Polarization Modulation in Ferroelectric Organic Field-Effect Transistors

The polarization modulation effect of the gate dielectric on the performance of metal-oxide-semiconductor field-effect transistors has been investigated for more than a decade. However, there are no comparable studies in the area of organic field-effect transistors (FETs) using polymer ferroelectric dielectrics, where the effect of polarization rotation by ${90}^{\circ }$ is examined on the FET characteristics. We demonstrate the effect of polarization rotation in a relaxor ferroelectric dielectric, poly(vinylidene fluoride trifluorethylene) (PVDF-TrFE), on the performance of small-molecule-based organic FETs. The subthreshold swing and other transistor parameters in organic FETs can be controlled in a reversible fashion by switching the polarization direction in the PVDF-TrFE layer. X-ray diffraction and electron microscopy images from PVDF-TrFE reveal changes in the ferroelectric phase and domain size, respectively, upon rotating the external electric field by ${90}^{\circ }$. The structural changes corroborate density-functional-theoretical studies of an oligomer of the ferroelectric molecule in the presence of an applied electric field. The strategies enumerated here for polarization orientation of the polymer ferroelectric dielectric are applicable for a wide range of polymeric and organic transistors.

Figure 1

Chemical structure and poling of PVDF-TrFE films. (a) Schematic representation of the chain conformation for the $\alpha$ and $\beta$ phases of PVDF-TrFE. Arrows denote the direction of the dipole moment. (b) Polarization versus voltage hysteresis curves for Al-PVDF-TrFE-Au capacitors at 1 kHz. (c),(d) Schematic of vertical and lateral poling of PVDF-TrFE, respectively. Arrows denote the polarization direction in the crystalline phases of the ferroelectric slab after poling.

Figure 2

Transfer characteristics of DNTT-based FETs. (a) Chemical structure of DNTT. (b) Transfer characteristics of DNTT FETs with V-poled, unpoled, and L-poled PVDF-TrFE. The black line depicts a linear fit to the subthreshold region. (c) Transfer characteristics of a vertically poled FET by applying various magnitudes of a lateral voltage. Green and blue curves depict devices where the lateral field is applied such that the polarization direction of the dielectric is oriented parallel to the external source-drain electric field. (d) Transfer curves of the same FET device shown in (c) after reversing the polarity of the lateral field, such that the polarization direction is oriented antiparallel to the source-drain electric field.

Figure 3

Schematic transfer curves of PVDF-TrFE-based FETs for different poling conditions. $V_{\mathrm{th}}$ may be positive or negative. A slightly positive $V_{\mathrm{th}}$ for the L-poled FET results in a higher current at zero gate bias condition (depicted by the circle). The dipole orientation in PVDF-TrFE with poling conditions are also shown.

Figure 4

Capacitance-voltage characteristics of PVDF-TrFE MIM capacitors. (a) Capacitance versus voltage curves for V-poled, L-poled, and unpoled PVDF-TrFE films. Arrows show the direction of the applied voltage. (b) Corresponding $\tan \delta$ values from the three films. (c) Capacitance versus voltage curves after L-poling of a V-poled film.

Figure 5

Structure and morphology of PVDF-TrFE as a function of poling. (a)–(c) Scanning electron micrographs from unpoled, V-poled, and laterally L-poled films. (d) Grazing-incidence XRD from unpoled, V-poled, and L-poled PVDF-TrFE films on Si substrate. The (hkl) indexing is based on the ferroelectric phase as discussed in the text. The inset shows the XRD from the V-poled film and after L-poling the same V-poled film. Deconvolution of two peaks at $2\theta$ (deg) ($\beta$ phase) and at ${18}^{\circ }$ for the paraelectric phase (PE) are shown for the laterally poled film. (e) Density-functional-theoretical calculations of the structural changes on a PVDF-TrFE oligomer enclosed in a dielectric medium upon the application of an external electric field. The color coding of the atoms is the same as in Fig. 1. Blue arrows indicate the direction of the dipole moment.