Charge density increase in submonolayer organic field-effect transistors

Interface confinement plays a central role in charge carrier accumulation and transport along the channel of organic field-effect transistors. Understanding the relevant interfacial interactions that affect the energy landscape experienced by carriers in the channel is of fundamental interest. Here we investigate charge transport in the submonolayer regime of pentacene transistors in which confinement arises due to the finite size of the interconnected semiconducting islands. In situ real-time electrical characterization is used to monitor the formation and evolution of the accumulation layer at the very early stages of growth. The morphology of the confining interfaces is controlled by growth conditions and pentacene coverage. Charge transport occurs when percolation pathways connecting source and drain electrodes are formed at a critical coverage. The displacement current across the oxide/semiconductor interface is observed starting from the onset of percolation (0.69 monolayer coverage). The analysis of the characteristics shows that already the submonolayer film fully screens the gate field and accumulates higher charge carrier density as compared to the monolayer film. We propose an electrostatic model to correlate the charge density to the characteristic length scale of the submonolayer film and the thickness of the dielectric layer. This explains charge mobility and threshold voltage of thin-film transistors in the submonolayer regime.

Figure 1

In situ growth and electrical characterization of pentacene transistor: (a) drain current $I_D$ at $V_{SG}=V_{SD}=-20\mathrm{V}$ as a function of nominal layer thickness Θ; (b) examples for transfer curves at various Θ acquired at $V_{SD}=-20\mathrm{V}$. Each point in plot (a) corresponds to a full transfer characterization as shown in (b).

Figure 2

Measurement of carrier accumulation in pentacene thin-film transistors: (a) potential waveform applied to $V_{SG}$ and resulting displacement current $I_G$ at $3\mathrm{ML}$ thickness;(b) schematic diagram showing the measured capacitor and equivalent circuit; (c) displacement current measured during growth of pentacene film with a deposition rate of 0.47 ML/min applying continuously the described waveform. The displacement current before pentacene percolation $(\Theta =0.69\mathrm{ML})$ results from the capacitance formed by gold source and drain electrodes.

Figure 3

Capacitance $C$ (a) and surface charge density σ (b) at $V_{SG}=30\mathrm{V}$ in the semiconducting film during growth as extracted from the displacement current measurements; (c) development of electrostatic model describing capacitance and charge density as a function of coverage in the submonolayer regime.

Figure 4

Morphology of submonolayer pentacene thin films: AFM topography at 0.5 ML (a) and 0.8 ML (b) coverage and corresponding height radial autocorrelation function (ACF) in (c) showing a characteristic minima at 195 nm.

Figure 5

Mobility µ (a) and threshold voltage $V_t$ (b) calculated from transfer characteristics acquired during growth of pentacene semiconducting layer at $V_{SG}=-20\mathrm{V}$ and $V_{SD}=-20\mathrm{V}$. Correction for the increased charge density in the submonolayer regime has been included. Mobility values as extracted from two transistors of different channel length L are shown.

Figure 6

Conformal mapping technique applied to submonolayer pentacene film: The two-corner structure in complex plane $z$ wherein Laplace equation is analytically solved is shown in (a). The area of interest is mapped upon the upper half in plane $w$ (b).