1. 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 thins
Gerry Lucovsky, NC State University
graduate students and post docs S. Lee, H. Seo, J.P. Long, C.L. Hinkle and L.B. Fleming
collaborators J.L. Whitten (ab-initio theory), J. Lüning (NEXAS, SSRL) , D.E. Aspnes (SE), M.D. Ulrich. J.E. Rowe (SXPS, NSLS-BNL)
outline
recent technology advances
introduction to spectroscopic techniques
conduction and valence band electronic states
intrinsic bonding defects in Ti, Zr and Hf elemental oxides
engineering solutions - HfO2, Hf silicates, tphy < 2 nm

2. spectroscopic detection of intrinsic defects in nano-crystalline transition metal elemental oxides scales of order (0.5 to 5 nm) in nano- and non-crystalline films
Gerry Lucovsky, NC State University
graduate students and post docs S. Lee, H. Seo, C.L. Hinkle and L.B. Fleming
collaborators J.L. Whitten (ab-initio theory), J. Lüning (NEXAS, SSRL) , D.E. Aspnes (SE), M.D. Ulrich. J.E. Rowe (SXPS, NSLS-BNL)
outline
recent technology advances
introduction to spectroscopic techniques: ab initio molecular orbital calculations for spectral feature assignments
conduction and valence band electronic states
intrinsic bonding defects in Ti, Zr and Hf elemental oxides: scales of order for d-state degeneracy, and defect removal
ternary Zr, Ti and Hf Si oxynitride alloys:
two qualitatively different approaches

3. recent technology advances
two different dielectrics have emerged as candidates for introduction at the 32 nm process node
nano-crystalline HfO2 (< 2 nm)
and
non-crystalline HfSiON
most significant published technology advances from SEMATECH group:Gennadi Bersuker, Pat Lysaght, Paul Kirsch, Manuel Quevedo, Chad Young, et. al
this 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

4. spectroscopic approaches near 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 - BNL
40 to 200 eV, S/N ~1000:1, resolution, ~0.15 eV
vis-vacuum UV spectroscopic ellipsometry - vis-VUV SE
1.8 to 6 eV, and 4 eV to ~ 8.5 eV

5. octahedral bonding of Ti with 6 O
zeroth order MO approximation
molecular orbital model valence band (SXPS, UPS) and conduction band (NEXAS) reveal d-state features
two contributions to lifting of d-state degeneracy
“crystal field” symmetry/coordination nano- and non-crystalline films
D[Eg(2) - T2g(3)] ~ eV
“Jahn-Teller” bonding distortions rutile and distorted CaF2
degeneracies removed Eg  2 states & T2g  3 states
d[Eg(2)]~d[T2g(3)] ~ eV
Ti molecular orbitals O
NEXAS O K1 edge conduct. band
UPS, SXPS valence band
states 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

6. x-ray absorption spectroscopy - a novel way to study conduction band d
x-ray absorption spectroscopy - a novel way to study conduction band d*-states in transition metal oxides
intra-atom core level spectra
Zr 1s Zr
5s*,p* + O 2p*
4d* s
(5s*,p*) + O 2p* p
5s*,p*, (Ti 4s*,p*)
(Ti 3d* s)
(Ti 3d* p)
Zr 3p
deep core level transitions terminate in empty localized 4d* and 5s*(p*) states
final states display local bonding symmetry of Zr (Ti) atoms
(Ti 2p)
inter-atomic spectra Zr Zr
5s*,p* + O 2p*
5s*,p* + O 2p* Zr Zr
4d*
4d* s s
(5s*,p*) + O 2p*
+ O 2p* s s Zr
Zr 4d* p + O 2p*, p
4d* p
(5s*,p*) + O 2p* p
core level and and band edge transitions terminate in similar Zr-O molecular orbital states
O 2p nb
vis and VUV spectroscopic ellipsometry
O 1s
similar transitions to Ti 3d*, etc.

7. conduction and valence band edge electronic states
TiO2; roadmap for other dielectrics: HfO2 and "Hf SiON"
band gaps, BG, scale with atomic d-state energies
D(Eg-T2g)av ~3 eV
oxide BG at. d-state
Ti: eV, eV
Zr: eV, eV
Hf: eV, eV
D(Eg-T2g)av ~2.1 eV
crystal field splitting fold coordination octahedral symmetry

8. Ti d-state degeneracy removal
O K1 edge inter-atomic O 2p + 3d, 4s and 4p
2nd derivative of absorption respective OK1 - L3 spectra
Ti L3 edge intra-atomic Ti 3d
Ti 3d, 4s, and 4p

9. Ti d-state features in O K1 (x-axis) and Ti L3 (y-axis) edges
slope of ~ 1 indicates
crystal field - average Eg-T2g splittings, and
Jahn-Teller term-splittings
essentially the same in O K1 and Ti L NEXAS spectra
L3 is intra-atomic transition
O K1 are projections of same d-states, but different transition matrix elements

10. comparison of average (C-F) d-state splittings valence and conduction band band edge states
O K1, empty conduction band states, filled valence band states
D(Eg,Tg): 3.0 eV, 2.1 eV
D(4s, 4p): 3.5 eV, 2.6 eV
anti-bonding states split more than bonding states

