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DOI: 10.1007/s00339-004-2806-8

Appl. Phys. A 79, 14391443 (2004)

Materials Science & Processing

Applied Physics A

p.r. dahoo

t. hamon

b. negulescu

p. rocher

m. tessier

a.wack

l. thomas

Evidence by spectroscopic ellipsometryof optical property change in pulsed laser

deposited NiO films when heated in airat Neel temperatureLMOV 45 AV Etats-Unis, 75035 Versailles Cedex, UMR 8634, France

Received: 6 October 2003/Accepted: 18 April 2004

Published online: 26 July 2004 Springer-Verlag 2004

ABSTRACTA spectroscopic ellipsometry (SE) study of Nickeloxide (NiO) films, an anti-ferromagnetic insulator, deposited

by pulsed laser ablation technique (PLD) on quartz substratehas been undertaken in the region of its Neel temperature(TN=523 K), when its structure changes from rock-salt toa rhombohedral one, owing to a slight contraction along the 111direction. The films were grown under optimal conditions ina MECA 2000 process chamber so as to reduce surface rough-ness to a minimum as shown by Atomic Force Microscope(AFM) and SE characterization. The optical properties havebeen investigated as a function of temperature in an ultra highvacuum (UHV) process chamber (in situ) and in air, under stan-dard atmospheric conditions (ex situ). We report in this studya change in the ellipsometric parameters of NiO in the visiblespectral range from 550 nm to 700 nm when the film is heatedin air in the region ofTNdifferently from that observed in ultra-

high vacuum. On the contrary, as expected, SmFeO3 studiedunder the same conditions in the same temperature range inair shows no such change in ellipsometric parameters in thisspectral region which corresponds to the spectral signature ofNiO. Results are discussed in terms of spectral lines involvingvibrational states of NiO molecule, when transitions occur be-tween its electronic ground state X3 and the excited states[16.0]30 and [16.0]

31. These results suggest that NiO oughtto be classed as a charge-transfer insulator rather than a MottHubbard one.

PACS07.60.Ds; 78.20.Ci; 81.15Fg

1 Introduction

Nickel oxide (NiO) grown in thin films, from thepoint of view of its chemical properties in terms of its structureand oxidation states, is an interesting catalytic material [1].NiOis also an interesting magnetic material in structures thatexhibits exchange bias such as in NiO/FeNi [2] bilayer. Inboth cases, the role of its electronic structure in determin-ing its reactivity and its magnetic property in thin films is ofimportance.

While the magnetic properties ofNiO are well known,the electronic structure is not yet well established, in par-

Fax: +33-1/39254481, E-mail: prd@physique.uvsq.fr

ticular the nature of the upper edge of the valence band ofO(2p) character which determines ifNiO should be classed asa charge-transfer insulator, as opposed to a MottHubbardinsulator where the conduction and upper valence edge are

of the same character [3]. As optical properties of materi-als in the UV-visible region are closely related to their elec-tronic structure, a spectroscopic ellipsometry (SE) study ofNiO films deposited by pulsed laser ablation technique (PLD)on quartz substrate has, thus, been undertaken in the regionof its Neel temperature (TN= 523K). At this temperature thestructure changes from the rock-salt structure to a rhombohe-dral one owing to a slight contraction along the 111 directionand the change in the electronic structure ofNiOis expectedto be more sensitive to SE investigation.

In this study, as described in Sect. 2, the optical propertiesof polycrystalline NiOfilms grown by PLD on quartz sub-strates exhibiting a strong texture whose orientation is along[111]direction (deposition temperature at about 400 C) [2]have been investigated by SE using the SE850 rotating an-alyzer ellipsometer from Sentech [4]. The optical propertieshave been investigated as a function of temperature in anultra high vacuum process chamber(UHV), and in air, understandard atmospheric conditions, these experimental config-urations being thereafter termed as in situ and ex situ, re-spectively. ForNiO, the ellipsometric parameters have beenstudied as a function of temperature in the range 300 K to565K. For comparison, a SmFeO3 film deposited on quartzsubstrate has also been studied by SE in the same tempera-ture range, ex situ in air. The choice ofSmFeO3is motivatedby its higher TN temperature (TN= 674 K) [6]. For in situstudy, thetemperature of thelatter wasbrought to 1073 K.Thecorresponding results are given and discussed in Sect. 3.

