Illumination and annealing characteristics of two-dimensional electron gas systems in metal-organic vapor-phase epitaxy grown AlxGa1−xN∕AlN∕GaN heterostructures

We studied the persistent photoconductivity (PPC) effect in AlxGa1−xN∕AlN∕GaN heterostructures with two different Al compositions (x=0.15 and x=0.25). The two-dimensional electron gas formed at the AlN∕GaN heterointerface was characterized by Shubnikov-de Haas and Hall measurements. Using optical illumination, we were able to increase the carrier density of the Al0.15Ga0.85N∕AlN∕GaN sample from 1.6×1012to5.9×1012cm−2, while the electron mobility was enhanced from 9540to21400cm2∕Vs at T=1.6K. The persistent photocurrent in both samples exhibited a strong dependence on illumination wavelength, being highest close to the band gap and decreasing at longer wavelengths. The PPC effect became fairly weak for illumination wavelengths longer than ∼530nm and showed a more complex response with an initial negative photoconductivity in the infrared region of the spectrum (λ>700nm). The maximum PPC efficiency for 390nm illumination was 0.011% and 0.005% for Al0.25Ga0.75N∕AlN∕GaN and Al0.15Ga0.85N∕AlN∕GaN samples, resp...