An external bias is applied to two samples containing composite surface structures, while recording an XPS spectrum. Altering the polarity of the bias affects the extent of differential charging in domains that are chemically or electronically different to create a charge contrast. By utilizing this charge contrast, we show that two distinct silicon nitride and silicon oxynitride domains are present in one of the composite samples. Similarly, we use this technique to show that titanium oxide and silicon oxide domains exist as separate chemical entities in another composite sample. © 2007 American Chemical Society

Dâna, A.

Ertas G.

Suzer, S.

Bilkent University Institutional Repository

Differentiation of Domains in Composite SurfaceStructures by Charge-Contrast X-ray PhotoelectronSpectroscopySefik Suzer,*,† Aykutlu Dâna,‡ and Gulay Ertas†Chemistry Department and Physics Department, Bilkent University, 06800 Ankara, TurkeyAn external bias is applied to two samples containingcomposite surface structures, while recording an XPSspectrum. Altering the polarity of the bias affects theextent of differential charging in domains that are chemi-cally or electronically different to create a charge contrast.By utilizing this charge contrast, we show that two distinctsilicon nitride and silicon oxynitride domains are presentin one of the composite samples. Similarly, we use thistechnique to show that titanium oxide and silicon oxidedomains exist as separate chemical entities in anothercomposite sample.X-ray photoelectron spectroscopy (XPS) is a powerful analyticaltechnique for characterizing different chemical/physical forms ofelements within the top 0-10-nm surface layers.1 In principle,determination of the chemical state of an element in a homoge-neous surface is straightforward. However, in practical analysis,one is faced with having to analyze a number of compositematerials that are heterogeneous from the viewpoint of micro-structure, chemical composition, or both. In general, a specimenfor XPS analysis should be conductive, and in the case ofcomposite materials, the electronic conductivity may not beuniform and differential charging may develop as a result ofphotoemission from insulating components. Although severaltechniques have been developed to minimize the charging,complete elimination is not always possible.2 The positive chargingis usually compensated by directed flow of low-energy electronsfrom an external unit (flood gun) to the sample, which undercertain circumstances, overneutralizes it and may even causenegative charging.Purposely generated negative charging, called controlledsurface charging (CSC), has been successfully used for derivingsome chemical/physical properties of various surface structures.3-7Lau and co-workers have utilized the CSC to extract structuralandelectricalpropertiesofultrathindielectricsonsemiconductors.8-11Thomas et al.12 were able to separate the surface spectrum (mainlysilicon dioxide) from the silicon substrate spectrum (consistingof contamination and silicon dioxide on silicon), by use of surfacecharging, and similar applications were also reported by Ermolieffet al.13 and Bell and Joubert,14 while Miller et al. applied thetechnique to separate the XPS signals of the fiber from theexposed matrix at fractured surfaces.15 Surface charging was alsoutilized for lateral differentiation of mesoscopic layers and fordepth profiling.6,7 One interesting application of the surfacecharging for composite surface structures was reported by Ohnoet al.16 for separate examination of the chemical nature of thecarbon fibers and the BMI resin, which enabled them to resolvethe carbon fiber and the resin phases in one specimen.In all of these studies, surface charging was controlled or variedvia a low-energy electron flood gun, whereas, as we attempt todemonstrate in the present contribution, useful information canalso be extracted by analysis of the positively charged surfaces.We control or enhance the positive charging by applying anexternal bias to the sample rod while recording the spectrum,similar to what has been reported by others.17-21 We were able* Towhomcorrespondenceshouldbeaddressed.E-mail: suzer@fen.bilkent.edu.tr.