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Optoelectronics with Carbon Nanotubes

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sufficient drain-source voltage is necessary to overcome the Schottky barriers at the injection site,<br />

and second, a high electric field must be created for exciton production.<br />

Even when CNTFETs are in the on state, small Schottky barriers normally remain at<br />

contacts and the I-VDS characteristic is not completely ohmic, especially for a small-diameter<br />

tube like the one shown in Figure III-5. In this device, which is made of the same SWNT as that<br />

in Figure III-4 but from a different segment, a non-ohmic transport behavior is apparent. The<br />

absolute value of the threshold voltage is below 2 V, followed by a linear I-V region. A decrease<br />

in slope occurs when the VDS is between 3 and 4 V when the current approaches 10 μA, and then<br />

the conductance picks up again above VDS = 5 V, a behavior similar to what was observed in<br />

Ref. 54. The low conductance (i.e., the small I-V slope) and the non-linearity below 2 V can be<br />

attributed to the resistance due to Schottky barriers. A very similar behavior is observed in the<br />

opposite polarity as VDS is decreased from 0 V to -7 V, which indicates that the contacts are<br />

symmetric in this device.<br />

It should be noted that virtually ohmic behavior <strong>with</strong>in a small VDS range is possible by<br />

choosing a larger-diameter semiconducting SWNT, appropriate choice of contact metals, and<br />

possibly some annealing. There remains a small intrinsic quantum resistance between the bulk<br />

contact and nano-scale SWNT (see Introduction and Ref. 59), but the resistance due to Schottky<br />

barrier dominates for our devices at a small VDS value because of the small diameter.<br />

Figure III-5. Drain current (IDS) as a function of applied drain-source voltage (VDS)<br />

<strong>with</strong> the device “on”. VDS was swept from 0 in both positive and negative directions.<br />

The device is <strong>with</strong> the same SWNT as Figure III-4, but from a different pair of source<br />

and drain contacts. The device channel length is the same as in Figure III-4.<br />

45

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