Masahiro Matsunaga

Conduction Mechanisms in CVD-Grown Monolayer MoS2 Transistors: From Variable-Range Hopping to Velocity Saturation.

We fabricate transistors from chemical vapor deposition-grown monolayer MoS2 crystals and demonstrate excellent current saturation at large drain voltages (Vd). The low-field characteristics of these devices indicate that the electron mobility is likely limited by scattering from charged impurities. The current-voltage characteristics exhibit variable range hopping at low Vd and evidence of velocity saturation at higher Vd. This work confirms the excellent potential of MoS2 as a possible channel-replacement material and highlights the role of multiple transport phenomena in governing its transistor action.

The magnetic Y-branch nanojunction: Domain-wall structure and magneto-resistance

We analyze the domain structure of magnetic Y-branch nanojunctions that are formed by the intersection of a pair of orthogonal nanoscale notches. Dependent upon the magnetization history, we show the possibility of localizing various domain-wall (DW) structures at different parts of these junctions, leading to measurable magneto-resistance changes. These changes in turn allow the investigation of DW resistance, separate from the complicating effects of anisotropic magneto-resistance. The capacity to electrically detect the presence of DW structures in different regions of the junction may eventually make these structures of interest for future investigations of DW-enabled logic.

High Current-Induced Electron Redistribution in a CVD-Grown Graphene Wide Constriction

Investigating the charge transport behavior in one-dimensional quantum confined system such as the localized states and interference effects due to the nanoscale grain boundaries and merged domains in wide chemical vapor deposition graphene constriction is highly desirable since it would help to realize industrial graphene-based electronic device applications. Our data suggests a crossover from interference coherent transport to carriers flushing into grain boundaries and merged domains when increasing the current. Moreover, many-body fermionic carriers with disordered system in our case can be statistically described by mean-field Gross-Pitaevskii equation via a single wave function by means of the quantum hydrodynamic approximation. The novel numerical simulation method supports the experimental results and suggests that the extreme high barrier potential regions on graphene from the grain boundaries and merged domains can be strongly affected by additional hot charges. Such interesting results could pave the way for quantum transport device by supplying additional hot current to flood into the grain boundaries and merged domains in one-dimensional quantum confined CVD graphene, a great advantage for developing graphene-based coherent electronic devices.

Imaging coherent transport in chemical vapor deposition graphene wide constriction by scanning gate microscopy

We use a scanning gate microscopy to perturb coherent transport in chemical vapor deposition (CVD) graphene wide constriction. Particularly, we observe conductance oscillations in the wide constriction region (W ∼ 800 nm) characterized by spatial conductance variations, which imply formation of the nanometer-scale ring structure due to the merged domains and intrinsic grain boundaries. Moreover, additional hot charges from high current can suppress the coherent transport, suggesting that the hot carriers with a wide spreading kinetic energy could easily tunnel merged domains and intrinsic grain boundaries in CVD-grown graphene due to the heating effect, a great advantage for applications in graphene-based interference-type nano-electronics.

Hot Carriers in CVD-Grown Graphene Device with a Top h-BN Layer

We investigate the energy relaxation of hot carriers in a CVD-grown graphene device with a top h-BN layer by driving the devices into the nonequilibrium regime. By using the magnetic field dependent conductance fluctuations of our graphene device as a self-thermometer, we can determine the effective carrier temperature at various driving currents while keeping the lattice temperature fixed. Interestingly, it is found that is proportional to I, indicating little electron-phonon scattering in our device. Furthermore the average rate of energy loss per carrier is proportional to ( ), suggesting the heat diffusion rather than acoustic phonon processes in our system. The long energy relaxation times due to the weak electron-phonon coupling in CVD graphene capped with h-BN layer as well as in exfoliated multilayer graphene can find applications in hot carrier graphene-based devices.

Shubnikov–de Haas measurements on a high mobility monolayer graphene flake sandwiched between boron nitride sheets

A flake of monolayer graphene was sandwiched between boron nitride sheets. Temperature dependent Shubnikov-de Haas measurements were performed to access how this technique influences the electronic properties of the graphene sample. The maximum mobility found in this configuration was approximately 105 cm2 Vs -1. From the phase of the oscillations a Berry phase β of 1/2 was obtained indicating the presence of Dirac fermions. We obtained Fermi velocities around [Formula: see text] m s-1 which imply hopping energies close to 2.5 eV. Furthermore, the carrier lifetime is typically higher than that found in graphene supported by SiO2, reaching values higher than 700 fs.

Imaging local transport property within MoS 2 transistors by scanning gate microscopy

Using scanning gate microscopy (SGM), we study local transport property of molybdenum disulfide (MoS2) transistors. The transistors were fabricated using domain-free monolayer-MoS2 crystals grown by chemical vapor deposition (CVD). We observed two different types of SGM responses in different transistors. One is observed along the contact edge and the other is observed within MoS2 channel. The former one is described as visualization of an injection barrier formed by ohmic contact. On the other hand, the latter one implies that there is a barrier at a boundary of two regions having a different electrical property. We will discuss these results including electron force microscopy and photoluminescence mapping in the presentation.

Shubnikov-de Haas Oscillations observed in high-mobility monolayer graphene encapsulated by h-BN

Sumamry form only given. Clear Shubnikov-de Haas Oscillations have been observed in a high-mobility monolayer graphene sandwiched between two sheets of h-BN. The maximum mobilities of the p- and n-type carriers are 1.2 × 10⁵ cm²/Vs and 1.1 × 10⁵ cm²/Vs at 0.3 K, respectively. The typical effective mass can be estimated as 0.030 and 0.031 in p-type and n-type regions, respectively. The effective mass and the lifetime as a function of the carrier concentration will also be discussed and compared to the values observed in a conventional low-mobility sample using a SiO₂ layer.

Anomalous conductance fluctuations in bilayer graphene in h-BN layers

In recent experiments, we have succeeded to fabricate a h-BN/graphene/h-BN heterostructure showing a good mobility reaching to 8 × 104 cm2/Vs. And then, we observed a transition between conductance fluctuations at low magnetic field less than 0.5 T and regular Shubnikov-de Haas oscillation at higher than the magnetic field. We analyzed the fluctuations compared to that of former graphene samples on SiO2 layer, especially focused on the amplitude and the carrier density dependence. We will discuss the anomalous behavior breaking the universality in the conductance fluctuations of the bilayer graphene in a quasi-ballistic transport regime.

Thermal current-induced charge redistribution in wide CVD-grown graphene constriction

Chemical vapor deposition (CVD) graphene would find great applications in industrial graphene-based electronics recently. Most importantly, the one-dimensional constriction of CVD graphene due to its grain boundaries and merged domains revealing interesting interference effects, like Aharonov-Bohm effect. Such interesting interference transport behavior can be revealed by low-temperature scanning gate microscopy (LT-SGM) and be numerical simulated about the dynamic transport point of view by mean-field Gross-Pitaevskii equation. In order to realize graphene-based quantum transport device, our results suggested that the supplying additional thermal current could flood into the grain boundaries and merged domains in one-dimensional quantum confined CVD graphene so as to suppress the interference effect, a great discovery for graphene-based materials coherent electronic devices.