high performance photocathodes for microscopy and accelerators 8th Oct. 2012 15:15~15:40 Photocathode Physics for Photoinjectors (P3) 2012　　 Development of high performance photocathodes for microscopy and accelerators ○ X.G. Jin1, F. Ichihashi1, A. Mano1, N. Yamamoto1, Y. Takeda1, M. Yamamoto2, T. Miyajima2, Y. Honda2, T. Uchiyama2, S. Matsuba3 1Nagoya University 2High Energy Accelerator Research Organization (KEK) 3Japan Atomic Energy Agency

Outline Transmission-type spin-polarized photocahtode for microscopy GaInP photocathode for ERL

Outline Transmission-type spin-polarized photocahtode for microscopy GaInP photocathode for ERL

Introduction Spin-polarized electron beams are drawing much attention in several types of electron microscopy for magnetic image. Spin-polarized LEEM observation Electron beam for microscopy requires two properties: High spin-polarization, (for high image contrast) High brightness (for short exposure time) A : Magnetic contrast M P   µ A  P : Spin-polarization of electrons SPLEEM Image  M : Magnetization of surface P  M W(110) Co : 4 ML Polarized electrons 2 m Conventional electron beam for SPLEEM: Spin-polarization=20~30% Brightness=1103 Acm-2sr-1 Exposure time=1-10 s Real-time imaging with a high contrast: Spin-polarization >80% Brightness >3105 Acm-2sr-1 Exposure time< 0.03 s

Semiconductor photocathode Vacuum Electron Conduction band Light Valence band Negative electron affinity surface

Semiconductor photocathode for spin-polarization GaAs-type semiconductor Strained superlattice Bulk Conduction band Circularly-polarized light ｘ Band splitting Heavy hole band Γ-point Γ-point Light hole band Up-spin electron Down-spin electron In GaAs type semiconductor, the heavy hole band and the light hole band degenerate at -point and both up- and down-spin electrons are excited simultaneously by circularly polarized light. If the valence bands are split, one type of electrons can be excited selectively. The band splitting allows excitation of one type of spin-polarized electrons.

GaAs/GaAsP strained superlattice Spin-polarization result Spin-polarized electrons Circularly polarized light 92% spin-polarization GaAs/GaAsP superlattice 12 pair GaAsP buffer layer GaAs substrate GaAs/GaAsP strained superlattice structure T. Nakanishi, Proceedings of LINAC (2002) 813. Using GaAs/GaAsP strained superlattice grown on GaAs substrate, we have achieved the highest spin-polarization of 92%.

by highly focused laser light irradiation High brightness by highly focused laser light irradiation I I: Electron beam current S: Electron generation area : Electron beam solid angle Brightness = S High brightness is obtained by reducing electron generation area. Conventional reflection-type New transmission-type Diameter 100 m Diameter 1~2 m The pump laser light is difficult to be focused, and the electron generation area is large. The pump laser light can be highly focused, and the electron generation area is very small.

GaP substrate Conventional reflection-type New transmission-type GaAs substrate Band gap energy: 1.42 eV GaP substrate Band gap energy: 2.26 eV Pump laser light energy: 1.4~1.8 eV GaAs band gap energy is smaller than the pump laser light energy, and irradiation through the substrate is impossible. On the other hand, GaP is transparent to pump laser light, and transmission-type photocathode is possible.

Transmission-type photocathode photocathode structure Growth conditions Growth method: MOVPE, Growth temperature： 660C, Reactor pressure: 76 Torr, V/III: 15, Source materials： TEG, TBP, TBA. Flow rates: For GaAs, TEG: 9.5 mol/min, TBA: 143 mol/min. GaAs/GaAsP superlattice GaAsP buffer layer GaAs or AlGaAs inter-layer For GaAsP, TEG: 9.5 mol/min, TBA: 28 mol/min, TBP: 114 mol/min. GaP substrate X.G. Jin et al. J. Appl. Phys. 108 (2010) #094509

