Tunable GaN/AlGaN ultraviolet detectors with built-in electric field

We present the optical and the electrical properties of GaN/AlGaN structures that can be used as tunable ultraviolet photodetectors. The photosensitivity spectrum of the structures can be tuned in the range 3.5-3.85 eV by external voltage. The photosensitivity of the sample in the 3.5 eV spectral range increases about 500 times when the reverse bias changes from -2 to -6 V. The effect can be explained by changes in the electric field in the GaN/AlGaN structure. The field is generated by the internal electric polarization and the external bias. We present also a numerical modeling of the electric field, the potential profiles, and the current flow in such structures. Our modeling shows that at low bias a two-dimensional (2D)-electron gas at the AlGaN/GaN interface screens the electric field generated by spontaneous polarization. The lack of the field stops transport of photoexcited holes. The holes that are accumulated on the interface causes fast electron recombination and reduces photocurrent. The external electric field can move the 2D-electron gas and increase photocurrent from the GaN layer.