Published papers

We present a new method for temperature distribution measurement with sub-100-nm spatial resolution using molecular modification on device surfaces. In this method, a Au nanosheet modified with self-assembled monolayers (SAM) is locally heated by Joule heating. The SAM molecules are desorbed due to the local heating, and the temperature is estimated from the molecular distribution evaluated by Kelvin probe force microscopy (KFM). The validity of the temperature estimation is confirmed by finite-element-analysis (FEA) results. This method is useful for visualizing thermal distribution in nanoscale devices such as metal nanosheet gas sensors.

Abstract Operating principle of a Pt thin-film H2 gas sensor was investigated by a combination of surface sensitive ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and bulk sensitive X-ray absorption fine structure (XAFS) techniques, which provided chemical and structure information under working conditions, coupled with electric resistivity measurements. It is evidenced that the sensor response was in a linear relation to both coverages of H and O atoms on the Pt surface. Moreover, the bulk structure of Pt remains unchanged under the H2­ exposure. These observations support that the resistivity change is associated with electron scattering in the near surface region.

High two-dimensional hole gas (2DHG) mobility (mu(2DHG) > 10000 cm(2)/Vs at T < 100 K) strained Ge/Si1-xGex structures with x = 0.5 and 0.65 were fabricated, and temperature dependence of their 2DHG mobility was obtained experimentally by the mobility spectrum analysis of the conductivity under magnetic fields. The theoretically calculated 2DHG mobility was compared to experimental data to determine the effective deformation potentials for scattering by acoustic and optical phonons. Using empirically confirmed parameters, the upper theoretical limit of room temperature 2DHG mobility mu(2DHG) in strained Ge as a function of strain was calculated. The possibility to achieve mu(2DHG) > 5000 cm(2)/Vs at room temperature is presented. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4723690]

The understanding of carrier transport in metal nanostructures is indispensable for the development of nanoelectronics. In particular, Pt nanostructures have been intensively studied to realize gas sensors based on adsorbate-induced surface electron scattering. Conventionally, electron scattering at the surface of metal nanostructures has been phenomenologically described by a single specularity parameter. In this work, surface electron scattering was quantitatively studied through molecular dynamics simulations, followed by density functional nonequilibrium Green's function calculations. Although the extracted specularity parameters qualitatively agreed with empirically treated diffusive scattering at the O-covered Pt surface and specular scattering at the H-covered Pt surface, our atomistic calculation revealed an increase in resistivity owing to H adsorption on thin Pt(111) nanosheets.

Cryo-CMOS circuit performance at 4K including both BEOL and FEOL characteristics has been investigated in a 65nm bulk CMOS for the first time. ON-current (Ion) of n/pMOSFET are improved +25%/+9% with excellent gate modulation (Ion/Ioff=∼109). Cu line/via resistances decrease with temperature due to reduction of phonon scattering, and -75%/-20% lower resistances are obtained at 4K. It is revealed that there is no inter-line capacitance change and no severe Joule heating effect (JHE) of Cu BEOL at 4K. The newly developed 4K-SPICE model including BEOL characteristics enables accurate CMOS circuit design at 4K, giving 5 ∼ 40% faster operation of RC line with clear dependence on driver-size and interconnect-load.

Abstract A nonvolatile resistive switching of NanoBridgeTM (NB) at 4 K has been demonstrated for realizing the Quantum-classical interface (QCI), in which the challenging of reset operation at cryogenic temperature is successfully achieved. The set voltage of the NB is increased with decreasing temperature, saturated around 150 K and to be 2.55 V at 4 K. The on-state resistances tuned at 1 k–5 kΩ show small temperature dependence down to 4 K due to high residual resistivity. The increased reset current of the NB at 4 K is compensated by the process optimization with thermal engineering and the increased I dsat of the select transistor at 4 K, resulting in stable switching. The low-power QCI featuring NBs is a strong candidate for controlling a large number of qubits at cryogenic temperature.

Immobilization of a catalytically active metal complex onto an electrode surface in highly dispersed form is crucial to assure high catalytic activity observed in solution. In this report, mononuclear CoII chlorin complexes were successfully immobilized on a conductive metal-oxide electrode by using electron conducting molecular wires comprising phenylene-ethynylene-based pi-conjugation covered by a linked permethylated alpha-cyclodextrin to enhance electrocatalysis for selective two electron reduction of molecular oxygen to hydrogen peroxide. First, the rotaxane-encapsulation effect of the molecular wire on electron transfer was examined by the comparison of electrochemical behaviors of ferrocene (Fc) molecules immobilized on an ITO substrate using the molecular wires with or without rotaxane encapsulation. Then, electrodes were modified with metalloporphyrinoids, i.e., RhIII(OEP)Cl (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin) and CoII chlorin through coordination with the molecular wires with pyridine moieties at the end. The electrocatalytic O2 reduction was performed with CoII chlorin immobilized on an electron-conductive metal-oxide substrate by molecular wires. The turnover fre-quency for H2O2 production using the CoII chlorin coordinated by molecular wires with rotaxane encapsulation was 331 +/- 75, which is significantly higher than that of 82 +/- 8 obtained for CoII chlorin immobilized by that without rotaxane encapsulation. These results clearly indicate that the molecular wires with rotaxane encap-sulation are beneficial for CoII chlorin complexes to exhibit high electrocatalytic activity and selectivity to H2O2.

