AP 5301/8301Instrumental Methods of Analysis

and Laboratory

Final Review

(except XPS and AES)

Prof YU Kin Man

E-mail: kinmanyu@cityu.edu.hk

Tel: 3442-7813

Office: P6422

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2

Teaching and Learning Questionnaire

(TLQ) - Course-end Evaluation,

Semester A, 2016/17

Course-end teaching and learning evaluation

for Semester A, 2016/17 will be carried out from

31 October to 4 December 2016

Prof K. M. Yu

Prof Paul K. Chu

Introduction: Materials CharacterizationThe broad and general process by which a material's structure and properties

are probed and measured. It is a fundamental process in the field of materials

science, without which no scientific understanding of engineering materials could

be ascertained.

The structure of a material is

determined by its chemical

composition and how it was

synthesized (processed)

A material’s properties will

determine what it can be used

for (applications) and the

performance of the final device.

At the CORE of this tetrahedron

is material characterization

Performance is the ultimate end use function of the material and is resulted from

properly tuning properties of materials by optimizing the structure down to the

atomic level through material processing (synthesis).

3

Materials characterization techniquesThere are many different ways to classify material characterization techniques:

─ What information they produce: crystal structure, electronic structure,

electrical conductivity, optical constants, chemical composition, nature of

defects, etc.

─ What probe they use: photons, electrons, ions, neutrons, x-ray, etc.

─ What they measure: ions, electrons, photons, etc.

4

Electron probe X-ray probe

Ion beam probe Optical probe

AP5301/AP83011. Introduction

2. Optical microscopy

3. Electron microscopy:

Scanning electron microscopy (SEM), scanning probe microscopy (SPM)

4. Electron microscopy:

Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), electron probe microanalysis (EPM)

5. X-ray diffraction

6. Electrical measurements:

Four point probe, Hall effect, Capacitance-voltage profiling, thermoelectric effect, minority carrier lifetime

7. Optical spectroscopy:

Spectrophotometry, Spectroscopic ellipsometry (SE), Photoluminescence (PL), Modulation spectroscopy

8. Secondary ion mass spectrometry (SIMS)

9. Auger electron spectroscopy (AES)

10. X-ray photoelectron spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA)

11. Ion beam analysis:

Rutherford backscattering spectrometry (RBS), hydrogen forward scattering (HFS), particle induced x-ray emission (PIXE)

5

6

SEM

TEM

SPM

XPS

AES

SIMS

RBS

EDS,WDS

EELSSTEM

diffraction

XRD

SAD

CBED

Spectro-

photometry

PL

SE

PR

4-pt

probe

Hall

CV/ECV

lifetime

OM

Microstructure

Crystalline defects

Mapping

Surface morphology

Elemental

composition

Depth profiles

Elemental mapping

Chemical bonding

Crystal structure

Chemical

compositionOptical properties

Band gap

Crystalline defects

Resistivity

Carrier conc.

Mobility

Minority carrier

lifetime

Dopant profiles

Chemical

analysis

AP5301/AP8301

Materials characterization: acronyms7

Microscopy

OM optical microscopy

NA numerical aperture

BF bright field

DF dark field

SCOM scanning confocal optical

microscopy

SEM scanning electron microscopy

SE secondary electron

BES backscattered electron

SPM scanning probe microscopy

STM scanning tunneling

microscopy

AFM atomic force microscopy

SNOM scanning near field optical

microscopy

TEM transmission electron

microscopy

SAD/ selected area electron

SAED diffraction

CBED convergent beam electron

diffraction

STEM scanning transmission

electron microscopy

EDS/ energy dispersive x-ray

EDX spectroscopy

X-ray techniques

WDS wavelength dispersive x-

ray spectroscopy

ADF annular dark field

imaging

HAADF high angle annular dark

field

EPMA electron probe

microanalysis

EELS electron energy loss

spectroscopy

XRF x-ray fluorescence

XRD x-ray diffraction

HRXRD high resolution x ray

diffraction

XAS x-ray absorption

XANES x-ray absorption near

edge structure

EXAFS extended x-ray

absorption fine structure

Electrical measurements

SRP spreading resistance

profiling

CV capacitance voltage

ECV electrochemical CV

Optical spectroscopy

SE spectroscopic ellipsometry

PL photoluminescence

PR photoreflectance

ER electroreflectance

TR thermoreflectance

Chemical analysis

SIMS secondary ion mass

spectrometry

ToF-SIMS time of flight SIMS

RBS Rutherford backscattering

spectrometry

HFS hydrogen forward

scattering

PIXE particle induced x-ray

emission

AES Auger electron

spectroscopy

SAM scanning Auger

microscopy

XPS x-ray photoelectron

spectroscopy

ARPES angle resolved

photoemission

spectroscopy

Optical vs. electron microscopy8

Microstructure of steel D2

(Metal Ravne Steel Selector)

Glass ceramic transmission microscope image

made with polarized light and full wave plate

Exfoliated molybdenum

disulfide on a perforated gridOptical Microscope

Electron Microscope

Atomic resolution TEM image of

nanocrystalline palladium. H. Rösner and C.

