Deep-Ultraviolet Photodetectors Based on Epitaxial ZnGa2O4 Thin Films.

A single-crystalline ZnGa2O4 epilayer was successfully grown on c-plane (0001) sapphire substrate by metal-organic chemical vapor deposition. This epilayer was used as a ternary oxide semiconductor for application in high-performance metal-semiconductor-metal photoconductive deep-ultraviolet (DUV) photodetectors (PDs). At a bias of 5 V, the annealed ZnGa2O4 PDs showed better performance with a considerably low dark current of 1 pA, a responsivity of 86.3 A/W, cut-off wavelength of 280 nm, and a high DUV-to-visible discrimination ratio of approximately 107 upon exposure to 230 nm DUV illumination than that of as-grown ZnGa2O4 PDs. The as-grown PDs presented a dark current of 0.5 mA, a responsivity of 2782 A/W at 230 nm, and a photo-to-dark current contrast ratio of approximately one order. The rise time of annealed PDs was 0.5 s, and the relatively quick decay time was 0.7 s. The present results demonstrate that annealing process can reduce the oxygen vacancy defects and be potentially applied in ZnGa2O4 film-based DUV PD devices, which have been rarely reported in previous studies.

Graphene-based electrodes for enhanced organic thin film transistors based on pentacene.

This paper presents 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and pentacene-based organic thin film transistors (OTFTs) with monolayer graphene source-drain (S-D) electrodes. The electrodes are patterned using conventional photolithographic techniques combined with reactive ion etching. The monolayer graphene film grown by chemical vapor deposition on Cu foil was transferred on a Si dioxide surface using a polymer-supported transfer method to fabricate bottom-gate, bottom-contact OTFTs. The pentacene OTFTs with graphene S-D contacts exhibited superior performance with a mobility of 0.1 cm(2) V(-1) s(-1) and an on-off ratio of 10(5) compared with OTFTs with Au-based S-D contacts, which had a mobility of 0.01 cm(2) V(-1) s(-1) and an on-off ratio of 10(3). The crystallinity, grain size, and microscopic defects (or the number of layers of graphene films) of the TIPS-pentacene/pentacene films were analyzed by X-ray diffraction spectroscopy, atomic force microscopy, and Raman spectroscopy, respectively. The feasibility of using graphene as an S-D electrode in OTFTs provides an alternative material with high carrier injection efficiency, chemical stability, and excellent interface properties with organic semiconductors, thus exhibiting improved device performance of C-based electronic OTFTs at a reduced cost.

Blending effect of 6,13-bis(triisopropylsilylethynyl) pentacene-graphene composite layers for flexible thin film transistors with a polymer gate dielectric.

Solution processible poly(4-vinylphenol) is employed as a transistor dielectric material for low cost processing on flexible substrates at low temperatures. A 6,13-bis (triisopropylsilylethynyl) (TIPS) pentacene-graphene hybrid semiconductor is drop cast to fabricate bottom-gate and bottom-contact field-effect transistor devices on flexible and glass substrates under an ambient air environment. A few layers of graphene flakes increase the area in the conduction channel, and form bridge connections between the crystalline regions of the semiconductor layer which can change the surface morphology of TIPS pentacene films. The TIPS pentacene-graphene hybrid semiconductor-based organic thin film transistors (OTFTs) cross-linked with a poly(4-vinylphenol) gate dielectric exhibit an effective field-effect mobility of 0.076 cm(2) V(-1) s(-1) and a threshold voltage of -0.7 V at V(gs) = -40 V. By contrast, typical TIPS pentacene shows four times lower mobility of 0.019 cm(2) V(-1) s(-1) and a threshold voltage of 5 V. The graphene/TIPS pentacene hybrids presented in this paper can enhance the electrical characteristics of OTFTs due to their high crystallinity, uniform large-grain distribution, and effective reduction of crystal misorientation of the organic semiconductor layer, as confirmed by x-ray diffraction spectroscopy, atomic force microscopy, and optical microscopy studies.

Stellar spin-orbit misalignment in a multiplanet system.

Stars hosting hot Jupiters are often observed to have high obliquities, whereas stars with multiple coplanar planets have been seen to have low obliquities. This has been interpreted as evidence that hot-Jupiter formation is linked to dynamical disruption, as opposed to planet migration through a protoplanetary disk. We used asteroseismology to measure a large obliquity for Kepler-56, a red giant star hosting two transiting coplanar planets. These observations show that spin-orbit misalignments are not confined to hot-Jupiter systems. Misalignments in a broader class of systems had been predicted as a consequence of torques from wide-orbiting companions, and indeed radial velocity measurements revealed a third companion in a wide orbit in the Kepler-56 system.

A sub-Mercury-sized exoplanet.

Since the discovery of the first exoplanets, it has been known that other planetary systems can look quite unlike our own. Until fairly recently, we have been able to probe only the upper range of the planet size distribution, and, since last year, to detect planets that are the size of Earth or somewhat smaller. Hitherto, no planets have been found that are smaller than those we see in the Solar System. Here we report a planet significantly smaller than Mercury. This tiny planet is the innermost of three that orbit the Sun-like host star, which we have designated Kepler-37. Owing to its extremely small size, similar to that of the Moon, and highly irradiated surface, the planet, Kepler-37b, is probably rocky with no atmosphere or water, similar to Mercury.

Kepler-36: a pair of planets with neighboring orbits and dissimilar densities.

In the solar system, the planets' compositions vary with orbital distance, with rocky planets in close orbits and lower-density gas giants in wider orbits. The detection of close-in giant planets around other stars was the first clue that this pattern is not universal and that planets' orbits can change substantially after their formation. Here, we report another violation of the orbit-composition pattern: two planets orbiting the same star with orbital distances differing by only 10% and densities differing by a factor of 8. One planet is likely a rocky "super-Earth," whereas the other is more akin to Neptune. These planets are 20 times more closely spaced and have a larger density contrast than any adjacent pair of planets in the solar system.