† Chemistry Department.‡ Physics Department.(1) Briggs, D.; Seah, M. P. Practical surface analysis. Part I. Auger and X-rayphotoelectron spectroscopy, 2 nd ed.; John Wiley & Sons Ltd.: London,England, 1996.(2) Cazaux, J. J. Electron Spectrosc. Relat. Phenom. 2000, 113, 15-33.(3) Sherwood, P. M. A. Surf. Sci. 2006, 600, 771-772.(4) Lau, W. M.; Wu, X. W. Surf. Sci. 1991, 245, 345-352.(5) Lau, W. M. J. Appl. Phys. 1989, 65, 2047-2052.(6) Shabtai, K.; Rubinstein, I.; Cohen, S. R.; Cohen, H. J. Am. Chem. Soc. 2000,122, 4959-4962.(7) Doron-Mor, H.; Hatzor, A.; Vaskevich, A.; van der Boom-Moav, T.; Shanzer,A.; Rubinstein, I.; Cohen, H. Nature 2000, 406, 382-385.(8) Lau, W. M. Appl. Phys. Lett. 1989, 54, 338-340.(9) Chan, R. W. M.; Kwok, R. W. M.; Lau, W. M.; Yan, H.; Wong, S. P. J. Vac.Sci. Technol., A 1997, 15, 2787-2792.(10) Chan, R. W. M.; Kwok, R. W. M.; Lau, W. M. J. Appl. Phys. 1996, 79, 3635-3639.(11) Lau, W. M.; Jin, S.; Wu, X.-W.; Ingrey, S. J. Vac. Sci. Technol., B 1990, 8,848-855.(12) Thomas, J. H., III; Bryson, C. E., III; Pampalone, T. R. J. Vac. Sci. Technol.,B 1988, 6, 1081-1086.(13) Ermolieff, A.; Deleonibus, S.; Marthon, S.; Blanchard, B.; Piaguet, J. J.Electron Spectrosc. Relat. Phenom. 1994, 67, 409-416.(14) Bell, F. H.; Joubert, O. J. Vac. Sci. Technol., B 1997, 15, 88-97.(15) Miller, J. D.; Harris, W. C.; Zajac, W. G. Surf. Interface Anal. 1993, 20,977-983.(16) Ohno, S.; Lee, M. H.; Lin, K. Y.; Ohuchi, F. S. J. Vac. Sci. Technol., A 2001,19, 1116-1120.(17) Gouzman, I.; Dubey, M.; Carolus, M. D.; Schwartz, J.; Bernasek, S. L. Surf.Sci. 2006, 600, 773-781.(18) Dubey, M.; Gouzman, I.; Bernasek, S. L.; Schwartz, J. Langmuir 2006, 22,4649-4653.(19) Johansson, G.; Hedman, J.; Berndtsson, A.; Klasson, M.; Nilsson, R. J.Electron Spectrosc. Relat. Phenom. 1973, 2, 295-317.(20) Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Electron Spectrosc. Relat.Phenom. 1973, 2, 441-447.(21) Havercroft, N. J.; Sherwood, P. M. A. Surf. Interface Anal. 2000, 29, 232-240.Anal. Chem. 2007, 79, 183-18610.1021/ac0613683 CCC: $37.00 © 2007 American Chemical Society Analytical Chemistry, Vol. 79, No. 1, January 1, 2007 183Published on Web 11/16/2006to apply this simple method (i) to analyze some dielectricalproperties of ultrathin silicon oxide layers, (ii) to separateotherwise overlapping XPS peaks of the same atom belonging todifferent chemical entities, and (iii) to derive information relatedto the proximity of the atoms within composite multilayers.22-24We now extend our application to detect the presence of domainsthat are chemically or electronically different in different compositesurface structures. For the first application, analysis of an annealedsilicon oxynitride sample is presented, and for the second one, asample containing both silicon oxide and titanium oxide ispresented.Silicon nitride has received a great deal of attention due to itssuperior mechanical and electrical properties for applications inmicroelectronics and optoelectronics. Some of these propertiesinclude better breakdown strength, higher electrical permittivity,improved barrier to impurity diffusion, and superior resistanceto plasma and radiation damage.25-29 However, most preparationroutes produce oxynitrides rather than the pure nitride. Then,the question arises as to whether oxides and nitrides are separateentities or domains, especially after thermal annealing. Distin-guishing the various chemical domains present in a given sampleis an analytical challenge, and partial success has been achievedby IR spectroscopy.30,31 In both