1. Epitaxy: Application to Polarized Emitters
Aaron Moy and Brian Hertog
SVT Associates, Eden Prairie, Minnesota
Acknowledgements:
US Dept. of Energy SBIR Phase I and II Grant
contract #DE-FG02-01ER83332
in collaboration with
SLAC Polarized Photocathode Research Collaboration (PPRC):
A. Brachmann, J. Clendenin, E. Garwin, S. Harvey,
R. Kirby, D.-A. Luh, T. Maruyama, R. Prepost, and C. Prescott

2. Outline Strained Layer Semiconductor for Polarized Electron Source
Epitaxial Crystal Growth
Methods of III-V Epitaxy
Metal organic chemical vapor deposition (MOCVD)
Molecular beam epitaxy (MBE)
Gas source molecular beam epitaxy (GSMBE)
Growth of Photocathodes Using GSMBE
Characterization of Material

3. Polarized Electron Emitters
Emission of electrons with specific spin
Applications
High energy physics, colliders
Spintronics
Motivation
Efficiency ~ P2I, where P=polarization, I= current
Increased efficiency, less machine time cost

4. Strained Layer Polarized Emitters
Photocathode emission
Circularly polarized light
Unstrained GaAs
50% max polarization
Compressively strained GaAs
lattice constant < 5.65 Å
valence band splitting
3/ /2 transition favored
100% max polarization

5. Creating Strained GaAs Layers
Heteroepitaxy
New layers will form based on previous lattice
Compressive strain
Growth on lattice with smaller lattice constant
Larger difference in lattice size increased strain force
GaAs 5.65 Å
GaAs0.64P Å
Compressively strained
GaAs on GaAs0.64P0.36
lattice constant 5.58 Å

6. Epitaxy Growth of thin film crystalline material where crystallinity
is preserved, “single crystal”
Atomic Flux
Bare (100) III-V surface,
such as GaAs
Deposition of crystal source
material (e.g. Ga, As atoms)

7. Epitaxy Result: Newly grown thin film, lattice structure maintained
Starting surface

8. Epitaxy Advantages of epitaxy- Improved crystallinity Reduced defects
Higher purity
Precise control of thickness
Precise control alloy composition
“Lattice matched” compounds
Abrupt or graded interfaces
Ability to engineer unique device structures
Nanostructures
Superlattices
Strained layers

9. III-V Compound Semiconductors
III IV V VI VII VIII

10. How Epitaxy Is Achieved
Two primary methods for thin film epitaxy-
Metal Organic Chemical Vapor Phase Deposition (MOCVD) (aka metal organic vapor phase epitaxy MOVPE)
Molecular Beam Epitaxy (MBE)
Differences in growth chemistry

11. Metal Organic Chemical Vapor Phase Deposition
Growth in “reactor”
Pressure 10s-100s of torr
Metal organic group III source material
Trimethyl Gallium Ga(CH3)3
Trimethyl Indium In(CH3)3
MO vapor transported H2 carrier gas
Hydride group V source gas
Arsine AsH3
Phosphine PH3
Thermal cracking at growth surface

12. MOCVD- Surface Chemistry
Basic layout of an MOCVD reactor

13. MOCVD- Gas Handling System

14. MOCVD Summary Growth rates 2-100 micron/hr high throughput
P-type doping
Zn (Diethyl Zinc), high diffusivity
C (CCl4, CBr4), amphoteric
Complex growth kinetics
delicate interaction between injected gasses, temperatures
High background pressure
Parasitic incorporation
Intermixing of atoms at interfaces

15. Molecular Beam Epitaxy (MBE)
Growth in high vacuum chamber
Ultimate vacuum < torr
Pressure during growth < 10-6 torr
Elemental source material
High purity Ga, In, As ( %)
Sources individually evaporated in high temperature cells
In situ monitoring, calibration
Probing of surface structure during growth
Real time feedback of growth rate

16. Molecular Beam Epitaxy
Growth Apparatus

17. MBE- In Situ Surface Analysis
Reflection High Energy Electron Diffraction (RHEED)
High energy (5-10 keV) electron beam
Shallow angle of incidence
Beam reconstruction on phosphor screen
RHEED image of GaAs (100) surface

18. MBE- In Situ Growth Rate Feedback
Monitoring RHEED image intensity versus time
provides layer-by-layer growth rate feedback

19. MBE- Summary Ultra high vacuum, high purity layers
No chemical byproducts created at growth surface
High uniformity (< 1% deviation)
Growth rates micron/hr
High control of composition
In situ monitoring and feedback
Mature production technology

20. MBE- System Photo

21. Gas Source MBE
Combines advantages of MBE with gas source delivery of group V atoms (as used in MOCVD)
PH3, AsH3 used for group V sources
Thermally cracked at injector into P2, As2 and H2
P2, As2 dimers arrive at growth surface along with Ga, In
MBE surface kinetics maintained

22. Gas Source MBE Advantages of GSMBE
PH3 a more mature method for phosphorus MBE growth
Improved dynamic range of switching state
As, P molecules travel around shutter in solid source MBE
Control of P, As flux by adjustment of gas flow
Can replenish group V source material without breaking vacuum
Disadvantages
Requires gas handling system
Requires extra vacuum pumping to remove hydrogen
Arsine and Phosphine highly toxic

23. Limits to Strained Layers: Critical Thickness
Strain forces increase with thickness
Strain reaches threshold, lattice relaxes
“Critical Thickness”
Layer thickness where relaxation occurs
Relaxed lattice- bulk crystal state
Thickness inversely proportional to strain (difference in lattice constant)
Misfit dislocations created
Scattering, absorption of photons
Non-uniformities
GaAs on GaAsP
Critical Thickness

24. Photocathode Polarized Emitters
Device Considerations
Strained GaAs layer
Highly p-type doped
Thick to provide enough emission current
Structure Growth
Uniform
Excellent crystallinity
Substrate for epitaxy
High quality
Robust

25. Strained Superlattice Photocathode
Strained GaAs on GaAsxP1-x
Multiple GaAs layers sandwiched by GaAsxP1-x
Each GaAs layer below critical thickness
Multiple GaAs layers to provide thick overall active volume for electron emission
Superlattice- repetition of thin layers
GSMBE for epitaxy
Thin layers (< 50 Å)
Utilizes phosphorus
Abrupt, uniform interfaces

26. GaAs(1-x)Px Graded Layer
Strained Superlattice Photocathode
Strained GaAs
GaAsP
30 A
GaAs Substrate
GaAs(1-x)Px Graded Layer
GaAs0.64P0.36
Buffer
Active Region
2.5mm
1000 A
x 16 pair

27. Strained Superlattice Photocathode by GSMBE
Growth details
Substrate heated to 580 °C to remove surface oxide
GaAs buffer layer grown at 1 micron/hr using AsH3 flow 3 sccm
GaAs -> GaAsP graded layer grown
Step graded GaAsxP1-x using six distinct compositions
Maintained AsH3 + PH3 = 4.5 sccm gas flow rate
GaAsP layer grown at 480 °C
Superlattice layer grown at 480 °C

28. Material Characterization- X-ray

29. Material Characterization- Photoluminescence

30. PL points for uniformity probe
Material Characterization- Photoluminescence
Half die,
PL points for uniformity probe

31. Conclusion Strained layers for photocathode applications
Molecular beam epitaxy successful method for photocathode growth
MBE growth parameters and structure can be refined to improve polarization of devices