Electronic structure of stoichiometric and Ar+-bombarded ZrO2 determined by resonant photoemission

PHYSICAL REVIEW BVOLUME S2, NUMBER 16 1S OCTOBER 1995-II Elec onic s uc u e o s oichiome ic and A +-bomba ded Z O2 de e mined by esonan pho oemission C. Mo an Can oblanco, E-28049 Mad id, Spain A. Fe nandez and A. R. Gonzalez-Elipe Ins i u o Ciencia de Ma e iales de Se illa, Consejo Supe io de In es igaciones Cien igcas, Uni e sidad de Se illa, Depa amen o de Quimica Ino ga'nica, P.O. Box 1115,E41071 -Se illa, Spain L. So iano Can oblanco, E-28049 Mad id, Spain A. S amp i and A. M. B adshaw F i z Habe I-ns i u -de Max Planck G-esellscha- , Fa aday eg 4-6, 14195Be lin, Ge many J.M. Sanz* Can oblanco, E-28049 Mad id, Spain (Recei ed 2May 1995; e ised manusc ip ecei ed 10July 1995) The elec onic p ope ies o he mally g own Z O& hin ilms be o e and a e A + bomba dmen ha e been s udied wi h esonan pho oemission spec oscopy using synch o on adia ion. Fo s oichiome ic Z 02 hin ilms he expe imen al alence-band spec a a e in good ag eemen wi h he calcula ed densi y o s a es o bulk Z 02. Fo bo h s oichiome ic and A +-bomba ded Z 02 hin ilms, esonan pho o- emission om he alence band was obse ed when he pho on ene gy was swep h ough he Z 4p ~4d ansi ion ene gy. The esonan p o ile was ound o exhibi amaximum a h =39 eV, ollowed by a second well- esol ed b oad maximum a ound 50 eV. The ea u e a 39 eV is consis en wi h esonan enhancemen o he Z 4d s a es and has been used o iden i y hose egions o he alence band wi h an impo an Z 4d admix u e. The esul s a e in good ag eemen wi h he calcula ed Z 4d pa ial densi y o s a es. The in ensi y inc ease obse ed a h -45 — 50 eV is ound o be associa ed wi h he nonbond- ing egion o he alence band, al hough ap ope in e p e a ion is needed. In addi ion, i was ound ha A + bomba dmen induces elec onic s a es in he band-gap egion and changes in he 02p alence band. Th ee dis inc emission bands we e iden i ied in he band gap as a unc ion o he A + dose. They a e associa ed wi h he o ma ion o oxygen acancies and mixed oxida ion s a es due o p e e en ial spu e ing o he oxygen a oms. Resonan pho oemission o hese A +-bomba ded ilms demons a es bo h he ca ionic cha ac e o he band-gap s a es and he inc ease o he ca ionic con ibu ion o he 0 2p alence band. I. INTRODUCTION The main pu pose o his pape is o in es iga e he elec onic s uc u e o he mally g own Z 02 hin ilms as well as he e ec s induced by A + bomba dmen using esonan pho oemission spec oscopy (RPES). The elec- onic band s uc u e o Z 02, a e ac o y ansi ion- me al oxide (TMO), is o g ea in e es om bo h he echnological and undamen al poin s o iew. I s wide use as ahigh-pe o mance ce amic o nuclea and high- empe a u e applica ions, as well as in gas sensing, elec o-op ical, hin- ilm, and ca alysis applica ions makes band s uc u e in o ma ion e y impo an o un- de s anding i s physical and chemical p ope ies. Pas in- es iga ions seem o ha e been o ien ed mainly owa ds p ac ical aspec s, ' a he han owa ds hese mo e un- damen al aspec s. 'Only a ew expe imen s ha e p obed he elec onic s uc u e o Z 02, 'and hei in- e p e a ion has elied on alimi ed numbe o band s uc- u e calcula ions. Mo eo e , he elec onic s uc u e o de ec s on ansi ion-me al oxides has also become a subjec o ex ensi e s udy. 'Low-ene gy ion bo n- ba dmen is o en used o c ea e de ec s in TMO's, bu , because o he di icul y o cha ac e iza ion, i s e ec on elec onic p ope ies has no ye been su Ficien ly s udied. Many spec oscopic s udies ha e shown ha oxygen a- cancies cause localized ene gy le els in he band gap. In mos cases i has p o en di icul , howe e , o ela e he obse ed elec onic s a es o speci ic de ec s, e en hough his has been a emp ed ecen ly in some heo e ical ap- p oaches. ''To ou knowledge only Sobole e al. ,using asel -consis en sca e ed-wa e me hod, 0163-1829/95/52(16)/11711(10)/$06.00 11 711 1995 The Ame ican Physical Socie y 11 712 C. MORANT e al. ha e ied o s udy he inAuence o oxygen acancies on he elec onic s uc u e o Z 02. The changes in compo- si ion and elec onic s uc u e induced in Z 02 by 3keV A + bomba dmen ha e been s udied by co e and alence-band x- ay pho oelec on spec oscopy (XPS) as epo ed in a ecen pape . The esul s showed ha he elec onic s uc u e was ai ly sensi i e o A + spu e - ing, a ea men ha p oduces emission ea u es in he band gap and o he changes in he O2p egion o he alence band (VB), which in u n we e asc ibed o he o ma ion o oxygen acancies and suboxides (i.e., educ- ion). Howe e , he ela i ely low pho oemission c oss sec ion o he VB p e en ed any de ini i e conclusions om being d awn as o he elec onic s uc u e o he s oichiome ic and A +-bomba ded solid and o he A +-modi ied su ace. The phenomenon o RPES in ansi ion me als and hei compounds has been he subjec o many ecen in- es iga ions. Ala ge esonance in he dpho oion- iza ion c oss sec ion is known o occu a pho on ene gies close o he h eshold o ap~d ansi ion. The eso- nance