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

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 strained GaAs on GaAs0.64P0.36 lattice constant 5.58 Å

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)

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

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

MBE- In Situ Growth Rate Feedback Monitoring RHEED image intensity versus time provides 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 GaAsP Critical 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

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

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

PL points for uniformity probe 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