Polarization ratio and effective mass in InP nanowires: Effect of crystallographic axis

Indium phosphide nanowires grown along different crystallographic axes-namely, the [001], [101], and [111] directions of zinc-blende structure-are investigated using a first-principles derived semi-empirical pseudopotential theory, aimed at understanding the effects of wire orientation on band structure, polarization ratio, and effective masses of semiconductor nanowires. Band energies over entire Brillouin zone are determined, and are found to exhibit different characteristics for three types of wires in terms of band dispersion and the location of orbital energy. A pronounced dispersion hump is revealed to exist in the lowest conduction band for the [001] and [111] wires, but not for the [101] wires. On the other hand, the [001] and [111] wires are shown to have very different orbital energy for the top valence state at the zone boundary X point-being -6.8 eV in the former and -6.2 eV in the latter. These differences provide specific and useful suggestion to encourage experimental determination of the band structure in InP nanowires. As another key result, we study the polarization ratio in wires of different orientations. Our calculations show that, given the same lateral size, the [111] wires yield the highest polarization ratio as compared to wires along the other two directions, while simultaneously possessing larger band-edge photoluminescence transition intensity. The [111] wires are thus suggested to be better suitable for optical device applications. Interestingly, we also found that polarization ratio displays a different size dependence than transition intensity does. More specifically, the polarization ratio is predicted to increase with the decreasing size, which is opposite to the behavior as exhibited by the optical transition intensity. The polarization ratios in the [101] and [111] wires of 11.7 A diameter are shown to approach the limit of 100%. In addition to polarization ratio, we further determine the electron and hole masses for wires of different crystallographic axes. For the [101] and [111] wires, the hole masses are revealed to be similar to 0.25, which are markedly smaller than the values (>= 1.0) along the same direction in bulk. This result demonstrates an interesting possibility of obtaining in nanowires a high hole mobility that is not available in bulk. An explanation for the anomaly in the hole mass is suggested and is associated with the existence of an electronic band transition.