Why Does Wurtzite Form in Nanowires of III-V Zinc Blende Semiconductors?

Abstract

We develop a nucleation-based model to explain the formation of the wurtzite (WZ) phase during the vapor-liquid-solid growth of free-standing nanowires of zinc-blende (ZB) semiconductors. Nucleation occurs preferentially at the edge of the solid/liquid interface, which entails major differences between ZB and WZ nuclei. Depending on the pertinent interface energies, WZ nucleation is favored at high liquid supersaturation. This explains our systematic observation of ZB during early growth.PACS numbers: 68.65. La,64.60.Qb,81.05.Ea,81.15.Kk,64.70.Nd Free-standing wires with diameters ranging from hundreds down to a few nanometers are nowadays commonly fabricated from a large range of semiconductor materials [1,2,3,4,5]. These nanowires (NWs) have remarkable physical properties and many potential applications. The present work deals with the epitaxial growth of NWs of III-V semiconductors on a hot substrate. Metal catalyst nanoparticles deposited on the substrate before growth define the wire diameter. According to the vapor-liquidsolid (VLS) growth mechanism, the atoms are fed from the vapor phase to the solid wire through this particle (or droplet), which remains liquid during growth [6].We consider III-V compounds which, under bulk form, adopt the cubic zinc-blende (ZB) crystal structure [7] (although some non-ZB high-pressure phases [8] may be metastable at atmospheric pressure [9]), leaving aside nitrogen-based NWs. We discuss the usual case of NWs grown on a [111]B (As-terminated) face of the ZB substrate. Probably the most surprising feature of these NWs is that, in contrast to their bulk counterparts, they often adopt the hexagonal wurtzite (WZ) structure. This was observed for most ZB III-V materials and growth techniques [1,3,4,10,11]. However, although often dominantly of WZ structure, the NWs usually contain stacking faults (SFs) and sequences of ZB structure. The coexistence of two phases is clearly a problem for basic studies as well as applications, so that phase purity control is one of the main challenges of III-V NW fabrication.The surprising prevalence of the WZ structure in III-V NWs has not been explained satisfactorily so far. Here, based on new experimental observations, we propose an explanation of the occurrence of the WZ structure and develop a model predicting quantitatively in which growth conditions it should form. We consider the specific case of gold-catalyzed GaAs NWs grown by molecular beam epitaxy (MBE) on a GaAs substrate but we expect our model and our conclusions to remain valid for any ZB III-V compound and any growth method.Let us start with briefly reviewing previously proposed explanations. Calculations give the difference δw in cohesive energy between ZB and WZ bulk GaAs as about 24 meV per III-V pair at zero pressure [7]. It has been argued that this favoring of the ZB form might be offset in NWs of small diameter by the large relative contribution to the total energy of either the lateral facets [12] or the vertical edges separating the latter [13] (provided the specifi...

Ferromagnetic GaAs/GaMnAs Core−Shell Nanowires Grown by Molecular Beam Epitaxy

“…35 The rather high flux ratio leads to a low Ga supersaturation of the catalyst droplets, which favors zinc blende as the crystal structure of the NWs. 36 It also ensures a high density and uniform distribution of the NWs. 37 After 4 h the growth of GaAs NW templates was stopped.…”

Raman spectroscopy of wurtzite and zinc-blende GaAs nanowires: Polarization dependence, selection rules, and strain effects

“…From the thermodynamic point of view, the surface energy of ͕1100͖ wurtzite planes should be lower than the ͕110͖ and ͕111͖A / B of zinc blende. 12 This means that for small radii nanowires, the wurtzite phase should be more stable than the zinc blende. In the case of GaAs, the critical radius, under which wurtzite is expected to be the most stable phase has been predicted to lie between 5 and 25.5 nm.…”

Structural and optical analysis of GaAsP/GaP core-shell nanowires

“…14͔͒ resulting in a metal-semiconductor alloy. Supersaturation of the Au alloy leads to island nucleation at a triple-phase-point, 15 proceeded by solid-phase semiconductor epilayer formation at the interface between the liquid alloy seed and the underlying substrate. VLS growth is continued, and, thus, a NW is grown, as long as sufficiently high supersaturation conditions are sustained.…”