Structural and electronic properties of GaP-AlP (001) superlattices

We present the results of ab initio pseudopotential calculations for studying the structural stability and the electronic structure of short-period (GaP${)}_{\mathit{m}}$(AlP${)}_{\mathit{m}}$ superlattices with m ranging from 1 to 6, composed of lattice-matched indirect-gap semiconductors. Both the bulk and epitaxial superlattices ordered in the CuAu-I, CuPt, and chalcopyrite structures are found to be unstable against phase segregation into their binary constituents at T=0. The bulk formation enthalpies are found to be similar to those for epitaxial superlattices grown on (001) GaP. The band gap of superlattices tends to decrease as the superlattice period increases. The ultrathin (001) superlattices with the superlattice period of m=1 and 2 show indirect-gap behavior while the direct band gap occurs for m≥3. Details of the electronic structure of superlattices are discussed based on the band-pushing and charge-confinement effects. The oscillator strength of the optical transition from the valence-band maximum to the conduction-band minimum state at the Γ point is found to be much stronger for even numbers of m. Both the monolayer and bilayer superlattices can be direct-gap semiconductors if substrates are selectively chosen with lattice constants above 5.48 and 5.47 Å, respectively.