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spectroscopic detection of intrinsic defects in nano-crystalline
transition metal elemental oxides scales of order (0.5 to 5 nm) for
nano- and non-crystalline thinsGerry Lucovsky, NC State
Universitygraduate students and post docs S. Lee, H. Seo, J.P.
Long, C.L. Hinkle and L.B. Flemingcollaborators J.L. Whitten
(ab-initio theory), J. Lning (NEXAS, SSRL) , D.E. Aspnes (SE), M.D.
Ulrich. J.E. Rowe (SXPS, NSLS-BNL)outlinerecent technology
advancesintroduction to spectroscopic techniquesconduction and
valence band electronic statesintrinsic bonding defects in Ti, Zr
and Hf elemental oxidesengineering solutions - HfO2, Hf silicates,
tphy < 2 nm
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recent technology advancestwo different dielectrics have emerged
as candidates for introduction at the 32 nm process
nodenano-crystalline HfO2 (< 2 nm) and non-crystalline
HfSiONmost significant published technology advances from SEMATECH
group:Gennadi Bersuker, Pat Lysaght, Paul Kirsch, Manuel Quevedo,
Chad Young, et. althis report: science base for quantifying
differences in electronic structure between these two classes of
materials intrinsic defects assigned to O-atom vacancies clustered
on nano-crystalline grain boundaries defect densities significantly
reduced when film thickness is ~2 nm
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spectroscopic approachesnear edge x-ray absorption - NEXAS -
SSRL 200 to 1200 eV x-rays, S/N ~ 5000:1, resolution, 0.1 eV soft
x-ray photoelectron spectroscopy - SXPS - BNL40 to 200 eV, S/N
~1000:1, resolution, ~0.15 eVvis-vacuum UV spectroscopic
ellipsometry - vis-VUV SE 1.8 to 6 eV, and 4 eV to ~ 8.5 eV
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molecular orbital model valence band (SXPS, UPS) and conduction
band (NEXAS) reveal d-state featurestwo contributions to lifting of
d-state degeneracy crystal field symmetry/coordination nano- and
non-crystalline filmsD[Eg(2) - T2g(3)] ~ 2.5 - 4 eVJahn-Teller
bonding distortions rutile and distorted CaF2degeneracies removed
Eg 2 states & T2g 3 statesd[Eg(2)]~d[T2g(3)] ~ 0.5-1 eV
octahedral bonding of Ti with 6 Ozeroth order MO approximation
Ti molecular orbitals Ostates labeled wrt to molecular symmetry
considerable mixing: 3d, 4s and 4p states however, atomic labeling
provides useful description of band edge electronic structure,
intrinsic defects
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x-ray absorption spectroscopy - a novel way to study conduction
band d*-states in transition metal oxidesinter-atomic
spectraZr5s*,p* + O 2p*Zr4d* s(5s*,p*) + O 2p* sZr4d* p(5s*,p*) + O
2p* pZr5s*,p* + O 2p*Zr4d* s+ O 2p* s Zr 4d* p + O 2p*, p O 1sO 2p
nbcore level and and band edge transitions terminate in similar
Zr-O molecular orbital statesvis and VUV spectroscopic
ellipsometrysimilar transitions to Ti 3d*, etc.
