Koji Kajimura

Homoepitaxial diamond films grown by step-flow mode in various misorientation angles of diamond substrates

Abstract Surface morphologies of device-grade homoepitaxial diamond films grown by microwave plasma chemical vapor deposition (CVD) with CH 4 /H 2 ratios and misorientation angles of diamond (001) substrates are discussed. We produced a map of surface morphologies as a function of both the misorientation angles of the diamond substrates and CH 4 /H 2 ratios. This map revealed that homoepitaxy under CH 4 /H 2 ratios greater than 0.05% was in good agreement with results reported in the literature. This could be explained in terms of migration length and CH 4 /H 2 ratios. On the other hand, when the CH 4 /H 2 ratios were lower than 0.15%, we found that the misorientation angle does not affect the surface morphology of the homoepitaxial diamond films so much, and we obtained atomically flat surfaces over the whole substrate area. The numerical result may be consistent with the fact that the migration length of precursors increases by orders of magnitude in the presence of a large amount of atomic hydrogen.

Stepped growth and etching of (001) diamond

Abstract We have observed, for the first time, step-flow etching of (001) diamond substrates using hydrogen plasma treatment and have succeeded in step-flow epitaxial growth of diamond films on these hydrogen plasma-treated substrates. By subjecting diamond (001) substrates to hydrogen plasma, the corrugated structure observed before the treatment almost disappears and atomic steps (several atoms high) running over the surface parallel to the [110] direction appear. This shows that lateral propagation of multiatomic steps takes place during etching. In the step-flow epitaxial growth, multi-atomic steps with a terrace width of approximately 8 nm run over the surface parallel to the [110] direction, suggesting that the migration length of precursors under plasma conditions is longer than previously reported for hot filament growth.

TUNNELING SPECTROSCOPY AND PAIRING SYMMETRY OF THE HIGH-TC SUPERCONDUCTORS

Abstract Influences of d-wave pairing symmetry on superconducting electronic states is studied by tunneling spectroscopy. Detailed experiments on spatial and orientational dependences of the surface electronic states of YBa2Cu3O7−δ thin films and preliminary results of the proximity effect observation in Au layer deposition on YBCO are presented. By comparing these experimental results with recent theoretical works, the existence of anomalous interference effects are verified in the electronic states of the high-Tc superconductor, due to the d-wave symmetry of the pair potential.

High quality homoepitaxial diamond thin film synthesis with high growth rate by a two-step growth method ☆

Abstract Homoepitaxial diamond films grown in the condition of CH 4 /H 2 ratio lower than 0.15% in a microwave-assisted plasma chemical vapor deposition system had excellent electrical and optical properties without any unepitaxial crystallites (UCs). Under such a low CH 4 concentration condition, however, the growth rate becomes too slow to obtain a useful thickness. In order to overcome this problem, we attempted a two-step growth method. In the first step the substrate surface was treated by homoepitaxial growth of diamond in the presence of 0.05% CH 4 in H 2 ; in the second step the CH 4 concentration was increased. By considering the origin of UCs with cross-sectional transmission electron microscope studies, it was found that this method is based on surface improvement of the initial substrate by means of ultra-low CH 4 concentration growth. This method was quite useful for obtaining high quality films, with high growth rate and reproducibility.

SCANNING TUNNELLING SPECTROSCOPY OF YBa2Cu3O7−δ THIN FILMS

Abstract Orientational dependence of the tunnelling spectroscopy of YBa2Cu3O7−δ (YBCO) films is investigated by scanning tunnelling spectroscopy. For (110)-oriented films, the surface structures estimated by reflection high energy electron diffraction (RHEED) showed high crystallinity. The tunnelling spectra of the corresponding films showed zero-bias conductance peaks which were uniformly distributed in space. The result is consistent with the theory based on the d 2 x 2 −y -wave symmetry of the pair potential. On the other hand, for (100)-oriented films, the surface structures observed by RHEED showed relatively low crystallinity. The tunnelling spectra of these films showed various types of spectra with strong spatial dependence. These results reflect the high sensitivity of the surface electronic states to the surface qualities of high-Tc superconductors.

Spatial uniformity of Schottky contacts between aluminum and hydrogenated homoepitaxial diamond films

Homoepitaxial diamond films prepared under low CH4 concentration conditions (CH4/H2 42 dots) prepared on the same film. This result indicates that the quality of the films with atomically flat surfaces over the whole area of the substrate is actually excellent in a viewpoint of chemical stability as well as electrical characteristic.

Pinning-controlled metal/semiconductor interfaces

We propose a new systematical method to control Schottky barrier heights of metal/semiconductor interfaces by controlling the density of the interface electronic states and the number of charges in the states. The density of interface states is controlled by changing the density of surface electronic states, which is controlled by hydrogenation. We apply an establishing hydrogen termination technique for the hydrogenation using a chemical solution, pH controlled buffered HF or hot water. The density of interface charges is changeable by controlling a metal work function. When the density of interface states is completely hydrogenated, the barrier height is determined simply by the difference between a work function of a metal (phi) m and an electron affinity of a semiconductor (chi) s. In such an interface with zero density of interface states, an Ohmic contact with a zero barrier height is formed when we select a metal with (phi) m < (chi) s. We have already demonstrated controlling Schottky and Ohmic properties by changing the hydrogenation on silicon carbide (0001) surfaces.

Pinning-controlled ohmic contacts: application to SiC(0001)

We propose a new systematic method to make ohmic contacts by reducing Schottky barrier heights by decreasing the density of interface electronic states. The decrease in the density of states releases the Fermi level of the interface from pinning. This results in the barrier height being determined simply by the difference between the work function of the metal and the electron affinity of the semiconductor. An atomically flat surface is a prerequisite for the decrease in the density of interface states. We apply an established hydrogen termination technique to passivate a semiconductor surface using a chemical solution, pH controlled buffered HF or boiling water. We have demonstrated this type of ohmic contacts without a Schottky barrier on silicon carbide (0001) surfaces without post-annealing.

Quasiparticle tunneling in d/I/d junctions

Abstract Quasiparticle tunneling properties of SIS junctions including d -wave superconductor are studied theoretically. Similarly to the cases of tunneling junctions formed between normal metals and d -wave superconductors, the existence of the surface bound states is shown to affect seriously on the properties of the tunneling spectra. Our theory explains the origin for experimental observations of the zero-bias conductance peaks in grain-boundary junctions made of high- T c superconductors.

Frequency dependence of the ultrasonic attenuation in superconducting niobium in the mixed state

Abstract Attenuation of longitudinal ultrasonic waves was measured in the frequency range from 25 to 700 MHz in a single crystal of high purity niobium. Ratio of the attenuation coefficients in the superconducting and normal states, αs/αn, was measured as a function of magnetic field, H, at various constant temperatures. For the magnetic field near the upper critical field, Hc2, the result was found to be composed of two parts: In the immediate vicinity of Hc2, the attenuation coefficient obeyed the relation, 1−(αs/αn)=A(Hc2−H), whereas the result at field rather below Hc2 was described by 1−(αs/αn)=C(Hc2−H)1/2, a relation of Makis pure limit theory. The above linear dependence on H hardly seems to obtain a plausible explanation at present. The observed value of C varied rather steeply around the frequency corresponding to ql⋍I, where q is the impressed wave number. The dip of the attenuation just above the lower critical field was also investigated. The depth of the dip was found to depend on the frequency as well as temperature, and the frequency dependence could not be accounted for by the existing proposals. Besides the above-mentioned results, the superconducting energy gap at 0°K without magnetic field was found to be smaller for higher frequencies.