Influence of spatial sputterig distribution on TCO
thin film properties
V. Tvarozek1, S. Flickyngerova1, I. Novotny1, A. Rehakova1,
P. Sutta2, M. Netrvalova2, L. Prusakova2, P. Ballo3, E. Vavrinsky1
1Department
of Microelectronics, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia
2Department New Technologies Research Center, The University of West Bohemia, Univerzitni 8, 306 14 Plzen, Czech Republic
3Department of Physics, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia
Technology
Introduction
Doped ZnO is a promising transparent
conducting oxide (TCO) and a wide band-gap semiconductor
for application in thin film solar cells and various optoelectronic
devices. ZnO thin films codoped by Al or Sc prepared by RF/DC
magnetron sputtering are dependent on the deposition
conditions [1]. It was also studied an effect of substrate
position and content of oxygen on the properties of ZnO:Al
films prepared by reactive co-sputtering from Zn and Al targets
[2] or RF magnetron sputtering from ceramic ZnO + 2 wt.%Al2O3
target [3].
Corning glass substrates were placed on different positions under the target in diameter of 152.4
mm (ZnO + 2 wt.% Al2O3) or 101.6 mm (ZnO + 2 wt.% Sc2O3), distance of target-substrate holder
was 40 mm. We have got the continual change of thin film thickness (so-called deposition
profile) in one deposition run. Experimental data of normalized deposition profiles (to the
maximal value in the center) fitted very well with computer simulations based on the Knudsen
cosine law of the particle emission.
teor6''
teor4''
exp6''
exp4''
1,0
Aim
Deposition profile
To accelerate our investigation of suitable thin film
properties of ZnO:Al and ZnO:Sc, we exploit both (i) the RF
diode sputtering where the bombardment of a growing film
during deposition with energetic particles of various types
(negative ions, reflected atoms, secondary electrons) has to be
taken into consideration [4] and (ii) spatial distribution of
sputtered particles given by configuration of substrates under
the target.
0,8
0,6
0,4
0,2
-100 -80 -60 -40 -20
0
20
40
60
80 100
Substrate position x [mm]
Modeling and simulations
peripheral sample
50 000
40 000
middle sample
10 000
0
30
32
34
36
2 [degrees]
38
40
100
Transmittance [%]
80
peripheral
sample
60
40
middle sample
20
0
200
400 600 800 1000 1200
Wavelength [nm]
Examples of XRD and
optical spectra
Optical band-gap [eV]
3,10
3,05
3,00
2,95
2,90
-80 -60 -40 -20 0
20 40 60 80
Position [mm]
(b)
Electron diffraction confirms
the hexagonal ZnO:Al phase
reveals
the
preferential
orientation (001) in normal to
the film plane direction. ED is
taken at beam perpendicular
to the film plane
-2
-4
-6
160
120
80
40
0
-80
-40
0
40
Position [mm]
1
10
0
10
-1
-2
10
0
-2
-4
-6
160
120
80
40
0
-80
80
-40
0
40
Position [mm]
80
Evolution of the electrical resistivity, biaxial lattice stress
and crystallite size vs. sample position
4” ZnO:Sc
3,10
94
92
90
88
86
-80 -60 -40 -20 0
20 40 60 80
Position [mm]
3,05
3,00
2,95
2,90
-80 -60 -40 -20 0
20 40 60 80
Position [mm]
94
92
90
88
86
-80 -60 -40 -20 0
20 40 60 80
Position [mm]
Optical properties
Plan view TEM micrograph of ZnO:Al thin films. The mean
grain size is approx. 50 nm for middle sample (a) 20 nm for
peripheral sample (b)
Cross-sectional TEM
micrograph
(bright
field image) of ZnO:Al
thin film reveals the
columnar structure
-2
10
0
Optical band-gap [eV]
TEM characterization of samples ZnO:Al
(a)
-1
6” ZnO:Al
Integral transmittance [%]
ZnO:Sc
Figure shows a cut in the plane (010) for
ZnO where an atom of Zinc (large magenta
sphere) is alternated by Scandium (large
blue sphere). Small red spheres mean
oxygen. It is shown that despite equal ionic
radius of Scandium and Zinc atoms, the
first one forms larger atomic volume in the
thin layer. This in fact could induce non
zero
magnetic
moment
which
is
experimentally observed.
10
Biaxial lattice stress [GPa]
20 000
10
0
10
Integral transmittance [%]
30 000
10
1
Size of crystallite [nm] Biaxial lattice stress [GPa]
60 000
Resistivity [cm]
70 000
4’’ ZnO:Sc
Resistivity [cm]
6’’ ZnO:Al
Size of crystallite [nm]
Figure shows a cut in the plane (010) of 32
atom supercel. The purple spheres are Zinc
atoms and the dark red spheres are Oxygen
atoms. Aluminum atom migrates from initial
position along dashed black arrow to the
stable position where is situated between
the two Oxygen atoms. The atom in the
stable position is shown in blue.
Intensity (counts)
ZnO:Al
4” ZnO:Sc
Conclusion
The different spatial distribution of structural/electrical/optical properties of ZnO:Al and ZnO:Sc
thin films (more or less corresponding to deposition profiles) was observed. This effect is caused
particularly spatial distributions of both fluxes, sputtered particles and energetic species (Ar ions
neutralized at the target and reflected from it, negative oxygen ions coming from sputtered targets
and secondary electrons) and their mutual ratios, which were responsible for both opposite effects
on thin film properties: an improvement of composition (e.g. breaking-up oxide compounds of Al,
Sc dopands and to replace Zn by them in the lattice) or the degradation of structure (e.g. to cause
extended defects as intersticials, lattice expansion, grain boundaries).
ZnO:Al films growing on the periphery of substrate holder showed smaller grains and crystallite
sizes (regions of coherent x-ray scattering), high resistivity, very high compressive lattice stresses
and a remarkable decrease of optical band-gap widths.
Properties of ZnO:Sc films were not influenced considerable by different substrate position. They
showed small sizes of crystallite, low comprehensive lattice stresses, relative high resistivity and
transparency.
[1]
[2]
[3]
[4]
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Jing-Chie Lin, Kun-Cheng Peng, Hsueh-Lung Liao, Sheng-Long Lee, Thin Solid Films 516 5349 (2007)
F. Couzinie-Devy, N. Baaeau, J. Kessler, Thin Solid Films 516 7094 (2008)
V. Tvarozek, I. Novotny, P. Sutta, S. Flickyngerova, K. Schtereva, E. Vavrinsky, Thin Solid Films 515 8756 (2007)
Corresponding author: vladimir.tvarozek@stuba.sk
Special thanks to Dr. I. Vavra for TEM analyses