Michele Amato,1 Thanayut Kaewmaaya,1 Alberto Zobelli,2 Maurizia Palummo,3,4 and Riccardo Rurali5
The design of electron device is based on the juxtaposition of semiconductors with different properties, typically regions of the same material with different doping characteristics. Other times, however, interesting effects can be obtained with junctions made of two different materials, thus called heterojunctions; and when the heterojunction is repeated in a periodic fashion, one ends up with a superlattice, a metamaterial with its own properties dictated by both the constituent materials and the periodicity of the repeated motif.
Recent progresses in the synthetic growth of semiconducting nanowires have given access to phases that in bulk are only observed under extreme pressure conditions. Silicon hexagonal polytypes, for instance, feature important differences in the electronic structure with the more common diamond structure, particularly for what the bandgap and the location of the conduction-band minima are concerned.
These advances make possible to envisage the so-called crystal phase superlattices or homojunctions, where different crystal phases of the same material, rather than different materials, are alternated in an ordered fashion.
In this work we have studied the electronic properties of individual hexagonal silicon nanowires and compared with the most common cubic counterparts. Most importantly, though, we have studied the band alignment in cubic-hexagonal silicon homojunctions, finding that they feature a type-II band offset, where electronics and holes localize on different sides of the junction. This, together with the fact that hexagonal silicon exhibit a larger absorption in the visible, suggest that these systems can have an important role as efficient photovoltaic materials.
1Centre de Nanosciences et de Nanotechnologies, Université Paris-Sud, France
2Laboratoire de Physique des Solides, CNRS, Université Paris-Saclay, France
3Dipartimento di Fisica, Università di Roma Tor Vergata, Italy
4INFN, Laboratori Nazionali di Frascati, Italy
5Institut de Ciència de Materials de Barcelona (ICMAB−CSIC), Spain