Band Offsets at Semiconductor Heterojunctions: Bulk or Interface Properties?

Abstract

The problem of whether band offsets at semiconductor interfaces are determined by bulk properties of the constituents or substantially affected by interface phenomena is critically readdressed. In particular, the conditions under which band offsets do not depend on the interface orientation are examined. State-of-the-art ab-initio pseudopotential calculations are performed for (GaAs)₃(AlAs)₃ grown in the (001), (110), and (111) directions. Our results are analysed through a novel definition of the interface charge distribution which does not snake any use of ideal reference interfaces: the dipole corresponding to such a distribution directly yields the potential drop across the interface. Our calculations give for the (001), (110), and (111) interfaces a band offset of 0.49, 0.51, and 0.49 eV respectively, thus indicating that orientation independence holds in this case. However, in the case of the (111) orientation, two inequivalent interfaces exist whose offsets slightly differ (0.07 eV); associated with this difference we also found a net interfacial charge accumulation at the two inequivalent interfaces (±2.8 ×10⁻⁴ electrons per unit surface cell). Our results are finally interpreted through a new model based on crystal symmetry and whose only ingredients are the bulk charge densities of the the two constituents. The model — though not reproducing the fine details of the (111) superlattice — is in excellent agreement with our first-principles results and with available experimental data.