Non-Charge Storage Based Nonvolatile Memory with IGZO Oxide Semiconductor and LiCoO_x Charge Trap & Ion Supplying Layer with Large Memory Window

Demand for developing advanced nonvolatile memories significantly increases as emerging data centric applications such as artificial intelligence, the Internet of Things, and automated systems have evolved [1]. Among various nonvolatile memories, charge storage-based flash-type memory has been the most representative. However, adopting charge storage-based nonvolatile memory has some reliability issues such as unintended shift of threshold voltage (<i>V_T</i>) or electrostatic coupling effects [2]. In this study, we investigated non-charge storage-based nonvolatile memory with IGZO channel layer and LiCoO_x as ion supplying layer and charge-trap layer. To demonstrate non-charge storage-based memory behavior, two thin-film transistors (TFTs) having different HfO₂ tunneling oxide thickness, i.e., IGZO/HfO₂(5 nm)/LiCoO_x and IGZO/HfO₂(2 nm)/LiCoO_x stacks, were prepared, and their memory performances were examined. The IGZO/HfO₂(5 nm)/LiCoO_x device showed the clockwise hysteresis in transfer curves with Δ<i>V_T</i> ~ 5 V due to electrical charging in the LiCoO_x layer upon applying the gate voltage sweep of -5 ~ +25 V in forward and backward direction. On the other hand, IGZO/HfO₂(2 nm)/LiCoO_x device with a thinner HfO₂ layer showed much enhanced memory window with Δ<i>V_T</i> up to approximately 20 V in clockwise hysteresis in the same measuring condition. The <i>V_T</i> shift could be induced by the electron charging in the LiCoO_x layer as well as the Li ion migration between IGZO and LiCoO_x layer that depends on the thickness of HfO₂ tunneling oxide. In particular, Li ions could pass through the 2 nm-thick HfO₂ tunneling oxide and modulate the channel conductance as dopants in the case of 2 nm-thick HfO₂ device, while only electron charging would induce the <i>V_T</i> shift in the case of 5 nm-thick HfO₂ device because Li ion cannot penetrate through 5 nm-thick HfO₂. Since Li ions act as p-type dopants in IGZO channel layer, the IGZO channel conductance decreases and <i>V_T</i> shifts positively during application of positive gate voltage as a result of Li ion migration from LiCoO_x layer to IGZO channel layer, in addition to the electrical charging of LiCoO_x layer. Thanks to the use of thinner HfO₂ tunneling oxide, both electron charging and Li ion migration in response to the applied gate voltage lead to the wide memory window more efficiently. This study demonstrates non-charge storage based nonvolatile memory characteristics through viable approach of introducing Li ion as dopants in order to improve the nonvolatile memory performance for advanced nonvolatile memory application.<br/><br/>References<br/>[1] Chen, A. (2016). A review of emerging non-volatile memory (NVM) technologies and applications. <i>Solid-State Electronics</i>, <i>125</i>, 25-38.<br/>[2] Hwang, C. S. (2015). Prospective of semiconductor memory devices: from memory system to materials. <i>Advanced Electronic Materials</i>, <i>1</i>(6), 1400056.