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conduction and valence band edge electronic statesTiO2; roadmap
for other dielectrics: HfO2 and "Hf SiON"D(Eg-T2g)av ~3 eVcrystal
field splitting 6-fold coordination octahedral symmetryD(Eg-T2g)av
~2.1 eVband gaps, BG, scale with atomic d-state energiesoxide BG
at. d-stateTi: 3.2 eV, -11.0 eVZr: 5.5 eV, -8.1 eVHf: 5.7 eV, -8.4
eV
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Ti d-state degeneracy removalO K1 edge inter-atomic O 2p + 3d,
4s and 4pTi L3 edge intra-atomic Ti 3d2nd derivative of absorption
respective OK1 - L3 spectraTi 3d, 4s, and 4p
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Ti d-state features in O K1 (x-axis) and Ti L3 (y-axis)
edgesslope of ~ 1 indicatescrystal field - average Eg-T2g
splittings, andJahn-Teller term-splittingsessentially the same in O
K1 and Ti L3 NEXAS spectraL3 is intra-atomic transition O K1 are
projections of same d-states, but different transition matrix
elements
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comparison of average (C-F) d-state splittings valence and
conduction band band edge statesO K1, empty conduction band states,
filled valence band statesD(Eg,Tg): 3.0 eV, 2.1 eVD(4s, 4p): 3.5
eV, 2.6 eVanti-bonding states split more than bonding states
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linear scaling (slope 1) between Ti L3, and both O K1 and
e2gives linear scaling (slope 1) between e2 and O K1O K1 edge -
wider spectral range, than lab VUV SE
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limitations on NEXAS approach p-state core hole life-times scale
inversely as Zeff2.5 (Slater rules) for M3,N3 absorptions - Zr 3p
to 4d, Hf 4p to 5d, too much broadening to resolve d-state
splittingshowever,J-T separations are easily obtained from O K1
edgegaussian fits and 2nd derivatives term splittings for Eg and
T2g compared with epsilon 2 (e2) from VUV-SE, SXPS valence band
spectra, and studied for different scales of order film
thickness
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intrinsic conduction band edge electronic structure O K1 edge in
XAS d-states - same splittings as conduction band d-states in SE
VUV e2 and anano-crystalline - 800C anneal Eg - 532.5, 533.5 (0.15
eV) T2g - 535.2, 536.3, 537.4 (0.15 eV) DEg=10.2 eV - D(T2g -
Eg)=2.70.2 eV epsilon 2 (e2) spectrum DEg = 0.80.2 eV D(T2g - Eg) =
2.30.2 eV
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summary - part I experimental determination of electronic
structure of conduction and valence band edge statesNEXAS - OK1 for
conduction band states SXPS - for valence band statesbonding and
localized nature of d-states molecular orbital description basis
for correlating spectral features with atomic states of transition
metal atoms of high-k dielectrics
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defect states - electron and hole injection into HfO2 through a
thin SiO2/SiON interfacial layer electron and hole
trapping/transport asymmetriesfirst studied by IMEC group confirmed
at NC Statemodel calculations for defects
Robertson/Schlugerspectroscopic studies TiO2 VB and e2 spectra --
defect state electronic structure
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Massoun et al., APL 81, 3392 (2002)Si-SiO2-HfO2 gate stackstraps
are in high-k material of stack 2x1013 cm -2 -- s ~ 1.5x10-17 cm-2
coulombic center - lower x-section than Pb centers in Si substrate
screened by high dielectric constant of HfO2Z. Xu et al., APL 80,
1975 (2002)substrate injection electrons gate injection electrons
electron trap ~0.5 eV below conduction band edge HfO2substrate
injection holes
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J-V asymmetry - IMEC modelcontinuity of eE - e(SiO2) ~ 3.9 <
e(HfO2)~20 asymmetry in potential distribution across stacktraps
accessible for injection from n-type substrate using mid-gap gate
metal - TiNtraps not accessible for injection from mid-gap metal --
TiN
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from M. Houssa, IOP, Chapter 3.4 Lucovsky group NCSUsubstrate
electron injectionelectron transportsubstrate hole injection hole
trapping>500xC-V -- surface potentials of Si substrate are
negative hole injection
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spectroscopic (SXPS, SE) identification of defect states in
