Transparent and conductive ZnO:Al prepared by
RF diode sputtering
K. Shtereva1), S. Flickyngerová2), P. Šutta3), M. Netrvalová3), I. Novotný2) and V. Tvarožek2)
1)
Department of Electronics, University of Rousse, Studentska 8, 7017 Rousse, Bulgaria
Department of Microelectronics, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia
West Bohemian University, New technologies – Research Centre, Univerzitni 8, 306 14 Plzen, Czech Republic
2)
3)
Motivation
Introduction
The unique material properties in combination with a great natural
abundance and low cost, make zinc oxide a promising transparent
conducting oxide (TCO) for application in thin film solar cells and
various optoelectronic devices [1, 2]. In addition it is an environment
friendly material.
Technology
 Thin
Development:
films ZnO:Al were prepared by RF
diode sputtering from ZnO + 2wt % Al2O3
target.
 high transparent and conductive ZnO:Al thin films for solar
cells and optoelectronic devices application
 It
is a plasma assisted deposition
method which involves a significant
energetic bombardment of neutral atoms,
ions and electrons on the growing film.
RF power and substrate temperature are
determining for the properties of
sputtered ZnO films [ 3 ].
Investigate correlations:
 structure
 surface morphology  electrical parameters
 optical parameters
A matter of great importance for these industrial applications is the
availability of a cost effective deposition technology. RF sputtering is
such method that offers a deposition at low temperatures, safety
advantages, and where the use of the toxic gases is avoided.
STRUCTURE
RT
200°C
100°C
300°C
Surface
morphology
(002)
(002)
(002)
(002) (101)
200 °C
200 °C
8000
400 W 600 W
 The average transmittance of ZnO:Al thin films, including
500
1500
1000
100 °C
RT
Intensity (cps)
Intensity (cps)
2000
Intensity (cps)
Optical properties
600
2500
6000
2000
100 °C
300 °C
500
400

300
200

100
300 °C
300 °C
RT
0
0
40
800 W
200 W
-110
42
-100
0
-90
-80
-70
38
40
 (degrees)
2 (degrees)
Corning glass substrate, varies from 82 ÷ 86 % with
increasing RF power
The sharp absorption edge appears at ~ 380 nm
wavelength and the enlargement of the sputter power
causes the blue shift of the cut – off wavelength
Optical bandgap Eg (~3.3 eV) widens with increasing RF
power and temperature
42
2 (degrees)
100
Corning glass
Transmittance [%]
80
Structure characterization
 XRD patterns show polycrystalline ZnO:Al thin films with a strong texture in the [001] direction perpendicular to the substrate
 2-dimensional XRD patterns display elliptic diffraction spots of identically orientated polycrystals
 The widths of azimuthal (002) line profiles (FWHM of the -scan ) decrease from 15 to 3.5° with increasing energy delivered to the
60
200 W
400 W
40
600 W
800 W
1000 W
20
growing film during the deposition
1200 W
 The up shift of the 2 with increasing RF powers and temperatures is a result of the increase of Al3+ substituents (Al3+ that
0
300
substitute for Zn2+ in the ZnO lattice) and a reduction of the interplanar distance, which changes the lattice distortion in ZnO:Al
films from compressive to tensile lattice stresses
400
500
900
1000
Wavelength [nm]
 Asymmetry of the (002) diffraction line indicates a region with heterogeneous structure at the substrate – film interface for films
grown at room substrate temperature, and completely diminished at higher substrate temperatures
 The RF power and temperature growth, result in the larger grains (growth from 60 to more than 200 nm) and better crystalline
structure (no line asymmetry)
Stress
1 + 2
[GPa]
FWHM 
[deg]
Grain size
<D>
[nm]
RT
- 5.01
14.4
57
3.9
400
RT
- 2.72
18.0
69
4.3
600
RT
- 1.36
15.7
77
4.6
800
RT
+0.04
14.1
120
6.7
800
100
+0.26
7.52
800
200
+1.29
3.26
- 0.21
8.75
25
3.3
RF
power
[W]
Substrate
temperature
[°C]
200
800
Line
(002)
(002)
600
(101)
Strain
<>x10-3
1 µm
The surface images of ZnO:Al prepared with 800 W RF power and 200ºC substrate temperature obtained by means of
SEM and AFM show a nanostructured surface
-1
n
1.0
4
H
2
0.5
-3
10
-3
n

2.0
6
1.5
3
1.0
-3
200
400
600
800
1000
1200
0


100°C
300°C
 The lowest resistivity (2 x 10-3 cm) and the highest mobility
200°C
100
200
300
10
-100
0
100
200
Hall measurement T [°C]
Substrate temperature Ts [°C]
RF power P [W]

-2
-3
10
Electrical properties
10-3 cm,
Minimum resistivity 2.6 x
as a result of the highest carrier concentration 2 x
is obtained for ZnO:Al grown at 1200 W RF power and RT
1020
cm-3
Conclusions
properties of ZnO:Al thin films were considerable
modified by RF power and substrate temperature. The RF
diode sputtering method can partially replace an influence of
substrate temperature on growing film by the increasing of
RF power, which becomes greater in the bombardment of
substrate by energetic secondary electrons and ions.
10
20
9
 The
Substrate temperature
RT
Resistivity  [ cm]
-2
10
2
20
6
-2
Mobility H [cm / Vs]
2

1.5
2.5
10
Resistivity  [ cm]
Mobility H [cm / Vs]
Resistivity  [ cm]
8
12
Concentration n [10 cm ]
H
-3
2.0
Concentration n [10 cm ]
10
and mobility 7.81
cm2/Vs,
The carrier concentration goes straightforwardly up (to 2.4 x 1020 cm-3) with increasing temperatures, as a result of the
increased Al3+ substituents into the films
ZnO:Al thin films deposited with 800W and 200ºC exhibit strong degenerated semiconductor behaviour, the resistivity
rises with the temperature at temperature-dependent Hall measurements
300
(12 cm2/Vs), carrier concentration (2 x 1020 cm-3) and
transmittance ( > 82% including the substrate) are obtained in
highly textured ZnO:Al films (widths of azimuthal line profiles
(002) has a minimum FWHM ~3.26º) prepared at high RF
power (800 W) and substrate temperature (200°C).
References
[1] Yoo J, Lee J, Kim S, Yoon K, Park I J, Dhunge S K, Karunagaran B, Mangalaraj D
and Yi J 2005 Thin Solid Films 480–481 213– 217
[2] Hüpkes J, Rech B, Calnan S, Kluth O, Zastrow U, Siekmann H and Wuttig M 2006
Thin Solid Films 502 286 – 291
[3] Tvarozek V, Novotny I, Sutta P, Flickyngerova S, Schtereva K and Vavrinsky E
2007 Thin Solid Films doi:10.1016/j.tsf.2007.03.125 (in press)