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 Al x Ga 1−x N / GaN heterostructures ͑x = 0.20͒ 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 ͑PL͒, photocurrent, and persistent photoconductivity ͑PPC͒ that in turn illuminated the samples with a blue ͑ = 470 nm͒ light-emitting diode ͑LED͒ and thereby induced a persistent increase in the carrier density and two-dimensional electron gas ͑2DEG͒ electron mobility. In sample A ͑Al 0.20 Ga 0.80 N / GaN/ sapphire͒, the carrier density increased from 7.59ϫ 10 12 to 9.9ϫ 10 12 cm −2 via illumination at 30 K. On the other hand, in sample B ͑Al 0.20 Ga 0.80 N / GaN/ SiC͒, the increments in the carrier density were larger than those in sample A, in which it increased from 7.62ϫ 10 12 to 1.23ϫ 10 13 cm −2 at the same temperature. The 2DEG mobility increased from 1.22ϫ 10 4 to 1.37ϫ 10 4 cm −2 / V s for samples A and B, in which 2DEG mobility increments occurred from 3.83ϫ 10 3 to 5.47ϫ 10 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 as well as a decay exponent ␤ 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 centers͒ 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 ⌬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.

Dislocation-governed current-transport mechanism in (Ni/Au)–AlGaN/AlN/GaN heterostructures

“…The DDs in the Al 0.22 Ga 0.78 N / AlN/ GaN heterostructures can be taken as equal to the DDs in the GaN layers. 3 There are three main types of dislocations that are present in the GaN epitaxial layers. 16,17 The pure edge dislocation with Burgers vector b = 1 3 ͗1120͘ ͑͗a͒͘, the pure screw dislocation with Burgers vec- tor b = ͗0001͘ ͑͗c͒͘, and the mixed dislocation with b = 1 3 ͗1123͘ ͑͗c + a͒͘.…”

“…3,7 The DD of the sample was investigated by the methods of high-resolution diffractometry. The DDs in the Al 0.22 Ga 0.78 N / AlN/ GaN heterostructures can be taken as equal to the DDs in the GaN layers.…”

“…These DDs are consistent with the values given for GaN epitaxial films in the literature. 3,4,9 Analysis of the forward bias I-V data indicated that the predominant current mechanism of the Al 0.22 Ga 0.78 N / AlN/ GaN heterostructure with high DDs in the intermediate bias voltage region that was investigated in this study was a dislocation-governed current-transport mechanism rather than the other currenttransport mechanisms.…”

“…5,6 The device structures are usually grown on highly lattice-mismatched substrates, such as sapphire, 7 SiC, 8 or Si. 3,9 It still remains difficult to obtain a high-quality GaN epilayer because of the large lattice mismatch and large difference in the thermal expansion coefficients between the GaN film, sapphire, and SiC substrate. This fact causes a high level of in-plane stress and threading dislocation density ͑DD͒ generation, as grown by metal-organic chemical vapor deposition ͑MOCVD͒ in the GaN epitaxial layer.…”

“…[1][2][3] Because of the large energy band gap, the applications of Al x Ga 1−x N are extensive, such as for visible-blind ultraviolet ͑UV͒ detectors, laser diodes, and short-wave light-emitting diodes. 4 High-quality AlGaN/GaN heterostructures have been shown to contain twodimensional electron gas ͑2DEG͒, which has attracted special interest due to its potential applications in high mobility transistors operating at high power and high temperature levels.…”