Epitaxy: Application to Polarized Emitters

Aaron Moy and Brian HertogSVT 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

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

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

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 1/2 transition favored

• 100% max polarization

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

GaAs0.64P0.36 5.58 Å

Compressively strainedGaAs on GaAs0.64P0.36

lattice constant 5.58 Å

Epitaxy

Bare (100) III-V surface,such as GaAs

Deposition of crystal sourcematerial (e.g. Ga, As atoms)

Growth of thin film crystalline material where crystallinityis preserved, “single crystal”

Atomic Flux

Epitaxy

Result: Newly grown thin film, lattice structure maintained

Starting surface

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

III-V Compound Semiconductors

III IV V VI VII VIII

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

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

MOCVD- Surface Chemistry

Basic layout of an MOCVD reactor

Surface chemistry-

MOCVD- Gas Handling System

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

Molecular Beam Epitaxy (MBE)

• Growth in high vacuum chamber

• Ultimate vacuum < 10-10 torr

• Pressure during growth < 10-6 torr

• Elemental source material

• High purity Ga, In, As (99.9999%)

• Sources individually evaporated in high temperature cells

• In situ monitoring, calibration

• Probing of surface structure during growth

• Real time feedback of growth rate

Molecular Beam Epitaxy

Growth Apparatus

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

0 20 40 60 80 100 120 140 160 180

Time (s)

RHEEDSignal GaAs Oscillations

AlAs Oscillations

InAs Oscillations

MBE- In Situ Growth Rate Feedback

Monitoring RHEED image intensity versus timeprovides layer-by-layer growth rate feedback

MBE- Summary

• Ultra high vacuum, high purity layers

• No chemical byproducts created at growth surface

• High uniformity (< 1% deviation)

• Growth rates 0.1-10 micron/hr

• High control of composition

• In situ monitoring and feedback

• Mature production technology

MBE- System Photo

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

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

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 GaAsPCritical Thickness

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

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

Strained GaAs

GaAsP

Strained GaAs

GaAsP

Strained GaAs

GaAsP 30 A

30 A

GaAs Substrate

GaAs(1-x)Px Graded Layer

GaAs0.64P0.36 Buffer

Active Region

2.5m

2.5m

1000 A

Strained Superlattice Photocathode

x 16 pair

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

Material Characterization- X-ray

Material Characterization- Photoluminescence

Material Characterization- Photoluminescence

Half die,PL points for uniformity probe

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