 2 to 5 μm, were obtained, the surfaces of which were covered with small crystal grains composed of fine facets. Electron emission from the PDPs was characterized by Fowler-Nordheim tunneling with low turn-on-field values (0.8 – 2.0 V/μm) and a low barrier height of 0.02 eV. An emission current greater than 5 μA was maintained for over 24 h in a cathode based on the developed PDPs. In contrast, single-crystalline diamond particles prepared for comparative purposes exhibited no emission up to 2.5 V/μm. Auger electron spectroscopy revealed that the surface oxygen content modified by annealing in air did not affect the emission properties. The macroscopic spatial...

Fabrication of 10-nm-scale nanoconstrictions in graphene using atomic force microscopy-based local anodic oxidation lithography

We performed local anodic oxidation (LAO) lithography on monolayer graphene and highly oriented pyrolytic graphite (HOPG) using atomic force microscope (AFM). Auger electron spectroscopic measurements in the oxidized area formed on the HOPG revealed that the number of oxygen atoms systematically increased with the bias voltage applied to the AFM cantilever |Vtip|, which demonstrates the tunability of the extent of oxidation with |Vtip|. By optimizing the LAO conditions, we fabricated monolayer graphene nanoconstrictions with a channel width as small as 10 nm, which is the smallest graphene nanoconstriction so far achieved by utilizing AFM lithography techniques.

Epitaxial growth of fcc-Ag(0 0 1) nanodots on MgO(0 0 1) substrates via Ti seed layer-assisted agglomeration

We have analysed the influence of Ti seed layer (2.0 nm thick) on the agglomeration of Ag films (4.0 nm thick) grown onto MgO(0 0 1) single crystal substrates by RF magnetron sputtering. The samples were deposited at room temperature and post-annealed at 200–450 °C for 4 h while still maintaining the chamber vacuum condition. The surface profile of the sample, as analysed using atomic force microscopy, confirms that the insertion of a Ti seed layer between the MgO substrate and Ag layer promotes the agglomeration process, forming the nanodot. Furthermore, the atomic concentration depth profile of the Ag/Ti/MgO film, as estimated by using angle-resolved x-ray photoelectron spectroscopy, suggests that the nanodot surface mainly consists of Ag. Moreover, x-ray diffraction studies prove that the initial deposition of the Ti seed layer onto MgO(0 0 1) prior to the Ag deposition yields high-quality face-centred cubic (fcc)-Ag(0 0 1) oriented epitaxial nanodots. Based on these results, it can be concluded that the Ti thin film acts as a seed layer, assisting the epitaxial growth of the Ag nanodot onto the MgO substrate.

High Transparency and Electrical Conductivity of SnO2:Nb Thin Films Formed through (001)-Oriented SnO:Nb on Glass Substrate

Nb-doped (101)-oriented SnO2 (SnO2:Nb) thin films were fabricated through postdeposition thermal annealing of (001)-oriented Nb-doped SnO films grown on glass substrates using pulsed laser deposition. Nb doping resulted in an anisotropic lattice deformation explained by Nb in Sn site. Resistivity, electron concentration, and mobility values of 3.7×10-3 Ω cm, 6.5×1019 cm-3, and 26 cm V-1 s-1, respectively, were achieved in the films at room temperature for a dopant concentration of 1 at. %. The optical band gap increased from 3.9 to 4.1 eV with Nb doping ≥1 at. %.

Oxidation of SnO to SnO2 thin films in boiling water at atmospheric pressure

We demonstrated that SnO is oxidized to SnO2 in boiling water. (001)-oriented SnO thin films were pulsed-laser deposited onto a glass substrate. The Sn valence number changed from (II) to (IV) by keeping SnO films in boiling water at atmospheric pressure for 5 h. Optical transparency of the obtained SnO2 films was greater than 95% in the visible light range. The SnO2 films possessed an amorphous structure, and exhibited dielectric properties. Atomic force microscopy and Fourier transform infrared spectroscopy revealed granular structures and the existence of –OH groups, which may account for the diffusion of oxidants within the film.

Fabrication of self-organized epitaxial fcc-Ag(110)-oriented nanorods by a Ti seed-layer-assisted thermal agglomeration method

We analyzed the effect of a Ti seed layer on the agglomeration of Ag films grown on MgO(110) substrates. The samples were deposited at room temperature and post-annealed at 350 °C for 4 h. Atomic force microscopy observations indicated that the insertion of the Ti seed layer between the substrate and Ag promoted the agglomeration, leading to nanorod formation. X-ray diffraction analyses showed that the Ag/Ti film grown on MgO(110) substrate yielded fcc-Ag(110)-oriented epitaxial nanorods. Additionally, typical optical absorbance spectra of the Ag nanorods were obtained in the Ag/Ti films.