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First Principles Studies of Defects in HfO 2 and at Si:HfO 2 Heterojunctions

Ph.D. Dissertation Proposal. First Principles Studies of Defects in HfO 2 and at Si:HfO 2 Heterojunctions. Chunguang Tang ( 唐 春光 ) (Bachelor Eng.: Univ. Sci. Tech. Beijing) (Master. Sci.: NUS) Chemical, Materials & Biomolecular Engineering Institute of Materials Science

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First Principles Studies of Defects in HfO 2 and at Si:HfO 2 Heterojunctions

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  1. Ph.D. Dissertation Proposal First Principles Studies of Defects in HfO2 and at Si:HfO2 Heterojunctions Chunguang Tang(唐春光) (Bachelor Eng.: Univ. Sci. Tech. Beijing) (Master. Sci.: NUS) Chemical, Materials & Biomolecular Engineering Institute of Materials Science University of Connecticut Principal Advisor: Prof. R. Ramprasad Associate Advisor: Prof. L. Shaw Associate Advisor: Prof. P. S. Alpay

  2. First transistor radio: 4 transistors. Quad core processor contains 820 million transistors # of transistors Introduction: device miniaturization

  3. K ~ 4 K ~ 30 high k (dielectric constant) transistor

  4. High k issues • Formation of interface phases (SiOx, silicate, Hf-Si) • Effects of oxygen point defects* • Point defects migration may contribute to interfacial phase formation • High oxygen pressure favors silica & low pressure favors Hf silicide Wong et al, Microelectronic Eng. (2006) Locquet et al, JAP (2006); Stemmer et al • * D. Y. Cho et al, APL, 86, 041913 (2005); X. Y. Qiu et al, APL, 88, 072906 (2006); S. Stemmer, JBSTB, 22, 791 (2004)

  5. High k issues • High leakage currents and low dielectric constant due to crystallization • As-deposited: amorphous (preferred) • Crystallizes at 400~500 °C • Doped HfO2 with alloying elements. • Si, Y, La, F, N • Increase crystallization temperatures • Stabilize higher k phases amorphous cubic tetragonal monoclinic k ~ 30 k ~ 29 k ~ 70 k ~ 16

  6. Proposed research plan • Undoped HfO2 • The formation and migration of O vacancies, O interstitials and Hf vacancies. • Their contribution to the interfacial phases. • Doped HfO2 • Dopants: Si, Y, La, F, N. • Effects of dopants on relative stabilities of various phases of HfO2. • Effects of dopants on O defect chemistry.

  7. 0 1 2 3 4 Computational Methods • Density Functional Theory (DFT) • Many nuclei-many electron problem  one electron problem • Supercell approach • Phase and structure information, defect energies • Computational times • Defect formation energies  16 days in one AMD 2.0 GHz processor (Supercell of ~230 atoms). • Migration energy calculation  45 days. E Emigr. Reaction path

  8. Completed Research • Bulk HfO2 results Table I: relative energies (eV) and lattice constants (Å) of bulk HfO2 * J. Wang, H. P. Li, and R. Stivens, J. Mater. Sci. 27, 5397 (1992) ** D. M. Adams, S. Leonard, D. R. Russel, and R. J. Cemik, J. Phys. Chem. Solids52, 1181 (1991) *** J. Adam and M. D. Rodgers, Acta Crystallogr. 12, 951 (1959)

  9. Eform Defects in bulk HfO2 + 1 O atom Eform = Evac+ (EO2)/2 - Eperf Table II: Formation energies (eV) of point defects in bulk HfO2

  10. O interstitial Formation and Migration* Si HfO2 Interfacial segregation: Thermodynamic driving force (decreasing Eform as interface is approached) Hf O Kinetic driving force, and O penetration into Si (decreasing Emigr as interface is approached) Experimental Emigr. of O interstitial in bulk Si: 2.44 eV** (2.26 eV, calculated) O interstitials could lead to the formation of SiOx * C. Tang & R. Ramprasad, Phys. Rev. B75, 241302 (2007); ** J. C. Mikkelsen, Appl. Phys. Lett. 40, 336 (1981).

  11. O Vacancy Formation and Migration* Si HfO2 Hf O Interfacial segregation: Aided by thermodynamic & kinetic driving forces O vacancies could lead to the formation of Hf silicide * C. Tang, B. Tuttle & R. Ramprasad, Phys. Rev. B76, 073306 (2007)

  12. Hf Vacancy Formation and Migration* Hf • Hf vacancies prefer the interface • Si strongly prefers to penetrate into HfO2 O Hf vacancies could lead to the formation of Hf silicate Si penetration * C. Tang & R. Ramprasad, Appl. Phys. Lett., 92, 152911 (2008)

  13. Abrupt Interface “SiOx” Interface “Hf-Si” Interface Accumulation of O Point Defects* Thermodynamics favors accumulation of point defects at interface, and consequently, the creation of Hf silicide or SiOx * C. Tang & R. Ramprasad, Appl. Phys. Lett., 92, 182908 (2008)

  14. Si doped HfO2 (SDH) C-SDH (1-x) HfO2 + xSiO2 + Ef = Hf1-xSixO2 t-SDH m-SDH • If Si > 12%  t-HfO2 most stable • The local chemistry of Si prefers SiO2 configuration m-SDH t-SDH c-SDH

  15. Y doped HfO2 (YDH) (1-x) HfO2+(x/2)Y2O3+Ef=Hf1-xYxO2-x/2 m-YDH Charge neutrality  2 Y atoms & 1 O vacancy c-YDH t-YDH • If Y > 12%, t-HfO2 and c-HfO2 more stable. • Similar stabilization phenomenon in c-YSZ for fuel cell application. • Instead of Y, positively charged O vacancies are identified as the major stabilizing factor.

