outline of presentation research objectives

1. length scales of order and defects in nano-crystalline and non-crystalline Hf-based high/medium-k gate dielectrics Gerry Lucovsky, NC State University, Dept. of Physics students and post doc S. Lee, JP Long, H Seo and L Fleming collaborators J Whitten and D Aspnes (NCSU), J Lüning (SSRL), G Bersuker and P Lysaght (Sematech) and M Ulrich (ARO/NCSU)

2. outline of presentation • research objectives • spectroscopic determination of band edge electronic structure conduction and valence band states intrinsic defects • effects of nano-crystalline grain size on band edge states band edge defects • an emerging medium-k non-crystalline on SiON/Si SiO2 look-alike!- tunneling reduced 102-103 at 1 nm EOT • recent results Hf-based dielectrics directly on Ge substrates SiO2 look-alike - lowest tunneling leakage!!

3. research objectives correlate electronic structure, pre-existing and stress-induced defects with different length scales of order nano-grain size and intermediate range order, respectively fornano- and non-crystalline Hf-based dielectrics  answer this question!! is there a higher k dielectric with reduced leakage that has defect properties qualitatively and quantitatively similar to SiO2 and/or Si oxynitrides currently in CMOS device?? Glen Wilk and I once believed that the answer would be either a Zr(GL) or Hf(GW) silicate but we were both wrong!! they are unstable wrt to chemical phase separation, but the answer is YES!!

4. relative d-state band energies, by NEXAS, SXPS, vis-VUV SE interpretation - symmetry adopted linear combinations (SALCs) of atomic states -- FA Cotton – 1962 monograph

5. symmetry adopted linear combinations (SALC) of atomic states define molecular orbitals for 6-fold coordinated Ti octahedral bonding of Ti with 6 O NEXASO K1 edge; SE Ti O 2p UPS, SXPS valence band conduction band and valence band states symmetry and bonding coordination of TM and O parentage of TM d-states in conduction band is easily resolved in NEXAS O K1 and SE, but valence band states are broader, but spectra are consistent with SALCs crystal-field (C-F) splitting symmetry and coordination dependent ~ 1.6 to 5 eV cooperative Jahn-Teller bonding distortions - TiO2 - anatase-like, monoclinic Hf(Zr)O2 - degeneracy removalJ-T term splittings Eg 2 states and T2g 3 states d[Eg(2)]~d[T2g (3)] ~ 0.5-1.2 eV

6. comparison - nano-crystalline TiO2 near NEXAS and Symmetry Adopted Linear Combination (SALC) anti-bonding MO conduction band states Ti 3d, 4s and 4p atomic contributions distinct features in NEXAS O K1 consistent with group theory / SALC MO description

7. SXPS deconvolution of valence band spectrum O 2p p non-bonding, Ti 3d p-states, Ti 3d, 4s and 4p s-states and band edge defect doublet s p nb p p s ordering of valence band states -- s-p-s-p, etc...

8. relative d-state energies of final conduction band states from NEXAS and spectroscopic ellipsometry NEXAS -- equivalent to vis-VUV SE for IVB, IIIB TM oxides wrt energy differences of conduction band d-states NEXASfaster and more direct can also be applied to ultra-thin dielectrics for devices (~2 nm )

9. O K1 edges of 900°C Ar annealed ZrO2 and HfO2 on Si with SiON interfaces - film thickness > 4 nm 7-fold coordinated in monoclinic structure sd3p3 SALC* MO labeling pseudo-cubic (8-fold) asymmetry in ZrO2, and features in HfO2J-T degeneracy removal p Egs (T2g + A1g + T1u) 7-fold coordinated Hf- black in HfO2 5d3 + 6s1 + 6p3 = 7- s states sd3 - tetrahedron- p3- pyramid -

10. band edge defects in HfO2 and ZrO2 - films > 4 nm thick vis-VUV SE SXPS 2 discrete defect states below conduction band edge 2 discrete defect states above valence band edge defect features in X-ray, visible, UV, VUV enhanced by f-sum rule

