Literaturereview
The discoveryof IBM was made in 1973 by Lee et al., who investigated mixing at thePd/SibyimplantationwithPions,andVanderWegetal.,whofoundtheformation palladiumsilicideforPd/SiafterArionbombardmen[38].Inthenearly50 yearssince then, much IBM data has been gathered for a variety of systems, including thin film makers, bilayer films, and multilayer films embedded in a substrate In literature, various parameters from experiments or theoretical models have been used to characterizethemixingprocess.Mostformsofthetheoryforcollisionalmixingpredict alineardependenceofmixingparametersonfluence(∅)andnuclearenergydeposited perionperunitlength(𝐹 𝐷 )inthetarget.Theeffectsofotherbeamparametersortarget properties,h o w e v e r , c o u l d b e c h a r a c t e r i z e d o n l y b y p r i m a r y p a r a m e t e r s t h a t are measurableusingexperimentaltechniques.Forabilayersysteminmixinganalysis,the number of impurity atoms/ion that have crossed the initial interface or the enhanced broadening of this interface are typically used to describe the degree of mixing In the following review, the main experimental findings will be discussed to give a concise summaryfor the dependence of bilayer’s IBM on various factors including ion energy and mass that form the subject of the present work.
Manycollisionalmixingmodelspredictthatmixingwillbelinearlydependenton ionfluence,thisrelationshiphasbeenexaminedforvariousconfigurations.Forinstance, in 1999, in order to investigate the IBM of Fe with alumina, a method to determine the interdiffusion coefficient (𝐷 𝑡 )using the RBS spectrum was employed [39] It was observed that ballistic effects
(cascade mixing) are the cause of the observed IBM ofFe/Al 2 O3inducedbyNe+andAr+ionsirradiation.Fortheirradiatedsamples,theplotof
𝐷𝑡v e r s u s i o n f l u e n c e∅fits with a straight line showing IBM resulting from recoil mechanisms N Bibic [40] has also conducted an experiment based on RBS and XRD analyses,andfoundthation-beammixingofTa/Sibilayersresultsinthelow- resistivityTaSi 2phase aftermultiplychargedArionirradiationatambienttemperature.Accordingto the Ta depth profiles, the thickness𝜎of the Ta–Si intermixed region is a function of the ion fluence∅ In all cases, the relationship between∆𝜎 2 and∅was linear This was supportedbythestudyoffluenceeffectsonAu–Nibilayersmixingby300keVKr + ions (D Datta,
2003) [41]; and IBM in Pt/Co bilayers by implanting with 600 keV Pt + ions (S.Balaji,2008) [42].ItshouldbenoticedthatthefluencedependenceofIBMvariesin the same way for diverse material configurations, and mixing in all of the aforementioned situations was done at or below a critical temperature.
Under 80 K, the IBM process typically does not depend on temperature, but in excessofaparticularcriticaltemperature,itdoesvarywithtemperature[43].Thecritical temperature for the majority of systems ranges between 160 K and 470 K, whereas inother systems, such as tungsten markers in aluminum and gold markers in SiO2,
IBMhasbeenfoundtobetemperatureindependentevenupto500K[44].Abovethecritical temperature, mixing changes as the temperature rises For instance, it enhances in the case of Sb markers in Al, and reduces for Ni markers in Si [45] In intermixing of metal/SiCinterfaceinducedbyheavyion(R.Nagel,2001)[46],Ni/SiC,Cu/SiC,Pt/SiC werei r r a d i a t e d by150 keVAr + ,attemperaturesrangingfrom77to773K.Allsystems exhibit ballistic mixing at the interface at low temperatures, which is well in line with theoretical prediction.I n t h e h i g h - t e m p e r a t u r e r e g i o n ( b e t w e e n 3 7 3 K a n d 7 7 3
K ) , t h e s y s t e m s h o w s d i f f e r e n t b e h a v i o r s I n w h i c h r a d i a t i o n e n h a n c e d d i f f u s i o n , ballistic mixing, and phase formation of Pt-Si were observed for Ni/SiC, Cu/SiC, and Pt/SiC systems, respectively More recently, in temperature-dependent IBM of amorphous SiOC/CrystallineFecomposite(QingSu,2016)[47],theamountofIBMafter298,473, 673, 873, and 1073 K irradiation was investigated Transmission ElectronMicroscopy and RBSdata didnotshowanymixingbetween Feand SiOCfor irradiation at473and 673K,andtherewasonlyaverylittleamountofmixing(2nm)followingirradiationat 298 K.T h e
In a given target, the range of mixing ions and, in turn, the energy density distribution are determined by their energy Generally, when more energy is being deposited at the interface, mixing proceeds more quickly In the mixing model byHaff andSwitkowski[48],i thasbeenestablishedthatIBMdependslinearlyonthedeposited energy (𝐹 𝐷 ).
An experimental proof for this was carried out by Tsaur et al [49], who study Pt marker in Si irradiated by 300 keV Xe ions Later, S.Matteson et al [50] examinedmixingofNi,Ge,Pd,Sn,SbandAumarkersinSiirradiatedwithNe,Ar,Kr, andXeions.Theyalsoobservedthatmixingincreasedlinearlywith𝐹 𝐷 Thedependence on deposited energy of mixing was supported by D Datta [51], in which, the Au–Ni bilayersystemwasinvestigatedusing300keVKr 2+ ions,mixingrateisproportionalto
𝐹 𝐷i n theballisticmodel.However,thereareseveralexceptionstothisrule,andthereis stilldebateonthemixingmodels.ThestudyonmixingofAumakersinSishowsanon- linear relation between deposited energy𝐹𝐷a n d i n t e r d i f f u s i o n c o e f f i c i e n t𝐷 𝑡 This deviation from mixing models might be caused by diffusion due to interstitial mechanism [52,53] In the study of J Conrad [54], the IBM of Pt/Ni systems was surveyedusingAr,Kr,Xe,Pbionswithenergiesof125-400keV.Despitethefactthat Pt/Ni was one among the systems of the global spike model [55], it did not follow the predictionofthismodelthatthemixingratewouldquadraticallyscalewiththedeposited energy.Moreover,themixingrateistoohightobeaccountedforbytheballisticmodel.
Consideration of local spike diffusion as the primary transport mechanism leads to the experimental data's best agreement.
It has also been demonstrated that IBM is not merely a function of ion mass. Indeed,ina studyof R.S.Averback et.al.[56],Pt/Sibilayers were irradiatedwithions ofmassfrom4amu(He)to131amu(Xe)atdifferenttemperatures.Itwasobservedthat
IBMdependsontheenergydensityofthecascadesproducedbytheirradiationparticles rather than scaling directly with the deposited damage energy during irradiation The heavier ions were more effective in causing mixing than the lighter ones In the IBM experimentsofPt/Nibilayers,J.Conrad[57]usedA r , Kr,X e , P b ionsate n e r g y range of 125 -
400 keV For all ions, the mixing rate𝑘was shown to scale linearly with𝐹 𝐷 The𝑘values for each ion tend to be constants and are yet not comprehended Mixing efficiencyissubstantiallyhigherthanwhatSigmundandGrass-ballisticMartin'smodel predicted, and it also did not match with the global spike model In the Pt/Ni system, intermixing in local spikes has been demonstrated to be the predominant mechanism. Anotherexperimentofmassdependence,Ta/SibilayerswereirradiatedwithAr,Kr,and
Xe ions to fluences of (0.5– 2.5) 3x10 16 ions/cm 2 [58] It was found that the mixed region's thickness grew rapidly with ion mass The measured mixing rates were explainedbyacompoundcreationmodelinthepresenceofthermalspikes.Pureballistic mixing can be ruled out to govern the mixing process When the ion mass was raised fromArtoKrorXe,localspikesshiftedtoglobalspikes Moreover,IBMofSb/Niwas surveyedwithirradiationofionHe,Ne,Ar,Xe,Pbatbeamenergiesfrom40to900keV byF.Shiet.al[59].ForionsrangingfromHetoXe,𝐹 𝐷s h o w n tobelinearscalingwith the mixing rate However, for the Pb irradiations, the mixing efficiency increase associatingthebeginningofanonlinearregime.Globalspikemixingisthepredominant process, and the purely ballistic model fails for all irradiations.