Kübel et al., Acta Mat., 2011, 59, 7380-7387.

Cross-section TEM image of MOCVD

grown InGaAs/GaAs quantum dot

superlattice solar cell (NREL)SEM micrographs of SMNb0.05%

Mat. Res. vol.6 no.2 São Carlos

Apr./June 2003.

Scanning electron microscopy (SEM) image

of as-grown p-type gallium nitride (p-GaN)

nanowire arrays on a silicon (111) substrate

Optical microscope9

Uses visible light as the

illumination source,

Has lateral resolution down to

0.1m

Used for almost all solids and

liquid crystals

Typically nondestructive

Mainly used for preliminary

direct visual observation

Microstructural features

observed: grains, precipitates,

inclusions, pores, whiskers,

defects, twin boundaries, etc.

Bright field (BF) and dark field (DF) imagingBright field (BF): using the full illumination of the light source

Dark field (DF): illuminating the sample with peripheral light by blocking the axial

rays, producing a dark, almost black, background with bright objects on it

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DF images are visually impressive but may be very sensitive to dirt and dust

located in the light path. The intensity of the illumination system must be high

Advanced optical microscopy11

Polarized light microscopy involves illumination of the

sample with polarized light for specimens that are visible

primarily due to their optically anisotropic

Phase contrast microscopy uses a special condenser

and objective lenses to convert phase differences (not

visible) into amplitude differences (visible)

Differential interference contrast (DIC) microscopy

enhances contrast by creating artificial shadows (pseudo

three-dimensional) using polarized light as if the object is

illuminated from the side

A fluorescence microscope uses fluorescence to

generate an image

─ Only allows observation of the specific structures

which have been labeled for fluorescence

Confocal microscopy adds a spatial pinhole at the

confocal plane to increase optical resolution and contrast

Scanning confocal optical microscopy (SCOM) is a

technique for obtaining high-resolution optical images with

depth selectivity. (a laser beam is used)

Blue-green Algae

Phase contrast DIC

Optical and electron microscopies: a comparison

Optical

microscopeTransmission electron

microscope (TEM)

Magneticlenses

detector

Light source Source of electrons

Condenser

Specimen

Objective

Eyepiece

Projector Specimen

Probe

scanning electron

microscope (TEM)

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Electron beam microscopy/spectroscopy13

(1-50keV)

The types of signals produced by a SEM include secondary electrons (SEs),

back-scattered electrons (BSEs), characteristic X-rays and photons

(cathodoluminescence) (CL), absorbed current (specimen current) and

transmitted electrons. Both SEs and BSEs are used for imaging

Scanning electron microscopy

SEM has a large depth of field, producing an

image that is a good representation of the

three-dimensional sample

SEM produces images of high resolution at a

high magnification.

SEM usually also equipped with analytical

capability: electron probe microanalysis

(energy dispersive x-ray analysis).

The higher magnification, larger depth of field,

greater resolution and compositional and

crystallographic information makes the SEM

one of the most useful instruments in various

fields of research.

Magnification Depth of Field Resolution

OM 4 – 1000x 15.5 – 0.19m ~ 0.2m

SEM 10 – 3000000x 4mm – 0.4m 1-10nm

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Secondary (SE) & backscattered (BSE) electrons15

SEs are low energy electrons (<50eV) produced

by inelastic interactions of high energy electrons

with core electrons

SE yield: 𝛿 = 𝑛𝑆𝐸/𝑛𝐵 >1 independent of Z but

depends on the angle of incidence. This gives rise

to topographic contrast of the specimen

Due to their low energy, only SE that are very

near the surface (<10nm) can exit the sample

and be examined (small escape depth).

Bright

Dark

BSE are produced by elastic

interactions (scatterings) of

electrons with nuclei of atoms and

they have high energy and large

escape depth

BSE yield: 𝜂 = 𝑛𝐵𝑆/𝑛𝐵 ~ increases

with atomic number, Z and thus can

be used to obtain images with

atomic number contrast

SEM magnification & resolution 16

Magnification in an SEM can be

controlled over a range of about 6 orders

of magnitude from about 10 to 500,000x.

