Surface Characterization Technique by LEED & RHEED

Pre p a re d b y

A n s h u m a n S a r k a r 11 3 0 5 0 0 6

U n d e r t h e C o u r s e I n s t r u c t o rD r. K . S u re s h B a b u

Assistant ProfessorCENTRE FOR NANO SCIENCE & TECHNOLOGY

PONDICHERRY UNIVERSITY

OverviewWhat is Electron Diffraction LEED & RHEED

Comparison of LEED & RHEED ConclusionRefferences

LEED

History

Basic Principle

Instrumentation

How to Use

Examples

Complication in LEED

Electron Diffraction

• The phenomenon associated with the interference processes which occur when electrons are scattered by atoms in crystals to form diffraction patterns.

• Types of Electron Diffraction:-

Electron Diffraction

Electron diffraction

Low Energy Electron Diffraction

LEED

Reflection High Energy Electron Diffraction

RHEED

Transmission High Energy Electron Diffraction

THEED

Introduction to Low energy electron diffraction-LEED

Low energy electron diffraction mainly used for surface characterization

• LEED = Low Energy Electron Diffraction

– Incoming electron beam (< 500 eV) is perpendicular to sample

Introduction to Reflection High energy electron diffraction-RHEED

• RHEED = Reflection High Energy Electron Diffraction

– Incoming electron beam (10~50keV) has glazing angle to sample

RHEED is an important method to achieve surface sensitivity

LOW ENERGY ELECTRON DIFFRACTIONLEED

What is LEED• Low-energy electron

diffraction, is a technique for the determination of the surface structure of crystalline materials by bombardment with low energy electrons (20-500eV) and observation of diffracted electrons as spots on a fluorescent screen.

• The pattern of spots contains information of surface structure and the spot intensity indicates reconstruction.

Fig: Schematic of a LEED

History• Low Energy Electron Diffraction (LEED) = e– in, e– out

(elastic)

• 1924: Discovered accidentally by Davisson and Kunsman during study of electron emission from a Ni crystal.

• 1927: Davisson and Germer found diffraction maxima for:– nl = D sinf where D = surface spacing, l = electron wavelength

• 1934: Fluorescent screen developed by Ehrenburg for data imaging.

• 1960: UHV technology enabled LEED of clean surfaces.

Instrumentation of LEED• A sample holder with the prepared

sample• An electron gun• A display system, usually a

hemispherical fluorescent screen on which the diffraction pattern can be observed directly

• A sputtering gun for cleaning the surface

• A number of highly transparent grids are placed in front of the screen.

Grid 1: retarding voltage

(selects only elastic electrons)

Grid 2: accelerating voltage(creates fluorescence on screen)

Information obtained from LEED• LEED is the principal technique for the determination of

surface structures. • It may be used in one of two ways: Qualitatively : where the diffraction pattern is recorded and

analysis of the spot positions yields information on the size, symmetry and rotational alignment of the adsorbate unit cell with respect to the substrate unit cell.

Quantitatively : where the intensities of the various diffracted beams are recorded as a function of the incident electron beam energy to generate so-called I-V curves which, by comparison with theoretical curves, may provide accurate information on atomic positions.

Structural Information by LEED

Diffraction Techniques

What is Real Space and Reciprocal Space?

Phys 661 - Baski

Reciprocal-Space (i.e. spacing of diffraction spots in nm–1)

Real Space (i.e. spacing of surface atoms in nm)

larger real-space smaller reciprocal-space

a

2G

a

Only Substrate

Substrate +Adsorbate

Substrate & over layer LEED pattern

Intensity Measurement by LEED

Structure determination procedureLEED pattern reflects the size and shape of the real space unit cell. But it says nothing about the positions of the atoms in the real space unit cell i.e the structure --------

This is why surface structure determination by LEED requires the measurement and analysis of intensities.

STEPS

Images obtained in LEED

Complications and other aspects of LEED

• Electron beam damage – sensitive molecular adsorbates.

• Domain structure If two domains with different structure coexist easy to ⇒

distinguish.But sometimes difficulties exist (e.g., 3 domains of p(2x1)

on fcc (111) = (2x2)

REFLECTION HIGH ENERGY ELECTRON DIFFRACTION

RHEED

What is RHEED? In order to extract surface structural information from the diffraction of high energy electrons, therefore, the technique has to be adapted and the easiest way of doing this is to use a reflection geometry in which the electron beam is incident at a very grazing angle - it is then known as Reflection high-energy electron diffraction (RHEED) RHEED theory is very similar to LEED.

Basic Principle of RHEED A high energy electron beam(10-

30Kev) is directed at the sample surface at a low incident angle(1-20).

The electrons are diffracted by the crystal structure of the sample being investigated

Then projected on a fluroscent screen mounted opposite the electron gun.

Fig:-Electrons hit the surface at different grazing angle

NB: The combination of grazing incidence and strong electron-substrate interactions reduces the penetration depth of incident electrons to a few monolayer's.

Instrumentation of RHEED• A sample holder with the

prepared sample.• An electron gun-The electron

gun generates a beam of electrons which strike the sample at a very small angle.

• Photo-luminescent detector screen- which collect the diffracted electrons & form the regular pattern on the screen.

• although modern RHEED systems have some additional parts to optimize the technique.

Figure:- The most basic setup of a RHEED system.

Images obtained in RHEED

RHEED diagram : SiC surface structure evolution with Si dose

Graphene growth by molecular beam epitaxy : RHEED diagram (160 eV)

What, if any, advantages does RHEED offer over LEED?

• In terms of the quality of the diffraction pattern absolutely none in RHEED.

• By using RHEED it is therefore possible to measure, and hence also to control, atomic layer growth rates(i.e to monitor the atomic layer-by-atomic layer growth) in Molecular Beam Epitaxy(MBE) growth of electronic device structures -this is by far and away the most important application of the RHEED technique.

EXPERIMENTAL DATA LEED RHEED

Range of elements All All

Destructive No, except in special cases of electron-beam damage.

Same as LEED.

Depth probed 4-20Å. 2-100Å .

Detection limits 0.1ML; atomic positions to 0.1Å .

Same as LEED.

Resolving power Typically 200Å; best systems 5mm

Same as LEED.

Lateral resolution Typically 0.1mm; best systems ~10mm.

200mm x 4mm; best systems 0.3nm x 6 nm.

Imaging capability No; need special instruments – LEEM.

No.

Main uses Analysis of surface crystallography .

Monitoring surface structure, in-situ growth .

Cost <75K€ 50k-200k€.

LEED is (still) the most frequently used surface structural method

References Surface Characterization By D.Brune & R. Hellborg.http://philiphofmann.net/surflec3/surflec014.htmlhttp://hpcrd.lbl.gov/~mezaOptimization Methods for Simulation-Based Problems in Nano Science By Juan Meza, Michel van Hove, Zhengji Zhao (Lawrence Berkeley National Laboratory)www.wikipedia.org

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