M. Grayson

GaS and GaSe Ultrathin Layer Transistors

Room-temperature, bottom-gate, field-effect transistor characteristics of 2D ultrathin layer GaS and GaSe prepared from the bulk crystals using a micromechanical cleavage technique are reported. The transistors based on active GaS and GaSe ultrathin layers demonstrate typical n-and p-type conductance transistor operation along with a good ON/OFF ratio and electron differential mobility.

Driving perpendicular heat flow: (p×n)-type transverse thermoelectrics for microscale and cryogenic peltier cooling

Whereas thermoelectric performance is normally limited by the figure of merit ZT, transverse thermoelectrics can achieve arbitrarily large temperature differences in a single leg even with inferior ZT by being geometrically tapered. We introduce a band-engineered transverse thermoelectric with p-type Seebeck in one direction and n-type orthogonal, resulting in off-diagonal terms that drive heat flow transverse to electrical current. Such materials are advantageous for microscale devices and cryogenic temperatures--exactly the regimes where standard longitudinal thermoelectrics fail. InAs/GaSb type II superlattices are shown to have the appropriate band structure for use as a transverse thermoelectric.

Transient photoresponse in amorphous In-Ga-Zn-O thin films under stretched exponential analysis

We investigated transient photoresponse and Hall effect in amorphous In-Ga-Zn-O thin films and observed a stretched exponential response which allows characterization of the activation energy spectrum with only three fit parameters. Measurements of as-grown films and 350 K annealed films were conducted at room temperature by recording conductivity, carrier density, and mobility over day-long time scales, both under illumination and in the dark. Hall measurements verify approximately constant mobility, even as the photoinduced carrier density changes by orders of magnitude. The transient photoconductivity data fit well to a stretched exponential during both illumination and dark relaxation, but with slower response in the dark. The inverse Laplace transforms of these stretched exponentials yield the density of activation energies responsible for transient photoconductivity. An empirical equation is introduced, which determines the linewidth of the activation energy band from the stretched exponential param...

Wafer-scale solution-derived molecular gate dielectrics for low-voltage graphene electronics

Graphene field-effect transistors are integrated with solution-processed multilayer hybrid organic-inorganic self-assembled nanodielectrics (SANDs). The resulting devices exhibit low-operating voltage (2 V), negligible hysteresis, current saturation with intrinsic gain >1.0 in vacuum (pressure < 2 × 10−5 Torr), and overall improved performance compared to control devices on conventional SiO2 gate dielectrics. Statistical analysis of the field-effect mobility and residual carrier concentration demonstrate high spatial uniformity of the dielectric interfacial properties and graphene transistor characteristics over full 3 in. wafers. This work thus establishes SANDs as an effective platform for large-area, high-performance graphene electronics.

Structure of a single sharp quantum hall edge probed by momentum-resolved tunneling

Momentum-resolved magnetotunneling spectroscopy is performed at a single sharp quantum Hall (QH) edge to probe the structure of integer QH edge modes. An epitaxially overgrown cleaved edge is shown to realize the sharp-edge limit with interchannel distances smaller than both the magnetic length and the Bohr radius where the Chklovskii soft-edge picture is no longer valid. The line shape of principal conductance peaks is explained, and an edge filling factor is determined from the peak position. A step in the dispersion is attributed to fluctuations in the QH ground energy.

Optimization of an on-chip active cooling system based on thin-film thermoelectric coolers

In this paper, we explore the design and optimization of an on-chip active cooling system based on thin-film thermoelectric coolers (TEC). We start our investigation by establishing the compact thermal model for the chip package with integrated thin-film TEC devices. We observe that deploying an excessive number of TEC devices and/or providing the TEC devices with an improper supply current might adversely result in the overheating of the chip, rendering the cooling system ineffective. A large amount of supply current could even cause the thermal runaway of the system. Motivated by this observation, we formulate the deployment of the integrated TEC devices and their supply current setting as a system-level design problem. We propose a greedy algorithm to determine the deployment of TEC devices and a convex programming based scheme for setting the supply current levels. Leveraging the theory of inverse-positive matrix, we provide an optimality condition for the current setting algorithm. We have tested our algorithms on various benchmarks. We observe that our algorithms are able to determine the proper deployment and supply current level of the TEC devices which reduces the temperatures of the hot spots by as much as 7.5 °C compared to the cases without integrated TEC devices.

Evaporative thinning: A facile synthesis method for high quality ultrathin layers of 2D crystals

The palette of two-dimensional materials has expanded beyond graphene in recent years to include the chalcogenides among other systems. However, there is a considerable paucity of methods for controlled synthesis of mono- and/or few-layer two-dimensional materials with desirable quality, reproducibility, and generality. Here we show a facile top-down synthesis approach for ultrathin layers of 2D materials down to monolayer. Our method is based on controlled evaporative thinning of initially large sheets, as deposited by vapor mass-transport. Rather than optimizing conditions for monolayer deposition, our approach makes use of selective evaporation of thick sheets to control the eventual thickness, down to a monolayer, a process which appears to be self-stopping. As a result, 2D sheets with high yield, high reproducibility, and excellent quality can be generated with large (>10 μm) and thin (∼ 1-2 nm) dimensions. Evaporative thinning promises to greatly reduce the difficulty involved in isolating large, mono- and few-layers of 2D materials for subsequent studies.

Thermal Sensing With Lithographically Patterned Bimetallic Thin-Film Thermocouples

A chromium-nickel thin-film thermocouple that is 50 nm thick is demonstrated on a semiconductor substrate as proof of concept for lithographically processed bimetallic on-chip temperature sensors. The Seebeck coefficient of the thin-film thermocouple is calibrated to be 10.37 μV/°C, reproducibly smaller than the bulk literature value by a factor of 3.98. The batch reproducibility of this thin-film Seebeck coefficient is demonstrated. The linear Seebeck response up to 90°C is calibrated with the help of a simple formula which accounts for the temperature variations of the reference thermocouple under large heat loads.

Tunneling in the quantum Hall regime between orthogonal quantum wells

We present experimental investigations of tunneling between two quantum wells forming a T-shaped structure. At zero magnetic field we observe a nonlinear tunnel characteristic with clearly pronounced negative differential resistance. With magnetic field B, both quantum wells can independently be set in a quantum Hall state. We demonstrate spectroscopy of Quantum Hall bulk states in one B field orientation, and spectroscopy of edge states with the orthogonal orientation. The observed features can be explained assuming that transverse momentum is conserved during tunneling.

Infrared Hall Effect in High-Tc Superconductors: Evidence for Non-Fermi-Liquid Hall Scattering

Infrared ( 20-120 and 900-1100 cm(-1)) Faraday rotation and circular dichroism are measured in high- T(c) superconductors using sensitive polarization modulation techniques. Optimally doped YBa2Cu3O7 thin films are studied at temperatures in the range ( 15<T<300 K) and magnetic fields up to 8 T. At 1000 cm(-1) the Hall conductivity sigma(xy) varies strongly with temperature in contrast to the longitudinal conductivity sigma(xx) which is nearly independent of temperature. The Hall scattering rate gamma(H) has a T2 temperature dependence but, unlike a Fermi liquid, depends only weakly on frequency. The experiment puts severe constraints on theories of transport in the normal state of high- T(c) superconductors.