Electronic fluctuation analysis of Resistive RAM based on TiOx material and Solution-processed ZnO thin film transistors

Abstract

This doctor dissertation describes an experimental study on switching mechanisms of the TiOx single layer and multilayer homo-junction and conductance characteristic of oxide semiconductor based solution-processed ZnO layer for thin film transistors (TFTs) from low frequency noise (LFN) analysis. At first, this dissertation explored the low frequency noise sources verse frequency for the TiOx/TiOy cell before and after electroforming steps to verify the conduction features induced by the evolution of bias-dependent conduction filaments. The lower 1/f noise at LRS is primarily related to the formation of conductive filaments, while the higher 1/f noise at HRS is attributable to the distinct combination factors of the top resistive layer and oxygen-driven redox reaction at the oxide interfaces. Therefore, these results expect that the fundamental nature of resistive switching behaviors may rely on the movements of oxygen ions or vacancies under bias. Second, this dissertation examined the conduction features of TiN/TiOx/Pt (Sample I) and Pt/TiOx/TiOy/Pt (Sample II) bipolar resistive switching elements by analyzing low-frequency noise sources after electroforming. The lower noise amplitudes of both samples in their LRSs reflect the formation of conductive filaments, while the higher 1/f noise in the HRS corresponds to the presence of additional noise sources induced at the interfaces between the oxide material and the oxygen reservoir. The different noise levels in the LRS and HRS between Samples I and II suggest the possible occurrence of different concentration of oxygen vacancies and types of filaments at their interfaces, leading to the presence of different resistance distributions in their HRSs. Finally, we introduced the origins of the LFN and time-domain random telegraph signal noise (RTN) features in solution-processed ZnO TFTs without/with (Sample A/ Sample B, respectively) adapting Al evaporation on the back channel of ZnO active layer. The noise of Sample A consists of typical trap centers due to unstable bonding at the SiO2/ZnO interface and additional traps due to weakly Zn-O bonding and pore trap in a space charge region arising from O2- ions absorption into back channel surface of the ZnO layer. The LFN of Sample A mainly is carrier fluctuations in all regimes and the RTN response also presents more dominant carrier emission (trap) time counting than capture (de-trap) time counting in a threshold voltage regime. In contrast, the enhanced noise feature of Sample C turns out to be a reduction of weak Zn–O bonding through Al substitution of Zn and a decrease in adsorbed oxygen through the passivation role of Al-evaporated ZnO TFTs. The LFN of Sample C is a mobility noise in regime I and the RTN characteristic follows the typical RTN behavior observed in a conventional Si-MOSFETs, in which only interface noise contribution is considered. Thus, we expect that the ability to control and improve the noise features of TFTs by proper Al evaporation will lead to practical applications of solution-processed oxide semiconductors.