The Role of Polarity in Nonplanar Semiconductor Nanostructures

Abstract

The lack of mirror symmetry in binary semiconductor compounds turns them into polar materials, where two opposite orientations of the same crystallographic direction are possible. Interestingly, their physical properties (e.g., electronic or photonic) and morphological features (e.g., shape, growth direction, and so forth) also strongly depend on the polarity. It has been observed that nanoscale materials tend to grow with a specific polarity, which can eventually be reversed for very specific growth conditions. In addition, polar-directed growth affects the defect density and topology and might induce eventually the formation of undesirable polarity inversion domains in the nanostructure, which in turn will affect the photonic and electronic final device performance. Here, we present a review on the polarity-driven growth mechanism at the nanoscale, combining our latest investigation with an overview of the available literature highlighting suitable future possibilities of polarity engineering of semiconductor nanostructures. The present study has been extended over a wide range of semiconductor compounds, covering the most commonly synthesized III−V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) and II−VI (ZnO, ZnTe, CdS, CdSe, CdTe) nanowires and other free-standing nanostructures (tripods, tetrapods, belts, and membranes). This systematic study allowed us to explore the parameters that may induce polarity-dependent and polaritydriven growth mechanisms, as well as the polarity-related consequences on the physical properties of the nanostructures.

Polarity-dependent nonlinear optics of nanowires under electric field

“…The polarity of vertical ZnO nanowires with respect to their growth direction was reported to depend on the growth mechanism, usually being Zn-polar for catalyst-free nanowire growth and O-polar for VLS nanowire growth. The important role of the catalyst droplet in determining nanowire polarity, and the mechanism that leads to it as opposed to catalyst-free growth have been recently reviewed in detail 9 . Moreover, other VLS-grown nanowires of compound semiconductors 14 tend to grow anion-polar like ZnO.…”

“…Specifically, polarity was found to influence chemical reactivity 3 , 4 , doping and impurity incorporation 5 – 7 , surface states and band-bending, which can strongly affect the nature of metallic contacts in functional systems, reflectivity 8 and the performance of devices based on polar orientations, such as LEDs and high-mobility transistors 1 (HEMTs). In nanowires, the polarity is a key property 9 , as it was found to affect morphology 10 , 11 , growth rates 12 and defect incorporation; and therefore has a significant impact on their electronic structure and optical properties 13 . Determination of the polarity of nanowires has been achieved by advanced transmission electron microscopy (TEM) methods, such as high-angle annular dark field (HAADF) and annular bright field (ABF) imaging along the nanowires 14 , 15 .…”

GaAs nanoscale membranes: prospects for seamless integration of III–Vs on silicon

“…In the case of non-planar polar semiconductor nanostructures the growth front tends to follow the {111} planes. 61 In the presence of an APB, the growth front evolves along different angles on both sides of the boundary, which explains the formation of kinks at the surface of the nanostructure. The presence of complex APBs at the merging point between polar crystals has been also observed in complex nanostructures such as V-shape nanofins, 61 vertical nanospades 62 and close to the nucleation seed of A-polar GaAs NWs.…”

“…61 In the presence of an APB, the growth front evolves along different angles on both sides of the boundary, which explains the formation of kinks at the surface of the nanostructure. The presence of complex APBs at the merging point between polar crystals has been also observed in complex nanostructures such as V-shape nanofins, 61 vertical nanospades 62 and close to the nucleation seed of A-polar GaAs NWs. 63 In addition, the complex defects appearing in this region also accumulate strain around them (medium-angle annular dark-field (MAADF) images revealing the accumulation of strain are shown in the ESI †).…”

“…APBs have been reported to act as potential barriers for carriers. 60,61 Therefore, the APB blocks the diffusion of excitons, creating two spatially separated regions of CL emission. APBs have also been shown to act as recombination centres.…”