Magnification results from the ratio of

the dimensions of the raster on the

specimen and the raster on the

display device,

Magnification=area scanned on the

monitor/area scanned on the specimen

The spatial resolution of the SEM

depends on

─ the size of the electron spot, which in turn

depends on both the wavelength of the

electrons and the electron-optical system

that produces the scanning beam.

─ the size of the interaction volume.

The resolution can fall somewhere

between less than 1 nm and 20 nm.

Low M; Large x 40m

High M; small x 7m

2500X15000x

Scanning probe microscopy (SPM)17

Scanning probe microscopy (SPM) is a branch of microscopy that forms images

of surfaces using a physical probe that scans the specimen.

Scanning tunneling

microscopy (STM): uses an

atomically sharp metallic tip

and records the minute

tunneling current (I) between

the tip and the surface. It maps

out the sample topography and

electrical properties.

Atomic force microscopy

(AFM): a cantilever with a sharp

tip is scanned over the surface.

Using the van der Waals

forces or contact forces

between a tip and the sample,

the sample topography or

mechanical properties can be

measured.

constant current mode constant height mode

STM: modes of operation18

constant height modeconstant current mode

STMs use feedback to keep the

tunneling current constant by

adjusting the height of the scanner at

each measurement point

the voltage applied to the

piezoelectric scanner is adjusted to

increase/decrease the distance

between the tip and the sample

The image is then formed by plotting

the tip height vs. the lateral tip

position.

Tunneling current is monitored as

the tip is scanned parallel to the

surface.

There is a periodic variation in the

separation distance between the tip

and surface atoms.

A plot of the tunneling current vs.

tip position shows a periodic

variation which matches that of the

surface structure-a direct "image"

of the surface.

STM vs. AFM19

STM AFM

Measures local electron density of states,

not nuclear positions─not true topographic

imaging

Real topographic imaging

High lateral and vertical resolution –because

of the exponential dependence of the

tunneling current on distance

Lower lateral resolution

Exponential dependence between tunneling

current and distance

The force-distance dependence in AFM is

much more complex

Probe electronic properties (LDOS –

including spin states)

Probe various physical properties: magnetic,

electrostatic, hydrophobicity, friction, elastic

modulus, etc

Generally applicable only to conducting

(and semiconducting) samples

Applied to both conductors and insulators

Writing voltage and tip-to-substrate spacing

are integrally linked

Writing voltage and tip-to-substrate spacing

can be controlled independently

Control brightness,

convergence

Control contrast

A disc of metal

Transmission electron microscopy20

TEM operation21

TEM offers two methods of specimen observation, diffraction mode and image

mode. The objective lens forms a diffraction pattern in the back focal plane with

electrons scattered by the sample and combines them to generate an image in the

image plane.

Whether the diffraction pattern or the

image appears on the viewing screen

depends on the strength of the

intermediate lens.

The image mode produces an image of

the illuminated sample area

In image mode, the post-specimen lenses

are set to examine the information in the

transmitted signal at the image plane of

the objective lens.

There are three primary image modes

that are used in conventional TEM work,

bright-field microscopy, dark-field

microscopy, and high-resolution electron

microscopy.

Polycrystalline materials

The electron diffraction pattern is a set of rings, with some spots depending on

the crystallite sizes.

Nano to Amorphous materials

As the crystal size get smaller (nm) the

rings get more diffuse and eventually

become halo-like when the material

becomes amorphous

TEM: diffraction pattern

Al single crystalPolycrystalline Pt

silicide (PtSi)

Silicon with epitaxial nickel

silicides ( Si - NiSi - NiSi2)

Polycrystalline nickel mono

silicide (NiSi) on top of

single crystalline silicon

(Si).

Amorphous GaNAsnanocrystalline GaNAs

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Advantages

TEMs offer very powerful magnification and resolution.

TEMs have a wide-range of applications and can be utilized in a variety of

different scientific, educational and industrial fields

TEMs provide information on element and compound structure.

Images are high-quality and detailed.

Chemical information with analytical attachments

Disadvantages

TEMs are large and very expensive (USD 300K to >1M)

Laborious sample preparation.

Operation and analysis requires special training.

Samples are limited to small size (mm) and must be electron transparent.

TEMs require special housing and maintenance.

Images are black and white .

TEM: advantages and disadvantages23

Comparison: SEM and TEM

TEM SEM

Electron beam Broad, static beam Beam focused to fine point and

scan over specimen

Electron path passes through thin specimen. scans over surface of specimen

Specimens Specially prepared thin

specimens supported on TEM

grids.

Sample can be any thickness and is

mounted on an aluminum stub.

