PhD Defense: "Engineering van der Waals materials for more-Moore and beyond-Moore devices"

Xuejun Xie

September 5th (Thursday), 12:00pm
Elings Hall, Rm 1605

Conventional transistor scaling is approaching the inevitable end. Two-dimensional (2D) materials or atomically-thin van der Waals materials, a group of layered crystals (such as graphene and MoS2) with unique properties, have emerged as promising candidates to extend transistor scaling and Moore’s law. However, such nanomaterials suffer from significant challenges for transistor applications induced by the van der Waals gap. My doctoral dissertation has been focused on addressing some of those critical issues such as synthesis of high-k dielectrics compatible with 2D semiconductors, as well as identifying beyond-Moore applications uniquely enabled by 2D materials. More specifically, I have explored the potential of 2D materials in creating quantum dot arrays desired for quantum computing, neuromorphic transistor for artificial intelligence (AI), and light emitting material for augmented reality (AR) targeted future displays.

First, for beyond-Moore applications, I will show that we have demonstrated the world’s first two-dimensional quantum dot superlattice with sub-nanometer size controllability and nanometer localization precision on a mono-atomic layer of MoS2 using e-beam irradiation; allowing tunable bandgap by design (Scientific Reports Top-100 in Materials Science 2017), which can help create large scale quantum dot based quantum computers. I have also utilized the 2D superlattice in the first demonstration of a 2D channel light-sensitive memristive transistor with short-term plasticity (IEDM 2017). I will show that a large quantum dot array essentially functions as gate controllable charge traps that create a light-sensitive memristive effect in a field-effect transistor at room temperature, which may help create novel electro-optical neuron links for large scale AI applications.

Second, analogous to Moore’s law for VLSI transistors, I will reveal a new scaling trend of pixels for the next-generation mobile electronic application, AR, which requires MicroLEDs for extremely high pixel density. For pixels made of MicroLEDs, the scaling down of the LED size leads to increasing sidewall leakage and limited extraction efficiency. I will show that these two problems can be simultaneously addressed by reducing the LED thickness to atomic-scale by employing 2D materials. In that regard, I will show that partial fluorination of graphene enables an atomically-thin light emitting material with bandgap tunable over a wide range, which opens up the possibility of creating full-color atomically-thin 2D MicroLEDs.

Finally, to drive the Moore’s law forward, I will present my work on a critical issue in transistors based on 2D materials. Transistors with 2D channel materials suffer from poor gate dielectric quality due to their dangling bond free surfaces. I will disclose a completely new high-k dielectric material formed with atomically-precise superlattice of 2D layered materials with a dielectric constant of ~40, EOT of 0.65 nm, and low leakage current density that’s applicable to transistors down to 15 nm gate length according to 2013 ITRS projections.

About Xuejun Xie:

photo of Xuejun Xie Xuejun Xie is a Ph.D. Candidate and Graduate Student Researcher in the Nanoelectronics Research Laboratory (NRL), Department of Electrical and Computer Engineering at University of California, Santa Barbara, CA, advised by Prof. Kaustav Banerjee. His doctoral research has played a key role in the demonstration of several breakthrough device innovations from the NRL, uniquely enabled by 2D materials. He has authored or coauthored around 20 peer-reviewed publications in high-impact journals and highly-selective conferences (including Nature, Nature Electronics, ACS Nano, Nano Letters, Scientific Reports, and IEDM), which have collectively received over 1,750 citations (as of Aug 2019) with an h-index of 13.

Hosted by: Kaustav Banerjee