Topics in Surface Science
Gao Xingyu
Office: S13 M01-12
Tel: 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?)
•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 will
deliver 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 Structure
surface tension, relaxation, reconstruction, and defects surface lattice
Experimental Techniques: LEED, LEEM, RHEED, PEEM, XD, FIM, RBS/IS,
SPM (STM, AFM, MFM), SEM, TEM, SEXAFS, PhD
•Surface Electronic Structure
Surface Potential and Work Function,
surface states (intrinsic and extrinsic), band bending
surface plasmons
Experimental Techniques: PES (XPS, UPS), Inverse Photoemission, EELS,
Kelvin Probe
•Surface Magnetic Properties:
some theoretical consideration
some magnetic phenomena
Experimental Techniques: VSM, MOKE, EMPA, SQUID, Ac susceptibility,
XMCD, XMLD, spin-resolved photoemission, MDAD
•Surface Vibrations
Surface lattice dynamics
Surface diffusion and surface melting
Experimental Techniques: HREELS, Atom and Molecular Beam
scattering,
•Adsorption of atoms and molecules
physisorption and chemisorption,
Desorption techniques: Kelvin Probe, PES (XPS,UPS),
surface segregation and epitaxial processes
film growth techniques: MBE, MS, PLD
Reference
1.
2.
3.
4.
5.
6.
7.
8.
Surface and Interfaces of Solids by Hans, Luth Springer-Verlag 2001
(4th. Edition)
Very good
Concepts in Surface Physics by M.-C. Desjonqueres, D. Spanjaard,
Springer , 1996
Introduction to Surface Physics (2nd Ed) by M. Prutton, Oxford
Science Publications 1994
Good for newcomer
Physics at Surfaces by A. Zangwill, Cambridge University Press 1988
Handbook of Surface Science by N. V. Richardson & S. Holloway
North-Holland 1996
Vol. 1. Physical Structure
Vol. 2. Electronic Structure
Vol. 3. Dynamics
Principles of Adsorption and Reaction on Solid Surfaces by Richard
I. Masel Hohn, Wiley & Sons, Inc. 1996
Surface Science An Introduction by J. B. Hudson, John Wiley & Sons,
Inc. 1998
Online source: http://www.uksaf.org/tutorials.html
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 ], p = 3.1416. When it assumes unity "sticking coefficient“:
Clean surface require UHV.
Lots of techniques require UHV, electrons, soft x-ray, etc.
UHV ensures that significant contamination by background gases
does not occur during an experiment !!!
Pressure (Torr)
Gas Density
(molecules m-3 )
Time / ML (s)
2 x 1025
10-9
10-3
3 x 1019
10-3
10-6
3 x 1016
1
10-10
3 x 1012
104
760 (ATM)
UHV system
chamber
Connection
pumps
Stainless Flanges apparatuses
vessel
& tubes
& valves manipulator
Rough
High
vacuum
UHV
UHV system
XPS chamber at SSLS
Bellows
Ion pump
From back
Rough pump Turbo pump
What should do to get UHV?
1.
2.
3.
4.
5.
6.
Right configuration
Close all the flanges
Make sure all the material
inside clean and without
incompatible materials
Start pumping in the right
sequence
Proper Baking
Degass all the filaments
Heater and fan
XPS chamber
during During
baking
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 problems
1.
2.
3.
4.
5.
Improper cleaning techniques
Incompatible materials
Leaks
Virtual leaks
Not proper baking
O-ring seals
For low vacuum
Metal knife-edge seals
For UHV
Pumps
Rotary pump
2-stage one
Basic one
How it works
Turbo pump
Sorption pump
Sublimation pump
Ion getter pump
Anode
cathode
cathode
Cryopump
High pumping speed
Most gases except He and H2
Cannot be used above 10 torr
Consume coolant as liquid N2
or He
Normally used together with
other pump
Diffusion pump
Vapor pressure determines the final pressure (depends on fluid)
Require cooling water
Robust, high speed, inexpensive and reliable
Oil decomposition, dirty
Gauges
Thermocouple
gauge
Constant
current heats
filament
Thermocouple
measure T of
filament
Gas sensitive
10-10-4 torr
Pirani gauge:
The temperature change
measured as resistance
Gas sensitive
102-10-4 torr
Ion gauge
Ionization rate of
the residual gas
10-4-10-11 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
-(U+Vcos(ωt))
(U+Vcos(ωt))
How it works?
Details
here!
There are two methods: varying ω and holding U and V
constant, or varying U and V (U/V) fixed for a constant ω.
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/Dp
I is the gas current, R is a constant and Dp 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?
Or how do you know the surface is clean
(without contamination)?
Chemical component analysis
(with surface sensitivity)