|
Since their initial proposal by Esaki and Tsu1 and the advent of MBE, the interest in semiconductor superlattices (SLs) and quantum well (QW) structures has continuously increased over the years, driven by technological challenges, new physical concepts and phenomena, as well as promising applications. A new class of materials and heterojunctions with unique electronic and optical properties has been developed. Here we focus on antimonide-based type-II superlattices, which involve infrared excitation of carriers and can be realized in several material systems.
The physical properties of the respective quantum well structures are strongly determined by the band discontinuities at the interface, i.e., the band alignment. An abrupt discontinuity in the local band structure is usually associated with a gradual band bending in its neighborhood, which reflects space-charge effects. The conduction- and valence-band discontinuities determine the character of carrier transport across the interfaces; therefore, they are the most important quantities that determine the suitability of present SLs or QWs for IR detector purposes. The presence of an additional SL periodic potential changes the electronic spectrum of a semiconductor in such a manner that the Brillouin zone is divided into a series of mini-zones, giving rise to narrow subbands separated by minigaps. Thus, SLs possess new properties not exhibited by homogeneous semiconductors. Surprisingly, the corresponding values for the band discontinuities of the conduction band DEc and the valence band DEv cannot be obtained by simple considerations. Band lineups based on electron affinity do not work in most cases when two semiconductors form a heterostructure. This is because of subtle charge-sharing effects that occur across atoms on the interface. There have been a number of theoretical studies that predict general trends in how bands line up. However, the techniques are quite complex and heterostructure designs usually depend on experiments to provide line-up information. It must be taken into account that electrical and optical methods do not measure the band offsets themselves but instead measure the quantities associated with the electronic structure of the heterostructure. The band offset determination from such experiments requires an appropriate theoretical model.
|
