First-principles Study on Intrinsic Defects of Ge in Strained Condition Jung-Hae Choi , Seung-Cheol Lee, and Kwang-Ryeol Lee Computational Science Center Future Fusion Technology Laboratory Korea Institute of Science and Technology [email protected]http://diamond.kist.re.kr/CSC 1~6, July, 2007 Singapore O-4-PO31 International Conference on Materials For Advanced Technologies 2007
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First-principles Study on Intrinsic Defects of Ge in Strained Condition Jung-Hae Choi, Seung-Cheol Lee, and Kwang-Ryeol Lee Computational Science Center.
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First-principles Study on Intrinsic Defects of Ge
in Strained Condition
Jung-Hae Choi, Seung-Cheol Lee, and Kwang-Ryeol LeeComputational Science Center
Future Fusion Technology LaboratoryKorea Institute of Science and Technology
International Conference on MaterialsFor Advanced Technologies 2007
MOSFET scaling down
Science 309, 388 (2005).
Physical limitations on scaling-down of conventional Si/SiO2 semiconductors various researches on next generation devices
Physical limitations on scaling-down of conventional Si/SiO2 semiconductors various researches on next generation devices
High-k gate oxide strained Si
Ge channel???
& Many others
Ge as a channel materials
• Higher mobility than Si
Application on high performance device
• Higher mobility than Si
Application on high performance device
• Unreliable oxide
• Difficulties of growing single crystals & their high cost
• Unreliable oxide
• Difficulties of growing single crystals & their high cost
DisadvantagesDisadvantages
graded SiGeGe film
Si substrate
Ge film
Si substrate
AdvantagesAdvantages
Ge
Si2 nm
Next generation MOS ?Strained !
Motivations
• Understanding and controlling the defect structures in the strained condition are the fundamental steps in solid state reactions such as crystal growth, processing and operation of devices, which accompany diffusion.
• Despite the rising importance of Ge and its similarities with Si, the intrinsic defects of Ge in strained condition are seldom characterized experimentally and theoretically.
• The calculation on the defect formation in Ge is controversial in terms of defect formation energy, atomic configurations, etc.
• Investigations of the strain effect on the vacancy formation was not performed yet.
Controversial results on the vacancy formation
Depend on• Code• Exchange-correlation scheme - parametrization• Number of atoms• Cutoff energy• Convergence of Relaxation • K-point sampling• Symmetry constraints• Spin• …….• …….
Si
Ge
Unstrained Ge
Purpose of this work
First-principles calculations- the dependency of vacancy formation energy on the strain - only on neutral vacancy
Strained Ge
Evunstrained
< 0
< 0
aGe = 5.66 Å aSi = 5.43 Å
Evstrained≠
Ge
?
Calculation condition using VASP
DFT scheme
Ecut = 300 eV
Exchange-correlation potential; LDA (CA)
Projector Augmented-Wave (PAW) potential
Brillouin zone sampling using Monkhorst-Pack technique
Tests of exchange-correlation potential on Si & Ge
aSi (Å) BSi (GPa) aGe (Å) BGe (GPa)(aSi-aGe)
/aGe
PAW-LDA 5.402 97 5.646 72 -0.043
PAW-PBE 5.468 88 5.783 61 -0.054
US-LDA 5.389 95 5.625 71 -0.042
US-PW 5.456 88 5.759 60 -0.053
Experimental 5.43 99 5.66 75 -0.041
PAW-LDA was selected !
Vacancy formation energy
• Eqv ; vacancy formation energy
• N ; number of atom
• EqN ; total energy of N atom system
• EqN-1 ; total energy of (N-1) atom system
• q ; charge state of vacancy
e ; EF relative to the VBM Ev
• Eqv ; vacancy formation energy
• N ; number of atom
• EqN ; total energy of N atom system
• EqN-1 ; total energy of (N-1) atom system
• q ; charge state of vacancy
e ; EF relative to the VBM Ev
Perfect structure One vacancy
Eqv
Vacancy formation energy
• Decrease of the vacancy formation energy of (~1.3 ev) by the compressive planar strain Easier formation of vacancies Fast diffusion and intermixing in Ge epitaxial layer on Si ??
• Effect of supercell size due to vacancy-vacancy interactions ; 2x2x2 supercell is not large enough
• Decrease of the vacancy formation energy of (~1.3 ev) by the compressive planar strain Easier formation of vacancies Fast diffusion and intermixing in Ge epitaxial layer on Si ??
• Effect of supercell size due to vacancy-vacancy interactions ; 2x2x2 supercell is not large enough