p ocess is usually explained as being due o he in- e e ence be ween he di ec pho oemission p ocess and au oioniza ion o ahighly localized exci ed s a e c ea ed by pho oabso p ion. Simila e ec s a e ound o he alence bands o TMO's e en when hose ha e a o mal dcon igu a ion: I is assumed ha he esonance is due o hyb idiza ion be ween he 02p and ca ion do bi als. In ac , RPES is used ex ensi ely as ame hod o isola e he d-s a e con ibu ion and o unde s and he VB s uc- u e o ansi ion-me al compounds and o he complex hyb idized sys ems. Fo ade ailed discussion he eade is e e ed o he e iew by Da is and he book by Hen ich and Cox. S udies o he esonan phenomenon i sel a e also in e es ing since he de ails o he esonance in some compounds a e no ully unde - s ood. Fo example, ex a esonances no di ec ly ela ed o he me al ds a es ha e been ecen ly obse ed in Ti02 (Re . 37) and MoS2, bu asa is ac o y explana ion has no ye been pu o wa d. In his pape we desc ibe aPES and RPES s udy o Z 02 hin ilms be o e and a e A + bomba dmen aimed a ob aining abe e unde s anding o i s alence- band s uc u e and o he changes induced by A + bom- ba dmen . In Sec. II we desc ibe some expe imen al de- ails and he sample p epa a ion. In Sec. IIIAwe p esen acompa ison o he expe imen al da a wi h he calcula ed densi y o s a es (DOS) o ob ain a eliable pic- u e o he alence-band s uc u e. The RPES s udy o he Z Q& alence band is p esen ed in Sec. IIIB. Ap e- limina y accoun o some o hese esul s appea ed else- whe e. 'Sec ion IIICis dedica ed o he e ec s induced by A + bomba dmen . In Sec. IIIDwe p esen and dis- cuss he RPES da a om ahighly educed su ace (i.e., Z O„) ob ained by A + bomba dmen o he o iginal su - ace. Asumma y and he conclusions a e p esen ed in Sec. IV. II. EXPERIMENTAL DETAILS Apolyc ys alline Z oil o 99.98%%uo pu i y om Good- ellow Me als has been used h oughou his wo k. The oil was subjec ed o he usual cleaning p ocedu es by insing in ace one, e hanol, and dis illed wa e be o e i was in oduced in o he UHV chambe . The su ace was hen cleaned by A + spu e ing and annealed unde he condi ions p e iously desc ibed in o de o a oid seg e- ga ion o impu i ies which hinde he oxida ion. The mally g own Z 02 hin ilms we e ob ained by hea - ing he oil a -450 K o longe han 30 min in an oxy- gen a mosphe e (I'o — 10 To ). In some cases, a e long pe iods o subsequen exposu e o he esidual gas some OH adso p ion was obse ed, bu could be easily e- mo ed by hea ing a 373 K o sho pe iods. In o de o induce su ace de ec s and changes in s oichiome y he sample was bomba ded wi h A + ions using an ion gun ope a ed a 0.5and 2keV and cu en densi ies o — 1pA cm The pho oemission expe imen s we e pe o med on he TGM3 monoch oma o a he Be lin synch o on adia- ion sou ce (BESSY) using pho on ene gies be ween 30 and 70 eV. A hese ene gies he escape dep h o he pho oelec ons is expec ed o be less han 10 A. The angle o incidence o he p-pola ized ligh was 45 wi h espec o he sample no mal. The spec a we e eco ded a no mal emission wi h an ADES-400 elec on ene gy analyze . The o e all esolu ion (monoch oma o + spec ome e ) was ypically 600 meV. O e iew spec a we e measu ed in he cons an - e a ding- a io mode, whe eas he alence-band spec a in he 0~E~ ~14 eV binding-ene gy ange we e aken in he cons an - analysis-ene gy mode wi h apass ene gy o 20 eV. Fo a mo e de ailed desc ip ion o he monoch oma o he eade is e e ed o he BESSYuse 's handbook. All he alence-band spec a p esen ed in his pape ha e been no malized o he pho on Aux as measu ed a a gold mi o a he en ance o he spec ome e chambe . In mos cases abackg ound has been sub ac ed om he spec a using s anda d me hods. III. RESULTS AND DISCUSSION A. Elec onic s uc u e o Z O& Figu e 1shows an o e iew spec um o he mally g own Z Oz eco ded a h =50 eV. The spec um shows he Z 4p double a 30.6eV, he b oad 02s peak a 21.7eV, and he alence band peaked a 5.3eV wi h a clea ly dis inguishable shoulde a highe ene gies. The binding ene gy is e e ed o he Fe mi le el o me allic zi conium. Figu e 2shows he 0— 14 eV binding-ene gy (E~) ange measu ed wi h h =35 eV. Fo compa ison we ha e in- cluded he o al densi y o s a es, as well as he pa ial ox- ygen and zi conium densi ies o s a es, calcula ed by So i- ano e aI. o cubic zi conia using he LSW me hod. The calcula ed DOS was ob ained in e ms o he Z and 02p pa ial DOS's weigh ed acco ding o hei espec- i e c oss sec ions. Fo amo e de ailed desc ip ion o his p ocedu e see Re . 8. The o e all shape o he expe - imen al cu e is well explained by he calcula ed bulk DOS. The alence band exhibi s wo dis inc s uc u es a -9and -6eV binding ene gy, which acco ding o he 52 EI.ECTRONIC STRUCTURE OF STOICHIOMETRIC AND A +-. . . 