Despite the importance of cryo CMOS technologies in quantum computing systems, the transient behaviors of cryo MOS transistors have been less studied. In this work, in advanced CMOS transistors we observed sub-us transient drain current (IdTrans) that was much greater than the static drain current (IdStatic) at 4 K (Fig. 6); the transient-to-static ratio r=IdTrans/IdStatic reached as large as 2.7 (Fig. 9), whereas r stays at one in the same device at 20 K. The observed transient characteristics are not due to the self-heating effects, but due to the long emission time of holes from acceptors at 4 K. After applying biases, more electrons flow into the channel than those in static conditions to mitigate the frozen acceptors. IdTrans. goes down to IdStatic because of gradual ionization of acceptors. We consider that the observed transient behavior needs to be considered in cryo MOSFET model to accurately design cryo LSI circuits.

Abstract The donor properties at the interface of the PtOx/ZnO Schottky diodes on hydrothermally grown ZnO were investigated. From the capacitance-voltage (C-V) characteristics of the diode and Hall effect measurements of the bulk substrate, we revealed that the carrier concentration at the ZnO Schottky diodes interface is much higher than that in the bulk. By utilizing the C-V characteristics and the deep level transient spectroscopy (DLTS), we demonstrated that the interfaces have two kinds of ionized donors; the shallow and deep donors, whose activation energies were 4 meV and 0.33 eV, respectively. The two donors were ionized near the interface, whereas only the low-concentration shallow donors were ionized in the bulk. The shallow and deep donors were assigned to hydrogen interstadials and E3 defects, respectively. Since both the donor types are related to hydrogens, annealing in the oxygen atmosphere was effective to greatly suppress the donor concentration at the diode interface.

A performance evaluation of cryogenic CMOS circuit at liquid-helium temperature (4.2K) is conducted using a standard 65nm bulk CMOS for quantum state controller (QSC) applications. The ON-current (Ion) of the core n/pMOSFET are increased by 25% and 9% with excellent gate modulation (Ion/Ioff=~109). The cryogenic characteristics of copper interconnects in the back end of the line (BEOL), including line and via resistances, capacitances, and Joule-heating effect (JHE) are accurately assessed. The interconnect and via resistances decrease with temperature due to a reduction of electron-phonon scattering, resulting in resistances that are 75% and 20% lower at 4.2K compared to those at room temperature(RT). No significant change in inter-line capacitance and no severe JHE are observed in the Cu BEOL at 4.2K. The developed cell libraries for Simulation Program with Integrated Circuit Emphasis (SPICE) model and the technology file, which includes RC interconnect parameters, enable precise design of CMOS circuits at 4.2K. This results in a demonstrated +18.3% increase in speed or -16% reduction in power consumption for ring-oscillator (ROSC) at 4.2K, aligning well with the simulation results obtained from the developed model.

Long-alkyl-chain alcohols are promising biofuel alternatives to fossil fuels; however, their detection has been a challenging task. In this study, the responses of nanoscale Pt-based sensors to aliphatic alcohols were examined, focusing on alkyl-chain-length and temperature dependencies. Atomistic simulations with a universal neural network potential were used to analyze the reaction and activation energies. Two types of Pt-based sensors were fabricated: a Pt nanosheet resistor sensor (PtNS) and a Pt nanoparticle-decorated graphene field-effect transistor sensor (PtNP-GFET). PtNP-GFET exhibited considerably higher sensor responses to 1-heptanol than to ethanol at 225 °C; moreover, PtNS showed the opposite alkyl-chain-length dependence compared to the response of PtNP-GFET at the same temperature. At a lower temperature of 150 °C, the sensor response toward ethanol and 1-heptanol in the case of PtNP-GFET was nearly identical, indicating that the response was primarily influenced by the concentration of hydroxyl groups. Atomistic simulation analysis revealed that the dehydrogenation reaction of the hydrocarbons near the hydroxyl groups was thermodynamically more favorable on the Pt147 nanocluster than on the Pt(111) surface. The dehydrogenation from the alkyl chain by PtNP was confirmed in alkanes; this dehydrogenation caused a higher sensor response of PtNP-GFET to 1-heptanol than to ethanol at 225 °C. Overall, employing nanomaterials, namely nanostructured Pt catalysts and a graphene-FET transducer, and using multiple sensing temperatures are key factors for realizing long-alkyl-chain alcohol recognitions. A low temperature of 150 °C is suitable for hydroxyl group sensing, while a high temperature of 225 °C is preferable for hydrocarbon group sensing.

