24G16
Low Resistive ZnO:Al Films with ZnO Buffer Layers Fabricated by Ar/N2
Magnetron Sputtering
Ar/N2マグネトロンスパッタによる低抵抗ZnO:Al膜の作製
Naho Itagaki1,2, Kazunari Kuwahara1, Kenta Nakahara1, Daisuke Yamashita1, Kunihiro Kamataki1,Hyunwoong Seo1, Giichiro Uchida1, Kazunori Koga1, Masaharu Shiratani1
板垣奈穂1,2, 桑原和成1, 中原賢太, 山下大輔1, 鎌瀧晋礼1, 徐鉉雄1, 内田儀一郎1, 古閑一憲1,白谷正治11

Kyushu University, Motooka 744, Fukuoka 819-0395, Japan.
PRESTO, Japan Science and Technology Agency, Sanbancho 5, Chiyoda-ku, Tokyo 102-0075, Japan1
九州大学システム情報科学研究院, 〒819-0395 福岡市元岡744
2JSTさきがけ, 〒102-0075 東京都千代田区3番町

2

Low resistive ZnO:Al (AZO) films with uniform spatial distribution have been obtained by utilizing bufferlayers fabricated by Ar/N2 magnetron sputtering. For 100 nm-thick AZO films, the averaged grain size ofAZO films with buffer layers is 65 nm, which is 1.8 times larger than that of the films without buffer layers.
This increase in the grain size of AZO films is due to the low grain density of buffer layers. As a result, theresistivity is drastically reduced. At the area facing the target erosion, the resistivity reduced from 2.27mΩ•cm for the films without buffer layers to 0.50 mΩ•cm for our films, consequently, the spatial
distribution of the resistivity is significantly improved. These results reveal that our method described hereis full of promise for fabrication of ZnO-based TCO materials.

1. Introduction
ZnO is a fascinating II–IV semiconductor withunique features and a wide application range. The
high electrical conductivity, high transparency tovisible light, and material abundance make ZnO
the most promising alternative to tin-doped
indium oxide (ITO) for transparent conductiveoxide (TCO). The resistivity of ZnO-based TCO,
however, is higher than that of ITO and exhibits
non-uniform spatial distribution. Since thecrystallinity of ZnO films strongly affects boththe carrier density and the mobility, it isimportant to control the crystallinity and its
spatial distribution. Recently, we have developed
a novel fabrication method of ZnO films utilizing
solid phase crystallization (SPC) via Ar/N2sputtering deposition of amorphous zincoxynitride (a-ZnON) films and subsequentannealing [1,2]. The resultant ZnO films havewell aligned crystal orientation and higher
crystallinity than the conventional sputteringfilms.
Here we utilize the SPC-ZnO films as bufferlayers for ZnO:Al (AZO) TCO films andinvestigate their effects on the crystallinity ofAZO films. Since the grain density of SPC films
is lower than that of the conventional films, thebuffer layers are expected to significantly
improve the crystal grain size of AZO films. We
also discuss the effects of buffer layers on theresistivity and its spatial distribution of AZO

films.
2. Experiment
Figure 1 shows the flow chart of the fabrication
method for AZO films on the buffer layers prepared
via SPC. The buffer layers were fabricated byannealing of a-ZnON films, which were depositedon quartz glass substrates by radio-frequency (RF)magnetron sputtering. The used targets were ZnO(2 inches in diameter). The sputtering gasses wereFig. 1. Flow chart of the fabrication method for AZO
films on the buffer layers prepared via SPC.

Ar and N2. According to X-ray diffraction (XRD)
analysis, the films were confirmed to be amorphousin nature. a-ZnON films were annealed at 200°C in
an oxygen atmosphere and were converted to ZnOfilms. In the experiments, the thickness of seedlayers was 10 nm, which was measured byspectroscopic ellipsometry and scanning electron
microscope (SEM). AZO films were deposited on
SPC buffer layers by RF magnetron sputtering withAZO targets (2wt% Al2O3). The used gas was Ar.
The substrate temperature was kept at 200°C. No
post annealing of AZO was performed. The crystal
structures of the films were examined by X-ray
diffraction (XRD) and transmission electron
microscopy (TEM). The electrical properties ofAZO films were evaluated by 4 point probe
measurements and hall-effect measurements.
3. Results and discussion
Low resistive AZO films have been obtained byutilizing SPC buffer layers. Figure 2 shows thespatial distribution of the resistivity of AZO films(100 nm thick). The resistivity is drasticallyreduced, for instance, from 1.20 mΩ•cm to 0.39mΩ•cm at the center (R = 0), which is attributed toimprovement of crystallinity of AZO films. From
the XRD measurements, it was observed that there
is a significant difference in the intensity and thefull width at half-maximum (FWHM) of ZnO (002)diffraction peak, that is, crystallinity and thecrystallite size, between with and without
SPC-buffer layers. TEM observation of the SPC
seed layers shows that this increase in grain size isattributed to the low nuclei density of buffer layers.
Furthermore, the fluctuation of the crystalline
orientation of AZO film prepared on SPC-ZnO
buffer layer is much smaller than that of the film
without buffer layer.
The SPC buffer layers also improve the spatial
distribution of the resistivity. For AZO films
without buffer layers, the resistivity at the areafacing the target erosion is 1.50 mΩ•cm, which ismuch higher than that at 60 mm away from theerosion (0.39 mΩ•cm). On the other hand, AZO
films on SPC buffer layers exhibit uniform
distribution of the resistivity of 0.51 - 0.25 mΩ•cm,respectively. These results reveal that controllingthe crystallinity at the initial stage of AZO
deposition is of great significance, especially at thearea facing the target erosion where the energetic
species impinging on the growing films can induce
a high density of nucleation with broad orientation
distributions.
From these results, we conclude that our method
described here is full of promise for fabrication ofFig. 2. Spatial distribution of the resistivity of
100-nm-thick AZO films with and without SPC
buffer layers.

ZnO-based TCO materials.
3. Conclusions
The effects of SPC-ZnO buffer layers on theproperties of ZnO-based TCO films have beenstudied. As a result, low resistive ZnO:Al (AZO)
films have been obtained. The resistivity is
drastically reduced from 1.50 mΩ•cm to 0.51mΩ•cm at the area facing the sputtering target
erosion, which is attributed to improvement ofcrystallinity of AZO films. The SPC buffer layers
also improve the spatial distribution of theresistivity. From these results, we conclude that ourmethod described here is full of promise forfabrication of ZnO-based TCO.
Acknowledgments
This work was partly supported by PRESTO
from the Japan Science and Technology Agency.
References[1] N. Itagaki, K. Kuwahara, K. Nakahara, D.
Yamashita, G. Uchida, K. Koga, and M. Shiratani,“Highly Conducting and Very Thin ZnO:Al Films
with ZnO Buffer Layer Fabricated by Solid Phase
Crystallization from Amorphous Phase”, Appl. Phys.
Express 4 (2011) 011101.
[2] K. Kuwahara, N. Itagaki, K. Nakahara, D.
Yamashita, G. Uchida, K. Kamataki, K. Koga, andM. Shiratani,“High quality epitaxial ZnO films
grown on solid-phase crystallized buffer layers” tobe published in Thin Solid Films (2011).