11 713 I) 12 10- Z 02, hu=508V EF 8- I— (0 Z4- LLI I— Z2- 0- I 50 IIIII 40 30 20 10 BINDING ENERGY (eV) FIG. 1. Pho oemission spec um measu ed a h =50 eV om s oichiome ic Z 02. ES os 0- I— GO Z UJ I— Z band s uc u e calcula ion co espond o he bonding O 2pc and nonbonding 02pm. o bi als, espec i ely, and con ain an impo an zi conium con ibu ion. The Z con ibu ion is mo e signi ican in he highe -binding- ene gy egion, whe e he bonding 02p o bi als a e ex- pec ed o ha e ala ge admix u e o Z o bi als, and il- lus a es he impo ance o he co alen in e ac ion (es- ima ed o be abou 24%) in he bonding o Z 02. The wid h o he alence band is es ima ed o be -4.8eV om he ene gy sepa a ion o he poin s o maximum slope on each side o he band. The alence-band max- imum (VBM) is loca ed a — 3.8eV below he Fe mi le el as de e mined by ex apola ion o he angen on he low-binding-ene gy side o he 02p band. This alue is in good ag eemen wi h ha de e mined by o he elec- on spec oscopies" and op ical gaps epo ed in he li e a u e. ''" Apa om he almos classic calcula ion o Mo inaga, Adachi, and Tsukuda based on adisc e e a ia ional (DV) Xa clus e me hod, a ious o he a emp s ha e been made o s udy he elec onic s uc u e o Z O2. Us- ing he linea -mu in- in-o bi als (LMTO) me hod Med ede a e QI. ha e de e mined he ene gy band s uc u es o Z 02 and o he TMO's in o de o in es i- ga e ends in he band gap and cohesi e ene gy. The band s uc u e o cubic and e agonal zi conia ha e been calcula ed by Jansen using densi y- unc ional heo y. The s uc u al and elec onic p ope ies o cubic and e agonal zi conia we e also explo ed by O lando e al. using ape iodic ab ini io Ha ee-Fock me hod. Zan- diehnadem, Mu ay, and Ching pe o med calcula ions o he h ee s able phases o Z 02 (i.e.,monoclinic, cu- bic, and e agonal) using he o hogonalized linea com- bina ion o a omic o bi als (OLCAO) me hod, and con- cluded ha he bonding is e y simila o he h ee c ys- al s uc u es wi h only mino di e ences in he wid h o he band and he band gap. In addi ion o he gene al good ag eemen a esul common o all hese calcula ions is he exis ence o an impo an co alen bonding con i- bu ion, which has been con i med expe imen ally wi h 0 ls x- ay abso p ion spec oscopy' (XAS) and esonan pho oemission. '" Ade ailed discussion o hese calcula- ions and hei compa ison wi h o he app oaches as well as wi h expe imen al pho oemission (PES) and b emss ahlung isoch oma (BIS) spec a can be ound in Re s. 8and 13. We no e ha he pho oemission in ensi y o he PES spec a epo ed he e does no all o ze o wi hin he band gap e en o he eshly p epa ed su ace. Weak emission in he band gap ex ends om he op o he alence band up o he Fe mi le el, showing amaximum a Ez =2.5eV. An expanded iew o hei con ibu ion is shown in Fig. 2. Usually, such bands a e asc ibed o apped elec ons in oxygen acancies and o he su ace de ec s' caused by he p epa a ion me hod used (i.e., hea ing a low oxygen p essu es). Howe e , by means o asel -consis en sca e ed-wa e me hod, Sobole e al. ha e p edic ed ha oxygen acancies in pe ec Z 02 c ys als a e expec ed o cause apped elec ons in local s a es wi hin he gap a binding ene gies be ween 1.5and 2eV, in good ag eemen wi h he band obse ed in his s udy. Ap e ious XPS s udy has quan i ied his e ec gi ing an elec on su ace densi y o 6X 10' cm which is equi alen o amonolaye o oxygen acancies. I will be shown below ha he in ensi y o his ea u e can be subs an ially inc eased by ion bomba dmen . B. Resonan pho oemission in Z IO2 Dos os 14 12 10 86420 BINDING ENERGY (eV) FIG. 2. Calcula ed bulk DOS (solid line) (see ex and Re . 8 o de ails) and expe imen al alence-band pho oemission spec- um (poin s) a h =35 eV o s oichiome ic Z O2. Lowe cu es: Calcula ed Z and 0pa ial densi ies o s a es (Re . 8). The p ocess o esonan pho oemission has been ex en- si ely used as ame hod o isola ing he ca ionic con i- bu ion o he alence band o ansi ion-me al com- pounds. The ene gy dis ibu ion cu es (EDC's) om he alence band o Z 02 we e hus measu ed a pho on ene - gies be ween 30 and 70 eV as shown in Fig. 3. They indi- ca e ha he ela i e in ensi ies o he wo ea u es ob- se ed in he alence band a e e y sensi i e o he pho- on ene gy due o a esonance p ocess associa ed wi h he Z 4p h eshold. Al hough he esonance mani es s i sel C. MORANT e al. III'LII'III'I'II Z 02 Eh(eV) F70 10- a) Z 02 EF 60 50 5- 45 42 40 39 38 36 33 30 ~iIII l 14 12 10 86420-2 BINDING ENERGY (eV) FIG. 3. Valence-band pho oemission spec a o Z 02 as a unc ion o he pho on ene gy. o e he whole alence band a pho on ene gies be ween 36 and 50 eV, he 02p bonding egion on he high- binding-ene gy side o he alence band is enhanced a pho on ene gies a ound 39 eV. In he case o he spec- um aken a h = 30 eV he unusually high in ensi y o he gap s a es is due o he con ibu ion o he Z 4p co e le els exci ed wi h second-o de ligh . In o de o s udy sepa a ely he beha io o he bond- ing and nonbonding con ibu ions o he alence band, we ollowed ap ocedu e p e iously used by Zhang, Jeng, and Hein ich in hei s udy o esonance e ec s in Ti02. Acco ding o hese au ho s he alence-band spec a can be analyzed in e ms o h ee Gaussians, labeled "bond- ing, ""nonbonding, "and "o e lapping, " he wid hs and ene gies o which a e summa ized in Table I o he whole se ies o spec a. A ypical i is shown in Fig. 4(a) o he VB spec um measu ed a h =50 eV. Ob iously he high- and he low-binding-ene gy componen s in ol e p edominan ly he bonding and nonbonding o bi als, e- spec i ely, whe eas he componen labeled "o e lapping" helps o ge abe e i o he alence band. Amo e quan i a i e insigh in o he esonance e ec s TABLE I. Bes - i pa ame e s binding ene gy (Eb) and ull wid h a hal maximum (FWHM) o he line shape o he alence band a di e en pho on ene gies. Th ee and wo Gaussians we e used o Z 02 and Z O, espec i ely. The mean, s anda d de ia ion (SD), and median alue a e gi en o each pa ame e . 