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