Meta-stable Sites in Amorphous Carbon Generated by Rapid Quenching of Liquid Diamond

1. Meta-stable Sites in Amorphous Carbon Generated by Rapid Quenching of Liquid Diamond

Seung-Hyeob Lee, Seung-Cheol Lee, Kwang-Ryeol Lee, Kyu-Hwan Lee, and June-Gunn Lee Future Technology Research Division, Korea Institute of Science and Technology, P.O. Box, 131 Cheongyang, Seoul, Korea

2. Introductions

Diamond like Carbon (DLC) High density and hardness Optically transparent Chemically inertness High�electronic quality Wide range of physical properties Lower deposition temperature Synonyms a-C:H : hydrogenated amorphous carbon a-C : amorphous carbon ta-C : tetrahedral amorphous carbon Application Protective coatings for semiconductor or mechanical device Electronic devices

3. Tetrahedral Amorphous Carbon (ta-C)

Properties Particular form of amorphous carbon High ratio of sp3 hybridization bonding (>80%) High hardness and wear resistance with optical transparency Smooth surface Similar properties of diamond Lower synthesis temperature ? Enable to various applications High residual compressive stress ? Poor adhesion Advantages Disadvantages In order to overcome the disadvantages, we should understand the structural properties of ta-C in atomic scale using computer simulation.

4. Amorphous Structure Generation

Methodologies that can make an amorphous phase Rapid quenching of liquid carbon Interstitial addition of carbon atom in diamond High-energy carbon ion bombardment In many cases, a new peak near the second peak was observed from radial distribution function (RDF). The new peak is considered to be a meta-stable site of carbon The role of the meta-stable site in the amorphous carbon structure has not been understood. We investigated the role and the structural dependence of the meta-stable site in amorphous carbon.

5. Purposes and Approaches of this work

Investigation of the meta-stable site of an amorphous carbon Calculation of the properties of crystal diamond to check the validity Generation of an amorphous carbon structure Rapid quenching of liquid carbon Ion bombardment into a crystalline carbon Using the simulation program XMD 2.5.29 with empirical potential proposed by Tersoff Investigation of meta-stable site of the amorphous carbon structure

6. Properties of Crystal Diamond

Calculated the crystal diamond Interatomic Potential : Tersoff Potential J. Tersoff, Phys. Rev. Lett., 61 (1988) 2879. Time Step : ~10-15 sec XMD ver. 2.5.29

7. Melting the Diamond Lattice

Methodology Forming the diamond lattice (lattice parameter = 3.565 �) Increasing the temperature from 0 K to 10000 K Heating rate : 1.25 K/fs The periodic boundary condition : X-Y-Z axis The crystal diamond melted between 7000 ~ 8000 K. Z Y X Suddenly increasing region 8000K 4000K 10000K 6000K

8. Amorphization of Carbon Structure

Methodology Melted the diamond structure lattice sufficiently (10000K) Decreasing the temperature with different cooling rate to 0 K. Instantaneous freezing to 0 K 1.25 K/fs ~ 6.25 K/fs In the amorphous structure, first and second nearest peak were observed at 1.52 � and 2.52 � We observed a small peak near the 2nd nearest peak, when the cooling rate is larger than 6 K/fs Substantial number of atoms were placed at a meta-stable site at about 2.2 � Spontaneous Quenching 6.25 K/fs 2.50 K/fs 1.25 K/fs

9. Carbon Atom Bombardment

Methodology Bombarding atom Beam energy : 1 ~ 100 eV (10, 40, 70, 100) Time step : 0.155 ~ 0.5 fs / Time interval : 500 fs Stabilization time : 10 ps Formation of a meta-stable site became significant as the kinetic energy increased The more smeared peaks implied the higher degree of disorder 100 eV 70 eV 40 eV 10 eV

10. Relaxation by Annealing

Methodology After spontaneous Quenching to 0K Annealing at an elevated temperature : 300 K ~ 1500 K Heating time : 2.4 ps As the annealing temperature was increased, the meta-stable phase peak at 2.2 � was significantly decreased The atoms occupied at the meta-stable site were relaxed by thermally activated process 1500 K 1200 K 900 K 600 K 300 K

11. Energy behavior

At the heat-up stage, the potential energy was decreased due to the atomic rearrangement when the heat-up temperature is sufficiently high. But total energy was kept almost constant because increasing temperature was cancelled out by the decreasing potential energy. At the holding stage, both total and potential energy decreased as the relaxation process continued Methodology After spontaneous Quenching to 0K Annealing at an elevated temperature to 300 K ~ 1500 K Heat-up time 2.4 ps / Relaxation (Holding) time 4 ps

12. Activation Energy CalculationFrom meta-stable to stable sites

Methodology In case of relaxation rate control system, the relaxation rate was characterized by decrease in the peak intensity at 2.2 � Linear dependence of logarithm of relaxation kinetics was thus governed by Arrhenius type reaction So, we calculate the activation barrier of the meta-stable site Activation energy( ) = 4.8x10-14 erg/atom

13. Summaries & Conclusions

Structure of amorphous carbon generated by rapid quenching of the liquid was investigated by MD simulation. We observed that a meta-stable site exists at 2.2 �, and the atomic population of the site increased as the quenching rate increased. The activation barrier of the site was 4.8 x 10 �14 erg/atom. We observed the similar meta-stable site when carbon atoms of high kinetic energy bombarded to the diamond lattice.