Near-midgap deep levels in Al0.26Ga0.74N grown by metal-organic chemical vapor deposition

Abstract

A deep level with an activation energy of 1.0 eV in n-type Al 0.26 Ga 0.74 N grown by metal-organic chemical vapor deposition was detected by deep-level transient spectroscopy ͑DLTS͒ with a sampling time window of several seconds. The deep-level density was 6 ϫ 10 15 cm −3 . At the temperatures around which the DLTS peaks were observed, capacitance transient was measured. Under the dark condition, a capacitance increase was observed, corresponding to the thermal emission of electrons from the level with 1.0 eV activation energy. After that, we observed a large capacitance increase under illumination with 2.3 eV photon energy. On the basis of potential simulation taking account of deep levels, we found that the photoinduced capacitance change arose from electron emission from additional near-midgap levels in energy ranging from E C − 1.5 to E C − 2. Deep levels in AlGaN induce not only serious degradation, such as drain current instability and lack of long-term reliability, in AlGaN/GaN-based transistors 1,2 but also significant reduction in light-emitting efficiency in ultraviolet light-emitting diodes and laser diodes using AlGaN materials. 3 Although photoluminescence ͑PL͒ or cathodoluminescence ͑CL͒ investigation has shown some characteristic luminescence peaks related to deep electronic levels in AlGaN and AlN, 3-7 electrical characterization on deep levels in AlGaN or AlGaN/GaN heterostructure has been reported only a few times. [8][9][10][11] In particular, the deep-level transient spectroscopy ͑DLTS͒ method only detected deep levels with activation energies under 0.9 eV, 10 in spite of the fact that a near-midgap level can act as a dominant recombination center rather than a level near conduction or valence band. To detect near-midgap levels in AlGaN, a temperature range should extend to 800 K in a typical DLTS measurement condition. In such very high temperatures, a Schottky or a p-n junction suffers from serious leakage and thermal junction breakdown. In this letter, we characterize near-midgap deep levels in AlGaN grown by metal-organic chemical vapor deposition ͑MOCVD͒ using a combination of DLTS and photoassisted capacitance transient methods.We used a Si-doped n-Al 0.26 Ga 0.74 N layer with a thickness of 1.0 m grown at 1000°C on a sapphire substrate by MOCVD. Hall measurements determined the typical values of electron concentration and mobility of the AlGaN layer at room temperature ͑RT͒ to be 2 ϫ 10 17 cm −3 and 100 cm 2 / V s, respectively. As a ring-shaped Ohmic contact, a Ti/Al/Ti/Au multilayer structure ͑20/50/20/150 nm͒ was deposited on the AlGaN surface, followed by an annealing at 800°C for 1 min in N 2 ambient. A circular Schottky electrode with a diameter of 200 m was fabricated at the center of the Ohmic ring by an electron-beam deposition of Ni.To try to detect deep levels with large activation energies in a temperature range as low as possible in the DLTS measurement, a time window of up to several seconds was used with an optimized signal sampling system and a temperaturesweeping rate...

Traps and Defects in AlGaN-GaN High Electron Mobility Transistors on Semi-Insulating SiC Substrates

“…In addition, I-V-T measurements were not able to detect current contributions from either defect-related trap states or from conduction along dislocations based on the Frenkel-Poole emission model. Our level 1 traps are due to point defects related to Group III-vacancy complexes or N anti-sites, whereas our level 2 traps are due to point defects related to formation of extended defects including dislocations [9,10]. Among various techniques used to investigate traps in these material systems, we chose a capacitance digital DLTS technique mainly because this technique provides precise information on traps by measuring changes in capacitance due to the presence of traps in the depletion region of metal-semiconductor junctions.…”

Capacitance–Voltage Characteristics of Al2O3/AlGaN/GaN Structures and State Density Distribution at Al2O3/AlGaN Interface

“…The reason for this is not clear yet, but there is a possibility that the high interface state density is related to oxygen incorporation into the AlGaN layer 32) or a high density of defect levels in the AlGaN layer. 33) To realize a stable and reliable gate control in MOS-type AlGaN/GaN HEMTs, it is necessary to reduce interface state density by further characterization of the Al 2 O 3 /AlGaN/GaN structure.…”

Electrical characteristics of AlGaN-GaN high electron mobility transistors and AlGaN Schottky diodes irradiated with protons

“…Densities and capture cross sections of both traps are 2.6 − 7.6×10 16 cm -3 and 7.8×10 -16 − 2.4×10 -14 cm 2 , respectively. It is most likely that our Level 1 traps are due to point defects related to Group III-vacancy complexes or N anti-sites, whereas our Level 2 traps are due to point defects related to formation of extended defects including dislocations [9,10]. Table 4.…”