1. FTIR Ellipsometry Study on RF sputtered Permalloy-Oxide Thin Films
Md Abdul Ahad Talukder1
, Yubo Cui1
, Maclyn Compton1
, Wilhelmus Geerts1
, Luisa Scolfaro1
,
Stefan Zollner2
1
Department of Physics, Texas State University, San Marcos, TX 78666
2
Department of Physics, New Mexico State University, Las Cruces, NM 88003
ABSTRACT
The optical properties of RF sputtered polycrystalline permalloy oxide (PyO) thin films were
studied in the infrared by variable angle ellipsometry. The dispersion of PyO shows a Lorentzian
dispersion peak at 381.5 cm-1
. We attribute this peak to the transverse optical phonon of PyO.
This peak is consistent with a rocksalt crystal structure for the Ni0.81Fe0.19O1- thin films.
INTRODUCTION
The transition metal-oxides of Ni [1] and Fe [2] are currently being investigated for
application in radiation-hard, low-energy, high-speed logic and resistive RAM devices (RRAM).
Combining both oxides in a single device might allow in a mean to adjust atom mobility,
morphology and texture, to optimize switching properties. We therefore investigate the
properties of Fe doped NiO [3]. This material maintains the rocksalt crystal structure up to about
2 at. % at atmospheric pressure [4,5]. At lower oxygen pressure the solubility of Fe increases
though up to 40 at. % at 1000o
C [4]. Dual ion beam sputtered permalloy-oxide (PyO) thin films
were shown to have a rocksalt crystal structure [6]. The texture of thin oxygen-depleted doped
NiO films is poor and very sensitive on deposition parameters such as oxygen flow rate,
substrate temperature, and deposition power, and often only one diffraction peak is observed in
an XRD 2 scan. Therefore in this paper we report on the far infrared phonon spectrum of RF
sputtered PyO films to confirm the crystal structure found by XRD.
EXPERIMENTAL PROCEDURE
Fused quartz purchased from Pella and SiO2 covered Si wafers that were roughened at the
back with a bead-blaster were used as substrates. Prior to loading them into an AJA sputter
system they were cleaned ultrasonically in water, acetone, and IPA. Deposition was done by
reactive RF magnetron sputtering (240 Watt) from a Permalloy target (Ni0.81Fe0.19) using a
sputter gas of 80% Ar and 20% O2 (p=10-3
Torr). The films were deposited at different substrate
temperatures using a heater powered by a halogen lamp. The heater was switched off
immediately after deposition and then let to cool down in the vacuum system over several hours.
The time it took for the 588o
C and 513o
C samples to cool down below 400o
C was less than 5 and
3 minutes. X-ray powder diffraction patterns were measured with a Panalytical Empyrean X-ray

2. diffractometer. Films deposited at room temperature on glass confirm the rocksalt crystal
structure with equal height (111) and (200) peaks and smaller but visible (220), (311), and (222)
peaks. At elevated deposition temperatures the films show a predominant (200) texture. The
(200) peak height and width (~0.75o
) was scattered with deposition temperature and sensitive to
the substrate cleaning procedure indicative of weak crystallinity. The sample’s chemical
composition was determined with EDAX in a FEI Helios Nano Lab 400 SEM. EDAX spectra
were measured at different electron beam energies and angles and compared to spectra calculated
with McXRayLite [7]. Adjustments were made to the model’s film thickness and concentration
until the calculated spectra were similar to the measured ones. The Ni to Fe atomic ratio was
similar to the target’s concentration and the O concentration was estimated to be less than 50%
indicating that our samples are surely not stoichiometric and contain a significant amount of O-
vacancies in addition to the Fe dopant. The samples and their substrates were characterized by
ellipsometry from 200 nm to 40 m using various Woollam ellipsometers. The  and  spectra
(250-1000 nm) measured at five different angles of incidence as determined with an M2000
ellipsometer were used to determine the film thickness and optical properties of the PyO. The
thickness was approximately 70-80 nm for all samples consistent with the deposition rate
measured by the crystal thickness monitor (1.2 Å/sec) and separate experiments performed with
a stylus profilometer. The IR ellipsometric quantities  and  were measured with a Woollam
FTIR-VASE rotating compensator ellipsometer at four angles of incidence from 0.03 to 1 eV.
Data were taken at 1466 different photon energies. The incident light was polarized at 45o
. The
total measurement time per sample was 3 hours.
DATA ANALYSIS
The measured ellipsometric quantities of the bare substrates were analyzed in CompleteEase
to determine the IR-optical properties of the SiO2. Two different approaches were used to
determine the optical dispersion of the SiO2: a wavelength by wavelength fit and a fit consisting
of 7 general oscillator peaks: 1 Lorentz and 2 Gaussian oscillators to describe the Si-O-Si
rocking mode peak, 1 Gaussian oscillator to describe the Si-O-Si bending mode, and 3 Gaussian
oscillators to describe the Si-O-Si stretching mode. Both approaches gave similar results for the
optical properties with the general oscillator model having small systematic errors near the base
and top of the various phonon peaks. The wavelength by wavelength model resulted in a more
noisy dispersion. Figure 1 shows the 1 and 2 of the quartz substrate with phonon peaks at 453,
807 and 1073 cm-1
. This data is consistent with literature [8]. The position of the low energy and
high energy phonon peaks of the oxide grown on silicon are slightly red and blue shifted
compared to the literature values (phonon peaks at 449, 807, and 1082 cm-1
) [8].
To determine accurate phonon energies from ellipsometric data on the PyO films, the
dielectric function of the PyO was modeled by a Lorentzian oscillator:
 
