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Institut für Angewandte Physik
Festkörperanalytik
Vorlesung „ Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik
FestkörperanalytikDünne Schichten
Organisatorisches
Johannes Heitmann Institut für Angewandte Physik Gellert-Bau, EG.17Tel.: 39 2590E-Mail: johannes.heitmann@physik.tu-freiberg.de
2
Vorlesungsfolien finden Sie unter:http://tu-freiberg.de/fakult2/angph/studium/
Nutzer: iapuserPasswort: iap0107
Vorlesung „Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik
Dünnschichtanalytik1. Einleitung
- Übersicht über unterschiedliche Techniken
- Vergleich von Auflösung, Empfindlichkeit und Eindringtiefe
- Grundlagen der Vakuumtechnik
2. Ionenbasierte Anregung
- IonenstrahlanalytikRBS, PIXE, NRA, ERDA; Microprobes
- Ionenbasierte Sputter prozesseSIMS, SNMS
3. Elektronenbasierte Anregung
3
- Scanning Electron MicroscopyElektronenangeregt Abbildung, EDX, STEM
- Transmissionselektronmikroskopie (TEM)Abbildung, EELS, EDX, HRTEM, Dunkelfeld
4. Röntgenanregung
- Detektion von Röntgenstrahlung (XRF), Photoelektronen (XPS)
5. Photonen
- Ellipsometrie
- Infrarotspektroskopie
- Raman
- Photolumineszenz
Vorlesung „Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik
Dünnschichtanalytik1. Einleitung
- Übersicht über unterschiedliche Techniken
- Vergleich von Auflösung, Empfindlichkeit und Eindringtiefe
- Grundlagen der Vakuumtechnik
2. Ionenbasierte Anregung
- IonenstrahlanalytikRBS, PIXE, NRA, ERDA; Microprobes
- Ionenbasierte Sputter prozesseSIMS, SNMS
3. Elektronenbasierte Anregung
4
- Scanning Electron MicroscopyElektronenangeregt Abbildung, EDX, STEM, Auger
- Transmissionselektronmikroskopie (TEM)Abbildung, EELS, EDX, HRTEM, Dunkelfeld
4. Röntgenanregung
- Detektion von Röntgenstrahlung (XRF), Photoelektronen (XPS)
5. Photonen
- Ellipsometrie
- Infrarotspektroskopie
- Raman
- Photolumineszenz
Vorlesung „Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik
Scanning probe microscopy
SPM (Scanning probe micrioscopy)
- Atomic force microscopy(AFM)
- Conductive atomic force
5
- Conductive atomic forcemicroscopy (c-AFM)
- Scanning tunnelingmircoscopy (STM
Vorlesung „Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik
Atomic force microscopy (AFM)
V
photo diodemirror
laser
piezo
feedback loop
6Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
tip
sample
Beispiele
7Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
Technical realization
Basic Components
measurement tip
movement of tip in all 3 directions
feedback loop
signal processing
8Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
many components same for STM, AFM, c-AFM, therefore often combined microscopes
...
signal processing
sample environment and sample holder
Cantilever & Tip
material often isolating (e.g. Si3N4)tip radius ≈ 10 .. 30 nm
spring constant of the cantilever: k = 0,005 - 50 N/m(for comparison spring in a ball point pen - 1000 N/mspring in a car - 10.000 N/m)
9Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
spring in a car - 10.000 N/m)
Technical Realization
The tips are sensitive ....
Measurement Tips
10page 10
... and therefore consumeables.
L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.3. Technical Realization
Cantilever & Tip control
11Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
PrinzipDistance-Force-Relation
repulsion
tip
distance
typical length ~ 1 nm
The atomic force microscopy is based on the measurement of the force between the tip and the sample.
12Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
Van-der-Waals bonding forcesdominate
forc
eo
n t
he
tip
distancetip - sample
SurfaceHow a surface looks like under ambient conditions ?
water layer
The surface of materials under ambient conditions has a thin water film on top, particularly, if the surface is hydrophilic.
This is often the case for oxides. In this case OH groups form the outer surface which favors the bonding of water molecules.
13Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
favors the bonding of water molecules.
The thickness of the water film depends on the properties of the surface and the humidity. Typical thicknesses on hydrophilic surfaces are 1.5 ... 3 nm.
