In this research, we have studied the morphology, electronic states and interface properties of the simplest and yet significant heteroepitaxial system. The atomic layer deposition (ALD) technique enabled us to produce various functional thin films in atomic level. The self-limited reaction serves as great stage for understanding the interaction between dissimilar atoms. By a combination of the two concepts, we designed a two-step reaction to grow a atomic layer of ionic NaCl.
The first principle calculation results indicate that molecular chlorine can spontaneously dissociative chemisorbs on the clean Si(100) surface forming p(2x1) chlorine-terminated surface without extra energy input. The nearest neighbor distance between surface chlorine atoms (3.86 Å) is very close to that in typical NaCl crystal (3.99 Å). Each chlorine atom received about 0.67 electrons from the bonded silicon atom, resulting in a polar surface. A sodium atom prefers to adsorb on the hollow site between surface chlorine atoms, and donates about 0.85 electrons to the nearby chlorine atoms. So the partial ionic bonding would be form to replace Si-Cl covalent bonding. However, the singly-adsorbed structure had never been observed in our STM images. By the NEB method, we figured out the diffusion barrier along and perpendicular to the dimer row is only about 0.5 eV. In other words, the single adsorption case can be observed only under very dilute sodium dosage or at low temperature. It tells us that the dim spot, we observed in STM images, is consisted of cluster formation.
We have employed several simulation methods to find possible NaCl 2D structure. Static calculation results indicated two sodium atoms adsorbing on adjacent hollow site would result in energy increasing. The result implies that the cluster must
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be form in other arrangement in the initial stages of heteroepitaxy. Furthermore, the AIMD result suggests the sodium and chlorine atoms tend to form (111) phasic NaCl clusters. Each cluster unit contains three sodium and one chlorine atom. The clusters also prefer to assemble along 2D plane instead of 3D stack. From an electrostatic point of view, the growth of NaCl clusters along its „(111)‟ direction would result in very strong surface dipoles. However, previous studies have reported that non-equilibrium growth shapes are also observed for a range of finite temperatures3,
105. Our simulated results indicate three possible formations of assembled clusters, the single cluster, 2D assembled clusters, and double-layer 3D stack they have only about 0.05 eV energy difference. They may correspond to the single dim spot, the p(2x2) and a collected area of dim spots in our STM images. Even though the topography of cluster is higher than other uncovered area, the clusters could correspond to dim spots in our STM images due to it is lacking of electron density of state under our scanning bias (-2 eV). With increasing sodium coverage (Na 0.5), the cluster tends to grow along the dimer row. When the clusters gathered over a certain density, it would transform into NaCl(111)-(100) concomitant structure. They spell out which two bonded in two different energy level, one in about -2 eV, another in about -4 eV, which two bonding types are correspond to three fold (111) and four fold NaCl, respectively. From this point of view, the continuous spectra shift of Na 2p and Cl 2p peak in XPS spectra is induced by structure phase transition. The amount of shifts corresponds well to the formation ratio of NaCl(111) to NaCl(100).To put it the other way around, the XPS data strongly support our theoretical model. When sodium coverage reached one monolayer, the STM images indicates NaCl(111)-(100) would entirely transform into large scale and uniform NaCl(100) monolayer with random corrugations. The XPS spectra also show that the surface silicon regains its dangling
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bond state; which means only Van Der Waals force is present between the interface of NaCl thin film and silicon substrate.
Finally, we demonstrated that the growth mechanism of Na on the Cl/Si surface follows Franck-van der Merve growth mode. As far as we know, the NaCl(100) monolayer is fabricated successfully for the first time. We also found unusual (100)-(111) concomitant phase in the system. In additional, we discovered that the thin NaCl films protected the dangling bond state of the surface silicon can be preserved for long time here. Such surface state usually has very high chemical reactivity and electron mobility. This phenomenon can someday become useful for device applications.