EiBrEE
EBrA
E
pp
ppp

  22
 (1)

3. Where  is the static dielectric constant, Ap is the peak height, Brp is the peak width and Ep is
the peak position. Although the data on some of the samples suggested that more than just a
single Lorentz oscillator would be necessary to describe the dielectric function in the IR, the
remaining unexplained features in the spectra after modeling with a simple Lorentz oscillator
were small and not present in all samples. The data between 0.03 and 0.5 eV were used for the fit
using the NCS fit weight. We assumed no surface roughness and used the ellipsometry thickness
determined from the NIRVISUV spectra. Figures 2 and 3 show the measured (colored solid) and
calculated (black thin solid) ellipsometry spectra assuming the PyO film has no dispersion in the
infrared. In the low-energy range below 0.09 eV, the resemblance is poor, because infrared
phonon absorption in PyO has been neglected. Our ellipsometry experiment clearly is very
sensitive to the phonon features in PyO. To obtain a better fit we assumed that a phonon would
modify the optical properties in the far infrared as described by equation (1) given. The dashed
lines in Figures 2 and 3 show the model data. The agreement with the measurement data (colored
solid) is good, suggesting that a phonon introduces a Lorentzian dispersion in the far infrared.
Figure 1: Optical Properties of SiO2 determined from measurements on fused quartz substrates.
Figure. 2: Psi as a function of angle of incidence for 513o
C PyO film on Si/SiO2(103nm).
-5
0
5
10
15
-10
-5
0
5
10
0.03 0.06 0.09 0.12 0.15 0.18
Epsilon2
Epsilon1
Energy [eV]
rocking
mode
stretching
mode
bending
mode
0
5
10
15
20
25
0.03 0.06 0.09 0.12 0.15 0.18
Psi[degrees]
Energy [eV]
60 degr.
65 degr.
70 degr.
75 degr.

4. Figure. 3: Delta as a function of angle of incidence for 513o
C PyO film on Si/SiO2(103nm).
The results of the data analysis are summarized in Tables I (PyO on quartz) and II (PyO on
Si/SiO2(103nm)). The 1st
column of Table I provides the PyO phonon position using the SiO2
dispersion determined from a wavelength by wavelength fit of the quartz substrate’s  and 
spectra. The 2nd
column provides the calculated PyO phonon position, width, and amplitude
using the SiO2 dispersion determined from a general oscillator fit of the quartz substrate’s  and
 spectra. For the PyO on Si/SiO2 films, the phonon peak properties were calculated using the
oscillator model of the fused quartz substrate (Table II column 2) and using an oscillator model
based on the  and  spectra of a Si/SiO2 substrate (Table II column 3).
The phonon peak position for the samples sputtered on fused quartz depends on the type of
model used for the SiO2. The general oscillator model for the SiO2 material shows a small bump
on the low energy site of the low energy phonon. Although this bump lowered the total MSE of
the SiO2 fit, a careful observation of the results show that there is no experimental data in that
wavelength range that justifies the existence of the bump. The bump causes the PyO phonon
peak to red shift approximately 2 cm-1
as shown in the 2nd
column of Table I. The bump has less
effect on the calculated PyO phonon frequency of the samples sputtered on the Si/SiO2 wafers as
the substrate is mainly consisting of Silicon (Table II column 2). We therefore position the
phonon peak of our samples near 381.5 cm-1
. This is consistent with the calculations done using
a more advanced general oscillation model for SiO2 that includes two GLAD oscillators. These
calculations were done using the WVASE32 software at Sandia National Lab where the FTIR
ellipsometry measurements were performed. The dependence of the phonon peak position on
deposition temperature is currently not understood. Note from Tables I and II that there is a
strong correlation between the positions of the phonon peak of the films sputtered at the same
temperature. Another thing we noticed in our experimental data is that although the peak width
and peak height vary quite a bit among samples sputtered at different temperatures, their product
seems to be almost constant.
120
130
140
150
160
170
180
190
0.03 0.06 0.09
Delta[degrees]
Energy [eV]
60 degr.
65 degr.
70 degr.
75 degr.

