Supplemental material
Tuning the optoelectronic properties of amorphous MoOx films by
reactive sputtering
André L. F. Cauduro,1,a) Zacarias E. Fabrim,2 Mehrad Ahmadpour,1 Paulo F.
P. Fichtner,2,3 Søren Hassing, 4 Horst-Günter Rubahn,1 and Morten Madsen1
1
NanoSYD, University of Southern Denmark, Alsion 2, 6400-Sønderborg, Denmark
2
PPGMicro- Graduate Program on Microelectronics, Universidade Federal do Rio
Grande do Sul, 91501-970 Porto Alegre, Brazil
3
Engineering School, Universidade Federal do Rio Grande do Sul, 91501-970 Porto
Alegre, Brazil
4
Institute of Chemical Engineering, Biotechnology and Environmental Technology,
University of Southern Denmark, Campusvej 55, 5230-Odense, Denmark
a)
Electronic mail: cauduro@mci.sdu.dk
1. Sputtering parameters
Molybdenum oxide films were deposited by DC-reactive sputtering by maintaining
the Argon partial pressure (pAr) constant at 2.44x10-3 mbar. The oxygen partial pressure
(pO2) was varied from 1.00x10-3 mbar up to 2.70x10-3 mbar by increasing the oxygen flow
into the chamber. Target soaking was performed with pure argon in the chamber for 5 min
followed by chamber stabilization prior to the deposition. The chamber was intentionally left
for at least 10 min in order to stabilize with the desired pressure and gas mixture before
opening the sample shutter. Four different sputtering powers were used in this investigation
(100 W, 150 W, 200 W and 250 W) and a graph of the deposition rate versus the sputtering
power is shown in Fig. S1(a). The deposition rate varies from roughly 0.3 Å/s at 100 W under
2.70x10-3 mbar of oxygen up to 3.6 Å/s at 250 W under 1.00x10-3 mbar of oxygen. In order
to understand more about the sputtering process, the deposition rate versus the oxygen partial
pressure was investigated thoroughly at 250 W and it is displayed in Fig. S1(b). The oxygen
flow was let into the chamber in steps of 1 sccm and it shows that at a pO 2 of about 1x10-4
mbar the deposition rate is as high as 4.70 Å/s. As the pO2 increases, a drop in the deposition
rate is observed as shown in Fig. S1(b). The decrease in deposition rate with increased
oxygen partial pressures indicates that oxygen molecules cause severe target poisoning
during the deposition process, and therefore MoOx is sputtered off the target in addition to
the metallic Mo.1,2
1
FIG. S1. Sputtering parameters used throughout the MoOx depositions showing (a) the deposition rate versus
the sputtering power and (b) the deposition rate versus the oxygen partial pressure at 250 W.
2. HRTEM analysis of the oxygen deficient as-deposited MoO2.57 sample
TEM/HRTEM analysis of the sample grown in oxygen deficient environment was
conducted in order to compare the microstructure of the thin-films grown under the extreme
cases (oxygen deficient and oxygen excess). Fig. S2(a) shows a HRTEM image of the asdeposited sample grown in an oxygen deficient environment, i.e. with an oxygen partial
pressure of 1.00x10-3 mbar. The HRTEM image reveals a similar amorphous microstructure
with lack of crystalline domains, as presented in the case of thin-films grown under oxygen
excess, i.e., with an oxygen partial pressure of 2.70x10-3 mbar. Selected Area Diffraction
(SAD) pattern is shown in Fig. S2(b) shows thick diffraction rings, typical for short-rangeorder non-crystalline semiconductors, i.e., amorphous materials.
FIG. S2. High resolution bright field TEM image of the as-deposited oxygen deficient MoOx film (pO2 =
1.00x10-3 mbar) and (b) SAD pattern indicating an amorphous structure of the film. The MoOx oxygen deficient
films were deposited on Si substrates for TEM and HRTEM analysis.
2
3. Total transmittance (%) and reflectance (%) spectra of MoOx as-deposited at 250 W
Total transmittance (%) of the MoOx films and the total reflectance (%) of the MoOx
films plus the substrates were obtained using a UV-VIS-NIR lambda 900 Perkin Elmer
spectrophotometer with a integrated sphere in order to collect both diffuse and specular
scattering. Both specular and diffuse scattering were considered for T (%) and R (%), giving
the absolute values for these measurements. Fig. S3 shows the variation of the optical
transmittance of the as-deposited films as a function of the oxygen partial pressure. The
substoichiometric MoO2.57 presents very little light transmittance, mostly due to the strong
absorption shown in Fig. 3.
FIG. S3. Absolute transmittance (%, specular + diffuse) of the as-deposited MoOx films. The stoichiometry’s
are indicated along with the oxygen partial pressures.
Absolute reflectance (%) of the MoOx films plus of the substrate (BK7 glass) are
displayed in Fig. S4. The reflectance shows very little changes with the oxygen partial
pressure, corroborating that the absorption of the films was the physical parameter that
caused the drastic changes on the optical transmittance.
FIG. S4. Absolute reflectance (%, specular + diffuse) of the MoOx films plus the substrate (BK7). The
discontinuity at 1.40 eV is an artifact of the measurement.
3
4. RBS spectra of the MoOx films as-deposited at 250 W
The RBS spectra shown in Fig. S5 indicate a change on molybdenum concentration of
about 4% between the sample grown with an oxygen partial pressure of 1.00x10-3 mbar (28%
of Mo) and the one grown with an oxygen partial pressure of 1.98x10-3 mbar (24% of Mo).
The stoichiometric MoO3 is observed at an oxygen partial pressure of 1.37x10-3 mbar.
FIG. S5. RBS measurements of the Si/MoOx/Au multilayer structures for the MoOx deposition series at 250 W.
The curves have been normalized with respect to the Au peak and fitted using SIMNRA. The Mo concentration
is estimated to be 28 at% (black solid line), 25 at% (red solid line) and 24 at% (dark blue line) for the case of the
samples grown under 1.00x10-3 mbar, 1.37x10-3mbar and 1.98.x10-3 mbar of oxygen, respectively. The gold
peak intensity has been rescaled by a factor of 5 to give a clear view of the whole spectrum, resulting in an
artificial discontinuity at channel number 425.
3. References
1
M. Sook Oh, B. Seob Yang, J. Ho Lee, S. Ha Oh, U. Soo Lee, Y. Jang Kim, H. Joon Kim,
and M. Soo Huh, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 30, 031501 (2012).
2
V. Nirupama, K.R. Gunasekhar, B. Sreedhar, and S. Uthanna, Curr. Appl. Phys. 10, 272
(2010).
4