Specimen stage Located halfway down column. At the bottom of the column.

Image formation Transmitted electrons collectively

focused by the objective lens and

magnified to create a real image

Beam is scanned along the surface

of the specimen to build up the

image

Image display On fluorescent screen. On TV monitor.

Image nature Image is a two dimensional

projection of the sample.

Image is of the surface of the

sample

Magnification Up to 5,000,000x ~250,000x

Resolution ~0.2 nm ~2-5 nm

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Scanning Transmission Electron Microscopy (STEM)

The rastering of the beam across the

sample makes these microscopes

suitable for analysis techniques such

as mapping by

energy dispersive X-ray (EDX)

spectroscopy

electron energy loss

spectroscopy (EELS)

annular dark-field imaging

(ADF).

By using a high-angle detector

(high angle annular dark-field

HAADF), atomic resolution

images where the contrast is

directly related to the atomic

number (z-contrast image) can be

formed. SAED =0.26o or ~6.4 mrads

I Z2

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X-rays

EDX detector

luminescence

In a STEM the electron beam is focused into a narrow spot which is scanned

over the sample in a rastering mode.

Electron probe microanalysis (EPMA)26

Energy-dispersive X-ray spectroscopy

(EDS, EDX, or XEDS) is an analytical

technique used for the elemental

analysis. In a SEM or STEM, the

incident electrons are used as the

excitation source creating characteristic

x-rays from different elements in the

target.

Electron energy loss spectroscopy

(EELS): Electrons lose energy through

inner-shell ionizations are useful for

detecting the elemental components of

a material

─ the detailed shape of the spectral

profiles gives information on the

electronic structure, chemical

bonding, and nearest neighbor

distances for each atomic species.

─ Quantitative elemental concentration

for the element 3≤𝑍≤35

EPMA: WDS vs EDS27

WDS EDS

Spectra acceptance One element/run Entire spectrum in one shot

Collection time > 10 mins Mins

Sensitive elements Better for lighter elements

(Be, B, C, N, O)

Resolution ~few eV ~130 eV

Probe size ~200 nm ~5 nm

Max count rate ~50000 cps <2000 cps

Detection limits 100 ppm 10000 ppm

Spectral artifacts rare Peak overlap

EPMA: EELS vs EDS28

EELS EDS

Energy resolution ~0.1 eV ~130 eV

Energy range 0-3000 eV 1-50 keV

Element range Better for light elements Better for heavy

elements

Ease of use Medium high

Spatial resolution Good beam broadening

Information Elemental, coordination,

bonding

Only elemental

Quantification Easy Easy

Peak overlap No Can be severe

X-ray powder diffraction29

X-ray powder diffraction (XRD) is a rapid analytical

technique primarily used for phase identification

of a crystalline material and can provide

information on unit cell dimensions.

The sample holder and the x-ray detector are

mechanically linked: the detector turns 2 when

the sample holder turns so that the detector is

always ready to detect the Bragg diffracted

beam 2d sin θ = 𝑛𝜆

Bragg-Brentano geometry

Applications:

Phase Composition of a Sample

Unit cell lattice parameters and Bravais lattice symmetry

Residual Strain (macrostrain)

Epitaxy/Texture/Orientation

Crystallite Size and Microstrain

XRD: alloy composition analysis30

ZnO is alloyed with ZnS to form

ZnO1-xSx alloy

Wurtzite ZnO (c=0.52 nm) and ZnS

(c=0.626 nm)

As x increases (more S substituting

in O sublattice), the lattice

parameter increases

Bragg law: 𝜆 = 2𝑑 sin 𝜃 increasing 𝑑means decreasing 𝜃.

Vegard's law: lattice parameter of a

solid solution of two constituents is

approximately equal to a rule of

mixtures of the two constituents'

lattice parameters

𝑐𝑍𝑛𝑂1−𝑥𝑆𝑥 = 𝑥𝑐𝑍𝑛𝑆 + (1 − 𝑥)𝑐𝑍𝑛𝑂

Composition 𝑥 can be derived from

the measured lattice parameter 𝑐.

