ECASIA special issue paper
Received: 22 August 2011
Accepted: 20 December 2011
Published online in Wiley Online Library: 7 February 2012
(wileyonlinelibrary.com) DOI 10.1002/sia.4847
Combined XPS and first principle study of
metastable Mg–Ti thin films†
I. J. T. Jensen,a* O. M. Løvvik,a,b H. Schreuders,c B. Damc and S. Diplasb,d
X-ray photoelectron spectroscopy (XPS) was employed to investigate Mg80Ti20 thin film samples prepared by magnetron
sputtering and density functional theory (DFT) calculations were performed on atomistic models with similar stoichiometry.
As Ti is known to be immiscible in Mg, the microstructure and atomic distribution of Mg–Ti thin films are not fully understood.
In this work, it was shown by DFT calculations that the density of states (DOS) depends strongly on whether Ti is arranged in
nano-clusters or if it is distributed quasi-randomly. The calculated DOS was compared to valence band spectra measured by
XPS, as a new way of indirectly probing short-range order of such thin films. The XPS results of Mg80Ti20 were found to
correspond best with the DOS calculated for the nano-cluster model, supporting the view that Ti forms small clusters in such
sputtered thin films. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: XPS; valence; DFT; density of states; thin films; metastable
Introduction
Thin films composed of Mg–Ti with hydrogen exhibit interesting
optical and electrical properties. Possible applications range
from coatings on solar collectors and smart windows to optical
hydrogen sensors and semiconductor devices. In order to utilize
this potential, a detailed understanding of their electronic structure
is important.
Ti is immiscible in Mg, but the two elements may be joined
together by non-equilibrium synthesis procedures like magnetron
sputtering of thin films.[1–3] Since the material in such films often lack
long-range order, it is difficult to identify their microstructure and
local crystal structure with standard methods. Previous investigations
of Mg–Ti–H thin films by Rutherford backscattering spectrometry,
X-ray diffraction (XRD) and transmission-electron microscopy
indicated the presence of a single phase in such films,[2,3] in spite of
the general tendency of Mg and Ti to phase separate. It was
concluded that if Ti had segregated by precipitation from Mg–Ti solid
solution, it must have formed small particles with sizes below the
XRD detection limit. Evidence for such chemical short-range ordering
was later found from simulations of optical isotherms obtained by
the hydrogenography technique[4] and further verified by a combination of XRD and extended X-ray absorption fine-structure spectroscopy.[5] Density functional theory (DFT) calculations confirmed that
the formation enthalpy per Ti atom was indeed lowered by up
to ~0.5 eV when Ti was arranged in nano-clusters rather than distributed quasi-randomly.[6]
As a continuation of our previous work,[6] the present study
compares calculated DOS to valence band spectra measured by
X-ray photoelectron spectroscopy (XPS). This is a new way of
indirectly probing the short-range order of sputtered thin films.
in 0.3 Pa of Ar, on single-crystal Si(100) substrates. The substrate
was kept at room temperature. In order to obtain homogenous
films the substrate was continuously rotated during sputtering.
The typical deposition rate was 2.2 Å/s at 150 Watt RF for Mg and
0.42Å/s at 131 Watt DC for Ti. A Pd (99.98%) caplayer of 1.5 nm was
sputtered with a rate of 0.58 Å/s at 25 Watt DC to prevent oxidation.
The samples were investigated by XPS using a KRATOS AXIS
ULTRADLD instrument with monochromatic Al Ka radiation
(hn =1486.6 eV) operated at 15 kV and 15 mA. The Pd layer was
removed by sputtering an area of 2 mm x 2 mm with a 500-V Ar + ion
beam delivering 100 mA of current. To minimise cratering effects and
avoid detection of Pd, the measurements were done using a ‘small
spot’ aperture with a 110-mm diameter. High-resolution spectra were
acquired with pass energy 20 eV and dwell time 200 ms. To monitor
the possible effect of oxidation during measurement, the data was
collected in 10 consecutive scans of about 20 min each which were
then added together to obtain the adequate signal-to-noise ratio.
An increasing amount of MgO was observed, but the change in the
valence spectrum did not seem to fundamentally alter the region
which was compared to the calculated data as it occurred at a higher
binding energy. Data processing was done using CasaXPS.[7]
DFT calculations at the PBE-GGA level[8] were performed for a
Mg81.25Ti18.75 composition using the Vienna Ab-Initio Simulation
* Correspondence to: I. J. T. Jensen, Department of Physics, University of Oslo, P/O
box 1048 Blindern, 0316 Oslo, Norway. E-mail: i.j.t.jensen@fys.uio.no
†
Paper published as part of the ECASIA 2011 special issue.
a Department of Physics, University of Oslo, P/O box 1048 Blindern, 0316 Oslo, Norway
b SINTEF Materials and chemistry, P/O box 124 Blindern, 0314 Oslo, Norway
Methodology
986
200 nm thick Mg80Ti20 films covered with 1.5 nm of Pd were
deposited in a UHV system (base pressure = 10–6 Pa) by DC/RF
magnetron co-sputtering of Mg (99.95%) and Ti (99.999%) targets
Surf. Interface Anal. 2012, 44, 986–988
c Materials for Energy Conversion or Storage (MECS), DelftChemTech, Faculty of
Applied Science, Technical University Delft, P/O Box 5045, NL-2600 GA Delft,
The Netherlands
d Center for materials science and nanotechnology, P/O box 1126 Blindern, 0318
Oslo, Norway
Copyright © 2012 John Wiley & Sons, Ltd.
Combined XPS and first principle study of metastable Mg–Ti thin films
Package.[9,10] The technical details can be found elsewhere.[6] To
obtain a composition and size comparable to the experimental
sample, a 4x4x2 Mg unit cell was used, containing 64 atoms.
