First Principles Studies on High-k Oxides and Their Interfaces with Silicon and Metal Gate

1. First Principles Studies on High-k Oxides and Their Interfaces with Silicon and Metal Gate Feng Yuan Ping (冯元平) Department of Physics National University of Singapore phyfyp@nus.edu.sg

2. CCP2006

3. www.mrs.org.sg CCP2006

4. G S D Outline • Introduction • Oxygen vacancy in HfO2 and La2Hf2O7 • Tuning of metal work function at metal gate and high-k oxide interface • Properties of high-k oxide and Si interface • Conclusion CCP2006

5. G S D CMOS Scaling ITRS roadmap shows the expected reduction in device dimensions CCP2006

6. SiO2 HK Oxide Why High-k oxides ? Gate CB Si • 1.2 nm (5 atomic layers) physical SiO2 in production of 90 nm logic technology node; 0.8 nm physical SiO2 in research of transistors with 15 nm physical Lg • Gate leakage is increasing with reducing physical SiO2 thickness. SiO2 layers <1.6 nm have high leakage current due to direct tunneling. Not insulating • SiO2 running out of atoms for further scaling. Will eventually need high-K Rober Chau, Intel CCP2006

7. Choice of High K Oxide CCP2006

8. Growth of ZrO2 on Si Interface Wang et al. APL 78, 1604 (2001) Wang & Ong, APL 80, 2541 (2002) CCP2006

9. Problems with High K oxides Among other problems, oxide has too many charge traps, and the threshold voltage (Vth) shifts from CMOS standards. CCP2006

10. Dynamic Charge Trapping Power law shift! Oxygen vacancy? Negative-U traps? Time evolution of threshold voltage Vthunder static and dynamic stresses of different frequencies, for (a) n-MOSFET, and (b) p-MOSFET. The Vthevolution has a power law dependence on stress time. C. Shen, H. Y.Yu, X. P. Wang, M. F. Li, Y.-C. Yeo, D. S. H. Chan, K. L. Bera, and D. L. Kwong, International Reliability Physics Symposium Proceedings 2004, 601. CCP2006

11. Hydrogen in HfO2 Formation energies for (a) interstitial H and H2 molecules, and (b) the VO-H complex. J. Kang et al., APL, 84, 3894 (2004). CCP2006

12. Bulk HfO2 P21/c Monoclinic Fm3m Cubic P42/nmc Tetragonal J. Kang, E.-C. Lee and K. J. Chang, PRB, 68, 054106 (2003) CCP2006

13. W L  X W K Cubic HfO2 Vasp Cutoff energy = 495 eV GGA Eg = 3.68 eV (direct) (Exp gap ~ 5.8 eV) Valence band = O 2p Conduction band = Hf d Peacock and Robertson, JAP (2002) CCP2006

14. Computational Details • DFT, planewave, pseudopotential method (vasp) • 2s and 2p electrons of O, 5d and 6s electrons of Hf are treated as valence electrons. • Cut off energy: 495 eV • 80 atom supercell (3x3x3 primitive cells) • Uniform background charge for charged vacancy CCP2006

15. Supercell

16. Total Energy CCP2006

17. Excothermic (0.32 eV) Excothermic (0.94 eV) Excothermic (0.38 eV) Energetics Negative-U Property! CCP2006

18. p+Poly-Si gate electron HK Si sub. electron hole n+Poly-Si gate HK Si sub. Vg > 0 Vg < 0 (a) (b) Charge Trapping Mechanism Negative bias for p-MOSFET Holes are injected to HK V0 V+ (meta-stable)  V++ Positive bias for n-MOSFET Electrons are injected to HK V0 V- (meta-stable)  V-- In both cases, when the gate bias is removed, no charges are injected to HK, all charges in the O traps will be de-trapped, the gate dielectric remains neutral CCP2006

19. Frequency Dependence of Vth Experimental and simulation results for n-MOSFET CCP2006

20. Formation Energy A. S. Foster, et al. PRB 65, 174117 (2002) Formation energy for neutral vacancy: 9.36 eV (O3) & 9.34 eV (O4) Present calculation: 9.33 eV (relative to O atom) CCP2006