Understanding the dopant properties in heavily doped nanoscale semiconductors is essential to design nanoscale devices. We report the deionization or finite ionization energy of dopants in silicon (Si) nanofilms with dopant concentration (N-D) of greater than 10(19) cm(-3), which is in contrast to the zero ionization energy (E-D) in bulk Si at the same N-D. From the comparison of experimentally observed and theoretically calculated E-D, we attribute the deionization to the suppression of metal-insulator transition in highly doped nanoscale semiconductors in addition to the quantum confinement and the dielectric mismatch, which greatly increase E-D in low-doped nanoscale semiconductors. Thus, for nanoscale transistors, N-D should be higher than that estimated from bulk Si dopant properties in order to reduce their resistivity by the metal insulator transition.

The continuous monitoring of hydrogen sulfide (H2S) in exhaled breath enables the detection of health issues such as halitosis and gastrointestinal problems. However, H2S sensors with high selectivity and parts per billion-level detection capability, which are essential for breath analysis, and facile fabrication processes for their integration with other devices are lacking. In this study, we demonstrated Au nanosheet H2S sensors with high selectivity, ppb-level detection capability, and high uniformity by optimizing their fabrication processes: (1) insertion of titanium nitride (TiN) as an adhesion layer to prevent Au agglomeration on the oxide substrate and (2) N2 annealing to improve nanosheet crystallinity. The fabricated Au nanosheets successfully detected H2S at concentrations as low as 5.6 ppb, and the estimated limit of detection was 0.5 ppb, which is superior to that of the human nose (8-13 ppb). In addition, the sensors detected H2S in the exhaled breath of simulated patients at concentrations as low as 175 ppb while showing high selectivity against interfering molecules, such as H2, alcohols, and humidity. Since Au nanosheets with uniform sensor characteristics enable easy device integration, the proposed sensor will be useful for facile health checkups based on breath analysis upon its integration into mobile devices.

The temperature dependence of the mobility of the two-dimensional electron gas (2DEG) in a silicon quantum well strained by Si0.7Ge0.3 relaxed buffer layer is determined precisely by a mobility spectrum analysis. The 2DEG mobility is 2780 cm(2)/V s at room temperature and, upon cooling, increases continuously to reach mu(2DEG) = 7.4 x 10(4) cm(2)/V s at 7K. A back gate installed on the sample changes the 2DEG concentration n successfully to establish mu(2DEG) alpha n(1.4) at the constant temperature T = 10K, implying that the scattering at such low temperature is limited solely by the remote ionized impurity scattering. Based on this finding, theoretical analysis of the temperature dependence of mu(2DEG) is performed based on the relaxation time approximation using 2DEG wavefunctions and subband structures determined self-consistently and including three major scatterings; by intravalley acoustic phonons, intervalley g-processes of longitudinal optical (LO) phonons, and remote ionized impurities. The calculation included only three fitting parameters, the shear deformation potential (Xi(u) = 9.5 eV), LO phonon deformation potential for g-process scattering (D-0 = 9.0 x 10(8) eV/cm), and sheet density of remote ionized impurities that have been determined by quantitative comparison with our experimental results. The temperature dependence of mu(2DEG) calculated theoretically show excellent agreement with experimentally determined mu(2DEG). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3702464]

Deformation potential (D-ac), which is one of the most important parameters determining the rate of electron-acoustic phonon scattering, in Si around MOS interfaces is thoroughly studied with regard to the dependences on surface carrier densities, back-gate biases, and device structures. It is demonstrated that D-ac increases sharply at the MOS interface. To investigate the impact of the increased D-ac on mu(e), thick body-channel SOI MOSFETs, where drain current flows in the entire SOI layers, was fabricated. The carrier transport experiments reveal that mu(e) of greater than 1100 cm(2) V-1 s(-1) is obtained in body-channel SOI MOSFETs with the SOI thickness of greater than 70 nm. By taking into account the D-ac profile around the MOS interface, experimental mu(e) of SOI MOSFETs is numerically reproduced over a wide range of SOI thicknesses. mu(e) of the body-channel SOI MOSFETs is also well reproduced using the same D-ac profile. Thus, it is concluded that D-ac increases sharply at the Si/SiO2 interface. The accurate modeling of the increased D-ac around the Si/SiO2 interface is indispensable for designing high-performance and/or low-power 3-D MOSFETs including FinFETs, extremely thin SOI MOSFETs, and nanowire MOSFETs, because these types of MOSFETs have greater interface-to-volume ratios.

As a candidate of highly-sensitive hydrogen sensors, Palladium (Pd)-decorated graphene sensors have been investigated. Thanks to high reactivity of Pd nanoparticles to hydrogen and high surface-tovolume ratio of two-dimensionally layered graphene, graphene decorated with Pd nanoparticles can be used as a highly sensitive hydrogen sensor. Although the conductivity of Pd-graphene sensor is known to be greatly changed in hydrogen atmosphere, the mechanisms of conductivity change have not been fully understood. In this work, both carrier concentration and mobility in hydrogen are investigated using Hall effect measurement. It is shown that relative change of carrier concentration is greater when carrier concentration is lower. On the other hand, mobility is not systematically changed as a function of carrier concentration. This mobility behavior is attributed to the competition between greater effective mass and shorter screening length at higher carrier concentration. Based on these observations, we develop strategy for achieving high sensitivity in Pd graphene hydrogen sensor.