Optoelectronics with Carbon Nanotubes
Optoelectronics with Carbon Nanotubes Optoelectronics with Carbon Nanotubes
commonly available metals, so the Fermi level aligns with the valence band edge, allowing for holes to tunnel easily into the conduction channel. Thus, the alignment of bandgap with the Fermi level of the contact metal determines the type of conduction that takes place. If the Fermi level aligns midgap with the CNT, both types of carriers can be injected from opposite contacts, in which case the device is defined to be ambipolar. In ambipolar devices, the back gate can be biased positive or negative to switch the majority carrier. On the other hand, in a unipolar device, either the conduction or valence band edge aligns with the Fermi level of the metal, so the current is carried mostly by electrons or holes, respectively. For such a device, only one polarity of the back gate will allow significant conduction. Figure I-8. Schematic band structure of the first conduction (Ec) and valence (Ev) bands in the “on” and “off” states. The source-drain bias (VDS) is the same in both cases. In the on-state, applied gate voltage changes the thickness of the Schottky barrier, allowing p-type carriers (denoted “h” for holes) to tunnel through. Figure I-8 illustrates how a CNTFET operates as a p-type device. In the “off” state, the p-type carriers, i.e., holes, at Fermi level cannot tunnel through the Schottky barrier into the conduction channel. By changing the band bending with an applied gate voltage (VGS), the barrier becomes thin, allowing the holes to tunnel from the contact into the channel. As is apparent from Figure I-8, the effective Schottky barrier thickness is the critical property that determines the conductivity of the device. Appenzeller, et al. demonstrated that thermally- 19
assisted tunneling through Schottky barrier is the dominant injection mechanism by comparing a simulation and measurements using carbon nanotube and boron nitride nanotube FETs 69 . Later, detailed imaging of band bending at contacts was conducted via spatially-resolved photoconductivity on ambipolar SWNT devices 70 , confirming the presence of Schottky barriers and characterizing them at different gate voltages. In this experiment, absorbed photons create excitons across the bandgap, which are separated by the electric field present at the Schottky barriers and detected as current. Current intensity as a function of position yields spatially- resolved electric field strengths, i.e. band potential profile along the channel. Figure I-9 compares ambipolar and unipolar conduction to show that ambipolar devices (i.e., exhibiting both p-type and n-type conduction) can be created by using a metal with a work function that aligns at the mid-gap of the nanotube. Furthermore, to achieve sufficient tunneling rates, it is usually desirable to use a large-diameter SWNT, i.e., a smaller-bandgap SWNT for lower Schottky barriers. Applied gate voltage can shift the band energies up and down to make electrons or holes the majority carrier, leading to a “V” shape in the current versus gate sweep as seen in Figure I-7 (b). If electrons and holes are injected simultaneously, they could recombine across the bandgap and emit light. In principle, this process could be highly efficient because, as we have already seen, SWNTs have direct bandgap. However, in a conventional ambipolar CNTFET, there is little control over position-dependent carrier concentration within the channel, which limits our ability to improve efficiency. A position-specific electrostatic doping offered by a split-gate can overcome this limitation, which will be discussed briefly below, and is the topic of Chapter IV. Figure I-9 also shows that the highest electric field regions exist next to the contacts. If injected carriers are accelerated to a sufficiently high kinetic energy, an additional inelastic scattering mechanism comes into play, i.e., creation of excitons by impact excitation. This means that even in unipolar transport, radiative recombination of excitons can occur under certain conditions. In the next section, we give a brief history of EL including both emission mechanisms in order to provide some background information for our work. 20
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commonly available metals, so the Fermi level aligns <strong>with</strong> the valence band edge, allowing for<br />
holes to tunnel easily into the conduction channel.<br />
Thus, the alignment of bandgap <strong>with</strong> the Fermi level of the contact metal determines the<br />
type of conduction that takes place. If the Fermi level aligns midgap <strong>with</strong> the CNT, both types of<br />
carriers can be injected from opposite contacts, in which case the device is defined to be<br />
ambipolar. In ambipolar devices, the back gate can be biased positive or negative to switch the<br />
majority carrier. On the other hand, in a unipolar device, either the conduction or valence band<br />
edge aligns <strong>with</strong> the Fermi level of the metal, so the current is carried mostly by electrons or<br />
holes, respectively. For such a device, only one polarity of the back gate will allow significant<br />
conduction.<br />
Figure I-8. Schematic band structure of the first conduction (Ec) and valence (Ev)<br />
bands in the “on” and “off” states. The source-drain bias (VDS) is the same in both<br />
cases. In the on-state, applied gate voltage changes the thickness of the Schottky<br />
barrier, allowing p-type carriers (denoted “h” for holes) to tunnel through.<br />
Figure I-8 illustrates how a CNTFET operates as a p-type device. In the “off” state, the<br />
p-type carriers, i.e., holes, at Fermi level cannot tunnel through the Schottky barrier into the<br />
conduction channel. By changing the band bending <strong>with</strong> an applied gate voltage (VGS), the<br />
barrier becomes thin, allowing the holes to tunnel from the contact into the channel. As is<br />
apparent from Figure I-8, the effective Schottky barrier thickness is the critical property that<br />
determines the conductivity of the device. Appenzeller, et al. demonstrated that thermally-<br />
19
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