21. Band Structures V0 CCP2006

22. BC plane Band Structures AC plane V-2 CCP2006

23. Relaxation of NN Hf atoms 2 1 V V  (b) (a) C2v Mode Breathing Mode CCP2006

24. Relaxation of NN Hf Atoms CCP2006

25. Effect of Lanthanum X. P. Wang et al. VLSI2006 Charge trapping induced Vth shift under constant voltage stress for HfO2, HfLaO with 15% and 50% La gate dielectric NMOSFETs. CCP2006

26. Td V4 C2V V3 Effect of La The formation energies of oxygen vacancies at varies sites in monoclinic HfO2 and pyrochlore HfLaO, calculated by ab initio total energy calculations. CCP2006

27. Summary • Oxygen vacancy in HfO2 has negative-U property. It is energetically favors trapping two electrons or two holes. • Oxygen vacancy is a main source of charge trapping in HfO2 and the origin for frequency dependence of dynamic charge trapping in HfO2 MOS transistors. • Large lattice relaxation for charged vacancies, due to strong electron-lattice interaction. • Oxygen vacancy has higher formation energy at Td site in La2Hf2O7. CCP2006

28. G S D Gate Material • Currently polycrystalline silicon (poly-Si) gate electrode is used. • Problems: • high gate resistance • boron penetration • Fermi level pinning • poor compatibility with high- gate dielectrics • increase of EOT due to gate depletion • Need metal gate! • Eliminates the gate depletion problem • Eliminates boron penetration problem • Reduces the gate sheet resistance • Generally more compatible with alternative gate dielectric or high-permittivity (high-k) gate dielectric materials than poly-Si. • The urgent need for alternative gate dielectrics to suppress excessive transistor gate leakage and power consumption could speed up the introduction of metal gates in complementary metal oxide semiconductor (CMOS) transistors. CCP2006

29. Issues • The integration of metal gate with high- gate dielectric requires the metal effective work functions to be within ±0.1 eV of the Si valence- and conduction-band edges for positive- (PMOS) and negative-channel metal-oxide-semiconductor (NMOS) devices, respectively. • However, to find two metals with suitable work functions and to integrate them with current semiconductor technology remains a challenge. CCP2006

30. Work Function of Metals Work function of several elemental metals in vacuum, on a scale ranging from the positions of the conduction band to the valence band of silicon. Metal work functions are generally dependent on the crystal orientation and on the underlying gate dielectric. CCP2006

31. Can we tune the metal workfunction? CCP2006

32. Transition Metal Monolayer/half-monolayer Tuning of Workfunction? Ni-m-ZrO2 Ni ZrO2 m = Au, Pt, Ni, Ru, Mo, Al, V, Zr and W (for half monolayer) m = Ni, V, and Al (for one monolayer) CCP2006

33. Bulk ZrO2 Very small lattice mismatch (<2%) CCP2006

34. Models Supercells for the Ni-m-ZrO2 interfaces, The interface is formed using c-ZrO2(001) and fcc Ni(001) surfaces. • with one monolayer metal m (m=Ni, V, and Al). • with half monolayer metal m (m=Au, Pt, Ni, Ru, Mo, Al, V, Zr and W) CCP2006

35. Computational Details • DFT, planewave, pseudopotential method (vasp) • Ultrasoft pseudopotential & GGA • Cut off energy: 350 eV • K points: 8x8x1 • In plane lattice constants constrained to that of c-ZrO2 • Electronic energy was minimized using a fairly robust mixture of the blocked Davidson and RMM-DIIS algorithm. Conjugate gradient method for ionic relaxation CCP2006

36. Density of States Spin resolved and atomic site-projected density of states (PDOS) for (a) Ni-Pt-ZrO2 interface and (b) Ni-Al-ZrO2 interface, with half monolayer of metal insertion. The PDOS for the Ni in the bulk region (Ni-bulk), interface metal m (Pt or Al), interface oxygen (O-Int.), and oxygen in the bulk region (O-bulk) are shown. CCP2006