∅andenergydepositedperunitdepth𝐹 𝐷h a s beenwellestablishedbybothexperimental studies using the primary RBS method and model-based calculations However, choosing a convenient mixing model depends on ion beam parameters and the target properties (or material configuration) Mixing degree do not depend directly on the factorsassamplestemperature,ionchargestate,ionenergy,orionmass.Effectsofthese parameters on mixing of different material configurations has been carried out experimentally Moreover, most investigations of ion-induced mixing have dealt with metal films on oxide, polymer, semiconductor, and metal substrates The mixing behavior and potential of ion mixing for modifying the interfacial properties of oxide/oxidesystemshaveyettobeadequatelydetermined.Inourknowledge,ithasbeen sporadicforstudyingonthechangingpropertiesofmixedlayers,aswellastheinfluence chemical composition on the mixing amount In the present dissertation, IBM for oxide/oxide systems is investigated experimentallyusing RBS method Mixing amountbetween TiO2a n d S i O 2l a y e r s w i t h d i f f e r e n t t h i c k n e s s e s w i l l b e s u r v e y e d a s a f u n c t i o n o f i o n e n e r g y a n d m a s s I n a d d i t i o n , t h e v a r i a t i o n i n o p t i c a l c o n s t a n t s o f t h e T i O 2/SiO2mixed area after implantation is examined by mean of Ellipsometry Spectroscopy method Based on XPS, changing in chemical compositions of the near surface layers are determined, for the first time, the dependence of the mixing amount on chemical changes will be studied as a function of ion energy.
Conceptofionbeammixing
Ion beam mixing is the process of atoms from several atomic species merging across an interface while being impacted byan ion beam When energetic ions interact with nuclei and electrons of a solid, their energy is deposited in the substance The formation of a moving atoms cascade is one of the effects of energy transfer to target atoms.Iftheionenergyishighenoughtopenetratebeyondtheinterfaceoftwomaterials
AandB,therecoilingatomsproducedclosetotheinterfacemayhavesufficientenergy tocrossit.IntermixingofAandBatomsintheinterfaceregionthereforeistheoutcome Figure1.1shows thesampleconfigurations thatare used typically intheIBM study In Fig.1.1a, a thin maker of element A is placed between two layers of material B The systemapproximatesthespreadingofimpurityAinamatrixmadeuplargelyofBatoms with the typical thickness of layer A is about 1 nm The second type of geometry is shown in Fig.1.1b Thin film of element A is evaporated onto substrate B During ion bombardment, A and B form a semi-infinite diffusion couple and are free to form continuoussolidsolutions,intermediatephasesorcompounds.Thethirdtypeofsample a)
AB AB design, shown in Fig.1.1c, is made up of alternate thin evaporated layers (multilayers) of A and B with an overall thickness less than the ion range To be merged with the oppositelayer,A(orB)atomsnowmustbedisplacedonlyafewinteratomiclengths.In this dissertation, the configuration of bilayer (Fig.1.1b) has been utilized for the IBMstudies.
Before describing the IBM process and the mechanisms theorized to contribute tomixing,itisusefultolookintotheunderpinningIBMphenomenanamelyinteraction ofionswithsolidsinthelowenergyrange,wheretheelasticscatteringisdominant,that isnowdiscussedinthenextsectionincludingkinematicofelasticcollisions,differential cross- section, andenergy loss process.
Atomiccollisionsinsolids
Kinematicofelasticcollisions
In elastic collisions, which are defined as that kinetic energy is conserved, the energy transfers and kinematics can be addressed by applying the principles of energy and momentum conservation to the interactions between two isolated particles [61]. Figure 1.2 shows the geometry and notation of the laboratory coordinate system for elastic collision between two different masses The equations express the conservation ofkineticenergyandmomentumparallelandperpendiculartotheincidentdirectionare as follows:
𝑀 1 𝑣 0 = 𝑀 1 𝑣 1 𝑐𝑜𝑠𝜃+𝑀 2 𝑣 2 𝑐𝑜𝑠𝛷, (1.2) 0=𝑀 1 𝑣 1 𝑠𝑖𝑛𝜃−𝑀 2 𝑣 2 𝑠𝑖𝑛𝛷 (1.3) There are numerous approaches to solve Equations (1.1) - (1.3) For example, in Equations (1.2) and (1.3), converting the first term on the right to the left side, squared and added, will remove𝛷, giving
To eliminate𝑣 2 , the equation (1.4) is substituted into Equation (1.1), the ratio of the particle’s velocity after and before the collision can be calculated as follows:
If𝑀 1 >𝑀2, the quantity under the radical in Equation (1.5) will be zero for𝜃 = 𝜃 𝑚 , where𝜃𝑚i s d e t e r m i n e d f r o m
(1.6)For𝜃 > 𝜃 𝑚 ,(and𝜃= 𝜋), 𝑣 1/𝑣0iseitherimaginaryornegative,botharenon-physical,so𝜃𝑚d e f i n e s t h e l a r g e s t s c a t t e r e d a n g l e
Fig.1.2.Diagramofelasticcollisionfortwodifferentmassesasshowninthelaboratory reference frame with scattering angles of center of mass reference frame [61].
For𝑀1< 𝑀2,anyvalueofθbetween0andπisacceptable,if theplussignin Equation (1.5) is selected,𝑣 1 /𝑣0has a positive value.I n t h i s e q u a t i o n , t h e m i n u s s i g n r e s u l t s i n n e g a t i v e𝑣1/𝑣0that is physically unrealistic In the case of plus sign, the ratio of projectile’s energies for𝑀1≤ 𝑀 2i s g i v e n b y
Asaresult,weareabletodeterminethek i n e m a t i c factorK=𝐸1⁄𝐸0,Equation(1.7)can be rewritten as:
(1.8)AccordingtoEquation(1.8),theratioofincidentiontotargetmassesandthescattering angleθare the only variables that affect the kinematic factor.
Differentialcross-section
Sincetherearesomanyinteractionsbetweentheionsandtargetatomsduringthe ion irradiation, answers to the questions of what the scattering angle and how much energy could be transferred are necessary This can be fulfilled by the fundamental parameterdifferential cross-section, which provides a measurement of either the probability of scattering an incident ion into some angles or the probability of transferring energy to a target atom.
