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

outline of presentationresearch objectivesspectroscopic determination of band edge electronic structure conduction and valence band states intrinsic defectseffects of nano-crystalline grain size on band edge states band edge defectsan 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!!

research objectives correlate electronic structure, pre-existing and stress-induced defects with different length scales of order nano-grain size and intermediate range order, respectivelyfor nano- 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!!

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

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, butvalence band states are broader, but spectra are consistent with SALCscrystal-field (C-F) splitting symmetry and coordination dependent ~ 1.6 to 5 eVsymmetry adopted linear combinations (SALC) of atomic states define molecular orbitals for 6-fold coordinated Ti cooperative Jahn-Teller bonding distortions - TiO2 - anatase-like, monoclinic Hf(Zr)O2 - degeneracy removalJ-T term splittings Eg 2 states and T2g 3 statesd[Eg (2)]~d[T2g (3)] ~ 0.5-1.2 eV

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

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 ordering of valence band states -- s-p-s-p, etc...

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 NEXAS faster and more direct can also be applied to ultra-thin dielectrics for devices (~2 nm )

O K1 edges of 900C Ar annealed ZrO2 and HfO2 on Si with SiON interfaces - film thickness > 4 nm7-fold coordinated in monoclinic structuresd3p3SALC* MO labeling pseudo-cubic (8-fold)asymmetry in ZrO2, and features in HfO2 J-T degeneracy removalp Eg s (T2g + A1g + T1u)7-fold coordinated Hf- black in HfO25d3 + 6s1 + 6p3 = 7- s statessd3 - tetrahedron - p3 - pyramid -

band edge defects in HfO2 and ZrO2 - films > 4 nm thick vis-VUV SE SXPS2 discrete defect states above valence band edge2 discrete defect states below conduction band edgedefect features in X-ray, visible, UV, VUV enhanced by f-sum rule

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 ZrO2films 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 nmdiagram verified by cathodo-luminescence CLS results

cathodo-luminescence spectra CLS electrons in - keVs - photons out from defect transitionsBrillson group at Ohio State Universitydeconvolution of spectrum for bulk excitation in thick HfO2mapping of Gaussians onto edge states - SXPS, SE, XAS41, 52, 63, 7

correlations between grain-size and defects nano-scale of order, lcoh (p-bonding) ~3-3.5 nm for Hf(Zr)O2 suppression of Jahn-Teller d-state degeneracy splittings when grain size

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 Osp-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 effectsprimitive unit cells (PUCs)1/nO-Hf-O1/n (n=3, 4) labeled as 1, 2 and 3

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

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 nmsofter edge for tphys = 2 nm compared to tphys = 3, 4 nmband tail defects - tphys = 2 nm discrete defects - tphys = 4 nm

differences in band edge defects for kinetic constraint300, 500C suppression J-T and broader features wrt 700&900C 300C band tail defect at VB edge discrete defects after 700C annealto obtain electrical defect densities, must apply f(N) sum rule matrix element enhancement, ~ 50-100x for discrete defects

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

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 HfO2substrate injection holes

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

from M. Houssa, IOP, Chapter 3.4 Lucovsky group NCSUsubstrate electron injection n-Si into SiO2/HfO2 F-P hopping DE~0.5 eV < CB HfO2substrate hole injection n-Si into SiO2/HfO2 interface>500xC-V -- surface potentials of Si substrate are negative!! hole injection

intrinsic band edge defect states grain-boundary defects suboxide bonding - trivalent Hf3+ ~/>5x1018 cm-3 - ~/>3x1012 cm-2 clustered on grain boundarieshole trapping into partially-occupied statestrap-assisted transport of electrons through empty states

band edge and paired band edge defects in HfO2occupied empty

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

low Si3N4 content Zr(Hf) Si oxynitrides chemically-separatehigh Si3N4 content Zr(Hf) Si oxynitrides stable against chemical-separationtetrahedrally-bonded Zr to O encapsulated in Si-N cagesZr (also Ti, Hf) Si oxynitrides extends EOT to ~0.7-0.8 nm

phase-separated Zr/Hf silicates75% SiO2SiO2 encapsulates Zr/HfO2 grains55-65% SiO2SiO2 partially encapsulates Zr/HfO2 grains20 % SiO2SiO2 "islands" encapsulated by connected Zr/HfO2 grainslimits Zr/HfO2 grain size

O K1 XAS spectra same before and after 900C annealbecause of overlap of O, N features in O K1 with Zr d-states must extract slitting by differentiationD E(p)-T2(s) = 2.2 eVcubic zirconia (Y2O3 alloy)D E(p) - T2g(s) = 4.8 eVratio = 2.20.3 eV ~ = 8/4 = 2

