Topics in Surface Science Gao Xingyu
Office: S13 M01-12Tel: 6516 2970 (office) 6516 1670 (SSLS)
http://www.physics.nus.edu.sg/~phygaoxy/
What is Surface?
A special interface between solid/liquid and vacuum Interface: a small number of atomic layers that separate two solids in intimate contact with one another
Why Surface?(do you know something interesting for surface?)(do you know something interesting for surface?)
•Many properties determined by the surface/interface
•Many processes happens mainly at surface (chemical reaction including catalysis, crystal growth, thermionic emission)
•Surface/interface may differ significantly from the bulk (Phase Density, Composition, Mechanical, Electronic, Magnetic properties…)
•Strong interrelation of surface /interface with many other research fields
•Surface as ultra-thin films presents 2-D model for theory (even 1D eg. Nanostructures - 3D thick film)
•Many technical applications (semiconductor device)
Surface can be unique !!!
What determines the properties
Atoms of which element?
What structure?
First principles calculation willdeliver most important properties.
Topics in Surface Science•Introduction
vacuum technique, preparation of clean surface
•Surface Chemical Composition AES, XPS, SIMS, XAS
•Surface Morphology and Physical Structuresurface tension, relaxation, reconstruction, and defects surface
latticeExperimental Techniques: LEED, LEEM, RHEED, PEEM, XD, FIM,
RBS/IS, SPM (STM, AFM, MFM), SEM, TEM, SEXAFS, PhD
•Surface Electronic StructureSurface Potential and Work Function,
surface states (intrinsic and extrinsic), band bendingsurface plasmonsExperimental Techniques: PES (XPS, UPS), Inverse
Photoemission, EELS, Kelvin Probe
•Surface Magnetic Properties:some theoretical considerationsome magnetic phenomena
Experimental Techniques: VSM, MOKE, EMPA, SQUID, Ac susceptibility, XMCD, XMLD, spin-resolved photoemission, MDAD
•Surface VibrationsSurface lattice dynamics
Surface diffusion and surface meltingExperimental Techniques: HREELS, Atom and Molecular Beam scattering,
•Adsorption of atoms and moleculesphysisorption and chemisorption,Desorption techniques: Kelvin Probe, PES (XPS,UPS), surface segregation and epitaxial processes film growth techniques: MBE, MS, PLD
Reference1. Surface and Interfaces of Solids by Hans, Luth Springer-Verlag 2001
(4th. Edition) 2. Concepts in Surface Physics by M.-C. Desjonqueres, D. Spanjaard,
Springer , 1996 3. Introduction to Surface Physics (2nd Ed) by M. Prutton, Oxford
Science Publications 1994 4. Physics at Surfaces by A. Zangwill, Cambridge University Press 1988 5. Handbook of Surface Science by N. V. Richardson & S. Holloway
North-Holland 1996 Vol. 1. Physical Structure Vol. 2. Electronic Structure Vol. 3. Dynamics
6. Principles of Adsorption and Reaction on Solid Surfaces by Richard I. Masel Hohn, Wiley & Sons, Inc. 1996
7. Surface Science An Introduction by J. B. Hudson, John Wiley & Sons, Inc. 1998
8. Online source: http://www.uksaf.org/tutorials.html
Very good
Good for newcomer
The way we mark your performance
• 50% on final test
• 35% on group work and workshop
presentations about certain topics
• 15% on tests in class
Vacuum technology
Rough (low) vacuum : 1 - 10-3 Torr
Medium vacuum : 10-3 - 10-5 Torr
High vacuum (HV) : 10-6 - 10-8 Torr
Ultrahigh vacuum (UHV) : < 10-9 Torr
(Pressure unit conversion)
Why UHV is necessary?
The incident flux is given by (atoms/m2s)
m - molecular mass [ kg ], k - Boltzmann's constant ( = 1.38 x10-23 J K-1 ), T - temperature [ K ], When it assumes unity "sticking coefficient“:
Clean surface require UHV.
Lots of techniques require UHV, electrons, soft x-ray, etc.
Pressure (Torr) Gas Density(molecules m-3 )
Time / ML (s)
760 (ATM) 2 x 1025 10-9
10-3 3 x 1019 10-3
10-6 3 x 1016 1
10-10 3 x 1012 104
UHV ensures that significant contamination by background gases does not occur during an experiment !!!
