Here we report the growth and the characterization of pillar microcavities, constituted by Bragg mirrors, which are composed by alternate layers of GaAs and aluminum oxide. The selective oxidation of the AlAs layers allowed to increase the refraction index mismatch between the GaAs and AlAs. As a consequence we obtained high reflectivity Bragg mirrors even with only three layers, a factor three less than usually necessary in the more common GaAs/AlAs Bragg reflectors. This not only simplifies the growth process, but also gives a reduction of the cavity mode volume. We have examined several MESAs with pillar diameter ranging from 1 to 10 mm, containing a system of InAs quantum dots. The quality factors of the cavities range from Q = 600-2000. Enhancement of the spontaneous emission rate of QDs was observed when the emission is on resonance with cavity modes. Purcell factors up to 2 were measured.