Ph.D. Public Defense

Strain based 2D Nonvolatile Memory Devices

Wenhui Hou

Supervised by Professor Stephen Wu

Wednesday, April 19, 2023
2:30 p.m.–3:30 p.m.

Computer Studies Building, Room 523

wenhui.jpegStrain engineering is a powerful technique to control materials properties. While its static form has long been used in the semiconductor industry to enhance the channel charge mobility of CMOS transistors, dynamic control of materials properties using strain in epitaxially grown 3D-bonded systems is more challenging due to defect formation and lattice match limitations. 2D materials bring new opportunities in strain engineering as they show high elastic strength, various strain sensitive properties, and can be bonded through van der Waals interaction with no lattice match restrictions.

In this thesis, I will first demonstrate that static strain engineering technique based on stressed capping layers can be used to strain 2D materials and based on which a multilayered transition metal dichalcogenide MoTe2 phase change memristor with record-high performance can be achieved. The device combines strain-induced phase transitions and electric-field induced phase transitions of MoTe2 together, with the former setting the initial semiconducting active region and the latter achieving the reversible resistive switching behavior.

Then I will demonstrate that a dynamic strain engineering technique based on the converse piezoelectric effect of ferroelectric substrate can also be used to strain 2D materials. And by combining thin film stressed capping layers as metal contacts and ferroelectric Pb(Mg1/3Nb2/3)0.71Ti0.29O3 (PMN-PT) as the gate dielectric, a three-terminal nonvolatile MoTe2 phase change transistor can be achieved, which can reversibly switch MoTe2 between the semimetallic phase and semiconducting phase using gate voltage controllable ferroelastic strain.

Finally, I will show that the thin film stressors can also form large strain gradients near the surface of the ferroelectric, causing a flexoelectric field comparable with the coercive field of PMN-PT. By controlling the film force of the thin film stressor, we can continuously tune the internal bias to control ferroelastic strain applied by the ferroelectric vs. applied electric field, thereby achieving control of ferroelastic non-volatility. This can be further used to achieve the full control of the non-volatile functionality of the MoTe2 phase change transistors.