5. Table I. Calculated PyO Phonon of RF sputtered PyO on fused quartz.
Tg SiO2 modeled with WvlByWvl
fit on quartz substrate (Ep, Brp)
SiO2 modeled by 7 peak general oscillator model
on quartz substrate (Ep, Brp, Ap, MSE, EpBrp)
588o
C 376.7 cm-1
, 40 cm-1
376 cm-1
, 60 cm-1
, 56, 23, 3360 cm-2
513o
C 384.7 cm-1
, 54 cm-1
382 cm-1
, 77 cm-1
, 45, 33, 3465 cm-2
433o
C 384.6 cm-1
, 70 cm-1
382 cm-1
, 86, cm-1
, 37, 18, 3182 cm-2
330o
C 381.3 cm-1
, 57 cm-1
379 cm-1
, 74 cm-1
, 43, 17, 3182 cm-2
167o
C 377.46 cm-1
, 43 cm-1
376 cm-1
, 92 cm-1
, 37, 16, 3404 cm-2
24o
C 382 cm-1
, 88 cm-1
376 cm-1
, 115 cm-1
, 28, 14, 3220 cm-2
average 381.12 cm-1
, 58.7 cm-1
378.5 cm-1
, 84 cm-1
, 41, xx, 3302 cm-2
Table II. Calculated PyO Phonon of RF sputtered PyO on Si/SiO2 (103nm).
Tg SiO2 modeled by 7 peak general
oscillator model on quartz substrate (Ep,
Brp, Ap, MSE, EpBrp)
SiO2 modeled by 7 peak general
oscillator model on Si/SiO2 substrate
(Ep, p, Ap, MSE, EpBrp)
588o
C 380.5 cm-1
, 112 cm-1
, 30, 15, 3371 cm-2
381cm-1
, 110 cm-1
, 30.4, 10, 3344 cm-2
513o
C 386 cm-1
, 72 cm-1
, 44, 15, 3168 cm-2
386 cm-1
,71 cm-1
, 44, 7, 3124 cm-2
433o
C 384 cm-1
, 72 cm-1
, 43, 13, 3096 cm-2
384 cm-1
, 72 cm-1
, 43, 5, 3096 cm-2
330o
C 383 cm-1
, 86 cm-1
, 37.6, 15, 3234 cm-2
384 cm-1
, 85 cm-1
, 37.9, 7, 3222 cm-2
24o
C 378cm-1
, 54 cm-1
, 17, XX, 3078 cm-2
379 cm-1
, 59 cm-1
, 55, 9, 3245 cm-2
average 382.3 cm-1
, 80 cm-1
, 42, xx, 3189 cm-2
382.8 cm-1
, 79 cm-1
, 42, xx, 3206 cm-2
CONCLUSIONS
We assign the Lorentzian peak at 381.5 cm-1
to the TO phonon of PyO. It is red shifted
compared to the TO phonon peak of single crystalline NiO [9,15] but has a higher frequency
than the TO phonon of rocksalt FeO [10]. The FTIR-phonon peaks of various transition metal
oxides are listed in Table III. All are inconsistent with the measured peak at 381.5 cm-1
,
confirming the rocksalt crystal structure of RF sputtered PyO thin films. Taking a weighted
average of the bunsenite and wustite phonon peaks, we expect the PyO phonon to be at 385-377
cm-1
which is in agreement with our results. The picture of a weighted average is consistent with
the theoretical calculations of Fe doped CoO by Wdowik et al. [11]. They used first principle
methods to calculate the effect of Fe on the lattice dynamics and found Fe to introduce locally its
own force constant splitting TO into modes corresponding to oxygen vibrating around Co and
Fe. The calculated vibration frequencies near Fe and Co are similar to the TO phonon
frequencies in wustite and Co-II-oxide. A model assuming the PyO dispersion in the far-infrared
as described by an effective medium approximation of two Lorentz oscillators at 390 and 325
cm-1
lowered the MSE of the fit, from 17 to 12 for the 588o
C sample on Si/SiO2. At this moment
it is not clear whether or not this improvement is due to the large measurement noise in the IR or
indicates evidence for the presence of both phonon peaks.

6. Table III. FTIR active Phonon peaks of various nickel and iron oxides (s=strong, w=weak).
Material Formula Phonon [cm-1
] reference
Magnetite Fe3O4 340 (w), 450 (w), 560 (TO,s) [12]
Hematite -Fe2O3 385 (w), 436 (TO,s)-459 (LO, w), 526 (TO, s) [13]
Maghemite -Fe2O3 440 (TO, w), 546-547 (TO, s) [13]
Nickel Ferrite NiFe2O4 438 (s), 676.2 (s), 339.4, 377 [14]
Bunsenite NiO 390-401 (TO, s) [9,15]
Wustite FeO 325 (TO,s), 534.7 (LO,w) [10]
ACKNOWLEDGMENTS
Work at Texas State was funded by DOD (HBCU/MI grant W911NF-15-1-0394) and work
at NMSU by the National Science Foundation (DMR-1505172). This work was performed, in
part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated
for the U.S. Department of Energy (DOE) Office of Science. MT and YC acknowledge financial
support from the Graduate College of Texas State University.
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