Comparison between air and UV/ozone surfaces passivation methods of GaAs (100) substrates

Air and UV/ozone exposure have been used for growing sacrificial thin oxide layers on HCl etched GaAs (100) surfaces. Passive films have been then desorbed by vacuum thermal cleaning in order to prepare suitable surfaces for molecular beam epitaxy (MBE) growth. Structure, thickness and chemical composition of passive films and desorbed surfaces have been studied by ellipsometry and angle dependent X-ray photoelectron spectroscopy (ADXPS). The results have indicated an increased chemical reactivity of both arsenic and gallium during UV/ozone exposure, compared to that of air exposure, that produces Ga (III) oxide enriched films. Furthermore, XPS results have also shown that the thermal desorption behavior are different. In particular, GaAs (100) after short term UV/ozone exposure and oxide film desorption, has an As/Ga surface atomic ratio close to unity and a level of carbon contamination below the XPS detectability. On the contrary, air exposed surfaces never have a stoichiometric composition and carbon is not completely removed by a vacuum heating.