Effects of Dielectric Material, HMDS Layer, and Channel Length on the Performance of the Perylenediimide-Based Organic Field-Effect Transistors.

It is well-known that the improvement in the performance of organic field-effect transistors (OFETs) relies primarily on growth properties of organic molecules on gate dielectrics, their interface behavior, and on understanding the physical processes occurring during device operation. In this work, the relation of varying the dielectric materials in an n-type OFET device based on 1,7-dibromo-N,N'-dioctadecyl-3,4,9,10-perylenetetracarboxylic diimide (Br2PTCDI-C18) molecule on a low-cost glass substrate at different channel lengths is reported, which is conceptually very important and fundamental in the context of device performance. Anodized alumina (Al2O3) along with dielectric films of polyvinyl alcohol (PVA) or polymethylmethacrylate (PMMA) was used to fabricate the devices and study their influence on various transistor properties. In addition, the effects of a thin hexamethyldisilazane (HMDS) layer on the performance of OFETs including their contact resistances were studied with the channel length variations. The devices with PVA dielectric material exhibited the maximum mobility values of 0.012-0.025 cm2 V-1 s-1 irrespective of varying channel lengths from 25 to 190 µm. The bias-stress measurements were recorded to realize the effects of the channel length and HMDS layer on the stability of the devices. The on/off ratios and electrical stabilities of these devices were enhanced significantly by modifying the surface of the PVA dielectric layer using a thin layer of HMDS. Similarly, in the case of PMMA dielectric layer, a drastic enhancement in the on/off ratio and bias-stress stability was observed. Characterization of all devices at different channel lengths using different dielectric materials permitted us to identify the effects of contact resistance on OFET devices. The stability of the devices in relation to the bias-stress measurements of devices by varying channel lengths and surface modification was systematically investigated. A careful analysis of oxide gate dielectrics modified with polymer-based dielectric materials, contact resistance, influence of thin HMDS layer on the electrical properties, and other parameters on top-contact bottom-gated configured n-type OFET devices is presented herein.

Enhanced environmental stability induced by effective polarization of a polar dielectric layer in a trilayer dielectric system of organic field-effect transistors: a quantitative study.

We report a concept fabrication method that helps to improve the performance and stability of copper phthalocyanine (CuPc) based organic field-effect transistors (OFETs) in ambient. The devices were fabricated using a trilayer dielectric system that contains a bilayer polymer dielectrics consisting of a hydrophobic thin layer of poly(methyl methacrylate) (PMMA) on poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) or polystyrene (PS) with Al2O3 as a third layer. We have explored the peculiarities in the device performance (i.e., superior performance under ambient humidity), which are caused due to the polarization of dipoles residing in the polar dielectric material. The anomalous behavior of the bias-stress measured under vacuum has been explained successfully by a stretched exponential function modified by adding a time dependent dipole polarization term. The OFET with a dielectric layer of PVA or PVP containing hydroxyl groups has shown enhanced characteristics and remains highly stable without any degradation even after 300 days in ambient with three times enhancement in carrier mobility (0.015 cm(2)·V(-1)·s(-1)) compared to vacuum. This has been attributed to the enhanced polarization of hydroxyl groups in the presence of absorbed water molecules at the CuPc/PMMA interface. In addition, a model has been proposed based on the polarization of hydroxyl groups to explain the enhanced stability in these devices. We believe that this general method using a trilayer dielectric system can be extended to fabricate other OFETs with materials that are known to show high performances under vacuum but degrade under ambient conditions.

Local diffusion induced roughening in cobalt phthalocyanine thin film growth.

We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))(1/2), with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, ß, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ t(ß), with growth exponent ß = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t(1/z) with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.