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
Motivation
Abstract
Optical Properties SiO2:
The optical properties of RF sputtered polycrystalline permalloy oxide (PyO: Ni0.8Fe0.2O1-d) thin films were studied in the infrared by variable angle ellipsometry. The IR- dispersion of PyO shows a Lorentz-like dispersion peak at cm-1. We attribute this peak to the transverse optical phonon of PyO.
Determined from
Wavelength by wavelength fit
General oscillator model consisting of 6 Gaussian peaks and 1 Lorentz peak
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).
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.
Introduction
Sputtered NiO and PyO films have the rocksalt crystal structure as confirmed by powder diffraction.
Works of others show that solubility of Fe in rocksalt NiO structure is below 2 %
TxState RRAM test chip
Data Analysis PyO
Experimental Procedure
Modeling of phonon by Lorentz oscillator:
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 – 600oC
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 600oC and 489oC samples to cool down below 400oC is estimated to be 50 and 25 minutes.
Substrate Preparation
PyO on quartz (Tg in C)
SiO2 modeled with WvlByWvl fit on quarts subst. (Ep, Brp)
SiO2 modeled by 7 peak general oscillator model on quartz subst. (Ep, Brp, Ap, MSE, EpBrp)
600oC
376.7 cm-1, 40 cm-1
376 cm-1, 60 cm-1, 56, 23, 3360 cm-2
489oC
384.7 cm-1, 54 cm-1
382 cm-1, 77 cm-1, 45, 33, 3465 cm-2
352oC
384.6 cm-1, cm-1
382 cm-1, 86, cm-1, 37, 18, 3182 cm-2
159oC
381.3 cm-1, 57 cm-1
379 cm-1, 74 cm-1, 43, 17, 3182 cm-2
56oC
cm-1, 43 cm-1
376 cm-1, 92 cm-1, 37, 16, 3404 cm-2
24oC
382 cm-1, 88 cm-1
376 cm-1, 115 cm-1, 28, 14, 3220 cm-2
average
cm-1
378.5 cm-1, 84 cm-1, 41, xx, 3302 cm-2
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)
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, Brp, Ap, MSE, EpBrp)
600oC
380.5 cm-1, 112 cm-1, 30, 15, 3371 cm-2
381cm-1, 110 cm-1, 30.4, 10, 3344 cm-2
489oC
386 cm-1, 72 cm-1, 44, 15, 3168 cm-2
386 cm-1,71 cm-1, 44, 7, 3124 cm-2
352oC
384 cm-1, 72 cm-1, 43, 13, 3096 cm-2
384 cm-1, 72 cm-1, 43, 5, 3096 cm-2
159oC
383 cm-1, 86 cm-1, 37.6, 15, 3234 cm-2
384 cm-1, 85 cm-1, 37.9, 7, 3222 cm-2
24oC
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
Optical properties:
D and Y spectra at four angles of incidence
M2000 Woollam RCE with CCD detection
spectral range: nm, 4 angles of incidence
Ellipsometric spectra were used to calculate optical properties and thickness of films.
IR measurements:
D and Y spectra at 3 or 4 angles of incidence
Woollam FTIR VASE RC-Ellipsometer
spectral range: 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.
A Lorentzian peak is found at around 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 ( cm-1).
Conclusions
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.
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), (s), 526 (s)
[2] Jubb el al., Appl. Mat. & Interf. 2 (2010)
Maghemite
g-Fe2O3
440 (w), (s)
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)
Bead Blaster Woollam FTIR Vase
Work at TxSTate was funded by DOD (HBCU/MI grant W911NF ) and a Research Enhancement grant, and work at NMSU by the National Science Foundation (DMR ). 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.