11. energy level diagram for band edge defects from NEXAS, vis-VUV SE and SXPS spectra for HfO2 same diagram - reduced band gap, 5.5 vs 5.7 eV for ZrO2 films are O-deficient - mixture of HfO2 (Hf4+) and Hf2O3 (Hf3+) defects described by clustered O-atom vacancies or equivalently Hf3+ states clustered at internal grain boundaries for nano-grains > 4 nm diagram verified by cathodo-luminescence CLS results

12. cathodo-luminescence spectra CLS electrons in - keV’s - photons out from defect transitions Brillson group at Ohio State University 2, 6 4 1, 5 3, 7 deconvolution of spectrum forbulk excitation in thick HfO2 mapping of Gaussians onto edge states - SXPS, SE, XAS

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

14. correlations between grain-size and defects nano-scale of order, lcoh (p-bonding) ~3-3.5 nm for Hf(Zr)O2suppression of Jahn-Teller d-state degeneracy splittings when grain size </~ 2 nm

15. length scale factor for inter primitive unit cell (PUC) coupling coherence of p-bonding - analog to super exchange in MnO Mn spins in adjacent PUCs coupled thorugh O’s PUC 1 2 3 p-bonding couples metal atoms, M, in neighboring PUCs through their nearest neighbor O-atoms p-bonding orbitals phase reversal of 2nd nearest neighbor O-atoms alternating phase of 2nd M-atoms (same phase reversing as for spins in A-F MnO) length scale, lcoh, for J-T splitting ~ 6-7 PUCs or ~3-3.5 nm for coherent p-bonding effects primitive unit cells (PUCs) 1/nO-Hf-O1/n (n=3, 4) labeled as 1, 2 and 3

16. kinetic and dimensional constraints suppress Eg - p-bond state J-T splittings in O K1 edge spectra kinetic as-dep. 300°C, 500 °C anneal no J-T splitting 700°C, 900°C anneals J-T splitting “dimensional” 2 nm thick film no J-T splitting 3 and 4 nm thick films J-T splitting

17. differences in band edge defects in HfO2 discrete states and sharp NEXAS edge for tphy > 3nm, band tail defects and softer edges for tphys ~ 2 nm softer edge for tphys = 2 nm compared to tphys = 3, 4 nm band tail defects - tphys = 2 nm discrete defects - tphys = 4 nm

18. differences in band edge defects for kinetic constraint 300, 500°C suppression J-T and broader features wrt 700&900°C 300°C band tail defect at VB edge discrete defects after 700°C anneal to obtain electrical defect densities, must apply f(N) sum rule matrix element enhancement, ~ 50-100x for discrete defects

19. electrical properties in HfO2 MOSCAPs with discrete band edge defects (tphys > 4 nm) C-V and J-V measurments

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

21. J-V asymmetry - IMEC model continuity of eE - e(SiO2) ~ 3.9 < e(HfO2)~20 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

22. EOT~7 nm EOT~1.7 nm from M. Houssa, IOP, Chapter 3.4 Lucovsky group NCSU substrate electron injection n-Si into SiO2/HfO2 F-P hopping DE~0.5 eV < CB HfO2 >500x substrate hole injection n-Si into SiO2/HfO2 interface C-V -- surface potentials of Si substrate are negative!! hole injection

23. intrinsic band edge defect statesgrain-boundary defects – suboxide bonding - trivalent Hf3+~/>5x1018 cm-3 - ~/>3x1012 cm-2 clustered on grain boundaries trap-assisted transport of electrons through empty states empty occupied hole trapping into partially-occupied states band edge and paired band edge defects in HfO2

24. electrical properties in Hf Si oxynitride MOSCAPs electrical measurements and X-ray stress

25. Zr (also Ti, Hf) Si oxynitrides extends EOT to ~0.7-0.8 nm Cav (0.33,0.33,0.33) = 2.9 Cav (0.30,0.30,0.40) = 3.2 intermed. phase Si3N4 0.33-0.4 fraction strained [SiN4/3]~0.20 below strain percolation limit however -- relative concentration of Hf/Si ~ 16% suggests a percolation of the low Cav high Si3N4 content Zr(Hf) Si oxynitrides stable against chemical-separation tetrahedrally-bonded Zr to O encapsulated in Si-N cages low Si3N4 content Zr(Hf) Si oxynitrides chemically-separate

