The etching behavior of n-GaP in aqua regia solutions

Abstract

The etching of GaP, whether in strong HNO₃ or in strong HCl is severely inhibited, whereas mixtures of HNO₃/HCl are commonly employed as etchants for chemical polishing. The etching mechanism of n-GaP in aqua regia (3HCl/1HNO₃) has been investigated. Aqua regia etching may occur by accomplishing both the following two processes :(1) oxidation and (2) dissolution. First, the nascent chlorine and nitrate ions obtained from the vigorous interaction between HCl and HNO₃, may easily oxidize the surface atoms of GaP, then nucleophilic attacked by Cl⁻ on the electron-poor trivalent gallium ions of the oxide lattice generally takes place over the passive surface. Moreover, the remaining phosphate or phosphorus oxide is very soluble. Ga and P atoms are thus carried away from the surface by the etchant system. In HNO₃-etch, the nitrate ion formed is a weak nucleophile and the amount of its production is very small. In the HCl-etch, Cl⁻ causes a direct attack on GaP substrate via an Ga^(I) chloro intermediate whose thermodynamics is very unfavorable. Thus, the experimental observations are in good agreement with the mechanistic concepts presented.