Epitaxial Y-stabilized ZrO2 on silicon Dynamic growth process and interface structure

Epitaxial Y-stabilized ZrO2films on silicon:Dynamic growth process and interface structureS.J.Wang and C.K.Ong a)Center for Superconducting and Magnetic Materials(CSMM),and Department of Physics,National University of Singapore,Singapore119260͑Received11December2001;accepted for publication5February2002͒We have studied the initial stage of the growth of yttria-stabilized zirconia͑YSZ͒films on nativelyoxidized͑100͒Si wafer by pulsed-laser deposition.X-ray photoelectron spectroscopy andhigh-resolution electron microscopy show that,for thefirst few monolayers of crystalline YSZdeposited on͑100͒Si,the dynamic processes appear to be the decomposition of SiO2to SiO,theformation of ZrO2,and the desorption of SiO.The native amorphous SiO2layer is removedcompletely with the continued deposition of YSZ.The atomically sharp and commensurate YSZ/Siinterface is suggested to have a sequence of Si–Si–O–Zr–O–.©2002American Institute ofPhysics.͓DOI:10.1063/1.1467970͔Developments in the semiconductor industry over thepast20years have dramatically reduced the feature size ofmetal-oxide-semiconductor transistors,and the gate oxideSiO2thickness has been scaled down to around2nm.1,2Tofurther the ambition of minimizing feature sizes,alternativegate dielectrics with much higher dielectric constant͑⑀͒are needed to replace the conventional SiO2(⑀silicaϳ4)3,4dielec-trics.Crystalline oxides on silicon as alternative gatedielectrics,5–9maintaining one-to-one atomic correspon-dence at the oxide/silicon interface without the formation ofan underlying SiO2layer,are expected to have excellentphysical properties and chemical stability.However,the is-sue concerning the dynamic growth process of crystallineoxide on silicon is a very crucial step and should be solvedfirst for the implementation of this structure.In this letter wepresent experimental results that reveal the dynamic growthmechanism of crystalline oxide on natively oxidized siliconwafer.Yttria-stabilized zirconia͑YSZ͒is chosen as the targetmaterial,as it has a sufficiently large electron barrier10andexcellent thermodynamic stability in contact with Si at tem-perature up to900°C.10–16The pulsed laser deposition sys-tem with a base vacuum of5ϫ10Ϫ7mbar is used to growYSZfilms on͑100͒Si.9,17All the Si substrates were cut fromone piece of as-purchased Si wafer used in chip manufactur-ing.This is to ensure that each substrate has an identicalnative oxide layer,surface morphology and crystalline orien-tation.As determined by high-resolution transmission elec-tron microscopy͑HRTEM͒analysis,the thickness of the na-tive SiO2is around 1.0–1.5nm.The Si substrates areultrasonically cleaned in dilute nitric acid and then thor-oughly rinsed.In this treatment the amorphous SiO2layerremains on the substrates.The deposition is done at a sub-strate temperature of730°C in an ambient oxygen pressureof6–8ϫ10Ϫ6mbar.This pressure is just above the transi-tion line from passive oxidation to active oxidation in theP–T phase diagram of silicon oxidation,and at the same timethe desorption of SiO can be expected to take place in this condition.18–20Laserfluence͑wavelength248nm͒of1.5J/cm2and laser repetition frequency of3Hz are set for allthe samples.YSZfilms are deposited for only5,10,20,and30s,respectively.For the30sfilm,the thickness is about1.5nm.As the deposition of YSZ is performed at a temperatureover700°C,one concern would be whether the thickness ofnative SiO2on substrate remains the same or increases.Fig-ures1͑a͒and1͑b͒show the x-ray photoelectron spectroscopy ͑XPS͒spectra obtained from