  16. Remaining research • Undoped HfO2 • Amorphous HfO2 and Si heterojunction; • Lower leakage current • High dielectric constant • Various charged states of O defects (VO0, VO+1, VO+2, iO0, iO-1, iO-2); • Formation energies

  17. Remaining research • Doped HfO2 • Effects of dopants on HfO2 stabilities (La, F, N); • Formation and migration energies of O defects close to and far away the dopants. • How they influence the behaviors of defects in HfO2 and Si heterojunctions

  18. Publication list • C. Tangand R. Ramprasad, "Oxygen defect accumulation at Si:HfO2 interfaces" , Appl. Phys. Lett., 92, 182908 (2008). • C. Tangand R. Ramprasad, "A study of Hf vacancies at Si:HfO2 heterojunctions" , Appl. Phys. Lett., 92, 152911 (2008). • C. Tangand R. Ramprasad, "Oxygen pressure dependence of HfO2 stoichiometry: An ab initio investigation" , Appl. Phys. Lett., 91, 022904 (2007). • C. Tang, B. R. Tuttle and R. Ramprasad, "Diffusion of O vacancies near Si:HfO2 interfaces: An ab initio investigation", Phys. Rev. B, 76, 073306 (2007). • C. Tangand R. Ramprasad, "Ab initio study of O interstitial diffusion near Si:HfO2 interfaces", Phys. Rev. B, 75, 241302(R) (2007). • B. R. Tuttle, C. Tangand R. Ramprasad, "First-principles study of the valence band offset between silicon and hafnia", Phys. Rev. B, 75, 235324 (2007). • R. Ramprasad and C. Tang, "Circuit elements at optical frequencies from first principles: a synthesis of electronic structure and circuit theories", J. Appl. Phys. 100, 034305 (2006). • Tang CG, Li Y, Zeng KY, Mater. Lett., 59, 3325, (2005). • Tang CG, Li Y, Zeng KY, Mater. Sci. Eng. A, 384, 215, (2004). • Li Y, Cui LJ, Cao GH, Ma QZ, Tang CG, Wang Y, Wei L, Zhang YZ, Zhao ZX, Baggio-Saitovitch E, Physica C, 314, 55, (1999). • Li Y, Wang YB, Tang CG, Ma QZ, Cao GH, SCI CHINA SER A, 40, 978, (1997). • Li Y, Tang CG, Ma QZ, Wang YB, Cao GH, Wei T, Wang WH, Zhang TB, Physica C, 282, 2093, (1997).

  19. Acknowledgment Committee members: Profs. Rampi Ramprasad, Leon L. Shaw and Pamir S. Alpay Profs. Puxian Gao and George A. Rossetti Group students: Ning, Luke, Tom, Ghanshyam and Hong Computational resources: IMS computation clusters; SGI supercomputer in SoE Funding: NSF & ACS-PRF

  20. Backup slides

  21. (P, T) dependence of O defects • DFT computations of O vacancy & interstitial formation energies as a function of defect concentration … combined with … thermodynamic model yields (P,T) dependence of stoichiometry Jiang et al Appl. Phys. Lett.87, 141917 (2005) Pick up T, find P to make formation energy 0, corresponding to equilibrium condition. C. Tang & R. Ramprasad Appl. Phys. Lett.91, 022904 (2007)

  22. T = 1200 K (P, T) dependence of interface morphology • Pressure changes could stabilize silicide or SiOx • Increase in T makes abrupt Si:HfO2 interface less stable T = 400 K “Hf-Si” Interface Abrupt Interface “SiOx” Interface

  23. Si doped HfO2 (SDH) (1-x) HfO2+xSiO2+Ef=Hf1-xSixO2 a1 a2 The local chemistry of Si prefers SiO2 configuration t-SDH m-SDH c-SDH

  24. Y doped HfO2

  25. Density Functional Theory Initial guess of wave function & electron density Set up Hamiltonian Energy, forces on atoms New electron density DE < Ebreak no yes end

  26. Accuracy of DFT • Structures (bond lengths, bond angles, lattice constants) predicted to within 1 % of experiments • Elastic properties (bulk & shear modulus, etc.) accurate to within 5% of experiments • Bond energies, cohesive energies within 10% of experiments • Relative energies (energy difference between FCC & BCC, for example) are accurate to within 2% • Band gaps are off by about 50% !!!

  27. Si:HfO2 heterostructure models Tetragonal HfO2-based Monoclinic HfO2 -based

  28. Why HfO2 Source: R. M. Wallace and G. D. Wilk, Crit. Rev. Solid State Mater. Sci. 28, 231

  29. Influence of defects on performance • Charges are trapped in defects, shifting threshold voltage and making operation unstable. • Trapped charges scatter carriers in the channel  lower carrier mobility • Cause unreliability (oxide breakdown)

  30. Effect of F • (APL 90, 112911) • Remove midgap states from Hf dangling bonds at HfO2/SiO2 interface; • Excessive F increase leakage current. • (APL 89, 142914) • Defect passivation

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