24P008-O
Effects of N2 Gas Addition to Sputtering Plasmas
on Properties of Epitaxial ZnO Films
スパッタリングプラズマへの窒素ガス添加のエピキシャルZnO薄膜物性に対する効果
Kazunari Kuwahara, Naho Itagaki, Kenta Nakahara, Daisuke Yamashita, Seo Hyunwoong,
Giichiro Uchida, Kunihiro Kamataki, Kazunori Koga, and Masaharu Shiratani
桑原 和成 1，中原 賢太 1，山下 大輔 1，徐 鉉雄 1，
内田 儀一郎1，鎌滝 晋礼1，古閑 一憲1，白谷 正治1，板垣 奈穂1,21

2

Kyushu University, Motooka 744, Fukuoka 819-0395, Japan.
PRESTO, Japan Science and Technology Agency, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan1
九州大学〒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5

Effects of N2/Ar flow rate ratio on properties of ZnO films fabricated via nitrogen mediated crystallization(NMC) have been studied. NMC-ZnO films are deposited by RF magnetron sputtering using Ar-N2 mixedgas. X-ray diffraction (XRD) analysis shows that the crystallinity of NMC-ZnO films is improved byaddition of a small amount of N2 to sputtering atmosphere (N2/(Ar+N2) flow ratio of 8%). By using theNMC-ZnO films as homo-buffer layers, ZnO films with high crystallinity are deposited by RF magnetron
sputtering. The full width at half-maximum of XRD patterns for ω scan of (002) plane are 0.061°, being
significantly small compared with 0.49° for the films without buffer layers. A further increase in N2/Ar
flow rate ratio deteriorates the crystallinity because excess N atoms in the films disarrange the crystal
structure of ZnO. The results indicate that utilizing NMC buffer layers deposited at an adequate N2 partialpressure is very promising to obtain epitaxial ZnO films with high crystallinity.

1. Introduction
Zinc oxide (ZnO) is one of the most fascinatingoxide semiconductors with unique features. Widedirect band gap (3.3eV), high exciton binding
energy (58meV), and material abundance makeZnO a promising material for optoelectronicdevices. For realizing such devices, a fabrication
method of high quality crystalline ZnO films arerequired. We have recently demonstrated a newfabrication method of ZnO films utilizing nitrogen
mediated crystallization (NMC), where the crystal
nuclei density can be controlled because thenitrogen atoms suppress crystallization of ZnOfilms. By using NMC-ZnO films as a buffer layer,
ZnO:Al (AZO) films with low resistivity have beenobtained [1, 2]. Furthermore, we have succeeded in
epitaxial growth of ZnO films on sapphire substrateby using the NMC method [3]. Here we investigate
relationship between the film properties ofNMC-ZnO films and N2 partial pressure in order tofurther improve the quality of ZnO films.
Furthermore, the crystallinity of ZnO films
fabricated on the NMC-ZnO buffer layers aredescribed with comparison to the ZnO films
fabricated by a conventional method.

2. Experimental
NMC-ZnO buffer layers were fabricated by RFmagnetron sputtering. The used gasses were N2 andAr, where N2/(N2+Ar) flow rate ratio was ranged
from 0 to 38%. The total pressure was 0.3 Pa. ZnO
ceramic targets (2 inches in diameter; purity >99.99%) were used. The applied RF power was 100
W and the deposition temperature was 650oC. The
thickness of buffer layers was 10 nm and 1 μm,
which were confirmed by scanning electron
microscopic images. On NMC-ZnO buffer layers,ZnO films were deposited by RF magnetron
sputtering at 700oC, where Ar-O2 mixed gas wasused and the total pressure was 0.67 Pa. The applied
RF power was 60 W. The film thickness was 1 μm.
The crystal structure was examined by X-ray
diffraction (XRD) with a four-circle texture
diffractometer (Bruker AXS D8 Discover).
3. Results and discussion
3.1 Nitrogen mediated epitaxy
Figure 1 shows FWHM of (002) rocking curves
for NMC-ZnO films deposited as a parameter of N2
partial pressure. The introduction of small amountof N2 drastically improves the crystallinity of thefilms. At N2/(N2+Ar) of 8%, FWHM of XRD
pattern decreases to a minimum value of 0.073°,

FWHM (deg.)