0- LU Z10- 0- I'I'I'I'1'I'II 14 12 10 86420-2 BINDING ENERGY (eV) FIG. 4. Analysis o he alence-band pho oemission spec- um a h =50 eV using h ee and wo Gaussians o Z O& (a) and Z O (b), espec i ely. can be gained by in eg a ing he a eas associa ed wi h he di e en componen s o he alence band and hus gen- e a ing spec a equi alen o he usual cons an -ini ial- s a e (CIS) cu es. The pho on ene gy dependence o he o al in ensi y o he alence band has hus been depic ed in Fig. 5 oge he wi h ha o he in ensi y o he bond- ing, nonbonding, and o e lapping componen s and o he emission band in he gap. The p o iles a e a he com- plex and qui e unlike he ideal Fano-like cu es. In he case o he o al alence-band in ensi y (cu e a) he p o61e is cha ac e ized by a ise a 36 eV wi h amax- imum a h =39 eV, and by asecond b oad maximum a ene gies be ween 45 and 50 eV. The in ensi y o he bonding componen (cu e b) eaches he main maximum I'I'I'I'I'I'I'I'I e 35— 30- 25- N20— I— 15 -~ 10- Eb {eV) Mean SD FWHM {eV) Median Mean SD Median 0- Bonding 8.74 Nonbonding 5.67 O e lap 7.14 Bonding 8.63 Nonbonding 6.51 0.06 0.07 0.07 0.09 0.20 Z 02 8.75 5.68 7.15 Z O 8.65 6.60 2.32 2.09 1.89 2.53 2.57 0.05 0.03 0.01 0.07 0.04 2.31 2.10 1.89 2.57 2.57 IIiIiI~I I I IiIsI 30 35 40 45 50 55 60 65 70 PHOTON ENERGY (eV) FIG. 5. Pho on ene gy dependence o he in eg a ed in ensi y o he o al alence-band emission (0) {cu e a) and o he band-gap emission {0)(cu e e) as well as o he bonding (A) (cu e b), nonbonding (0)(cu e c), and o e lapping (Q) (cu e d) componen s. 52 ELECTRONIC STRUCTURE OF STOICHIOMETRIC AND A +-. . . 11 715 a app oxima ely 39 eV ollowed by ab oad pla eau which ex ends up o 50 eV. On he o he hand, he eso- nance p o ile o he nonbonding componen (cu e c) con- sis s o asmall bump a 39 eV and ag adual ise up o -50 eV o all o a highe ene gies, so ha in his case he maximum enhancemen o he in ensi y is p oduced a pho on ene gies a ound 45— 50 eV. The o e lapping componen (cu e d) shows no clea e idence o esonan beha io , bu shows a he cons an in ensi y up o 50 eV pho on ene gy. The p o ile co esponding o he s a es in he band gap (cu e e) sugges s aweak esonance a 39 eV, bu because o he low in ensi y and possible e o s in he backg ound sub ac ion p ocedu e an unambiguous cha ac e iza ion is di icul . The lack o asys ema ic pic- u e in he obse a ion o he wo esonances (i.e.,a 39 and 50 eV) may be linked o he ac ha none o he peaks used in he i ing p ocedu e ac ually co espond o pu e bonding o pu e nonbonding o bi als. In o de o ollow he changes obse ed in he alence-band shape in ameaning ul way, i is use ul o compu e he 39/36 eV di e ence spec um by sub ac ing he EDC measu ed a 36 eV (o esonance) om ha measu ed a 39 eV (on esonance). The esul is labeled b in Fig. 6. The cu e labeled aco esponds o an XPS alence-band spec um (i.e.,h = 1253.6eV) om Re . 26. Fo compa ison we ha e included se e al cu es la- beled om c o which ep esen he Z and 0PDOS's ob ained by di e en me hods. Cu es cand a e he calcula ed Z and 02p pa ial densi y o s a es, espec- i ely (see Fig. 2) scaled by he ela i e pho oemission c oss sec ions a h = 1253.6eV. Since he Z 5s and Z 5p con ibu ions a e negligible, we will use cu e c o ep esen he Z 4d densi y o s a es. Cu es dand ea e in ended o ep esen he 02p densi y o s a es. Cu e d I'I'I'I'I'I'I'I'I Z 02 a) XPS Z 4d b) c) 02p d) I'I'I'I'I'I'I'I'I 14 12 10 86420-2 BINDING ENERGY (eV) FIG. 6. Cu e a: XPS spec um o he alence band om s oichiome ic Z 02. Cu e b: Z 4d con ibu ion o he alence band as de i ed om he esonance a h =39 eV. Cu e c: Calcula ed Z pa ial densi y o s a es ( om Re . 8). Cu es d : 02p con ibu ion — o he alence band as de e - mined om (cu e d) pho oemission a h =70 eV, (cu e e) he XPS spec um (a) a e sub ac ion o ib), and (cu e Pelec- onic s uc u e calcula ions (Re . 