Electrical characteristics of organic field effect transistor with high-k gate oxide for flexible display

Abstract

Organic materials have been used in applications such as organic field effect transistors (OFETs), active matrix - organic light-emitting displays (AM-OLEDs), polymer random access memory (PoRAM), and radio frequency identification (RFID) due to their flexibility, low cost, and easy process. However, there are still many critical issues such as high power consumption, high operating voltage, low mobility, threshold voltage shift, instability, and charge trapping. Thus, many researchers have being studied to solve the problems. Of the problems, high power consumption and high operating voltage are the most critical issues to realize the mobile and portable device. In this study, we preferentially focus on lowering the operating voltage of the device using the high dielectric constant (high-k) layer. Furthermore, we aim to improve the performance of device such as mobility, sub-threshold swing, threshold voltage, and current modulation, on the basis of low operation voltage and low power consumption, which accomplished from high capacitance characteristics. In chapter 1, we explain the introduction of organic FET, issues of organic FET, and the requirement of high-k material. Moreover, prior to evaluate the performance of the device with high-k layer, the electrical characteristics of device with the low dielectric constant (low-k) SiO2 as a gate oxide is also investigated. Despite the high mobility, the low performance such as threshold voltage of approximately  7 V, sub-threshold swing of about 4 V/decade, on/off ratio of 104 are observed at pentacene FET with 1000Å SiO2. This performance is not appropriate for mobile and portable devices, which requires the threshold voltage below about 2 V and sub-threshold swing below about 1 V/decade [Table 1.2]. Thus, the application of high-k material is required to low operating voltage as a gate oxide of devices. In chapter 2, prior to the application of high-k layer in organic FET, the electrical characteristics of metal oxide semiconductor (MOS) capacitor are measured and analyzed. HfO2 and Al2O3 as a gate oxide have a high dielectric constant and a low leakage current density due to the high capacitance. Moreover, it is believed that Al2O3 is suitable to be used as a gate oxide compared to that of HfO2, considering the lower leakage current density. And ultimately, HfO2 and Al2O3 gate oxide would be evaluated for suitability of gate oxide in organic FET in chapter 3, 4, and 5. In chapter 3, we studied the electrical characteristics of a pentacene FET formed by the hydrogen (H2) and nitrogen (N2) mixed gas treatment of Al2O3. After the treatment, the mobility and subthreshold swing were observed to be significantly improved by the decreased hole carrier localization at the interfacial layer between the gate oxide and pentacene channel layers. H2 gas plays an important role in removing the defects of the gate oxide layer at low temperature. In chapter 4, the dependence of the crystal structure and mobility of pentacene on HfO2 after octadecyltrichlorosilane (OTS) surface modification is investigated. OTS monolayers play an important role in decreasing the surface energy of HfO2 film to below about 45 mN/m. In addition, they significantly affect the crystal structure, dislocation distribution, and ordering of pentacene molecules. As a result of OTS SAMs treatment, the mobility of pentacene devices was improved by the presence of a single thin-film phase. In chapter 5, we investigate the interface trap density of pentacene FET with Al2O3 after channel engineering. After OTS and gas treatment, the performance of the device such as operating voltage, sub-threshold swing, mobility, and current modulation was remarkably improved. From the Gp/ω peak, interface trap density is remarkably reduced by channel engineering. In light of results in chapter 3, 4, and 5, it is believed that Al2O3 is suitable as a gate oxide of organic FET and gas treatment is suitable as channel engineering, considering the cost, process, and electrical characteristics. In chapter 6, we evaluated that the reliability of the pentacene FET after electrical and time stress. Gas treated Al2O3 was used as a gate dielectric layer. According to the two stress parameters of electrical and time stress, threshold voltage shift was observed. The mobility and subthreshold swing were significantly decreased due to hole carrier localization and degradation at the channel layer. In particular, it is observed that electrical stress is a more critical factor in the degradation of mobility than time stress caused by H2O and O2 in the air. In chapter 7, we report the effect of consecutive electrical stress on the performance of pentacene FET with gas treated Al2O3 and HfO2. After the electrical stress, the threshold voltage, which strongly depends on bulk defects, was remarkably shifted to the negative direction, while, the other performance characteristics of device such as on-current, transconductance, and mobility, which are sensitive to interface defects, were slightly decreased. This result implies that the defects in the bulk layer are significantly affected compared to the defects in the interface layer. Thus, it is important to control the defects in the pentacene bulk layer in order to maintain the good reliabilities of pentacene devices. In chapter 8, we investigate the reliability of pentacene FET depending on mechanical bending. After mechanical bending, the performance of the device is gradually degraded. In particular, leakage current density is critically affected. High-k layer with thin thickness of 10 nm s was required to reduce the leakage current density. When rigid HfO2 layer is mechanically bended, the organic layer on the top of high-k layer must be also need to endure the tensile and compressive strain by mechanical bending at high-k layer. It is observed that the performance of device is relatively maintained.