Pitch-Induced Bandgap Tuning in Self-Catalyzed Growth of Patterned GaAsSb Axial and GaAs/GaAsSb Core–Shell Nanowires Using Molecular Beam Epitaxy

Abstract

We report on the pitch-induced bandgap tuning in GaAsSb (axial) and GaAs/GaAsSb (core–shell) patterned vertical nanowire (NW) arrays grown by Ga-assisted molecular beam epitaxy on Si(111). Red shifts in the range of 40–50 meV in the 4 K micro-photoluminescence (μ-PL) spectral peaks have been observed, for NW arrays with pitch length variation from 200 to 1200 nm, in the axial and core–shell configurations. The variation in the PL peak intensity closely follows the optical absorption dependency on the pitch length of the NW array computed using finite dimension time domain simulation. A semiempirical mass conservation of the growth-species-based-model has been proposed encompassing different material pathways. The secondary fluxes re-emitted from the side facets of the neighboring NWs contribute substantially toward the growth for smaller pitch lengths, while those from the oxide surface dominate at larger pitch lengths for high V/III beam equivalent pressure ratios. Excellent agreement between the experimental and simulated results have been observed for the pitch dependent axial and radial NW dimensions of both the axial and core–shell configured GaAsSb NWs. This study shows great promise for the applicability of patterned NWs for band gap tuning by simply varying the NW array pitch length.

A Two-Step Growth Pathway for High Sb Incorporation in GaAsSb Nanowires in the Telecommunication Wavelength Range

“…First, an increase in the number density of the Ga droplet with a decrease in temperature 37 is expected from reduced mobility of Ga adatoms. However, it also causes the Ga droplet volume to be smaller 37 , which can alter the contact angle between the droplet and the substrate surface 38 as well as the amount of growth species intercepted by the droplet 23 . The reduction of the NW density with decrease in growth temperature therefore suggests that the reduced desorption of the Ga adatoms from the substrate may balance the reduction in droplet volume to some extent, but it cannot compensate for other effects.…”

“…The larger initial volume of some droplets is likely to have resulted in longer NWs than the average in the present growth. It is to be noted that radial growth which is enhanced by the presence of Sb in the growth environment follows the vapor-solid (VS) mechanism 23 .…”

“…In contrast, axially configured GaAsSb NWs yield nearly planar-defect-free nanowires due to the predominance of the thermodynamically driven vapor-liquid-solid (VLS) growth mechanism, which leads to improved optoelectronic quality nanostructures 12 . Even though there is an extensive body of literature on Au-catalyzed and Au-free axial GaAsSb NWs 12–23 , the majority of the work on self-assisted axial GaAsSb NWs is mainly focused on structural and electronic characterization. Despite the fact that in the axial configuration an Sb composition as high as 93 at.% has been reported 9 , Sb compositions beyond 30 at.% have a detrimental impact on NW morphology and characteristics, namely uneven growth, tapered morphology, multiple facets, compositional gradient, thick parasitic islands of GaAsSb and poor PL emission 9, 12, 19–21 .…”

III–V ternary nanowires on Si substrates: growth, characterization and device applications

“…There are already reports that describe detailed characterizations and investigation of the optical properties of GaAsSb NWs on Si substrates grown via CVD [78] and MBE. [27][28][29]52,[79][80][81][82][83][84][85][86] As mentioned earlier, the Sb content, for structures grown on Si, is kept at a relatively low level. In several cases of growth on patterned Si substrates, where VLS growth occurred, the size of the pattern plays an important role in the morphology and elemental composition of the structures.…”

“…7a-c). [83] It is worth mentioning that the change in the dimensions of the NWs reduces the wetting angle between the droplet and the NW, a phenomenon which affects the elemental distribution of Sb in the axis of the NW. [80] Both the height and the diameter of the NWs increase with increasing dimensions of the pitch.…”