Experimental methods in

physics

Local probe microscopies I

Scanning tunnelling microscopy (STM)

Jean-Marc Bonard

Academic year 09-10

1. Scanning Tunneling

Microscopy

1.1. Introduction

Image of surface reconstruction on a clean Gold

(Au(100)) surface

“usual” !scopies

! optical !scopy

! transmission electron !scopy

! projection !scopy

! Object probed by a source" Integral observation of the image

" Magnification determined by# Focal length of the lenses

#Distances between source, object and observation screen

" Resolution limited by#Wavelength of probe particles

# Lens aberrations

# Source coherenceSchmid and Fink, APL 70, 2679 (1997)

Local probe !scopies

! Interaction between a probe and the sample" Tunnelling current (STM)

" Atomic force (AFM), magnetic, electrostatic, …

" Luminescence (SNOM)

" Secondary electrons (SEM)

! Image formed by scanning the probe on the sample" Magnification determined by size of scanned surface

" Resolution limited by# Probe size

# Sensitivity of the detection

1. Scanning Tunneling

Microscopy

1.2. Principle of STM

STM, first experiments:

Young’s “topografiner” (1971)

! Young’s goal: detecting atomic steps" Strong dependence of field emission current with tip-sample distance

" “Topografiner”# x-y-z scanner to raster the sample

# Profiles at constant current: topography

" Resolution too low to detect atomic steps

" Project abandoned…

Young, Physics Today 11, 42 (1971)Young et al., PRL 27, 922 (1971) Optical grating, 180 lines/mm

The first STM –

Binnig and Rohrer

! 1982" Binnig and Rohrer take Young’s work one step further

" Goal: local spectroscopy of electronic properties

" First constant current scans on a CaIrSn4 surface

" Mono-, bi-, triatomic steps

! Key differences" Tunnelling regime (field emission for Young)

" Mechanical isolation

" Vibration damping!

Binnig et al., PRL 49, 57 (1982)

The first STM –

Binnig and Rohrer II

! 1983" Binnig and Rohrer study the Si(111)(7x7) surface# At that time, structure not known with certitude

#Unit cell with 49 atoms

" First observation in direct space

! Atomic resolution!

" Greeted with great caution…

" …Nobel prize in 1986

Binnig et al., PRL 50, 120 (1983)

Quantum contact

Tunnelling regime

Ohmic contact

Lang

, PR

B 3

6, 8

173

(198

7)

The tunnelling effect

! Transmission probability of electrons between two materials > 0" Low barrier width#High electric field: field emission, I ! exp(V)

# Small distance between electrodes:STM, I ! V

! Quantum effect" Overlap between wavefunctions of electrons at tip and sample

" Applied field " 0# Tunnelling current: I ! exp(-z), I ! V

" Contact# Atomic chain (quantum contact, one conduction path)

#Ohmic contact

Tunnelling current

" First order perturbation

# !: initial state; ": final state

#M: matrix element

# Elastic tunneling effect between an occupied state and an unoccupied state

" Spherical tip, with only states

##: local density of states (LDOS)

# T: transmission coefficient

# k: depends on V and workfunctions of both materials; k # 1Å-1

It ! "(r r ,Ef ) #T (Ef ,V ) ! "(

r r ,Ef ) # e

$2 kz

It ! $µ," f(Eµ)[1-f(E"+eV)]|Mµ,"|2 %(Eµ-(E"+eV))

E"

$E = eV

Eµ

"$It depends strongly on z!

#$Very high z resolution

#$Typical tunnelling currents between 10pA and 1nA

1. Scanning Tunneling

Microscopy

1.3. Instrumentation

Instrumentation

! An STM is composed of…" Probe tip

" x-y-z scanners

" Electronics

" Vibration damping

" Vacuum chamber# STM is not necessarily done under UHV• Electrochemical STM (tunnelling in a liquid)

• STM in ambient atmosphere (surface cleaneliness?)

" Options# Cryostat

#Magnet

# Surface preparation tools (ion gun, electron diffraction setup, …)

Probe preparation

! Au, W, Pt-Ir wire…

! Coarse sharpening" Fracture, cutting…

" Electrochemical sharpening# Tip with ~0.1!m radius of curvature

! Fine sharpening

" High voltage pulse (~5V)# Transfer of atoms between probe and sample

# Probe ends with one (a few) atom(s)

Scanning I

! Specifications" Resolution of 0.05Å

" Full course of 10nm - 1!m (10!m?)

" Linear behavior of displacement as a function of voltage

! Piezoceramics

" Dilatation/contraction under applied voltage

" Typ. 2Å/V

! x-y-z “scanner”

" Tube tripod

" Sticks tripod

" x-y-z tube

Scanning II

! Binnig and Rohrer design" Coarse approach: piezo slab and electrostatic clamps

" Scanning: sticks tripod

Control electronics

! Constraints" Tunnelling current between 10pA and 1nA# Low noise electronics

#No ground loops

" Scanning#Resolution in x,y of 1Å

" Approach and measurement#Resolution in z of 0.05Å

# Feedback: ln(It) ! z

Vibration damping

! Binnig and Rohrer" Damping springs (3 stages)

" Damping through Foucault currents

! “Pocket-size” STM" Copper slabs isolated with elastomer half-rings

! In general…" “Pocket-size” damping

" Suspension of UHV chamberon springs or on a damping table

Other challenges

! Cryostat" He cryostat: %4K (typ. 20K)

" 3He cryostat: ~250mK

" Vibrations, thermal shifts?