UHV system
chamber pumps
Stainless vessel
Flanges& tubes& valves
apparatuses RoughHigh vacuumUHV
manipulator
Connection
UHV system
XPS chamber at SSLS
From back
Ion pump
Turbo pumpRough pump
Bellows
What should do to get UHV?
1. Right configuration2. Close all the flanges3. Make sure all the material
inside clean and without incompatible materials
4. Start pumping in the right sequence
5. Proper Baking 6. Degass all the filaments
XPS chamber during During baking
Heater and fan
Construction Materials which are compatible with UHV
•OFHC copper, also pure copper •Be-Cu alloy, phosphor bronze •304 SS (non-magnetic), 310 series SS, 340 SS (magnetic) •Teflon (gassy, compressible) •MACOR (machinable glass composite) •6061 Al (essentially pure aluminum), 2024 Al (harder alloy) •quartz, pyrex (gassy) •alumina (careful with glazed ceramics) •molybdenum, tungsten •"mu-metal" magnetic shielding (Co, Ni, Fe) •polyimide (Vespel) •Sn-Ag solder
Materials which should be avoided
•Zn, Cd: especially be careful of fasteners, bolts •Brass •Certain solders (eutectics have high vapor pressures
Common vacuum problems1. Improper cleaning techniques2. Incompatible materials3. Leaks4. Virtual leaks 5. Not proper baking
O-ring seals
Metal knife-edge seals
For low vacuumFor low vacuum
For UHVFor UHV
Pumps
Rotary pump
Basic one
2-stage one
How it works
Turbo pump
Sorption pump
Sublimation pump
Ion getter pump
cathode
cathode
Anode
Cryopump
High pumping speedMost gases except He and H2
Cannot be used above 10 torrConsume coolant as liquid N2 or HeNormally used together with other pump
Diffusion pump
Vapor pressure determines the final pressure (depends on fluid)Require cooling waterRobust, high speed, inexpensive and reliableOil decomposition, dirty
Gauges
Thermocouple gaugeConstant current heats filamentThermocouple measure T of filamentGas sensitive10-10-4 torr
Pirani gauge:
The temperature change measured as resistanceGas sensitive102-10-4 torr
Ion gauge
Ionization rate of the residual gas10-4-10-11 torr
How to find a leak? Simple ways for low vacuum leak 10-6-7 one can try acetone spray to check the variation of ion gauge pressure
Most efficient: RGA (residual Gas analyzer)1. filament and anode 2. Quadrupole 3. ion detector. Quadrupole is the essential part of RGA
How it works?How it works?
There are two methods: varying ω and holding U and V constant, or varying U and V (U/V) fixed for a constant ω.
(U+Vcos(ωt))
-(U+Vcos(ωt))
Details here!
Typical mass spectrum
The mass spectra of the residual gas in the chamber tells essential information about the vacuum system (with water,…?)Helium gas was widely used to help RGA locate precisely the leak point
Basic theory of Vacuum
The final pressure of the chamber is determined by the outgassing rate of the chamber and the pumping speed. The pumping equation is
vAv = Vv/kT(dp/dt + Sp/Vv)
where Av and Vv is the inner surface area and volume of the chamber, respectively, Sp is the pumping speed and v is the outgassing rate.
In the real cases, one need also consider C the finite conductance of the tubes through which the gas is pumped.
C=I*RT/p
I is the gas current, R is a constant and p is the pressure difference through the tube. For a single tube, C is only determined by mean free path length of the gas and the dimension of the tube. The tubes have a sum rules for conductance like the electric capacitors.
The efficient pumping speed
1/Seff=1/S + 1/Cs
How to prepare clean surface
Cleavage in UHV:
Many materials like some alkali halides (NaCl etc.), oxides (ZnO etc.),and some semiconductor (Ge, Si etc.) It happens along certain low index axis.
Ion bombardment and Annealing:Surface bombarded by noble gas ion and then annealing with many cycles.
One kind of sputter gun
Heating:Temperature near melting point, for example Si(111) at 1370 K remove C and O
Chemical processes:with acids such as HF and other chemical solution to make fresh surface then move into UHV
Film growth:Grow new film with high purity in UHV condition using MBE, etc.
How to check cleanness?How to check cleanness?Or how do you know the surface is clean Or how do you know the surface is clean (without contamination)?(without contamination)?
Chemical component analysis(with surface sensitivity)