  In this paper we show how to link the electronic structures of two III-V
systems, one a direct gap material, GaAs, and the other an indirect gap
material, GaP, from their bulks right down to the shape of thin nanowires. GaAs
and GaP bulk and nanowire systems are studied in the zincblende and wurtzite
structures both free of strain and subjected to biaxial strains perpendicular
to the [111]/[0001] direction. We provide an interpretation of the band
structure of nanowires, grown along the [111] (zinc-blende structure) and the
[0001] (wurtzite structure) directions, in terms of the bulk band structures of
the corresponding binary compounds. The procedure reveals the origin of the
valence and conduction valleys relevant to determine the nature (direct or
indirect) of the band gaps and the kind (direct and pseudodirect) of the
valence to conduction transitions. Thus, by calculating only the bulk bands it
is possible to describe the behavior of the nanowire bands even for very thin
nanowires. The effects on the band structures due to biaxial strain are
analogously analyzed, providing for bulk GaP the first results in literature.
The role of confinement, and surface relaxation, in determining the nanowire
electronic structure of thin nanowires are analyzed separately revealing that
the change in the nature of the band gap is due mainly to surface relaxation
effects, not confinement. We show that the change for indirect/direct of the
gap from the bulk to the 1D systems is mainly due to the competition between
the energies of bulk conduction valleys which are differently inuenced by
confinement and strain. While the main effect of confinement is to open all
gaps it is not necessarily the main cause of the direct/indirect change in the
nature of the electronic gap as instead is usually claimed in the literature.
