7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Fabrication of ZnO based transparent conducting oxides by sputtering
combined with solid phase crystallization : a way to meet the future demand fortransparent electrodes
Naho Itagaki, Yoshiharu Wada, Hisato Yabuta and Masaharu Shiratani
Graduate School of Information Science and Electrical Engineering, Kyushu University
e-mail: itagaki@ed.kyushu-u.ac.jp

Photovoltaic (PV) market is now expanding atunprecedented pace as we strive to climate change
impacts. However, the most widely used transparent
electrode in PV devices, In2O3:Sn (ITO), has a key
drawback that it contains rare metal and is not sufficient
to meet the future demands. ZnO based transparent
conducting oxides (TCOs) thus have regained a hugeinterest as the alternatives because of the wide band gap,
the high doping efficiencies, and most of all, the materialabundance. The resistivity of the films fabricated byconventional sputtering, however, is not low enough dueto the poor crystal quality, still limiting the replacement ofITO electrodes. Here we report a new fabrication methodthat enables us to make ZnO TCOs with low resistivity
comparable to that of ITO, where a seed layer is firstly
fabricated via solid phase crystallization (SPC), andZnO:Al electrode is then deposited on it [1,2]. SPC has
been known to be a powerful method to achieve highcrystal quality because crystal growth proceeds underthermal equilibrium conditions. Though this method was
hardly applicable to ZnO due to the difficulty in forming
the amorphous (a-) phase, we have realized it by sputter
deposition of a-ZnON film, which itself is fabricable only
under non-equilibrium condition, followed by annealing
to desorb nitrogen and to promote crystallization under athermal equilibrium condition.
We prepared 10-nm-thick seed layers on quartz glass
substrates by sputtering deposition of ZnON in Ar/N2 atroom temperature, followed by air annealing at 300–600°C. Then 30-nm-thick ZnO:Al films were depositedby sputtering on the seed layers in Ar at 200°C.
We have succeeded in fabrication of a-ZnON films
and thus in fabrication of ZnO films via solid phasecrystallization. Extended x-ray absorption fine structure(XAFS) measurements revealed that the SPC brings about
more ordered atomic arrangements in the grain interiors,causing the drastic improvement in the crystal quality. We
also found that using such films as seed layers brought
about improvement in electrical properties of ZnO:Alfilms. The resistivity of 30-nm-thick ZnO:Al films on theseed layers is low of 8×10-4 Ω·cm, 40% lower than that ofconventional ZnO:Al films. Here the seed layers werefabricated via 600°C annealing of fully-amorphous ZnONfilms.
Next, aiming to further reduce the resistivity ofZnO:Al films and to lower the SPC temperature, wesynthesize seed layers prepared via SPC of not fully
amorphous but fractionally crystallized (fc-) ZnON films,

where the degree of crystallization is controlled bycontrolling the geometric relationship between thesputtering targets and the substrates. Since the crystal
nucleation finishes during deposition and atoms do nothave to overcome a higher energy barrier to nucleate
during SPC, the new seed layers are expected to bringabout lower SPC temperature as well as to enable us tocontrol the nucleation and the subsequent crystal growth
independently.
The resistivity of ZnO:Al films on the seed layers
fabricated via 300°C anneal of fc-ZnON films issurprisingly low of 3.2×10-4 Ω·cm,. This value is fourtimes lower than that of conventional one fabricatedwithout seed layers and still 2.5 times lower than that ofabove mentioned one using SPC seed layers fabricated via
600°C anneal of a-ZnON. Atomic force microscopy
measurements revealed that the seed layers grown fromfc-ZnON have more uniform grain size distribution thanthat grown from a-ZnON have, considered to be due to theshorter period of nucleation occurring, that is, thenucleation might occur only during the film deposition as
we expected. In fact, we observed that the activationenergy of crystal growth during SPC of fc-ZnON is lowof 0.22 eV, about one half of that of a-ZnON, indicatingthat no nucleation occurred, and crystals just greworiginating from nuclei embedded in the fc-ZnON during
SPC. The resultant seed layers grown from fc-ZnON films,
therefore, possess smoother surfaces and higher crystal
quality despite the low anneal temperature, and these are
the reasons why 30-nm-thick ZnO:Al films withsurprisingly low resistivity has been realized by using thenew seed layers.
We believe our findings offer a new pathway tomeet huge TCO demands in the coming decades.
The authors are grateful to Dr. Hirosawa and Dr.
Setoyama of Kyushu. Synchrotron Light Research Center(SAGA Light Source) for XAFS measurements. This
work was partially supported by JSPS KAKENHI Grant
Numbers JP21H01372, JP21K18731, and JP22H05000.
References[1] N. Itagaki, et. al., Appl. Phys. Express 4, 011101(2011).
[2] I. Suhariadi, et. al., ACS Appl. Nano Mater. 3, 2480
(2020).