TiO2energy of defect state with respect to valence band edge
analysis of SXPS spectrum defect state (peak) 2.40.2 eV above VB
edgeanalysis of e2 - band gap of 3.2 eV defect state at 2.50.2 eV
above VB edgeTiO2 valence band & defect states TiO2 conduction
band & defect states
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SXPS valence band spectra qualitative similarities between TiO2
and HfO2 symmetry driven reversal of Eg and T2g statesHfO2 - mid
1019 cm-3 -- TiO2 - high 1019 to low 1020 cm-3) greater departurses
(d) from stoichiometry in TiO(2-d)
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Robertson et al., IEEE Trans DMR, 5, 84 (2005)O-atom
mono-vacancy defects do not describe exp. results states too close
to Si conduction band edge~4.2-4.5 above valence band edge of
Zr(Hf)O2 ~ 2 eV below lowest d-state feature in Hf(Zr)O2vacancy
(VO) and interstitial (IO) O-atom defects in ZrO2 similar results
for HfO2K. Xiong, J. Robertson, S.J. Clark, APL 87 (2005)adds two
more charge states for O-atom vacancies VO- and VO2+
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Schluger groupno mono-vacancy states at valence band
edgeRobertson, et al.Schluger group
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Ti3+ in Ti(H2O)63+model for intrinsic O-vacancy defects in TiO2
comparison with hydrated Ti3+ ion spectrum classic example in
Molecular Orbital Theory texts - Ballhausen and GrayEp(wrt VB) ~
2.4 eV, D ~ 1 eVproposal: Ti3+ in TiO2 O-divacancies clustered
along grain-boundaries
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clustered vacancy model for TiO2TiO(2 - d) = b(TiO2) + a(Ti2O3)b
+ 2a = 12 - d = 2b + 3aa = db = 1 - 2dconcentration of Ti3+ = 2dif
defects are divacancies, then 2d ~ 10-3 or 2-3x1019 cm-3similar
calculations apply to ZrO2 and HfO2 and defect states are labeled
accordinglydivacancies - clustered on grain boundaries two Ti, Zr
or Hf atoms of each divacancy defect nearest neighbors to 2 missing
O-atoms
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HB Gray, and HB Gray and CJ Ballhausenmodel for Ti3+ defect
states in TiO2 band gapmodel calculation TiO2 valence &
conduction bandsdegeneracy in T2g defect state removed by J-T
distortion (as in Ti2O3)
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SXPS and UPS valence band spectra of TiO2Ti 3d-state
contributions to VBSXPS valence band spectra for HfO2 and UPS
valence band spectra of ZrO2qualitative similar spectra 4d and 5d
statesUPS He IHfO2SXPS 60 eVrange of UPSreliable
data6p6s6d3/26d5/2O2ppnbHf3+photoelectron countsbinding energy
(eV)Eg symmetryTi3+ T2g Zr3+
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O K1VUV SEPC
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comparison between Zollner and NCSU VUV SE measurements and
analysis
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summary - part II spectroscopic detection of band edge defects
valence band edge - SXPS, conduction band edge - O K1 NEXAS, VUV SE
and PCnot described by mono-vacancy calculations of Robertson (and
Schluger) energy of formation (>5-8 eV) much too high for
concentrations > 1019 cm-3defect states in TiO2 identified by
analogy with hydrated Ti3+ ion in solution, and Ti2O3 band gap Ti3+
in divacancies clustered along grain boundariessimilar assignments
for HfO2 and TiO2 intrinsic defectsgrain boundary defect model
supported by measurements of TJ King for HfO2 as function of
annealing as crystal size grows, grain boundary density decreases
and defect signature in VUV SE is reduced
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as annealing temperature is increased band edge, and discrete
defect concentrations are each reduced
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scales of order for electronic structure/defect formationgrain
boundaries are well-defined when crystallite size extends to
several (~3-4) primitive cell unitscell dimensions are typically ~
0.5 nm, so that critical dimension for defect formation is ~1.5 nm
to 2 nmexperimental observations by SEMATECH and STM defect
densities are significantly reduced when physical film thickness is
reduced below 2 nmis there a spectroscopic signature for this
change in crystallite size? yes, relative strengths of p and
s-anti-bonding states in OK1 spectra
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s-bonding is intra-primitive cell in character it therefore