XPS and IR techniques, an averagestructure of the sample examined is determined, and the pres-ence/absence of domains with different chemical compositionscannot be verified easily. Due to the predominantly amorphousnature of the oxynitride structures, diffraction techniques cannotbe of help either.Titanium and silicon mixed oxides are encountered in variouscatalytical and photocatalytical applications, antireflection coatings,filters, mirrors, etc.; and similar questions related to domains arealso of importance.32-36 For some coating applications, control ofthe refractive index is the most crucial issue, and two approacheshave been used in order to get intermediate refractive indices:(i) use mixed oxides with a high and low index material,37,38 and(ii) stack very thin layers of pure materials to achieve any indexprofile.39 These two methods require good control of the formationof mixed material or of interfaces, and it is important to knowhow the interfaces are formed. Using a combination of XPS andFT-IR techniques, the formation of SisOsTi bond or SiO2/TiO2in a single phase material depending on the Ti content in themixture was claimed by different groups.33,38,40,41 In all thesestudies, separate phases were only identified by the behavior ofthe O 1s core level peak to indicate that at least two phases coexist.The appearance of an extra shoulder in the O 1s spectra of the(22) Ulgut, B.; Suzer, S. J. Phys. Chem. B 2003, 107, 2939-2943.(23) Suzer, S. Anal. Chem. 2003, 75, 7026-7029.(24) Karadas, F.; Ertas, G.; Suzer, S. J. Phys. Chem. B 2004, 108, 1515-1518.(25) Chaiyasena, I. A.; Lenahan, P. M.; Dunn, G. J. Appl. Phys. Lett. 1991, 58,2141-2143.(26) Bustillo, J.; Chang, C.; Haddad, S.; Wang, A. Appl. Phys. Lett. 1991, 58,1872-1874.(27) Dunn, G. J. J. Appl. Phys. 1989, 65, 4879-4883.(28) Dunn, G. J. Appl. Phys. Lett. 1988, 53, 1650-1651.(29) Yount, J. T.; Lenahan, P. M.; Krick, J. T. J. Appl. Phys. 1994, 76, 1754-1758.(30) Hernandez, M. J.; Garrido, J.; Martinez, J.; Piqueras, J. Semicond. Sci.Technol. 1997, 12, 927-932.(31) Zhang, B. R.; Yu, Z.; Collins, G. J.; Hwang, T.; Ritchie, W. H. J. Vac. Sci.Technol., A 1989, 7, 176-188.(32) Gracia, F.; Holgado, J. P.; Caballero, A.; Gonzalez-Elipe, A. R. J. Phys. Chem.B 2004, 108, 17466-17476.(33) Gracia, F.; Yubero, F.; Espinos, J. P.; Holgado, J. P.; Gonzalez-Elipe, A. R.;Girardeau, T. Surf. Interface Anal. 2006, 38, 752-756.(34) Mergel, D.; Buschendorfa, D.; Eggerta, S.; Grammesb, R.; Samsetc, B. ThinSolid Films 2000, 371, 218-224.(35) Gracia, F.; Holgada, J. P.; Gonzalez-Elipe, A. R. Langmuir 2004, 20, 1688-1697.(36) Abad, J.; Rogero, C.; Mendez, J.; Lopez, M. F.; Martin-Gago, J. A.; Roman,E. Appl. Surf. Sci. 2004, 234, 497-502.(37) Demiryont, H. Appl. Opt. 1985, 24, 2647-2650.(38) Gallas, B.; Brunet-Bruneau, A.; Fisson, S.; Vuye, G.; Rivory, J. J. Appl. Phys.2002, 92, 1922-1928.(39) Gluck, N. S.; Sankur, H.; Heuer, J.; DeNatale, J.; Gunning, W. J. J. Appl.Phys. 1991, 69, 3037-3045.(40) Lassaletta, G.; Fernandez, A.; Espinos, J. P.; Gonzalezelipe, A. R. J. Phys.Chem. 1995, 99, 1484-1490.(41) Larouche, S.; Szymanowski, H.; Klemberg-Sapieha, J. E.; Martinu, L.;Gujrathi, S. C. J. Vac. Sci. Technol., A 2004, 22, 1200-1207.Figure 1. Si 2p, O 1s, and N 1s regions under +10 and -10 V dc bias for SiO2 samples, one of them containing ∼6-nm and the other∼300-nm oxide layer together with a ∼300-nm composite siliconoxynitride sample.184 Analytical Chemistry, Vol. 79, No. 1, January 1, 2007samples with intermediate compositions was accepted as evidencefor the presence of SiO2 and TiO2 as separate phases.33,42 Thepresence of SisOsTi, confirming the presence of a single phasecontaining both Si and Ti was also claimed by FT-IR spectra ofSiO2, TiO2, and mixtures. However, the FT-IR data do not provideenough evidence for the formation of a single phase or separatephases in these samples,41 