Review
Structural, optical and electrical properties of AZO/Cu/AZO tri-layer films
prepared by radio frequency magnetron sputtering and ion-beam sputtering
Tianlin Yang
a
,
b
,
*
, Zhisheng Zhang
a
, Shumei Song
b
, Yanhui Li
b
,
MaoShui Lv
b
, Zhongchen Wu
b
, Shenghao Han
b
a
School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, PR China
b
Department of Space Science and Applied Physics, Shandong University at Weihai, Weihai 264209, Shandong, PR China
article info
Article history:
Received 24 January 2008
Received in revised form 24 May 2008
Accepted 24 May 2008
Keywords:
AZO
Tri-layer films
Ion-beam sputtering
Transparent conducting films
abstract
Highly conducting tri-layer films consisting of a Cu layer sandwiched between Al-doped ZnO (AZO)
layers (AZO/Cu/AZO) were prepared on glass substrates at room temperature by radio frequency (RF)
magnetron sputtering of AZO and ion-beam sputtering of Cu. The tri-layer films have superior photo-
electric properties compared with the bi-layer films (Cu/AZO, AZO/Cu) and single AZO films. The effect of
AZO thickness on the properties of the tri-layer films was discussed. The X-ray diffraction spectra show
that all films are polycrystalline consisting of a Cu layer with the cubic structure and two AZO layers with
the ZnO hexagonal structure having a preferred orientation of (0 0 2) along the c-axis, and the crystallite
size and the surface roughness increase simultaneously with the increase of AZO thickness. When the
AZO thickness increases from 20 to 100 nm, the average transmittance increases initially and then de-
creases. When the fixed Cu thickness is 8 nm and the optimum AZO thickness of 40 nm was found,
a resistivity of 7.92 10
5
U
cm and an average transmittance of 84% in the wavelength range of visible
spectrum of tri-layer films have been obtained. The merit figure (F
TC
) for revaluing transparent electrodes
can reach to 1.94 10
2
U
1
.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Transparent conducting oxide (TCO) films are extensively used
for a variety of applications in microelectronic systems, such as
liquid crystal displays, organic light emitting diodes, and solar cells
[1–5]. Indium tin oxide (ITO) and doped ZnO thin films have been
investigated extensively because their interesting electrical and
optical properties make them suitable for many applications
[6–10]. However, the resistivity of these thin films is still high,
which limits the use as transparent electrodes for color filters in
super twisted nematic liquid crystal displayers (STN-LCDs) [11].In
order to improve the electrical and optical properties of transparent
conducting films, the tri-layer films consisting of a metal layer and
semiconductor layers have been studied, such as ZnS/Ag/ZnS [12],
ITO/Metal/ITO [13], ITO/Ag/ITO [14], ITO/Au/ITO [15], ZnO/Cu/ZnO
[16] and ZnO/Ag/ZnO [17]. For ITO/Ag/ITO (I/M/I) tri-layer films, ITO
lacks the high refraction required to reduce the strong reflection
from the Ag layer [12]. Because of the high refraction of ZnO, ZnO/
Metal/ZnO tri-layer films with high transmittance and conductivity
have been studied [16,17]. But to our knowledge, there are few
reports on AZO/Cu/AZO tri-layer transparent conducting films.
According to the parallel circuit model [13], compared with ZnO/
Cu/ZnO tri-layer films, AZO/Cu/AZO tri-layer films with further low
resistivity can be expected due to AZO layers with low resistance.
Moreover, in many reports, the metal layer was prepared by RF
magnetron sputtering or vacuum heat evaporation [12,16]. Com-
pared with ion-beam sputtering, these depositing methods are
disadvantageous in growing high quality metal layers, and the
thickness of metal layers cannot be controlled precisely. In our
work, the Cu layers were prepared by ion-beam sputtering. The
optimum thickness (8 nm) of the Cu layer for AZO/Cu/AZO tri-layer
films is obtained, which will be reported elsewhere.
In this paper, bi-layer (Cu/AZO, AZO/Cu) and tri-layer (AZO/Cu/
AZO) films were prepared on glass substrates at room temperature
by RF magnetron sputtering of AZO and ion-beam sputtering of Cu.
The bi-layer and tri-layer films were investigated comparatively.
The effect of AZO thickness on the structural, optical and electrical
properties of AZO/Cu/AZO tri-layer films was analyzed in detail.
2. Experimental details
Bi-layer and tri-layer films were deposited on glass substrates by
RF magnetron sputtering using a ceramic oxide AZO target (7.6 cm
diameter, Al
2
O
3
:ZnO ¼ 3:97 wt.%) and ion-beam sputtering using
*
Corresponding author. School of Electronic and Information Engineering,
Tianjin University, Tianjin 300072, PR China.
E-mail address: ytlin@sdu.edu.cn (T. Yang).
Contents lists available at ScienceDirect
Vacuum
journal homepage: www.elsevier.com/locate/vacuum
0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.vacuum.2008.05.029
Vacuum 83 (2009) 257–260