Polycrystalline GaAs layers have been deposited on sapphire substrates by the AsCl3-Ga-H2 vapour transport method, and treated with caesium and oxygen. A layer 0·6 μm thick when operated as a photocathode has shown a luminous sensitivity of 70 μA lm−1, in transmission. Thicker layers have shown lower sensitivities. Spectral response curves are given for layers 0·6 and 0·8 μm thick and show yields out to 1·4 ev.An attempt has been made to relate the results to an elementary theory for transmission photocathodes.

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