Photocathode Materials for Photoinjectors

         
 

metallic photocathodes

low Quantum Efficiency

long lifetime

 

nonmetallic photocathodes

high Quantum Efficiency

short lifetime

  
On the list of known types of photocathodes, metallic photocathodes and semiconductor photocathodes are the most popular used in the photoinjectors. Semiconductor cathodes have higher quantum efficiency (QE), but shorter life time than metallic cathodes. Especially the NEA photocathodes ( for example GaAs ) has a QE of more than 10%. But almost all of the semiconductor photocathodes are extremely sensitive to the contamination, and the working condition for them is ultra high vacuum better than 10-6 Pa.The relevant properties for a photocathode are: spectral response, operational lifetime, temporal response, saturation level, damage threshold, voltage hold-off, the transverse energy spread of the emitted electron beam, dark current, and the maximum average current density.

Metallic photocathodes are the most robust cathodes among the known photocathodes. Most of them have long lifetime and the convenience that they are air-transportable. But the main disadvantage for metal is that the QE is too low because of their high reflectivity and the shallow escape depth, even after special treatments.

 

Table: Materials of metallic photocathodes

Fam.

Element

No

Work function

(eV)

Threshold

(nm)

Specials

Lab
IIA Mg 12 3.66 339 used in normal conducting RF guns BNL
IIA Ca 20 2.9 427    
IIA Ba 56 2.5 -    
IB Cu 29 4.3 288 used in normal conducting RF guns BNL, SLAC
IIIB Y 39 2.9 427    
VB Nb 41 4.0 310 Nb wall in SC cavity used as cathode BNL
Lathanoide Sm 62 2.7 459    
modified Mg - - - Mg implanted with Cs+ ions PKU

Among these nonmetallic materials, semiconductor cathodes have higher quantum efficiency (QE), but shorter life time than metallic cathodes. Thermionic cathodes are investigated as cathode for RF injector using a heating temperature below the usual operating temperature for thermionic emission. Ferroelectric photocathodes and ceramic superconducting cathodes are not very well-known in their photoelectric performance.

Table: Properties of semiconductor photocathodes

 

Material

Ea+Eg

(eV)

Threshold

(nm)
Alkali-halide CsI 6.4 209
CsI-Ge 5.0 248
Alkali-antimonide Cs3Sb 2.0 620
K3Sb 2.3 539
Na2KSb 2.0  
K2CsSb - 620
Alkali-telluride Cs2Te 3.5 354
CsKTe    
Rb2Te 4.1 302
RbCsTe    
K2Te    
negative electron affinity (NEG) GaAs(Cs)    

Thermionic cathodes are investigated as cathodes for RF Injector using heating temperatures below the usual operating temperature for thermionic emission. Ferroelectric photocathodes and ceramic superconducting cathodes are not very well-known in the photoelectric performance. From ion-implanted cathodes there is not enough information available to describe them well. For ferroelectric photocathode a quantum efficiency of QE~6·10-4 at 355 nm wave length was found. The other performance is unknown. Ceramic superconducting were investigated but their photoelectric performance is still unknown. Ion-implanted photocathodes were produced by implanting Cs ions to a depth of 30 nm into a metal, which is the mean-free path of visible light in metals. Implantations into Ag, Au, W, and Mg were carried out.The quantum efficiency can be increased by one order of magnitude. The lifetime is longer than that of a semiconductor photocathodes. Another significant result is, that photoelectrons can be generated with ion-implanted photocathodes by green laser light.

Table: Properties of thermo-photocathodes

Material

QE

Advantages

Disadvantages

Trioxide cathode

0.1% @ 355 nm

Air transportable, can be rejuvenated by heating to 700°C, the work function is about 2 eV

long response time, need slight heating to keep the QE from degrading too quickly

B-type thermionic dispenser

3.5x10-4@ 266 nm

 

need slight heating to keep the QE from degrading too quickly

LaB6

0.1% @ 355 nm