ß-Ga2O3nanotube arrays for high-performance self-powered ultraviolet photoelectrochemical photodetectors.

Self-powered ultraviolet (UV) photodetectors (PDs) are critical for future energy-efficient optoelectronic systems due to their low energy consumption and high sensitivity. In this paper, the vertically alignedß-Ga2O3nanotube arrays (NTs) have been prepared on GaN/sapphire substrate by the thermal oxidation process combined with the dry etching technology, and applied in the UV photoelectrochemical photodetectors (PEC-PDs) for the first time. Based on the large specific surface area ofß-Ga2O3NTs on GaN/sapphire substrates and the solid/liquid heterojunction, the PEC-PDs exhibit excellent self-powered characteristics under 255 nm (UVA) and 365 nm (UVC) light illumination. Under 255 nm (365 nm) light illumination, the maximum responsivity of 49.9 mA W-1(32.04 mA W-1) and a high detectivity of 1.58 × 1011Jones (1.01 × 1011Jones) were achieved for theß-Ga2O3NTs photodetectors at 0 V bias. In addition, the device shows a fast rise/decay time of 8/4 ms (4/2 ms), which is superior to the level of the previously reported self-powered UV PEC-PDs. This high-performance PEC-PD has potential applications in next-generation low-energy UV detection systems.

High Power Figure-of-Merit, 10.6-kV AlGaN/GaN Lateral Schottky Barrier Diode with Single Channel and Sub-100-µm Anode-to-Cathode Spacing.

GaN-based lateral Schottky barrier diodes (SBDs) have attracted great attention for high-power applications due to its combined high electron mobility and large critical breakdown field. However, the breakdown voltage (BV) of the SBDs are far from exploiting the material advantages of GaN at present, limiting the desire to use GaN for ultra-high voltage (UHV) applications. Then, a golden question is whether the excellent properties of GaN-based materials can be practically used in the UHV field? Here, UHV AlGaN/GaN SBDs are demonstrated on sapphire with a BV of 10.6 kV, a specific on-resistance (RON,SP ) of 25.8 mΩ cm2 , yielding a power figure-of-merit (P-FOM = BV2 /RON,SP ) of 4.35 GW cm-2 . These devices are designed with single channel and 85-µm anode-to-cathode spacing, without other additional electric field management, demonstrating its great potential for the UHV application in power electronics.

The influence of an AlN seeding layer on nucleation of self-assembled GaN nanowires on silicon substrates.

Gallium nitride (GaN)-based nanowires (NWs) have attracted much attention for the fabrication of novel nanostructured devices. In this paper, the influence of an AlN seeding layer on the nucleation of self-assembled GaN NWs grown by plasma-assisted molecular beam epitaxy (MBE) on Si (111) substrates has been investigated. Not only is the formation of a two-dimensional compact GaN layer at the bottom of the NWs suppressed, but also a high density of vertically aligned well-separated GaN NWs originating from GaN islands are successfully obtained after introducing annealing and nitridation processes. Scanning electronic microscope and transmission electron microscope measurements show that the NWs have a high crystalline wurtzite structure nearly free of dislocations and stacking faults and the NW diameter remains constant over almost the entire length. Due to the temperature-dependent diffusion length of Ga adatoms during the nucleation process, the formation of well-separated NWs relies on the distribution and morphology of the underlying AlN seeding layer. Moreover, the SiNx layer served as mask to inhibit coalescence at the nucleation sites. The developed growth processes and the obtained results provide a viable path facilitating the use of MBE growth techniques to fabricate III-nitride NW-based materials and related devices on Si substrates.

Study on the self-absorption of InGaN quantum wells at high photon density.

In this paper, the self-absorption of InGaN quantum wells at high photon density is studied based on a rectangular ridge structure. The ridge structure was fabricated based on a standard GaN-based blue LED wafer grown on (0001) patterned sapphire substrate. The high-density photons were obtained by a high-power femtosecond laser with high excitation of 42kW/cm2 at room temperature. Based on the analysis of the photoluminescence intensities of the InGaN quantum wells, we found that the absorption coefficient of the InGaN quantum wells varies with the background photon density. The results revealed that the final absorption coefficient of the InGaN quantum well decreases with the increase of photon density, which can be 48.7% lower than its normal value under our experimental conditions.

[Strain in GaN epi-layer grown by hydride vapor phase epitaxy].

In the present paper, strain in GaN epitaxial layer grown by hydride vapor phase epitaxy (HVPE) was investigated by means of high-resolution X-ray diffraction (HRXRD), Raman spectra and photoluminescence (PL) measurements. Both the biaxial in-plane and out-of-plane strains (of the order of -10(-4) and 10(-4), respectively) and the hydrostatic strain component (of the order of -10(-5)) were extracted from HRXRD measurements. These values agreed well with the ones computed from the blue-shift of E2 Raman mode and the near-band-edge PL peak. The results showed that strains in GaN layer were superposed by the biaxial strain and hydrostatic strain.

The Study on the Lasing Modes Modulated by the Dislocation Distribution in the GaN-Based Microrod Cavities.

Low-threshold lasing under pulsed optical pumping is demonstrated in GaN-based microrod cavities at room temperature, which are fabricated on the patterned sapphire substrates (PSS). Because the distribution of threading dislocations (TDs) is different at different locations, a confocal micro-photoluminescence spectroscopy (µ-PL) was performed to analyze the lasing properties of the different diameter microrods at the top of the triangle islands and between the triangle islands of the PSS substrates, respectively. The µ-PL results show that the 2 µm-diameter microrod cavity has a minimum threshold of about 0.3 kW/cm2. Whispering gallery modes (WGMs) in the microrod cavities are investigated by finite-difference time-domain simulation. Combined with the dislocation distribution in the GaN on the PSS substrates, it is found that the distribution of the strongest lasing WGMs always moves to the region with fewer TDs. This work reveals the connection between the lasing modes and the dislocation distribution, and can contribute to the development of low-threshold and high-efficiency GaN-based micro-lasers.

A Selective Etching Route for Large-Scale Fabrication of ß-Ga2O3 Micro-/Nanotube Arrays.

In this paper, based on the different etching characteristics between GaN and Ga2O3, large-scale and vertically aligned ß-Ga2O3 nanotube (NT) and microtube (MT) arrays were fabricated on the GaN template by a facile and feasible selective etching method. GaN micro-/nanowire arrays were prepared first by inductively coupled plasma (ICP) etching using self-organized or patterning nickel masks as the etching masks, and then the Ga2O3 shell layer converted from GaN was formed by thermal oxidation, resulting in GaN@Ga2O3 micro-/nanowire arrays. After the GaN core of GaN@Ga2O3 micro-/nanowire arrays was removed by ICP etching, hollow Ga2O3 tubes were obtained successfully. The micro-/nanotubes have uniform morphology and controllable size, and the wall thickness can also be controlled with the thermal oxidation conditions. These vertical ß-Ga2O3 micro-/nanotube arrays could be used as new materials for novel optoelectronic devices.