26. phase-separated Zr/Hf silicates SiO2 is black Zr/HfO2 is white/grey 20 % SiO2 SiO2 "islands" encapsulated by connected Zr/HfO2 grains limits Zr/HfO2 grain size 55-65% SiO2 SiO2 partially encapsulates Zr/HfO2 grains 75% SiO2 SiO2 encapsulates Zr/HfO2 grains

27. E T2 O K1 XAS spectra same before and after 900°C anneal because of overlap of O, N features in O K1 with Zr d-states must extract slitting by differentiation D E(p)-T2(s) = 2.2 eV cubic zirconia (Y2O3 alloy) D E(p) - T2g(s) = 4.8 eV ratio = 2.2±0.3 eV ~ = 8/4 = 2

28. analysis of spectral data tetrahedrally-bonded Zr (Ti,Hf) in high Si3N4 Si oxynitrides ls ~1 nm order of bonding Zr-O-Si-N "viewed" from O in O K1 and N in N K1 Cav ~ 3.0 due to chemical ordering/broken constraints on Si

29. spectroscopic detection of defects by soft X-ray XPS valence band spectroscopy 16% Si3N4 alloy shows high defect level alloy spectroscopically consistent with high level of defects observed in C-V trace 40% Si3N4 alloy shows sharper valence band edge and lower defects spectroscopically consistent with high level of defects observed in C-V trace

30. J-V measurements sharp minimum in gate leakage, direct tunneling consistent with low level of trap-assisted tunneling measurements at VU have demonstrated i) defect levels in 40% alloy films comparable to SiO2 ii) only positively charged defects by X-ray stressing iii)rate of defect level generation approximately same as SiO2

31. preview radiation induced defects (DK Chen, VU, later in program) high Si3N4 D(Vfb) vs stress time for high/low Si3N4 H SiON with Vg= 1.5 V injected charge 104 s ~7 x 1015 C/cm2 low Si3N4 these plots indicate improved performance of high Si3N4 HfSiON wrt i) low Si3N4 HfSiON, and ii) Hf silciates high Si3N4 total-dose-induced midgap voltage shiftsDVmg's for high Si3N4 HfSiON compared with larger DV's for Hf silicates Hf silicates

32. HfO2 and TiO2 on Ge – no detectable Ge-N interfacial transition regions approach resonantatom-specific near edge X-ray absorption spectroscopy (NEXAS) and vis-UV spectroscopic ellipsometry O K1 spectra MO anti-bonding states with Hf, Ti d, s and p contributions - 2 to 6 nm thick films N K1 spectra “buried” nitrided interfaces between Si (SiON) and Ge (Ge-N) and HfO2 and TiO2 as function of process temperature comparisons O K1 and vis-UV SE emphasis of differences/process induced changes in band edge defects Ge(100) and Ge(111)

33. buried interface studies - resonantatom-specific near edge X-ray absorption spectroscopy (NEXAS) HfO2 and TiO2 films are "transparent" to N K1 X-rays and to decay products after x-ray excitation

34. N K1 spectra - remote plasma assisted nitridation of Ge (100) (b) buried interfaces for 2 nm and 6 nm thick HfO2 films after 800C anneal. (a) Ge substrate nitridation for all Ge samples strong N-feature for all buried as-deposited interfaces no N-feature after 800C anneal - for both 2 & 6 nm

35. O K1 spectra for HfO2 on Ge(100) with Ge-N interface as-deposited at 300C - after an 800C 1 min anneal in Ar (a) 2 nm thick (b) 6 nm thick broad, but different spectra as-deposited -- no J-T Eg splitting similar sharp spectra after 800Canneal -- J-T Eg splitting

36. O K1 spectra for HfO2 on Si(100) with SiON interface as-deposited at 300C - after a 900C 1 minute anneal in Ar (a) 2 nm (b) 6 nm no J-T degeneracy removal some sharpening after anneal dimensional constraint > kinetics J-T degeneracy removalafter 800C annealkinetic constraint - as-dep