2 Experimental details

The NiO andSmFeO3samples have been preparedby PLD as described in [5]. In the case ofNiO, samples,a shadow mask following the technique developed by Tachikiet al. [7] has been used to get rid of micron-sized particulatesand thus reduce surface roughness [2]. The ellipsometric pa-rameters have been measured, ex situ [4] and in situ [8], withan ellipsometer described thereafter, as a function of tempera-ture. Temperature is varied in the range 300K to 565K ex situin air for both samples.NiO and SmFeO3. For in situ meas-

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urements, temperature is varied from300 Kup to603 KforNiOand up to1073 Kfor SmFeO3. Alignment in the UHVchamber is checked with an optical system designed forin situgrowth characterization by SE [8].

SE study of the films are performed ex situ with a Multi-ple Angle Spectroscopic Ellipsometer (MASE) from Sentech(SE850) operating in the rotating analyzer (RA) mode. The

spectral range varies from250 nmto850 nmand the angle ofincidence can be manually varied from55 to85 in steps of5. The analyzer can be operated in Fast (F) mode with the an-alyzer rotating continuously or in Stepping (S)mode. In eithermode, intensity measurement of the full spectrum is acquiredby means of a diode array with a resolution of1.2 nm, for 8spectra in F mode (in steps of45) and a maximum of50 spec-tra in S mode.The ellipsometric parameters defined, for eachwavelength kby the real part of:

(k)= tanexp(i)=Rp(k)

Rs(k)(1)

where Rpand Rs are the complex Fresnel Reflection Coeffi-

cients (FRC) for light polarizedparallel (p) and perpendicular(s)to the plane of incidence, respectively are measured in thetemperature ranges given above. Note that is related to theoptical thickness of the sample by the formula:

=2ndcos(i)

k(2)

where nis the complex refractive index of the film,dits thick-ness and ithe angle of incidence of the light beam.

Except for the simple case of a film on a subtrate forwhich analytical inversion is possible, numerical analysis isnecessary to characterize a sample from measured data [9].Numerical inversion consists in the minimization of the non

linear function (x)described below and requires consider-able mathematical analysis of the theoretical model built torepresent the film grown on a substrate. Analysis is performednumerically using the software implementing SE850 by thesimplex algorithm [9]. The latter allows for the optimizationof adjustable parameters in the model representing the samplewith respect to the acquired ellipsometric data. Such param-eters are for instance the complex refractive index and thethickness of the film and effective complex refractive index atair-film and film-substrate interfaces.

From the two real numberstanmes(k)andcosmes(k)measured by ellipsometry at each wavelength k, one com-putes a square function(x) defined by:

(x)=1

Nm1

k=1..N

J(k,x) (3)

where N is the number of experimental points and m thenumber of parameters to be adjusted. The function J(k,x)isexpressed as:

J(k,x)={tanmes(k) tanthe(k,x)}2

+{cosmes(k) costhe(k,x)the}2 (4)

wherekis the kthwavelengthofthelightand xavectorofthemelements to be determined. Thetwo numbers, tanthe(k,x)and costhe(k,x), are calculated from (1) given above.

For SE analysis, the films have been modelled as twolayers on quartz substrate, the top layer consisting ofNiOorSmFeO3and voids. The latter layer is modelled followingBruggemann effective medium approximation (BEMA) [911] according to the formula:

i=1,2

fii

i+ 2= 0 (5)

whereis the composite dielectric function and iand fi thedielectric function and the volume fraction respectively forthe ith constituent. The dielectric function is related to thecomplex index of refraction by the formula:

n2i = i . (6)

3 Results and discussion

Figures 1 to 4a and b display the ellipsometric pa-rameterscos()and tan()measured at different tempera-tures, respectively ex situ in air and in situ in UHV for NiO

(Figs. 1 and 2) and SmFeO3 (Figs. 3 and 4) samples. Ex-cept for the curves on Figs. 1 and 2, those on the other fig-ures vary only slightly and almost superimpose in the spec-tral range from300 nmto 850 nm. These slight changes areinterpreted in terms of both complex refractive index andthickness change through expansion or sometimes surfaceeffects [4]. For clarity, the ordinates have, thus, been giventhe same offset on all the curves, that is are moved upwardsfrom the abscissas axis. No offset has been given to abscis-sas. The curves of the ellipsometric parameterscos()andtan()are periodic continuous functions of the wavelength with maxima and minima amplitudes varying with ascan be seen on Figs. 1 to 4a and b. Note that the ellipso-metric parameters of the NiO films slightly differ becauseof different film thicknesses determined following the pro-cedure described above in Sect. 2. ForNiO in air the thick-ness measured has a mean value of1692 nm (roughness19nm). The in situ samples thickness is1962 nm(rough-ness 15nm).

