Using current-voltage (I-V), capacitance-voltage (C-V), and electric-field-modulated Raman measurements, we report on the unique physics and promising technical applications associated with the formation of Schottky barriers at the interface of a one-atom-thick zero-gap semiconductor (graphene) and conventional semiconductors. When chemical-vapor-deposited graphene is transferred onto n-type Si, GaAs, 4H-SiC, and GaN semiconductor substrates, there is a strong van-der-Waals attraction that is accompanied by charge transfer across the interface and the formation of a rectifying (Schottky) barrier. Thermionic-emission theory in conjunction with the Schottky-Mott model within the context of bond-polarization theory provides a surprisingly good description of the electrical properties. Applications can be made to sensors, where in forward bias there is exponential sensitivity to changes in the Schottky-barrier height due to the presence of absorbates on the graphene, and to analog devices, for which Schottky barriers are integral components. Such applications are promising because of graphene's mechanical stability, its resistance to diffusion, its robustness at high temperatures, and its demonstrated capability to embrace multiple functionalities.

GaMnN thin films were synthesized using gas-source molecular-beam epitaxy. Mn concentrations between 3 and 12 at. % were investigated. No evidence of second-phase formation was observed by powder x-ray diffraction or high-resolution cross section transmission electron microscopy in films with 9% or less Mn. The films were n type as determined by capacitance–voltage or Hall analysis. Magnetic characterization performed using a squid magnetometer showed evidence of ferromagnetic ordering at room temperature for all samples. In agreement with theoretical predictions, material with 3% Mn showed the highest degree of ordering per Mn atom. At 320 K, the samples show a nonzero magnetization indicating a TC above room temperature.

There is renewed emphasis on development of robust solid-state sensors capable of uncooled operation in harsh environments. The sensors should be capable of detecting chemical, gas, biological or radiation releases as well as sending signals to central monitoring locations. We discuss the advances in use of GaN-based solid-state sensors for these applications. AlGaN/GaN high electron mobility transistors (HEMTs) show a strong dependence of source/drain current on the piezoelectric polarization-induced two-dimensional electron gas (2DEG). Furthermore, spontaneous and piezoelectric polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors to detect gases, polar liquids and mechanical pressure. AlGaN/GaN HEMT structures have been demonstrated to exhibit large changes in source–drain current upon exposing the gate region to various block co-polymer solutions. Pt-gated GaN Schottky diodes and Sc2O3/AlGaN/GaN metal-oxide semiconductor diodes also show large change in forward currents upon exposure to H2. Of particular interest is detection of ethylene (C2H4), which has strong double bonds and hence is difficult to dissociate at modest temperatures. Apart from combustion gas sensing, the AlGaN/GaN heterostructure devices can be used as sensitive detectors of pressure changes. In addition, large changes in source–drain current of the AlGaN/GaN HEMT sensors can be detected upon adsorption of biological species on the semiconductor surface. Finally, the nitrides provide an ideal platform for fabrication of surface acoustic wave (SAW) devices. The GaN-based devices thus appear promising for a wide range of chemical, biological, combustion gas, polar liquid, strain and high temperature pressure-sensing applications. In addition, the sensors are compatible with high bit-rate wireless communication systems that facilitate their use in remote arrays.

Single ZnO nanorods with diameters of ∼130nm were grown on Au-coated Al2O3 substrates by catalyst-driven molecular beam epitaxy. Individual nanorods were removed from the substrate and placed between Ohmic contact pads and the current–voltage characteristics measured as a function of temperature and gas ambient. In the temperature range from 25to150°C, the resistivity of nanorods treated in H2 at 400°C prior to measurement showed an activation energy of 0.089±0.02eV and was insensitive to the ambient used (C2H4,N2O,O2 or 10% H2 in N2). By sharp contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors.

An improved process for graphene transfer was used to demonstrate high performance graphene enabled vertical organic field effect transistors (G-VFETs). The process reduces disorder and eliminates the polymeric residue that typically plagues transferred films. The method also allows for purposely creating pores in the graphene of a controlled areal density. Transconductance observed in G-VFETs fabricated with a continuous (pore-free) graphene source electrode is attributed to modulation of the contact barrier height between the graphene and organic semiconductor due to a gate field induced Fermi level shift in the low density of electronic-states graphene electrode. Pores introduced in the graphene source electrode are shown to boost the G-VFET performance, which scales with the areal pore density taking advantage of both barrier height lowering and tunnel barrier thinning. Devices with areal pore densities of 20% exhibit on/off ratios and output current densities exceeding 10(6) and 200 mA/cm(2), respectively, at drain voltages below 5 V.