! Environment

" UHV chamber

" STM under air (adsorbates?)

" Electrochemical STM (tunnelling in a liquid)

! Other useful add-ons" Auger spectrometer, ion gun, electron diffraction (surface preparation)

" Sputtering setup (deposition/growth)

" Magnetic field Röder et al., Thin Solid Films 264, 230 (1995)

1. Scanning Tunneling

Microscopy

1.4. Imaging basics

Point defects on Cu(111), possibly impurity atoms, and

scattered surface state electrons (Crommie et al., IBM)

STM imaging I

! Tunnelling current" Proportional to local electronic density of states at the Fermi level

" Constant current images# Constant electronic LDOS

#Defects, steps: topography

! Example: Al(111) and adsorbed C

" Steps: 2.34Å

" C atoms: apparent height of ~0.2Å

STM imaging II

! Image depends on z

! C on Al(111)" High apparent height or…

Brune et al., Europhys. Lett. 13, 123 (1990)

"$Transparent atoms!#$Destructive interference between probe and sample wavefunctions

#$Redistribution of electronic charge and modification of LDOS

STM imaging III

! Image depends on V" Electronic structure of surface

" Tunnelling current initiates from occupied states (or goes to empty states)#Density of states of corresponding electronic levels

" Si(111)(7x7)# Empty states: dangling bonds of upper atoms

# Filled states: bonds between first and second layers

" GaAs(110)# Empty states: on Ga atoms

# Filled states: on As atoms

GaAs(110)Si(111)(7x7)

STM imaging IV

! Cu(111)" Steps: ~2Å

" Adsorbates: ~0.2Å

" Lines, circles around adsorbates: ~0.05Å

! What are these structures?

" Atomic arrangements?

" Defects?

" Surface electrons!#Quasi-2D electron sea

# Interference between incident and reflected electrons– at atomic steps– at defects

Crommie et al., Surf. Rev. Lett. 1, 127 (1995)

1. Scanning Tunneling

Microscopy

Annex 1 –

Nomenclature of surface structures and

reconstructions

25

Preparing a surface

! Metal/semiconductor: well-defined crystalline structure

" In theory, one should be able to form a surface of arbitrary orientation

" In practice: only a few orientations are energetically favourable

! Preparation

" Cleavage

" Machining

" Chemical etching

" Ion bombardment

" Vacuum annealing…

1st layer2nd layer

face-centered cubic crystal (fcc)

26

High symmetry surfaces

! Most common surfaces

" High density of atoms

" High number of neighbouring atoms

! Notation: Miller indices

fcc structures (Cu, Pt, Si…)(100)

(110)

(111)STM image of a Cu(111) surface

27

Surface reconstruction

! Lowering of the free surface energy

" Relaxation

" Reconstruction

!$Notation (Wood): Size and orientation of unit cell of the reconstruction with respect to the 2D unit cell

(2 x 2)

c(2 x 2) or(&2 x &2)R45

c: centredR: rotation

28

Surface reconstruction

GaAs c(2 x 4) Si(111)(7 x 7)

29

Vicinal surface

! Surface with an orientation that is very close to a high symmetry surface

" Low indices surface with periodic terraces

" Diffusion/segregation studies…

Pt(997)

1. Scanning Tunneling

Microscopy

Annex 2 – “Tip” electron microscopies

Electron field emission

! Emission of “cold” electrons" First observed in 1897

" Applying an electric field on a sample renders the surface potential barrier triangular, with a slope that depends on the applied field

" Significant probability of crossing of surface barrier by tunnelling effect for F % 2 V/nm

" Observed on sharp tips (field amplification)

" Non-linear behavior between local applied field and emitted current

-

+

V

EFD(EF)

D(E)

E

D(E)

V

I

1/V

ln(I/V )2

F = 0 V/nm

F ! 2 V/nm

F

Field emission microscopy

! Observation of a metallic tip during field emission" Sharp tip with low radius of curvature: high field enhancement

" High sensitivity to local protuberances and work function

" Adsorption/desorbtion studies

" Diffusion studies e–

V– +

Field emission microscopy

! Example: NO+H2 on Ir

" Clean tip: emission from (110)

"With NO+H2 partial pressure

# Change of emission on (110)

# Short lowering of work function on (100) planes: probable presence of NHx or O

#Oscillatory local chemical reactions

Cobden et al, Surf. Sci. 402, 155 (1998)

+–

Field ion microscopy (1956)

! Ionisation of He atoms (F%2V/Å)" Sharp tip: field enhancement

" High electric field at atoms located at edges of terraces

" He atoms ionized near these atoms

" Ions follow the electric field lines to the observation screen

First images with atomic resolution

Field evaporation microscopy:

“Atom Probe”! Field evaporation" High electric field can lead to atom evaporation

#Time-of-flight measurement: determination of mass of atom

#Comparison of images before and after evaporation: position of atom

" 3D probe of atomic composition

Miller, Mater. Charact. 44, 11 (2000)

Superalloy 708precipitate and grain boundary

12 nm