Increasing x

(0002) Diffraction peaks of ZnO1-xSx alloy

XRD: crystallite size31

Crystallites smaller than ~120nm create broadening of diffraction peaks

this peak broadening can be used to quantify the average crystallite size of

nanoparticles using the Scherrer equation

─ contributions due to instrument broadening should be known by using

a standard sample (e.g. a single crystal)

Scherrer equation:

𝐵 2𝜃 =𝐾𝜆

𝐿 cos 𝜃

where B is the 2𝜃 FWHM

peak broadening in radian,

𝜆 is the wavelength of the x-

ray used, L is the grain size

and K~0.9

XRD: lattice strain32

No Strain

do

2

Uniform Strain: (d1-do)/do

d1

d strain

2Peak moves, no shape changes

Non-uniform StrainPeak broadens

Broadeing 2 2 tan

d

bd

2

d1constant

XRD: preferred orientation

Preferred orientation of crystallites can create a variation in diffraction peak

intensities that can be

qualitatively analyzed using a 1D diffraction pattern (powder pattern)

quantitatively analyzed by a pole figure which maps the intensity of a single

peak as a function of tilt and rotation of the sample

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Resistivity: the four point probe34

The four point probe is commonly used to

determine the resistivity of semiconductor

samples (wafers)

The outer 2 probes are connected to a current

source

The two inner probes are high impedance

voltage sensors

The sample thickness 𝛿 is assumed to be

constant

Hall Effect35

The Hall coefficient for electrons is

𝑅𝐻 =ℰ𝐻,𝑒

𝐽𝑥𝐵= −

1

𝑛𝑒(negative)

For hole carriers: 𝑅𝐻 =ℰ𝐻,ℎ

𝐽𝑥𝐵=

1

𝑝𝑒(positive)

The Hall effect describes the behavior of

free carriers in a semiconductor when an

electric and a magnetic field are applied.

The van der Pauw method is a technique

commonly used to measure the resistivity and

the Hall coefficient of a sample of any arbitrary

shape

The free carrier concentration 𝑛 =1

𝑒𝑅𝐻is given by: 𝑛 =

1

𝛿

𝐵

𝑒∆𝑅24,13

With known resistivity and carrier concentration, the mobility is given by:

𝜇 =1

𝑛𝑒𝜌=2ln(2)

𝜋𝐵

∆𝑅24,13𝑅12,34 + 𝑅23,41

1

𝑓

Variable Temperature Hall Effect

At high temperature: Intrinsic

─ 𝑛𝑖 ∝ 𝑒−𝐸𝑔/2𝑘𝐵𝑇

─ slope=−𝐸𝑔

2𝑘𝐵𝑇

Medium Temperature: 𝑘𝐵𝑇 ≫ 𝐸𝑑─ Extrinsic or saturation regime

─ 𝑛 = 𝑁𝑑 − 𝑁𝑎

Low temperature: freeze out region

─ 𝑛 ∝ 𝑒−𝐸𝑑/2𝑘𝐵𝑇

─ slope=−𝐸𝑑

2𝑘𝐵𝑇(half slope)

─ At even lower temperature

𝑛 ∝ 𝑒−𝐸𝑑/𝑘𝐵𝑇 (full slope)

─ The concentration at which the half slope turns into full slop corresponds to 𝑁𝑎

For a semiconductor sample with both donors and acceptors 𝑁𝑑 𝑎𝑛𝑑 𝑁𝑎. A

variable temperature Hall effect measurement plotted as ln 𝑛 𝑣𝑠.1

𝑇─Arrhenius

plot can tell us a lot of information

T

Arrhenius Plot

Half slope

Full slope

Variable temperature Hall effect measurements provide a convenient way to obtain donor or acceptor binding energies.

36

Electrical measurements: a comparison37

4 point probe Hall effect CV ECV thermopower

Information

obtained

Resistivity Free carrier

concentration,

mobility

Net ionized dopant

concentration

Net ionized dopant

concentration

Seebeck coefficient

Conduction

type (n or p)

No Yes Yes Yes Yes

Sample size From mm to wafer 0.5 to 2 cm square 0.5 to 2 cm square >0.5 cm >0.5 cm

Depth

profiling

No No Yes Yes Mo

Destructive Somewhat No Need to form a

Schottky contact

Yes No

Equipment

cost

Low Low to high

(from <20kUSD to

>200k USD)

Low High

(>150k USD)

Low to medium

SpectrophotometrySpectrophotometry is the quantitative

measurement of the reflection or

transmission properties of a material as a

function of wavelength in the spectral

range of visible light (Vis), near-ultraviolet

(UV), and near-infrared (NIR). It is more

commonly called UV-Vis-NIR

spectroscopy.

38

Transmission and reflection data are

combined to find the absorption coefficient

𝛼 𝜆

𝐼𝑇(𝜆) = (𝐼𝑜 𝜆 − 𝐼𝑅(𝜆))𝑒−𝛼(𝜆)𝑥

𝛼 𝜆 =1

𝑥ln

𝐼𝑜 𝜆 − 𝐼𝑅(𝜆)

𝐼𝑇(𝜆)

From the absorption coefficient of a solid (thin

film), the electronic properties can be derived.

