[Title] Amorphous semiconducting oxides; from the fundamental perspective
[Speaker] Prof. Seungwu Han
[Photo]
[Affiliation] Department of Material Science and Engineering, Seoul National University
[Contact] Building 33, room 319, Seoul National University
[Tel] +822-880-1541
[Fax] +822-880-1541
[Email]
[Curriculum Vitae]
[Education]
> 2000 Ph.D., Seoul National University (Physics)
> 1998 M.S., Seoul National University (Physics)
> 1996 B.S., Seoul National University (Physics)
[Positions]
> Postdoctor, Center for Strongly Correlated Materials Research, Seoul National University, 2000-2001
> Postdoctor, Princeton Materials Institute, Princeton University, 2001-2003
> Full-time lecturer, Dept. of Physics, Ewha Womans University, 2003-2004
> Assistant Professor, Dept. of Physics, Ewha Womans University, 2004-2008
> Associate Professor, Dept. of Physics, Ewha Womans University, 2008-2009
> Associate Professor, Dept. of Materials Science and Engineering, Seoul National University, 2009-2013
> Professor, Dept. of Materials Science and Engineering, Seoul National University, 2013-
[Research Interests]
> Ab initio material modeling, amorphous semiconducting oxides, organic electronics, high-throughput material screening
[Abstract]
There have been growing interests in transparent semiconducting oxides (TSOs) such as ZnO, In2O3, and Ga2O3 and their compound structures in crystalline or amorphous structures due to their optical transparency and high electrical conductivity, enabling various applications toward optoelectronic devices. Recently, studies on amorphous compounds of TSOs such as InGaZnO4 (IGZO) have been attracting a great deal of interests due to their application as high-mobility channel layer in TFT. Even though many researches have emphasized that the spherical symmetry and the overlap of metal s orbitals are primarily responsible for the high electron mobility, full microscopic understanding on the electronic structure and transport mechanisms has not been revealed yet.
In this presentation, we report a tight-binding (TB) calculation to reveal the microscopic origin of the conduction bands of TSOs. We explicitly consider oxygen s and p orbitals in addition to metal s orbitals. The result of TB model shows that the conduction bands of TSOs are depicted by the dispersion relation of massive Dirac particles (E= √(ε^2+γ^2 k^2 )) and the contribution of oxygen p orbital becomes progressively significant with increasing energy. Considering TB result, we evaluate the electron mobility of single crystalline ZnO employing Boltzmann transport equation. It is found that the mobility in ZnO based on the dispersion relation of massive Dirac particle has a minimum value at the carrier density of 1019cm-3 which is in good agreement with the previous experiment. From the detailed analysis, the upshift at higher carrier densities is a result of the increase in the group velocity and the suppressed electron scattering that is characteristic of the linear band. [1] In addition, we also evaluate the Hall mobility of single crystalline IGZO (c-IGZO). We note that Ga and Zn randomly occupy the same crystallographic sites in c-IGZO. In order to obtain the scattering rates determined by the cation disorder (CD), we employ the virtual crystal approximation (VCA) in which the c-IGZO is treated as a periodic system with the random perturbation at the Ga/Zn sites. Besides the CD scattering, ionized impurity and polar optical phonon scattering are also considered in computing the Hall mobility. It is revealed that the calculated Hall mobility is mainly limited by CD scattering. Furthermore, the increases of the Hall mobility with temperature and carrier density are found, in good agreement with the experimental data. The microscopic origin for these counter-intuitive behaviors are well explained by the underlying band structures.
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