Optimization of the InAsxP1−x–Cs2O Photocathode

Zinc-doped InAsP liquid epitaxial layers with bandgaps between 0.4 and 1.34 eV were grown on InAs and InP substrates. The grown layers were 2–4-μ thick with mirror-smooth as-grown surfaces. Preliminary phase diagram calculations based on Darken's quadratic formalism to describe the ternary liquid in equilibrium with the pseudobinary solid are in good agreement with the bandgaps of the grown layers determined by photoluminescence. The InAsxP1−x–Cs2O heterojunction barrier height as a function of composition has been measured using photoemission. For InAs the barrier is at 1.24 eV, and it decreases with decreasing arsenic concentration to a value of 1.16 eV for InAsP with a 1.27-eV bandgap. For InAsxP1−x samples with bandgaps in the range 1.17–1.34 eV, high escape probabilities and efficient photoemission were observed. A typical cleaned (not cleaved) sample with a bandgap of 1.19 eV has a sensitivity of 600 μA/lm, 70 μA with a lumen source through a 2540 ir filter, a quantum efficiency of 1.5% at 1.06 μ, and a Γ escape probability of 0.08. This is the most sensitive infrared photocathode yet produced. All processing steps seem compatible with tube production. The effects of the heterojunction barrier are clearly visible with this material. The escape probability drops by an order-of-magnitude when the InAsxP1−x bandgap is reduced to 0.05 eV below the barrier.