Transmission-type electron gun 20 kV Electron Gun Laser light Photocathode Brightness measurement chamber Electron beam Electron beam Mott Analyzer： Spin-polarization measurement Transmission-type electron gun was designed and fabricated in Nagoya University

Effect of inter-layer ＴＥＭ image ＴＥＭ image Without inter-layer 50 nm GaAs GaAsP g [004] GaAs/GaAsP superlattice GaAsP buffer Thickness modulation GaP substrate 100 nm With inter-layer ＴＥＭ image GaAs/GaAsP superlattice g [004] GaAsP buffer GaAs inter-layer GaP substrate 100 nm

Spin-polarization measurement With inter-layer 90% Spin-polarization (%) Without inter-layer 64％ Wavelength (nm) The maximum spin-polarization of 90% was achieved in transmission-type photocathode.

Brightness measurement = r2 L2 (R-r)2 I: electron beam current r: electron beam source radius on photocathode R: electron beam radius L: length between photocathode and knife Laser spot image on photocathode back side Laser light Laser spot profile Intensity (a.u) FWHM 1.3 m L=531 mm 1.9 mm 10 m Position (m) Laser spot diameter: 1.3 m, Electron beam current: 3.2 A, Electron beam diameter: 1.9 mm. The brightness is 1.3107 Acm-2sr-1 (@20 kV).　It is by 10000 times higher than that of the conventional reflection-type photocathode.

Progression of Photocathode 2008 2010 2011 Spin-polarized electrons Spin-polarized electrons Spin-polarized electrons GaAs/ GaAsP Strain compen sated SL GaAs/ GaAsP SL GaAs/ GaAsP SL GaAs layer AlGaAs layer GaP Sub. GaP Sub. GaP Sub. A.R. coating Laser light Laser light Laser light Spin-polarization: 90%; Brightness: ~107 Acm-2sr-1 Quantum efficiency: 0.1% Spin-polarization: 90%; Brightness: ~107 Acm-2sr-1 Quantum efficiency: 0.4% Spin-polarization: 90%; Brightness: ~107 Acm-2sr-1 Quantum efficiency: 0.5% (without A.R. coating) X.G. Jin et al. APEX 1 (2008) #045002 X.G. Jin et al. JJAP 51 (2012) #108004 X.G. Jin et al. submitted for publication

New SPLEEM Transmission-type electron gun system LEEM Electron gun Screen Spin rotator Sample

SPLEEM images Specimen : Co 10 ML on W(110) substrate View area: 6 m Exposure time : 0.02 sec 0.04 sec 0.2 sec 1 m M. Suzuki et al. Appl. Phys. Express 3 (2010) #026601 Clear magnetic image observed with exposure time 0.02sec. Comparison: 1~10 s in conventional spin-polarized LEEM.

Spin-polarized TEM image Transmission-type Gun M. Kuwahara, S. Kusunoki, X.G. Jin et al. Appl. Phys. Lett. 101 (2012) #033102. TEM with transmission-type spin-polarized photocathode is being developed at Nagoya University.

Conclusion Transmission-type GaAs/GaAsP superlattice photocathodes were designed and fabricated. A super-high brightness (1.3107 Acm-2sr-1 ) and a high spin-polarization (90%) of electron beam was achieved. Using SPLEEM with transmission-type photocathode, clear magnetic image was observed with exposure time 0.02 s. TEM with transmission-type spin-polarized photocathode is being developed at Nagoya University.

Outline Transmission-type spin-polarized photocahtode for microscopy GaInP photocathode for ERL

ERL image 500 kV Electron Gun cERL facility has been developed in KEK.

Targets for electron beam @Gun High average current: 10~100 mA Low emittance: 0.1~1 mm.mrad Fast response: t90<10 ps Drive laser: 532 nm 16 ps(FWHM) pulse Laser

Semiconductor photocathode Vacuum e- Energy relaxation Conduction band e- e- Photons: h Band gap (Eg) Valence band Energy relaxation Negative electron affinity surface (h - Eg) contributes to heat semiconductor photocathode.

