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2004-10-16 1

Molecular Beam Epitaxy (MBE)

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Outline

To give some information about:! Background of MBE

! MBE growth processes! Views inside representative

Commercial MBE Systems

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What is Epitaxy ?

Epitaxy = “Epi” + “taxis”

upon arranged

Epitaxy = the arrangement of atoms

on an ordered substrate

! Liquid Phase Epitaxy (LPE)

! Vapor phase Epitaxy (VPE)

! Molecular Beam Epitaxy (MBE)

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What’s MBE?Three factors to define MBE technique

! Thermal-energy molecular or atomic beams! Substrate with elevated temperature

!

High vacuum (10E-3~10E-9 Torr)or Ultral-high vacuum environment

(p ≤ 1.33x10E-7 Pa or 10E-9 Torr)

“A versatile technique for growing thin epitaxial structures made of semiconductors, metals or insulators”---defined by Dr. Morton B. Panish.

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Surface processes occurringduring film growth

! Surface Adsorption

!

Surface Dissociation &Migration

!

Lattice Incorporation! Thermal Desorption

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Island forming duringMBE growth

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 A commercial MBE system delivered by VG Semicon

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Schematic illustration of a MBE system

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Illustration of the Deposition Chamber 

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Important Issues in MBE!

Stoichiometry! Doping

! Vacuum Level

! Lattice Matching – or nearly so

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 Advantages of MBE Technique

!

Clean growth environment! Precise control of the beam fluxes

and growth condition

! Easy implementation of in situdiagnostic instruments

!

Compatibility with other high vacuumthin-film processing methods (metal

evaporation, ion beam milling, ion implantation)

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Side view of anentry lock system

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Cassette transfer mechanism

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Depositionchamber 

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The MechanicalShutters

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 A Manipulator used in Deposit ion Chamber 

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Front view of Effusion Cells Lock system

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The Structure of an Effusion Cell

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Diagnostic instruments for MBE

! Auger Electron Spectroscopy(AES, SEM)

! Ellipsometry

! Laser interferometric method! Reflection High-Energy Electron

Diffraction (RHEED)

! Surface Acoustic Wave Devices(Quartz Crystal Microbalances)

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Illustration of how RHEED is used in a MBE system

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Ref lec t ion HighRef lec t ion HighRef lec t ion HighRef lec t ion HighRef lec t ion HighRef lec t ion HighRef lec t ion HighRef lec t ion High

--------EnergyEnergyEnergyEnergyEnergyEnergyEnergyEnergy

Elec t ron Di f f rac t ionElec t ron Di f f rac t ionElec t ron Di f f rac t ionElec t ron Di f f rac t ionElec t ron Di f f rac t ionElec t ron Di f f rac t ionElec t ron Di f f rac t ionElec t ron Di f f rac t ion(RHEED)(RHEED)(RHEED)(RHEED)(RHEED)(RHEED)(RHEED)(RHEED)

Sc reen ImageSc reen ImageSc reen ImageSc reen ImageSc reen ImageSc reen ImageSc reen ImageSc reen Image

RHEED

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RHEED

Photo from http://www.k-space.com/ 

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Find ing Grow t h Rat esFind ing Grow t h Rat esFind ing Grow t h Rat esFind ing Grow t h Rat esFind ing Grow t h Rat esFind ing Grow t h Rat esFind ing Grow t h Rat esFind ing Grow t h Rat es

w i t h RHEEDw i t h RHEEDw i t h RHEEDw i t h RHEEDw i t h RHEEDw i t h RHEEDw i t h RHEEDw i t h RHEED

!! 22 --d grow t h oc c urs oned grow t h oc c urs oneat omic m ono layer a t aa t omic m ono layer a t at i m et i m e

!!

Sm oot h surfac e g ivesSm oot h surfac e g ivespeak s in RHEEDpeak s in RHEEDin tens i tyin tens i ty

!! Per iod o f RHEEDPer iod o f RHEED

int ens it y osc i l la t ionsin t ens it y osc i l la t ionsc or responds t o thec or responds t o thet ime o f g row t h fo r onet ime o f g row t h fo r one

layer  layer  

RHEED vs time

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RHEED vs time

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Issues for Solar Apps! Epitaxy is Ideal State

! Defects are Recombination Sites

! MBE is relatively slow 

! May Prefer Compromise toAchieve best Performance/Cost

!

Probably More suited to NanoCircuits

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The End

Upcoming events:Oct 14th: Review for Test 1

Oct 18th: Test 1

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