8). is he alence band o Z 02 measu ed a h =70 eV (see Fig. 3), which is expec ed o e iec he 02p PDOS since he a io o he 02p and Z 4d pho oemission c oss sec- ions is &30 a ha pho on ene gy. "Cu e eis he e- sul o he sub ac ion o he Z con ibu ion as ep esen ed by spec um b om he expe imen al XPS alence band (i.e.,cu e a) (see de ails below). Resonan pho oemission o he Z 4d s a es in Z 02 is expec ed o occu when he pho on ene gy is uned o e he Z 4p ~48 ansi ion ene gy a — 31 eV. The 4p~4d ansi ion c ea es an exci ed s a e [Z 4p 4d']' which emains highly localized on he ca ion si e. The exci ed s a e can hen elax in adi ec ecombina ion p ocess which in ol es he Z 4d s a es pa icipa ing in he alence band; i.e.,[Z 4p 4d 'VB"]~Z 4p 4d VB" '+e, which is he same inal s a e as in con en ional pho oemission. In his manne he e is a possible in e e ence be ween bo h p ocesses, gi ing ise o he esonan enhancemen o hose pa s o he spec- um wi h an impo an ca ionic cha ac e . In he p esen s udy Fig. 6clea ly demons a es ha his is he case o he esonance obse ed a h =39 eV. The good ag eemen exis ing be ween he 39/36 eV di e ence spec- um, cu e b, and he calcula ed Z 4d PDOS, cu e c, clea ly con i ms ha he phenomenon can be used o iden i y he Z -de i ed s a es in he alence band. Fu - he mo e, he di e ence spec um bp o ides expe imen- al e idence ha he pa ial co alen cha ac e o he Z - 0bond is la ge in he bonding egion han in he non- bonding pa o he alence band. The e o e, he bonding con ibu ion should esona e mo e s ongly han he non- bonding one in good ag eemen wi h he expe imen al ob- se a ion (see Fig. 5). The esonance a h =39 eV is hen consis en wi h he esonan pho oemission o he Z 4d s a es and wi h he assump ion o apu e in a-a omic elaxa ion o he exci ed s a e. I is impo an o men ion he e ha he esonan pho- oemission o he ds a es a 39 eV occu s a an 8.4eV highe ene gy han he Z 4p binding ene gy (E~=30.6 eV) as measu ed by pho oemission (see Fig. 1). Simila delays be ween he h eshold (Z 4p binding ene gy) and he esonance ene gies ha e also been obse ed in me al- lic Z (— 10 eV), 'as well as in he zi conium ni ides Z N (— 10 eV), and Z 3N4( — 11.5eV), and seem o be acommon obse a ion in he ligh e ansi ion me als and hei compounds. In gene al, he binding ene gy o he [4p 4d "+' ]*exci ed s a e measu ed om esonan pho oemission p o iles ag ees well wi h he esul s o o h- e measu emen s such as op ical abso p ion o elec on- ene gy-loss spec oscopy (EELS). Fo Z 02, he Z 4p co e losses ha e been shown o exhibi amaximum in en- si y a ound 42 eV,"in easonable ag eemen wi h he ene gy measu ed by esonan pho oemission. Figu e 6also allows he compa ison be ween he spec- um assigned o he Z 4d s a es and he alence-band spec um ob ained by XPS. In e es ingly, i is obse ed ha he XPS spec um is mo e in ense a ound he op o he alence band (i.e., he nonbonding egion). Clea ly his is due o he 02p-de i ed s a es, because he ca ionic con ibu ions om Z 5s and Z 5p s a es a e negligible. The O2p s a es canno be neglec ed: al hough hei c oss 11 716 C. MORANT e al. 52 sec ion a XPS ene gies is ou imes lowe han ha o he Z 4d o bi als, he much la ge 02p pa ial DOS compensa es. We ha e a emp ed o ex ac he 02p con ibu ion om he expe imen al XPS alence band by sub ac ing he Z 4d con ibu ion de e mined by eso- nan pho oemission. The esul is labeled as ein Fig. 6 and shows good ag eemen wi h he calcula ed pa ial 0 2p densi y o s a es depic ed in he same igu e as . This ag eemen is no su p ising because he di e ence was ob ained by sub ac ing ap ope ly scaled di e ence spec um, o which he scaling ac o was de e mined by ap e ious i ing o he expe imen al XPS alence band in e ms o he calcula ed pa ial densi ies o s a es and he espec i e c oss sec ions as abula ed by Yeh and Lin- dau, so ha he ela i e 02p/Z 4d con ibu ions o he alence band we e de e mined acco ding o heo e i- cal calcula ions. A p esen we do no ha e asa is ac o y explana ion o he b oad esonance occu ing a an ene gy nea 50 eV. Recen ly, Heise and co-wo ke s ''pe o med a RPES and pho oelec on di ac ion (XPD) s udy o he TiOz(110) su ace o de e mine he Ti 3d and 02p alence-band pa ial densi ies o s a es. The RPES e- sul s show di e en displacemen s o he esonance maxi- Ina o di e en ea u es o he VB, which he au ho s sugges a e associa ed wi h di e en symme y- esol ed ini ial and inal s a es, al hough he de ails o he assign- men emain unclea . Assuming apu ely in a-a omic di ec ecombina ion mechanism, he esonance could be associa ed wi h ansi ions o he Z 4p — +Z 5sp ype, whe e a4p elec on is exci ed in o he emp y 5sp band al- lowing a esonance wi h he ca ionic Z 5s s a es in ol ed in he alence band o Z 02. Ob iously, he Z 5p s a es should no play any ole since ansi ions 4@~5p a e di- pole o bidden. In ac , his mechanism was p e iously used o explain ex a esonances in RPES expe imen s on Ti02 (Re . 