Study on the characteristics of Zn-based oxide semiconductor and its 3D stacked device for flash memory applications

Abstract

Amorphous oxide semiconductors (AOSs) have attracted a great amount of attention due to their potential as an alternative for silicon based semiconductors in thin film transistor (TFT) for next generation electronic devices such as display, high mobility device, and memory applications. In particular, Zn-based oxide semiconductors, such as zinc-tin-oxide (ZTO) or indium-gallium-zinc-oxide (IGZO), are subject of intensive research for TFT channel materials owing to its reasonably high electron mobility (>10cm2/Vs), tunable electrical conductivity, and high uniformity in large area. Since the AOSs are composed of post-transition-metal cations which have a large wave function overlap with neighboring atoms in the s-orbitals, the electrical properties are not significantly altered from the atomically regular crystalline material even when in the amorphous structure. As a result, a high carrier density and mobility can be simultaneously obtained. In this regard, the amorphous nature and high carrier mobility of ZTO and IGZO thin films attract a great deal of attention for this approach. In addition, the high performance of ZTO and IGZO TFTs also allows them to be easily integrated with Si-based devices even at low processing temperature, which is another forte of this concept. However, despite these promising factors, research on applications for ZTO or IGZO has mostly been focused on the development of thin-film transistors for display or planar-type devices. Furthermore, while there have been many efforts to improve the electrical performance of ZTO or IGZO, only a few results have been reported about the device scaling or novel device applications. Although the issues related with scaling have not been highlighted for display devices, a proper understanding of the overall conduction mechanism is very important for new applications such as logic and memory devices. Therefore, it is very important to establish a concrete understanding of the device performance in terms of cation atomic composition of AOS thin films. Furthermore, to be implemented in novel devices applications, various device structures should be suggested and their electrical characteristics should be investigated. First, this study experimentally examined the physical and electrical characteristics of ZnxSnyOz (ZTO) thin films grown by metal organic chemical vapor deposition (MOCVD) method with various Zn/Sn atomic compositions. The corresponding defect structures of the deposited films were investigated in detail using negative bias illumination stability (NBIS) analysis in the thin film transistor (TFT) structure. Among the films with the different Zn/Sn atomic compositions, the film with Zn/Sn atomic composition of ~ 50/50 showed the best electrical performance. After applying NBIS stress for 1000 sec, the transfer curves of the Zn-rich ZTO TFT showed a hump, but the transfer curves of the Sn-rich ZTO TFT exhibited a parallel shift into the negative bias direction. These phenomena were attributed to the difference in the oxygen vacancy energy states generated in the ZTO band gap by light illumination. The Zn- and Sn-related oxygen vacancies generated deep donor like trap states at ~0.3 eV and shallow states at ~0.1 eV from the conduction band (or mobility) edge, respectively, which were identified by the quantitative simulations of the transfer curves of the TFTs. Second, two serially connected and vertically integrated amorphous-In2Ga2ZnO7 thin film transistors (V-TFTs) with ~600 and 400-nm channel lengths were fabricated. Top and bottom V-TFTs showed well-behaved transfer characteristics with an Ion/Ioff ratio of ~108 and a sub-threshold swing of ~0.6 V/dec., which are much improved results compared with the previous report on single-layer V-TFTs. Electrical performances of two V-TFTs were cross-checked, and they showed certain influences from the other device depending on operation conditions, which was attributed to charge trapping in the gate dielectric layer during gate voltage sweeping. V-TFT with thermally grown SiO2 showed negligible charge trapping behavior. Finally, double-layered vertical integrated charge trap memory TFTs (CTMTs) using a-ZTO deposited by MOCVD were fabricated and examined electrical characteristics. The memory layers of fabricated device consist of blocking oxide, charge trap layer, and tunneling oxide as SiO2, Si3N4, SiO2, respectively. The memory operation characteristics of a-ZTO charge trap memory TFT were attributed to electron trapping, hole trapping, and charged oxygen vacancy (Vo2+) diffusion. Its memory window was ~ 2.5 V after 10 years. Meanwhile, the Vth shifts and retention between program/erase (P/E) states ere much different by different P/E operation mechanism. The retention characteristics of erase were superior to that of program. Also, the charge spreading in the vertical devices occurred in charge trap layer form program state to erase state. In this dissertation, fundamental material characteristics of AOSs were investigated and based on the understanding of its characteristics, the novel devices structures were proposed and fabricated. By analyzing the operation characteristics of fabricated devices, novel devices application of AOSs TFTs were suggested.