New material for photoemission electron source: semiconductor alloy InGaAsP grown on GaAs substrate

Wide-bandgap epitaxial In_xGa_1−xAs_yP_1−y layers grown on GaAs substrates were investigated as a material for photoemission electron sources for the first time. By variation of x and y, both thick (of about one μm) lattice-matched unstrained layers and thin (of about 0.2 μm) lattice-mismatched strained layers with direct energy gap from 1.78 to 1.92 eV were grown. The layers were chemically treated in a glove-box, loaded into UHV via a loading chamber without exposure to air, heat cleaned, and activated to the state of negative electron affinity (NEA). For InGaAsP it was possible to reach the NEA-state by an activation with cesium only, without oxygen. The estimated e-folding lifetime of InGaAsP photocathodes operating at DC current of 400 μA was above one thousand hours in the regime with automatical additional cesiations and above one hundred hours without additional cesiations. Lifetime limitation factors are discussed. Strain-induced splitting of the valence band up to 32 meV was observed in the lattice-mismatched films; this observation gives prospects for an increase of spin polarization beyond the 50% limit.