Epitaxy: Application to Polarized Emitters

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 1/2 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 GaAs0.64P0.36 5.58 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 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 < 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

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 0.1-10 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. GaAsP 30 A Strained GaAs 30 A Active Region 1000 A GaAs0.64P0.36 Buffer GaAsP 2.5mm Strained GaAs GaAs(1-x)Px Graded Layer 2.5mm GaAsP Strained GaAs GaAs Substrate Strained Superlattice Photocathode 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. 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