pst'05 (xith workshop on polarized source and target)1 generation of polarized electrons by...

PST'05 (XIth Workshop on Polarized Source and Target) 1

Generation of Polarized Electrons Generation of Polarized Electrons by Filed Emissionby Filed Emission

M. KuwaharaM. Kuwahara A A, T. Nakanishi, T. Nakanishi A A, S. Okumi, S. Okumi A A, M. Yamamoto, M. Yamamoto A A, , M. MiyamotoM. Miyamoto A A, N. Yamamoto, N. Yamamoto A A, K. Yasui, K. Yasui A A, T. Morino, T. Morino A A, ,

R. SakaiR. Sakai A A, K. Tamagaki, K. Tamagaki A A, K. Yamaguchi, K. Yamaguchi B 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 --


MotivationMotivation

Spin polarized electronsSpin polarized electrons Necessary for high energy physicsNecessary for high energy physics

» Linier collider project (ILC project)Linier collider project (ILC project) Powerful application for material sciencesPowerful application for material sciences

» Spin electron microscopySpin electron microscopy（（ SP-LEEM, Spin-SEM, TEM, …SP-LEEM, Spin-SEM, TEM, … ））

» Electron beam holography Electron beam holography considered a spin effect considered a spin effect

Photocathode developmentsPhotocathode developmentsby GaAs-GaAsP strained superlatttice by GaAs-GaAsP strained superlatttice

Polarization ~90% @QE 0.5%Polarization ~90% @QE 0.5% Generation of multi-bunch beam (by overcoming SCL effect)Generation of multi-bunch beam (by overcoming SCL effect)

Few problems are still remainedFew problems are still remained Low emittance and long life time of photocathodeLow emittance and long life time of photocathode

1.1.Low Emittance and High Brightness Polarized eLow Emittance and High Brightness Polarized e-- beam beam2.2.ExtrExtraction of Polarized eaction of Polarized e-- beam without NEA surface problem beam without NEA surface problem


MethodMethod

１．１． Low emittance spin polarized electron Low emittance spin polarized electron

[i] spin polarizing[i] spin polarizing →→ GaAs type semiconductorGaAs type semiconductor

[ii] low emittance[ii] low emittance →→ cross section of beam: very small cross section of beam: very small

２．２． NEA surface lifetime problemNEA surface lifetime problem　　　　 ((by avoiding NEA surface)by avoiding NEA surface)

Using a tunneling effect Using a tunneling effect by a high gradient at the surfaceby a high gradient at the surface

→ → Field EmissionField Emission

Field emission from very small area of

the top

Using tip-GaAs

(the feature is needle like)


Generation of spin polarized electronsGeneration 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


PhotocathodePhotocathode

・・ PhotocathodePhotocathode sample (GaAs tip)sample (GaAs tip)

・・ Fabrication of tip-GaAsFabrication of tip-GaAs

SEM images (left:×25k, right:×100)

ratio

temperature

H3PO4:H2O2:H2O=10:1:1Temperature 20℃

H3PO4:H2O2:H2O=5:1:1Temperature -1℃

H3PO4 etching solution’s condition, mixing ratio and temperature

(p-GaAs substrate, Zn-dope:2×1019cm-3)

Height : ～ 10mRadius : ～ 25nm


ApparatusApparatus

Laser

Tsunami(SPTsunami(SP) ) Pulse-LaserPulse-Laser 　　（（ 532nm,5W seed532nm,5W seed ））

wavelengthwavelength 730nm730nm ～～850850nmnm

Pulse widthPulse width ～～ 20 20 ppss

repetitionrepetition 81.25 Hz81.25 Hz

Model3900(SPModel3900(SP 社社 ) ) CW-Laser CW-Laser （（ 532nm,5W seed)532nm,5W seed)

wavelengthwavelength 730nm730nm ～～950950nmnm

Electron gun 70keV PES70keV PES （ I-V characteristics and

polarization measurement ） Mott-scattering polarization analyzerMott-scattering polarization analyzer Vacuum pressure Vacuum pressure ：： 3×103×10-11 -11 TorrTorr Field gradient at photocathodeField gradient at photocathode ：： 0.60.6MV/m @70kVMV/m @70kV

20kV DC-gun 20kV DC-gun （ I-V characteristics ） 20kV-DCgun, variable gap separation20kV-DCgun, variable gap separation

Field gradient at photocathode ~ Field gradient at photocathode ~ 4.84.8MV/mMV/m (@20kV, gap=3.2mm)(@20kV, gap=3.2mm)

2020kV DC-gunkV DC-gun

7070keV PESkeV PES

Ti:Sapphire Laser


Experimental results (1) - Experimental results (1) - I-V characteristicsI-V characteristics - - Behaviors Behaviors

; under impressing high gradient and illuminating circular light; 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 ( E=3.4MV/m @Flat)

well fit

Fitting curve is estimated by WKB approximation.