Parallel calculations were performed for Ti distributed quasirandomly and arranged in nano-clusters. In the quasi-random
calculations, the Ti atoms were distributed in the cell at random,
while the model of segregated Ti was constructed by using sheets
of four Ti atoms stacked together in the a direction. Structural
optimizations were performed using the standard hexagonal
structure of Mg as a starting point. Volume, lattice parameters
and atom positions were allowed to change simultaneously. For
comparisons with the XPS spectrum the calculated DOS was
scaled using photoionization cross sections from Yeh & Lindau.[11]
Results and discussion
DFT calculations of the density of states (DOS) revealed significant
differences for the two arrangements of Ti in Mg. In the quasirandom model, the calculated DOS had one dominant peak
centred around the Fermi level, while the nano-cluster model gave
two peaks, one on each side. Fig. 1 shows the total DOS for each
model, along with total DOS for Mg and Ti separately. In both models, the dominant contribution to the total DOS is the Ti d orbital
(not shown). Comparing the total DOS of Ti in the Mg–Ti mixture
to the calculated DOS of pure Ti, it becomes clear that in the
nano-cluster environment, Ti resembles elemental Ti, while in the
quasi-random arrangement, the Ti DOS is significantly distorted.
This corresponded well with our previous work,[6] where it was
found to be energetically favourable for Ti to segregate into
nano-clusters; which allowed it to simulate elemental state. The difference between quasi-random and nano-cluster is much less pronounced for Mg. The location of the valence peak compared to the
Fermi level is related to the charge transfer between Mg and Ti:
When Ti is distributed quasi-randomly, it attracts charge from Mg,
Figure 2. The calculated density of d states for compounds with increasing
amount of Ti compared to elemental Ti.
which results in the peak centred at 0 eV. Ti in nano-clusters,
however, forms Ti-Ti bonds which bind the electrons more tightly
and results in a peak centred below 0 eV. In Fig. 2, the d states
of three different compositions, Mg81.25Ti18.75, Mg93.75Ti6.25 and
Mg98.44Ti1.56, are shown together with elemental Ti. The Mg98.4Ti1.56
composition, with only one Ti atom in the 64 atom unit cell is totally
without features. The Mg81.25Ti18.75 composition, with 12 Ti atoms,
resembles the elemental Ti, as already discussed. The Mg93.75Ti6.25
composition, with 4 Ti atoms, has similar features, but is shifted
about 0.5 eV towards higher energies. This demonstrates again that
even for such small clusters, Ti is able to form states similar to those
of pure bulk Ti. The difference between one Ti atom alone and just
a few Ti atoms in a nano-cluster is pronounced.
The striking differences between the calculated DOS of the nanocluster and quasi-random models encouraged a direct comparison
between the calculated results and the XPS valence spectrum of a
Surf. Interface Anal. 2012, 44, 986–988
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/sia
987
Figure 1. Calculated DOS for the nano-cluster (top) and quasi-random model (bottom) for composition Mg81.25Ti18.75. The local DOS for Mg (right)
and Ti (left) are compared to the total DOS and the elemental states.
I. J. T. Jensen et al.
real sample. Even if it is well-known that DFT calculated spectra at
this level of theory (GGA) generally are only qualitatively matching
experimental data (errors can be in the eV range), this could be sufficient to distinguish important features in the experimental spectra. It should be noted that collecting such data from the Mg–Ti
sample was not without practical challenges. The rapid oxidation
of Mg and Ti required the deposition of a Pd capping layer, which
had to be sputtered away in situ. Even in UHV, the sample oxidized
during data acquisition. The experiment setup was aiming to minimise the effect of these obstacles as explained in the methodology
section. Fig. 3 shows the Mg 2p and Ti 2p spectra. Both Mg and Ti
were clearly detected in the sample, with some presence of oxide
seen on the high binding energy side of the peaks. Fig. 4 shows
the calculated results together with the XPS valence spectrum,
which has a peak located at approximately 1 eV. The resolution
of the experimental spectrum is much inferior to that of the calculated data due to instrumental effects. Moreover, the signalto-noise ratio of the XPS data is limited by the acquisition time,
and the inevitable presence of oxides will serve to broaden out
single phase features of the valence band spectrum. Bearing
these conditions in mind, the experimental data still seems to
follow the DOS calculated for nano-clusters of Ti, rather than
the quasi-random one. This is mainly manifested by the presence of a DOS maximum at ~1 eV below the Fermi level in very
close agreement with the experimental data. Obviously the
XPS spectrum will only show states below the Fermi level, but
for a peak located exactly at the Fermi level a steeper decline towards 0 eV should be expected.
Conclusions
Figure 3. Ti 2p (left) and Mg 2p (right) XPS core level spectra for Mg80Ti20.
In this work, DFT calculations showed that arranging Ti in nanoclusters within Mg leads to significant differences in the DOS as
compared to Ti being distributed quasi-randomly in Mg. In both
cases, the main contributor to the total DOS was the Ti d state.
When Ti was allowed to segregate into nano-clusters in Mg,
the Ti DOS started to resemble that of pure Ti, while in the
quasi-random arrangement, the Ti DOS was severely distorted.
The calculated DOS was compared to valence band spectra
measured by XPS, as a way of investigating the microstructure
of Mg–Ti thin films. The XPS results were found to correspond
best to the DOS calculated for the nano-cluster model, further
supporting the view that Ti forms small clusters in such sputtered
thin films.
References
Figure 4. Total DOS calculated for the nano-cluster and the quasi-random
model for Mg81.25Ti18.75 together with the experimentally obtained XPS
valence spectrum for Mg80Ti20.
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Surf. Interface Anal. 2012, 44, 986–988