Microstructural characterization of GaAs-Al x Ga 1-x As core-shell nanowires grown by Au-catalyst assisted MOVPE
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Microstructural characterization of GaAs-AlxGa1-xAs core-shell nanowires grown by Aucatalyst assisted MOVPE D. Altamura1,2,*, I. Miccoli1, P. Prete3, N. Lovergine1, L. Tapfer2 1 Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Via Monteroni, I-73100 Lecce, Italy 2 ENEA, Unità Tecnica Tecnologia dei Materiali Brindisi (UTTMATB), S.S. 7 “Appia” km 706, I-72100 Brindisi, Italy 3 Istituto per la Microelettronica e Microsistemi (IMM), CNR, Via Monteroni, I-73100 Lecce, Italy * present address: Istituto di Cristallografia (IC), CNR, Via G. Amendola 122, I-70126 Bari, Italy. ABSTRACT In this work, we report on the microstructural and morphological characterization of IIIV semiconductor nanowires (NWs) epitaxially grown on (111)B-GaAs substrates by Au-catalyst assisted metalorganic vapor phase epitaxy. As-grown dense (108-109 cm-2) arrays of few-micron long vertically-aligned (i.e. parallel to the crystallographic axis) GaAs, AlxGa1-xAs and core-shell GaAs-AlxGa1-xAs NWs were investigated, carrying out HRXRD measurements on different (hkl) reflections and by recording reciprocal space maps (RSMs) around the materials (111) reciprocal lattice points (relps). We show that NW diffraction peaks are visible in the RSM by means of characteristic halos. In the case of GaAs NWs, the halo is located at the (111) relp indicating that the NWs are grown along the direction and parallel to the axis of the GaAs substrate. On the contrary, for AlxGa1-xAs NWs or intentional core-shell GaAsAlxGa1-xAs NWs the halo is displaced (along the momentum transfer normal to the surface, Qz) with respect to the GaAs (111) relp due to the elastic lattice strain associated with the compositional variation, e.g. the Al molar fraction in the AlxGa1-xAs alloy, within the nanostructures. INTRODUCTION Nanowires (NWs) based on III-V compound semiconductors are of particular interest for energy conversion (photovoltaics) [1], opto-electronics [2], and sensor applications. NWs are the latest technological solution in the field of pseudomorphically-strained single-crystal semiconductor heterostructures for optoelectronic device fabrication. Defect formation through strain relaxation in large mismatched structures can indeed lead to significant degradation of the active materials optical properties, and hence limit options in device design. On the other hand NWs can be reliably grown as dislocation-free heterostructures on large mismatched substrates, as strain can be more easily elastically accomodated than in planar systems [3]. The critical thickness for dislocation formation will depend on the specific geometry of the NWs, in particular their radius [4]. Moreover, in the case of core-shell NWs (also known as radial heterostructures) strain is distributed between the core and the shell, allowing a significant flexibility in heterostructure design and tailoring of device properties [5]. High-resolution TEM techniques are usually employed for a full microstructural characterization of single NWs, but as-obtained information are intrinsically
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