37. Ni m Oxide Si Schottky Barrier Heights CCP2006

38. p-type Schottky Barrier Height • p-type SBH is obtained using the “bulk plus lineup” procedure, using the average electrostatic potential at the core (Vcore) of ions in the “bulk” region as reference energy DEb the difference between the Fermi energy of Ni and the energy of the valence band maximum (VBM) of the oxide, each measured relative to Vcore of the corresponding “bulk” ions, DV is the lineup of Vcore through the interface. • DEb is adjusted by quasiparticle and spin-orbital corrections (0.29 eV for Ni, +1.23 eV to the valence-band maximum of ZrO2,  overall correction of 0.94 eV). CCP2006

39. Vcore Average electrostatic potential at the cores (Vcore) of Ni (filled dark circle) and Zr (open circle) as a function of the distance from the interface for Ni-m-ZrO2 interfaces (m= Au, Ru, Ti) with half monolayer metal insertion. Breaks were introduced in the vertical axis (Vcore) between - 41 eV and -36 eV. CCP2006

40. n-type Schottky Barrier Height • where Eg is the energy gap of the dielectric • The experimental band gap of 5.80 eV was used. • The SBH can also be estimated directly from the difference between the Fermi energy and the energy corresponding to the top of the valence band given in the PDOS of oxygen in the bulk region. Results obtained using the two methods are in good agreement (within 0.1~ 0.2 eV). CCP2006

41. Results CCP2006

42. SBH Tunability Range of tuning: 2.8 eV! CCP2006

43. n-type Schottky Barrier Height n-SBHs of Ni-m-ZrO2 interfaces are shown as a function of electronegativity (Mulliken scale) of m. The straight line is a least-squares fit to data points shown in filled squares (Al and W were not included). CCP2006

44. Workfunction of Ni(001) with m Work functions of Ni(001) with half monolayer of metal m coverage are shown as a function of electronegativity (Mulliken scale) of m. The straight line is a least-squares fit to data points shown in filled squares. CCP2006

45. Bulk Ni Bulk ZrO2 Ni m O Mechanism? • Contribution from the tails of the metallic wave functions which tunnel into the oxide band gaps or metal induced gap sates can be ruled out, due to short delay length (~0.9Å) which is nearly independent of the interlayer metal. • Interface dipole can contribute significantly to band alignment between the metal and oxide. • Ionic m-O bonds • Charged metal layer and its image CCP2006

46. Gap States Penetration of electronic density of the gap states into the ZrO2 of Ni-m-ZrO2 interfaces. Position of the surface oxygen is set to z = 0 Å. CCP2006

47. Method Structure p(eV) n (eV) DFT-GGA XPS IPEa O-t Zr-t O-v O-rich O-deficient 2.13 3.80 2.92 2.60 3.36 2.2 3.67 2.00 2.88 3.20 2.44 3.2 Interface bonding dependent SBH: experimental evidence (in-situ XPS) Afanas'ev et al. JAP 91, 3079 (2002). CCP2006

48. Interface bonding dependent SBH: experimental evidence (in-situ XPS) CCP2006

49. Summary • A scheme for tuning the Schottky barrier height or workfunction of metal gate – high-k dielectric interface was proposed and has been experimentally confirmed. • By including a monolayer or half monolayer of transition metal between the metal gate and high-k dielectric, a tunability as wide as 2.8 eV can be achieved. • There exists a linear correlationship between the Schottky barrier heights / workfunction and the electronegativity • Preliminary experimental results with m=Al agree with prediction. CCP2006

50. Acknowledgement Y F Dong Physics Department, NUS Y Y Sun Physics Department, NUS S J Wang Institute of Materials Research & Engineering A Huan Institute of Materials Research & Engineering M F Li Dept of Electrical & Computer Engineering, NUS Institute of Microelectronics CCP2006