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Română (Romanian)GaN Nanostructuring
Due to its excellent properties: wide direct bandgap, high thermal stability, high electron mobility and saturation velocity, gallium nitride (GaN) is a suitable material for a wide range of applications in optoelectronics and highpower/high-frequency devices, such as light emitting diodes (LEDs) for solid-state lighting and visible light communications (VLC), and radar applications.
The experiments were carried out on 2-inch diameter HVPE grown n-GaN single crystalline templates. The 300-μm thick wurtzite-phase GaN substrates were (0001)-orientated. EC etching was carried out in stirred 0.3M HNO3 or 3.5 M NaCl aqueous solution for 15 min under different voltage biases.
The anodic etching of the N-face of the wafer in HNO3 electrolyte starts with the formation of a porous layer with the thickness around 2 μm with most of pores propagating perpendicularly to the wafer surface. Underneath this layer, one can distinguish a complex structure consisting of porous pyramids with dimensions in the range of tens of microns, as it can be seen in the image on the right.
The EC etching in HNO3 electrolyte on the of the wafer starts at some nucleation points determined by surface defects and imperfections and proceeds in radial directions at the initial phase of the EC process. Porous structures with spatially modulated degree of porosity can be obtained by changing the anodization voltage during the pore growth process.
HVPE-grown GaN can be efficiently porosified in a 3.5 M NaCl electrolyte under 15 V anodic bias. A clear formation of porous circular/hexagonal rings is observed at the surface. Such morphologies with alternation of regions with high and low degrees of porosity are typical for the N-surfaces subjected to EC or photoelectrochemical etching. The porous structures develop from the surface to the bulk, as illustrated by a cross-sectional image shown below, and the pores penetrate rather deep in the wafer, similarly to the case of etching in the HNO3 electrolyte, resulting in the generation of pores oriented perpendicularly to the wafer surface in deeper regions exhibiting a more uniform conductivity.
The HR-STEM images acquired show two lattice spacings between adjacent planes along reciprocal perpendicular directions that were found to be of 0.28 nm and 0.26 nm. These values correspond to spacing between adjacent (1-100) and (0001) planes of the wurtzite GaN lattice, respectively.
The results presented above are obtained in collaboration with the group headed by Prof. Dr. Ion Tiginyanu, President of The Academy of Sciences of Moldova.