Ultrabright and Ultrafast III–V Semiconductor Photocathodes

Crucial photoemission properties of layered III–V semiconductor cathodes are predicted using Monte Carlo simulations. Using this modeling, a layered GaAs structure is designed to reduce simultaneously the transverse energy and response time of the emitted electrons. This structure, grown by molecular beam epitaxy and activated to negative electron affinity, is characterized. The measured values of quantum efficiency and transverse energy are found to agree well with the simulations. Such advanced layered structures will allow generation of short electron bunches from photoinjectors with superior beam brightness.

Figure 1

Process of photoemission from (a) activated $p$-doped GaAs, (b) layered structure with 100 nm intrinsic GaAs. The various electron processes (indicated $A-H$) are described in the text.

Figure 2

Simulations of (a) QE, (b) response time, and (c) MTE as a function of the thickness of intrinsic GaAs layer for NEA of 100 and 200 meV. A thin layer of intrinsic GaAs on heavily $p$-doped GaAs causes the response time and MTE to reduce, changing them in a favorable way. The reduction in QE is unfavorable but can be tolerated for use in a photoinjector.

Figure 3

(a) QE and (b) MTE as a function of incident photon energy for the two samples. The simulated results agree well with the experiment. The layered cathode shows a reduced QE with respect to the control sample. However, the QE is still greater than 1% in the green, exceeding the QE requirement of most photoinjectors [7]. NEA is fixed for a particular sample. It should be noted that the photon energy range above 2.4 eV is outside the validity of the 3-valley model for GaAs and cannot be simulated.