an as-cleaned substrate and a substrate which has been heated in the deposition chamber at730°C for5min in the deposition pressure but without thedeposition of YSZ,paring Fig.1͑b͒withFig.1͑a͒,the intensity of the Si2p peak at103.6eV,corre-sponding to the binding energy of SiO2,does not increasea͒Electronic mail:phyongck@.sgFIG.1.Si2p core-level XPS spectra of samples without YSZ deposition͑a͒as deposited,͑b͒heated,and with YSZ deposition for5͑c͒,10͑d͒,20͑e͒,and30͑f͒s,respectively.APPLIED PHYSICS LETTERS VOLUME80,NUMBER148APRIL200225410003-6951/2002/80(14)/2541/3/$19.00©2002American Institute of Physics Downloaded 23 Nov 2002 to 137.132.3.12. Redistribution subject to AIP license or copyright, see /aplo/aplcr.jspsignificantly;this implies that the heating treatment does not result in considerable further oxidation of the Si surface.In the series of samples where YSZ thin films are deposited on the substrates for 5,10,20,and 30s,an evolution of the Si 2p peaks in both intensity and position in their XPS spectra ͓Figs.1͑c ͒–1͑f ͔͒can be paring the Si 2p peaks in Figs.1͑b ͒and 1͑f ͒,we find that the intensity from Si is dramatically reduced in the latter.This reduction is due to the substrate being covered by a layer of ZrO 2film that attenuates the photoelectron signals.Although the thickness of ZrO 2layer is different for dif-ferent samples and it is hard to extract the variation of SiO 2thickness from the intensity variation by simulation,the bond information can be obtained from the shift of the peak loca-tions.The Si 2p peak corresponding to SiO 2shifts towards lower binding energy,from 103.6eV ͓Fig.1͑b ͔͒to 102.8eV ͓Fig.1͑f ͔͒.This can be attributed to two causes:one is the oxygen loss for SiO 2layer;another one is the formation of silicate.21–23In order to check this kinetic process,we turn to O 1s and Zr 3d core-level spectra to examine the nature of oxide formed on the silicon,shown in Figs.2and 3,respectively.Before the deposition of YSZ,both native substrate and heated substrate present only a peak near 533eV ,corre-sponding to the O 1s binding energy for SiO 2.The slight shift of the O 1s peak 0.3eV towards higher binding energy,together with the enhancement of the Si 2p peak intensity shown in Fig.1͑b ͒,are attributed to the incorporation of a small amount of oxygen atoms into the substrate surface dur-ing the heating process.However,after YSZ is deposited for 5s,the peak at 533eV disappears and a peak at 530.5eV emerges.With increasing YSZ thickness,this peak keeps on shifting towards lower energy and fixes its position at 529.4eV after deposition of 2–3ML of YSZ ͑20–30s of deposi-tion ͒.The binding energy of O 1s in ZrO 2is 529.4eV .The O 1s binding energy for silicate is in the range 531.1–532.0eV .21–23The jump of peak to 530.3eV for the 5s sample in Fig.2could be considered as the combination of a dominat-ing O 1s peak from ZrO 2,and a weak O 1s peak from oxygen deficient SiO x (x Ͻ2)and the silicate.With contin-ued YSZ deposition,the peak shape becomes sharper and the peak shifts to lower binding energy at 529.4eV ,correspond-ing to that of ZrO 2.In addition,as seen from the HRTEM image in Fig.4͑a ͒,the YSZ films are found to grow epitaxi-ally on ͑100͒Si substrate.From the film surface to the inter-face,the films show uniform atomic pattern,cubic YSZ structure,which had been confirmed by the electron diffrac-tion pattern and the x-ray diffraction pattern.24The epitaxial relationship between YSZ and silicon is YSZ ͑001͒ʈSi ͑001͒and YSZ ͓100͔ʈSi ͓100͔.Figure 4͑a ͒shows a magnified HRTEM interface image of crystalline YSZ film on ͑100͒Si wafer.It can be clearly seen that