Terahertz (THz) radiation has uses from security to medicine [1]; however, sensitive roomtemperature detection of THz is notoriously difficult [2]. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism: photoexcited carriers rapidly thermalize due to strong electron-electron interactions [3,4], but lose energy to the lattice more slowly [3,5]. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating [6,7] or dissimilar contact metals[8] produces a net current via the thermoelectric effect. Here we demonstrate a graphene thermoelectric THz photodetector with sensitivity exceeding 10 V/W (700 V/W) at room temperature and noise equivalent power less than 1100 pW/Hz 1/2 (20 pW/Hz 1/2 ), referenced to the incident (absorbed) power. This implies a performance which is competitive with the best room-temperature THz detectors [9] for an optimally coupled device, while time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those [7,10]. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.Graphene has unique advantages for hot-electron photothermoelectric detection. Gapless graphene has strong interband absorption at all frequencies. The electronic heat capacity of single-layer graphene is much lower than in bulk materials, resulting in a larger change in temperature for the same absorbed energy. The photothermoelectric effect has a picosecond response time, set by the electronphonon relaxation rate. [10,11]. Hot electron effects have been exploited in graphene for sensitive bolometry in THz and millimeter-wave at cryogenic temperatures, by using temperature-dependent resistance in gapped bilayer graphene [12], which is sizable only at low temperature, or noise thermometry [13], which requires complex RF electronics. In contrast, our photothermoelectric approach is temperature insensitive and produces an observable dc signal even under room temperature conditions.To realize our graphene hot electron thermoelectric photodetector we generate an asymmetry by contacting graphene with dissimilar metals using a standard double-angle evaporation technique as shown in Figs. 1a-e (also see Methods). Fig. 1f shows optical and atomic-force micrographs of our monolayer graphene device. Two metal electrodes, each consisting of partially overlapping Cr and Au regions, contact the monolayer graphene flake. The 3 µm × 3 µm graphene channel is selected to be shorter than the estimated electron diffusion length [14]. Fig. 1g shows the schematic of our detector in cross section. Figs. 1h-k illustrate the principle of operation: Electrons in graphene are heated by the incident light and the contacts serve as a heat sink, resulting in a non-uniform electron temperature T(x)as a function of position x within the device (Fig. 1h)...

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Structural investigations of organometallic vapor phase epitaxy grown ␣-GaN films using high-resolution transmission electron microscopy and scanning force microscopy have revealed the presence of tunnel-like defects with 35-500 Å radii that are aligned along the growth direction of the crystal and penetrate the entire epilayer. These defects, which are termed ''nanopipes,'' terminate on the free surface of the film at the centers of hexagonal growth hillocks and form craters with 600-1000 Å radii. Either one or two pairs of monolayer-height spiral steps were observed to emerge from the surface craters which allowed us to conclude that nanopipes are the open cores of screw dislocations. The measured dimensions of the defects are compared to Frank's theory for the open-core dislocation. © 1995 American Institute of Physics.Gallium nitride and its related alloys ͑AlGaN and InGaN͒ are important wide band-gap semiconductors that have potential applications in both short-wavelength optoelectronic and high power/high frequency devices. 1 It is widely accepted that both the external efficiency and reliability of light emitting devices are sensitive to the type and density of extended defects in the material. Nitride films deposited on sapphire, which is poorly matched to GaN both in terms of lattice parameter and thermal expansion coefficient, typically exhibit dislocation densities in the 10 10 cm Ϫ2 range. 2,3 Most of these dislocations are of pure edge type forming low angle ''twist'' boundaries. Recently, we have reported observations of another type of defect which could have a profound impact on characteristics of high voltage power devices. 4 These defects, referred to as nanopipes, are long, faceted empty pipes which thread through the entire thickness of the GaN epilayer. The radii of the pipes are in the 35-500 Å range and they appear to propagate along the c axis of the film. A similar defect is frequently observed in another wide band-gap semiconductor with a hexagonal crystal structure, namely, silicon carbide. 5 The present study, which combines high-resolution transmission electron microscopy ͑HRTEM͒ and scanning force microscopy ͑SFM͒, provides evidence that the nanopipes occur at the cores of screw dislocations. SFM images show that spiral steps emerge from the crater formed where the screw dislocations intersect the surface. These spiral steps create hexagonal growth hillocks which eventually lead to a nonplanar surface morphology. Although we believe that these observations can be understood in terms of Frank's theory for open-core dislocations, we note several quantitative discrepancies. 6 The 2.8-m-thick ␣-GaN epilayers described here were grown at 1040°C on ␣-Al 2 O 3 (0001)substrates in an inductively heated, water cooled, vertical organometallic vapor phase epitaxy ͑OMVPE͒ reactor. 7 An AlN buffer layer was first deposited at 450-500°C using 1.5 mol/min triethylaluminum, 2.5 standard liters per minute ͑slm͒ NH 3 and 3.5 slm H 2 flows. After annealing in 2.5 slm NH 3 and 3.5 slm H 2 for 10 min at ...

