The discovery process and the mechanism of p-type surface layer accumulation of hydrogen terminated diamond surfaces have been reviewed from the point of surface characterization and carrier formation near the subsurface. Since diamond is an insulating material, the surface density is as high as the order of 1013 cm−2 that is enough for high performance field effect transistors to be realized. The surface dipole formed by hydrogen-carbon bonds has a crucial role for the surface band structure of diamond. Also important is several types of negatively charged adsorbates necessary for charge neutrality condition and hole accumulation. New models for carrier emergence such as a transfer doping model have been discussed.
Diamond has unique physical and chemical properties such as the highest thermal conductivity. Especially, the negative electron affinity (NEA), where the conduction band minimum is higher than the vacuum level, is highlighted for a long time as to be one of the best candidates for the electron emitters. We have developed growth techniques of high quality diamond films, and total photoelectron emission yield spectroscopy (TPYS) equipment to be able to investigate NEA on diamond. Now it is possible to determine NEA quantitatively, and characterize electronic properties of surface and bulk of diamond by means of NEA. According to these results, we find that free-exciton derived electron emission plays an important role in electron emission from diamond with the NEA surface, and finally realizes a NEA electron emitter with diamond p-n junction diodes. The diamond semiconductor, that makes it possible to study “excitonics” at room temperature, will be one of the most exciting materials for the next surface science.
Hydrogen surface termination is widely used as a p-type doping in diamond semiconductors, but the p-type conduction mechanism is still controversial. In this study, we found an energy barrier for holes between the gate and the two-dimensional hole channel on the hydrogen-terminated diamond surface from FET characteristics. Separately we confirmed an interfacial layer between the gate metal layer and hydrogen-terminated diamond surface from cross-sectional transmission electron microscopic observation. We conclude that during metal evaporation on hydrogen-terminated diamond surface, metal atoms diffuse through point defects in the subsurface layer, and eventually the interfacial layer forms there.
Conductive boron-doped diamond (BDD) electrodes are very attractive electrode materials because of their superior electrochemical properties such as wide potential window, low background current, chemical inertness, and mechanical durability. We have studied the application of diamond electrodes as electrochemical sensors for the detection of several types of biochemical species or environmental pollutants. For example, free chlorine, metals (Zn, etc), and proteins including cancer markers could be detected by using the diamond electrodes with very high sensitivity and high stability. Furthermore, modified diamond electrodes such as ion-implanted diamond electrodes and diamond electrodes with micro-meter size were also prepared in order to improve the electrochemical properties. Here, several examples for electrochemical applications by using the boron-doped diamond electrodes were introduced.
Phosphorous (P)-doped CVD diamonds are expected to be good n-type semiconductor diamonds. However, the resistivity of P-doped intrinsic diamonds is very high due to a large ionization energy of impurity phosphorus. When phosphorus is doped in a large amount (≥∼1020cm−3), the resistivity reduces with a pseudo-n-type conductivity at temperatures above 400 K. These intrinsic n-type and pseudo-n-type P-doped diamond single crystal surfaces have been studied recently by electron spectroscopic methods. The surface electronic energy states of both intrinsic n-type and pseudo-n-type diamond surfaces have been examined by electron spectroscopic methods. The mechanism of field emission from the pseudo-n-type diamond surface has been elucidated.
Photolysis of perfluoroazooctane with diamond and related materials, such as diamond powders, films, diamond-like carbon (DLC) and nanodiamond films, led to a chemical modification of the surface to introduce perfluorooctyl functional groups, confirmed by means of FT-IR, XPS, Raman and TOF-SIMS measurements. The diamond and related materials modified with fluorine moieties showed an improvement of frictional property and a reduction of surface energy evaluated by contact angle to water, compared with pristine diamond, DLC and nanodiamond films. The results on the values of water contact angle depending on irradiation time are consistent with those of F/C ratio of fluorinated diamond and related materials by monitoring with XPS.
Diamond that shows low or negative electron affinity has good electron emission properties. The authors are developing a diamond electron source, which is suited for electron optics instruments such as electron beam exposure systems and electron microscopes. As a result of thermionic emission evaluations, an electron emission current of practical level (116 μA at 600oC) was obtained. The energy spread of an electron beam generated by thermionic emission was measured as 0.23 eV (FWHM), the value of which was lower than those of a LaB6 cathode and a ZrO/W cathode measured together. The result of field emission evaluations, a practical beam current (459 pA) and stability (6%rms for 10 h) for scanning electron microscope (SEM) was obtained. The energy spread of over 200 pA of a high current electron beam generated by field emission was about the same as that of a tungsten field emission cathode. It was found that an unprecedentedly high current and high convergence electron source can be made by using diamond.
We improved the present X-ray photoelectron diffraction apparatus for Differential Photoelectron Holography (DPH) in a laboratory scale experiment. The new rotating anode (Dual target) of which surface is covered with stripes of Mg and Al and two new artificial multi-layer toroidal mirrors realized the high-intensity and dual energy X-ray source.
Nanoscale boundary lubrication and interfacial friction are studied using computer simulations based on extended Frenkel-Kontorova models. Several peculiar properties of static and kinetic frictional forces and the dynamics of lubrication are discussed. Some possible methods to control frictional forces and dynamics of lubrication are proposed and discussed in relation to the mechanism of nanomachines.
We review first-principles theoretical simulations of surface vibrational spectra such as infrared absorption spectroscopy and high-resolution electron energy loss spectra. We compare theoretical and experimental vibrational spectra for two cases, i.e., n-alkane on Cu(100) and methylthiolate on Au(111) and see the predictive power of the first-principles method for vibrational spectra.