Research

Transport spectroscopy:

We have a special technique to measure electron energy (Fermi energy) as a function of electron density.  The measured electron energy vs density data provide band gap, effective mass, and Fermi velocity, which are important fundamental electronic properties and critical to design of diverse electronic and optical applications.  We also do manipulating electronic structure of materials for practical purposes, and probing the reconstructed electronic structure.

Novel quantum states:

We are interested in various quantum phenomena such as quantum Hall effect and topological/quantum spin Hall states in two-dimensional materials.  In such nanoscale materials due to the reduced dimensionality quantum effect can be pronounced.  Using our non-local Fermi energy measurement technique we are capable of direct probing of energy of such quantum states.  We design novel condensed matter systems, study new quantum phenomena, and explore practical nanoelectronics using emerging nanoscale materials and  physics.  This research includes developing new method of nano fabrication and precise characterization techniques.

Exploring the physical limits of 2D materials and addressing the challenges of their industrialization:

2D materials show great promise as potential replacements for silicon, primarily because they do not suffer from performance degradation caused by miniaturization. Nevertheless, the broader implementation of 2D semiconductors faces certain challenges, such as high contact resistance and Fermi level pinning issues. These limitations pose obstacles to the practical utilization of 2D semiconductors in various fields, particularly in the development of CMOS circuits and systems because p-type 2D semiconductor options are highly restricted. 

We have recently achieved realization of ambipolar characteristics of 2D MoS2 and InSe by developing advanced van der Waals contacts that relieve the Fermi-level pinning without forming defects at the surface of semiconductors. This allowed us to invent unprecedented strategies to achieve superior ambipolar characteristics and complementary reconfigurable multifunctionality. We also aim to demonstrate circuits composed of such reconfigurable devices and validate the practical scalability of our devices.

Low-power nanoscale electronics:

We are developing new types of low-power nanoscale devices.  One of our major interests is achieving low-power transistors such as quantum tunneling transistors. The advanced van der Waals contacts will particularly contribute to the realization of tunneling transistors based on 2D semiconductors. The idea of a tunneling transistor capable of a subthreshold swing < 60 mV/dec was first presented in the 1950s, but has not been commercialized until now. The challenges in the realization of tunneling transistors are primarily related to difficulties in implementing an ideal sharp p-n junction without impurities and interface traps. Our accomplishment on van der Waals contacts, featuring perfect interfaces and efficient electrostatic control to 2D semiconductors, holds tremendous potential for the realization of ideal tunneling transistors for ultra-low-power circuits and systems. 

The research on the low-power switching technology can be also extended to the development of advanced sensors with superior sensitivity. We will employ the methodology for efficient electrical modulation of 2D materials to various sensing electronics to achieve reduced off-state current (e.g., dark current) and quick response. We are also developing quantum sensors with exceptional sensitivity by employing the switching technology into quantum materials.