Potential mapping of pentacene thin-film transistors using purely electric atomic-force-microscope potentiometry

Nakamura, Masakazu; Goto, Naoyuki; Ohashi, Noboru; Sakai, Masatoshi; Kudo, Kazuhiro

doi:10.1063/1.1891306

Cited by 73 publications

(45 citation statements)

References 12 publications

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“…31 nm)/Cr (ca. 2 nm) were used as the S/D electrodes, and pentacene thin films, fabricated by using the molecularbeam-deposition technique, [8,9] were used as the active organic semiconductor. The width and length of our transistors was 5000 and 22.9 lm, respectively.…”

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confidence: 99%

High‐Performance Bottom‐Contact Organic Thin‐Film Transistors with Controlled Molecule‐Crystal/Electrode Interface

Nakamura

Sakai

et al. 2007

Advanced Materials

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Organic thin-film transistors (OTFTs) are emerging as an inexpensive alternative to amorphous silicon devices because of their many attractive features, such as a simple fabrication process, low cost, and mechanical flexibility. Recently, the performance of OTFTs has been significantly improved by modifying the dielectric surface and/or source and drain (S/D) electrodes, [1] or by using novel dielectric materials. [2][3][4][5] However, high-performance OTFTs were mostly achieved in top-contact (TC) configurations [6] and not in bottom-contact (BC)configurations. As organic active materials are sensitive to chemical wet and ion-beam processes, shadow-mask evaporation is usually used to form S/D electrodes in TC configurations. Such a process, however, is incompatible with largescale integration and does not allow one to produce a channel length shorter than a few tens of micrometers. It is therefore desirable to use a BC configuration, which is compatible with fine lithography processing, for promising applications such as high-resolution flexible displays. The performance of OTFTs is mainly dependent on i) molecular ordering in the organic active layer, ii) charge-injection ability of the source electrode, and iii) charge transport at the interface between the organic active layer and the dielectric layer; the inferior performance of BC-OTFTs compared with TC counterparts is believed to mostly derive from factors (i) and (ii). The step formed by S/D electrodes, in particular thick S/D electrodes on a dielectric layer, disturbs the continuous growth of a single-phase domain in the organic active layer. [7] Additionally, electrode edges with a relatively irregular shape may cause disordered growth of organic crystals adjacent to the electrodes. Despite this, not much effort is required to overcome the inherent drawbacks. A simple approach to completely eliminate the electrode-induced effects in BC-OTFTs is to embed the S/D electrodes in the dielectric layer and planarize the S/D electrodes, making the S/D electrodes and dielectric layer coplanar.In this Communication, we report, for the first time, planarized BC-OTFTs (pBC-OTFTs) with S/D electrodes embedded in the dielectric layer (Fig. 1a); this led to a preferential orientation of organic crystals adjacent to the S/D electrodes and a continuous growth of organic crystals across the S/D edges. Our pentacene pBC transistors on SiO 2 /Si showed a superior performance in controlling conventional BC-OTFTs (cBC-OTFTs) (Fig. 1d) and TC-OTFTs, and the mobility was comparable with the best value ever reported for TC-OTFTs made with similar materials.We investigated bottom-gate pBC-OTFTs and cBC-OTFTs built on a SiO 2 /Si platform. In the present study, Pt (ca. 31 nm)/Cr (ca. 2 nm) were used as the S/D electrodes, and pentacene thin films, fabricated by using the molecularbeam-deposition technique, [8,9] were used as the active organic semiconductor. The width and length of our transistors was 5000 and 22.9 lm, respectively. We tested eight devices for each type of transi...

