Fabrication of p-Type 2D Single-Crystalline Transistor Arrays with Fermi-Level-Tuned Van Der Waals Semimetal Electrodes

Abstract
High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T’-phase semimetal contact electrodes. By transforming polycrystalline 1T’-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T’ semimetals and 2H semiconductors. Work function modulation of 1T’-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·μm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 μA/μm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

Professor Soon-Yong Kwon in the Department of Materials Science and Engineering and the Graduate School of Semiconductor Materials and Devices Engineering at UNIST, in collaboration with Professor Zonghoon Lee, has embarked on a pioneering research endeavor focusing on the development of high-performance p-type semiconductor devices, utilizing molybdenum ditelluride (MoTe2)—a compound renowned for its unique properties. This pioneering technology holds great promise for application in the next-generation complementary metal oxide semiconductor (CMOS) industry, where ultrafine technology is crucial.

CMOS devices are based on the complementary bonding of p-type and n-type semiconductors. Known for their low power consumption, CMOS devices are widely used in everyday electronic devices such as PCs and smartphones. While silicon-based CMOS is prevalent, there has been growing interest in two-dimensional materials as potential candidates for future semiconductors due to their thin structure. However, challenges arise during the manufacturing process when forming three-dimensional metal electrodes on these materials, leading to various defects at the interface.

Figure 1. In situ control of MoTe2 polymorphs during wafer-scale synthesis.

In this research endeavor, spearheaded by Professor Kwon’s team together with Professor Lee’s team, they focused on developing high-performance p-type semiconductor devices utilizing MoTe2—a compound known to exhibit unique properties. By employing chemical vapor deposition (CVD) techniques that facilitate thin film formation through chemical reactions, the researchers have successfully synthesized large-area 4-inch MoTe2 wafers with remarkable purity.

The key innovation lies in controlling the work function by depositing a three-dimensional metal onto a two-dimensional semi-metal—effectively modulating barrier layers that prevent charge carriers from entering. Moreover, this approach leverages three-dimensional metals acting as protective films for two-dimensional metals—resulting in improved yields and enabling transistor array device implementation.

“The significance of our research extends beyond MoTe2,” explained Sora Jang (Combined MS/PhD Program in Materials Science and Engineering, UNIST). “The device manufacturing method developed can be applied to various two-dimensional materials, opening doors for further advancements in this field.”

This study has been jointly carried out by Professor Soon-Yong Kwon (Co-corresponding author), Professor Zonghoon Lee (Co-corresponding author), Dr. Seunguk Song (Co-first author) from the University of Pennsylvania, Dr. Aram Yoon (Co-first author), and Sora Jang (Co-first author).

The results of this groundbreaking research have been published ahead of their official publication in the online version of Nature Communications on August 7, 2023. This study has been supported by the 2020 research Funds of UNIST, Institute for Basic Science, and National Research Foundation (NRF) of Korea, funded by the Ministry of Science, ICT (MSIT).

Journal Reference
Seunguk Song, Aram Yoon, Sora Jang, et al., “Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes,” Nat. Commun., (2023).