Jung-Hae Choi, Hyo-Shin Ahn, Seung-Cheol Lee & Kwang-Ryeol Lee Third Conference of the Asian Consortium for Computational Materials Science (ACCMS-3) PC43 08-11, Sep., 2005 Beijing, China Stress reduction behavior in metal incorporated amorphous carbon films ; First-principle approach Jung-Hae Choi, Hyo-Shin Ahn, Seung-Cheol Lee & Kwang-Ryeol Lee Future Technology Research Division Korea Institute of Science and Technology Seoul, KOREA choijh@kist.re.kr http://diamond.kist.re.kr/DLC

Tetrahedral amorphous carbon (ta-C) films High fraction of tetrahedral (sp3) bonding Advantages - High hardness and wear resistance Optical transparency Chemical inertness - Smooth surface - Bio-compatibility Hard coatings Hard disk Disadvantages - High residual compressive stress (6~20 GPa) → poor adhesion Delamination Before deposition After deposition Substrate bending M. W. Moon et al., Acta Mater., 50 219 (2002).

Motivation reduction of residual compressive stress in W-incorporated a-C:H films prepared by a hybrid process composed of ion-beam deposition and magnetron sputtering 1.9 at % W W atoms are fully dissolved in a-C matrix Mechanism ? Not fully understood yet !!! A.-Y. Wang et al., APL 86 111902 (2005).

Purpose of this work Amorphous carbon Diamond ; distorted sp3 ; ideal sp3 bonding 109.5o ≠109.5o Amorphous carbon ; distorted sp3 + sp2, sp bonding Known as a primary cause of the residual stress in a-C structure to elucidate the dependency of total energy of the system on the bond angle & the electron density distribution and its effects on the stress reduction behavior of a-C films

Calculation condition by DMOL3 DFT scheme Exchange-correlation potential; GGA (PBE) Atomic orbital; double-zeta polarization basis set Cutoff radius of atomic orbitals; 9 Å All electron calculation Spin consideration

Tetrahedron bond model C 109.5o Me tetrahedral bonding of carbon(or Me)-carbon structure relaxation total energy calculation ; reference state DEC-C DEMe-C 90o~ 130o C 90o~ 130o Me Bond angle distortion bond distance relaxation total energy calculation

Total energy change by the bond angle distortion Increase in total energy drastically decreases by Me-incorporation. Metal atoms having a filled d-shell (underlined atoms) show lower increase in total energy by the bond angle distortion. Al shows a similar behavior with noble metals.

Formation energy of Me-C tetrahedron DEfM-C = (EtotM-C + EatomC) - (EtotC-C + EatomM) Me Me Higher formation energy of Me-C tetrahedron  weaker Me-C bond  less angular dependency of total energy

Isosurface of electron density; C-C-tetrahedron 0.5 1.5 1.0 90o C 1.5 1.0 0.5 Inset values are the electron density [Å-3] of the isosurface

Iso-e- density surface ; Au-C-109 Isosurface of electron density; Au-C-tetrahedron Iso-e- density surface ; Au-C-109 109.5o Au 0.2 0.4 0.5 90o Au 0.2 0.4 0.5 Inset values are the electron density [Å-3] of the isosurface

Isosurface of electron density right before it is separated Ag 0.40 Ar 0.01 Cd 0.36 Au 0.40 Al 0.45 Zn 0.45 Cu 0.53 Pd 0.58 C 1.50 109.5o

Isosurface of electron density right before it is separated V 0.63 Ti 0.64 Ni 0.67 Si 0.72 Mn 0.70 W 0.70 Cr 0.72 Mo 0.72 Co 0.76 Fe 0.82 C 1.50 109.5o

electron density right before its isosurface is separated (res) Weaker bonding Lower angular dependency of total energy  stress reduction Lower res Lower shape anisotropy of electron density

Larger atom incorporation in a-C structure d 0.0 % 2.0 % 5.0 % Si C MD of a-C:Si deposition : Dimensional increase along the surface normal by Si incorporation  may release the strain S.- H. Lee et al.. PA15 in ACCMS-3 Induce strain energy ? Maybe not significant !

Summary The stress reduction behavior in metal-incorporated amorphous carbon films was investigated by the first-principle calculation. Present calculations evidently show that the metal incorporation reduces the directionality of the bond, which results in the reduction of the residual stress caused by bond angle distortion in amorphous carbon network. The pivotal action of the metal atoms dissolved in the carbon matrix would be more significant when noble metals having filled d-shells, such as Au, Ag, or Cu, are incorporated. These atoms have a weak and more isotropic bond with carbon atoms as confirmed by the electron density distribution. Interestingly, Al also shows a very weak dependence of the energy on the bond angle. Electron density distribution of Al incorporated tetrahedron shows the similar behavior to that of noble metal-incorporated tetrahedron.