Commonly, the amount of particles scattered throughdifferentangles𝜃 𝑐i s representedintermsofangulardifferentialscatteringcross section. Consider a beam of ions collides with a target, the scattered ions are collected by a detector (Fig. 1.3) Each ion in the beam with a unique impact parameter will be scattered at a different angle The differential𝑑𝑛𝜃is defined as the number of ions get intothedetectorofarea𝛥𝑎perunittime,betweenangles𝜃 𝑐a n d 𝜃 𝑐+ 𝑑𝜃 𝑐 Thedetector’s solid angle𝛥𝛺thus is given by
𝑑𝛺 𝐼 0 𝑑𝛺 (1.10) where,incaseof𝛥𝑎→0,𝛥𝛺→𝑑𝛺.Theterm𝑑𝜎(𝜃 𝑐 )/𝑑𝛺isthedifferentialscattering cross section per unit solid angle, and(𝑑𝑛𝜃 𝑐 )/𝑑𝛺i s t h e n u m b e r o f p a r t i c l e s s c a t t e r e d i n t o t h e a n g u l a r r e g i m e b e t w e e n 𝜃 𝑐a n d 𝜃 𝑐+ 𝑑𝜃𝑐p e r u n i t s o l i d a n g l e , p e r u n i t t i m e I n t h ecasewheretheinteractionispurelyCoulombic,theprojectileandtargetnucleusare regarded as pure nuclei The differential cross section is given by d () 2 1 d c 4 E c s i n 4 (/ 2) c , (1.11) where𝛼 = 𝑍1𝑍 2 𝑒 2 ,𝐸 𝑐 = 𝑀 2 /(𝑀 1 +𝑀 2 )𝐸 0 Equations (1.10) and (1.11) are the Coulomb angular differential scattering cross sections, also known as the Rutherforddifferential cross section From these Equations, we see that bothdσ(θ c )/dθ ca n d dσ(θ c )/dΩincrease asθ cd e c r e a s e s T h i s s u g g e s t s t h a t s c a t t e r i n g e v e n t s w i t h n a r r o w angleshavethehighestcrosssections.AtransformingEquation(1.11)tothelaboratory frame of reference yields is given by d() Z Ze 2 2 1((M1/M2)sin) cos
Energylossprocess
An energetic ion will undergo a series of collisions with the nuclei and electrons when it penetrates a solid Depending on the energy, mass of the ion, and the substrate material,energyoftheincidentionlosesatrateof𝑑𝐸/𝑑𝑥ofafewtoahundredelectron volts per nanometer Mechanisms of energy loss are commonly divided into two different categories: (1) nuclear collisions, in which energy is transmitted as translator motion to a target atom as a whole; and (2) electronic collisions, in which the moving particle loses its kinetic energy by exciting or ejecting atomic electrons Hence, the energy loss rate𝑑𝐸/𝑑𝑥is given by
Nuclear collisions can cause large, discrete energy losses and skew the ion's trajectory by a significant angle This process produces lattice disorder due to the displacementofatomsfromthelattice,andthebackscatteringevents(willbediscussed in chapter II) For electronic collisions, they result in substantially lower energy losses per collision,littleiontrajectorydeflection,andnegligiblelattice disorder The energy- loss mechanisms varyrapidlywith the velocity and atomic number𝑍1incident ion: for lowvelocityand high𝑍 1 thenuclearstoppingis dominant, whereaselectronicstopping takes over for high velocity and low𝑍1.
Figure 1.4 shows the relative share of both terms in Eq (1.13) for the total stoppingpower𝑆versusionvelocitythroughoutabroadenergyrange.Anotablerateof nuclear stopping identifies low ion energies in the range of a few hundred keV or less This is evident from the maximum in the left corner of region I in Fig.1.4 The energy lost by a moving particle per unit length traveled in the target is known as the nuclear stopping power or nuclear energy-loss rate This energy loss can be obtained from the scattering cross-section [61].
Both elastic and inelastic interactions could be created at the greater energy in region I (1 MeV - 10 MeV), with inelastic collisions gradually taking over The interactionoftheincidentionandthetarget’selectronscauseselectronicstoppinginthis lowenergyregime;thestoppingiscomparabletoaviscousdragforceandisproportional totheionvelocity.Theenergylostbyincidentionsisdissipatedintothermalvibrations of the target atoms through the electron cloud As the ion velocity rises in region II of Fig.1.4(10MeVto100MeV)thechargestateoftheionincreases.Theionistravelling therefasterthantheaverageorbitalspeedoftheelectronsinthetargetatomshells.Bohr's theory of stopping power, which is based on classical considerations, can therefore be used to explain electronic stopping caused by the ion-target interaction [62].
Due to the ion interaction time in the vicinity of the target atoms is shorter, theelectronicstoppingpowerdecreasesagainwithenhancementofionvelocityatextremelyhig h ion velocities (region III of Fig 1.4) This high energyrange is also known as fast collision regime.
Fig.1.4.TheionstoppingpowerSwithnuclearandelectroniccomponentsasafunction of ion velocity [63]
Low-energyionmodificationofsolidsandIBMprocess
Recoilmixing
Theincidention'skinematicenergywillbetransferredwhentheioncollideswith theatomsclosetoanA/Binterface.AsseeninFig.1.6b,thetargetatomsrecoilintothe bulk B, sometimes very far from where they started This process, known as recoil implantationorrecoilmixing,isthemostbasictypeofballisticmixingandresultsinthe transportation of atoms byrepeated single impact events between the incident ions and targetatoms.Therecoilshouldtravelthemaximumrangepossibleforeffectivemixing; this occurs when the incident ion and the target atom collide directly (𝜃 = 0).T h e p r o b a b i l i t y o f a h e a d - o n c o l l i s i o n i s v e r y s m a l l , w h i l e t h e m a j o r i t y o f c o l l i s i o n s b e i n g s o f t ( 𝜃 > 0).In comparison to a head-on collision, the recoil generated by such soft collisions will have substantially less energy, a smaller range, and their trajectory will notbeintheforwarddirection.Asaresult,therewillnotbemanytargetatomsinvolved in mixing bythe mechanism of recoil implantation Recoil mixing has been the subject of theoretical analyses by Kelly et al (1980) and Littmark et al (1980) According to these models, there is a low possibility of primary recoil implantation compared to secondary recoils, and it typicallycannot account for the mixing amount in most of thecases.
Cascademixing
During ion irradiation and implantation, in addition to recoil mixing and other ballisticprocesses,enhancedatomicmixingcanhappenasmultipledisplacementatoms occur from a single ion In the multiple displacements, a knock-on-atom process that continued by initially displaced target atom (primary recoil) also moves secondary recoils,whichinturndisplaceadditionalatoms.Themultipledisplacementsequenceof collision events iscommonlyreferred to as a collision cascade.In contrast to the recoil implantation process, which involves an atom receiving a significant kinematic energy in a single displacement, atoms in a collision cascade undergonumerous displacement and relocation events Each incident ion creates a distinct volume across the interface thatiscomposedofbothatomsAandB,asseeninFig.1.6b.Thecontinuousmixedarea in Fig 1.6c is the result of the overlap of these cascades with increasing ion fluence. Cascademixingisthetermusedtodescribeatomicmixingthatoccursasaresultofsuch several uncorrelated low-energy displacements.
