Anomalous Strain Relaxation in Core–Shell Nanowire Heterostructures via Simultaneous Coherent and Incoherent Growth

Abstract

Nanoscale substrates such as nanowires allow heterostructure design to venture well beyond the narrow lattice mismatch range restricting planar heterostructures, owing to misfit strain relaxing at the free surfaces and partitioning throughout the entire nanostructure. In this work, we uncover a novel strain relaxation process in GaAs/InGaAs core-shell nanowires that is a direct result of the nanofaceted nature of these nanostructures. Above a critical lattice mismatch, plastically relaxed mounds form at the edges of the nanowire sidewall facets. The relaxed mounds and a coherent shell grow simultaneously from the beginning of the deposition with higher lattice mismatches increasingly favoring incoherent mound growth. This is in stark contrast to Stranski-Krastanov growth, where above a critical thickness coherent layer growth no longer occurs. This study highlights how understanding strain relaxation in lattice mismatched nanofaceted heterostructures is essential for designing devices based on these nanostructures.

Determination of indium content of GaAs/(In,Ga)As/(GaAs) core-shell(-shell) nanowires by x-ray diffraction and nano x-ray fluorescence

“…Depending on the amount of lattice mismatch, this strain can relax plastically or accommodate elastically accompanied by lattice deformation. In a previous publication on such GaAs/(In,Ga)As core-shell NWs, Lewis et al [37] did not observe any plastic strain relaxation up to an In content of 40%. Thus in the present study we can safely analyze the results for samples 1 and 2 assuming entirely elastic relaxation.…”

“…. The NWs of samples 1 and 2 exhibit smooth shells whereas the morphology of NWs with high nominal In content on sample 3 is characterized by the presence of mounds resulting in a rough shell surface, as observed before [37] (overview SEM images of all three samples are shown in the Supplemental Material (SM) 1 [39]). In addition, both images display a dropletlike feature at the NW top.…”

“…The three investigated samples were grown by Ga-assisted molecular beam epitaxy (MBE) on Si (111) substrates. Details of the growth are described elsewhere [12,37]. For sample 1, a prepatterned substrate was used, resulting in an ordered array of NWs [38], while the other two NW ensembles are random.…”

“…The three different samples differ mostly in the nominal In content of the shells, which was varied by changing the In/(In + Ga) flux ratio. A previous study of similar samples indicated that the crystal structure of these NWs is predominantly zinc blende with a small wurtzite content [37]. The shell thickness, t, and In shell content, x, of each sample under investigation are listed in Table I along with the NW number density and other information that will be introduced later.…”

Correlated Nanoscale Analysis of the Emission from Wurtzite versus Zincblende (In,Ga)As/GaAs Nanowire Core–Shell Quantum Wells

“…28 However, little is known on the influence of the crystal phase on the band structure of important ternary alloys such as (In,Ga)As. 29 Beyond the influence of crystal phase, engineering the optical properties of NW-based QWs requires a detailed un-derstanding of how composition, QW width, and the associated interface strain impact the band structure, [2][3][4]30,31 necessitating an integrated approach to nanoscale characterization.…”