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confidence: 99%

High‐Performance Bottom‐Contact Organic Thin‐Film Transistors with Controlled Molecule‐Crystal/Electrode Interface

Nakamura

Sakai

et al. 2007

Advanced Materials

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“…The Au is supposed to be scattered by the edge of the shadow mask or residual gas during the vacuum evaporation. This steeper potential slope is therefore due to the electrical damage in pentacene induced by the trace amount of Au [18]. In this case, the apparent mobility was estimated to be about 75% of the intrinsic one.…”

Section: Extrinsic Limiting Factors Of Carrier Mobility In Pentacene mentioning

confidence: 95%

“…Figures 2(a) and 2(b) show atomic-force-microscope potentiometry (AFMP) [3,4,[17][18][19] images taken on a pentacene TC-TFT grown at relatively high temperature, 50°C. The applied drain and gate voltages correspond to the linear regime of the OTFT.…”

Section: Extrinsic Limiting Factors Of Carrier Mobility In Pentacene mentioning

confidence: 99%

Carrier Mobility in Organic Thin-film Transistors: Limiting Factors and Countermeasures

Nakamura

Matsubara

2014

J. Photopol. Sci. Technol.

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Device physics in organic thin-film transistors (OTFTs) have been extensively studied along with the efforts to improve device performance and to develop their applications. Among many promising organic semiconducting materials, pentacene has been a benchmark in this research field. Understanding the bottlenecks of carrier transport in pentacene OTFTs is therefore important to maximize the performance of OTFTs. In this paper, we summarize our knowledge on the bottlenecks against carrier transport in practical polycrystalline organic thin films: extrinsic factors that disturb the carrier transport in OTFTs, intrinsic structure and properties, equations to express the overall carrier mobility in polycrystalline films, and influence of surface chemical modification on the crystallographic and electronic structures.

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“…The shape of the potential profile has been experimentally observed in the operating OFET by using scanning Kelvin probe microscopy (SKPM), giving linear and superlinear distributions corresponding to the linear and the saturation regions of FET output characteristics, respectively. 18 The potential profile also gives detailed information of charge transport, such as contact resistance, [18][19][20][21] charge trapping at grain boundaries, [20][21][22] as well as charge concentration dependence of the mobility. [23][24][25] However, SKPM provides no spectroscopic information of charge carriers.…”

Section: Introductionmentioning

confidence: 99%

Electron spin resonance observation of charge carrier concentration in organic field-effect transistors during device operation

et al. 2013

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Charge carrier concentration in operating organic field-effect transistors (OFETs) reflects the electric potential within the channel, acting as a key quantity to clarify the operation mechanism of the device. Here, we demonstrate a direct determination of charge carrier concentration in the operating devices of pentacene and poly(3-hexylthiophene) (P3HT) by field-induced electron spin resonance (FI-ESR) spectroscopy. This method sensitively detects polarons induced by applying gate voltage, giving a clear FI-ESR signal around g = 2.003 in both devices. Upon applying drain-source voltage, carrier concentration decreases monotonically in the FET linear region, reaching about 70% of the initial value at the pinch-off point, and stayed constant in the saturation region. The observed results are reproduced well from the theoretical potential profile based on the gradual channel model. In particular, the carrier concentration at the pinch-off point is calculated to be β/(β + 1) of the initial value, where β is the power exponent in the gate voltage (V gs ) dependence of the mobility (μ), expressed as μ ∝ V β−2 gs , providing detailed information of charge transport. The present devices show β = 2.6 for the pentacene and β = 2.3 for the P3HT cases, consistent with those determined by transfer characteristics. The gate voltage dependence of the mobility, originating from the charge trapping at the device interface, is confirmed microscopically by the motional narrowing of the FI-ESR spectra.

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Potential mapping of pentacene thin-film transistors using purely electric atomic-force-microscope potentiometry

Cited by 73 publications

References 12 publications

High‐Performance Bottom‐Contact Organic Thin‐Film Transistors with Controlled Molecule‐Crystal/Electrode Interface

High‐Performance Bottom‐Contact Organic Thin‐Film Transistors with Controlled Molecule‐Crystal/Electrode Interface

Carrier Mobility in Organic Thin-film Transistors: Limiting Factors and Countermeasures

Electron spin resonance observation of charge carrier concentration in organic field-effect transistors during device operation

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