PhD thesis - Evans Research Group - University of Wisconsin-Madison

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8flat band voltage in an OFET is operated in accumulation mode.Transistors operate in the linear regime when the difference between the gate voltage andthreshold voltage is much larger than the source-drain voltage. In this regime, I Dlinincreases linearly with V G according to:WIDlin= μ Ci( VG−VT) VD(1.3)LIn the linear regime, the conductivity of carriers induced in the channel depends on thecharge concentration and the mobility according to the equation.σ □ =1/R s =neµ (1.4)Here, σ □ is sheet conductance and n represents two dimensional density of charge carriers.R s is sheet resistance. In the linear regime, when V G >V T , the charge carriers induced aremobile and the density can be expressed as:Cgd ( VG−VT)n = (1.5)eIn the linear regime, mobility, µ, can be expressed by:1R 1= =ne Cssμ (1.6)gd1dRdVGIn equation 1.6, µ was calculated based on the assumption that µ is independent of twodimensional density of mobile carriers induced by V G . In some cases, the mobility ofcharge carriers depends on the gate voltage, which is contradictory to the assumption [41,43]. As gate voltage increases the mobility increases in previously reported literature [44].This gate voltage dependence of mobility has been predicted by multiple trapping andrelease (MTR) [45] and charge transport models based on hopping between localized states
9[46]. The charge transport mechanism of carriers in organic semiconductors is not entirelyclear.Contact resistance between the metal electrodes and organic semiconductors complicatesmeasurements of mobility. Contacts between metal and semiconductor were assumed to beohmic in equation 1.3. In MOSFETs, contact resistance can be reduced by doping thesemiconductor near the electrodes and the current voltage characteristics in the linear (eq.1.3) and saturation regime (eq. 1.2) can be widely applied. In OFETs, however, manystudies demonstrated that a significant voltage drop is present near the metalsemiconductorcontacts [41, 47-48]. In this thesis, we studied how contact resistance canaffect the mobility in OFETs.1.2.2 Device fabricationOur research used OFETs with the bottom-contact geometry fabricated usingphotolithography to define source and drain electrodes. A thermally grown 200 nm-thickoxide was used as a substrate (Fig. 1.3(a)). We spincoated an oxidized Si substrate withphotoresist (PR) at 4000 rpm for 30 sec (Fig. 1.3(b)). The sample was baked for 2 min at115°C on a hot plate. The PR was exposed to ultraviolet (UV) light for 12 sec (Fig. 1.3(c)).The chemical structure of the PR exposed to UV was changed and the PR was easilyremoved by dipping the sample into a developer (MF321) for 3 min (Fig. 1.3(d)). Then,metal layers were deposited by e-beam evaporation (Fig. 1.3(e)). A Cr adhesion layer wasdeposited on the patterned substrates then a thick Au layer was deposited on the Cr layer.
Page 1: ELECTRONIC AND STRUCTURAL PROPERTIE
Page 6 and 7: vTable of ContentsAbstract………
Page 8 and 9: 7.4 Electrical properties of rubren
Page 10 and 11: 2the interface. For efficient charg
Page 12: 4change the surface energy. The sur
Page 15: 7A larger negative gate voltage ind
Page 19 and 20: 11double bonds of carbon-carbon bon
Page 21 and 22: 131.3.2.1 PentaceneThe pentacene mo
Page 23 and 24: 15rubrene molecule in the gas phase
Page 25 and 26: 17[17] H. Ishii, K. Sugiyama, E. It
Page 27 and 28: 19[46] M. Vissenberg and M. Matters
Page 29 and 30: 21Chapter 2Channel Formation in Sin
Page 31 and 32: 23This value was comparable to thos
Page 33 and 34: 25The sheet resistance in the four-
Page 35 and 36: 27representing mobility of pentacen
Page 37 and 38: 29to 1.2 ML. After the coverage exc
Page 39 and 40: 31trap density and the two dimensio
Page 41 and 42: 33experiment. The drain current at
Page 43 and 44: 35Increasing the pentacene coverage
Page 45 and 46: 37(a)sheet conductance (1/Ω)2.5 10
Page 47 and 48: 392.5(b)). The change in mobility a
Page 49 and 50: 412.7 References[1] J. M. Shaw and
Page 51 and 52: 43[33] J. Takeya, C. Goldmann, S. H
Page 53 and 54: 45surface normal in the two phases.
Page 55 and 56: 47next higher orbital. Each peak ca
Page 57 and 58: 49Here, θ is the photon incidence
Page 59 and 60: 51plane of the surface we would obs
Page 61 and 62: 53deposited at a low deposition rat
Page 63 and 64: 5521.5M B/M TF10.500.01 0.1 1Deposi
Page 65 and 66: 57Chapter 4Functional Self-Assemble
Page 67 and 68:
59Einμmol= N ( )(4.1)εε d0 molHe
Page 69 and 70:
61Fig. 4.1 shows AFM images of an a
Page 71 and 72:
63(a) (a)(b)Flourinated SAMPentacen
Page 73 and 74:
65Debye and the length of the molec
Page 75 and 76:
67the literature [24, 26]. The turn
Page 77 and 78:
69form the conducting channel. A po
Page 79 and 80:
71(a)(b)80-NH 2+C 60SAM60- drain cu
Page 81 and 82:
732 10 -8 -20 0 20 40 60-drain curr
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75As summarized in Table 4.1, scan
Page 85 and 86:
77drain voltage. The change in the
Page 87 and 88:
79Pentacene thin films deposited on
Page 89 and 90:
81[2] S. H. Kim, J. H. Lee, S. C. L
Page 91 and 92:
83Chapter 5Self-assembled Dipolar C
Page 93 and 94:
85Fig. 5.2: XPS spectra of function
Page 95 and 96:
87In Fig. 5.4(b), the drain current
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89directions. In the scan from 50 V
Page 99 and 100:
91reaction of the device in the DR1
Page 101 and 102:
93100 V, the polarization of the DR
Page 103 and 104:
95gate voltage can cause the photog
Page 105 and 106:
97Chapter 6Ambipolar Rubrene Thin F
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99from the C-H bending bands (out o
Page 109 and 110:
101the size of rubrene islands.6.2.
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103region is 0.67 based on the perc
Page 113 and 114:
105three dimensional growth of rubr
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107(a)(b)0.000110 -80.0001210 -78 1
Page 117 and 118:
109(a)(b)-1 10 -10 0-20 V-30 V-2 10
Page 119 and 120:
111Phys. Rev. Lett. 93, 086602 (200
Page 121 and 122:
113Chapter 7Enhanced Hole Mobility
Page 123 and 124:
115(a)(b)SourceRubrenePolystyreneDr
Page 125 and 126:
117(a)(b)Fig. 7.3: AFM image of rub
Page 127 and 128:
119small average thickness play a v
Page 129 and 130:
121electrodes is much smaller than
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PhD thesis - Evans Research Group - University of Wisconsin-Madison

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