Takafumi Kamimura

Depletion-mode Ga2O3 metal-oxide-semiconductor field-effect transistors on β-Ga2O3 (010) substrates and temperature dependence of their device characteristics

Single-crystal gallium oxide (Ga2O3) metal-oxide-semiconductor field-effect transistors were fabricated on a semi-insulating β-Ga2O3 (010) substrate. A Sn-doped n-Ga2O3 channel layer was grown by molecular-beam epitaxy. Si-ion implantation doping was performed to source and drain electrode regions for obtaining low-resistance ohmic contacts. An Al2O3 gate dielectric film formed by atomic layer deposition passivated the device surface and significantly reduced gate leakage. The device with a gate length of 2 μm showed effective gate modulation of the drain current with an extremely low off-state drain leakage of less than a few pA/mm, leading to a high drain current on/off ratio of over ten orders of magnitude. A three-terminal off-state breakdown voltage of 370 V was achieved. Stable transistor operation was sustained at temperatures up to 250 °C.

Band alignment and electrical properties of Al2O3/β-Ga2O3 heterojunctions

The band alignment of Al2O3/n-Ga2O3 was investigated by x-ray photoelectron spectroscopy (XPS). With a band gap of 6.8 ± 0.2 eV measured for Al2O3, the conduction and valence band offsets at the interface were estimated to be 1.5 ± 0.2 eV and 0.7 ± 0.2 eV, respectively. The conduction band offset was also obtained from tunneling current in Al2O3/n-Ga2O3 (2¯01) metal-oxide-semiconductor (MOS) diodes using the Fowler-Nordheim model. The electrically extracted value was in good agreement with the XPS data. Furthermore, the MOS diodes exhibited small capacitance-voltage hysteresis loops, indicating the successful engineering of a high-quality Al2O3/Ga2O3 interface.

Protein Sensor Using Carbon Nanotube Field Effect Transistor

The effect of the selective adsorption of pig serum albumin as an antigen on the electrical properties of a carbon nanotube channel field effect transistor on which anti-pig serum albumin was immobilized as an antibody by physisorption in phosphate buffer solution has been investigated. We have succeeded in real-time detection of the adsorption of pig serum albumin on anti-pig serum albumin as a decrease in the conductance of the carbon nanotube channel field effect transistor, by a label-free process.

Room-Temperature Single-Hole Transistors Made Using Semiconductor Carbon Nanotube with Artificial Defects near Carrier Depletion Region

We have succeeded in observing the single hole transistor characteristics in position-controlled grown carbon nanotubes (CNTs) with artificial defects formed by a chemical process. Coulomb blockade characteristics were observed around the hole depletion region even at room temperature. At a low temperature, however, the Coulomb blockade characteristics were observed both in hole and electron transport regions.

Air Stable n-type Top Gate Carbon Nanotube Filed Effect Transistors with Silicon Nitride Insulator Deposited by Thermal Chemical Vapor Deposition

The air stable n-type carbon nanotube channel filed effect transistor (CNT-FET) with the top gate structure was successfully fabricated using the silicon nitride gate insulator deposited by the thermal chemical vapor deposition. The effects of the silicon nitride insulator on the electrical properties of the CNT-FET have been investigated. The p-type characteristics of the CNT-FET can be converted to the n-type characteristics in high yield of 90% only by depositing the silicon nitride insulator. The drain current is as high as few µA order. The n-type top gate CNT-FET stably operated even in ambient air.

Electrical Heating Process for p-Type to n-Type Conversion of Carbon Nanotube Field Effect Transistors

A new electrical heating process was proposed which converts carbon nanotubes from p-type to n-type by desorption of oxygen molecules around the carbon nanotubes. By applying a large voltage of ~40 V between the source and drain electrodes in vacuum, a large drain current of ~20 µA flows the carbon nanotube channel and heats it, which cause oxygen molecules to desorb from the surface of the carbon nanotube. After applying electrical heating, the carbon nanotube field effect transistors clearly showed n-type properties. Moreover, they recovered p-type properties when exposed to ambient air.

Non Contact Atomic Force Microscope Electrical Manipulation of Carbon Nanotubes and Its Application to Fabrication of a Room Temperature Operating Single Electron Transistor

We demonstrate that a non-contact atomic force microscope (AFM) can be used to cut and nick carbon nanotubes (CNTs) by applying negative bias to the metal-coated AFM tip when it is very close to the CNT. The voltage needed to cut the CNT completely varied from -8 to -15 V, while the nicking voltage ranged from -6 to -8 V. This technique was applied to make a nanoscale single CNT device. Unwanted CNTs were cut first to leave only one CNT connecting the electrodes. Finally, tunneling barriers defined by two small nicks were created on the CNT to fabricate a single electron transistor. The final device shows Coulomb oscillation and Coulomb diamond characteristics at room temperature.

Carbon Nanotube Fabry?Perot Device for Detection of Multiple Single Charge Transitions

By fabricating a few charge storages around the single-walled carbon nanotube (SWNT) channel of a Fabry–Perot quantum interference device (FPD), we have succeeded in detecting the single-charge transition from the channel to the charge storages. In addition, storage energy was controlled by an applied bias. An abrupt discrete switching of the source-drain current was observed in the electrical measurements of the SWNT FPD. These random telegraph signals (RTS) are attributed to charge fluctuating traps charge near the conduction channel. Moreover, drain current with RTS increased stepwise with increasing applied gate voltage, which is attributed to modulation of the channel by a single charge induced in the charge storage near the channel by the applied gate voltage. The current-switching behavior associated with the occupation of individual charge traps was analyzed statistically.

Growth Control of Carbon Nanotube using Various Applied Electric Fields for Electronic Device Applications

The control of the growth direction of a carbon nanotube was accomplished by applying an electric field during the growth of the carbon nanotube. The effects of two types of applied bias, one is a constant DC bias, and the other is a ramp bias, on the control of the growth direction were examined. By maintaining a constant DC bias we could control the growth direction of the carbon nanotube, however, the bridging ratio between the two electrodes was as small as 35%. We suppose that this low bridging ratio may be caused by the etching effect of hydrogen. When a ramp bias was applied, bridging ratio tended to increase with the slope of ramp bias. Under optimal conditions, the bridging ratio reached a value as high as 95%.

Raman Scattering of Single-Walled Carbon Nanotubes Implanted with Ultra-Low-Energy Oxygen Ions

Single-walled carbon nanotubes (SWNTs) implanted with ultra-low-energy oxygen (O+) ions have been studied by means of Raman scattering experiments. The relative intensities of the D-band related to some defects increase with the O+ dose in the Raman spectra of the implanted samples. Although no recognizable shift of phonon energies due to the ion implantation has been observed, the intensities of the Stokes and anti-Stokes lines originated from radial breathing modes of SWNTs which exhibit various behaviors due to degree of the implantation, indicating that the resonant energies are changed. To explain these findings, the structure and the electronic states of SWNTs with substitutional O impurities have been studied by applying theoretical calculations based on the first principle method and a tight-binding method. As a result, it has been suggested that the incorporation of oxygen atoms at carbon sites is plausible, and the experimental observation can thus be consistently explained on the basis of the O impurities in SWNTs.