Hydrophilic materials (e.g. noble metals) the thickness is reduced to about 1 ... 4 monolayers of water.
atth
etip distance
tip - sample
Contact mode
14page 14L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.2. AFM 2.3.2.2. Physical Processes
forc
e
• constant deflection of the cantilever• sample contact through the water film• force range nN ... µN
Contact mode
advantages • high scan speed • especially suitable for ratherrough samples
• very good spatial resolution
15Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
Contact mode
disadvantages • lateral forces may distort the image• forces rather large due to the capillary forces of the water film• deterioration of the surface possible
16Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
1 µm scan 2 µm scan
forc
eat
the
tip distancetip - sample
Non-Contact mode
17page 17L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.2. AFM 2.3.2.2. Physical Processes
forc
e
• forced vibration of the cantilever close to the resonance frequency
• oscillation amplitude ≤ 10 nm• approach to the surface changes frequency, hence, the amplitude as well
Non-Contact mode
Advantages: • small deterioration of the sample• practically no lateral forces• high spatial resolution
disadvantages • often usable only in HV/UHV
18Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
NaCl surface
• often usable only in HV/UHV• therefore not useable forbiologic sample
forc
eat
the
tip distancetip - sample
Intermittent Mode
19page 19L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.2. AFM 2.3.2.2. Physical Processes
forc
e
• forced vibration of the cantilever withresonance frequency
• oscillation amplitude 20 .. 100 nm• forces in the range ≤ 200 pN• change of the amplitude by approachingthe sample surface
Intermittent Mode
advantages • small deterioration of samples• works in liquids as well • usable for biologic samples
disadvantages• rather strong load of the cantilever• reduced spatial resolution
20Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
• reduced spatial resolution
structure of chromosoms
Influence of the Tip Radius
The tip radius and the tip aspect determines the smallest resolvablesurface features.
21Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
Stiffness Measurements Contact Mode
Special Application
22page 22L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.2. AFM 2.3.2.4. Application Examples
topography force modulation
Special ApplicationFriction Force Microscopy
topography
23Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
topography
friction force
Self-assembly of alkanethiol on gold
STM - Scanning Tunneling Microscopy
nA
Itip
piezo actor
24page 24
L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.1. Basic Principle & Result
Icontrol feedback loop
R
∆I -> ∆V
examples
Copper surface
25Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
Fe on Cu (4K)
Ni (011) surface
7x7 reconstructed Si(111)
Basic principle
The function of the STM needs a description on the basis of quantum physics.
Tip Sample
STM - Scanning Tunneling Microscopy
26page 26
Quantenmechaniktunnel
currentClassical
Mechanics
L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.2. Physical Processes
Parameters Influencing the Tunneling Current
Sample Tip
Ene
rgy
voltage U appliedat the tip (positive)
WF,S WF,T
Basic principle
27page 27L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.2. Physical Processes
Ene
rgy
Distance
at the tip (positive)
Distance d
( )SFT
WdUI,
exp ⋅−⋅∝
Parameters Influencing the Tunneling Current
Sample Tip
Ene
rgy
voltage U appliedat the tip (positive)
Basic principle
28page 28L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.2. Physical Processes
Ene
rgy
Distance
at the tip (positive)
Distance d
Furthermore, the current density is influenced by the density of states (DOS) in the tip and in the sample
Basic principleParameters Influencing the Tunneling Current
29page 29L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.2. Physical Processes
NiAu
For example, the density of electronic states of Au is smaller in comparison to Ni. To keep the current constant the tip has to be approached closer to the surface at the position of the Au atom.
Basic principleSTM Tunneling Current
typical relation between tunneling current and distance sample - tip
Tun
nelin
g C
urre
nt /
nA
Half Atomic Radius
30Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
the exponential correlation results in an extreme sensitivity of the current vs. distance
Distance / nm
Tun
nelin
g C
urre
nt /
nA
Technical RealizationMeasurement Tips
conditions - conducting tip (and sample!)- extreme small tip radius (~ 10 nm) - chemically stable
most common tip material:tungsten tips, produced with
31page 31
tungsten tips, produced with an electrochemical etching process in NaOH
Technical RealizationMovement of the Tip and Feedback Loop
fixed: constant height z fixed: constant current i
32page 32
L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.3. Technical Realization
current i
position x position x
position z
Environmental Conditions
STM only for a small number of materials possible under ambient conditionsexamples: graphite, noble metals
the majority of applications requires UHV
Technical Realization
33page 33L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.3. Technical Realization
Example: surface reconstruction Si(7x7)
Example
34page 34L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.4. Application Examples
ExampleExamples: details of thin film growth
Tb on W(110)(200 x 150 nm2)
blue - H adsorption sitesyellow/green - stacking faults in Tb
35Vorlesung „Festkörperanalytik″
Johannes Heitmann, Institut für Angewandte Physik
yellow/green - stacking faults in Tb
step height: 0.28 nm
AFM•Morpholgy sensetive in nm scale(2D) + Angström resoltution in heightc-AfM•use of conductive tips•local current measurementspossibleSTM
Summary
36page 36L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM
STM •atomic resolution in three dimension on surfaces
• limitation to conducting/metallic samples
• investigation of details of the electron distribution in thematerial
• possibility to manipulate atoms on the surface
48 Fe atoms on Cu (111)
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