Thermally Driven Intrinsic and Extrinsic Doping Mechanisms in Amorphous Oxide Semiconductors

Abstract

Amorphous oxide semiconductors (AOSs) have been considered as one of the most promising materials for implementation of next-generation electronic devices that are flexible, transparent, and large-area applicable due to their novel properties. AOSs have long-range film uniformity induced by long-range structural disorder and show excellent electron mobility comparable to the corresponding crystalline oxide semiconductors. However, the use of AOSs in electronic devices has been hindered by the lack of controllability of electrical properties as well as stability, which is induced by various electronic states of dopant in subgap region. Although electron mobility is relatively insensitive to structural disorder, electronic states of both intrinsic and extrinsic dopants are affected by the local atomic structure. As a result, dopants form various electronic states and irregular doping efficiency is observed. Moreover, degree of structural disorder tends to decrease because amorphous structure is thermodynamically metastable, which is referred as structural relaxation (SR). Thus, doping efficiency of dopants as well as distribution of subgap states could change by thermal stress. Therefore, understanding the continuous change of doping efficiency of intrinsic and extrinsic dopants driven by thermal stress is necessary for controlling the electrical properties and improving the of stability of AOSs. The objective of this thesis is to unravel the intrinsic and extrinsic doping mechanisms in AOSs with respect to thermal history and to provide guidelines for not only delicate control of electrical properties, but also creation of new functionality in AOSs. Before investigating thermally driven doping mechanisms in AOSs, it is necessary to regulate the additional reactions in AOSs such as redox reactions. In this study, novel metal/AOSs/metal structured devices are designed to prevent unwanted reactions of AOSs with the ambient. Based on the devices, changes in electrical properties of AOSs induced by intrinsic atomic rearrangement as well as extrinsic dopant migration were investigated. First, concentration of oxygen vacancy (VO) as an intrinsic donor in amorphous In-Ga-Zn-O (a-IGZO) was modulated by solely SR. As annealing temperature increases from 300 °C to 450 °C, concentration of VO in the shallow donor state 1000 time increases. The SR-driven intrinsic doping effect depends strongly on the annealing temperature but not on the annealing time. The Arrhenius activation energy of the SR-driven doping effect is 1.76 eV, which is similar to the bonding energies in a-IGZO. Free volume in a-IGZO decreases during SR and VO in either deep-donor or electron-trap states consequently transforms into shallow-donor state. The second focus is to identify the electronic states of extrinsic Cu dopant in AOSs. Amorphization of Cu-based metal oxides have induced peculiar electrical characteristics with loss of p-type characteristics of Cu dopant in the corresponding crystalline oxides. Therefore, unravelling the doping mechanism of Cu in AOSs is essential to determine the exact electronic states of Cu in AOSs. In the early stage of annealing, Cu dominantly diffuses into a-IGZO through the free volume and acts as an electron donor and increases electrical conductivity of a-IGZO. Moreover, resistive switching (RS) characteristics are generated in Cu-doped a-IGZO due to the electrochemical migration of Cu at the free volume. With further annealing, substitutional Cu becomes predominant which prefers In to Ga or Zn. After annealing, inter-diffused Cu and In form crystalline Cu-ln-O clusters in a-IGZO. Cu-In-O clusters not only form bulk-heterogeneous pn junction, but also give rise to negative differential resistance behavior in a-IGZO. RS performance can be modulated by Cu doping concentration at the free volume as well as the formation of Cu-In-O clusters. This study reported thermally-driven intrinsic and extrinsic doping mechanism in AOSs without any reactions of AOSs with the ambient using the novel metal/AOSs/metal structured devices. A systematic study on electrical conduction mechanism analysis of the devices, microstructural and chemical analysis provided useful information for understanding the changes in electronic state of intrinsic and extrinsic dopants according to the structural location and suggested that extrinsic doping control gives rises to new-functionality in AOSs such as resistive switching in addition to the modulation of electrical conductivity.