Field Emission from Metals into Alkali Halide Crystals

The transient photocurrents associated with the movement of photoelectrons in additively colored alkali halide crystals, and the accompanying growth of positive space charge at the cathode leading to field emission, has been previously calculated and verified by room-temperature measurements. These experiments have been extended to low temperatures and the theory has been expanded to include the final photocurrents produced by the space-charge enhanced field.From measurements of the initial current, its time constant, and of the final current, a field strength at the cathode can be derived and correlated to the final current density. Fowler-Nordheim plots yield a straight line, as expected for field emission, but the order of magnitude of the calculated field and the "apparent" work function are too small to be reasonable. The work function furthermore exhibits a dependence on light intensity. The field emission should be nearly independent of temperature, and this is verified. The photocurrent decreases from room temperature to -130^∘{}C by a factor of only two; this decrease can be attributed to the lowered mobility of the electrons in the crystal. Final photocurrent vs voltage curves for various metals and composite surfaces indicate that the photocurrent is controlled by the ionized F-centers in a thin surface layer of the crystal and that the image forces of these centers create the high fields required for the emission.