ZnO nanowire (NW) visible-blind UV photodetectors with internal photoconductive gain as high as G ∼ 10 8 have been fabricated and characterized. The photoconduction mechanism in these devices has been elucidated by means of time-resolved measurements spanning a wide temporal domain, from 10 -9 to 10 2 s, revealing the coexistence of fast (τ ∼ 20 ns) and slow (τ ∼ 10 s) components of the carrier relaxation dynamics. The extremely high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the NW surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime, as evidenced by the sensitivity of the photocurrrent to ambient conditions. Surprisingly, this mechanism appears to be effective even at the shortest time scale investigated of t < 1 ns. Despite the slow relaxation time, the extremely high internal gain of ZnO NW photodetectors results in gain-bandwidth products (GB) higher than ∼10 GHz. The high gain and low power consumption of NW photodetectors promise a new generation of phototransistors for applications such as sensing, imaging, and intrachip optical interconnects.Because of its wide band gap (E g ) 3.4 eV), low cost, and ease of manufacturing, ZnO is emerging as a potential alternative to GaN in optoelectronic applications, 1 including light-emitting diodes, laser diodes, and photodetectors for the UV spectral range. In the past decade, the demonstration of a large variety of functional ZnO nanowire (NW) devices such as field effect transistors, 2,3 optically pumped lasers, 4,5 and chemical and biological sensors 6 have aroused growing interest in this material. 7 In particular, ZnO NW photodetectors and optical switches have been the subject of extensive investigations. [8][9][10][11][12][13][14][15][16][17][18] Despite the abundant research on NW photoconduction, 19 the two main factors contributing to the high photosensitivity of such nanostructures have been scarcely recognized: (1) the large surface-to-volume ratio and the presence of deep level surface trap states in NWs greatly prolongs the photocarrier lifetime; (2) the reduced dimensionality of the active area in NW devices shortens the carrier transit time. Indeed, the combination of long lifetime and short transit time of charge carriers can result in substantial photoconductive gain. [20][21][22] In this letter, we present ZnO NW photodetectors with large photoresponse; upon UV illumination at relatively low light intensities (I ∼ 10 µW/cm 2 ), the current in ZnO NWs increases by several orders of magnitude, which translates to a photoconductive gain of G > 10 8 . To elucidate the photoconduction mechanism that involves fast carrier thermalization and trapping at the NW surface and electronhole recombination at extended and localized states, we have studied the photoconductivity of ZnO NWs by time-resolved measurements and in different ambient conditions (e.g., in air or under vacuum). A physical model was developed to illustrate the origin of the photoconductive gain in ...

We report Al 2 O 3 /In 0.53 Ga 0.47 As MOSFETs having both self-aligned in situ Mo source/drain ohmic contacts and self-aligned InAs source/drain n + regions formed by MBE regrowth. The device epitaxial dimensions are small, as is required for 22-nm gate length MOSFETs; a 5-nm In 0.53 Ga 0.47 As channel with an In 0.48 Al 0.52 As back confinement layer and the n ++ source/drain junctions do not extend below the 5-nm channel. A device with 200-nm gate length showed I D = 0.95 mA/μm current density at V GS = 4.0 V and g m = 0.45 mS/μm peak transconductance at V DS = 2.0 V. Index Terms-InAs source/drain, InGaAs MOSFET, migration-enhanced epitaxial regrowth, source/drain regrowth, III-V MOSFET.

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Scanning gate microscopy, in which a conductive probe tip in an atomic force microscope is employed as a local, nanoscale top gate contact, has been used to characterize local carrier and current modulation effects in a 45nm diameter InAs semiconductor nanowire grown by metal organic chemical vapor deposition. Measurement of current flow in the nanowire as a function of tip position reveals that for both positive and negative tip bias voltages, carrier and current modulation is strongest when the probe tip is near the source and drain nanowire contacts, reaching a global maximum approximately 100–200nm distant from the source contact and a secondary maximum a similar distance from the drain contact and decreasing at greater tip-contact distances. This effect is explained, with verification by numerical simulation, as a consequence of the capacitance between the tip and the source and drain contacts as a function of tip location. Measurement of transconductance as a function of tip position reveals that the transconductance is approximately 80%–90% greater near the source contact than at the center of the nanowire.