𝑥

𝐼𝑜

𝐼𝑅

𝐼𝑇

Optical Absorption Measurement𝑥

𝐼𝑜

𝐼𝑅

𝐼𝑇

ZnO thin film

𝛼 𝜆 =1

𝑥ln

𝐼𝑜 𝜆 − 𝐼𝑅(𝜆)

𝐼𝑇(𝜆)

𝛼hν 1/𝑛 = 𝐴 hν − 𝐸𝑔To determine if the material has a direct

or indirect gap

plot 𝛼hν 1/𝑛vs. hν Notice that we can linearly

extrapolate 𝛼hν 2vs. hν (𝑛 = 1/2) to

obtain a band gap 𝐸𝑔 = 3.3 𝑒𝑉

𝛼hν 1/2vs. hν (𝑛 = 2) does not result

in a straight line

ZnO is a direct gap material.

39

Spectroscopic ellipsometry (SE)

The measured signal is the change in polarization as the incident radiation (in a known state) interacts with the material structure of interest (reflected, absorbed, scattered, or transmitted).

The polarization change is quantified by the amplitude ratio, 𝚿, and the phase difference, 𝚫.

𝜌 =𝑅𝑝

𝑅𝑠= 𝑡𝑎𝑛Ψ𝑒𝑖Δ = 𝑓(𝒏𝒊, 𝒌𝒊, 𝒕𝒊)

𝑤ℎ𝑒𝑟𝑒𝑅𝑝

𝑅𝑠is the Fresnel reflection coefficient

𝑅𝑝 =𝐸𝑝(𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑)

𝐸𝑝(𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡); 𝑅𝑠 =

𝐸𝑠(𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑)

𝐸𝑠(𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡)

40

The sample has a

layered structure and

each layer 𝑖 has optical

constants (𝑛𝑖 , 𝑘𝑖) and a

thickness 𝑡𝑖 .

SE: advantages and limitations

Non-destructive technique

Film thickness measurement, can measure down to <1 nm

Can measure optical constants 𝑛 and 𝑘 for unknown materials

─ Absorption coefficient, band gap, carrier concentration, mobility, effective

mass, etc.

Can also measure film composition, porosity and roughness

Absolute measurement: do not need any reference.

Rapid measurement: get the full spectrum (190nm up1700nm) in few seconds

Can be used for in-situ analysis

Small equipment footprint: do not require a lot of lab space

Can only measure flat, parallel and reflecting surfaces

Some knowledge of the sample is required: number of layers, type of layers, etc.

SE is an indirect measurement: does not give directly the physical parameters

A realistic physical model of the sample is usually required to obtain useful

information

41

Photoluminescence42

Spectral feature Material parameter

Peak energy Compound identification

Band gap/electronic levels

Impurity or exciton binding energy

Quantum well width

Impurity species

Alloy composition

Internal strain

Fermi energy

Peak width Structural and chemical quality

Quantum well interface roughness

Carrier or doping density

Peak intensity Relative quantity

Polymer conformation

Relative efficiency

Surface damage

Excited state lifetime

Impurity or defect concentration

Photoreflectance: principles Changes in reflectivity

∆𝑅

𝑅can be related to the perturbation of the dielectric

function of the material, 휀 = 휀1 + 𝑖휀2:

∆𝑅

𝑅= 𝛼 휀1, 휀2 ∆휀1 + 𝛽 휀1, 휀2 ∆휀2

where 𝛼 and 𝛽 are the Seraphin coefficients, ∆휀1 and ∆휀2 represent

photo-induced changes of the real and imaginary parts of the dielectric

function, respectively.

43

The imaginary part 휀2 changes

slightly in electric field, resulting

in a sharp resonance ∆휀2 exactly

at the energy of the optical

transition.

It can be shown that in a case of

the bulk crystal, the shape of

dielectric function changes is of

the third derivative of the

unperturbed dielectric function.

Secondary ion mass spectrometry (SIMS)

SIMS is generally used for surface, bulk,

microanalysis, depth profiling, and impurity

analysis. The technique involves bombarding

the surface of a sample with a beam of ions,

thus emitting secondary ions. These ions are

later measured with a mass spectrometer to

determine either the elemental or isotopic

composition of the surface of the sample.

44

negative, positive, and neutral

charges with kinetic energies ranging

from zero to a few hundred eV.Cs+, O2+, Ar+ and Ga+

at energies ~ 1-30 keV

Sputtered species:

Monatomic and

polyatomic particles of

sample material (+ve,

-ve or neutral)

Re-sputtered primary

species (+ve, -ve or

neutral)

Electrons

photons

Secondary ion yields: primary ion beams

Oxygen works as a medium which strips off electrons from the speeding

sputtered atoms when they leave surface, while Cesium prefers to load an

electron on the sputtered atoms.

