PST'05 (XIth Workshop on Polarized Source and Target)1 Generation of Polarized Electrons by Filed Emission M. Kuwahara A, T. Nakanishi A, S. Okumi A, M. Yamamoto A, M. Miyamoto A, N. Yamamoto A, K. Yasui A, T. Morino A, R. Sakai A, K. Tamagaki A, K. Yamaguchi B A: Graduate School of Science, Nagoya University B: Department of Electronic Engineering, The University of Electro-Communications Morning Session II -- Polarized Electron Beam II --

PST'05 (XIth Workshop on Polarized Source and Target)2 Motivation Spin polarized electrons Spin polarized electrons Necessary for high energy physics Necessary for high energy physics »Linier collider project (ILC project) Powerful application for material sciences Powerful application for material sciences »Spin electron microscopy （ SP-LEEM, Spin-SEM, TEM, … ） »Electron beam holography considered a spin effect Photocathode developments Photocathode developments by GaAs-GaAsP strained superlatttice Polarization 0.5% Polarization 0.5% Generation of multi-bunch beam (by overcoming SCL effect) Generation of multi-bunch beam (by overcoming SCL effect) Few problems are still remained Few problems are still remained Low emittance and long life time of photocathode 1.Low Emittance and High Brightness Polarized e - beam 2.Extr action of Polarized e - beam without NEA surface problem

PST'05 (XIth Workshop on Polarized Source and Target)3 Method １． Low emittance spin polarized electron [i] spin polarizing → GaAs type semiconductor → GaAs type semiconductor [ii] low emittance → cross section of beam: very small → cross section of beam: very small ２． NEA surface lifetime problem (by avoiding NEA surface) (by avoiding NEA surface) Using a tunneling effect by a high gradient at the surface → Field Emission Field emission from very small area of the top Using tip-GaAs (the feature is needle like)

PST'05 (XIth Workshop on Polarized Source and Target)4 Generation of spin polarized electrons Bulk-GaAs has degeneracy of electron bands at  Polarization: max. 50% By strained or super-lattice structure GaAs, the degeneracy at  point can be separated, Polarization > 50% enable In fact, Polarization ~ 90% by strained supper-lattice structure Under illuminating circular light to GaAs semiconductor. Selective excitation from valence band to conduction band. (conserving the helicity) Left figure shows a principled basis that excitation process of spin polarized electrons in GaAs semiconductor. Basis of generating spin polarized electrons

PST'05 (XIth Workshop on Polarized Source and Target)5 Photocathode ・ Photocathode sample (GaAs tip) ・ Fabrication of tip-GaAs SEM images (left:×25k, right:×100) ratio temperature H 3 PO 4 :H 2 O 2 :H 2 O=10:1:1 Temperature 20 ℃ H 3 PO 4 :H 2 O 2 :H 2 O=5:1:1 Temperature -1 ℃ H 3 PO 4 etching solution’s condition, mixing ratio and temperature (p-GaAs substrate, Zn-dope:2×10 19 cm -3 ) Height : ～ 10  m Radius : ～ 25nm

PST'05 (XIth Workshop on Polarized Source and Target)6 Apparatus Laser Tsunami(SP) Pulse-Laser （ 532nm,5W seed ） wavelength 730nm ～ 850nm Pulse width ～ 20 p s repetition Hz Model3900(SP 社 ) CW-Laser （ 532nm,5W seed) wavelength 730nm ～ 950nm Electron gun 70keV PES 70keV PES （ I-V characteristics and polarization measurement ） Mott-scattering polarization analyzer Mott-scattering polarization analyzer Vacuum pressure ： 3× Torr Vacuum pressure ： 3× Torr Field gradient at photocathode ： Field gradient at photocathode ： 20kV DC-gun 20kV DC-gun （ I-V characteristics ） 20kV-DCgun, variable gap separation 20kV-DCgun, variable gap separation Field gradient at photocathode ~ 4.8MV/m Field gradient at photocathode ~ 4.8MV/m gap=3.2mm) gap=3.2mm) 20kV DC-gun 70keV PES Ti:Sapphire Laser

PST'05 (XIth Workshop on Polarized Source and Target)7 Experimental results (1) - I-V characteristics - Behaviors Behaviors ; under impressing high gradient and illuminating circular light I-V characteristic → F-N(Fowler-Nordheim) plot Tunneling effect through a surface barrier (Field emission) Photon-excited electrons were extracted by F.E.mechanism Not observe by GaAs without tip QE vs. Photon energy at high gradient field ( well fit Fitting curve is estimated by WKB approximation. Demonstrated the tunneling yield depending on an excitation energy. Field-Emission is observed

