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

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28Fig. 2.1: (a) Drain current and (b) threshold voltage as a function of pentacene coverageduring the formation of a bottom-contact pentacene FET. The current in (a) is measured atV G = -100 V during a series of gate voltage scans from -70 V to -100 V with a source-drainvoltage of -50 V.Current began to flow through the channel at a coverage of 0.77 ML. The drain currentincreased rapidly with the coverage of pentacene as the coverage increases from 0.77 ML
29to 1.2 ML. After the coverage exceeded 1 ML, the drain current was not sensitive to thecoverage of pentacene.The onset of current at a particular coverage observed in current vs. coverage plots canbe explained by the geometric percolation of pentacene islands. The steep slope in the plotafter the percolation threshold supports the geometrical change between islands. At thepercolation threshold a current path has been formed by a geometric connection ofpentacene islands. In a continuous two-dimensional system, the percolation threshold of apentacene transistor is expected to occur when the area occupied by pentacene islandsreaches 67% of the total channel area in the device [19]. This threshold value varies onlyslightly with changes in the distribution of island sizes and deviations in shape away fromcircular islands [20]. In terms of the electronic properties of the transistor, the current pathbetween the source and drain is formed at the percolation threshold. Similar phenomenahave been observed in the growth of metal clusters on SiO 2 and in the formation of rubrenethin film transistors [21, 22]. In both of these cases, however, the formation of threedimensional islands at low coverages complicates the geometric description of thepercolation transition.In a number of in situ experiments, the coverage at which current began to flow wasdistributed between 0.69 ML and 1.3 ML. We suggest that this phenomemnon is due to thehigh threshold voltage of the devices. Threshold voltage has been linked to the trap density[23, 24]. According to Podzorov et al., below the threshold voltage, charges injected frommetal electrodes are trapped in the localized states which are located more than a few k B Tabove the highest occupied molecular orbital (HOMO) level. The trapping states are
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 and 16: 7A larger negative gate voltage ind
Page 17 and 18: 9[46]. The charge transport mechani
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: 27representing mobility of pentacen
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
Page 83 and 84: 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
Page 97 and 98:
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
Page 107 and 108:
99from the C-H bending bands (out o
Page 109 and 110:
101the size of rubrene islands.6.2.
Page 111 and 112:
103region is 0.67 based on the perc
Page 113 and 114:
105three dimensional growth of rubr
Page 115 and 116:
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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