there is no amorphous in-terfacial oxide at the interface.The simulated HRTEM image of crystalline YSZ on silicon ͓in Fig.4͑b ͔͒is well in agree-ment with experimental HRTEM image,indicating that the film on silicon is cubic YSZ film.Based on the HRTEM cross-section images of the YSZ/Si interface in Fig.4͑a ͒and the priority sequence for forming different stable phases,25the atomic structure of the YSZ/Si interface is proposed to be –Si–O–Zr–O–as shown in Fig.4͑c ͒.Therefore,we think,when the deposited film is very thin ͑such as 5and 10s samples ͒and the native SiO 2layer has not been completely removed,the deposited YSZ reacts with part of SiO 2to form Zr–silicate;the Si–O–Zr and Si–O bonds in the silicate and SiO 2contribute greatly to the XPS O 1s peaks.But in the oxygen deficient environment,the Zr ions will absorb the oxygen atoms in the silicate layer,and the silicate will de-compose with the continued deposition of YSZ to form stable ZrO 2.11Recent experiment indicates there is no sili-cate forming during subsequent rapid thermal annealing of ZrO 2in high oxygen pressure.15Thus the formation of ZrO 2is an irreversible process,and the original amorphous SiO 2is removed with the progress of film deposition.As we noted before,all the films are fabricated onna-FIG.2.O 1s core-level XPS spectra of substrates and the samples with YSZdeposition.FIG.3.Zr 3d core-level XPS spectra of the samples with YSZdeposition.FIG.4.͑a ͒An original HRTEM cross-section image of crystalline YSZ on ͑100͒Si;͑b ͒simulated HRTEM image of crystalline ZrO 2on silicon ͑viewed from the direction of Si ͓110͔͒;͑c ͒schematic atomic structure of the interface ZrO 2/Si.͑The white lines represent the position of the YSZ/Si interface.͒Downloaded 23 Nov 2002 to 137.132.3.12. Redistribution subject to AIP license or copyright, see /aplo/aplcr.jsptively oxidized silicon wafers.The thickness of native oxideis about1.5nm.However,no amorphous SiO2layer can beobserved at the YSZ/Si interface.Therefore,we think thisphenomenon is due to the following chemical reaction at theinitial stage of YSZ deposition on natively oxidized Si wa-fers:ZrϩSiO2→Zr–silicate→ZrO2ϩSiO or ZrϩSiO2→ZrO2ϩSiO,followed by the desorption process of SiO because of low ambient oxygen pressure and hightemperature.20This view is further supported by Zr3d spec-tra and HRTEM analysis.In contrast to the change in the Si2p and O1s bindingenergies,the XPS spectra of Zr3d5/2͑183.2eV͒and Zr3d3/2͑185.6eV͒peaks only show enhanced intensity but without energy shift͑Fig.3͒with the variation offilm thickness.The distance between two peaks does not change for different films.These two peaks correspond to the binding energy of Zr–O in ZrO2structure.There is no sign of Zr–Si bonds ͑e.g.,from ZrSi2͒appearing in the XPS spectra.13As to the variation of peak shapes for different samples,it can be at-tributed to the contribution of Si–O–Zr bonds when the de-positedfilm is very thin,which has been discussed above.From experimental and simulation studies on the laser-generated plume,26it is known that at an ambient gas pres-sure of10Ϫ6mbar,the species in the plume could reach akinetic energy as high as103eV.When Zr ions with kineticenergy of102–103eV impinge on the Si substrate surface,Zr ions may easily travel through a few monolayers of SiO2and cause collision-induced bond breaking or vibration,which consequently induce the chemical reactions describedabove.The ambient oxygen pressure and substrate tempera-ture in our experiment are controlled in critical regions suchthat the formation of zirconium silicide(ZrSi2)is suppressedbut allows the SiO,an intermediate product of