Optoelectronics with Carbon Nanotubes
Optoelectronics with Carbon Nanotubes Optoelectronics with Carbon Nanotubes
et al. also observed a linear red-shift in E11 emission corresponding to |ρ| = 0.25 e/nm at |VDS| = 6 V 109 . As seen in the schematic in Figure IV-1 (c), the recombination region is un-gated and is therefore undoped. We are keeping the back gate at 0 V and we attempt to inject both types of carriers at roughly equal rates in order to maximize the emission, so to the first approximation, the total charge carrier density is kept zero, leading to the absence of a peak shift for the “free- exciton” peak. On the other hand, we observe the total red-shift of 23 meV for the LX peak. Using the scaling relation between ΔE11 and Δ|ρ| derived from the PL data in Ref. 109, we obtain the local charge carrier density increase of Δ|ρ| = 0.19 e/nm, comparable to the global gate effect observed by Steiner et al. and Freitag et al. mentioned above. While we presently do not know the physical details of the localization potential, this information may help eliminate certain types of local fluctuations in future study. In addition, emission peak shift in EL has never been investigated systematically. Our devices show no E11 peak shift for the free excitons, which should enable future studies to explore the effect of dielectric environment and of electrostatic doping in EL. In fact, we have some preliminary data (not shown) showing that tuning the global back gate could change the E11 emission intensity in EL from CNT p-n junction by up to an order of magnitude. In Figure IV-5 we compare the ambipolar emission from a diode with the unipolar emission from a back-gated field-effect transistor (FET) made out of the same (long) SWNT. The FET emission amplitude is 4 times smaller than the emission from the diode, although IDS is 12.5 times higher. It is also much broader (180 meV) and exhibits a slightly asymmetric lineshape, consistent with our unipolar emission characteristics from Chapter III. The FET device was operated in the reverse bias regime, with VGS < 0 < VDS and |VGS| > |VDS|. (See Figure IV-5 inset for the schematic band structure of the operating condition.) In this regime, holes are the majority carriers and generate electron-hole pairs by impact excitation. Most electron-hole pairs are generated at the peak field Fmax near the drain electrode (see inset) and we estimate a lower limit of Fmax > VDS/tox ~ 25 V/μm, where we use the gate oxide thickness to the emission linewidth we find that, under those biasing conditions, the dominant contribution is due to mixing of exciton and continuum states in the high electric field. 77 t ox as the screening length 125 . When estimating the contribution of different broadening mechanisms
We claimed in Chapter III that in the unipolar case discussed, the lifetime shortening due to the field ionization was the dominant mechanism for the bias-dependent part of broadening. Using the same formalism and the values d = 1.4 nm and F = 25 V/μm, the effect of ionization is estimated to be 38 meV, which comprises only a small fraction of the 180 meV width observed in this case. The inset of Figure IV-5 shows a simulation 124 of the field dependence of the optical absorption of a 1.4 nm diameter tube. At zero field (0 V/μm; green line), as it is approximately the case in the CNT diodes, there is no absorption in the energy range between the E11 exciton and the onset of the weak band-to-band absorption. The absorption and hence the emission are dominated by the E11 excitonic transition. In the FET (25 V/μm; red line), however, due to the high electric field, the exciton wavefunction mixes with the band-to-band continuum, which leads to spectral weight transfer from the excitonic peak to the continuum. The band-to-band absorption moves into the forbidden region and merges with the E11 exciton peak, resulting in a strongly broadened, asymmetric lineshape. At 25 V/μm, the simulated absorption extends over an energetic range of more than 150 meV. Due to the high electron temperatures in those devices 87 , we expect the emission spectrum to be of comparable width as the absorption. Additional broadening mechanisms, such as field ionization already discussed, Auger recombination 79, 129, 146 and phonon broadening 115, 120 , will increase the width even further. 78
- Page 39 and 40: on quartz, which remains a signific
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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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- Page 129 and 130: 85. Marty, L.; Adam, E.; Albert, L.
- Page 131 and 132: 112. Steiner, M.; Freitag, M.; Pere
- Page 133 and 134: 140. Grüneis, A.; Saito, R.; Samso
We claimed in Chapter III that in the unipolar case discussed, the lifetime shortening due<br />
to the field ionization was the dominant mechanism for the bias-dependent part of broadening.<br />
Using the same formalism and the values d = 1.4 nm and F = 25 V/μm, the effect of ionization is<br />
estimated to be 38 meV, which comprises only a small fraction of the 180 meV width observed<br />
in this case.<br />
The inset of Figure IV-5 shows a simulation 124 of the field dependence of the optical<br />
absorption of a 1.4 nm diameter tube. At zero field (0 V/μm; green line), as it is approximately<br />
the case in the CNT diodes, there is no absorption in the energy range between the E11 exciton<br />
and the onset of the weak band-to-band absorption. The absorption and hence the emission are<br />
dominated by the E11 excitonic transition. In the FET (25 V/μm; red line), however, due to the<br />
high electric field, the exciton wavefunction mixes <strong>with</strong> the band-to-band continuum, which<br />
leads to spectral weight transfer from the excitonic peak to the continuum. The band-to-band<br />
absorption moves into the forbidden region and merges <strong>with</strong> the E11 exciton peak, resulting in a<br />
strongly broadened, asymmetric lineshape. At 25 V/μm, the simulated absorption extends over<br />
an energetic range of more than 150 meV. Due to the high electron temperatures in those<br />
devices 87 , we expect the emission spectrum to be of comparable width as the absorption.<br />
Additional broadening mechanisms, such as field ionization already discussed, Auger<br />
recombination 79, 129, 146 and phonon broadening 115, 120 , will increase the width even further.<br />
78
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