Exploration of Trap Levels in GaN and Al0.2Ga0.8N Layers by Temperature-Dependent Photoconductivity Measurement

The optoelectronics properties of Gallium Nitride and its alloys are attracting increasing interest due to the potential application of these materials for UV photodetectors and high electron mobility transistors. However, the persistent photoconductivity (PPC) in the GaN based photoconductive devices affects the light sensitive characteristics and with temperature these photoinduced effects are different in ternary alloys such as that of the AlxGa1-xN layers. In order to study the temperature dependent PPC effect, we have performed photoconductivity measurements on two heterostructures constituting unintentional n-type GaN and n-type Al0.2Ga0.8N layers on GaN buffer on Mo back-coated c-plane sapphire substrate using UV monochromatic light. Low temperature PPC measurements have shown that trap levels exist in both AlGaN and GaN layers. In our samples PPC decay behavior in n-Al0.2Ga0.8N layer was found to better than the n-GaN layer at low temperatures. Broad distribution of trap (defect) levels with energies lower than the bandgap energies were observed in both n-Al0.2Ga N-0.8 layer and GaN layer as evident from room temperature photoluminescence spectra. In ternary nitrides, the presence of 2DEG channel dominates the photoresponse at low temperature whereas in binary nitrides, the photoresponse is majorly affected by the trap levels. This study enabled us to propose AlxGa1-xN based UV photodetector for low-temperature applications