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

In the present study, we reported the results of the investigation of electrical and optical measurements in Al(x)Ga(1-x)N/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 Al(0.20)Ga(0.80)N/GaN heterostructures. The related electrical and optical properties of Al(x)Ga(1-x)N/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 (lambda=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 (Al(0.20)Ga(0.80)N/GaN/sapphire), the carrier density increased from 7.59x10(12) to 9.9x10(12) cm(-2) via illumination at 30 K. On the other hand, in sample B (Al(0.20)Ga(0.80)N/GaN/SiC), the increments in the carrier density were larger than those in sample A, in which it increased from 7.62x10(12) to 1.23x10(13) cm(-2) at the same temperature. The 2DEG mobility increased from 1.22x10(4) to 1.37x10(4) cm(-2)/V s for samples A and B, in which 2DEG mobility increments occurred from 3.83x10(3) to 5.47x10(3) 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 tau as well as a decay exponent beta 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 Al(0.20)Ga(0.80)N/GaN/sapphire and Al(0.20)Ga(0.80)N/GaN/SiC heterojunction samples are 343 and 228 meV, respectively. The activation energy for the thermal capture of an electron by the defects Delta E changed with the substrate materials. Our results show that the substrate material strongly affects the electrical and optical properties of Al(0.20)Ga(0.80)N/GaN heterostructures. These results can be explained with the differing degrees of the lattice mismatch between the grown layers and substrates. (C) 2008 American Institute of Physics.