In more detail, Fig 1.7 depicts the ballistic interaction of an energetic ion with a solid.The figureshows single-ion/single-atomrecoilevents,surfacesputtering,andthe developmentof a collision cascade involving a significant number of displaced atoms.Thecascadesareshownatanearlystageofdisplacement,whenthedisplacedatomsare occupying interstitial locations surrounding a core of a vacant lattice site.
Mostrecoilsarecreatedclosetothelowestenergyrequiredtodisplaceatoms,𝐸 𝑑 , according to calculating of the mean energy of the atoms in cascade The incident particle's initial momentum is quickly lost due to the low energy stochastic nature of these displacement events, and the overall motion of the atoms in a collision cascade becomes isotropic An atomic redistribution caused by this isotropic motion can be representedmathematicallyasarandomwalkwithstepsthataredeterminedbythemean range of an atom with an energy close to𝐸 𝑑 The diffusion equation expresses theeffectivediffusivityD casfor acollisioncascadeinducedrandom-walkprocess(Andersen1979) is given by
𝐷 𝑐𝑎𝑠 𝑡6 , (1.14) where𝑑𝑝𝑎(𝑥)is the number of cascade-induced displacements per atom at distance𝑥,and< 𝑟 2 >isthemeansquaredrangeof thedisplacedtargetatoms.The𝑑𝑝𝑎produced by a specific ion fluence can be expressed as
𝑁istheatomicdensity.Hence,theeffectivediffusioncoefficientduetoballisticcascade mixing given by
Sigmund and Gras-Marti (1981) [66] developed a more thorough theoretical formulation of collision mixing based on linear transport theory The mass differencebetween the target atomsM 2a n d t h e i o n M 1 is also considered in this formulation Thecalculation's result for the effective diffusion coefficient, which measures how an impurity profile spreads as a result of ion irradiation in a homogeneous matrix, is determined by lj 𝐹 𝐷 (𝑥)
(1.21) whereljis a dementionless parameter with a value of 0.608, is a mass-sensitive kinematic factor given by[4𝑀 1 𝑀 2 /(𝑀 1 +𝑀 2 ) 2 ]1/2 In the case of𝑀 1= 𝑀 2 , = 1, two models become verysimilar The main featuresof the models are that the effective diffusion coefficient scales with the damage energy,𝐹𝐷, and ion fluence∅, also this expression does not contain any temperature- dependent terms.
In order to investigate the mixing and changes in interfacial properties of theTiO 2 /SiO2s y s t e m s i n d u c e d b y i o n i m p l a n t a t i o n , t h e R u t h e r f o r d
B a c k s c a t t e r i n gSpectrometry (RBS), Ellipsometry Spectroscopy (ES), and X-ray Photoelectron Spectroscopy (XPS) methods have been used The techniques can be classified into major–ionimplantationandRBS;andauxiliary–XPSandESmethods.I n thischapter, ashortdescriptiononphysicalconceptsofthetechniqueswillbegiven,followedbythe experimental conditions.
In order to decrease light reflection and increase light transmission for solar panels, anti-reflection coatings have become increasinglyimportant and popular. According to reports, the difference in reflective indices will cause a reflection loss of around 8% at the glass-air interface [67] In order to reduce the energy loss caused by reflection, one or more layers of thin films with a moderate refractive index and film thickness are typically coated on the glass surface to achieve an anti-reflection effect based on light interference cancellation The refractive index of the ideal single-layer ARCisequaltothesquarerootoftherefractiveindexofthesubstrate.However,dueto the constrained antireflection waveband range of the single-layer ARC, multilayer preparation is gaining popularity [68] To date, a variety of materials, includingTiO 2 /SiO2, ZnO/SiO2,
Al2O3/TiO2, etc., have been employed [69] Since TiO2/SiO2i sthemostwidelyused material,itwas chosen tostudymixingof theARCsysteminthisdissertation The combination of SiO2a n d T i O 2film exhibits a number of benefits,including excellentstability, photocatalytic activity, non-toxicity, thermal stability, andchemical resistance The low refractive index SiO2layer functions as an anti-reflective coating, while the TiO2l a y e r i m p r o v e s t h e f i l m ' s s e l f - c l e a n i n g c a p a c i t y D i f f e r e n t t e c h n i q u e shavebeenusedtodepositTiO2/SiO2filmsincludingsol-gel,c h e m i c a l vapor deposition, chemical spray pyrolysis, atomic layer deposition, pulsed laser deposition, screen printing,h y d r o l y s i s , a n d s p u t t e r i n g [ 6 9 , 7 0 ]
Forthepurposeofextendingunderstandinginmixingoftheoxide/oxidebilayer,40 samples of TiO2/SiO2/Si structure were measured in the present work Firstly, thesamples were irradiated with noble gas ions to formthe mixing process The thickness ofsample’slayerswaschosensothattheprojectedrangeoftheionsisnottoofarfromtheinterfaceof theTiO2andSiO2layers,butstillapproachingtheoptimalconfigurationofanARCsystem[71].Thiswillopt imizetheirradiationtimewhilekeepingthedesired mixing level After implantation, the samples were examined by Ellipsometry Spectroscopy method to determine thickness of layers and the variation in optical constants.I n n e x t s t e p , c h a n g i n g i n c h e m i c a l c o m p o s i t i o n s o f t h e n e a r s u r f a c e l a y e r s a r e s u r v e y e d b a s e d o n X P S m e t h o d , t h e d e p e n d e n c e o f t h e m i x i n g a m o u n t o n t h e s e c h a n g e s w i l l b e s t u d i e d a s a f u n c t i o n o f i o n e n e r g y D u e t o t h e i n f l u e n c e o n m a t e r i a l c r y s t a l l i n e , w e u s e d t h e R B S m e t h o d i n t h e l a s t s t e p t o i n v e s t i g a t e t h e v a r i a t i o n i n t h i c k n e s s ofthelayers.Thereby,mixingprocesswascharacterizedandquantifiedbased on the depth profiles of elements In addition, the mixing mechanism and the effects induced by this phenomenon will be interpreted in term of ion transportation using SRIM simulation.