PST'05 (XIth Workshop on Polarized Source and Target)8 Estimation of electron affinity  [Estimation of  by the QE– data] [Estimation of  by F-N plot data] Tunneling yield T (WKB approximation) is written by The solid line is obtained by least-squares fitting in left figure. Therefore,  is estimated as Here, field enhancement factor is 51 (calculated by POISSON) for the tip feature (curvature is 50nm, distance is 200  m) →0.282 eV →0.226 eV Assumption: proportional to a tunneling yield of surface barrier F-N plot is written as, ←Fowler-Nordheim equation By the gradient of F-N plot  =1.64×10 -2  2/3 Consistent with each result

PST'05 (XIth Workshop on Polarized Source and Target)9 Experimental results (2) - Spin Polarization - Polarization of tip-GaAs Polarization of tip-GaAs 1) Polarization : 20 ～ 40% ≧ Bulk-GaAs’ Polarization 2) tip-GaAs Polarization was higher than NEA/Bulk- GaAs’ at shorter wavelength  < 760nm (1.6eV) We succeed in generation of spin polarized electrons by field emission ESP and QE spectrum under irradiating circular light. In order to compare, NEA/Bulk-GaAs polarization is also drown. Corresponding with the rising edge of Q.E. Spin polarization did not get worse, while F.E. mechanism was substituted for NEA

PST'05 (XIth Workshop on Polarized Source and Target)10 Difference of each polarization Generation process Generation process ① absorption and excitation ② diffusion and transport ③ escape into vacuum In transmission to the surface In transmission to the surface dependent on excitation energy (Phenomena of hot-electron) (Phenomena of hot-electron) [ １ ] Scattering in drifting process LO phonon scattering, impurity scattering [ ２ ] Spin flip in scattering BAP-process, DP-process, EY-process BAP-process, DP-process, EY-process Spin relaxation time becomes smaller with rising electron energy Spin relaxation time 75~85ps （ 850to880nm), Band-gap 865nm(1.43eV) （ different point between NEA and F.E. ）

PST'05 (XIth Workshop on Polarized Source and Target)11 Difference of each polarization Process in extracting into vacuum Process in extracting into vacuum Tunneling yield is sensitive to the excitation energy Tunneling yield is sensitive to the excitation energy drifting electron ： drifting electron ： Energy dispersion becomes wider in transport process by some scattering. Energy dispersion becomes wider in transport process by some scattering. Polarization of higher energy part : High polarization Polarization of higher energy part : High polarization lower energy part : Low polarization (cause by scattering) lower energy part : Low polarization (cause by scattering) Surface tunneling is like a filter effect of polarization. Higher energy part of electrons can be extracted dominantly.  : narrow, Pol : high High energy part is mainly extracted into vacuum.  Polarization becomes higher (cut off of depolarization part ) Fig. Generation process of spin polarized electrons with field emission. Blue color density means value of spin polarization.

PST'05 (XIth Workshop on Polarized Source and Target)12 Conclusion Achievements : We demonstrated that F.E. can be used for PES Achievements : We demonstrated that F.E. can be used for PES as a substitute for using NEA surface. Probability of miniaturization, integration and applications Probability of miniaturization, integration and applications for accelerators, microscopy, holography and so on. Extraction of polarized electrons by F.E. : O.K. Extraction of polarized electrons by F.E. : O.K. Electrons extracted by F.E. have higher polarization. Electrons extracted by F.E. have higher polarization. It is thought that filter effect of tunneling process in surface. It is thought that filter effect of tunneling process in surface. Lifetime （ long lifetime compared with NEA surface （ NEA~1week → F.E.>1month ） ） Lifetime （ long lifetime compared with NEA surface （ NEA~1week → F.E.>1month ） ） Problem ： Work function, fine structure, surface contamination Problem ： Work function, fine structure, surface contamination Stability and uniformity of current Stability and uniformity of current Field emission characteristic (operation voltage, field enhancement) Field emission characteristic (operation voltage, field enhancement) Extract more high current Extract more high current We can confirm that spin polarized electrons can be extracted by F.E., and demonstrate the fundamental characteristics.

PST'05 (XIth Workshop on Polarized Source and Target)13 Difference from NEA/Bulk-GaAs (2) Depolarization effect of spin polarized electron in a GaAs crystal. Depolarization effect of spin polarized electron in a GaAs crystal. （ In the case of hot-electrons ） DP-process （ >10meV ） DP-process （ >10meV ） （ p-dope>10^18 cm -3, 10meV 以下では BAP 機構が主たる効 果） LO phonon scattering. LO phonon scattering. SO splitting >341meV (in GaAs) SO splitting >341meV (in GaAs) Spin polarization of electrons diffusing into the surface (calculated by DP-process ）