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Appendix: Computation Conditions common setting
Program: Vienna Ab-initio Simulation Package (VASP) Pseudopotential: PAW-GGA-PBE
Cut off energy for plane wave: 300 eV (22.06Ry) Without Spin
Structural relaxation (from an unreasonable starting guess)
k-point grid : 4x4x1 (Monkhorst-Pack)
Structural relaxation (from pre-converged starting guess )
k-point grid : 4x4x1 (Monkhorst-Pack)
MAXMIX = 80 # keep dielectric function between ionic movements IBRION = 1 # use RMM-DIIS algorithm for ions
NFREE = 10 # estimated degrees of freedom of the system
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Static calculation (after Structural relaxation)
generating more precise charge density and wavefunction for 1. DOS 2.Bader 3.Work function 4. Electron localization function
5.XPS simulation 6.STM simulation
RWIGS=0.370 1.312 1.111 1.757 # specify project radius(A)
####################### 2.All electron(for Bader) #######################
LAECHG=.T. #output all electron
#(increase FFT mesh: NGFX(Y)(Z)=2*NGX(Y)(Z) for better charge resolution)
########################3.workfunction (dipole)########################
LVTOT=.T. #output LOCPOT
#################### 4.electron localization function######################
LELF=.TRUE. #output ELFCAR
#############################5.core level #############################
ICORELEVEL =1 # 0,1,2(approx final/ initial /acc. final state) CLNT = 1, 2, 3…..# processing N-th species of atom
CLN = 1, 2, 3... # N quantum number
CLL = 0, 1, 2, 3 # orbital quantum number ( 0=s,1=p,2=d)
CLZ = 1 # (How many electron removed from core,0.5 for transition state)
#############################6.STM ################################
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LPARD =.T. # output PARCHG
NBMOD = -3 # calculate the range window .vs. Fermi level EINT =-1 # project window ( eV)
Nudged Elastic Band
k-point grid : 1x1x1 (Monkhorst-Pack) INCAR
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Molecular Dynamics
k-point grid : 1x1x1 (Monkhorst-Pack) INCAR
LREAL= A # fully automatic optimization of projection operator ALGO = Fast # RMM-DIIS for electron
ISMEAR = 0 # Gaussian smear for low K-point SIGMA = 0.2 # Band width
################################MD################################
IBRION = 0 # For MD calculation
SMASS = -3 # -3 : micro canonical ensemble (NEV)
# -2: constant velocity(for calculating Morse potential) # -1: Annealing
# >=0: canonical ensemble (NTV) with Nosé-mass POTIM = 2 # step size (fs)
TEBEG = 300 # initial temperature TEEND = 300 # final temperature NSW = 1000 # 1000 ionic steps LCHARG = .F. # don't write CHGCAR LWAVE = .F. # don't write WAVECAR
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Curriculum vitae LI, Hong-Dao
李宏道
PERSONAL DETAILS - Date of birth: Dec 14, 1976 - Civil state: married
SKILLS
- Thin film process
- Ultra-High-Vacuum (UHV) operation
- X-ray Photoelectron Spectroscopy (XPS) measurements - Scanning Tunneling Microscope (STM) measurements - First principle calculations
- Linux operation
- C, Matlab, and Fortran program design
EDUCATION
- Fu Jen Catholic University (FJU)
Undergraduate degree in department of physics, 2000-2004 - National Central University (NCU)
Master degree in institute of space physics, 2004-2006 - National Chiao Tung University (NCTU)
Ph. D. degree in institute of physics 2006- 2011
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EXPERIENCE
- Operating Ultra-High-Vacuum system and XPS at National Synchrotron Radiation Research Center (NSRRC)
- Training semiconductor processing technology (VLSI) at National Nano Device laboratories (NDL)
- CMRFG member (Workshop on First-Principles Computational Material Physics)
PUBLICATIONS
1. C.-T. Lou, H.-D. Li, J.-Y. Chung, D.-S. Lin, T.C. Chiang
Oscillations of Bond Character and Polarization at the NaCl/Ge(100) Interface during Cyclic Growth.
Phys Rev. B 80, 195311 (2009)
2. Ming-Feng Hsieh, Hong-Dao Li, Deng-Sung Lin and Karina Morgenstern Formation, binding, and stability of O-Ag-CO 2 -Ag-O compounds on Ag(100) Investigated by Low Temperature Scanning Tunneling Microscopy and Manipulation.
J. Phys. Chem. C, 2010, 114 (33), pp 14173–14179 (2010)
3. Hong-Dao Li, Chan-Yuen Chang, Ling-Ying Chien, Shih-Hsin Chang, T.-C.Chiang and Deng-Sung Lin.
Adsorption and abstraction reactions of HCl on a single Si(100) dangling bond Phys. Rev. B 83, 075403 (2011)
4. Jen-Yang Chung, Hong-Dao Li, Wan-Heng Chang, T. C. Leung, and Deng-Sung Lin.
Sodium chloride on Si (100) grown by molecular beam epitaxy.
Phys. Rev. B 83, 085305 (2011)
5. Ying-Hsiu Lin, Hong-Dao Li, Horng-Tay Jeng, and Deng-Sung Lin
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Energetics and Interactions of Mixed Halogen Adsorbates on the Si(100) Surface.
J. Phys. Chem. C, DOI: 10.1021/jp201251j (2011)
6. Hong-Dao Li, Ying-Hsiu Lin, Horng-Tay Jeng, and Deng-Sung Lin
Two-dimensional Sodium chloride thin film on Si(100) grown by Atomic Layer Deposition.
J. Phys. Chem. C (has been accepted for publication) (2011)
7. Hong-Dao Li, Ying-Hsiu Lin, Horng-Tay Jeng, and Deng-Sung Lin Core Level Shifts of Halogen adsorbed Si(100) and Ge(100) surface.
J. Phys. Chem. C (reviewing) (2011)