and none of the previous approachescan provide indisputable evidence about the presence or absenceof different domains or phases. As we will demonstrate below,our method, by controlling the charging capacity of differentcomponents in composite films, provides a clear experimental toolfor differentiation of domains that are chemically/electricallydistinct.EXPERIMENTAL SECTIONA Kratos ES300 electron spectrometer with Mg KR X-rays(nonmonochromatic) was used for XPS measurements. In thestandard geometry, the sample accepts X-rays at 45° and emittedphotoelectrons at 90° with respect to its surface plane are analyzed.Samples were electrically connected to the sample holder, whichwas grounded or externally biased with a dc power supply.Resolution of our spectrometer is slightly better than 0.80 eV asmeasured in the Ag 3d peaks. Since we extract binding energydifferences by fitting the entire set of peaks, we estimate our errorin measuring the binding energy differences to be better than0.1 eV. FT-IR spectra of the samples were obtained using a BrukerTensor FTIR system operating in the mid-infrared region, i.e.,400-4000 cm-1.Silicon nitride films were deposited on p-type silicon wafer oforientation (100) using disilane (2% in nitrogen) and ammonia asreactant gases in a PECVD reactor (PlasmaLab 8510C) at a sampletemperature of 350 °C and a process pressure of 1000 mTorr underan applied rf power of 10 W. Afterward, the samples were annealedin air at 1100 °C for 10 min. The composite SiOx + TiOy samplewas obtained by depositing a thin layer of titanium onto a siliconsubstrate.RESULTS AND DISCUSSIONAs we have shown in our previous publications, the siliconoxide dielectric layer develops an overall positive charge whichis reflected in the Si 2p, O 1s, and SiKLL Auger peaks. This is bestdemonstrated in thin (<10 nm) oxide layers on a silicon waferwhen both the silicon and silicon oxide Si 2p peaks are observableby XPS, and the measured binding energy difference betweenthem exceeds the true chemical shift of 3.2 eV.1 We have alsoshown that the measured difference can be controlled by anexternal bias as reproduced again in Figure 1a for a samplecontaining ∼6-nm thermal oxide layer.22,24 As can be gathered fromthe figure, the Si0 peak is not influenced by the bias but both theSi4+ and the O 1s peaks are affected by as much as 0.7 eV afteraltering the polarity of the bias. Under positive bias, more of theneutralizing low-energy electrons are attracted to the sample hencereducing the extent of the positive charge. For a thicker oxidesample, only the peaks of the oxide layer are accessible. As shownin Figure 1b, both the Si 2p and the O 1s peaks are broadened(due probably to enhanced charging), but again, both peaksdisplay similar shifts under external bias as opposed to the caseof the oxynitride sample of a comparable thickness as shown in(42) Brunet-Bruneau, A.; Fisson, S.; Gallas, B.; Vuye, G.; Rivory, J. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3738, 188-196.Figure 2. Si 2p and O 1s regions of XPS spectrum of compositesiliconoxynitride sample upon annealing at 1100 °C with +10 and-10 V external dc biases applied to the sample rod.Figure 3. Part of the FT-IR spectra of the composite siliconoxyni-tride sample before and after annealing at 1100 °C.Analytical Chemistry, Vol. 79, No. 1, January 1, 2007 185Figure 1c. Oxygen is present in all samples, but nitrogen is onlypresent in the oxynitride sample and can be assigned to N3-.1 Inthe oxynitride sample, we detect differences in the positions ofboth the Si 2p (0.2 eV) and the O 1s (0.4 eV) peaks, and nodifference can be detected in the N 1s peak position, afterchanging the polarity of the bias. We take this as a spectralsignature of the presence of at least two chemically differentdomains. One of the domains is probably pure silicon nitride, sincethe N 1s corresponds to that