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Nucleation and growth of nanostructures and films
Seongshik (Sean) Oh
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Outline Introduction and Overview 1. Thermodynamics and Kinetics of thin film growth 2. Defects in films 3. Amorphous, Polycrystalline and Epitaxial Films Vacuum Film Deposition Techniques 1. Physical Vapor Deposition (PVD) Evaporation : Thermal and Electron-Beam Sputtering: RF and DC Magnetron Pulsed Laser Deposition (PLD) Molecular Beam Epitaxy (MBE) 2. Chemical Vapor Deposition (CVD)
Plasma-Enhanced CVD (PE-CVD) Atomic Layer Deposition (ALD) More about MBE
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Introduction and Overview
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What is a "thin film" ?
thin = less than about 1000 nm thick film = layer of material on a substrate
Applications: • Microelectronics - electrical conductors, electrical barriers, diffusion barriers . . . • Sensors: magnetic sensors, gas sensors, … • Optics - anti-reflection coatings • Corrosion protection, Wear resistance, etc • Tailored materials with new properties
Special Properties of Thin Films: different from bulk materials Thin films may be: •not fully dense •under stress •different defect structures from bulk •quasi - two dimensional (very thin films) •strongly influenced by surface and interface effects
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Typical steps in making thin films: 1. Emission of particles from source (heat, high voltage . . .) 2. Transport of particles to substrate 3. Condensation of particles on substrate
Simple model for thin film growth
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Detailed Steps in Film Formation 1. thermal accommodation (substrate should be cold, and impinging material should have low energy) 2. binding (Physisorption and Chemisorption) 3. surface diffusion (Larger than bulk diffusion) 4. nucleation 5. island growth 6. coalescence 7. continued growth
Nucleation and Growth occurs mostly on step edges because of higher bonding energy
Highly ordered thin films can be grown at much lower temperatures than for bulk, because of the larger surface diffusion (Step 3 above).
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1. Island growth (Volmer - Weber) form three dimensional islands source: film atoms more strongly bound to each other than to substrate
and/or slow diffusion
2. Layer by layer growth (Frank - van der Merwe)
generally highest crystalline quality source: film atoms more strongly bound to substrate than to each other
and/or fast diffusion
3. Mixed growth (Stranski - Krastanov) initially layer by layer then forms three dimensional islands
Three different growth modes
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Thermodynamics and Kinetics of thin film growth: Thin film growth is not a equilibrium process!
Two factors determining growth of thin films: 1. Thermodynamics (Equilibrium Condition, Gibbs Free Energy and Phase Diagram) Can the intended solid phase be formed at the growth temperature? 2. Kinetics (Rate and Diffusion) How fast are materials arriving and diffusing?
Artificial superlattice is the best example of manipulating Kinetics and Thermodynamics
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Three types of crystalline defects 1. Planar defects: Grain boundaries: Weak link for diffusion or corrosion
2. Line defects: Dislocations: Causing Stress (Tension and Compression)
3. Point defects: Interstitial, Vacancy, Impurities
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Thin film types based on crystallinity: 1. Amorphous: No-crystalline structures, so no crystalline defects:
Common insulators such as amorphous SiO2
2. Polycrystalline: Lots of grain boundaries: Most elemental metals grown near room temperatures
3. Epitaxial (Single-Crystalline): No grain boundaries: Requires high temperatures and slow growth rate: high quality thin films such as
III-V semiconductor films and complex oxides
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Vacuum Film Deposition Techniques
1. Physical Vapor Deposition (PVD) Evaporation : Thermal and Electron-Beam Sputtering: RF and DC Magnetron Pulsed Laser Deposition (PLD) Molecular Beam Epitaxy (MBE) 2. Chemical Vapor Deposition (CVD)
Plasma-Enhanced CVD (PE-CVD) Atomic Layer Deposition (ALD)
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pressure tm
1 atm 2 x 10-9 sec
10-6 torr 2 seconds
10-9 torr 31 minutes
Mean free path in vacuum How long does it take to form a single complete layer of gas on a surface ? Assume sticking coefficient of one.
We need good vacuum for thin film growth
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Evaporation (Pbase < ~10-6 Torr)
Thermal Evaporation for non-refractory materials
E-beam Evaporation for refractory materials
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Sputtering: (1~10 mTorr of Ar) • DC for conducting materials • RF for insulating materials • Magnetron sputtering is most popular due to high rate and low operation pressure
Magnetron sputtering for high density of plasma near target
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Pulsed Laser Deposition (PLD) Good for multielemental materials (Pgas < 1 Torr)
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Molecuar Beam Epitaxy (MBE) Most sophisticated growth method (Pbase < 10-8 Torr)
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Chemical vapor deposition (CVD) need precursors, which react and/or decompose on the substrate surface to produce the desired deposit.
