Graphene, a two-dimensional layer of carbon atoms, is a promising building block for a wide range of optoelectronic devices owing to its extraordinary electrical and optical properties, including the ability to absorb ~2% of incident light over a broad wavelength range. While the RC-limited bandwidth of graphene-based photodetectors can be estimated to be as large as 640 GHz, conventional electronic measurement techniques lack for analysing photocurrents at such frequencies. Here we report on time-resolved picosecond photocurrents in freely suspended graphene contacted by metal electrodes. At the graphene-metal interface, we demonstrate that built-in electric fields give rise to a photocurrent with a full-width-halfmaximum of ~4 ps and that a photothermoelectric effect generates a current with a decay time of ~130 ps. Furthermore, we show that, in optically pumped graphene, electromagnetic radiation up to 1 THz is generated. our results may prove essential to build graphene-based ultrafast photodetectors, photovoltaic cells and terahertz sources.

We report on photocurrent and photoconductance processes in a freely suspended p-doped single GaAs nanowire. The nanowires are grown by molecular beam epitaxy, and they are electrically contacted by a focused ion beam deposition technique. The observed photocurrent is generated at the Schottky contacts between the nanowire and metal source-drain electrodes, while the observed photoconductance signal can be explained by a photogating effect induced by optically generated charge carriers located at the surface of the nanowire. Both optoelectronic effects are sensitive to the polarization of the exciting laser field, enabling polarization dependent photodetectors. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3193540͔Semiconductor nanowires have attracted considerable attention for the past few years because of their compelling electronic, mechanical, and optical properties. 1-10 A very suitable and versatile technique for nanowire growth is the direct synthesis on a substrate. [11][12][13][14] The fabrication of III-V semiconductor nanowire based devices by such a bottom-up approach ensures the rational use of materials, as the nanowires can be obtained in principle on any substrate. Here, we report on the optoelectronic properties of photodetectors based on single p-doped GaAs nanowires grown by molecular beam epitaxy ͑MBE͒ with the so-called vapor-liquid-solid method using Ga droplets as self-catalysts. [15][16][17] The nanowires are electrically contacted by metal electrodes using a focused ion beam ͑FIB͒ deposition technique. 18,19 We experimentally identify two dominating optoelectronic processes in the metal-GaAs nanowire-metal photodetectors. On the one hand, there is a photocurrent generated at the Schottky contacts between the GaAs nanowires and the metal sourcedrain electrodes, as recently shown for CdS nanowires. 8 On the other hand, we observe a photoconductance effect, when illuminating the GaAs nanowire far away from the contacts. We interpret the photoconductance effect to arise from band bending effects caused by surface states on the nanowire surface. In particular, optically generated excess electrons are trapped at the surface, where they act as a negative gating voltage on the p-doped nanowires ͑photogating effect͒. At the same time, the optically excited free excess holes raise the Fermi energy of the hole gas within the nanowires ͑photodoping effect͒. Both effects can raise the conductance of the semiconductor circuits. 20,21 We demonstrate that both photocurrent and photoconductance effects are sensitive to the orientation of linear polarized light. The photoconductance ͑-current͒ varies by ϳ35% ͑ϳ15%͒ for the photon polarization being parallel or perpendicular to the direction of the GaAs nanowires. Hereby, the metal-GaAs nanowiremetal circuits act as polarization-sensitive photodetectors, 2 which can be integrated into electronic circuits by the FIBdeposition technique in a very versatile way.Starting point are GaAs nanowires, which are grown by MBE on a SiO 2 covered ͑111͒-oriente...

In order to clarify the temporal interplay of the different photocurrent mechanisms occurring in single GaAs nanowire based circuits, we introduce an on-chip photocurrent pump-probe spectroscopy with a picosecond time resolution. We identify photoinduced thermoelectric, displacement, and carrier lifetime limited currents as well as the transport of photogenerated holes to the electrodes. Moreover, we show that the time-resolved photocurrent spectroscopy can be used to investigate the drift velocity of photogenerated carriers in semiconducting nanowires. Hereby, our results are relevant for nanowire-based optoelectronic and photovoltaic applications.