𝑶𝟐+ ions beam:

─ During secondary emission the Me-O

bonds break thus generating 𝑀𝑒𝑛+

𝑪𝒔+ ions beam:

─ Increased availability of electrons

leads to increased negative ion

formation especially for elements with

high electron affinity.

45

Secondary ion yield depends critically on the primary ion beam species. Typically

𝐶𝑠+and 𝑂2+ ion beams are used in SIMS measurements.

Selection of primary ions:

Inert gas (Ar, Xe, etc.)

─ Minimize chemical modification

Oxygen

─ Enhance positive ions

Cesium

─ Enhance negative ions

Liquid metal (Ga)

─ Small spot for enhanced

lateral resolution

SIMS: secondary ion yield46

Secondary ion current of species 𝑚

𝐼𝑠𝑚 = 𝐼𝑝𝑦𝑚𝛼

+𝜃𝑚𝜂

𝐼𝑝 = primary particle flux

𝑦𝑚 = sputter yield

𝛼+ = ionization probability to positive ions

𝜃𝑚 = fractional concentration of m in the layer

𝜂 = transmission of the analysis systemIon yield is influenced by

─ Matrix effects

─ Surface coverage of reactive elements

─ Background pressure

─ Orientation of crystallographic axes with respect to the sample surface

─ Angle of emission of detected secondary ions

The number of secondary particles (atoms/ions) emitted by the surface for each

impinging primary ion is defined as sputtering yield and can range between 5

and 15. The fraction of ionized emitted particles is called secondary ion yield

and ranges typically between 10-4 to 10-6.

In SIMS, it is the secondary ions that are eventually detected

SIMS: modes of operation47

Static SIMS: 0.1-10 keV ions are

employed, with current surface

densities in the nA/cm2 range,

Under these conditions the total

erosion of the sample first

monolayer (1 nm) may take even

an hour.

Dynamic-SIMS: 10-30 keV ions,

with current surface densities in

the A-mA/cm2 range, are used.

Under these conditions the sample

is eroded continuously and the

acquired mass spectra enable the

monitoring of constituting elements

along the sample depth (depth

profiling).

According to the primary ion energy and current, the SIMS technique can be

divided into two variants:

Profiling

Material removal

Elemental analysis

Ultra surface

analysis

Elemental or

molecular analysis

Analysis completed

before significant

fraction of molecules

destroyed

Static SIMS48

Positive ion TOF mass spectrum of polydimethylsiloxane

contaminated polyethylene terephthalateSilicon wafer contaminated with copper, iron and chromium

Range of elements H to U: all isotopes

Destructive Yes, if sputtered long enough

Chemical bonding Yes

Depth probed Outer 1 to 2 monolayers

Lateral resolution Down to below 100 nm

Imaging/mapping Yes

Quantification Possible with suitable standard

Mass range Typically up to 1000 amu, 10000 amu (ToF)

Main application Surface chemical analysis, organics, polymers

Dynamic SIMS49

Range of elements H to U: all isotopes

Destructive Yes, material removed during sputtering

Chemical bonding In rare cases only

Depth probed Depth resolution 2-30 nm, probe into m below surface

Quantification Standard needed

Accuracy 2%

Detection limits 1012-1016 atoms/cm3 (ppb-ppm)

Imaging/mapping Yes

Sample requirements Solid; vacuum compatible

Near surface B depth profiles

from a 2.2 keV BF implant in

Si using different energies O2+

primary beam

Comparison: static and dynamic SIMS50

SIMS can be used to determine the composition of organic and inorganic

solids at the outer 5 nm of a sample.

Can generate spatial or depth profiles of elemental or molecular

concentrations.

These profiles can be used for elemental mapping.

To detect impurities or trace elements, especially in semiconductors.

Secondary ion images have resolution on the order of 0.5 to 5 μm.

Detection limits for trace elements range between 1012 to 1016 atoms/cc.

Spatial resolution is determined by primary ion beam widths, which can be as

small as 100 nm.

SIMS: summary

SIMS is the most sensitive elemental and isotopic surface

microanalysis technique (bulk concentrations of impurities of around

1 part-per-billion). However, very expensive.

