The lifetime of GaAs photocathodes can be greatly improved by introducing Li in the Cs+NF3 activation process. The surface activation layer of such photocathodes is studied by synchrotron radiation photoemission and is compared with GaAs photocathodes activated without Li. The charge distributions of N, F, and Cs experience significant changes when Li is added in the activation. In addition, the presence of Li causes NFx molecules to take an orientation with F atoms on top. All these changes induced by Li hold the key for the lifetime improvement of GaAs photocathodes.
Surface photovoltage effect on clean and negative electron-affinity surfaces of GaAs and its superlattice AIP Conf.
A method by which negative electron affinity GaAs photocathodes can be made to recover their photoyield following decay is reported. The source of decay can be either an oxidizer in the background gas or gas introduced through deliberate dosing. Conventional methods employ the use of the alkali Cs as the photoyield recovery agent. This work describes an alkali application-free method, similar to electron stimulated desorption, for photoyield recovery.
Amorphous silicon and amorphous silicon germanium photoemitters grown via radio frequency plasma enhanced chemical vapor deposition have been investigated to determine their usefulness as photoinjector electron sources. Characterized properties include activation process, wavelength dependent photoyield, germanium induced photoyield shift, background gas and ion sensitivities, and average transverse emission energy. While overall lower in yield than the GaAs photoemitter, many of their properties are comparable. The amorphous silicon photoemitter robustness upon gas and ion exposure is superior to that of GaAs. The combined properties of amorphous silicon germanium photoemitters make them ideal candidates for low cost photoinjector sources or as protective photoemitting layers on more sensitive photogenerating materials.
AThe GaAsP/GaAs superlattice (SL) structure has been widely recognized as the most efficient spin polarized electron source with 90°0 maximum polarization and more than 1% quantum efficiency [1]. The main spin depolarization mechanisms in these structures are: interband absorption smearing due to bandedge fluctuations, hole scattering between the heavy hole (HH) and light hole (LH) states that causes a broadening of the LH band, spin precession due to an effective magnetic field generated by the lack of crystal inversion symmetry and spin orbit coupling i.e. the Dyakonov-Perel mechanism (DP) and electronhole scattering. The last 2 mechanisms are material related and they take place during the transport of electrons in the photocathode active region. By decreasing the transport time of the electrons in the lOOnm cathode active region, all depolarization mechanisms are suppressed and the total number of scattering events also decreases. Based on the scattering rates in GaAs as a function of energy, it is possible to minimize the total number of scattering events when electrons have energies in the 0.05-0.1eV region. Electrons can acquire such energies inside the active region during the presence of accelerating fields of 0.1-V/lOOnm. For such fields, the cathode area can be completely depleted when the doping concentration of the active region is _1017cm-3.A metal grid was deposited on lOOnm GaAs based with different structure and doping concentrations. The grid formed a Schottky contact with the semiconductor film. I-V curves, quantum efficiency (QE) and spin polarization measurements were performed for various samples. The results are presented in this paper. 2 -0 4 02 00 0 4 05 0 Bias Vcotage (V) Bi'S Volt tiage (V) Figure 1. Spin polarization enhancement due to applied electric field on 1017cm-3 p-doped lOOnm (a)GaAs/GaAsP sample with Re grid and (b) GaAs sample with W grid. The spin polarization of the photogenerated electrons was measured when circularly polarized laser light was illuminating the samples.Further improvements in observing the bias effect on the polarization can be achieved by fabricating more stable Schottky contacts at the cathode surface and using samples with high polarization such as strained GaAsP/GaAs superlattices.
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