N-incorporation in MOVPE-grown GaNAs/GaAs planar and nanowire structures using tertiarybutyl-hydrazine

Dilute nitride III-V compounds (III-N-As) are candidates for application to high efficiency intermediate bandgap solar cells (IBSCs). Doping of group-III arsenides and phosphides crystals with nitrogen (N) atoms, in substitution of group-V atoms, leads to the introduction of N-impurity localized states and the formation of an intermediate band-gap at energies below that of the host material CB edge. The combination of IBSC functionality with the unique physical properties associated with core-multishell nanowires-based devices would effectively allow photovoltaic cells with unprecedentedly high conversion efficiencies, simpler junction geometry, and low materials usage. Planar GaNAs/GaAs epitaxial samples were grown on (100)-, (111)B- and (110)GaAs substrates. The latter two orientations relate respectively to the substrate orientation employed for the VLS self-assembling of III-V nanowires, and the sidewall orientation of GaAs nanowires over which GaNAs shells are overgrown. Planar and nanowire samples were characterized by high resolution XRD (HRXRD) and photoluminescence (PL) to determine the heterostructure elastic strain and N content (x) of GaNxAs1-x layers/shells. For each sample both symmetric and a-symmetric HRXRD reflections were measured. No diffraction peaks ascribable to a GaNAs alloy could be detected instead, for samples grown on (111)B substrates, indicating negligible N-incorporation (x<0.01%) for (111)B-oriented epilayers. 7K PL confirmed the formation of the GaNAs intermediate gap in quantitative agreement with N-compositions, as determined by HRXRD. Present results allowed to determine the solid-vapor distribution curve for N incorporation in GaNAs using tBuN2H3 and compare with recent data in the literature. Axial strain and N-content in GaAs-GaNAs core-shell nanowires were finally, determined based on the heavy- and light-hole free-exciton blue-shift of GaAs in the nanowire PL spectra, as induced by the GaAs-GaNAs lattice mismatch.