6.4: Photoemission images of cesium coated p- and n-type GaN

We report photoemission images of a GaN p-n junction patterned and etched to reveal both n-type and p-type regions. Immediately following Cs deposition at low temperature, the n-type areas produced more emission current than did the p-type areas and required only low energy (3 eV) photons to do so. After heating to 375°C the p-type areas became dominant when illuminated with high energy photons (4.8 eV), however the n-type areas continue to dominate when the specimen is illuminated with 3 eV photons. After further heating to 423°C portions of the Cs-coated n-type areas continue to produce emission from 3 eV photons. This behavior can be interpreted as the result of the formation of a surface state due to Cs interacting with the n-type areas, and photoemission directly from the Cs-induced state into vacuum. If this model is correct, it implies that the emission is relatively prompt, since there is no transport in the bulk material.