Electronic consequences of lateral composition modulation in semiconductor alloys
Physical Review B (Condensed Matter)
Lateral composition modulation ͑CM͒ is a periodic, position-dependent variation in alloy composition occurring in the substrate plane, perpendicular to the growth direction. It can be induced by growing sizemismatched short-period AC/BC superlattices ͑SL͒. Here we study the electronic structure induced by such lateral composition modulation in GaAs/InAs, GaP/InP, and AlP/GaP, in search of optical properties relative to the corresponding random alloys. We investigate in detail the properties of
... the properties of ͑a͒ pure CM without any SL, ͑b͒ pure SL without any CM, and ͑c͒ the combined CMϩSL system. The systems are modeled by constructing a large supercell where the cation sublattice sites are randomly occupied in the lateral ͑vertical͒ direction according to the composition variation induced by CM ͑SL͒. The atomic structure and strain induced by CM and SL are explicitly taken into account using an atomistic force field. This approach is found to be crucial for an accurate description of the microscopic strain in CMϩSL systems. The electronic structure is solved using specially constructed empirical pseudopotentials and plane-wave expansion of the wave functions. We find that ͑i͒ CM in GaAs/InAs and GaP/InP systems induces type-I band alignment ͑electrons and holes localized in the same spatial region͒, while CM in AlP/GaP is shown as an example exhibiting type-II band alignment. ͑ii͒ CM and SL both induce significant contributions ͑which add up nearly linearly͒ to band-gap redshift with respect to random alloy. CM in GaP/InP is found to induce larger band-gap redshifts than in GaAs/InAs due to larger band offsets in the former system. ͑iii͒ The symmetry of electronic states at the valence band maximum is sensitively affected by CM: the lowest energy optical transitions exhibit strong polarization where transitions polarized perpendicular to the CM are favored, while transitions polarized parallel to the CM are surpressed by being shifted to higher energy. These observations, as well as the magnitude of the predicted band-gap redshift, agree with available experimental data, and suggest that control of composition modulation during growth might be used to tailor band gaps, carrier localization, and transition polarizations relative to random alloys.