37) and MoSz. The p o ile o he esonance indica es ha in ha case he [Z 4p 5s']* exci ed s a e lies 9eV abo e he [Z 4p d']* s a e, in good ag eemen wi h p e ious XAS expe imen ' which loca e he Z 5' band -9eV abo e he emp y dband. Howe e , he cal- cula ed Z 5s PDOS does no appea o be in ense enough o gi e ise o he obse ed enhancemen . Un o una ely, he EELS spec a o Z 02 do no show any dis inguish- able ea u e which could be associa ed wi h he esonance a 45 eV; hus ei he he co esponding exci ed s a e is no a ailable wi h EELS o he exci a ion c oss sec ion is e y low. C. A + bomba dmen e ec s Figu e 7shows alence-band spec a in he 0— 14 eV binding-ene gy ange, aken a h =39 eV (i.e.,on eso- nance), o he ini ial Z Oz su ace (lowes cu e) and a e a ious deg ees o A + bomba dmen . In addi ion, he band-gap egion (4 eV ~EiI ~0) a e sub ac ion o he main O2p alence band is displayed on an expanded scale on he igh -hand side o Fig. 7. The igu e clea ly shows ha he elec onic s uc u e o Z O2 is sensi i e o bomba dmen wi h A + ions o 2keV and ha he spec- oscopic changes in oduced by low-ene gy spu e ing 2keV A + Z 02 15 Spu e ing ime (min. )I EF ','.EF I ~, I~ I0 I I I .i I 130 i~'~— 50 Ie ~~ ~~ i l —13 ~~ ~~ j 0 I~IcI~giIaI~II~II, I 10 505432 10-1-2 BINDING ENERGY (eV) FIG. 7. Pho oemission spec a (h =39 eV) o he alence band o Z 02 (le ) and band-gap emission on an expanded scale ( igh ) as a unc ion o he spu e ing ime wi h 2keV A + ions. begin o appea immedia ely upon bomba dmen . The spec a ob ained om a esh Z 02 su ace as well as a e he i s h ee minu es o spu e ing exhibi a b oad s uc u e o cons an shape in he band gap, he in- ensi y o which inc eases wi h spu e ing ime, and whose shape emains unchanged. I s emission maximum is ound a E~=2.5eV and i s o igin was explained in Sec. IIIAin e ms o oxygen acancies. A highe A + doses he s uc u e is obse ed o de elop wo addi ional ea u es a ound 1.7and 3.7eV which also g ow upon u - he A + bomba dmen . In o de o ollow he di e en ea u es sepa a ely he emission in he band gap was i s analyzed by accoun ing o he b oad emission a 2.5eV. Di e ence spec a we e gene a ed by emo ing he maximum possible con ibu- ion o he o iginal s uc u e (i.e.,0min) om he spec a a di e en spu e ing imes, so ha he o he wo ea u es could be clea ly dis inguished. These di e ence spec a a e displayed in he inse o Fig. 8labeled wi h he scaling ac o Fwhich was used o each spec um. Abo e 3min spu e ing ime (i.e.,— 1X10' ions cm ) wo peaks a 3.8and 1.2eV can be clea ly dis inguished, and hei g ow h can be ollowed as he spu e ing p o- cess p oceeds. The in eg a ed in ensi ies o he espec i e ea u es ha e been depic ed in Fig. 8as a unc ion o he spu e ing ime, oge he wi h he o al in ensi y o he emission band in he gap. The in ensi y o he whole emission band induced by A + ions o 0.5keV has also been included o compa ison. The igu e shows ha he in ensi ies o he di e en ea u es and o he whole emis- sion band inc ease p og essi ely un il as a iona y s a e is eached o spu e ing imes abo e 60 min (i.e.,2.2X10' ions cm ). Wha is he o igin o he h ee di e en gap emissions which can be p oduced by A + bomba dmen ? The i s ype is he b oad emission a 2.5eV binding ene gy, 52 ELECTRONIC STRUCTURE OF STOICHIOMETRIC AND A +-. ..11 717 M8 7- Cg ~6- I— M5- Z'. LLI I— Z' (0 I—3- 0 UJ LL 2 Cl Spu enng Time (min. )."'. E, F 130 '.: !' ~~3.S IS .-' 2.3 0)~- 14 e4 2 0-2 ~BINDING ENERGY (e ) ke I— ( ) LLj ~0 0 o eo ~o eo 10 ~3 I 0~ oe 0~ o~ oe .. 'Z 02 b o~ 0'I 0~ I'I'I'I'I'I'I'I'I 14 12 10 86420-2 BINDING ENERGY (eV) 0- 50 100 150 FIG. 9. Valence-band pho oemission spec a om bo h s oichiome ic Z 02 and educed Z O measu ed a h =39 eV. SPUTTERING TIME (ITiin) FIG. 8. In ensi y o he band-gap emission as a unc ion o he spu e ing ime wi h 2keV (~)and 0.5keV (0) A + ions. Fo 2keV A + ions he in ensi ies o h ee di e en con ibu- ions a 1.2eV (V), 2.5eV (o), and 3.8eV (4) ha e been in- cluded. Inse : band-gap emission a e sub ac ion o he spec- um a 0min scaled wi h a ac o F. which o ms a e any deg ee o educ ion o he su ace and is p esen e en on a esh Z 02 su ace. I has been assigned o elec ons apped on oxygen acancies ha popula e he 4d o bi als o adjacen Z a oms. The nea- su ed ene gy o his band is in good ag eemen wi h he calcula ions by Sobole e al. using asel -consis en sca e ed-wa e me hod. Fu he mo e, Munnix and Sch nei s' ha e also shown ha an emission band a ound 1.3eV obse ed in Ti02 could be accoun ed o by sub- su ace oxygen acancies. In ac , i seems o be agen- e al inding in TMO's ha oxygen acancies induce elec- onic s a es in he band gap. ' A highe A + dose he inc ease in he concen a ion o oxygen acancies p oduces mo e complex de ec s uc- u es and e en signi ican changes o s oichiome y. This is p obably he o igin o he wo sha p ea u es ob- se ed only a e hea y educ ion o he su ace (i.e., high-de ec -densi y su aces). The high-binding-ene gy ea u e a 3.8eV we assign o bulk Z -0 acancy com- plexes associa ed wi h he o ma ion o lowe oxida ion s a es (i.e.,Z +, Z +). Fu he mo e, he band can be ela ed o an op ical abso p ion a 3.31 eV obse ed in cubic zi conia and a ibu ed o six old-coo dina ed Z + by Azzoni and Palea i using elec on pa amagne ic eso- nance. In con as , he weakes band obse ed a 1.2 eV equi es s ill hea ie educ ion o be unambiguously dis inguished. I is en a i ely asc ibed o he o ma ion o small clus e s o Z + and e en Z in he limi ing species o he spu e ing p ocess when A + bomba dmen p oduces much highe de ec densi ies, and mo e elec- ons popula e he Z 4d o bi als. In