In the spectral regions ranging from 300 nm to 550nmand700 nmto850 nm, only very slight changes are observedfor ex situ NiO cos() and tan() curves. These changesare interpreted as above [4]. For this reason, the latter curvesare shown from 550 nm to 700 nmonly, on Fig. 1a and brespectively. When temperature is raised, then the continu-ous character of the curves in this region starts to changeabove380 K with oscillations appearing within the spectralrange from590 nmto650 nm. The latter can be more clearlyseen on Fig. 5 for cos() curves. At 380K, one observesa smooth continuous curve within experimental noise. Af-terwards, from412 K onwards up to 565 K, the oscillationsbecome more apparent and one can easily depict on Fig. 5four regions with the oscillations centered at 589 nm,600 nm,612 nmand 627 nm. The spacings between the oscillationsas measured range from 2 to3 nm, beyond the resolution of1.2 nm of the diode array used for detection. Then, consid-ering the work of E.J. Friedman-Hill and R.W. Field [12]who analysed 13 electronic bands ofNiO in the region ofthe visible spectrum, between550 nmand 625 nm, we sug-gest that these oscillations are due to spectral lines corres-

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DAHOO et al. Evidence by spectroscopic ellipsometry of optical property change in pulsed laser deposited NiO films 1441

FIGURE 1 aTemperature effect on cos()curves in the range 550 nm to 700 nm for NiO in air.304 K, 323 K, 380 K,450 K, 485 K and + 530 K.bTemperature effect on tan()curves in the range 550 nm to 700 nm for NiO in air. 304 K, 323 K, 380 K, 450 K, 485 K and + 530 K

FIGURE 2 aTemperature effect on cos()curves in the range 350 nm to 850 nm for NiO in vacuum. 380 K, 453 K, 493 K, 523 K and 603 K.bTemperature effect on tan()curves in the range 350 nm to 850 nm for NiO in vacuum. 380 K, 453 K, 493 K, 523 K and 603 K

ponding to electronic transitions of the NiO molecule at thesurface of the heated film. The formation ofNiOat the tem-perature around412 Kis consistent with the temperature of400K at which fner and Zaera [1] report NiO thin film (threeNiOmonolayers) formation, from oxidation ofNisurface bymolecular oxygen 99.99%pure. At low energies, the elec-tronic structure ofNiO molecule is dominated by the singlycharged zero-orderNi+O structure because of the low ion-ization potential of nickel [12]. The NiO orbitals are formedby overlap between the3dand 4sorbitals of theNi(3d84s2)atom and the2porbitals of theO(2p4) atom. The electronicground state is a 3 state arising from a {24422} elec-tron configuration with a vibrational frequency of840 cm1

and a bond length of0.1627 nm. In the state-naming con-vention of the energy levels, it was shown in [12], that theexcitedstates arebest described in he frame of Hunds case (a)

coupling scheme of angular momentum (spin_orbit) althoughsome states are nearer Hunds case (c) as the ground statefor example. Following this convention then the lines cen-tered at 627 nm and600 nm are assigned to transitions ofNiOmolecule between the electronic ground state X3 and theexcited state [16.0]31, with the quantum vibrational num-bers v =0 and v =1 respectively, and those at 589 nmand600 nm to transitions ofNiO molecule between the electronicground state X3 and the excited state [16.0]30 with thequantum vibrational numbers v=0 and v=1 respectively.No such oscillations are present on the ex situ SmFeO3 el-lipsometric parameter curves on Fig. 3a and b because theycorrespond to electronic transitions characteristic of theNiOmolecule. They do notappear on situ NiO and SmFeO3curvesshown on Figs. 2 and 4a and b either, because in UHV no oxi-dation of the surface can occur.

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1442 Applied Physics A Materials Science & Processing

FIGURE 3 aTemperature effect on cos() curves in the range 300 nm to 850 nm for SmFeO3in air.