Demonstrated the tunneling yield depending on an excitation energy.

Field-Emissionis observed


Estimation of electron affinity 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

2/3

3

24exp)( Zz eE

mT

428.1710.1

Here, field enhancement factor is 51 (calculated by POISSON) for the tip feature (curvature is 50nm, distance is 200m)

→0.282 eV

→0.226 eV

Assumption: proportional to a tunneling yield of surface barrier

SEE

Ilnln2

1054.1ln1085.6ln

62/39

2

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

Consistent with each

result


Experimental results (2) - Experimental results (2) - Spin PolarizationSpin Polarization - -

Polarization of tip-GaAsPolarization of tip-GaAs1) 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 We succeed in generation of spin generation of spin polarized electrons polarized electrons by field emission 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


Difference of each polarizationDifference of each polarization

Generation processGeneration process①①absorption and excitationabsorption and excitation

②②diffusion and transportdiffusion and transport

③③escape into vacuumescape into vacuum

In transmission to the surface In transmission to the surface dependent on excitation energydependent on excitation energy

((Phenomena of hot-electron)Phenomena of hot-electron)

[[ １１ ] ] Scattering in drifting processScattering in drifting process

LO phonon scattering, impurity scatteringLO phonon scattering, impurity scattering

[[ ２２ ] ] Spin flip in scatteringSpin flip in scattering

BAP-process, BAP-process, DP-process,DP-process, EY-process EY-process

Spin relaxation time becomes smaller with rising electron energy Spin relaxation time becomes smaller with rising electron energy

Spin relaxation time 75~85ps Spin relaxation time 75~85ps （（ 850to880nm), 850to880nm), BBand-gap and-gap 865nm(1.43eV)865nm(1.43eV)

（（ different point between NEA and F.E.different point between NEA and F.E. ））


Difference of each polarizationDifference of each polarization

Process in extracting into vacuumProcess in extracting into vacuum Tunneling yield is sensitive to the excitation energyTunneling yield is sensitive to the excitation energy

drifting electrondrifting 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 : Polarization of higher energy part : High polarizationHigh polarization

　　　　　　　　　　　　 lower energy part : lower energy part : Low polarizationLow polarization (cause by scattering)(cause by scattering)

Surface tunneling is like a filter effect of polarization.

Higher energy part of electrons can be extracted dominantly. : narrow, Pol : high

2/3exp)( ZzT

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.


ConclusionConclusion

Achievements : Achievements : We demonstrated that F.E. can be used for PES We demonstrated that F.E. can be used for PES as a substitute for using NEA surface.as a substitute for using NEA surface.

Probability of miniaturization, integration and Probability of miniaturization, integration and applications applications for accelerators, microscopy, holography and so on.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 Lifetime （（ long lifetime compared with NEA surfacelong lifetime compared with NEA surface

（（ NEA~1week NEA~1week → → F.E.>1monthF.E.>1month ））））

Problem Problem ：： Work function, fine structure, surface contaminationWork function, fine structure, surface contamination Stability and uniformity of currentStability and uniformity of current Field emission characteristic Field emission characteristic (operation voltage, field enhancement)(operation voltage, field enhancement) Extract more high current Extract more high current

We can confirm that spin polarized electrons can be extracted We can confirm that spin polarized electrons can be extracted by F.E. , and demonstrate the fundamental characteristics.by F.E. , and demonstrate the fundamental characteristics.


Difference from NEA/Bulk-GaAs (2)Difference from NEA/Bulk-GaAs (2)

Depolarization effect of spin polarized electron in a Depolarization effect of spin polarized electron in a GaAs crystal.GaAs crystal.

（（ In the case of hot-electronsIn the case of hot-electrons ）） DP-process DP-process （（ >10>10meVmeV ））

（（ p-dope>10^18 cmp-dope>10^18 cm-3-3 , , 1010meVmeV 以下では以下では BAPBAP 機構が機構が主たる効果）主たる効果）

LO phonon scatteringLO phonon scattering.. SO splitting >341meV (in GaAs)SO splitting >341meV (in GaAs)

dxxAdxxAk

aPP ggk

k

pi

2/12/1

1

3 expexp1

1)(

Spin polarization of electrons diffusing into the surface (calculated by DP-process ）

pst'05 (xith workshop on polarized source and target)1 generation of polarized electrons by...

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