51

Advantages and weaknesses of SIMS

Advantages Weaknesses

Excellent sensitivity, especially for

light elements

Destructive method

High surface sensitivity Element specific selectivity

Depth profiling with excellent depth

resolution (nm) (dynamic)

Standards needed for quantification

Good spatial resolution (<1-25 m) Sample must be vacuum compatible

Small analyzed volume (down to

0.3m3) so little sample is needed

Sample mist have a flat surface

Information about the chemical

surface composition due to ion

molecules (static)

High equipment cost (>1M-3M USD)

Elements from H to U can be

detected with excellent mass

resolution

52

SIMS and other techniques

* = yes, with compensation for the effects of sample charging

Characteristic AES XPS SEM/EDS SIMS

Elemental range Li and higher Z Li and higher Z Na and higher Z All Z

Specificity Good Good Good Good

Quantification With calibration With calibration With calibration Correction

necessary

Detection limits

(atomic fraction)

10-2 to 10-3 10-2 to 10-3 10-3 to 10-8 10-3 to 10-8

Mass resolution element element element <isotope

Lateral resolution (m) 0.05 ~1000 0.05 1

Depth resolution (nm) 0.3-2.5 1-3 1000-50000 0.3-2

Organic samples No Yes Yes* Yes

Insulator samples Yes* Yes Yes* Yes*

Structural information Elemental Elemental and

Chemical

Elemental Elemental and

Chemical

Destructiveness Low

High (profiling)

Very Low

High (profiling)

Medium High(dynamic)

Medium(static)

Ion Beam Analysis: an overview

Incident Ions

Absorber foil

Elastic recoil detection

Rutherford backscattering

(RBS), resonant scattering,

channeling

Particle induced x-ray

emission (PIXE)

Nuclear Reaction Analysis

(NRA) p, , n, g

Ion Beam

Modification

Defects generation

Inelastic Elastic

54

RBS: basic concepts

Kinematic factor: elastic energy transfer from a projectile to a target

atom can be calculated from collision kinematics

mass determination

Scattering cross-section: the probability of the elastic collision

between the projectile and target atoms can be calculated

quantitative analysis of atomic composition

Energy Loss: inelastic energy loss of the projectile ions through the

target

perception of depth

These allow RBS analysis to give quantitative depth distribution of

targets with different masses

55

Strengths & weaknesses of RBS Simple in principle

Fast and direct

Quantitative without standard

Depth profiling without chemical or physical sectioning

Non-destructive

Wide range of elemental coverage

No special specimen preparation required

Can be applied to crystalline or amorphous materials

Simultaneous analysis with several ion beam techniques Poor lateral

resolution (~0.5-1mm)

Moderate depth resolution (>50Å)

No microstructural information

No phase identification

Poor mass resolution for target mass heavier than 70amu (PIXE)

Detection of light impurities more difficult

Data may not be obvious: require knowledge of the technique

56

Hydrogen Forward Scattering (HFS)

Charles Evans and Assoc., RBS APPLICATION SERIES NO. 3

Quantitative hydrogen and deuterium profiling

Good sensitivity (~0.01at% of H)

Can be perform simultaneously with RBS and PIXE

Profiling with any light element in solid (using heavy ion beam, ERD)

Generally known as Elastic Recoil Detection (ERD)

57

Particle Induced X-ray Emission (PIXE)

Ion Channeling

Kobelco Steel Group

59

Channeling: Impurity Lattice Location

0 0.2 0.6 1.0Distance from atom row, r/ro

3

2

1

Flu

x

0.2

0.4

0.6

0.8y/y1

=1.0

0.0

Channel center

ro

r

Atomic row

Strengths of Ion Beam Analysis Techniques

Simple in principle

Fast and direct

Quantitative (without standard for RBS)

Depth profiling without chemical or physical sectioning

Non-destructive

Wide range of elemental coverage

No special specimen preparation required

Can be applied to crystalline or amorphous materials

Simultaneous analysis with various ion beam

techniques (RBS, PIXE, NRA, channeling, etc.)

61

Elemental detection techniques62

Factors EDS SIMS RBS AES XPS

Probe Electrons Ions (O, Cs) Ions (He) Electrons X-ray

Measured species X-rays Sputtered ions Scattered ions Electrons Electrons

Vacuum (Torr) <10-6 10-6-10-10 <10-5 10-6-10-10 10-6-10-10

Acquisition time minutes Seconds Minutes Seconds Minutes

Depth profiling No Yes (dynamic

SIMS)

Yes Yes, with

sectioning

Yes with

sectioning

Detection limits

(atomic fraction)

~10-3 10-3 to 10-8 10-3 to 10-6 10-2 to 10-3 10-2 to 10-3

Lateral resolution

(mm)

0.05 1 500-1000 0.05 ~1000

Elemental range Z>6 All Z Z>6 Z>3 Z>3