International lecture of Spintronics

Organized by Graduate Program in Spintronics (GP-Spin) andERATO Spin Quantum Rectification (ERATO-SQR)“Control of phonon transport byformation of Al2O3 interlayer in
Al2O3-ZnO superlattice thin films”
SPEAKER: Professor Dr. Sang-Kwon Lee

Department of Physics, Chung-Ang University, Seoul, Republic of Korea

January 16 (Tue.) 2018
14:00～15:00

Venue:IMR International Center of Educational
Research, 2nd floor, seminar room 1, KatahiraCampus
金属材料研究所国際教育研究棟2階セミナー室１

Contact
ERATO-SQR HQ sqr-erato@wpi-aimr.tohoku.ac.jp Phone: 022-217-6238
Presentation will be conducted in English. Registration not required.

Control of phonon transport by formation of Al 2 O 3 interlayer in Al 2 O 3 ZnO superlattice thin films
Presenting Author: Prof. Sang-Kwon Lee
Department of Physics, Chung-Ang University, Seoul 06974, Republic of Korea

Abstract
Recently, significant progress has been made in increasing the figure-of-merit (ZT) of various
nanostructured materials, including thin-film and quantum dot superlattice structures. Studies havefocused on the size reduction and control of the surface or interface of nanostructured materials sincethese approaches enhance the thermopower and phonon scattering in quantum and superlattice
structures. Currently, bismuth-tellurium-based semiconductor materials are widely employed forthermoelectric (TE) devices such as TE energy generators and coolers, in addition to other sensors, foruse at temperature under 400 K. However, new and promising TE materials with enhanced TEperformance, including doped zinc oxide (ZnO) multilayer or superlattice thin films, are also requiredfor designing solid-state TE power generating devices with a maximum output power density and forinvestigating the physics of in-plane TE generators. In my talk, I will present an experimental
observation and coherent phonon transport characteristics the Al 2 O 3 /ZnO (AO/ZnO) superlattice thinfilms, which were prepared by atomic layer deposition (ALD), and the evaluation of their bothelectrical and photon transport properties at the temperature range from 40 to 300 K. The
measurement setup and technology for the in-plane thermoelectric (TE) properties of the thin films
was also presented. All the in-plane TE properties, including Seebeck coefficient (S), electricalconductivity (σ), and thermal conductivity (κ) of the AO/ZnO superlattice (with a 0.82-nm-thick AOlayer) and AO/ZnO film (with a 0.13-nm-thick AO layer) were evaluated in the temperature range
from 40 to 300 K, and the measured S, σ, and κ were -62.4 and -17.5 µV/K, 113 and 847 (Ω⋅cm)-1,and 0.96 and 1.04 W/m⋅K, respectively, at 300 K. Consequently, the in-plane TE ZT factor ofAO/ZnO superlattice films was found to be ~0.014, which is approximately twice enhanced than thatof AO/ZnO films (ZT of ~0.007) at 300 K. At the end of my talk, I will also present our on-going
research works, including out-of-plane phonon transport and their energy generators of these materials.
In this topic, we, for the first time, directly measured all cross-plane thermal electric properties,
including thermal conductivity, electrical conductivity, and Seebeck coefficient, of the AO/ZnO
superlattice films in the temperature range from 80 to 500 K using newly presenting technique using
an integration of sandwiched superlattice films between two separate heaters as a heating source andelectrodes with two Cu foils. From cross-plane TE measurement, 6-cycled AO/ZnO superlattice films
show the best results among the samples. Our finding in ZT of ~0.45 at 500 K for AO/ZnO
superlattice films present the best results among previously reported studies in the same field of thematerials with high energy conversion efficiency of ~8.4%.