ac , he o ma ion o mixed oxida ion s a es (i.e.,Z +, Z +, Z +, and Z ) has been demons a ed by XPS. The ca ionic cha ac e o all hese band-gap s a es is con i med by he esonan pho oemission spec a shown below. Al hough he mos signi ican e ec o he A + bom- ba dmen is he g ow h o he emission ea u es in he gap, he 02p alence band is also obse ed o unde go changes. Figu e 9compa es he alence-band pho oemis- sion spec a o he s oichiome ic and se e ely educed su aces. In gene al, we obse ed adec ease in he alence-band wid h and an inc ease in i s in ensi y due o he p esence o addi ional educed ca ions, which in- c ease he dcon ibu ion. The alence band maximum mo es 0.45 eV away om he Fe mi le el (i.e, band bend- ing) as aconsequence o he educ ion o he su ace. In ac , i is well es ablished ha elec ons apped in oxy- gen acancies occupy localized emp y o bi als o he ca - ions, pushing up he Fe mi le el and bending he whole elec onic s uc u e o highe binding ene gies. Figu e 10 shows pho oemission spec a o Z O a e i s exposu e o inc easing amoun s o 02 a empe a u es be ween 298 and 473 K(as labeled). The igu e shows a he clea ly ha an oxygen exposu e o 100 La 298 K causes asigni ican dec ease in he in ensi y o he gap s a es. On he con a y, p ac ically no change is ob- se ed in he shape o he 02p band. Only a e expo- su es abo e 300 La 423 Kdoes he alence-band spec- um eco e he shape o ha o Z 02. Thus, al hough gene a ion o s a es in he band gap and modi ica ion o he alence-band shape occu simul aneously du ing A + bomba dmen , he second e ec emains e en a e emo- al o he oxygen acancies by 02 adso p ion a low em- pe a u es. This ac sugges s ha he shape could be e- la ed o he amo phiza ion and loss o he h ee- dimensional o de o Z 02 by A + bo nba d nen . Fu - he mo e, acco ding o hese esul s he modi ied s uc- u e cha ac e ized by ahighe co alency (see below) is p ese ed un il he s uc u e is annealed abo e 423 K. In o de o obse e any possible inAuence om he ion ene gy we pe o med asimila de ailed s udy o he e ec s induced by 0.5keV A + ions. The e ec s a e qual- i a i ely e y simila o hose discussed abo e o 2keV A + ions, bu no so in ense. A e asimila spu e ing ime we obse ed ha he s eady s a e eached a 0.5keV consis s o an al e ed laye wi h conside ably ewe de- ec s han ha ob ained by spu e ing wi h 2keV A + ions (Fig. 8). In he ollowing esonan pho oemission ex- pe imen s on he educed su ace using 0.5keV ions a e desc ibed. 11 718 C. MORANT e al. ~I~III~I~I~I~ EF +100 L02 473K ~4 +100 L02 423K +100 L02 298K A ' 2keV I'I'IIII'I 14 12 10 86420 BINDING ENERGY (eV) ~I FIG. 10. Pho oemission spec a (h =39 eV) o he alence band o Z O (2 keV A + ions) a e oxygen exposu e a in- c easing empe a u es. D. Resonan pho oemission o Z O„ Figu e 11 shows an o e iew pho oemission spec um aken a e bomba dmen wi h 0.5keV A + ions o 135 min (i.e.,s eady s a e). Compa ed wi h he spec um o Z 02 in Fig. 1 he e ec s o A bomba dmen a e clea - ly obse able in all he bands including he Z 4p, he 0 2s, he 02p, and he emission band in he gap. The Z 4p peak is b oadened, sugges ing ha Z species ha ing alence s a es lowe han 4+ a e now p esen . The Z 4p/0 2s in ensi y a io inc eases upon ion bomba dmen , con i ming he p e e en ial loss o oxygen. An es ima ion Z Ox EF h (eV)~E 'la jW~ 60 50 o he su ace composi ion in e ms o he Z 4p and 02s in ensi ies gi es an a e age s oichiome y Z O» which is in good ag eemen wi h he a e age composi ion (-Z O) o he al e ed laye caused by 3keV A + ions, as de e - mined by XPS. In Fig. 12 we show he alence-band pho oemission spec a o Z O„ in he 0— 15 eV binding- ene gy egion, measu ed a pho on ene gies in he ange 30~h ~60 eV. On he igh -hand side o he igu e, he band-gap emission has been depic ed on an expanded scale. Again, he in ensi y and shape o he gap s a es measu ed a h = 30 eV a e s ongly a ec ed by he con- ibu ion o he Z 4p co e le els exci ed wi h second- o de ligh and will no be conside ed in he quan i a i e analysis. E ec s associa ed wi h he esonan p ocess a e clea ly obse able and ha e been quan i a i ely analyzed in a simila manne o ha o s oichiome ic Z 02 in Sec. IIIB. Figu e 13 shows he dependence o he pho on en- e gy wi h espec o he in ensi y o he alence band and he emission in he band gap. Again we show he beha io o he espec i e in ensi ies o wo Gaussians la- beled as "bonding" and "nonbonding. "In his case, howe e , he alence-band spec a could be analyzed in e ms o only wo o such cu es, he wid hs and ene gy posi ions o which a e summa ized in Table I. A ypical i is shown in Fig. 4(b). The Gaussian labeled as non- bonding is assigned o he lowes -binding-ene gy co n- ponen , whe eas he bonding one has been assigned o he highes -binding-ene gy componen . The esonance p o iles o Fig. 13 show abeha io simi- la o hose measu ed o s oichiome ic Z 02. In pa ic- ula , all he p o iles a e cha ac e ized by wo maxima a ound 40 eV and 45— 50 eV. Whe eas he in ensi y o he bonding componen eaches i s main maximum a 40 eV, we no e ha he esonance p o ile o he nonbonding componen shows abump a ha ene gy ollowed by an ~10 C 8 CQ 6 g) 4- UJ z I— 0- Z O„, hU=50eV EF 15 I 10 50 45 42 41 —40 38 37 36 ~o ~'!M~+ /0 C w ~Q ) IIIII~I 54321 0-1-2 IIII I II 50 40 30 20 10 BINDING ENERGY (eV) FICx. 11. Pho oemission spec um measu ed a h =50 eV om Z O& a e bomba dmen wi h 0.5keV A + ions (135 min). BINDING ENERGY (eV) FICx. 12. Pho oemission spec a o he alence band (le ) and he band-gap emission ( igh ) om Z O„(see Fig. 11)as a unc- ion o he pho on ene gy. 52 ELECTRONIC STRUCTURE OF STOICHIOMETRIC AND A +-. . . 