III-V semiconductors Energy gap (eV) Lattice constant (Å) 532 nm (2.33 eV) Ga0.52In0.48P Eg: 1.9 eV Energy gap (eV) Eg: 1.42 eV Lattice constant (Å) Lattice constant (Å)

GaInP photocathodes 5-nm GaAs 5-nm GaAs 5-nm GaInP GaAs Activity layer SI GaAs (001) Substrate SI GaAs (001) Substrate SI GaAs (001) Substrate Zn dopant concentration of GaAs, GaInP cap layer: 6.01019 cm-3. Zn dopant concentration of GaAs, GaInP activity layer: 1.51018 cm-3.

Measurement system Emittance measuremet: Gun voltage: 100 kV DC gun Emittance measuremet: Gun voltage: 100 kV Measurement method: Waist scan Current of electron beam: 2~3 nA Response measurement: Gun voltage: 100 kV Pulse duration of the laser: 2 ps Current of electron beam: 2~3 nA

GaInP activity layer photocathodes showed higher values QE. Quantum efficiency QE measurement: 523 nm Pump laser 100 V Electron beam Photo- cathode GaAs activity with GaAs cap GaInP activity with GaAs cap GaInP activity with GaInP cap QE @523 nm 9% 14% 11% GaInP activity layer photocathodes showed higher values QE.

Mean Transverse Energy MTE （meV) GaAs with GaAs cap GaInP with GaAs cap GaInP with GaInP cap

Electron scattering Photocathode Vacuum CaP Activity layer layer Conduction band Photons: h Valence band CaP layer Activity layer Activity and cap layers have different effects on excited electrons.

Excited electrons were distributed on constant energy surface. In activity layer KY Constant energy surface E = (h - Eg) KZ  KX Excited electrons were distributed on constant energy surface.

Electrons lose energy and approach to  point. In activity layer KY KZ  KX Electrons lose energy and approach to  point. Absorption length and thickness of activity layer affect MTE.

In cap layer NEA value [meV] D.A. Orlov et al., APL 78 (2001) 2721 In band bending region, electrons were scattered elastically, and emitted with larger transverse energy.

Band effect on MTE GaAs activity layer with GaAs cap layer Absorption constant for 544 nm GaAs: 8104 /cm, GaInP: 2.5  104 /cm Conduction band 0.12 eV Band bending Band bending Vacuum level Eg:1.42 eV Eg:1.90 eV Vacuum level Valence band 0.36 eV GaAs GaAs cap GaInP GaInP cap GaAs activity layer with GaAs cap layer GaInP activity layer with GaInP cap layer

Surface layer effect on MTE Conduction band Band bending Band bending Eg:1.90 eV Eg:1.90 eV Vacuum level Valence band GaInP GaAs cap GaInP GaInP cap GaInP activity layer with GaAs cap layer GaInP activity layer with GaInP cap layer

Response time GaInP activity layer with GaAs cap layer Wavelength: 532 nm Fitting functions: Af Tauf (ps) As Taus (ps) Tzero (ps) SigmaT (ps) t90 (ps) 204419.4 1.13317 404.716 6.233265 -3.40053 2.218946 6.5165

Conclusion GaInP based photocathodes were developed. GaInP activity layer with GaAs cap layer photocathode showed a higher QE. The GaInP based photocathode showed a small value of mean transverse energy that is almost the same with That of GaAs photocathode. A short pico-second electron pulse was achieved from GaInP based photocathode.

Comparison among electron sources Reflection-type GaAs bulk Transmission-type GaAs/GaAsP 107 LaB6 emitter (applied to LEEM) Brightness (Acm-2sr-1) 105 Reflection-type GaAs bulk (applied to Spin-Polarized LEEM) Reflection-type GaAs/GaAsP 103 20 40 60 80 100 Spin-polarization (%)

Response time 600 nm GaInP with GaAs cap layer Fitting functions: Af Tauf (ps) As Taus (ps) Tzero (ps) SigmaT (ps) t90 (ps) 204419.4 1.13317 404.7169 6.233265 -3.40053 2.218946 6.5165