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)