11 719 45— l/l 40 35— lg 30— I— 25— Z', UJ I- 20— Z'. 15— 10— 0I~I~ I ~II~I~I 30 35 40 45 50 55 60 PHOTON ENERGY (eV) FIG. 13. Pho on ene gy dependence o he in eg a ed in ensi- y o he o al alence band (~)(cu e a) and he band-gap emission (~)(cu e e) as well as o he bonding (A) (cu e b) and nonbonding (0)(cu e c) componen s. absolu e maximum a -50eV. The esonance a 40 eV is associa ed wi h he enhancemen o he Z 4d s a es in- ol ed in he alence band, while he o igin o he eso- nance a 45— 50 eV is unknown. The esonan beha io o he s a es in he band gap is mo e clea ly obse ed in his case due o he inc eased in ensi y in his spec al e- gion. The maximum a 40 eV clea ly con i ms he ca ion- ic cha ac e o hese s a es. All he p o iles show ha he esonance a 40 eV has b oadened signi ican ly compa ed wi h he sha p peak obse ed o s oichiome ic Z Oz. As in Sec. III8we calcula ed di e ence cu es in o - de o poin ou hose egions o he spec a which a e enhanced when he ene gy o esonance is eached. The esul ing cu e ob ained a e sub ac ing he pho oemis- sion spec a measu ed a 36 eV (o esonance) om ha measu ed a 40 eV (on esonance) is shown in Fig. 14. This igu e also includes he XPS alence-band spec um Z O„ XPS '~ I'I'I'I'I'I'I' 'I 14 12 10 86420-2 BINDING ENERGY (eV) FIC . 14. Z 4d con ibu ion o he alence band as de e - mined om he esonances a h =40 eV. Fo compa ison we ha e included he XPS spec um o he alence band o Z O„ om Re . 26. o Z Oz bomba ded wi h 3keV A + ions published in Re . 26. Acco ding o he in e p e a ion gi en abo e he 40/36 eV di e ence spec um should e lec he Z 4d pa ial densi y o s a es in Z O; un o una ely, he e a e no band s uc u e calcula ions o compa ison. Compa - ing wi h he equi alen spec um o Z Oz (see Fig. 6) in- dica es asigni ican inc ease in he Z 4d con ibu ion, mainly in he low-binding-ene gy egion o he alence band (i.e., he nonbonding egion), as a esul o he spu e ing. The inc ease in he ca ionic con ibu ion clea ly sugges s asigni ican inc ease in co alency due o he p e e en ial loss o oxygen a oms and he co espond- ing educ ion o he ca ion. This e ec is also con i med by compa ing he 40/36 eV di e ence cu e, labeled Z 4d in Fig. 14, wi h he XPS alence band o Z O ob- ained a e bomba dmen wi h 3keV A + ions. Con- a y o he compa ison made in Fig. 6 o Z 02, we ob- se e in he case o Z O easonable ag eemen be ween he XPS alence-band spec um and he Z 4d con ibu- ion ep esen ed by he di e ence cu e 40/36 eV. This indica es ha he 02p con ibu ion o he alence band o Z O is quan i a i ely less impo an han o Z 02. The small di e ences should be accoun ed o by he 02p con ibu ion. Acompa ison be ween he analysis o he XPS alence band o Z 02 (Fig. 6) and Z O (Fig. 14) leads o he conclusion ha whe eas he 02p componen is signi ican ly educed in in ensi y he Z 4d con ibu ion inc eases o e he wid h o he alence band bu mo e s ongly in he nonbonding egion. Al hough he ca ion- ic componen is p obably o e es ima ed, he da a indica e ha he c ea ion o oxygen acancies and he hea y educ ion o he oxide by A + bomba dmen causes a signi ican dec ease in he numbe o ionic Z -0 bonds and aconside able inc ease in co alency o he modi ied su ace. IV. SUMMARY AND CONCLUSIONS The elec onic s uc u e o he mally g own Z 02 hin ilms be o e and a e A + bomba dmen was s udied wi h esonan pho oemission spec oscopy using synch o- on adia ion. Good ag eemen has been ound be ween he alence-band spec a o he mally g own Z 02 hin ilms and he calcula ed densi y o s a es o bulk Z 02. Fo bo h s oichiome ic and A +-bomba ded Z 02 hin ilms, esonan pho oemission om he alence band was obse ed when he pho on ene gy was uned h ough he Z 4p — +4d ansi ion ene gy. A a he complex depen- dence o he o al alence-band in ensi y on he pho on ene gy was ound. The esonance p o ile exhibi s amax- imum a h = 39 eV ollowed by asecond b oad s uc u e a 45— 50 eV. The in ensi y enhancemen obse ed a 39 eV is consis en wi h he esonan pho oemission o Z 4d s a es in ol ed in he alence band. In ac , he phenomenon has been used o iden i y hose egions o he alence band wi h an impo an Z 4d cha ac e and also o es ima e he deg ee o Z 4d/0 2p hyb idiza ion. The second esonance a pho on ene gies be ween 45 and 50 eV is associa ed wi h he nonbonding egion o he alence band; howe e , i s o igin emains unexplained. In addi ion, we ha e s udied he changes ha occu in