Ionimplantation
Ionimplantationisaprocessinwhichenergeticionsareinjectedinto asubstrate to produce particular mechanical, chemical, optical properties, or electrical in specific areas.Thereare severalcollisions between an acceleratingparticleandthetargetatoms when it enters a solid The ion is thereafter deflected in arbitrary directions and loses energy as a result of nuclear and electronic interactions until its energy falls below20eV.Anyfurthermotioniscausedbyslowthermaldiffusionbecausetheionstopsmovingat this point and becomes stuck [72] The ion specie, ion’s energy and mass; atomic number of the target, and the degree of crystallinity all affect where an ion settles in a target.Ifthetargetisamorphoustheionsofamonoenergeticbeamwillstoprandomly, andtheimplantedionswillbescatteredinanapproximatelyGaussianpattern[73].The average projected range𝑅𝑝a n d i t s s t a n d a r d d e v i a t i o n∆𝑅𝑝d e f i n e t h i s d i s t r i b u t i o n
B e c a u s e t h e t a r g e t a t o m s a r e u n i f o r m l y s p a c e d a p a r t i n s i n g l e c r y s t a l t a r g e t s , t h e i o n r a n g e i s v e r y d i f f e r e n t , a n d i o n s c a n g o f a r t h e r t h a n𝑅 𝑝i f t h e y a r e g u i d e d a l o n g o n e o f t h emaincrystallographicaxesorplanes.Theionsareconsideredtobechanneledinsuch asituationandareguidedbythepotentialwallsproducedbytherowsandplanesofthe latticeatoms[74,75,76].Duetotheprecisecontrolitprovidesoverthedopingleveland layer thickness,ionimplantation has proven tobe superior than diffusioninthe fieldof integrated circuit technology It can also be used to dope specific areas using masking techniquesandhasgoodreproducibility.Ionimplantationisaviolenttechniquebecause of its collisional character, but being a non-equilibrium process, it causes radiation damageorcrystallinedisorder[77,78].Thisdamagemayoccasionallybeundesiredand eliminated byanannealingcycle,althoughitisfrequentlyadvantageous IBMisone of the ion implantation’s fascinating application, where material properties can be created andalteredbyradiationdamage.Thismethodisespeciallyinterestedinproducingstable compounds,robustimitationalloys,andsuper-saturatedalloys[79].Moreover,itmight improvethewearorcorrosionresistanceofmetals[80].Insemiconductors,IBMisused as a technique for combining contacts,m e t a l l a y e r w i t h a s e m i c o n d u c t o r t o p r e p a r e e l e c t r i c a l , a n d i t h a s b e e n s h o w n t o b e e f f e c t i v e f o r d i s p e r s i n g i m p u r i t i e s b e f o r e f i l m f o r m a t i o n [ 8 1 ]
Fortheaimsofpresentstudy,twogroupsofTiO2/SiO2/Sistructureswithdifferent layer thickness were surveyed Mixing of the TiO2/SiO2s y s t e m s w a s i n d u c e d b yimplantationthesampleswithfourdifferentspeciesofnobleionsNe + ,Ar + ,Kr + andXe + at four different energies of 100, 150, 200 and 250 keV For each implantation, the fluence of the incident ion beam was fixed at3 × 10 16 (ions/cm 2 ).B a s i c i r r a d i a t i o n d a t a aregatheredtogetherinTable1.Usingofnoblegasionsallowedforsolelyphysical structure modification of the samples because they did not cause any chemical binding withthetargetatomsduringinteraction.Theenergyfortheseionspecieswaschosento ensurei n t e r a c t i o n b e t w e e n t h e i o n s a n d t h e a t o m s i n s a m p l e s a t a n d b e y o n d the
U N I M A Sion implanter at the disposal of Maria Curie-Skłodowska University (MCSU)
[82] Figure2.1showsthegeneralviewoftheUNIMASimplanterseeingfromthesideofthe accel power supply side – in the first plan.
Table1.Thesampleshavebeenusedinthepresentstudy,whichwereun-irradiatedand irradiated by Ne + , Ar + , Kr + , Xe + ions at four different energies 100, 150, 200, and 250keV.
Fig.2.1.ThegeneralviewoftheUNIMASimplanterseeingfromthesideoftheaccel power supply side – in the first plan. species 100 150 200 250
The scheme of the UNIMAS implanter is shown in Fig 2.2 Ions beams were produced using the ARC discharge ion source in the configuration without an internal evaporator [82] The feeding gas was introduced to the plasma chamber using a dosing valve Ion beam was initially extracted through an extraction opening (~ 0.7 mm in diameter) using voltage of 25 kV and then formed by a standard lens triplet and then mass-separatedusing90 0 magneticsectorfield(fieldstrengthupto0.5T,radius0.5m) A separated beam of diameter ~ 0.5 cm was then accelerated to its final energy(energy beam accuracy better than 1%) and swept in both vertical and horizontal directions in order to provide irradiation spatial homogeneity better than 1% Irradiations were performed at room temperature (no additional heating or cooling during the bombardment) The samples were fixed to the sample holder using the conducting carbonadhesivetape.Thebasepressureintheirradiationchamberwas10 -6 mbarduring the process - it was provided by a turbomolecular vacuum pump The ion fluence was measured using the charge integrator with accuracy of 1% The maximum irradiation current density was in order of1𝜇𝐴/𝑐𝑚 2
Fig.2.2.TheschemeoftheUNIMASimplanter.Z–ionsource,L–electrostaticlenses, KF– FaradayCup,M –ionbeammonitor,MS–s e p a r a t i n g electromagnet,Sz–slit,A – accelerator tube, ST –i o n b e a m s t o p p e r , S M – i o n b e a m m o n i t o r i n g s y s t e m , T – t a r g e t , K 1 – t a r g e t c h a m b e r , K 2 – C h a m b e r f o r p a r t i c l e i o n b e a m i n d u c e d r a d i a t i o n s t u d i e s ( c u r r e n t l y d e c o m i s s i o n e d ) , M O – m o n o c h r o m a t o r , V – v a c u u m p u m p
SRIMcalculation
The Stopping and Range of Ions in Matter (SRIM) is a set of tools that allows simulation of the movement of ions in matter as well as many other properties of ions interaction with matter [83] Radiation materials science and ion implantation research both frequently use this program This software's major uses are for calculating ion energy attenuation and ion range in materials, for analyzing the physical changes that ion implantation makes to a material, and for calculating sputtering which involves removing atoms from ionic-induced materials at any energy When the basic input parameters,suchasiontype,energylevel,andmaterialstructureareenteredintoSRIM, itcalculatesandproducesresultssuchasthedistributionofionsinmatter,atomicimpact casts in target, content and distribution of displaced atoms, holes; sputtering, ionization and phonon formation in the material; and energy loss resulting from ions' interactions withatomicnucleiandelectrons.Singleionsareassumedtochangedirectionwhenthey enter a target as a resultof binarynuclear collisionsandtravelalong straightfree-flight trajectories in between collisions Nuclear and electronic energy losses cause it to lose energy, and the subcascades are produced by lattice atoms recoiling along ion trajectories When the collisional energyis high enough, the target atomwill be moved from its lattice position sufficiently to preclude random thermal recombination with its vacancy Recombination of point defects can also occur during successive irradiation when the damage region overlaps [62] This recombination, however, is not taken into account by SRIM, it merely shown the total number of lattice atoms that have been displacedwithoutconsideringtheirfinallocation.Asaresult,fewerdisplacedatomswill actually be present in the lattice than was predicted Moreover, SRIM always assumes thecollisionwasmadeonanewtarget,meaningthereisnotraceofthecollisionhistory fromearliercollisions.Itshouldbenotedthatthedirectionalcharacteristicsofthecrystal lattice are disregarded in SRIM calculations since the target is considered to as amorphous with atoms at random locations.
SRIM calculations have been used in the present study to predict the mean penetration depths of ions, target damages, and the stopping powers along the ion trajectory.T h e c a l c u l a t i o n s u t i l i z e t h eDetailed calculation with full damage cascadesmode using 99999 ions The ion mixing process at the TiO2/SiO2i n t e r f a c e i s d e s c r i b e dbythepresenceofdisplacedTiandSiatomsaswellastheholescreatedatthetransition region.
The degree of atomic mixing varies with the mass and energy of the incoming ion could be predicted by the changes in displaced atoms and the generated vacancies In addition, the atomic mixing process and the it’s degree could be interpreted by the variationintheionenergylossandprojectedrange.Fig.2.3showsascreenshotofSRIMsimulationresults for100-keVXeirradiationin30-nmTiO2/130-nmSiO2/Sitargetwithan ion distribution window being displayed.
