Etching of wide-bandgap chemically resistant semiconductors : An electrochemical study

Abstract

The wide-bandgap semiconductors, SiC and GaN, are important for a whole range of (opto)electronic and other applications. Etching of these chemically very resistant materials poses problems in device technology. This thesis describes an electrochemical approach to etching. In addition, the use of p-type SiC as a photocathode for water splitting is described. For the (photo)electrochemical dissolution of SiC two etching systems are considered: acidic fluoride and alkaline solutions. The anodic current-potential curve of SiC in KOH solution shows a typical active/passive transition. The kinetics of the dissolution reaction were elucidated and interesting applications were identified. These include defect-selective, anisotropic and material-selective etching. Anodic etching of SiC in acidic fluoride solution, as in KOH solution, occurs for the p-type semiconductor in the dark and for the n-type semiconductor under illumination. What is striking for acidic solution is the growth of a micron-thick porous silicon oxide at positive potential. Electropolishing of p-type SiC is possible, while porous etching is observed for n-type 4H and 6H-SiC under illumination. The (photo)electrochemistry of n-type epitaxial GaN in alkaline peroxy¬disulphate (S2O82-) is described. The results form the basis for a consideration of the photoetching of the semiconductor. Three approaches are discussed: (i) photoanodic etching in which the potential of the semiconductor is fixed by a voltage source, (ii) photogalvanic etching in which the semiconductor is short circuited to the counter electrode (no voltage source), (iii) electroless photoetching (without a counter electrode). By using a two compartment cell, we showed that GaN short-circuited to a noble metal, acts as a photogalvanic cell. The factors determining the etching kinetics and surface morphology have been elucidated. It is shown that SiC is an interesting cathode for the hydrogen evolution reaction. Illuminated p-type SiC short-circuited to a platinum electrode in alkaline solution, splits water: hydrogen is formed at the semiconductor and oxygen at the metal. Surprisingly, the hydrogen can be stored in the semiconductor. These results offer interesting perspectives for hydrogen production from solar energy.