Naoki Okuyama

Electromigration of In Ultrathin Film on Si(111)

The electromigration of a pad of In ultrathin film with a thickness of several monolayers on a Si(111) 7×7 surface was investigated by scanning Auger electron spectroscopy. Indium atoms moved to the cathode in the ultrathin film contrary to conventional electromigration in bulk materials and in thin films and had a mobility of 8.5×10-5 cm2/(Vs) at 80°C. This is three orders of magnitude greater than the mobility of ordinary films. The morphology of the ultrathin film in the process of the movement was studied particularly carefully and proved to be associated with the growth mode of the In film on Si(111).

Electromigration of Ag Ultrathin Films on Si (111) 7×7

Scanning Auger electron spectroscopy observation revealed that a patch of Ag thin film with a thickness of several monolayers spread on the cathode side over the clean surface of Si(111) 7×7 with the application of dc current to the Si substrate. The spread-out layer had the constant thickness of 1.0 monolayer (=7.8×1014 atom/cm2) with √3×√3 structure. On the anode side the edge of the patch made no significant movement but a mixed phase of √3×√3 and 3×1 with a thickness of 0.8 monolayer appeared and extended from the edge toward the cathode. The mass transport was correlated with changes in the structure of the overlayer.

Formation of MIS Structure in Organic Cell Al/Pb-Phthalocyanine/ITO

The effects of ambient oxygen and air on the current vs voltage characteristic of the sandwich-type cell Al/Pb-Phthalocyanine/ITO (indium tin oxide) were investigated. While the effect of pure oxygen was small, exposure to air caused drastic changes in the characteristic: the linear relation with extremely low resistance observed in vacuum changed into a nonlinear characteristic with enhanced resistivity suggesting a tunnelling conduction, followed by a rectifying and photovoltaic characteristic. The extremely low resistance is a result of the formation of Al spikes penetrating though the PbPc to the ITO during preparation. The rectification and photovoltaic properties are considered to be due to a Schottky barrier with an oxide layer sandwiched between the Al and the PbPc, that is, an MIS-like barrier.

Structure and electrical properties of potassium-graphite-dimethylsulfoxide ternary intercalation compound

Abstract The chemical composition, the stage number, and the identity period, I c , are determined for ternary intercalation compounds prepared by the cathodic reduction of graphite in solution of KI in dimethylsulfoxide (DMSO) and their electrical properties are investigated. Gravimetry reveals that the chemical composition of the compounds is expressed as KC n (DMSO) 3 with n ≥ 24. Stepwise changes in the potential of graphite electrode on reduction are found to correspond to the formation of stage 2 compound KC 24 (DMSO) 3 with I c of 18.4 A and stage 4 compound KC 48 (DMSO) 3 with I c of 25.1 A. The spacing between adjacent carbon planes containing intercalate layer is 15.0 A. The electrical conductivity along the layer plane exhibits a significant increase followed by saturation with intercalate concentration. Compared with the corresponding binary compound KC n , the role of the organic molecule in the ternary compound is found to enhance the conductivity at the initial intercalation.

Investigation on the Microstructure of Hydrogenated Amorphous Carbon Films by Electron Energy-Loss Spectroscopy

Structure characterization of the hydrogenated amorphous carbon (a-C:H) films prepared by plasma CVD has been carried out by electron energy-loss spectroscopy (EELS). The EELS data are successfully resolved to the bulk contributions and the surface ones by using the curve fitting. The EELS spectra are discussed on the basis of the energy-loss function of graphite. The annealing may cause the increase in number of small linear or chainlike clusters of fused sixfold carbon rings rather than growth of their compact clusters.

Electronic Properties of Cs-Covered Surfaces of Graphite (0001)

The electronic properties of Cs-covered graphite surfaces were studied by low-energy electron diffraction, Auger-electron spectroscopy and work function measurements. The behaviors of an additional structure appearing with the adsorption of Cs on the high-energy side of the C KVV (272 eV) Auger spectra are interpreted in terms of the density of states (DOS) of the graphite π conduction band and the extra DOS induced by the alkali atom. The change in work functions is ascribed to a Fermi-level shift due to charge transfer from the Cs to the graphite π conduction band and the potential difference between the graphite surface and the Cs overlayer. Examination of the work function shows that the charge transfer per adsorbate is unity up to a relative coverage of θ=0.35 and thereafter decreases monotonically to a value of 0.35 at the full coverage θ=1.

Photovoltaic Properties of Lead Phthalocyanine Contacts

Measurement was made of the spectral sensitivity of the short-circuit photocurrent in lead phthalocyanine (PbPc) film cells sandwiched between several varieties of electrode materials. It was deduced from the spectral sensitivity of thick cells with thickness of the PbPc film more than 1 µm that the magnitude of the barrier height in the contact was in the following order; (PbPc-Al)>(PbPc-SnO2)>(PbPc-Au)>(PbPc-In2O3). All of these barriers bend downward in the PbPc toward each contact. The maximum spectral sensitivity was observed to be 2.4 mA/W in the cell In2O3/PbPc/Al. To improve the sensitivity, thin cells with thickness around 0.1 µm were prepared and examined with the result of unexpectedly low sensitivities in the cells with Al electrode.

Plasma-enhanced chemical vapor deposition method to coat micropipettes with diamond-like carbon

This article provides a simple method for coating glass micropipettes with diamond-like carbon (DLC) through plasma-enhanced chemical vapor deposition. The apparatus uses a cathode that is a thin-metal-coated micropipette itself and an anode that is a meshed cylinder with its cylinder axis along the micropipette length. To produce a uniform plasma and prevent a temperature increase at the tip due to ion collision concentration, we investigated the effect of the height and diameter of the meshed cylindrical anode on the plasma. Intermittent deposition is also effective for inhibiting the temperature rise and producing high quality DLC films. Measured Raman spectra and electric resistivity indicate that a DLC film suitable for use as an insulating film can be produced on the micropipette. This coating method should also be useful for other extremely small probes.

Diamond-like carbon coating on micropipettes

Based on glass micropipettes which are widely used in bioengineering and medicine, various microsensor probes could be produced by recent microfabrication techniques. Conductive surfaces of these sensor probes mostly need electrical insulating films coated on them, but it has been difficult to coat a high-quality and strong insulating film on a microacute probe such as a micropipette, especially on its tip. Therefore, we have employed diamond-like carbon (DLC) as an insulating film and developed a DLC coating method based on plasma chemical vapor deposition method. In the deposition apparatus, the cathode is the micropipette itself and the anode is a mesh cylinder with a central focus on the micropipette. In order to prevent the growing films from transformation due to high temperature at the tip, the voltage between the pair of electrodes is impressed intermittently. Raman spectrum and electrical resistivity measured here indicate that the deposited film is DLC and it can be worked well as an insulating film. The DLC coating method could be useful in microprobe fabrications.

Removal of a hydrogenated amorphous carbon film from the tip of a micropipette electrode using direct current corona discharge

Micropipette electrodes are fabricated by coating glass micropipettes first with metal and then with hydrogenated amorphous carbon (a-C:H) as an electrical insulator. Furthermore, at the tip of the micropipette electrode, the deposited a-C:H film needs to be removed to expose the metal-coated surface and hollow for the purposes of electrical measurement and injection. This paper describes a convenient and reliable method for removing the a-C:H film using direct current corona discharge in atmospheric air. The initial film removal occurred at an applied voltage of 1.5-2.0 kV, accompanied by an abrupt increase in the discharge current. The discharge current then became stable at a microampere level in the glow corona mode, and the removed area gradually extended.