Synthesis and study of VO2 ultra-thin films

(Performed by Jihoon Kang)


Project description

(V. Eyert, Ann. Phys. 119 (2002))
✔ To synthesize high quality VO2 ultra-thin films by Plasma Enhanced Atomic Layer Deposition(PEALD) technique.

✔ To confirm the separation of the Structural Phase Transition (SPT) and the Metal Insulator Transition (MIT) in the ultra-thin films.

✔ To study the physical properties and the strain effect of the ultra-thin films for better understanding of the MIT.

Why? The reasons of the research

(L. Foliga, Ph.D. thesis(2011))
✔ Vanadium Dioxide (VO2) is an archetypal strongly correlated electron system that undergoes phase transitions from a rutile (R) metal into a monoclinic (M1) insulator at a critical temperature near room temperature (Tc ~ 340 K)
✔ The nature of this metallic phase transition has been debated for nearly four decades, and continues to be a controversy. It is commonly accepted that Mott-Hubbard and/or Peierls pictures are two main mechanisms for metal insulator transitions in VO2.
✔ We will attempt to resolve this controversial issue by decoupling structural and electronic phase transitions by growing homogeneous 2-dimensional VO2 ultra-thin films using the Plasma Enhanced Atomic Layer Deposition (PEALD) technique.

Why is the project important?

✔ Many studies concerning the MIT mechanism in VO2 thin films has been done. However, due to the poor quality and inhomogeneity of the films, a clear description is still lacking.

• To provide a solution for the previous roadblocks in ultra-thin film synthesis
• To unravel the nature of the MIT in VO2
• To provide of a thorough study of the 2-dimensional VO2 system

Optical modulation spectroscopy for VO2 phase transition

(Performed by Dr. Sangheon Park)

(Courtey of Dr. Sangheon Park at ASDNL)

Project description

(Image from Introduction to Solid State Physics, C.Kittel)
✔ Identification of the differences in the characteristics of the phenomenon of VO2 phase transition by electrical or thermal external stimuli.

✔ By analyzing the difference of characteristics before and after phase transition by modulation spectroscopic method, we will identify the understanding of interaction between phase transition and external stimulus.

✔ It is expected that it can make a decisive contribution to understand the interaction between before and after phase transition and external stimulus.

The reasons for the research

(Courtey of Dr. Sangheon Park at ASDNL)
✔ Since conventional spectroscopic methods that have been studied so far are not methods for analyzing microscopic differences, conventional spectroscopic methods differ from the methods to be studied in this study.
✔ There is no analysis using this modulation spectroscopy for the interaction between phase transition of VO2 and external stimuli. Therefore, there is a difference in research that no one has tried.

Strengths of Modulation Spectroscopy.
• Localized features can be identified in the photon energy region of the inter-band transitions.
• Reduces unwanted background effects.
• Improved weak features that are not well visible in normal reflection or transmission lines, making them observable.

What will be found?

✔For example, there are two hypotheses for phase transitions due to external electric fields:
One is the joule heating model and the other is the electric-field induced breakdown model.
✔ In the Joule heating model, the joule heating by an external electric field causes the transition temperature to rise. On the other hand, in the case of the electric-field induced breakdown model, the electrons constrained by the vanadium dimer are released freely by strong external electric field
✔ Especially in the case of electric-field induced breakdown, the spectroscopic feature is expected to be observed by unbound electrons.
✔ However, this signal is not expected to be enough large, and the conventional spectroscopic methods that have been studied so far are not methods that can be used to analyze minute differences. Therefore, the conventional spectroscopic method differs from the method.

◎ The final goal of the study is to identify the differences in the modulation spectroscopic characteristics before and after phase transition, which is the key point of this study.

Why is the project important?

(Courtey of Dr. Sangheon Park at ASDNL)
✔ If we understand the difference in the interaction between VO2 phase transition before and after and external stimulus, we expect to be able to provide more information on the understanding of the phase transition mechanism.
✔ It is a very reasonable method, which is off the usual spectroscopic method, for studying the material which induces phase transition by external stimuli such as VO2 by measuring the modulation spectroscopic signal according to the external stimuli.

Synthesis of M2 phase in VO2

(Performed by Minkyun Sohn)

(Courtey of Mr. Minkyun Sohn at ASDNL)

Project description

(EPL(Europhysics Letters), Volume 109, Number 2 (2015))
✔ There are two main claims in VO2 MIT mechanisms, Peierls Transition & Mott Transition.

✔ In Mott Transition mechanism, M2 phase is main point.

✔ To find MIT mechanism in VO2, people are trying to synthesize stable M2 phase of VO2 at room temperature.

✔ There are no papers that introducing stable M2 phase of VO2 thin film without any other phase combined at room temperature.
M2 Phase
(Courtey of Mr. Minkyun Sohn at ASDNL)
• Mott Transition
• Synthesis Method
• Anti-ferromagnetism

What was found?

(1.Natuer Physics, 9, 661-666(2013) 2.Scientific Reports, 15 march (2016), 3.Nano Lett, 11, 3207-3213(2011), 4. Physical Review B 85, 020101(R) (2012))
✔ Recently, there have been many reports about strain effects in VO2. Some work focuses on strain via lattice mismatch between substrate’s lattice constant and VO2 lattice constant, while other work focuses on external strain (Pulling, Pushing, Bending).
(Pysical Review B, 94, 085105 (2016))
✔ Only few papers reported that synthesized M2 phase combined with M1(monoclinic) phase.
✔ They focus on strain over lattice mismatch between TiO2 substrate’s lattice constant and VO2 lattice constant.

Why is the project important?

✔ It is hard to synthesize stable M2 phase at room temperature
✔ Studying the M2 phase is the key information to studying the phase transition mechanism in vanadium dioxide
✔ This is why we try to synthesize stable M2 phase at room temperature without any other phase combined