310 K, 380 K,485 K and

530 K. bTemperatureeffect on tan()curves in the range 300 nm to 850 nm for SmFeO3in air. 310 K, 380 K, 485 K and 530 K

FIGURE 4 aTemperature effect on cos()curves in the range 500 nm to 850 nm for SmFeO3 in vacuum. 462 K, 621 K, 763 K and 901 K and 1073 K.bTemperature effect on tan()curves in the range 500 nm to 850 nm for SmFeO 3in vacuum. 462 K, 621 K, 763 K and 901 K and1073 K

Considering the fact thatNiOformation occurs at a lowertemperature (412K) than TN(523K) for which a change inthe structure ofNiOfilm occurs as described above, then ox-idation of the surface starts before this structural change andmay not be correlated to the latter. Moreover, from 300 Kto400K, theNiO film expands and one does not expect bandoverlapping to occur and a transition of the type insulator-metal as observed at 2.5 Mbar [13] for NiO may be ruledout. From the measured ellipsometric parameterscos()andtan()shown on Figs. 1a, b and 5, one can deduce that thesurface does not exhibit any metallic character at all tem-peratures (no periodicity is observed for reflecting metallicsurface). However, the fact thatNiOas a film on the NimetalorNiO molecule at the NiO film surface, forms at the sametemperature is surprising but not impossible if one consid-

FIGURE 5 Oscillations on cos()curves in the range 560 nm to 660 nmfor NiO in air. A 380 K, B 412 K, C 485 K, D 530 K and E 565 K

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DAHOO et al. Evidence by spectroscopic ellipsometry of optical property change in pulsed laser deposited NiO films 1443

ers that surface temperature is a crucial parameter. On thosegrounds, the mechanism that leads to formation ofNiO mono-layers upon oxidation ofNi metallic surface [1] may not bedifferent from that resulting in the formation ofNiO on thefilm surface.

The ionic character of the NiO bond suggests that Ni+

ions forms at the surface in the presence ofO2 at 400K,

althoughNi2+

ions are present in the film. The optical prop-erties of solid phase NiO have been interpreted in terms oftransitions involving either or both3d8 levels of nickel anion(Ni2+) or2p6 levels of the oxygen cation (O2) [14]. Bandcalculations forNiOyield2p(O2) and4s(Ni2+) bands (thelowest-lying completely empty band). For3d8 levels ofNi2+,they are either considered as highly localized 3d wavefunc-tions characterized by local site symmetry or an extremelynarrow 3dband characterized by translational symmetry [15].One possible interpretation proposed to explain the opticalproperties of solid phase NiO is the transition from3d8 ini-tial states (assumed in the ground state) to4sband leaving the3d7 hole in various states of excitation (3d8 3d74p) with

expected photoconductivity due to excitation of electrons inthe 4s band. A second possible interpretation is a localized ex-citation involving oxygen2pand Ni 3dstates, without anyphotoconductivity because of localized excitations. In thefirstcase there is the creation ofNi2+ (3d74p) states involving onlyNistates whereas in the second case there is the creation ofNi+ (3d84p) states involving Ni and O states. If one takes intoaccount the temperature dependenceofNiO formationandtheassumption that the same mechanism leads toNiOformationat the surface of both metallic NiandNiOinsulator, then oneought to consider that theoptical properties of solid phase NiOis due to3d8 3d74p transitions, rather than2p 3d84pones. The latter may be an explanation for the formation ofNiOat the surface of the heated films although no metalliccharacter is evidenced by ellipsometry. Then these consider-ations are in favour of localized Ni dorbitals rather than anextremely narrow d band.NiOought, therefore, to be classedas a charge-transfer insulator.

FurtherworkisinprogresstomodeltheopticalconstantofNiO in that particular case [16].

4 Conclusion

This work shows that ellipsometry may proveuseful as a spectroscopic technique for surface character-ization of films subject to reactive environment as shownby the results reported for NiO film heated in air. Be-sides the change in optical constants, the optical transitionsof molecules present at the surface may observed. In thiswork, these results suggest that NiO ought to be classedas a charge-transfer insulator rather than a MottHubbardone.

ACKNOWLEDGEMENTS The authors thank MBuwaNzenguet, an undergraduate student for the work done in modelling the NiOfilm.

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