First Principles Studies of Defects in HfO First Principles Studies of Defects in HfO 2 and at Si:HfO Si:HfO 2 Heterojunctions Heterojunctions Chunguang Tang (唐 唐唐 ) (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 Ph.D. Dissertation Proposal
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First Principles Studies of Defects in HfO 2 and at Si:HfO 2 Heterojunctions
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First Principles Studies of Defects in HfOFirst Principles Studies of Defects in HfO22 and at Si:HfOSi:HfO22 Heterojunctions Heterojunctions
Chemical, Materials & Biomolecular EngineeringInstitute of Materials Science
University of Connecticut
Principal Advisor: Prof. R. Ramprasad Associate Advisor: Prof. L. Shaw Associate Advisor: Prof. P. S. Alpay
Ph.D. Dissertation Proposal
Introduction: device miniaturization
# of
tr
ansi
stor
s
First transistor radio: 4 transistors.
Quad core processor contains 820 million transistors
high k (dielectric constant) transistor
K ~ 4
K ~ 30
High k issuesHigh 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
Locquet et al, JAP (2006);Stemmer et al
Wong et al, Microelectronic Eng. (2006)
* D. Y. Cho et al, APL, 86, 041913 (2005); X. Y. Qiu et al, APL, 88, 072906 (2006); S. Stemmer, JBSTB, 22, 791
(2004)
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
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.
Computational MethodsComputational 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.
rrrVm iiieff
2
2
2
0 1 2 3 4
Reaction path Emigr.E
Completed Research• Bulk HfO2 results
E/HfO2 a (calc./expr.) b (calc./expr.) c (calc./expr.)
c-HfO2 0.25 5.06/5.08* =a =a
t-HfO2 0.16 5.06/5.15** =a 5.14/5.29
m-HfO2 0 5.14/5.12*** 5.19/5.17 5.30/5.29
* 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. Solids 52, 1181 (1991)*** J. Adam and M. D. Rodgers, Acta Crystallogr. 12, 951 (1959)
Table I: relative energies (eV) and lattice constants (Å) of bulk HfO2
Defects in bulk HfO2
Eform + 1 O atom
Eform = Evac+ (EO2)/2 - Eperf
3-fold site 4-fold site
O interstitial 1.7 2.5
O vacancy 6.6 6.5
Hf vacancy 6.1
Table II: Formation energies (eV) of point defects in bulk HfO2
O interstitial Formation and Migration*O interstitial Formation and Migration*
Interfacial segregation:Thermodynamic driving force (decreasing Eform as interface is approached)
Kinetic driving force, and O penetration into Si (decreasing Emigr as interface is approached)
O interstitials could lead to the formation of SiOx
* C. Tang & R. Ramprasad, Phys. Rev. B 75, 241302 (2007); ** J. C. Mikkelsen, Appl. Phys. Lett. 40, 336 (1981).
Si HfO2
Hf
O
Experimental Emigr. of O interstitial in bulk Si: 2.44 eV** (2.26 eV, calculated)
O Vacancy Formation and Migration*O Vacancy Formation and Migration*
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. B 76, 073306 (2007)
Si HfO2
Hf
O
Hf Vacancy Formation and Migration*Hf Vacancy Formation and Migration*
• Hf vacancies prefer the interface
• Si strongly prefers to penetrate into HfO2
* C. Tang & R. Ramprasad, Appl. Phys. Lett., 92, 152911 (2008)
Hf vacancies could lead to the formation of Hf silicate
Hf
O
Si penetration
Accumulation of O Point Defects*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)
2. The local chemistry of Si prefers SiO2 configuration
Y doped HfO2 (YDH)
(1-x) HfO2+(x/2)Y2O3+Ef=Hf1-xYxO2-x/2
1. If Y > 12%, t-HfO2 and c-HfO2 more stable.
2. Similar stabilization phenomenon in c-YSZ for fuel cell application.
3. Instead of Y, positively charged O vacancies are identified as the major stabilizing factor.
m-YDH
c-YDH
t-YDH
Charge neutrality 2 Y atoms & 1 O vacancy
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
• 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
Remaining research
Publication list1. C. Tang and R. Ramprasad, "Oxygen defect accumulation at Si:HfO2 interfaces"
, Appl. Phys. Lett., 92, 182908 (2008). 2. C. Tang and R. Ramprasad, "A study of Hf vacancies at Si:HfO2
heterojunctions" , Appl. Phys. Lett., 92, 152911 (2008). 3. C. Tang and R. Ramprasad, "Oxygen pressure dependence of HfO2
stoichiometry: An ab initio investigation" , Appl. Phys. Lett., 91, 022904 (2007). 4. 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). 5. C. Tang and R. Ramprasad, "Ab initio study of O interstitial diffusion near
Si:HfO2 interfaces", Phys. Rev. B, 75, 241302(R) (2007). 6. B. R. Tuttle, C. Tang and R. Ramprasad, "First-principles study of the valence
band offset between silicon and hafnia", Phys. Rev. B, 75, 235324 (2007). 7. 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).
8. Tang CG, Li Y, Zeng KY, Mater. Lett., 59, 3325, (2005). 9. Tang CG, Li Y, Zeng KY, Mater. Sci. Eng. A, 384, 215, (2004). 10. 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). 11. Li Y, Wang YB, Tang CG, Ma QZ, Cao GH, SCI CHINA SER A, 40, 978, (1997). 12. Li Y, Tang CG, Ma QZ, Wang YB, Cao GH, Wei T, Wang WH, Zhang TB, Physica C, 282, 2093,
(1997).
AcknowledgmentAcknowledgment
Group students:
Ning, Luke, Tom, Ghanshyam and Hong
Committee members:
Profs. Rampi Ramprasad, Leon L. Shaw and Pamir S. Alpay
Profs. Puxian Gao and George A. Rossetti
Computational resources:
IMS computation clusters; SGI supercomputer in SoE
Funding:
NSF & ACS-PRF
Backup slides
(P, T) dependence of O defects(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 alAppl. Phys. Lett. 87, 141917 (2005)
22
ln2
1 0OOperfectdefect
formationdefect PkTEEE
C. Tang & R. RamprasadAppl. Phys. Lett. 91, 022904 (2007)
Pick up T, find P to make formation energy 0, corresponding to equilibrium condition.
(P, T) dependence of interface morphology(P, T) dependence of interface morphology
• Pressure changes could stabilize silicide or SiOx
• Increase in T makes abrupt Si:HfO2 interface less stable