Beyond extending Moore’s Law scaling laws for semiconductor industry, the two-dimensional (2D) materials, as thin as a few layers of atoms, will enhance the development of photonics, optoelectronics, biosensing, energy storage, and photovoltaics. Researchers from National Cheng-Kung University (NCKU) and National Synchrotron Radiation Research Center (NSRRC) now can extend Moore's Law scaling beyond the 3-nanometer node by choosing semiconducting atomic monolayer of transition-metal dichalcogenides (TMDs) on a multiferroic oxide substrate. They demonstrated a non-volatile lateral pn diode by integrating a monolayer WSe2 sheet with the BiFeO3 (BFO) multiferroic substrate for high-performance rectifying behavior as core of logic applications.   

As diode approaches the atomic scale, any imperfections at the diode interface are expected to have great impacts and significantly degrade the diode functionality and performance, according to professor Chung-Lin Wu at the National Cheng-Kung University. In order to successfully achieve a defect-free diode interface, it is necessary to develop an efficient method to bring p-type and n-type monolayer semiconducting WSe2 into contact to form rectifying pn diode, which is a fundamental building block of electronic and optoelectronic devices. Thus, the manipulation of carrier density for semiconducting TMD is crucial to control the electrical properties and create the 2D electric/optoelectric devices. Usually this involves laterally manipulating carrier density by gate biasing that can only be operated in a high-bias (VG>10 V) instead of a standard diode setup and thus leaves the TMD diode far from device functionality and application. Here, for the first time, by utilizing a locally reversed ferroelectric polarization of the supporting BFO, they obtain an additional degree of freedom to strongly modify the electron confinement without the help of biased gate. The researchers’ proof of concept used “ferroelectric doping,” with electrons, instead of atoms, serving as the dopant. The WSe2 pn diode is demonstrated with both optical and electrical methods in an ambient environment and synchrotron radiation excited scanning photoelectron micro-spectroscopy.

The institute conducted the research in collaboration with scientists from National Tsing-Hua University (Department of Materials Science and Engineering, Prof. Yi-Hsien Lee) and National Chio-Tung University (Department of Materials Science and Engineering, Prof. Ying-Hao Chu). Nano-manipulation of multiferroic oxide substrate and diode characterization came from Prof. Yi-Chun Chen (Department of Physics, National Cheng-Kung University) and Prof. Tze-Ming Chen (Department of Physics, National Cheng-Kung University), respectively.