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 Cruses, NM 88003
The optical properties of RF sputtered
polycrystalline permalloy oxide (PyO:
Ni0.8Fe0.2O1-δ) thin films were studied in
the infrared by variable angle
ellipsometry. The IR- dispersion of PyO
shows a Lorentz-like dispersion peak at
381.5 cm-1
. We attribute this peak to
the transverse optical phonon of PyO.
Abstract
Resistive Random Access Memory is a
new type of non-volatile memory device.
The RRam is based on reversible soft
breakdown of an oxide layer. NiO is
proposed as a RRam switching
material. The additional Iron can raise
the resistance of the NiO and enhance
the performance of the device.
Work at TxSTate was funded by DOD (HBCU/MI grant W911NF-15-1-0394) and a Research
Enhancement grant, and work at NMSU by the National Science Foundation (DMR-1104934). 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.
Data Analysis PyO
Experimental Procedure
Substrate Preparation
Motivation
Determined from
1.Wavelength by wavelength fit
2.General oscillator model consisting of 6 Gaussian peaks and 1
Lorentz peak
PyO on quartz (Tg
in C)
SiO2 modeled with WvlByWvl fit on
quarts subst. (Εp, Brp)
SiO2 modeled by 7 peak general oscillator
model on quartz subst. (Ep, Brp, Ap, MSE, EpBrp)
600o
C 376.7 cm-1
, 40 cm-1
376 cm-1
, 60 cm-1
, 56, 23, 3360 cm-2
489o
C 384.7 cm-1
, 54 cm-1
382 cm-1
, 77 cm-1
, 45, 33, 3465 cm-2
352o
C 384.6 cm-1
, 70.17 cm-1
382 cm-1
, 86, cm-1
, 37, 18, 3182 cm-2
159o
C 381.3 cm-1
, 57 cm-1
379 cm-1
, 74 cm-1
, 43, 17, 3182 cm-2
56o
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
378.5 cm-1
, 84 cm-1
, 41, xx, 3302 cm-2
PyO on Si/SiO2
(Tg in C)
SiO2 modeled by 7 peak general
oscillator model on quartz subst. (Ep,
Brp, Ap, MSE, EpBrp)
SiO2 modeled by 7 peak general oscillator model
on Si/SiO2 subst. (Ep, Βρp, Ap, MSE, EpBrp)
600o
C 380.5 cm-1
, 112 cm-1
, 30, 15, 3371 cm-2
381cm-1
, 110 cm-1
, 30.4, 10, 3344 cm-2
489o
C 386 cm-1
, 72 cm-1
, 44, 15, 3168 cm-2
386 cm-1
,71 cm-1
, 44, 7, 3124 cm-2
352o
C 384 cm-1
, 72 cm-1
, 43, 13, 3096 cm-2
384 cm-1
, 72 cm-1
, 43, 5, 3096 cm-2
159o
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
We list the phonon peaks of various transition metal oxides. The
phonon peaks of nickel ferrite, magnetite, hematite, and maghemite
are inconsistent with the measured peak at 382 cm-1
, confirming the
rocksalt crystal structure of RF sputtered PyO thin films.
Conclusions
Introduction
1. Sputtered NiO and PyO films have
the rocksalt crystal structure as
confirmed by powder diffraction.
2. Works of others show that solubility
of Fe in rocksalt NiO structure is
below 2 %
Deposition:
• reactive RF magnetron sputtering (240 Watt)
• Py target (20 at.% Fe, 80% Ni)
• sputter gas: 80% Ar and 20% O2 (p=10-3
Torr)
• AJA system with background pressure < 10-7
Torr
• substrates: fused quartz (Pella), Si/SiO2(103 nm)
• Tsub: RT – 600o
C
The substrate 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 600o
C and 489o
C samples to cool down below
400o
C is estimated to be 50 and 25 minutes.
Optical properties:
• ∆ and Ψ spectra at four angles of incidence
• M2000 Woollam RCE with CCD detection
• spectral range: 225-1000 nm, 4 angles of
incidence
Ellipsometric spectra were used to calculate optical
properties and thickness of films.
IR measurements:
• ∆ and Ψ spectra at 3 or 4 angles of incidence
• Woollam FTIR VASE RC-Ellipsometer
• spectral range: 0.03-1 eV, 3-4 angles of incidence
Chemical Composition: EDS spectra measured at
different electron beam energies and electron beam
angles were modeled in McXrayLite. The NiFe ratio
of the target was reproduced. The films were not
stochiometric and contained a significant amount
of oxygen vacancies. Electric transport properties
confirm the oxygen vacancies.
Optical Properties SiO2:
Fused quartz substrate
with phonon peaks at
453, 807 and 1073 cm-1
,
and Si/SiO2 with phonon
peaks at 449, 807, and
1082 cm-1
. (experimental
results of dry oxide Kirk
et al. (1988) 457, 810,
1076 cm-1
; theory:
Matsubara et al. (2011)
435, 800, 1054 cm-1
).
A Lorentzian peak is found at around 381.5 cm-1
which we assign to
the TO phonon of PyO. It is red shifted compared to the TO phonon
peak of single crystalline NiO [4,5] but has a higher frequency than the
TO phonon of rocksalt FeO [6]. It is close to the weighted average of
the FeO [6] and NiO [4,5] phonons (377-385 cm-1
).
Modeling of phonon by Lorentz oscillator:
Material Formula Phonon [cm-1
] reference
Magnetite Fe3O4 340, 350, 550 [1] Schleger el al. J. de Phys. 41 (1980)
Hematite
a-Fe2O3
385 (w),436-459 (s),
526 (s)
[2] Jubb el al., Appl. Mat. & Interf. 2 (2010)
Maghemite g-Fe2O3 440 (w), 546-547(s) [2] Jubb el al., Appl. Mat. & Interf. 2 (2010)
Nickel Ferrite NiFe2O4 438, 676.2, 339.4, 377 [3] Shimada el al., J.of All. and Comp. 379 (2004)
Bunsenite
NiO 390
[4]Willet-Giess el al., J. Vac. Sc. Techn. 33 (2015)
[5] Satoh el al., Phys. Stat. Sol. B 106 (1981)
Wustite
FeO 325, 534.7
[6] Hofmeister el al., Mon. Not. R. Astron. Soc. 245
(2003)
Two different substrates were used in
the study. Fused quartz and Silicon
wafers with a thin wet SiO2 layer (103
nm) on top.
Roughing
Bead blasted the back surface of the
substrates to scatter the light and avoid
back reflection: for 10 s on quartz or 30
s on Si/SiO2: at 45 degrees,1 meter
distance from gun
Cleaning
Water -> acetone -> Isopropyl alcohol
each for 5 min in a sonicator, spinner,
clean and dry again(acetone ->
Isopropyl alcohol -> N2 gun in the
cleanroom)
Bead Blaster Woollam FTIR Vase
TxState
RRAM test
chip