Negative photoconductivity due to intraband transitions in GaN/AlN quantum dots

In-plane photoconductivity (PC) measurements in a GaN/AlN quantum dots (QDs) layer show a TM-polarized infrared (IR) peak, at 0.9 eV, and a visible-UV (vis-UV) peak, at 2.8 eV. Based on the energy and polarization dependence, the 0.9 eV is associated with the polarized S to Pz intraband transition within the QDs. The IR PC turns from positive PC (PPC) to negative PC (NPC) as temperature is raised, increasing exponentially from 50 to 300 K. Vis-UV radiation renders PPC at all temperatures. Combined with vis-UV radiation, the IR PC is negative even at low temperatures. Based on these observations, we suggest a model in which IR excited carriers in the QD layer are coupled to deep levels (DLs) in the AlN barrier and turn immobile, i.e., NPC is observed. Vis-UV radiation re-excites them into the QDs, resulting in PPC. At lower temperatures coupling into the DL becomes inefficient, thus, IR excitation results in PPC. This model was translated into rate equations. Simulations based on these rate equations reproduce well the experimental results.