Demin Cai

A practical route towards fabricating GaN nanowire arrays

GaN nanowire (NW) arrays have been fabricated by the electrodeless photoelectrochemical (PEC) etching method for the first time. Under appropriate conditions, the etching process is just a dislocation-hunted process, in which the etching solution “digs down” along the threading dislocations, resulting in the formation of GaN NWs by preferentially etching away the defective parts of GaN with dislocations and retaining the flawless parts. The NWs have a density of 1∼2 × 107 cm−2, diameters ranging from 150 nm to 500 nm, and corresponding lengths ranging from 10 μm to 20 μm. Transmission electron microscopy (TEM) indicates that these GaN NWs possess few dislocations. High resolution X-ray diffraction (HRXRD) and micro-Raman measurements show that these GaN NWs are stress-free. Room temperature cathodoluminescence (CL) measurements show a single near-band-edge emission at 367 nm with a full width at half maximum (FWHM) of 8 nm from the NWs, indicating a high optical quality. Additionally, negative piezoelectric current pluses are generated from the GaN NWs when the conductive atomic force microscope is scanned cross the arrays in contact mode. Such GaN NW arrays are promising building blocks for exploring nanodevices with excellent performance.

Structure, Stress State and Piezoelectric Property of GaN Nanopyramid Arrays

GaN nanopyramid (NP) arrays have been fabricated by a convenient electrodeless photoelectrochemical etching method. Transmission electron microscopy measurement indicates that these NPs are composed of crystalline GaN surrounding a dislocation. High-resolution X-ray diffraction and the micro-Raman spectrum reveal a highly compressive stress relaxation in the NPs compared with compressed GaN subfilm. Additionally, negative piezoelectric current pluses are generated from the GaN NPs when the conductive atomic force microscope scans cross the arrays in the contact mode. The result demonstrates that the GaN NP arrays are a promising candidate for nanogenerators.

Evolution of threading dislocations in GaN epitaxial laterally overgrown on GaN templates using self-organized graphene as a nano-mask

Growth of high-quality GaN within a limited thickness is still a challenge, which is important both in improving device performance and in reducing the cost. In this work, a self-organized graphene is investigated as a nano-mask for two-step GaN epitaxial lateral overgrowth (2S-ELOG) in hydride vapor phase epitaxy. Efficient improvement of crystal quality was revealed by x-ray diffraction. The microstructural properties, especially the evolution of threading dislocations (TDs), were investigated by scanning electron microscopy and transmission electron microscopy. Stacking faults blocked the propagation of TDs, and fewer new TDs were subsequently generated by the coalescence of different orientational domains and lateral-overgrown GaN. This evolution mechanism of TDs was different from that of traditional ELOG technology or one-step ELOG (1S-ELOG) technology using a two-dimensional (2D) material as a mask.

Direct growth of GaN on sapphire with non-catalytic CVD graphene layers at high temperature

Today, heteroepitaxial GaN films on sapphire have focused on conventional two-step growth process using low temperature GaN buffer layer. Here, we show the direct growth of GaN films on sapphire by using a graphene layer at high temperature, which simplified the GaN growth process. The graphene is directly synthesized on non-catalytic sapphire substrate by chemical vapor deposition without problematic transfer processes, using C2H4 as a carbon source at the temperature of 1200 oC. The synthesized graphene has been characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM). We have compared the GaN grown on sapphire with and without graphene. The single crystal, smooth surface GaN films has been obtained on sapphire with graphene, and the nucleation of GaN films has been discussed. The GaN films illuminated high near-band-edge emission and good ultraviolet photosensor. It demonstrates that graphene is a potential, useful buffer layer for heteroepitaxy of high quality GaN films.