Among its many outstanding properties, graphene supports terahertz surface plasma wavessub-wavelength charge density oscillations connected with electromagnetic fields that are tightly localized near the surface [1, 2]. When these waves are confined to finite-sized graphene, plasmon resonances emerge that are characterized by alternating charge accumulation at the opposing edges of the graphene. The resonant frequency of such a structure depends on both the size and the surface charge density, and can be electrically tuned throughout the terahertz range by applying a gate voltage [3,4]. The promise of tunable graphene THz plasmonics has yet to be fulfilled, however, because most proposed optoelectronic devices including detectors, filters, and modulators [5][6][7][8][9][10] desire near total modulation of the absorption or transmission, and require electrical contacts to the graphene -constraints that are difficult to meet using existing plasmonic structures. We report here a new class of plasmon resonance that occurs in a hybrid graphene-metal structure.The sub-wavelength metal contacts form a capacitive grid for accumulating charge, while the narrow interleaved graphene channels, to first order, serves as a tunable inductive medium, thereby forming a structure that is resonantly-matched to an incident terahertz wave. We experimentally demonstrate resonant absorption near the theoretical maximum in readily-available, large-area graphene, ideal for THz detectors and tunable absorbers. We further predict that the use of high mobility graphene will allow resonant THz transmission near 100%, realizing a tunable THz filter or modulator. The structure is strongly coupled to incident THz radiation, and solves a fundamental problem of how to incorporate a tunable plasmonic channel into a device with electrical contacts. In order to be applied in practical optoelectronic devices, graphene terahertz plasmonic resonators must be connected to an antenna, transmission line, metamaterial, or other electrical contact, in order to sense or apply a voltage or current, or to improve the coupling to free-space radiation. The conductive boundary screens the electric field and inhibits the accumulation of charge density at the opposing edges of the graphene channel, thus disrupting the traditional graphene plasmon mode. Until now, there was no experimental evidence that two-dimensional plasmons could be confined with conductive boundaries.In this letter, we demonstrate a new type of plasmon resonance in metal-contacted graphene, and we use analytic calculations, numerical simulations, and THz reflection and transmission measurements to confirm the principle of operation. These plasmon modes shows strong coupling to incident terahertz radiation, so that maximal absorption in graphene can be achieved at a resonance frequency that is gate-tunable. We also introduce an equivalent circuit model that predicts the resonant frequency, linewidth, and impedance matching condition of the fundamental plasmon mode, and can be used for d...

We report on an experimental demonstration of graphene-metal ohmic contacts with contact resistance below 100 Ω µm. These have been fabricated on graphene wafers, both with and without hydrogen intercalation, and measured using the transmission line method. Specific contact resistivities of 3 × 10−7 to 1.2 × 10−8 Ω cm2 have been obtained. The ultra-low contact resistance yielded short-channel (source-drain distance of 0.45 µm) HfO2/graphene field effect transistors (FETs) with a low on-resistance (Ron) of 550 Ω µm and a high current density of 1.7 A/mm at a source-drain voltage of 1 V. These values represent state-of-the-art (SOA) performance in graphene-metal contacts and graphene FETs. This ohmic contact resistance is comparable to that of SOA high-speed III–V high electron mobility transistors.