  We present an inductively coupled-plasma reactive-ion etching process that
simultaneously provides both a high etch rate and unprecedented selectivity for
gallium phosphide (GaP) in the presence of aluminum gallium phosphide
(Al$_\textrm{x}$Ga$_{1-\textrm{x}}$P). Utilizing mixtures of silicon
tetrachloride (SiCl$_4$) and sulfur hexafluoride (SF$_6$), selectivities
exceeding 2700:1 are achieved at GaP etch rates above 3000 nm/min. A design of
experiments has been employed to investigate the influence of the inductively
coupled-plasma power, the chamber pressure, the DC bias and the ratio of
SiCl$_4$ to SF$_6$. The process enables the use of thin
Al$_\textrm{x}$Ga$_{1-\textrm{x}}$P stop layers even at aluminum contents of a
few percent.

Applied Physics;Materials Science