of the Si3N4. The other one cannotbe assigned to pure oxide, since the shift in the Si 2p is only halfof the shift in the O 1s. In order to gain more insight, we attemptedto mathematically decompose the Si 2p peak into two componentsas shown in Figure 2: one belonging to pure nitride, and the otherto the oxynitride to support our conclusion that these are the twochemically different domains (silicon nitride and silicon oxyni-tride). We should also stress that shifts observed in Figures 1and 2 are measured only after the sample is annealed at 1100 °C.The presence of different domains is also partially supported byFT-IR spectra of the silicon oxynitride film recorded before andafter annealing as illustrated in Figure 3. Before annealing, thespectrum has a main absorption peaks around 870 cm-1 and at1068 cm-1, which are associated with the absorption peaks of SisNO and SisO bonds, respectively.31 After annealing, the absorp-tion peak shifts toward 840 cm-1, which is characteristic of theSisN bond.Ti 2p, Si 2p, and O 1s regions of the XPS spectra of the othercomposite film containing both titanium and silicon oxides on asilicon substrate, recorded when the sample is subjected to +10and -10 V external bias, are shown in Figure 4. Whereas, nodifference can be detected in Ti 2p upon reversal of the polarityof the bias, as much as 0.6 eV difference can be induced in the Si2p peak. Accordingly, the O 1s peak can be successfully fitted totwo components: one exhibiting no shift that can be assigned astitanium oxide domains, and the other, which exhibits 0.6 eVdifference, to that of the silicon oxide domains. Hence, bycontrolling their charging capacity, we enforce domains to beseparated and identified (charge contrast).CONCLUSIONSIn this study, we demonstrate that by imposing a simpleexternal voltage stress to the sample rod, while acquiring XPSdata, we are able to control and measure the potential(s) developedas a result of charging in and on different domains. This simplecharge-contrasting procedure introduces to XPS a capability todifferentiate and determine the presence or absence of chemically/electrically distinct domains in composite surface structures.ACKNOWLEDGMENTThis work was partially supported by TUBA (Turkish Academyof Sciences) and by TUBITAK (The Scientific and TechnologicalResearch Council of Turkey) through the Grant 105M094.Received for review July 26, 2006. Accepted October 10,2006.AC0613683Figure 4. Ti 2p, Si 2p, and O 1s regions of the XPS spectrum of a TiO2/SiO2 composite sample recorded while applying -10 and +10 Vexternal bias.186 Analytical Chemistry, Vol. 79, No. 1, January 1, 2007

Differentiation of domains in composite surface structures by charge-contrast x-ray photoelectron spectroscopy

An external bias is applied to two samples containing composite surface structures, while recording an XPS spectrum. Altering the polarity of the bias affects the extent of differential charging in domains that are chemically or electronically different to create a charge contrast. By utilizing this charge contrast, we show that two distinct silicon nitride and silicon oxynitride domains are present in one of the composite samples. Similarly, we use this technique to show that titanium oxide and silicon oxide domains exist as separate chemical entities in another composite sample

Suzer, Sefik

Dana, Aykutlu

Ertaş, Gülay

OpenMETU (Middle East Technical University)

Analytical Chemistry

Differentiation of domains in composite surface structures by charge-contrast X-ray photoelectron spectroscopy

https://open.metu.edu.tr/bitstream/handle/11511/45808/index.pdf

Differentiation of domains in composite surface structures by charge-contrast x-ray photoelectron spectroscopy

Abstract

Similar works

Full text

Available Versions

Bilkent University Institutional Repository

OpenMETU (Middle East Technical University)