Outline

1. Effect of Oxygen Vacancies and Interfacial Oxygen Concentration on Local Structure and Band Offsets in a Model Metal-HfO2-SiO2-Si Gate Stack Eric Cockayne Ceramics Division, NIST, Gaithersburg Blanka Magyari-Kope Yoshio Nishi Electrical Engineering Dept., Stanford U.

2. Outline • Create atomistic models for layers and interfaces in a gate stack • Calculate band structures for these models • Study effect of modifying interfaces on band offsets • Study effect of defects on band offsets

3. Atomistic models for gate stacks Realistic models: • disorder • dangling bonds • amorphous SiO2 • suboxide SiOx layer (Giustino, • Bongiorno & Pasquarello, J. Phys. • Cond. Matter 17, S2065 (2005)). • possibly amorphous HfO2 • sufficient thickness of each layer Estimate: need thousands of atoms Capability: hundreds of atoms Compromise: keep layers relatively thick; use idealized crystalline components stacked “epitaxially” (Gavartin and Shluger, Microelectr. Engr. 84, 2412 (2007)).

4. Strategy: find crystalline structures with similar cross sections find atomistic models for interfaces from literature if possible “splice” together the models to create complete stack Re-relax at fixed volume, using density functional theory (DFT). metal: Pt 110 surface 0.554 nm x 0.480 nm semiconductor: Si 001 surface 0.543 nm x 0.543 nm interfacial SiO2: cristobalite 001 surface 0.497 nm x 0.497 nm high-k dielectric: HfO2 monoclinic 100 surface 0.529 nm x 0.517 nm metal: Pt 110 surface Overall cross section: 0.545 nm x 0.500 nm

5. Interface structures Si-SiO2: check phase interface O (Tu & Tersoff PRL 84, 4393 (2000)) SiO2-HfO2: 322 model (Sharia, Demkov, Bersuker & Lee PRB 75, 035306 (2007)).

6. HfO2-Pt (Gavrikov et al., J. Appl. Phys. 101, 014310 (1007).) Pt-Si .

7. Pt-Si-SiO2-HfO2-Pt gate stack model • Comments • VASP used • DFT, ultrasoft pseudopotential methods, PAW formalism • 287 eV plane wave cutoff; 2x2x1 k-point grid • Designed with inversion symmetry • Repeats “back to back” • Justification: avoid metal-vacuum surface in model • Strain favors in-plane bc orientation of HfO2 (100, not 001) • First full layer of O within HfO2 4-fold coordinated. • HfO2-Pt interfacial O layer 4-fold coordinated type

8. Calculated band structure for stack with O occupancy 0.75 Pt Si SiO2 HfO2 Pt HfO2 SiO2 Si Pt

9. Comparative band structure of fully reduced and fully oxided HfO2-Pt interface

11. Approximately 0.009 e nm dipole moment per interfacial O

15. Conclusions • (Pt)-Si-SiO2-HfO2-Pt stacks can be modeled using crystalline phases, sharp interfaces, and minimal strain with a 0.55 nm x 0.50 nm cross section. • Oxidation of HfO2-Pt interface raises energies of HfO2 conduction and valence bands equally; valence band offsets change 2.3 eV from metallic to fully oxidized interface • Oxygen vacancies: at level of LDFT; gap state lies below the Fermi level (neutral vacancy) • Although vacancy formally neutral, significant band bending occurs.