1010.10510 15 20 25 30 35 40

N /Ar+N (%)
22Fig. 1. FWHM of (002) rocking curves of ZnO
films on NMC-ZnO buffer layers. The buffer layers
were deposited as a parameter of N2/ (N2+Ar) flowrate ratio.

3.2 Epitaxial ZnO films on NMC-ZnO buffer layers
Figure 2 shows (002) rocking curves of ZnO
films on NMC-ZnO buffer layers deposited atN2/(N2+Ar) of 8%. For comparison, the rocking
curve of ZnO films without buffer layers is also
shown in Fig. 2. There are significant differences in
the intensity and the FWHM of the diffracted peak
between with and without buffer layers. TheFWHM of ZnO films on buffer layers is 0.061o
which is much smaller than that of ZnO films
without buffer layers (0.49o). The results indicatethat utilizing NMC buffer layers deposited atadequate N2 partial pressure is very promising toobtain epitaxial ZnO films with high crystallinity.

w/ buffer layer
FWHM 0.061

Intensity (arb. units)

being significantly small compared with 0.40° forthe films fabricated without N2. A further increasein N2/Ar flow rate ratio deteriorates the crystallinity.
as a result, FWHM increases to 3.9° at N2/(N2+Ar)of 38%. In the case of high N2/(N2+Ar) flow rate
ratio (N2/(N2+Ar) > 8%), the crystalline qualities
are deteriorated since excess N atoms in the films
disarrange the crystal structure of ZnO. While in thecase of low N2/(N2+Ar) flow rate ratio (N2/(N2+Ar)< 8%), the amount of nitrogen adatoms on the film
surface during deposition is too small to suppress
crystallization of ZnO films, leading to high andrandomly oriented crystallization. Thus we haveobtained high quality epitaxial ZnO films withsmall fluctuation of the crystal orientation under theoptimum nitrogen concentration (8% in the gasmixture).

15.5

w/o buffer layer
FWHM 0.49
16

16.5

17deg.

17.5

18

18.5

Fig. 2. XRD (002) rocking curves of ZnO films on
NMC-ZnO buffer layers. FWHM of ZnO films
without buffer layers is also shown.

4. Conclusions
We report here the effects of N2 partial pressure
on properties of ZnO films fabricated via NMC. We
have shown that the N2 partial pressure strongly
influences the crystalline qualities of NMC-ZnOfilms on sapphire (001) plane. The film qualities
have been drastically improved by using
N2/(N2+Ar) of 8%. By using the NMC-ZnO filmsas homo-buffer layers, ZnO films with highcrystallinity are fabricated. FWHM of XRD
patterns for (002) rocking curves for ZnO films
with buffer layers is 0.061o, being significantly
small compared with 0.49o for the films without
buffer layers. From these results, we conclude thatutilizing NMC buffer layers at adequate N2 partialpressure is very promising to obtain epitaxial ZnO
films with high crystallinity.
References[1] N. Itagaki, K. Kuwahara, K. Nakahara, D. Yamashita,
G. Uchida, K. Koga, and M. Shiratani “HighlyConducting 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] N. Itagaki, “Novel fabrication method for oxide
semiconductors via atomic-additive mediatedcrystallization” Proc. IEEE TENCON 2010 (2010)
T2-7.2A.
[3] K. Kuwahara, N. Itagaki, K. Nakahara, D.Yamashita,
G. Uchida, K. Kamataki, K. Koga, and M. Shiratani“High quality epitaxial ZnO films grown onsolid-phase crystallized buffer layers” to bepublished in Thin Solid Films (2011).