Raman spectroscopy of MBE-grown ZnTe-based nanowires

Zn1-xMexTe (Me = Mg, Mn) nanowires (NWs), with Mg content up to x = 0.75, and Mn content up to x = 0.6, have been investigated by Raman scattering. NWs were grown on GaAs substrates by catalytically enhanced molecular beam epitaxy (MBE). In the whole composition range the NWs crystallize in the zinc-blende structure with their axis (< 111 > direction) aligned along the < 111 > direction of the GaAs substrate, independently of the crystallographic orientation of this substrate. The NWs' diameters vary in the range from 30 to 70 nm, and their typical length is 1000-2000 urn. Raman scattering was measured in "near" resonant condition with the excitation energy close to the direct energy gap of the NW material. This enabled the observation of several LO phonon replicas on a collection of NWs and of up to three phonon replicas (2LO and 3LO) on a single NW. The dependence of the Raman spectra on the Me concentration shows the behavior typical of bulk Zn1-xMexTe mixed crystal, which confirms the incorporation of Me2+ ions into the cation substitutional sites of the ZnTe matrix of the NWs. For the composition range x < 0.2, the high crystalline quality of the investigated mixed crystal NWs, characterized by the width of the observed Raman structures, is comparable to that of pure ZnTe NWs. The crystalline quality of NWs degrades slightly when the Me content rises above x = 0.2. This effect can be attributed to an increasing number of defects or stacking faults in the mixed crystal NWs with higher compositions. Zn1-xCdxTe NWs (with x < 0.2) were also grown and investigated for comparison. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim