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2008

2008. First-principle study of thin metallic films annealed on crystalline «high- к » oxide. Fabien Fontaine-Vive Pierre-Yves Prodhomme Philippe Blaise. LN3M project "Multi-scale software for material modelling". CEA LETI-MINATEC. Fabien Fontaine-Vive.

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2008

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  1. 2008 First-principle study of thin metallic films annealed on crystalline «high-к» oxide Fabien Fontaine-Vive Pierre-Yves Prodhomme Philippe Blaise Presentation name - Speaker name

  2. LN3M project"Multi-scale software for material modelling" CEA LETI-MINATEC Presentation name - Speaker name Fabien Fontaine-Vive

  3. Simulating electronic nanodevices A multi-scale approach from ab-initio to Monte-Carlo is needed MOS 45nm TEM image by A.M. Papon (Léti-MINATEC) Presentation name - Speaker name Fabien Fontaine-Vive

  4. MOS transistor evolution in microelectronics • Downscaling limitation: Leakage current due to tunneling effect => replacing SiO2 by high-k (high permittivity) dielectric HfO2 • BUT unpredictable threshold voltage of the metal gate beyond Schottky model => • First approach, oxide “pin” the metal Fermi level • - Surface states (Bardeen1) • - MIGS (Metal Induced Gap States) (Louie2, Hobbs3) • - surface defects, charge trap (Spicer4) • The distribution of the gap states is characteristic of the oxide, ONLY ! 1 Phys. Rev., (1947), 71, 717 2 Phys. Rev. B, (1977), 15, 2154 3 IEEE Trans. Elect. Devices, (2004), 51, 971 4 J. Vac. Sci. Tec., (1979), 16, 1422 5 Phys. Rev. Lett., 1984, 52, 461 6 Phys. Rev. B, 2006, 74 • Not sufficient, realistic simulations => depend on interfacial dipoles • Interface structure (Tung5) • - surface stoechiometry (Fonseca6) Presentation name - Speaker name Fabien Fontaine-Vive

  5. Experimental inputs W on HfO2 X-Ray Diff. by S. Allegret (Léti-MINATEC) Presentation name - Speaker name Fabien Fontaine-Vive

  6. Construction of the metal-oxide interfacial supercell, two approaches: • stacking arbitrary crystalline surfaces together, suitable for close crystallographic arrangement Ex: cubic metal staking on cubic oxide Cubic metal crystal = + Supercell relaxation Cubic HfO2 Fmax ~ 0.05-0.1 eV/A But ! How stacking different crystalline phases like a cubic phase stacking on a monoclinic highk oxyde m-HfO2 Presentation name - Speaker name Fabien Fontaine-Vive

  7. Epitaxial approach: depositing metal atoms on the oxide surface, thermal annealing T -> 0 K N atoms of metal N relaxation, N*1ps Vo= -Vz.z = Monoclinic HfO2, Surface orientation 001 z Relaxed interface structures (very small Fmax, < 0.01 eV/A), with the previous method, Fmax > 5 eV/A Presentation name - Speaker name Fabien Fontaine-Vive

  8. on 2 types of m-HfO2 substrate 2 types of metal Matching surfaces: Small misfit for W bcc 110 oriented surface (3*a,1*b) on m-HfO2 001 surface (3*a, 1*b) body centred cubic (bcc) W && hexagonal compact (hcp) Ti Monoclinic HfO2 c Monoclinic supercell Orthorhombic supercell vacuum vacuum b a 2 slabs of (3a,1b) m-HfO2 001 Supercell: α=90 deg, β=90, γ=90 Supercell parameters: α=90 deg, β=99.25, γ=90 Presentation name - Speaker name Fabien Fontaine-Vive

  9. Deposition of W atoms on m-HfO2 inorthorhombic supercell Why ab-initio ? * Creation / rupture of bonds (ionic, covalent, hydrogen ….) => suitable for inorganic materials (but also for organic, bio….) * Parameter-free * Very powerful method to predict properties of crystalline systems Preferable positions for metal atoms (W & Ti): Hafnium sites Relaxation engine: ab-initio (DFT) molecular dynamics combined with thermal annealing, 1-2 picosec for each deposition SIESTA code, LDA, Ceperley-Alder functional, Trouiller-Martins pseudopotentials, DZP orbitals Presentation name - Speaker name Fabien Fontaine-Vive

  10. Deposition of thin films of 4 metallic layers Ex: hcp 110 phase of Hafnium On m-HfO2 001, in monoclinic supercell, films of W and Ti are hexagonal compact-like, 110 oriented W Ti On m-HfO2 001, in orthorhombic supercell, films of W and Ti are hexagonal-diamond-like, 110 oriented Presentation name - Speaker name Fabien Fontaine-Vive

  11. Relaxing cell parameters of (W/HfO2) W hcp-like 110 on m-HfO2 001 W bcc 111 on ~ortho-mono HfO2 Phase transition W hex-dia 110 on m-HfO2 001 W hex-dia 110 on m-HfO2 001 No phase transition Presentation name - Speaker name Fabien Fontaine-Vive

  12. Relaxing cell parameters of (Ti/HfO2) No phase transition Stability of the (metal/HfO2) interfaces = stability of the metal Presentation name - Speaker name Fabien Fontaine-Vive

  13. Interfacial structures and energies of W/HfO2 4 types of construction, 4 types of relaxed interfaces (T=0K) 1 monolayer W_hd + 3 ML W_bcc 110 / m-HfO2 001 4ML W_hd 110 / m-HfO2 001 4ML W_hcp 110 / m-HfO2 001 4ML W_bcc 111 / ~o-mHfO2 001 Stack energy (eV) bcc/o+14.7 bcc/o+9.9 bcc/o bcc/o+6.8 Formation energies of metal bulk structures (eV/atom) bcc+0.77 bcc+0.47 bcc = -12.55 Surface energy of metal films (eV/atom) hcp 110=0.21 hd 110=0.03 bcc 111=0.16 Tri~tetra (2Hf-O-1W) Tri (2Hf-O-1W) Octa (4W-O-2Hf) Tri (2Hf-O-1W) Tetra (2Hf-O-2W) Tri (2Hf-O-1W) ~Tetra (2Hf-O-2W) ~Tri (2Hf-O-1W) Interfacial layer structure Interfacial energies (eV) hd + 2.61 hd = -14.96 hd + 2.10 hd + 3.75 Presentation name - Speaker name Fabien Fontaine-Vive

  14. Interfacial structures and energies of Ti/HfO2 4ML Ti_hd 110 / m-HfO2 001 4ML Ti_hcp 110 / m-HfO2 001 Stack energy (eV) hcp/m hcp/m+1.1 Formation energies of metal bulk structures (eV/atom) hcp = -7.48 hcp+0.03 Surface energy of metal films (eV/atom) hcp=0.17 hd=hcp Octa (4Ti-O-2Hf) Tri (2Hf-O-1Ti) Tetra (2Hf-O-2Ti) Tri (2Hf-O-1Ti) Interfacial layer structure Interfacial energies (eV) hcp = -21.17 hcp+3.66 Presentation name - Speaker name Fabien Fontaine-Vive

  15. Crystallographic compatibilities ? Monoclinic HfO2 c b a W hcp 110 / m-HfO2 001 W bcc 111 / o-mHfO2 W hd 110 / m-HfO2 001 Interfacial structure W / HfO2 (and Ti / mHfO2) Interfacial structure mHfO2 (001) / mHfO2 (001) Tetragonal (2Hf-O-2Hf) +Trigonal (2Hf-O-1Hf) ~Tetra (2Hf-O-2W) ~Tri (2Hf-O-1W) Tetra (2Hf-O-2W) Tri (2Hf-O-1W) Octa (4W-O-2Hf) Tri (2Hf-O-1W) Presentation name - Speaker name Fabien Fontaine-Vive Fabien Fontaine-Vive

