Optoelectronics with Carbon Nanotubes
Optoelectronics with Carbon Nanotubes Optoelectronics with Carbon Nanotubes
(see also Figure IV-2 (b)). This is in contrast to previous studies of EL from CNTs 75 71, 73, 76, 85, 87, 97, 111, 117 , which exhibit current thresholds of >1 μA for light emission. The CNT diodes thus constitute threshold-less nano-scale light-emitters. The localized exciton LX also rises linearly at low currents but saturates as the current exceeds 100 nA. Saturation of exciton emission in CNTs is a characteristic signature of Auger-mediated exciton-exciton annihilation 79, 129, 146 , which is known to be strongly enhanced in tightly confined 1D systems 147 . It sets in when more than one exciton is present in the tube, i.e. when the electron-hole pair injection rate IDS/2q (q is the electron charge) exceeds the inverse carrier lifetime τL -1 . Therefore, τL = 2q/100 nA 3 ps, which is in agreement with the τL estimated from the EL efficiency above. The sudden saturation further suggests that τA LX
observed by increasing the VGS values, although a large uncertainty in both peaks does not allow us to draw conclusions from quantitative analysis. (a) (b) Figure IV-4. Characteristics of a two-peak CNT p-n junction device. The dotted lines are a linear fit for each peak in both figures. (a) Peak positions for X and LX peaks as a function of VDS. The X position changes less than 1 meV per 1 V in VDS. (b) FWHM for X and LX peaks as a function of VDS. The difference between the slopes is not statistically significant. Figure IV-4 (a) shows the change in peak position for X and LX as VDS is increased. We see that there is little change for X (ΔEX/ΔVDS = - 0.8 ± 0.6 meV/V), whereas LX red-shifts somewhat with respect to VDS (ΔELX/ΔVDS = -4.5 ± 1 meV/V). Red-shifts of PL and EL peaks as a result of drain-induced doping was also observed by Freitag et al. 109 . This is analogous to the red-shifts of E33 excitation energy observed under gate field in Ref. 148 . Doping within the channel is thought to change the dielectric screening between electrons and results in bandgap renormalization and a change in exciton binding energy. According to the study by Walsh et al. using different dielectric materials (ε = 1 to 1.78), the increased screening results in the reduction of both of these effects, leading to the combined effect of tens of meVs in red-shift for the E22 transition 149 . A similar effect was observed by Steiner et al. for the E33 excitation energy reduction of up to 20 meV at |VGS| = 4 V, corresponding to the induced charge density of |ρ| = 0.2 e/nm 148 . The red-shift for E11 is expected to be slightly smaller, 150 but the effect should also be an over all reduction in transition energy and by the same order of magnitude. In fact, Freitag 76
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- Page 123 and 124: Bibliography 1. Avouris, P.; Chen,
- Page 125 and 126: 30. Miyauchi, Y.; Maruyama, S., Ide
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observed by increasing the VGS values, although a large uncertainty in both peaks does not allow<br />
us to draw conclusions from quantitative analysis.<br />
(a) (b)<br />
Figure IV-4. Characteristics of a two-peak CNT p-n junction device. The dotted<br />
lines are a linear fit for each peak in both figures. (a) Peak positions for X and LX<br />
peaks as a function of VDS. The X position changes less than 1 meV per 1 V in VDS.<br />
(b) FWHM for X and LX peaks as a function of VDS. The difference between the<br />
slopes is not statistically significant.<br />
Figure IV-4 (a) shows the change in peak position for X and LX as VDS is increased. We<br />
see that there is little change for X (ΔEX/ΔVDS = - 0.8 ± 0.6 meV/V), whereas LX red-shifts<br />
somewhat <strong>with</strong> respect to VDS (ΔELX/ΔVDS = -4.5 ± 1 meV/V). Red-shifts of PL and EL peaks<br />
as a result of drain-induced doping was also observed by Freitag et al. 109 . This is analogous to<br />
the red-shifts of E33 excitation energy observed under gate field in Ref. 148 . Doping <strong>with</strong>in the<br />
channel is thought to change the dielectric screening between electrons and results in bandgap<br />
renormalization and a change in exciton binding energy. According to the study by Walsh et al.<br />
using different dielectric materials (ε = 1 to 1.78), the increased screening results in the reduction<br />
of both of these effects, leading to the combined effect of tens of meVs in red-shift for the E22<br />
transition 149 . A similar effect was observed by Steiner et al. for the E33 excitation energy<br />
reduction of up to 20 meV at |VGS| = 4 V, corresponding to the induced charge density of |ρ| =<br />
0.2 e/nm 148 . The red-shift for E11 is expected to be slightly smaller, 150 but the effect should also<br />
be an over all reduction in transition energy and by the same order of magnitude. In fact, Freitag<br />
76
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