11. linear scaling (slope 1) between Ti L3, and both O K1 and e2
gives linear scaling (slope 1) between e2 and O K1
O K1 edge - wider spectral range, than lab VUV SE

12. 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 splittings
however,
J-T separations are easily obtained from O K1 edge
gaussian 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

13. 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 a
nano-crystalline - 800°C anneal Eg , (0.15 eV) T2g , 536.3, (0.15 eV)
DEg=10.2 eV - D(T2g - Eg)=2.70.2 eV
epsilon 2 (e2) spectrum DEg = 0.80.2 eV D(T2g - Eg) = 2.30.2 eV

14. NEXAS - OK1 for conduction band states SXPS - for valence band states
summary - part I
experimental determination of electronic structure of conduction and valence band edge states
NEXAS - OK1 for conduction band states SXPS - for valence band states
bonding 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

15. defect states - electron and hole injection into HfO2 through a thin SiO2/SiON interfacial layer electron and hole trapping/transport asymmetries
first studied by IMEC group confirmed at NC State
model calculations for defects Robertson/Schluger
spectroscopic studies TiO2 VB and e2 spectra -- defect state electronic structure

16. Si-SiO2-HfO2 gate stacks
Massoun et al., APL 81, 3392 (2002)
Z. Xu et al., APL 80, 1975 (2002)
substrate injection holes
substrate injection electrons
gate injection electrons
traps are in high-k material of stack 2x1013 cm s ~ 1.5x10-17 cm coulombic center - lower x-section than Pb centers in Si substrate screened by high dielectric constant of HfO2
electron trap ~0.5 eV below conduction band edge HfO2

17. J-V asymmetry - IMEC model
continuity of eE - e(SiO2) ~ 3.9 < e(HfO2)~ asymmetry in potential distribution across stack
traps accessible for injection from n-type substrate using mid-gap gate metal - TiN
traps not accessible for injection from mid-gap metal -- TiN

18. substrate electron injection substrate hole injection
from M. Houssa, IOP, Chapter Lucovsky group NCSU
substrate electron injection
electron transport
>500x
EOT~7 nm
EOT~1.7 nm
substrate hole injection
hole trapping
C-V -- surface potentials of Si substrate are negative  hole injection

19. spectroscopic (SXPS, SE) identification of defect states in TiO2
TiO2 valence band & defect states TiO2 conduction band & defect states
Ti3+
T2g
energy of defect state with respect to valence band edge
analysis of SXPS spectrum defect state (peak) 2.4±0.2 eV above VB edge
analysis of e2 - band gap of 3.2 eV defect state at 2.5±0.2 eV above VB edge

20. SXPS valence band spectra qualitative similarities between TiO2 and HfO symmetry driven reversal of Eg and T2g states
HfO2 - mid 1019 cm-3 -- TiO2 - high 1019 to low 1020 cm-3) greater departurses (d) from stoichiometry in TiO(2-d)

21. Robertson et al., IEEE Trans DMR, 5, 84 (2005)
K. Xiong, J. Robertson, S.J. Clark, APL 87 (2005)
vacancy (VO) and interstitial (IO) O-atom defects in ZrO similar results for HfO2
adds two more charge states for O-atom vacancies VO- and VO2+
O-atom mono-vacancy defects do not describe exp. results states too close to Si conduction band edge
~ above valence band edge of Zr(Hf)O ~ 2 eV below lowest d-state feature in Hf(Zr)O2

22. no mono-vacancy states at valence band edge
Schluger group
no mono-vacancy states at valence band edge
Robertson, et al.

23. proposal: Ti3+ in TiO2 O-divancies clustered along grain-boundaries
model for intrinsic O-vacancy defects in TiO comparison with hydrated Ti3+ ion spectrum classic example in Molecular Orbital Theory texts - Ballhausen and Gray
Ti3+ in Ti(H2O)63
proposal: Ti3+ in TiO O-divancies clustered along grain-boundaries
Ep(wrt VB) ~ 2.4 eV, D ~ 1 eV

24. clustered vacancy model for TiO2
TiO(2 - d) = b(TiO2) + a(Ti2O3)
b + 2a = 1
2 - d = 2b + 3a
a = d
b = 1 - 2d
concentration of Ti3+ = 2d
if defects are divacancies, then d ~ 10-3 or 2-3x1019 cm-3
similar calculations apply to ZrO2 and HfO and defect states are labeled accordingly
divacancies - clustered on grain boundaries two Ti, Zr or Hf atoms of each divacancy defect nearest nieighbors to 2 missing O-atoms

25. model for Ti3+ defect states in TiO2 band gap
model calculation TiO2 valence & conduction bands
degeneracy in T2g defect state removed by J-T distortion (as in Ti2O3)
HB Gray, and HB Gray and CJ Ballhausen

