The persistent photoconductivity effect in AlGaN/GaN heterostructures grown on sapphire and SiC substrates

Abstract

In the present study, we reported the results of the investigation of electrical and optical measurements in AlxGa1−xN/GaN heterostructures (x=0.20) that were grown by way of metal-organic chemical vapor deposition on sapphire and SiC substrates with the same buffer structures and similar conditions. We investigated the substrate material effects on the electrical and optical properties of Al0.20Ga0.80N/GaN heterostructures. The related electrical and optical properties of AlxGa1−xN/GaN heterostructures were investigated by variable-temperature Hall effect measurements, photoluminescence (PL), photocurrent, and persistent photoconductivity (PPC) that in turn illuminated the samples with a blue (λ=470 nm) light-emitting diode (LED) and thereby induced a persistent increase in the carrier density and two-dimensional electron gas (2DEG) electron mobility. In sample A (Al0.20Ga0.80N/GaN/sapphire), the carrier density increased from 7.59×1012 to 9.9×1012 cm−2 via illumination at 30 K. On the other hand, in sample B (Al0.20Ga0.80N/GaN/SiC), the increments in the carrier density were larger than those in sample A, in which it increased from 7.62×1012 to 1.23×1013 cm−2 at the same temperature. The 2DEG mobility increased from 1.22×104 to 1.37×104 cm−2/V s for samples A and B, in which 2DEG mobility increments occurred from 3.83×103 to 5.47×103 cm−2/V s at 30 K. The PL results show that the samples possessed a strong near-band-edge exciton luminescence line at around 3.44 and 3.43 eV for samples A and B, respectively. The samples showed a broad yellow band spreading from 1.80 to 2.60 eV with a peak maximum at 2.25 eV with a ratio of a near-band-edge excitation peak intensity up to a deep-level emission peak intensity ratio that were equal to 3 and 1.8 for samples A and B, respectively. Both of the samples that were illuminated with three different energy photon PPC decay behaviors can be well described by a stretched-exponential function and relaxation time constant τ as well as a decay exponent β that changes with the substrate type. The energy barrier for the capture of electrons in the 2DEG channel via the deep-level impurities (DX-like centers) in AlGaN for the Al0.20Ga0.80N/GaN/sapphire and Al0.20Ga0.80N/GaN/SiC heterojunction samples are 343 and 228 meV, respectively. The activation energy for the thermal capture of an electron by the defects ΔE changed with the substrate materials. Our results show that the substrate material strongly affects the electrical and optical properties of Al0.20Ga0.80N/GaN heterostructures. These results can be explained with the differing degrees of the lattice mismatch between the grown layers and substrates.