Ultrathin organic thin-film transistors : investigating hybrid gate dielectrics and stable semiconductor monolayers

Abstract

Organic electronics is an emerging field of research which includes the investigation of novel materials such as organic semiconductors and the development of devices such as organic thin-film transistors (TFTs). The unique properties of organic semiconductors, such as the ability to process them at relatively low temperatures, enable the development of potential applications of organic TFTs in flexible and wearable electronics such as rollable and foldable displays, conformable sensors and electronic skin. In order to facilitate the portable and lightweight nature of flexible electronics by powering them with small batteries or solar cells, a low operating voltage and an overall low power consumption are some of the main requirements of organic TFTs. High-capacitance gate dielectrics, such as hybrid gate dielectrics with an ultrathin metal oxide and an organic self-assembled monolayer (SAM), are an essential choice towards fulfilling these operation requirements. Organic TFTs are fabricated by depositing different materials as thin films by a variety of processes, and the individual film properties of the different components influence the overall electrical characteristics of organic TFTs. The main contribution of this thesis is to establish a correlation between the material properties of the individual components and the electrical properties of the organic TFTs, and moreover, suitably modify the fabrication process to achieve better electrical characteristics in organic TFTs. In this thesis, hybrid gate dielectrics consisting of an ultrathin aluminum oxide (AlOx) film and a phosphonic acid SAM are investigated. The AlOx films are fabricated by exposing the surface of the underlying aluminum gate electrode to an oxygen plasma, and the SAMs are processed from solution. Phosphonic acid molecules with an alkyl or a fluoroalkyl chain with different chain lengths have been chosen to form the SAMs. Two small-molecule organic semiconductors are selected as the active material in the organic TFTs: dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT). The significance of both components of the hybrid gate dielectric in simultaneously achieving a leakage-current density of 10-7A/cm2 as well as an operating voltage of 3 V has been established. The thickness of the AlOx films was measured by Transmission Electron Microscopy and the films were characterized by Electron Energy Loss Spectroscopy and Atom Probe Tomography to determine the thin-film composition. Depending on the parameters of the oxygen-plasma process, AlOx films with a thickness ranging between 4 nm and 7 nm were fabricated, and consequently, organic TFTs with a gate-dielectric capacitance between 1 μF/cm2 and 1.6 μF/cm2. In particular, charge carrier mobilities ranging from 1.8 to 2.3 cm2/Vs were obtained for a number of favorable combinations of the plasma power and plasma duration that produce AlOx films with a small surface roughness and thus promote the formation of high-quality SAMs and well-ordered DNTT films on these gate dielectrics. The influence of the thickness of the SAM by employing different chain-length phosphonic acid molecules on different TFT characteristics such as the threshold voltage, gate-leakage current, charge-carrier mobility and the subthreshold swing has been examined in DNTT and DPh-BTBT TFTs. By employing the medium-chain-length phosphonic acid molecules, an optimum charge carrier mobility of 2 cm2/Vs for the DNTT TFTs, and a turn-on voltage of 0 V for the DPh-BTBT TFTs was achieved. The growth and morphology of the organic semiconductor DNTT on different gate-dielectric surfaces was observed by Scanning Electron Microscopy and Atomic Force Microscopy and was correlated with surface properties of the SAMs and the electrical characteristics of TFTs based on those films. The stability of ultrathin films with a thickness of one-two molecular monolayers of the organic semiconductor DNTT was investigated, and spontaneous morphological changes occurring in the films were observed and correlated with the stability of organic TFTs based on these films. The structural reconfiguration of the ultrathin DNTT films and the degradation of the charge-carrier mobility of organic TFTs based on these films were prevented to a certain extent by cryogenic cooling and in-situ encapsulation. The hybrid gate dielectric with its two components, the organic semiconductor and the gate dielectric-semiconductor interface are the focal points in this thesis. The material and surface properties of the individual components of the gate dielectric have been correlated with the film properties of the organic semiconductor and further with the electrical characteristics of organic TFTs.