Supporting information for
Indium tin oxide/InGaZnO bilayer stacks for enhanced mobility and optical
stability in amorphous oxide thin film transistors
By Yoon Jang Chung1,2, Un Ki Kim1, Eun Suk Hwang1, and Cheol Seong Hwang1*
Affiliation
1
Department of Materials Science and Engineering and Inter-university Semiconductor
Research Center, Seoul National University, Seoul 151-742, Republic of Korea
2
Thin Film Materials Group, Korea Research Institute of Chemical Technology, 141 Gajeong-Ro,
Yuseong-gu, Daejeon 305-600, Korea
*Corresponding author: cheolsh@snu.ac.kr
Figure S1: AFM topographic images of ITO films with various thicknesses. (a), (b), (c),
and (d) correspond to ITO films with 2nm, 5nm, 9nm, and 15nm thickness, respectively.
Here, the scale bars for all cases are 200nm. As can be seen from the figures, all these films
show dense structures with extremely smooth surfaces. (e) shows line profiles taken from
arbitrary regions in each of the films. As can be seen from figure, all films show RMS
roughness values smaller than ~1nm, showing that even at extremely thin thicknesses the ITO
films exist in the form of continuous films rather than aggregates of islands.
Fig. S2. 3D AFM images of the films mentioned in figure S1. Again (a), (b), (c), and (d)
correspond to cases where the ITO films are 2nm, 5nm, 9nm, and 15nm, respectively. The
topographic features of the film is better highlighted in these images, which show typical
columnar grains found in ITO thin films. These images further support the observation that
continuous films can be grown even at extremely thin thicknesses.
Figure S3. Representative spectral data of (a) Zn 2p, (b) Sn 3d, (c) In 3d, (d) Ga 2p, and (e)
O1s peaks measured during the depth profiling of an IGZO/ITO bilayer stack film. As can be
seen from the figures, the chemical integrity of each element is sustained until the ITO layer
is reached, where the In 3d and O 1s peaks show corresponding changes accordingly. In the
case of the O 1s peak, a significant shift occurs in the peak profile at elongated etch times,
which is due to the detection of the underlying substrate SiO2 layer once both IGZO and ITO
films are etched away. As mentioned in the manuscript, these peak profiles were fitted with
Thermoscientific K-alpha software in coordination with proper relative sensitivity factors for
each peak to determine the chemical composition given in figure 1 (c).
Figure S4. (a) 4 point probe and (b) Tauc plot data on ITO films with various thicknesses. As
can be seen from (a), it is clear that there is a certain threshold (~5nm) where ITO films turn
too metallic to be suppressed by factors such as annealing. The sheet resistance of the 2nmthick film could not be measured by the 4 point probe due to their high resistivity, which also
turned out to be the case for the 5nm-thick film after annealing. This type of behavior is in
good agreement with previous reports on ultrathin ITO thin films. [1,2] The X’s in the upper
left corner of the figure denote the contact error observed for these cases. This is most likely
due to the fact that the ITO layer is an oxide material. When the film is too thin (2nm case),
contributions from the surface region are comparable to those from the bulk region, and
overall the film shows high resistance even before annealing is performed. Since surface
regions are prone to react with oxygen species, they will tend to have a lower oxygen
vacancy concentration which in turn leads to a lower carrier concentration than bulk regions,
and an increased contribution from these regions would lead to an increase in resistance. In
contrast, when the film is sufficiently thick enough (>10nm), the bulk region dominates the
film and it shows low resistance values even after annealing is performed. As can be expected,
the 5nm case shows intermediate characteristics with metallic values in its pristine state but
high resistance once it experiences annealing, which would decrease overall oxygen vacancy
concentration in the film and hence weaken metallic contributions from the bulk region and
increase high resistance contributions from the surface region at the same time. The Tauc plot
in (b) shows that there seems to be no absorbance for the ITO films at photon energies
smaller than ~3eV, supporting that the films grown here are very dense and most likely have
very little defects. Also, extrapolating the curves shows that there is almost no variance in the
optical band gap values of the films even when they are very thin.
References
[1] E. Guo, H. Guo, H. Lu, K. Jin, M. He, G. Yang, Appl. Phys. Lett., 98, 011905 (2011)
[2] S. N. Luo, A. Kono, N. Nouchi, F. Shoji, J. Appl. Phys., 100, 113701 (2006)