Experimental evidence of difference in surface and bulk compositions of Al xGa 1-xAs, Al xIn 1-x As and Ga xIn 1-x As epitaxial layers grown by molecular beam epitaxy

Abstract

The surface composition of Al xGa 1-xAs, Al xIn 1-xAs and Ga xIn 1-x As epitaxial layers grown by moleculer beam epitaxy (MBE) has been determined in situ by X-ray photoelectron spectroscopy (XPS). The comparison of the values deduced from XPS with bulk compositions resulting from X-ray diffraction and RHEED oscillations leads to the conclusion that the most weakly bound group III element (i.e. Ga in Al xGa 1-xAs, In in Al xIn 1-xAs and Ga xIn 1-xAs) segregates at the alloy surface. The key rôle of of the surface temperature is illustrated in the case of Al xGa 1-xAs alloys for which a large range of growth temperatures are possible: a significant difference between surface and bulk compositions is observed between 620 and 680°C reaching a maximum near 650°C. This influence of the growth temperature is discussed in the framework of the previously suggested correlation between Ga segregation and the so-called forbidden temperature zone in MBE growth of Al xGa 1-xAs.