Alexey Nikiforov

Development of AlGaN-based graded-index-separate-confinement-heterostructure deep UV emitters by molecular beam epitaxy

The authors report on the growth, structure, and emission properties of AlGaN double heterostructures having a graded-index-separate-confinement-heterostructure design. These devices were grown on the Si-face of 6H-SiC substrates by plasma-assisted molecular-beam epitaxy. The active region of the device consists of 75-nm thick Al0.72Ga0.28N film, confined by two 50-nm thick compositionally graded AlxGa1−xN films (x = 1–0.8 and x = 0.8–1) and two AlN cladding layers. X-ray diffraction and transmission electron microscopy provide evidence that the compositionally graded AlGaN layer may also be serving as a strain transition buffer, by blocking threading defects in the vicinity of the AlN/AlGaN heterointerface. Polarization dependent photoluminescence studies indicate that the emission from these structures at 257 nm is transverse magnetic polarized. Simulation studies indicate that the vertical confinement of the optical mode in these structures is 32.5% and simulations of the band structure indicate the formation of a p-n junction resulting from polarization-induced doping. Electron-beam pumping of these structures provides evidence of the onset of stimulated emission at room temperature.

Photoelectrochemical corrosion of GaN-based p-n structures

Direct water photoelectrolysis using III-N materials is a promising way for hydrogen production. GaN/AlGaN based p-n structures were used in a photoelectrochemical process to investigate the material etching (corrosion) in an electrolyte. At the beginning, the corrosion performs through the top p-type layers via channels associated with threading defects and can penetrate deep into the structure. Then, the corrosion process occurs in lateral direction in n- type layers forming voids and cavities in the structure. The lateral etching is due to net positive charges at the AlGaN/GaN interfaces arising because of spontaneous and piezoelectric polarization in the structure and positively charged ionized donors in the space charge region of the p-n junction.

Investigation of direct water photoelectrolysis process using III-N structures

GaN, GaN/AlGaN and GaN/InGaN-based structures were used to study water photoelectrolysis in KOH-based electrolyte, measurement of current-potential characteristics, investigation of electrode corrosion and for hydrogen generation. The corrosion process of p-n AlGaN/GaN structure starts in the p-layers, spreads via vertical channels associated with threading defects, and continues laterally along the n-layers, where large local hollows and voids were observed. The H2 production rate of 0.3-0.6 ml/cm2×h was measured for n-GaN structure.

Photo-Electrochemical Etching in the Process of Direct H2 Generation by Illumination of GaN-Based Material Structures Immersed in Water

Hydrogen is considered a promising candidate for energy storage. We investigated the cleanest way for hydrogen production by direct photo electrolysis of water with GaN/AlGaN based p-n structures used as working electrodes. Besides the H2 production rate, an important consideration is the material etching (corrosion) that accompanies the photo-electrochemical process. The GaN-based structures were grown on sapphire substrates by the chloride hydride vapor phase epitaxy and used as a photo anode immersed into an aqueous electrolyte. For a p-n GaN/AlGaN structure we observed a H2 production rate of 0.6 mL/cm2×h. Corrosion of the electrode proceeds in two steps. First, there is a near vertical etching process, which is associated with defects in the material and penetrates deep into the structure. Subsequently, the process involves etching of n-type layers in lateral direction resulting in the formation of voids and cavities. The lateral etching is due to net positive charges arising from the spontaneous and...