TiO2/130-nm SiO2/Si target.
RutherfordBackscatteringSpectrometry(RBS)–anIBAmethod
As was mentioned, the methods that use ion beam for analyzing the features, properties of the materials, in which ion beams are accelerated to the proper energy by the charge particle accelerators, are called by Ion Beam Analysis – IBA The principle ofthismethodsbasedontheinteractionoftheionwiththeatomsintheanalyzedmaterial
[84].WhenthesamplesarebombardedbytheionsintheenergyrangeofMeV,theions make collisions with atoms and the nuclei by different mechanisms lead to emission of X rays, charge particles, neutron, or Gamma rays Detection of these events allows to obtain the information of the elemental composition, their concentration and depth distributioninthe samples.IBAisthegeneralterminologyof theanalytical methodsas Rutherford Backscattering Spectrometry (RBS), Elastic Recoil Detection (ERD), NuclearReactionAnalysis(NRA),andParticleInducedX-rayEmission(PIXE).These methods have been widely used in a variety of scientific fields for various types of material by their advantages, such as allowing to determine depth profiles of the elements,analyzingthenearsurfacelayersofmaterialatdepthrangeof𝜇m,identifying the impurities in matrix solid, and investigating the defects in crystal by channeling technique.
As a result, IBA techniques have gained many potencies, especially when used together In material science, these characterizing techniques have been used to address a very broad range of issues including basic elemental diffusion, corrosion, reaction kinetics, or the mixing of multilayers, etc [64].
The main analysis method has been used in this work to characterize sample structureandqualifymixingprocessistheRBSmethod.Inprinciple,RBSbasedonthe elastic collision between ions and target atoms (as mentioned in section 1.3) In which, an ion beam (typically MeV He) is directed onto a solid sample, enters the sample, scatters on atomic nuclei and travels back out to be detected, showing an energy distribution in form of spectra [85] The RBS spectra thus enables to answer the three fundamental quantitative characterization questions: what is in the sample? (mass analysis)howmuchisthere?(probabilityofcollisions)andwhereisit?(depthanalysis).
Three basic concepts enter into the description of RBS consist of kinematics factor, energy loss and scattering cross-section, each confers to the special possibilities.
Consideranelasticinteractionbetweenanion(ofmass𝑚,energyE)andatarget nucleus (of massM,at rest) The energies of the incident ion and the recoil atom at scatteringangleθcouldcouldbedeterminedbytheconservationprinciples.Kinematicfactor
𝐾is defined as the ratio of ion energybefore (𝐸0) and after (𝐸1) collision (see equation 1.8).Fortabulatedvalueof𝐾andknownvaluesofm,θcould,and𝐸0,theproduct𝐾𝐸0g i v e s t h e energyof theparticlesthatare backscattered fromthesurface atoms(of mass𝑀) of atarget.Thesignalforthisenergy,whichisassociatedwithsurfaceedgeinRBSspectra, will not be changed with different incident angles A heavier atom will produce a stronger backscattering of ions than a lighter atom will The target species thus can be determined in this manner.
Dependingontheionenergyandthetargetmaterial,theions arenotonlyscatter offtheatomsfromthetarget'ssurfacebutarealsodetectedfromscatteringeventsinthe bulkofthetargettoamaximumdepthofseveralthousandangstroms.Duetothelossof energy traversingthe material,𝑑𝐸/𝑑𝑥, the ions that backscatter from atoms below the surface of the target will be detected with increasinglylower energies at greaterdepths As a result, the width of a backscattering spectrumpeak is preciselyproportional to the thicknessofelementalfilm.Theenergywidthofthepeakthuscanbeusedtodetermine a thin film's thickness by
(2.1) where∆𝐸is the width oft h e p e a k ,[𝜀]is the stopping cross section factor which is a function ofθcouldand the stopping cross sections𝜀, and𝑁is the atomic density The thickness of an extremely thin film can also be determined by
𝑡= 𝐴𝑐𝑜𝑠𝜃 𝜎𝞨𝑄𝑁 , (2.2) where𝐴is the total integrated number of counts under the spectrum peak,𝜃is the scatteringangle,𝜎isthescatteringcrosssectionoftheelement,𝛺isthesolidangleof thedetector,and Qisthetotalchargecollected Foranelementalfilm,both approaches provide nearly the same results.
Thedifferentialscatteringcross-section𝑑𝜎/𝑑𝑄,whichisbasedontheRutherford scatteringformulation(seeequation1.12),providestheexpected yieldof backscattered particlesinacertainsetupforaspecifiednumberof scatteringcentersinthetarget.Itis clearthatheaviertargetsarebetterscattersforagivenprojectile,andthatbackscattering yield rises for projectiles with light atomic numbers Moreover, the yield changes as1/𝐸 2 , and for gradually decreasingprojectile energy, it rises quickly Similar to this, if the scattering angle is lowered, a sharp rise in yield is shown In order to increase yield andsensitivity,Rutherfordbackscatteringspectrometrytypicallyemployslowerenergy and heavier mass ion beams.
UsingRBS,amaximumanalyticaldepththatcouldbeachievedisafew𝜇mwith the incoming particle being He + ions and about 20𝜇m with the coming particle being theprotons.Theobtaineddataallowstoaccuratelydeterminethechangesinthecontent oftheelementswithdepthbytheresolutionatnmrange.Therefore,notonlybeingable toidentifyinformationofahomogeneousthinlayer,RBScanalsobeappliedtoanalyze multi-layer thin structures and the variation of the interface between layers The analytical sensitivity is a few percent for light elements and can reach a few part per millionths (ppm) for heavy elements The experiments of RBS is quite simply and quickly performed, without needs of standard samples.
In this work, the RBS experiments were carried out using ion beams accelerated by a Van de Graaff accelerator (Fig 2.4a) at the EG-5 group, Frank Laboratory of Neutron Physics, JINR, Dubna, Russia This system allows to accelerate ions up to the energyinrangeof0.9–3.5MeVwiththeenergyspread 0.T h e amount of waveenergylostto the material isindicated by the extinction coefficient, related to the absorption coefficient𝛼as follows
Light that propagates in a medium can be characterized using the complex refractive index𝑁defined by:𝑁=𝑛–𝑖𝑘,with𝑛is the refractive index, and𝑘is the extinctioncoefficient.
An electromagnetic wave influences the material by generating polarization within the material This is described by the dielectric constant𝜀and is closely related to the complexrefractiveindex𝑁.FromMaxwell’sequationsforconductors,𝑁≡ 𝑛 − 𝑖 𝑘 is defined as𝑁 ≡ 𝜀.² ≡ 𝜀.
InteractionofLightandMaterials
When light hits a surface separating two materials with different refractive indexes, or oblique incidence, different phenomena occur The direction of the propagation of light may change due to either refraction or reflection The most important phenomena for ellipsometry are transmission (refracted wave) and reflection (reflected wave) Snell’s law proves that the angle of the refracted light is dependent of therefractiveindexofbothmedia.Snell’slawisdescribedbythefollowingequation:
𝑁𝑖𝑠𝑖𝑛𝜃𝑖=𝑁𝑡𝑠𝑖𝑛𝜃𝑡,where,𝑁𝑖and𝑁𝑡representthecomplexrefractiveindicesofthetwo mediai a n d t , a n d𝜃 𝑖a n d𝜃 𝑡t h ea n g l e o f i n c i d e n c e a n d a n g l e o f t r a n s m i t t e d l i g h t respectively (Fig 2.8a).