37. 2 nm thick TiO2 on Ge(100) as-deposited at 300C - after an 800C 1 minute anneal in Ar (a) N K1 spectra (b) O K1 spectra. nitrogen removal - 800Canneal same effect as for HfO2 filmsalso for N-loss for 6 nm TiO2 spectral changes - 800Canneal features sharper - edge hwhmhwhm on SiON ~0.7 eV

38. comparison of O K1 spectra for HfO2 in contact with non-crystalline SiON interfacial transition region on Si in direct bonding contact with Ge after elimination of Ge-N interfacial transition region significant Eg differences qualitatively different p-bonding Hf-O-Hf-O...... HfO2 HfO2 SiON Si substrate Ge substrate

39. changes in absorption (bottom) are qualitatively the same as changes in CLS (top) comparisons between e2 of HfO2 and TiO2and CLS responseas-deposited on GeN with interfacial transition layer and after 800°C indirect bonding contact on Ge(100) and Ge(111) surfaces

40. comparisons between e2 of HfO2 and TiO2as-deposited on GeN with interfacial transition layer and after 800°C indirect bonding contact with Ge onboth Ge(100) and Ge(111) surfaces qualitatively similar changes – as-deposited to annealed 800°C

41. preliminary results first try I-V for HfO2 and Hf SiON on Ge after anneal difference between Ge(111) and Ge(100) for HfO2 higher Ge(111) symmetry more column-like growth habit  more leakage checking by TEM 10-30x lower tunneling than for HfO2Hf SiON better dielectriceven though keff is smaller by ~1.5-2 Eb and m*e are larger Jdt = ~exp(-ak[Eb me*]0.5)

42. summary of experimental results length scale, lcoh - coherent Hf dp – O pp p-bond inter-PUC coupling phase-sep. silicates and type I lcoh ~ 2 nm or < 5 PUCs no J-T splittings and band-tail defects type II lcoh > 3 nm or > 6 PUCs J-T splittings and discrete band-edge defects J-T splitting 1.2 eV Hf Si oxynitrides- high Si3N4 % no inter-group Hf dp-O pp-bonding no phase-separation, but only for small range of compositions very low tunneling, ~12 (to 16-18) positive charge defects - X-ray stress HfO2 and Hf silicates both positive/negative charged defects asymmetric h/e trapping +/- defects X-ray stress

43. direct bonding on Ge(100/111) electricals being studied and measurements extended to Ge(111) GeO and GeN removed during annealresonant X-rays reveal buried interface substrate specific p-bonds Ge (100/111) SiON interface Ge-direct different T2gp

44. plans for next year comparisons of HfO2 and ZrO2 with different length scales on Si/SiON and Ge substrates Ge work with include deposition of in-situ homo-epi Ge on Ge to improve surface quality and hetero-epi of Ge on Si, and GaAs NEXAS, SXPS and SE spectroscopy J-V and C-V compare electrical stress SILC and N and PBTI with x-ray stress

45. i) d-state electronic structure of elemental TM oxides symmetry adopted linear combinations (SLACs) of atomic orbitals – FA Cotton, 1962 text ii) conduction band or anti-bonding states in NEXAS same relative energies as in vis-VUV SE iii) band edge defects in SXPS, NEXAS and SE matrix element enhancement of 50-100 - N-sum rule 0.5 eV widths compared with 24-40 eV for band to band iv) d-state degeneracies suppressed - nano-grains < 3 nm,band edge defects are changed from discrete states to band tails and defects reduced > 20-50x defect densities as low as mid-1011 cm-2band tails - asymmetric - like discrete states possible show stopper for CMOS -- different NBTI and PBTI's v) dielectrics that work Hf Si oxynitrides, high Si3N4 stable to 1100 C, compatible with poly Si (Si,Ge) gates defects and defect precursors ~ same as SiO2, very low tunneling current, k's to 16-18 are possible in Hf,Ti Si oxynitrides