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

spectroscopic detection of defects by soft X-ray XPS valence band spectroscopy16% 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

J-V measurements sharp minimum in gate leakage, direct tunneling consistent with low level of trap-assisted tunnelingmeasurements at VU have demonstratedi) defect levels in 40% alloy films comparable to SiO2ii) only positively charged defects by X-ray stressing iii) rate of defect level generation approximately same as SiO2

preview radiation induced defects (DK Chen, VU, later in program)D(Vfb) vs stress time for high/low Si3N4 H SiON with Vg = 1.5 V injected charge 104 s ~7 x 1015 C/cm2high Si3N4Hf silicatestotal-dose-induced midgap voltage shifts DVmg's for high Si3N4 HfSiON compared with larger DV's for Hf silicateslow Si3N4these plots indicate improved performance of high Si3N4 HfSiON wrt i) low Si3N4 HfSiON, and ii) Hf silciates

HfO2 and TiO2 on Ge no detectable Ge-N interfacial transition regionsapproachresonant atom-specific near edge X-ray absorption spectroscopy (NEXAS) and vis-UV spectroscopic ellipsometryO K1 spectra MO anti-bonding states with Hf, Ti d, s and p contributions - 2 to 6 nm thick filmsN K1 spectra buried nitrided interfaces between Si (SiON) and Ge (Ge-N) and HfO2 and TiO2 as function of process temperaturecomparisons O K1 and vis-UV SE emphasis of differences/process induced changes in band edge defects Ge(100) and Ge(111)

buried interface studies - resonant atom-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

N K1 spectra - remote plasma assisted nitridation of Ge (100)(a) Ge substrate nitridation for all Ge samples(b) buried interfaces for 2 nm and 6 nm thick HfO2 films after 800C anneal.strong N-feature for all buried as-deposited interfacesno N-feature after 800C anneal - for both 2 & 6 nm

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

O K1 spectra for HfO2 on Si(100) with SiON interface as-deposited at 300C - after a 900C 1 minute anneal in Ar (a) 2 nm (b) 6 nmno J-T degeneracy removal some sharpening after anneal dimensional constraint > kineticsJ-T degeneracy removal after 800C anneal kinetic constraint - as-dep

2 nm thick TiO2 on Ge(100) as-deposited at 300C - after an 800C 1 minute anneal in Ar (a) N K1 spectra (b) O K1 spectra.nitrogen removal - 800C anneal same effect as for HfO2 films also for N-loss for 6 nm TiO2spectral changes - 800C anneal features sharper - edge hwhm hwhm on SiON ~0.7 eV

Si substrateGe substrateSiONHfO2HfO2comparison of O K1 spectra for HfO2 in contact with non-crystalline SiON interfacial transition region on Siin direct bonding contact with Ge after elimination of Ge-N interfacial transition regionsignificant Eg differences qualitatively different p-bonding Hf-O-Hf-O......

comparisons between e2 of HfO2 and TiO2 and CLS response as-deposited on GeN with interfacial transition layer and after 800C indirect bonding contact on Ge(100) and Ge(111) surfaceschanges in absorption (bottom) are qualitatively the same as changes in CLS (top)

qualitatively similar changes as-deposited to annealed 800Ccomparisons between e2 of HfO2 and TiO2 as-deposited on GeN with interfacial transition layer and after 800C indirect bonding contact with Ge on both Ge(100) and Ge(111) surfaces

preliminary resultsfirst try I-V for HfO2 and Hf SiON on Ge after annealdifference 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 HfO2 Hf SiON better dielectric even though keff is smaller by ~1.5-2 Eb and m*e are largerJdt = ~exp(-ak[Eb me*]0.5)

summary of experimental results length scale, lcoh - coherent Hf dp O pp p-bond inter-PUC couplingasymmetric h/e trapping +/- defects X-ray stress

direct bonding on Ge(100/111) electricals being studied and measurements extended to Ge(111)substrate specific p-bonds Ge (100/111) SiON interface Ge-direct different T2g p

plans for next yearcomparisons of HfO2 and ZrO2 with different length scales on Si/SiON and Ge substratesGe work with include deposition of in-situ homo-epi Ge on Ge to improve surface quality andhetero-epi of Ge on Si, and GaAsNEXAS, SXPS and SE spectroscopy J-V and C-Vcompare electrical stress SILC and N and PBTI with x-ray stress

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 SEiii) 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-2 band tails - asymmetric - like discrete states possible show stopper for CMOS -- different NBTI and PBTI'sv) 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