(Color online) Source-drain current as a function of back gate voltage. The conduction through the nanotube increases at large values of both negative and positive gate voltages. Inset: Atomic force microscopy (AFM) image of an long single wall carbon nanotube transistor device.
(Color online) Photocurrent measured through the nanotube channel as a function of gate voltage. The photocurrent switches sign near . Inset: Schematic of the energy bands with initial band bending at the interface.
(Color online) Schematic band diagrams showing band bending in the silicon substrate for the as fabricated device (a) and under laser illumination (b). For typical oxide charge densities, the initial band bending is . Under illumination, the positive oxide charge is partially screened by photogenerated electrons that become trapped at the interface, reducing the surface potential from . The resulting photovoltage is given by Eq. (3).
Calculated magnitude of photovoltage as a function of the number of photoexcited carriers, assuming a bulk doping density of and an initial surface potential of . The solid line includes only the effect of the initial band bending. The dotted line includes the effect of the Dember voltage (see appendix). Inset: Calculated normalized electric field at the interface as a function of the surface potential and injection ratio. The dotted line indicates the value of the field as determined from the initial charge in the silicon .
(Color online) Numerically calculated derivative from the data in Fig. 1. The form of the derivative is extremely similar to the measured photocurrent in Fig. 2 where the -channel conduction is increased and the -channel is suppressed.
Measured photocurrent as a function of the incident laser power on a long channel nanotube device. The nonlinearity of the intensity dependence indicates that the photocurrent does not arise from a direct process.
Ratio of photogenerated electrons in the silicon substrate as a function of position. The circles are the experimentally measured values, extracted by relating the raw photocurrent vs position in the upper inset to the intensity dependence shown in Fig. 6. The line is a fit to Eq. (5) which includes the direct generation of carriers beneath the nanotube as well as the diffusion of carriers when the laser illuminates away from the nanotube. Upper inset: Photocurrent as a function of position perpendicular to the long axis of nanotube. Photocurrent is measured up to away from the nanotube. Lower inset: scanning electron microscopy (SEM) image of the nanotube device, where the dotted line indicates the position of the nanotube. The arrow shows the position and direction of the scanned laser.
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