1. SUPERLATTICE PHOTOCATHODES: An Overview Tarun Desikan PPRC, Stanford University Tarun@Stanford.EDU

2. OUTLINE Spin polarized electrons quick study Uses and requirements Semiconductor polarized electron sources The need for strain Simulation of superlattice band structures Method and results Superlattice characterization X-ray diffraction and photoluminescence Results

3. USES OF POLARIZED ELECTRONS High-energy physics Surface analysis and imaging SPLEEM and SPSPM “Quantum” applications Computing, cryptography Single-electron devices Spintronics Spin-polarized currents Enhanced performance

4. REQUIREMENTS IN HEP High polarization Increases effective luminosity of Collider >90% High charge 10 13 electrons per train High speeds Picosecond micro-bunches Semiconductor photocathodes

5. SEMICONDUCTOR SOURCE Photo-excitation by polarized laser beam HH -> CB populates one spin state LH -> CB populates the other spin state Maximum polarization = 50% HHSOLH E CB kEgEg +1/2 -1/2 +1/2 -1/2 +3/2-3/2 +1/2 -1/2 m j = S 1/2 P 3/2 P 1/2 ++ --

6. NEGATIVE ELECTRON AFFINITY SURFACE Polarized e - tunnel through to NEA material and escape Atomically clean surface at UHV CB VB Circularly polarized laser photons Polarized electronsTunneling current GaAsCs 2 O

7. STRAINED PHOTOCATHODES HH and LH no longer degenerate at k=0 HH -> CB populates one spin state, LH -> CB does not occur Maximum polarization = 100% E CB HHSOLH k EgEg +1/2 -1/2 +1/2 -1/2 +3/2-3/2 +1/2 -1/2 m j = S 1/2 P 3/2 P 1/2 ++ --

8. SAMPLE STRUCTURE GaAs Substrate GaAs (1-x) P x Graded Layer GaAs 0.64 P 0.36 Buffer Active Region 25  m 1000 A

9. SUPERLATTICE PHOTOCATHODES Critical thickness (~100 A) limits the size of strained active region Practical limit is ~1000 A Active region partially relaxes Multiple quantum wells Strained material sandwiched between unstrained layers Strained region thickness < critical thickness Band engineering

10. SUPERLATTICE BAND CALCULATIONS kp transfer matrix method Chuang (UIUC), David Miller, Jim Harris (Stanford) 1 234N+1N+2

11. SUPERLATTICE BAND CALCULATIONS Must account for CB, HH, LH and SO CB decoupled HH, LH and SO interact Matrix solution to Schrödinger's equation 8x8 Hamiltonian Strain effects incorporated into Hamiltonian Boundary conditions Reach MATLAB noise floor

12. SINGLE QUANTUM WELL SIMULATION

13. MULTIPLE QUANTUM WELL SIMULATION

14. SIMULATION RESULTS Must use consistent parameters Easy wrap-around scripts Spot trends Compare with experiments? HH–LH Splitting Effective Band Gap E LwLw

15. X-RAY DIFFRACTION High-resolution XRD to analyze crystal Study layer attributes Thickness Composition Strain Tilt Vendor specifications

16. XRD THEORY Bragg’s Law: n* = 2*d*sin(  ) All lattice planes contribute to Bragg diffraction (004), (224), (113) planes commonly used Every layer contributes a Rocking Curve peak Repeating series of thin layers causes additional peaks  d

17. ROCKING CURVES

18. RECIPROCAL SPACE MAP 22  cps

19. RECIPROCAL SPACE MAP 22  cps

20. SAMPLE STRUCTURE GaAs Substrate GaAs (1-x) P x Graded Layer GaAs 0.64 P 0.36 Buffer Strained GaAs 25  m 1000 A

21. OTHER CHARACTERIZATION TOOLS Photoluminescence Band structure analysis Check simulation predictions SIMS Doping profile Destructive

22. STRAINED SUPERLATTICE SVT-3682 Strained GaAs GaAsP Strained GaAs GaAsP Strained GaAs GaAsP 30 A GaAs Substrate GaAs (1-x) P x Graded Layer GaAs 0.64 P 0.36 Buffer Active Region 25  m 1000 A

23. BAND STRUCTURE SIMULATION

24. BAND STRUCTURE Photoluminescence confirms the simulation prediction GaAsPGaAsGaAsPGaAsGaAsP CB1 HH1 LH1 1.65 eV 0.86 eV

25. LAYER THICKNESSES (004) scan [above] as well as (224)

26. SVT-3682 ANALYSIS Well Width = Barrier Width = 32 A Phosphorus fraction in GaAsP = 0.36 Strained GaAs does not relax significantly GaAs Substrate GaAs (1-x) P x Graded Layer GaAs 0.64 P 0.36 Buffer Active Region ActualIdeal a GaAs a GaAs 0.64 P 0.3 6

27. SVT-3682 PERFORMANCE Quantum Efficiency Polarization

28. SVT-3682 PERFORMANCE Peak polarization of ~86% A record at SLAC High QE > 0.2 % is great No charge limit A great photocathode! Repeatable?

29. CONCLUSIONS High performance superlattice photocathodes fabricated using GaAs/GaAsP Further improvement by optimizing parameters Need to test validity of band structure simulations Extend simulation model to calculate polarization