GaAs/(Ga,Al)As/GaAs transmission photocathodes were fabricated by a multiple layer liquid epitaxy technique using GaAs substrates. The GaAs substrate was subsequently removed by bubble etching. Transmission mode white light sensitivity of 340 μA/lm and an electron diffusion length of 5 μm or more can be routinely achieved. The reflection sensitivity was found to be lower than that of a similarly prepared layer grown directly on the GaAs substrate. The spectral response, when analyzed with the yield formula, indicated negligible recombination at the GaAs / (Ga,Al)As interface. Also, despite the almost perfect lattice match between GaAs and AlAs, some bowing of the GaAs / (Ga,Al)As samples was observed, suggesting the presence of internal stresses. A simplified formula is derived for the transmission quantum yield which enables graphic solution of surface recombination velocity at the photocathode input surface and the ratio of photocathode thickness to the electron diffusion length without tedious computer calculations.

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