Effect of Oxygen Vacancies and Interfacial Oxygen Concentration on Local Structure and Band Offsets in a Model Metal-HfO 2 - SiO 2 -Si Gate Stack Eric Cockayne Ceramics Division, NIST, Gaithersburg Blanka Magyari-Kope Yoshio Nishi Electrical Engineering Dept., Stanford U.

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

Atomistic models for gate stacks (Gavartin and Shluger, Microelectr. Engr. 84, 2412 (2007)). Realistic models: disorder dangling bonds amorphous SiO 2 suboxide SiO x layer (Giustino, Bongiorno & Pasquarello, J. Phys. Cond. Matter 17, S2065 (2005)). possibly amorphous HfO 2 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”

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 the ory (DFT). metal: Pt 110 surface nm x nm semiconductor: Si 001 surface nm x nm interfacial SiO 2 : cristobalite 001 surface nm x nm high-k dielectric: HfO 2 monoclinic 100 surface nm x nm metal: Pt 110 surface Overall cross section: nm x nm

Si-SiO 2 : check phase interface O (Tu & Tersoff PRL 84, 4393 (2000)) SiO 2 -HfO 2 : 322 model ( Sharia, Demkov, Bersuker & Lee PRB 75, (2007)). Interface structures

HfO 2 -Pt (Gavrikov et al., J. Appl. Phys. 101, (1007).) Pt-Si.

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 HfO 2 (100, not 001) First full layer of O within HfO 2 4-fold coordinated. HfO 2 -Pt interfacial O layer 4-fold coordinated type Pt-Si-SiO 2 -HfO 2 -Pt gate stack model

Calculated band structure for stack with O occupancy 0.75 Pt Si SiO 2 HfO 2 Pt HfO 2 SiO 2 Si Pt

Comparative band structure of fully reduced and fully oxided HfO 2 -Pt interface

Approximately e nm dipole moment per interfacial O

Conclusions (Pt)-Si-SiO 2 -HfO 2 -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 HfO 2 -Pt interface raises energies of HfO 2 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.