Optoelectronics with Carbon Nanotubes

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(a) (b) (c) Figure IV-1. Device structure and electronic characteristics. (a) Schematic drawing of the CNT infrared LED. (b) Electrical device characteristics for different biasing conditions. Solid red line: VGS1 = -8 V, VGS2 = +8 V; The CNT is operated as a diode and shows rectifying behavior. Dashed green line: VGS1 = VGS2 = -8 V. The CNT behaves as p-type resistor. The silicon bottom-gate was grounded during the measurements. (c) Bandstructure of the CNT diode when it is biased in forward direction (VDS > 0). Electrons and holes are injected into the intrinsic region and recombine partially radiatively and partially non-radiatively. After Ref. 11. 69
3. Electroluminescence mechanism and characteristics The left image in Figure IV-2 (a) depicts the infrared emission from a device when the gate electrodes are biased at VGS1 = -8 V, VGS2 = +8 V and a constant current of IDS = 240 nA is driven through the SWNT. As discussed in the previous chapter, electrically excited light emission from semiconducting CNTs can be produced under ambipolar 73, 76 or unipolar operation. In the former case, both electrons and holes are injected simultaneously into the tube and their radiative recombination generates light. In the second case, a single type of carriers, i.e. either electrons or holes, accumulate kinetic energy in a high-field region <strong>with</strong>in the device to generate excitons by means of impact excitation 71, 84, 85 . The fact that no light is emitted when our devices are operated under unipolar conditions (VGS1 = VGS2 = -8 V; hole current) – right image in Fig. IV-2 (a) – shows that they are ambipolar light-emitters. This is the behavior we would generally expect an LED to exhibit. It is essential when fabricating devices that the back- gate sweep show a good ambipolar behavior (not shown), i.e., both types of carriers must be injected into the channel efficiently, in order for the devices to show this type of emission. Since the recombination rate in the ambipolar case is limited by minority carriers, this gives additional evidence for the ambipolar nature of our observation. Figure IV-2 (b) shows that the emission intensity is proportional to the current (i.e. carrier injection rate), which also confirms that the device is operating as an ambipolar emitter. Recall that the EL intensity from impact excitation in unipolar emission grows as the second to third power of the current, as the increased field makes the exciton generation more efficient (Figure III-12). Moreover, the emission threshold <strong>with</strong> respect to the current is zero, and the signal is still detectable at IDS as low as ~10 nA in many of our devices. This is in contrast to all previous EL studies 71, 76, 83, 111 , where typically two orders of magnitude higher current levels are required to obtain light emission of comparable intensity. Moreover, the voltage drop across the intrinsic region is in the order of the bandgap (~1 V; see Figure IV-1 (c)), and therefore also 5–10 times smaller than in other devices, resulting in an up to 1000 times smaller power dissipation overall. Under typical operation conditions, we estimate a power density of only ~0.1 W/m in the tube, compared to 10–100 W/m in other devices. It is hence clear that the CNT diodes are operated in an entirely different regime from all other electrically-driven CNT light-emitters to date. In fact, the power density is comparable to what is typically used in photoluminescence (PL) 70 71, 85
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Stony Brook University The official
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Stony Brook University The Graduate
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diodes nonetheless show a rectifyin
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Table of Contents Abstract ........
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Chapter I List of Figures Figure
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Figure ‎V-3 Source-drain electric
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My parents have been steadfast supp
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This work explores both fundamental
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In fact, we typically have no knowl
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(a) (b) Figure I-2 (a) Energy dispe
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(a) (b) metallic Figure I-4. One-di
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optical absorption peaks for differ
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elax rapidly to lower non-radiative
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The disorder-induced band (D-band)
Page 29 and 30:
The saturation limit for semiconduc
Page 31 and 32: (a) (b) Figure I-7. (a) A schematic
Page 33 and 34: assisted tunneling through Schottky
Page 35 and 36: majority carriers that gain enough
Page 37 and 38: conventional ambipolar emission. Th
Page 39 and 40: on quartz, which remains a signific
Page 41 and 42: Chapter II Methods 1. Materials One
Page 43 and 44: diameter (< 2 nm) SWNTs at IBM T. J
Page 45 and 46: devices were annealed in vacuum at
Page 47 and 48: 3. Experimental set-up The optical
Page 49 and 50: off wavelengths of 2150 nm, 2000 nm
Page 51 and 52: Chapter III Unipolar, High-Bias Emi
Page 53 and 54: Figure III-1. Semi-log plot of drai
Page 55 and 56: wetting with CNTs 57 and a relative
Page 57 and 58: Figure III-4. (Main panel) Electrol
Page 59 and 60: By plotting the current as a functi
Page 61 and 62: phonon temperature in broadening. S
Page 63 and 64: found multiple tubes bound together
Page 65 and 66: where i is the phonon mode, Tsub is
Page 67 and 68: The main panel of Figure III-10 sho
Page 69 and 70: the effect following Perebeinos’
Page 71 and 72: the optical phonon population is no
Page 73 and 74: (a) Figure III-13. (a) Spectra from
Page 75 and 76: DOP = I║ / (I┴ + I║) = 0.77.
Page 77 and 78: inding energy for perpendicular exc
Page 79 and 80: 3. Conclusions We have examined the
Page 81: In a split-gate scheme, a new level
Page 85 and 86: After calibrating our detection sys
Page 87 and 88: (a) (b) Figure IV-3. Electrolumines
Page 89 and 90: observed by increasing the VGS valu
Page 91 and 92: We claimed in Chapter III that in t
Page 93 and 94: Let us finally comment on the effic
Page 95 and 96: Chapter V The Polarized Carbon Nano
Page 97 and 98: (a) (b) Figure V-1. (a) SEM image o
Page 99 and 100: oth electrons and holes can be inje
Page 101 and 102: In the reverse direction (i.e., neg
Page 103 and 104: 4. Electroluminescence characterist
Page 105 and 106: and drain pads (marked “S” and
Page 107 and 108: (a) (b) Figure V-8. (a) EL intensit
Page 109 and 110: We attribute the observation of the
Page 111 and 112: (a) (b) (c) Figure V-9. Electrolumi
Page 113 and 114: measurements (i.e. additional chemi
Page 115 and 116: (a) (b) Figure V-10. Full-width at
Page 117 and 118: mechanisms are the same for differe
Page 119 and 120: experiment, solid line: cosine squa
Page 121 and 122: emission observed at higher current
Page 123 and 124: Bibliography 1. Avouris, P.; Chen,
Page 125 and 126: 30. Miyauchi, Y.; Maruyama, S., Ide
Page 127 and 128: 56. Chen, Z.; Appenzeller, J.; Knoc
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
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Optoelectronics with Carbon Nanotubes

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