Transition from negative to positive photoconductivity in p-type Pb1-xEuxTe films

We investigated the photoconductivity effect in p-type Pb1-xEuxTe films for x = 0.01, 0.02, 0.03, 0.05, and 0.06 at T = 300 K. The measurements revealed a clear transition from negative to positive photoconductivity as the Eu content x is increased at room temperature. This transition is related to the metal-insulator transition that occurs due to the disorder originated from the introduction of Eu atoms and it is an Anderson transition. Our investigation found that, from the potential application point of view, the sample x = 0.06 is more suitable, i.e., it presents an almost noise-free signal and the higher photoconductivity amplitude response. The photoconductive amplitude response for the sample with x = 0.06 was investigated further in the temperature range of 77-300 K and, surprisingly, multiple additional transitions were observed with amplitudes that reached around 200 times the original value before illumination. We show that this anomalous behavior is a consequence of the generation and recombination rates between the bands and the 4f level and a defect level located inside the band gap.