  16. Crystallographic compatibilities ? Monoclinic HfO2 XRD Thermal annealing c b a Without thermal annealing Interfacial structure WNx / mHfO2 Interfacial structure mHfO2 (001) / mHfO2 (001) W2N / mHfO2 WN hex 110 / mHfO2 001 Presentation name - Speaker name Fabien Fontaine-Vive Fabien Fontaine-Vive

  17. Valence Band Offset and metal work function W/HfO2 1 monolayer W_hd +3 ML W_bcc 110 / m-HfO2 001 4ML W_hcp 110 / m-HfO2 001 4ML W_hd 110 / m-HfO2 001 4ML W_bcc 111 / o-HfO2 001 Band alignement method of Van de Walle & Martin + Many-body (GW) corrections (ABINIT code) VBO exp = 3.4 eV, (P+) VBO=4.0 eV (N+) VBO=3.5 eV (P+) Wf bcc 111 exp = 4.5 eV Wf bcc 110 exp = 5.2 Wf bcc on HfO2 exp= 5.0 (P+) Vacuum work function of metallic films (eV) Wf bcc 110 calc = 5.1 Wf hcp 110= 4.1 For Titanium, Wf = 3.4 Wf hd 110=4.4 For Ti Wf = 4.1 Polymorphism (or/and poly-orientation! ) of the metallic films could explain the wide range of work functions Presentation name - Speaker name Fabien Fontaine-Vive

  18. Electronic properties of W/HfO2 and Ti/HfO2 interfaces surface of isodensity at the Fermi level (HOMO density) W Ti HfO2 Charge transfer at the metal/oxide interface due to evanescent metal wavefunctions in the oxide => creation of interfacial dipole Presentation name - Speaker name Fabien Fontaine-Vive

  19. Interface Ti / W / m-HfO2: stabilizing meta-stable phases ? 2 ML Ti_hcp 1 ML Ti_hcp + 1ML Ti_bcc Phase transition 4 ML W_hcp 4 ML W_bcc 1 ML o-HfO2 2 ML m-HfO2 1 ML m-HfO2 Natural phases, not natural phases 1 ML Ti_hcp + 2ML Ti_bcc 3 ML Ti_hcp Phase transition 3 ML W_bcc + 1ML W_hcp 4 ML W_hcp 2 ML m-HfO2 2 ML m-HfO2 Presentation name - Speaker name Fabien Fontaine-Vive

  20. These meta-stable phases exist in reality ? For W, transition phase bcc -> double hcp at P > 6.5 Mbar Ruoff et al., PRB, 1998 Titanium metal at high pressure: Synchrotron experiments and ab initio calculations Rajeev Ahuja et al., PRB 2004 hcp -> (Temperature) ω -> bcc transition phase under pressure Growth of face-centred-cubic titanium on aluminium (fcc) S K Kim et al., J. Phys. Cond. Matt., 1996 Presentation name - Speaker name Fabien Fontaine-Vive

  21. Substrate HfOx (O-rich), deposition of Ti atom Formation of TiO2 octahedral structure Presentation name - Speaker name Fabien Fontaine-Vive

  22. Conclusion on metal-oxide interfaces • Metal-oxide interfaces = key-lock structure => ab-initio can design • Ab-initio thermal annealing favors the apparition of meta-stable interfaces and relaxing cell dimensions favors the way back to the metal natural phase (depend on the thickness metal/oxide) • Real interface with high-k oxide monoclinic HfO2 = • after the first atomic layer deposition => metal sites = Hafnium sites (=> ms-film in hexagonal structure, metal orientation determined by the oxide orientation) • structure of upper films determined by the deposition techniques • stability of interfacial films depend on the degree of incommensurability between metal and oxide. In W/HfO2, a possible coexistence of W bcc 111 (+1-2 monolayer of o-HfO2) and W bcc 110 (+m-HfO2) • MO electronic properties and interactions determined on 2 monolayers Presentation name - Speaker name Fabien Fontaine-Vive

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