the reaction,to be easily evaporated from substrate surface.Thus theamorphous oxide layer on the substrate is removed immedi-ately when Zr ions impinge on the substrate and a commen-surate interface between ZrO2and Si is formed.The experi-mental results discussed above are the direct evidence thatthe dynamic processes are de-oxidization processes fromSiO2to SiO x(xϽ2),the desorption of SiO,and the forma-tion of crystalline ZrO2.In summary,we present clear evidence on the growthmechanism of commensurately crystalline YSZ on nativelyoxidized Si wafer.The dynamic processes at the initial stageappear to be the decomposition of SiO2to SiO,the formationof ZrO2,and the desorption of SiO.The native SiO2is re-moved immediately when Zr atoms impinge upon the sub-strate surface,while the SiO2layer can be an oxygen sourceto form initial monolayers of ZrO2.As a result of the excel-lent structural properties,the crystalline YSZ on Si show much low leakage current.9The description of the dynamic growth mechanism of crystalline YSZ can be applied to fab-ricate other dielectricfilms on silicon or SiGe substrate by other techniques,such as sputtering.The project is supported by the National University of Singapore.The authors would like to thank S.Y.Xu,J.Y. Lin,and C.H.Sow for valuable discussions.The authors also would like to acknowledge W.C.Tjiu and J.W.Chai for HRTEM and XPS analysis in IMRE.1D.A.Buchanan,IBM J.Res.Dev.43,245͑1999͒.2D.A.Muller,T.Sorsch,S.Moccio,F.H.Baumann,K.Evans-Lutterodt, and G.Timp,Nature͑London͒399,758͑1999͒.3G.D.Wilk,R.M.Wallace,and J.M.Anthony,J.Appl.Phys.89,5243͑2001͒.4A.I.Kingon,J.P.Maria,and S.K.Streiffer,Nature͑London͒406,1032͑2000͒.5R.A.McKee,F.J.Walker,and M.F.Chisholm,Science293,468͑2001͒. 6R.A.McKee,F.J.Walker,and M.F.Chisholm,Phys.Rev.Lett.81,3014͑1998͒.7K.Eisenbeiser,J.M.Finder,Z.Yu,J.Ramdani,J.A.Curless,J.A.Hall-mark,R.Droopad,W.J.Ooms,L.Salem,S.Bradshaw,and C.D.Over-gaard,Appl.Phys.Lett.76,1324͑2000͒.8S.A.Chambers,Y.Liang,Z.Yu,R.Droopad,J.Ramdani,and K.Eisen-beiser,Appl.Phys.Lett.77,1662͑2000͒.9S.J.Wang,C.K.Ong,S.Y.Xu,P.Chen,W.C.Tjiu,J.W.Chai,A.C.H. Huan,W.J.Yoo,J.S.Lim,W.Feng,and W.K.Choi,Appl.Phys.Lett.78, 1604͑2001͒.10J.Robertson,J.Vac.Sci.Technol.B18,1785͑2000͒.11K.J.Hubbard and D.G.Schlom,J.Mater.Res.11,2757͑1996͒.12G.D.Wilk,R.M.Wallace,and J.M.Anthony,J.Appl.Phys.87,484͑2000͒.13T.S.Jeon,J.M.White,and D.L.Kwong,Appl.Phys.Lett.78,368͑2001͒.14M.Copel,M.Gribelyuk,and F.Gusev,Appl.Phys.Lett.76,436͑2000͒. 15J.-P.Maria,D.Wicaksana,A.I.Kingon,B.Busch,H.Schulte,E.Gar-funkel,and T.Gustafsson,J.Appl.Phys.90,3476͑2001͒.16W.J.Qi,R.Nieh,B.H.Lee,L.Kang,Y.Jeon,K.Onishi,T.Ngai,S. Banerjee,and J.C.Lee,Tech.Dig.-Int.Electron Devices Meet.,145͑1999͒.17P.Chen,S.Y.Xu,W.Z.Zhou,C.K.Ong,and D.F.Cui,J.Appl.Phys.85, 3000͑1999͒.18J.Seiple and J.P.Pelz,Phys.Rev.Lett.73,999͑1994͒.19F.M.Ross,J.M.Gibson,and R.D.Twesten,Surf.Sci.310,243͑1994͒. 20R.Tromp,G.W.Rubloff,P.Balk,and R.K.LeGoues,Phys.Rev.Lett.55, 2332͑1985͒.21J.J.Chambers and G.N.Parsons,Appl.Phys.Lett.77,2385͑2000͒.22G.Lucovsky,G.B.Rayner,Jr.,D.Kang,G.Appel,R.S.Johnson,Y. Zhang,D.E.Sayers,H.Ade,and J.L.Whitten,Appl.Phys.Lett.79,1775͑2001͒.23M.J.Guittet,J.P.Crocombette,and M.Gautier-Soyer,Phys.Rev.B63, 125117͑2001͒.24S.J.Wang,C.K.Ong,L.P.You,and S.Y.Xu,Semicond.Sci.Technol. 15,836͑2000͒.25Thermochemical Properties of Inorganic Substances,2nd ed.,edited by O. K.Nacke,O.Kubaschewski,and K.Hesselmann͑Springer,Berlin,1991͒. 26Pulsed Laser Deposition of Thin Films,edited by D.B.Chrisey and G.K. Hubler͑Wiley,New York,1994͒.Downloaded 23 Nov 2002 to 137.132.3.12. Redistribution subject to AIP license or copyright, see /aplo/aplcr.jsp。