Fig.2.8.Reflectionandrefractionof lightaccordingtoSnell’slaw(a), andtheplaneof incidence (b) [93].
In ellipsometry, the electric field vector is expressed in the (s, p, z) coordinate system, rather than the three dimensional Cartesian (x, y, z) coordinate system This coordinate system simplifies the notation used to describe incident light The z-axis describesthedirectionoftheincidentwave,whiles-polarizedlighthastheelectricfield vector perpendicular to the plane of incidence, p-polarized light parallel to the plane of incidence.(German: s = senkrecht = perpendicular; p = parallel) This is illustrated inFigure2.8.b.Whenlightreflectsofforrefractsonasamplesurfaceatobliqueincidence, the p- and s- polarized light behave differently upon interacting with the sample The electric field vector is split into a p- and s- electric field vectors for incident, reflectedand refracted light, respectively notated as: Eip, Eis, Etp, Ets, Erpa n d E rs The amplitude coefficients for p- and s-polarized light is defined byFresnel equations consist of rp, rs, rp, rs Where rp(s) is the reflection coefficient and tp(s) the transmission coefficient forp(s) polarized light These values are utilized to characterize light reflection and transmission in terms of amplitude and phase variations.
EllipsometryMeasurements
Ellipsometrymeasuresthechangeofpolarizationoflightforeachwavelengthby thinfilmsanddeterminetheψandΔandΔvalues.Throughaproperopticaldispersionmodel, these angles are modeled to obtain the optical constants and thickness of thin films In ellipsometry, Δ is considered as the difference in phase difference between the phase difference of p and s incoming waves and of the outgoing wave, and Ψ is the relative amplitude ratio.
Inarealsituation,multiplemediumsshouldbeconsideredandmultipleinterfaces influence the reflected light measured by ellipsometry As a result, ellipsometry offers information on all media The ratio of the resultant reflected wave to the amplitude of the incident wave can be calculated by the total reflection coefficient for p-parallel polarized light (𝑅 𝑝 ) and (𝑅 𝑠 )
𝑝 1+𝑟 𝑝 12 𝑟 𝑝 23 𝑒 −𝑖2𝛽 𝑠 1+𝑟 𝑠 12 𝑟 𝑠 23 𝑒 −𝑖2𝛽 (2.5) where𝑟 𝑥( 𝑥 + 1)is the Fresnel coefficient for media𝑥and𝑥 + 1, calculated from the Fresnelequations,and𝛽isthethicknessfactordescribingtheinfluenceofthethickness on the polarization state of the electromagnetic wave.𝛽is calculated using:
𝛽=2𝜋𝑁2𝑐𝑜𝑠𝜃𝑟/𝜆, (2.6) wheredisthethicknessofthemedium,withλtheconsideredwavelength,𝑁thecomplexrefractive index and𝜃𝑟t h e a n g l e o f t h e r e f r a c t e d w a v e
Fig.2.9 Reflectionand refractionof lightateachinterface leading tomultiple beamsin a thin film [93].
It should be notice that Eq 2.4 describes the sample homogeneous isotropic environment, flat layer with two opposite parallel walls and homogeneous isotropic substrate In a real situation, multiple mediums should be considered and multiple interfaces influence the reflected light measured by ellipsometry As a result,ellipsometry offers information on all media The following are the main equipment needed to gather ellipsometry data: light source, polarization generator, sample,polarization analyzer, and detector Common ellipsometer configurations include rotating analyzer (RAE), rotating polarizer (RPE), rotating compensator (RCE), and phase modulation (PME) [92] In the present study, the RAE configuration was used.Fig 2.10 is an overview of an RAE operation.
Light from the source is polarized in the PSG by a linear polarizer oriented to provide both p- and s-electric fields The light reflects from the sample, changing the polarizationtogenerallyanellipticalstate.Theellipticallypolarizedlighttravelsthrough the rotating analyzer to the detector Detector transforms light into an electronic signal toascertainthepolarizationof the reflectedlight.To determine the polarization change caused by the sample reflection, this data is compared to the known input polarization. This is how Psi (Ψ) and Delta (Δ) are measured in ellipsometry.
In the present study, the ES experiments were conducted at the Institute of Electron Technology in Warsaw, Poland using the RAE configurations The ellipse of theanglesΨ(λ)andΔ(λ)was measuredwiththelightwavelengthfrom250nmto1100 nm,withthestepof1nmatsixdifferentincidentangles(i.e.,theanglebetweendirection ofincidentlightbeamandthenormalofthesamplesurface),namely70.0 0 ,72.0 0 ,74.0 0 ,
76.0 078.0 0, and80.0 0 Onceallthese SEexperimentshaddone,allthemeasuredangles Ψ(λ) and Δ(λ) were used as input to calculate the spectra of Ψ(λ) and Δ(λ) using the Multiple- angle-of-incidence Ellipsometry(MAIE) method [90] In order to analyze theopticalparametersoftheirradiatedTiO 2 /SiO2/Sisystems,afour-layeropticalmodelwas constructed.ItconsistsofaSisubstrate,aSiO2l a y e r ,TiO2l a y e r ,andaninterfacelayer between SiO2a n d T i O 2 It was assumed that all layers are homogeneous, and theboundaries between the materials are sharp The thickness, and concentration of the compoundsofthemateriallayersarefreeparameters,whosevaluesweredeterminedbyfittingtothe experimentalΨ(λ)andΔ(λ)spectra.Knowingthevaluesofalltheparametermodels, the refractive index𝑛, and extinction coefficient𝑘, of the investigated samples were deduced using the effective medium approximation (EMA) method [91].
X-rayPhotoelectronSpectroscopy(XPS)method
X-rayphotoelectronspectroscopy(XPS)isasurface-sensitiveanalyticalmethod that bombards a material's surface with X-rays and measures the kinetic energy of the released electrons [92] The surface sensitivity of this technique and its capacity to extract chemical state information from the elements in the sample are two of its key qualitiesthatmakeiteffectiveasananalytical method.SoftX-raysareusedin theXPS methodtoexcite thecore and valence electronsof surface atoms.If theX-rayenergyis high enough, photoelectrons are released from the substance, and the instrument measures their kinetic energies (𝐾𝐸) The photoelectric effect, which represents this excitation process, is shown in Figure 2.11 Based on binding energy (𝐵𝐸), which iscalculated in relation to the Fermi level (E Fermi ) of the individual atoms, differencesbetweenchemicalelementsinthenearsurfaceregionarefound.Thekeyequationofthe photoeffect mechanism that combines the𝐾𝐸and 𝐵𝐸of the photoelectron is given by:
𝐾𝐸=ℎ𝜗−𝐵𝐸−∅𝑠𝑝𝑒𝑐𝑡𝑟𝑜𝑚𝑒𝑡𝑒𝑟, (2.7) whereℎ𝜗represents the energy of the absorbed photon, and∅𝑠𝑝𝑒𝑐𝑡𝑟𝑜𝑚𝑒𝑡𝑒𝑟i s t h e w o r k f u n c t i o n o f t h e s p e c t r o m e t e r E v e n w h e n t h e F e r m i l e v e l s o f t h e s p e c t r o m e t e r a n d t h e s a m p l earelinedupinelectricalcontact,theirassociatedworkfunctionsarenotthesame.Theworkfunctionsdifference, which isreallythecontact potentialbetweenthesample and the spectrometer, accelerates or decelerates an electron as it travels to the analyzer.
