Susumu Takabayashi

Formation of Diamond-Like Carbon Films by Photoemission-Assisted Plasma-Enhanced Chemical Vapor Deposition

Diamond-like carbon (DLC) films, which are an amorphous carbonaceous allotrope composed of sp2 carbon, sp3 carbon, and hydrogen, were prepared by photoemission-assisted plasma-enhanced chemical vapor deposition (PA-PECVD). The electrical behavior during film growth monotonically depended on the methane source gas concentration. Raman analysis of the films suggests that a DLC film grown at a high methane concentration condition contains a small number of graphitic domains, decreasing amorphicity of the film. In contrast, at a low concentration, the methane molecules were transformed into sufficiently fragmented radicals, forming a lot of graphitic nuclei and increasing the amorphicity. However, the variations of the relative dielectric constant, breakdown strength, and optical bandgap exhibited extreme values at an intermediate methane concentration. Thus, the two growth modes give different DLC films with varying combinations of electrical and optical characteristics.

Graphene-channel FETs for photonic frequency double-mixing conversion over the sub-THz band

We report on photonic frequency double-mixing conversion utilizing a graphene-channel FET (G-FET). Optoelectronic properties of graphene are exploited to perform single-chip photonic double-mixing functionality, which is greatly advantageous in future broadband technological conversion between optical fiber and sub-terahertz wireless communications. A 1-GHz modulation signal on a 125-GHz carrier is electrically input to the gate, whereas a 1.58-μm dual wavelength CW laser beam having a frequency difference of 112.5 GHz impinges on the G-FET. The G-FET works as a photomixer generating a 112.5-GHz local signal which is then electrically mixed to the 1-GHz modulated 125-GHz carrier signal, resulting in the down-conversion of the 1-GHz signal to a 12.5-GHz intermediate frequency (IF) signal.

Infrared Spectroscopic Study of Hydrogenation Process of Si(100) Surface During Hydrogen Plasma Exposure

We used infrared spectroscopy to investigate the hydrogenation process of Si(100) surface by hydrogen plasma exposure with the substrate bias of -150 V at different temperatures between room temperature and 200 °C. An amorphous layer is generated by exposure at room temperature, following the generation of both hydrogen insertion states and hydrogen terminated vacancies. By increasing the substrate temperature during exposure, generation of SiH2 in the amorphous layer is suppressed. Si vacancy terminated with three hydrogen atoms (VH3) is prominently generated by the plasma exposure at more than 150 °C, following generation of two hydrogen insertions into the Si-Si bonds (IH2).

Dielectric-Tuned Diamondlike Carbon Materials for an Ultrahigh-Speed Self-Aligned Graphene Channel Field Effect Transistor

The ‘DLC-GFET’, a graphene-channel field effect transistor with a diamondlike carbon (DLC) top-gate dielectric film, is presented. The DLC film was formed ‘directly’ onto the graphene channel without forming passivation interlayers using our photoemission-assisted plasma-enhanced chemical vapor deposition (PA-CVD), where the plasma was precisely controlled by significant photoemission from the sample with quite low electric power, minimizing plasma damage to the channel. The DLC-GFET exhibits clear ambipolar characteristics with a slightly positive shift of the neutral points (Dirac voltages). Relatively high transconductances were obtained as 14.6 (8.8) mS/mm in the n (p) channel modes, respectively, with a thick DLC gate dielectric of 48 nm and a long gate length of 5 μm, promising vertical scaling-down to improve the high-frequency performance. The positive shift of the Dirac voltage is due to unintentional hole doping from an oxygen species like H2O in the DLC film into the graphene channel, suggesting that a modulation-doped DLC structure with a δ-doped oxygen (nitrogen) species for the p (n) mode will overcome high access resistance.