Ternary and quaternary alloys of III-Arsenide semiconductor nanowires are attracting significant interest as building blocks of future optoelectronic devices [1]. Amongst other advantages they feature tunable bandgap energies by varying the alloy composition [2]. Recently, InGaAs nanowires were synthesized by vapor-liquid-solid (VLS) growth using Au nanoparticle catalysts. However, pure wurtzite (WZ) and zinc blende (ZB) crystal structures could only be achieved for few InxGa1-xAs compositions and an inhomogeneous content of In was observed along the nanowires [2].
This study looks into optimizing the growth conditions of InxGa1-xAs nanowires to achieve highly uniform structures, featuring a wide compositional range (0<x<1). Metal-organic-chemical-vapour-deposition (MOCVD) was used to grow the nanowires on a GaAs (111)B substrate, which in turn, was prepared using selected-area-epitaxy (SAE). Our transmission electron microscopy (TEM) results show that GaAs has a predominantly ZB structure whereas InAs nanowires have a WZ crystal structure. For GaAs nanowires, we observe a change in twin defect density along the length of the nanowires, which is strongly dependent on growth temperature and nanowires’ diameter. For example, the base of 370 nm in diameter GaAs nanowires, obtained at 770 ºC and V/III=20, has a very high twin defect density. However, moving toward the tip of these nanowires the twin defect density decreases sharply, resulting in ZB twin segment thickness of up to 150 nm.
TEM analysis of InGaAs nanowires shows a trend from a predominantly WZ phase for higher In content to a predominantly ZB phase for higher Ga content. We also observe that for a given nominal V/III ratio, the In/Ga composition within the nanowires was dependent on two factors: the growth temperature and the pitch size. A higher Ga content in InGaAs nanowires can be achieved by using higher growth temperatures and smaller pattern pitch sizes of less than 1 µm.