Three different precursor combinations to make SiO2 thin film SiH4 + O2 → SiO2 + 2H2 SiCl2H2 + 2N2O → SiO2 + 2N2 + 2HCl Si(OC2H5)4 → SiO2 + byproducts
Plasma-Enhanced CVD: P<~1 Torr)
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Atomic Layer Deposition: Self-limited CVD process
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More about MBE
MBE is special because you can monitor and control film growth at atomic scale using RHEED
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Reflection High Energy Electron Diffraction (RHEED)
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RHEED pattern gives information on real space morphology:
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RHEED oscillation provides atomic-level thickness control
22
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1970s: Birth of Molecular Beam Epitaxy
First MBE, Bell lab 1970
Modulation doping in GaAs/Al(Ga)As, Bell lab 1978
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1980s: Atomic-layer engineering in III-V semiconductors
• Discovery of RHEED oscillations in Ga(Al)As growth atomic layer-by-layer growth
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1990s: Atomic-layer engineering in complex oxides
Sr(La)O
Ca
Sr(La)O
CuO2 CuO2
Bi2O2+
Bi2O2+
• • •
• • •
1 2
3 4 1
1 2
3 4
Atomic-layer-by-layer growth of a layered superconductor
15.4
Å
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Our system (COAL MBE) (W132)
1. Flexible source to substrate distance 2. Vertical geometry and easy access to all components 3. Small source to substrate angle (good for uniformity) 4. Newly developed simple differential pumping schemes 5. Highly-optimized computer control
Rutgers MBE
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What are we working on now?
Growth of a topological insulator (Bi2Se3)
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Crystal structure of Bi2Se3 Van der-Waals bonding between Se bi-layers
4.2 Ǻ
~10 Ǻ
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Si(111) Substrate Preparation
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SiO2 free Si surface (thermal heating)
Si (111)
Needs to remove amorphous native oxide
20˚C < T < 700 ˚C 700˚C < T < 860 ˚C T > 860 ˚C
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(7x7) to (1x1) of Si(111) surface (7x7) occurs only if the native oxide is completely removed
(1x1) T > 860 ˚C (7x7) T < 860 ˚C
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No UV Ozone cleaning UV Ozone cleaned for 5 min
No extra spot Extra spot: Carbon contamination
STM image (SiC clusters)
Dirty surface results in SiC clusters
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Bi2Se3 Growth on Si (111)
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(1) Bi2Se3 growth @ 350 ˚C
t = 0 min
t = 1 min
t = 3 min
Bad
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(2) Bi2Se3 growth @ 250 ˚C
t = 1 min
t = 0 min
t = 3 min
Bad
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Two-Step Growth Process
(1) Outgas Substrate
(2) Cool to 110oC
TSub
Time
a-SiO2
Si
Al2O3
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Two-Step Growth Process
(1) Outgas Substrate
(2) Cool to 110oC
(3) Deposit 3QL at 110oC
TSub
Time
(3) 3QL
Si
Al2O3
a-SiO2
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Two-Step Growth Process
(1) Outgas Substrate
(2) Cool to 110oC
(3) Deposit 3QL at 110oC
(4) Slowly anneal to 220oC
TSub
Time
(3) 3QL
Si
Al2O3
a-SiO2
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Two-Step Growth Process
(1) Outgas Substrate
(2) Cool to 110oC
(3) Deposit 3QL at 110oC
(4) Slowly anneal to 220oC
(5) Deposit required QLs at 220oC
TSub
Time
(3) 3QL
(5) n-QL
SiO2
Si
Al2O3
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Two-Step Growth Process
(1) Outgas Substrate
(2) Cool to 110oC
(3) Deposit 3QL at 110oC
(4) Slowly anneal to 220oC
(5) Deposit required QLs at 220oC
(6) Cool to Room Temperature
TSub
Time
(3) 3QL
(5) n-QL
Si
Al2O3
SiO2
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RHEED oscillation in the intermediate growth condition: Layer-by-layer growth Best growth condition: Step-flow
growth no RHEED oscillation
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Growth of other systems
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1.0 34.0 67.0 100.0 133.0 time (s)
Intensity
a
a
a a a a
Start of supercell
CTO surface
End of supercell
CTO Surface After 1 ML BTO After 1 ML STO
M. Warusawithana and J. N. Eckstein
1 ML
BTO
2 ML
BTO 1 ML
STO
2 ML
STO 1 ML
CTO
2 ML
CTO
0 100 200 300 400 time (s)
After 0.5ML STO
Spec
ula
r sp
ot
inte
nsi
ty
(arb
itra
ry u
nit
s)
0%
90%
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TiO2LaO
CuO2Ca
CuO2
SrO
SrOBiO1+δ/2
Sr.75La.25OCuO2
CaCuO2
TiO2
SrO
Sr.75La.25O
BiO1+δ/2
BiO1+δ/2
BiO1+δ/2
1
2
1 2 3
4 5
6 7
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Growth of Epi-Re/Epi-Al2O3/Poly-Al
4×10-6 Torr O2, Al 10-6 Torr O2
Epitaxial Re/Al2O3
Re
@ 850 C Al
Amorphous AlOx
@ RT
Epitaxial Al2O3
@ 800 C
Polycrystalline Al
@ RT