Etching characteristics of Al_xGa_1-xAs in (NH₄)₂S_x solutions

The etching of GaAs and Al_xGa_1-xAs have been characterized in (NH₄)₂S_x solution with various excess sulfur concentrations and for temperatures ranging from 20 to 60°C. The etch rate varies with the concentration of excess sulfur and is highest at 60°C using a 4% excess sulfur solution. The etch rate of Al _xGa_1-xAs increases exponentially with increasing Al mole fraction. Activation energies of 19.8 and 15.9 kcal/mole are obtained for GaAs and Al_0.3Ga_0.7As in 4% (NH ₄)₂S_x, respectively. These high values and the linear time dependence of etch rates signify that the etching process of Al _xGa_1-xAs in (NH₄)₂S _x solutions is predominantly reaction-rate limited. Possible chemical processes involved in the etching and the formation of a passivating sulfur layer are discussed.