COL 11(Suppl.), S10201(2013) CHINESE OPTICS LETTERS June 30, 2013
Pulsed DC magnetron sputtering of transparent
conductive oxide layers
Astrid Bingel
1,2∗
, Kevin F¨uchsel
1,2
, Norbert Kaiser
2
, and Andreas T¨unnermann
1,2
1
Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universit¨at Jena,
Max-Wien-Platz 1, 07743 Jena, Germany
2
Fraunhofer Institute for Applied Optics and Precision Engineering,
Albert-Einstein-Str. 7, 07745 Jena, Germany
∗
Corresponding author: astrid.bingel@uni-jena.de
Received December 10, 2012; accepted December 26, 2012; posted online May 9, 2013
A comprehensive material study of d ifferent transparent conductive oxides (TCOs) is presented. The layers
are deposited by pulsed direct current (DC) magnetron sputtering in an inline spu ttering system. Indium
tin oxide (ITO) films are studied in detail. The optimum pressure of 0.33 Pa (15Ar:2O
2
) p roduces a 300-
nm thin film with a specific resistivity ρ of 2.2×10
−6
Ωm and a visual transmittance of 81%. Alternatively,
ZnO:Al and ZnO:Ga layers with thicknesses of 200 and 250 nm are deposited with a minimum resistivity
of 5.5×10
−6
and 6.8×10
−6
Ωm, respectively. To compare the optical properties in the ultraviolet (UV)
range, th e opt ical spectra are modeled and the band gap is determined.
OCIS codes: 310.0310, 220.0220, 160.0160.
doi: 10.3788/COL201311.S10201.
Transparent conductive oxides (TCOs) are used in a
wide field of application due to their extraordinary ma-
terial properties. They combine a metal-like conduc-
tivity with a high transpa rency in the visual spectral
range. Therefore they are well-suited for the application
as transparent electrodes in organic light-emitting diodes
(OLEDs), flat panel displays, and sola r cells
[1−3]
. In the
latter case, a promising concept is the semiconductor-
insulator-semiconductor (SIS) solar cell
[4,5]
. Here, the
TCO acts as a transparent electrode and simultaneously
induces the inversion of the material. Owing to its recti-
fying properties, SIS devices can be used for light sensors
as well.
In general, indium tin oxide (ITO) films provide the
best compromise between e lec trical conductivity and vi-
sual transparency. Also, its relatively large band gap
might enable sensor applications in the ultraviolet (UV)
sp e ctral rang e . However, the high costs of indium forced
the investigation of many kinds of substitutes. A promis-
ing candidate is doped zinc oxide, especially ZnO:Al,
because the electrica l properties are meanwhile co mpa-
rable to that of ITO but it is nontoxic and cost-saving.
Another option is the use of gallium-doped zinc oxide
(ZnO:Ga).
To ensure high quality films as well as moderate pro-
duction costs, inline pulsed direct current (DC) mag-
netron sputtering is a very good deposition pr ocess for
producing thin TCO films. This letter presents the elec-
trical, optical, and structural characterization, as well
as the individual o ptimization of different TCO materi-
als. Additionally, simulations of the transmittance a nd
reflectance spec tra are carried out to obtain the optical
constants that contain information about the band gap
of the materials.
The TCO laye rs were all depos ited by pulsed DC
magnetron sputtering. The plant that was used is an
MRC903 inline sputtering system with a sputter down
geometry. The deposition was carried out dynamically
meaning that the substrate carrier was moved below
the three available target s tations with a velocity of
approximately 130 cm/min. The film thickness co uld
be varied by the number of passes. The target to sub-
strate distance amounts to 60 mm and the targets have
a size of 380×120 (mm). The sputter pulse frequency
was kept constant to 100 kHz and the operating power
was 1 500 W. Before starting the depos itio n process, the
chamber was evacuated to a bas e pressure of 9×10
−5
Pa. The process gases Ar and O
2
were inset by mass
flow controllers. A direct substrate hea ting within the
sputtering chamber was not available. However, the sub-
strates could be heated within the loadlock of the system
by halogen lamps. This provides a rapid annealing for
approximately 7 min at temperatures up to 350
◦
C before
and after the deposition.
In this letter, different TC O materials were deposited.
ITO was used with a target composition of In
2
O
3
:SnO
2
87:13 wt.-%. The do pant compound in the ZnO:Al
2
O
3
and ZnO:Ga
2
O
3
targets was 2 wt.-% Al
2
O
3
and 6 wt.-%
Ga
2
O
3
, respectively.
The electrical film characterization was done by linear
four point probe measurements as well as Hall measur e -
ments in van- der-Pauw geometry. The optical trans-
mittance and reflectance spectra were recorded using
a PerkinElmer Lambda 850 spectrometer. The s urface
morphology of the films was observed using a “ΣIGMA”
(Carl Ze iss) scanning electron microscope (SEM).
In order to achieve maximum efficiency of the opto-
electronic devices that contain the TCO layers, it is very
impo rtant to optimize at first the material itself. For
that pur pose, ITO films were deposited on 1” glass sub-
strates from a target with a co mpounding of 87 wt.-%
In
2
O
3
and 13 wt.-% SnO
2
. The film thickness was kept
constant at about 300 nm in order to ensure the com-
parability of the results, since it is well known that the
sp e cific resistivity shows a decrease with film thickness
[6]
.
The sputtering of thin ITO films without substrate heat-
1671-7694/2013/S10201(5) S10201-1
c
2013 Chinese Optics Letters