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Electron microscopy and cathodoluminescence (CL) microanalysis were used for a comparative study of porous layers fabricated by electrochemical etching of n-GaP and n-InP substrates in aqueous solutions of sulfuric and hydrochloric acids. Both the CL and morphology of porous layers were found to depend upon the anodic current density. At high current density (100 mA/cm 2) anodization leads to the formation of so-called current-line oriented pores while at low current densities the pores grow along 111 crystallographic directions. The porosity relief was found to give rise to spatial modulation of the CL intensity. The composition microanalysis proved the stoichiometry of porous GaP and InP skeletons, although we found considerable traces of oxygen in porous GaP layers. Self-induced voltage oscillations giving rise to a synchronous modulation of the diameter of pores and CL intensity were evidenced.

Porous CdSe layers have been produced by anodic etching of crystalline substrates in a HCl solution. Anodization under in situ UV illumination resulted in the formation of uniformly distributed parallel pores with a diameter of 30 nm, stretching perpendicularly to the initial surface. At the same time, pronounced nonuniformities in the spatial distribution of pores were evidenced in samples subjected to anodic etching in the dark. Gain of luminescence was observed in some porous regions and attributed to the formation of ring microcavities for light in the porous network.

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A comparative study of the anodization processes occurring at the GaAs(111)A and GaAs(111)B surfaces exposed to electrochemical etching in neutral NaCl and acidic HNO3 aqueous electrolytes is performed in galvanostatic and potentiostatic anodization modes. Anodization in NaCl electrolytes was found to result in the formation of porous structures with porosity controlled either by current under the galvanostatic anodization, or by the potential under the potentiostatic anodization. Possibilities to produce multilayer porous structures are demonstrated. At the same time, one-step anodization in a HNO3 electrolyte is shown to lead to the formation of GaAs triangular shape nanowires with high aspect ratio (400 nm in diameter and 100 µm in length). The new data are compared to those previously obtained through anodizing GaAs(100) wafers in alkaline KOH electrolyte. An IR photodetector based on the GaAs nanowires is demonstrated.

In this paper, we report on results of a systematic study of porous morphologies obtained using anodization of HVPE-grown crystalline GaN wafers in HNO3, HCl, and NaCl solutions. The anodization-induced nanostructuring is found to proceed in different ways on N- and Ga-faces of polar GaN substrates. Complex pyramidal structures are disclosed and shown to be composed of regions with the degree of porosity modulated along the pyramid surface. Depending on the electrolyte and applied anodization voltage, formation of arrays of pores or nanowires has been evidenced near the N-face of the wafer. By adjusting the anodization voltage, we demonstrate that both current-line oriented pores and crystallographic pores are generated. In contrast to this, porosification of the Ga-face proceeds from some imperfections on the surface and develops in depth up to 50 μm, producing porous matrices with pores oriented perpendicularly to the wafer surface, the thickness of the pore walls being controlled by the applied voltage. The observed peculiarities are explained by different values of the electrical conductivity of the material near the two wafer surfaces.