26. SXPS and UPS valence band spectra of TiO2
SXPS valence band spectra for HfO2 and UPS valence band spectra of ZrO2
HfO 2 6d
SXPS 60 eV
5/2
100
1000 10 4
-16
-14
-12
-10 -8 -6 -4 -2
ZrO 2 6d
3/2 6s
O2p p 6p nb
UPS He I
Ti3+
T2g
photoelectron counts
Hf3+
Eg symmetry
Zr3+
range of UPS
reliable data
binding energy (eV)
Ti 3d-state contributions to VB
qualitative similar spectra d and 5d states

27. band edge defect spectral features ZrO2 XAS, VUV SE, PC
defect state
O K1
VUV SE PC

28. comparison between Zollner and NCSU VUV SE measurements and analysis

29. similar assignments for HfO2 and TiO2 intrinsic defects
summary - part II spectroscopic detection of band edge defects valence band edge - SXPS, conduction band edge - O K1 NEXAS, VUV SE and PC
not described by mono-vacancy calculations of Robertson (and Schluger) energy of formation (>5-8 eV) much too high for concentrations > 1019 cm-3
defect states in TiO2 identified by analogy with hydrated Ti3+ ion in solution, and Ti2O3 band gap Ti3+ in divacancies clustered along grain boundaries
similar assignments for HfO2 and TiO intrinsic defects
grain 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

30. defect states
as annealing temperature is increased band edge, and discrete defect concentrations are each reduced

31. scales of order for electronic structure/defect formation
grain boundaries are well-defined when crystallite size extends to several (~3-4) “primative” cell units
cell dimensions are typically ~ 0.5 nm, so that critical dimension for defect formation is ~1.5 nm to 2 nm
experimental observations by SEMATECH and STM defect densities are significantly reduced when physical film thickness is reduced below 2 nm
is there a spectroscopic signature for this change in crystallite size? yes, relative strengths of p and s-anti-bonding states in OK1 spectra

32. s-bonding is intra-primative 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 “primative” cell bonding unit
s-bonds within the cell - localized on intra-cell atoms
p-bonding is inter-primative cell in character
occurs on a scale > nm
p1-[O-p1-p1'-Ti-p1'-p2-O]-p2-p3'-Ti-, etc
p1-O-p1-p1'-{Ti-p1'-p2-O-p2-p3'-Ti}-, etc
p1, p2, p3, etc., are different O-atom p-bonds
p1', p2', p3'. etc., are different Ti-atom p-bonds
{.....} indicates the coupling of primative cells via O-p-bonds O p2 bond couples 2 different primative cells

33. chemical phase separation: Zr silicates - thickness > 20 nm
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
530
532
534
536
538
540
absorption (arb. units)
photon energy (eV)
as-deposited
300 o C
900
anneal
Zr 4d* 4d
3/2
5/2 D
E ~ 0.5 eV
~30% ZrO 2
SRO
IRO
0.5 eV spectral shift 1/2 of D(4d3/2/Eg) of nano-crystalline-ZrO2
degeneracy removal ~3 nm nano-crystallite grains
3 nm
Zr silicate x~ after 900°C anneal

34. change in energy of first peak in nano-crystalline ZrO2 as a function of film thickness

35. films of ~ 2 nm each show low defect densities for EOT < 1 nm
thickness dependence of lowest p-state as function of thickness ZrO2, high ZrO2 content silicate alloy
films of ~ 2 nm each show low defect densities for EOT < 1 nm

36. scales of order for SiO2
ab initio calculation for ~ 1 nm cluster gives excited states that correlate with absorption spectrum for thick 10 nm SiO2
fix energy gap of 8.90.1 eV with derivative feature at eV and e2 peaks are in good agreement with O K1

37. cluster for electronic transition calculations when relaxed by CI gives the experimental bond angle, bond angle distribution, and IR effective charges
physical size
~ 1 nm
Si* are effective potentials that ensure there is no dipole moment, and that core levels of Si and O within cluster are correct

38. O K1 edges of SiO2 spectra for films as thin as 1
O K1 edges of SiO spectra for films as thin as 1.5 nm are essentially the same as those of films10 nm thick
molecular 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

39. application to SiO correlation between OK1 and reflectivity spectrum of Herb Phillip for non-crystalline fused silica and crystalline alpha quartz
e2 peaks
est. from plot
(±0.2 eV)
10.13 eV
12.35 eV
13.92 eV
16.55 eV
is there a linear correlation between features in reflectivity and O K1 edge?
spectral peaks at same energies in non-crystalline and crystalline SiO2

40. radiation effects in nano-crystalline HfO2 and non-crystalline Hf silicate MOSCAPs
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 defects
(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

41. summary
band edge electronic structure NEXAS O K1 edge “replicates: conduction band states SXPS gives valence band states
defect 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 SE
scale 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-atoms
SiO2 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

42. Hf SiON - decreased hole trapping in rad testing (later in review)
Ti, Zr and Hf trapped in Si3N4-SiO2 matrix no chemical phase separation to 1100°C EOTs to 32 nm node
Zr SiON 40% Si3N4
high-lighted d-state splittings in Zr and Ti SiON consistent with 4-fold coordination of Zr and Ti
Hf SiON - decreased hole trapping in rad testing (later in review)