By using step-graded Al compositions of epitaxial AlGaAs, we can form
a tapered oxidation front due to the combination of rapid lateral oxid
ation in a higher Al content layer and slow transverse oxidation into
adjacent lower Al content layers. A study of the oxidation behaviour o
f this epitaxial aperture layer demonstrates the feasibility of tapere
d dielectric apertures which have been predicted to provide much lower
losses in vertical cavity lasers.