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-KopeYoshio 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 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 atomsCapability: 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 stackRe-relax at fixed volume, using density functional theory (DFT).

metal: Pt 110 surface 0.554 nm x 0.480 nmsemiconductor: Si 001 surface 0.543 nm x 0.543 nminterfacial SiO2: cristobalite 001 surface 0.497 nm x 0.497 nmhigh-k dielectric: HfO2 monoclinic 100 surface 0.529 nm x 0.517 nmmetal: Pt 110 surface

Overall cross section: 0.545 nm x 0.500 nm

Si-SiO2: check phase interface O (Tu & Tersoff PRL 84, 4393 (2000))

SiO2-HfO2: 322 model (Sharia, Demkov, Bersuker & Lee PRB 75, 035306 (2007)).

Interface structures

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

Pt-Si.

CommentsVASP usedDFT, ultrasoft pseudopotential methods, PAW formalism287 eV plane wave cutoff; 2x2x1 k-point gridDesigned with inversion symmetryRepeats “back to back”Justification: avoid metal-vacuum surface in modelStrain 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

Pt-Si-SiO2-HfO2-Pt gate stack model

Calculated band structure for stack with O occupancy 0.75

Pt Si SiO2 HfO2 Pt HfO2 SiO2 Si Pt

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

Approximately 0.009 e nm dipole moment per interfacial O

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.