occurs on a 0.5 nm scale[O-s1-s1'-Ti-s2'-s2-O]-s3-s3'-Ti-, etc s1,
s2, s3, etc., are different O-atom s-bonds s1', s2', s3'. etc., are
different Ti-atom s-bonds[....] indicates the primitive cell
bonding units-bonds within the cell - localized on intra-cell
atomsp-bonding is inter-primitive cell in character occurs on a
scale > 1.5-2.0 nmp1-[O-p1-p1'-Ti-p1'-p2-O]-p2-p3'-Ti-,
etcp1-O-p1-p1'-{Ti-p1'-p2-O-p2-p3'-Ti}-, etcp1, p2, p3, etc., are
different O-atom p-bonds p1', p2', p3'. etc., are different Ti-atom
p-bonds[....] indicates the primitive cell bonding unit{.....}
indicates the coupling of primitive cells via O-p-bonds O p2 bond
couples 2 different primitive cells
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chemical phase separation: Zr silicates - thickness > 20 nm3
nm0.5 eV spectral shift 1/2 of D(4d3/2/Eg) of
nano-crystalline-ZrO2degeneracy removal ~3 nm nano-crystallite
grainsZr silicate x~0.25 after 900C anneal
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change in energy of first peak in nano-crystalline ZrO2 as a
function of film thickness
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thickness dependence of lowest p-state as function of thickness
ZrO2, high ZrO2 content silicate alloyfilms of ~ 2 nm each show low
defect densities for EOT < 1 nm
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scales of order for SiO2ab initio calculation for ~ 1 nm cluster
gives excited states that correlate with absorption spectrum for
thick 10 nm SiO2 fix energy gap of 8.90.1 eV with derivative
feature at 529.6 eV and e2 peaks are in good agreement with O
K1
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Si* are effective potentials that ensure there is no dipole
moment, and that core levels of Si and O within cluster are
correctcluster for electronic transition calculations when relaxed
by CI gives the experimental bond angle, bond angle distribution,
and IR effective charges
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O K1 edges of SiO2 spectra for films as thin as 1.5 nm are
essentially the same as those of films10 nm thickmolecular Si-O-Si
units are coupled by s-bonds of four-fold coordinated Si atoms via
different orbitals very from p-coupling of Ti, Zr and Hf elemental
oxides
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application to SiO2 correlation between OK1 and reflectivity
spectrum of Herb Phillip for non-crystalline fused silica and
crystalline alpha quartzspectral peaks at same energies in
non-crystalline and crystalline SiO2is there a linear correlation
between features in reflectivity and O K1 edge?e2 peaksest. from
plot(0.2 eV) 10.13 eV12.35 eV13.92 eV16.55 eV
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(a) midgap voltage and (b) flatband voltage shifts for total
dose irradiations for Hf silicate capacitors with 4.5 nm EOT gate
dielectrics - SiO2 like midgap voltage shift vs. does for 67.5 nm
HfO2 and Al2O3 ALD dielectrics on 1.1 nm Si oxynitride for
different annealing - effects different than SiO2 due to
grain-boundary defectsradiation effects in nano-crystalline HfO2
and non-crystalline Hf silicate MOSCAPs
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summary band edge electronic structure NEXAS O K1 edge
replicates: conduction band states SXPS gives valence band
statesdefect states are vacancies clustered on grain boundaries of
nano-crystalline oxides densities >1019 cm-3 and easily detected
by O K1, SXPS and vis-VUV SEscale of order for suppression of band
edge p-state degeneracy removal is ~2 nm two engineering solutions
for 32 nm node ultra thin HfO2, Hf silicate alloys (well done
SEMATECH!!)scales of order for p- and s-bonding differentiated by
extent to which primative unit cells are coupled through
O-atomsSiO2 is qualitatively different scale of order for
conduction band resonance excitons is 1 nm demonstrated by ab
initio theory and verified by experiment - O K1 edge
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high-lighted d-state splittings in Zr and Ti SiON consistent
with 4-fold coordination of Zr and TiHf SiON - decreased hole
trapping in rad testing (later in review)Ti, Zr and Hf trapped in
Si3N4-SiO2 matrix no chemical phase separation to 1100C EOTs to 32
nm nodeZr SiON 40% Si3N42.2 eV for 4-fold, ~4 eV for 8(7) fold