Fig.2.11.Schematicoftheelectron'senergyindicatingtheabsorptionofaphotonand emission of a photoelectron 2p level [93].
The element and orbital from which the photoelectron was emitted are noted on photoelectronpeaks.Forinstance,"O1s"referstoelectronsthatleaveanoxygenatom's
1sorbital.Anyelectronsemittedfromthesamplewithbindingenergiessmallerthanthe energy of the x-ray source can be seen using the XPS technique An electron's binding energyisapropertyof thematerialandisunaffectedbytheX-raysourcethatejectedit The binding energy of photoelectrons will not change when experiments are run with variousX- raysources,butthekineticenergyofthephotoelectronsthatarereleasedwill vary, as shown by Eq. 2.7 This general equation makes it obvious that the binding energies of the released electrons affect the kinetic energies that are measured by the spectrometer.Adepictionofthenumberofelectronsdetectedversustheirkineticenergy characterizesatypicalXPSspectrum.Thisspectrumtellsushowtheenergyofelectrons is distributed throughout a material The Fermi energy is used as the "natural" zero reference point forsolids inactual data accumulation.Each elementthat existsinoron thesurfaceofthematerialcreatesacharacteristicsetofXPSpeaksatdistinctivebinding energyvalues that directlyidentifyeach element These characteristic peaks are related to the atom's electron configuration, such as 1s, 2s, 2p, 3s, etc XPS has been used to analyze the surface of practically any substance from plastics to fabrics, dirt and semiconductors All elements with order numbers 3 and higher can be measured, however hydrogen and helium cannot be found because their orbitals' diameters are so small that their electrons' photoemission cross sections are nearlyzero It turns out that the inner electrons are most susceptible to being knocked out by X-ray energies.
ItiscrucialtorememberthatXPSonlydetectselectronsthathavereallyescaped into the instrument's vacuum The photo-emitted electrons that have escaped into the instrument'svacuumarethosethatcamefromthe material'stopfewnanometers.Allof the deeper photo-emitted electrons, produced as the X-rays pierced the material by1 to 5 micrometers, are inelastically lost before escaping The probabilities of an electron interactingwithasubstanceishigherthanthatofaphoton.Theelectron'spathlengthis on the scale of tens of Ångstrửms, but the photon's path length is on the order of micrometers. Consider a volume element of the sample with thickness𝑑𝑧at a depth𝑧underthesamplesurface.Thephotoelectronsthatareemittedatanangleθtothesample surface's normal enter the detector and add to the spectrum The probability that a photoelectron will escape from the sample and enter semi-infinite spacewithout losing energy is:
𝑑𝑧, byphotoionization produces the intensityof photoectrons𝑑𝐼and assuming that the thickness of the sample is much larger than few Ångstrửms then we can calculate the intensity of the electrons emitted from the depth𝑑by following integral:
(2.9) where𝛼is a coefficient depending on photoemission crosssection incident X-ray flux, angle between photoelectron path and analyzer sample axis and others The Beer- Lambertrelationshipisthenusedtodeterminetheintensityofelectrons𝐼𝑑e m i t t e dfrom alldepthsgreaterthan𝑑inadirectionnormaltothesurface.Theelectronsthatcanleave thesurfacewithoutlosingenergyandcontributetothepeaksinthespectraarethosethat originate within tens of Ångstrửms below the solid surfaces The remaining electrons that undergoinelastic processes, suffer energy loss, and either contribute to secondary emission or enhance the spectral background This is the primary cause of the XPS method's high surface sensitivity.
Fig.2.12: The electron mean free path versus their kinetic energy for variety ofm e t a l s [94].
As indicated in Fig 2.13a and 2.13b, XPS measurements were taken for the present dissertation using the UHV (ultra-high vacuum) equipment at the Faculty of Chemistry, MCSU in Lublin, Poland Firstly, the sample is introduced through a prechamber that is in contact with the outside environment This prechamber is closed and pumped to low vacuum The sample is then placed into the main chamber, which hasan ultra-highvacuumenvironment.Ultra-highvacuum(10 −9 mbar)was maintained throughoutt h e a n a l y t i c a l p r o c e s s t o e n s u r e t h e p h o t o e l e c t r o n s t r a v e l e d t h e f a r t h e s t distance feasible along their mean path and to prevent contamination of the sample surface Fluids and other outgassing materials cannot be examined under low pressure inultra-highvacuum,makingthemunsuitableforXPScharacterization.AnX-raybeam isusedtoirradiatethesample'ssurface.Byreflectingfromabentquartzcrystal,X-rays can be efficiently monochromatized to create so-called monochromatic X-ray sources. Monochromatic X-rays have the advantage of having a narrower natural beam width than the unfiltered X-rayline, which enhances the resolution of the photoelectric peaks in the XPS spectra.
Electrostatic transfer lens transport the photoelectrons from the X-ray-excited material to the electrostatic hemispherical mirror analyzer An electron detector, a hemispherical deflector with entrance and exit slits, and a multi-element electrostatic input lens make up a traditional hemispherical analyzer The deflector, which has two concentric hemispheres (of radius𝑅 1 and𝑅2), is the analyzer's main component These hemispheres are maintained at a potential difference of𝛥𝑉when the constant analyzer energymodeisused.Thefunctionsoftheelectrostaticlensisdecelerationandfocusing thephotoelectronsontotheentranceslit.Electronsenteringtheanalyzerwithanenergy
𝐸0a n d a r a d i u s𝑅 0= ( 𝑅 1 +𝑅 2 )/2, follow a circular path with constant radius This energy𝐸0is defined as the pass energy The chosen pass energyand the analyzer's size determine the potentials appliedto the inner and outer hemispheres On the end of the analyzer the electrons hit the electron detector, and their energy is measured We can efficientlyrecordthephotoemissionintensityversusthephotoelectronkineticenergyby scanning the lens retarding potential.
Inthiswork,XPSmethodwasusedtostudyexperimentallyinfluenceofchangesin chemical composition induced by ion irradiation on mixing amount of TiO2/SiO2systemsasfunctionofionenergy.XPSspectrawererecordedintheenergyrangeof450 eV
- 462 eV, this energy range represents the binding energy of the electrons Ti 2p. a) b)
Fig 2.13 UHV ultra-high vacuum system in Faculty of Chemistry, MCSU in Lublin, Poland (a), and the simplified schematic (b) [96].
The Thermo Scientific equipped with a monochromatic Al Kradiation source (E= 1486.6eV).Alanodeswasusedbecauseofadominant,strongresonanceintheX-ray spectrum. For the Al X-ray, a doublet arises from the 2p1/2, 2p3/2→ 1 s e l e c t r o n i c r e l a x a t i o n ThesearesocalledKαhν)1,2l i n e s TheanalyserwasoperatedintheCAE modewithapassenergyof20eV.Thismethodprovidesdetectionlimitsto~0.1%atomic,is very surface sensitive (top