Excitonic effects in GaN stacking faults and crystal phase quantum wells
Résumé
While III-V semiconductor nanowires crystallize into a wurtzite geometry, instead of the zinc-blende crystal structure more common in the bulk materials, they usually exhibit along their length large densities of cubic material insertions [1]. Although usually regarded as detrimental to the carrier transport properties, the polytypism of III-V nanowires has recently attracted a huge interest from the scientific community. Planar ZB inclusions in the WZ phase of a III-V semiconductor should behave as shallow type-II quantum wells (QWs), where electrons are confined in the ZB layers. This prediction thus allows for a new kind of bandgap engineering, where the bandgap along a structure depends only on the crystal phase of a single material. Despite the huge progresses in the growth of these so-called crystal phase quantum wells, there have been, so far, only a few attempts at modeling the emission properties of these structures. In addition, up until now, excitonic effects have been systematically neglected, based on the assumption that due to the type-II band alignment between the wurtzite and zinc-blende and phases, the binding energy of the exciton should be small compared to the bulk case [2]. In this work, we therefore compute by envelope function calculations the emission properties of excitons confined in wurtzite / zinc-blende crystal phase quantum wells. We account not only for the Coulomb interaction between electron-hole pairs, but also for the polarization discontinuities at the wurtzite / zinc-blende interfaces. Although our modeling procedure can be applied to any III-V material, we address specifically the case of GaN, as there is, for this material, a general agreement between the zinc-blende / wurtzite valence and conduction band offsets determined experimentally and those obtained theoretically [3]. Our calculations first reveal that contrary to the assumptions made so far, excitons bound to basal stacking faults, three monolayers thick cubic inclusions in wurtzite material, show an increased binding energy compared to the bulk case. We then describe the coupling between adjacent quantum wells and discuss the consequences on the quantum well emission properties of the large spatial extent of the exciton wave function. Finally, comparing the result of our calculations with available experimental data [4] suggests the absence of built-in electric fields in the narrowest quantum wells, which we relate to the overlap between interface charge density regions. [1] R. E. Algra et al., Nature 456, 369 (2008); P. Caroff et al., Nature Nanotechnology 4, 50 (2009). [2] L. Zhang et al., Nano Lett., 10, 4055 (2010); G. Jacopin et al., J. Appl. Phys. 110, 064313 (2011). [3] C. Stampfl et al., Phys. Rev. B 57, R15051 (1998). [4] P. Corfdir et al., J. Appl. Phys. 105, 043102 (2009); P. P. Paskov et al., J. Appl. Phys. 98, 093519 (2005).