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Atomic scale strain relaxation in axial semiconductor III-V nanowire heterostructures

Combination of mismatched materials in semiconductor nanowire heterostructures offers a freedom of bandstructure engineering that is impossible in standard planar epitaxy. Nevertheless, the presence of strain and structural defects directly control the optoelectronic properties of these nanomaterials. Understanding with atomic accuracy how mismatched heterostructures release or accommodate strain, therefore, is highly desirable. By using atomic resolution high angle annular dark field scanning transmission electron microscopy combined with geometrical phase analyses and computer simulations, we are able to establish the relaxation mechanisms (including both elastic and plastic deformations) to release the mismatch strain in axial nanowire heterostructures. Formation of misfit dislocations, diffusion of atomic species, polarity transfer, and induced structural transformations are studied with atomic resolution at the intermediate ternary interfaces. Two nanowire heterostructure systems with promising applications (InAs/InSb and GaAs/GaSb) have been selected as key examples

Optical properties of wurtzite/zinc-blende heterostructures in GaN nanowires

The optical and structural properties of wurtzite GaN nanowires containing zinc-blende GaN inclusions of different thicknesses are investigated. Micro-photoluminescence spectra of single nanowires exhibit a series of narrow emission peaks with linewidth as low as 0.8 meV in the interval 3.1-3.42 eV. The peak energy blue-shifts with increasing excitation power following a ∼I1/3 law due to the progressive band filling and to the screening of the internal field. The quantum confinement in these type-II crystal phase heterostructures was simulated in the framework of a one-dimensional effective mass model, accounting for the internal electrical polarization of the wurtzite GaN. The predicted transition energies are in good agreement with the energy statistics realized on more than 30 single nanowire emission spectra

Optical characterization of AlGaN/GaN quantum disc tructures in single nanowires

International audienceWe report a systematic study of the luminescence properties of AlxGa1-xN/GaN single and multi quantum disc structures in single nanowires with Al fraction xAl varying from 0.05 to 1. These quantum structures are situated on the top of GaN nanowires grown in the polar [0001] direction. Nanowires are synthesized by Plasma Assisted Molecular Beam Epitaxy under nitrogen rich growth conditions at a substrate temperature of T = 780 °C on Si (111) substrates. The PL energy of single and multiquantum disc systems is explained in terms of Al content in the barriers, band bending at the upper polar surface, and strain relaxation in the heterostructure region

Origin of energy dispersion in AlxGa1−xNÕGaN nanowire quantum discs with low Al content

Individual GaN nanowires containing AlxGa1−xN/GaN quantum discs QDiscs with Al content x16% have been investigated by microphotoluminescence, transmission electron microscopy, and theoretical modeling. Single quantum discs show narrow emission lines with a linewidth as low as 3 meV at energies above the GaN band gap while the emission of nanowires containing multiple quantum discs shows multiple peaks with total spectral broadening that depends on the Al content in the barrier. As assessed by simulations of the quantum confinement based on a three-dimensional effective-mass model, the main factors influencing the spectral dispersion are: i strain relaxation in the QDiscs, strongly affected by the presence of a lateral AlGaN shell with a progressively changing thickness formed during the barrier growth; ii monolayer fluctuations in the QDisc thickness

Photoluminescence polarization properties of single GaN nanowires containing Al(x)Ga(1-x)N/GaN quantum discs

The polarization anisotropy of single GaN nanowires containing Al(x)Ga(1-x)N/GaN multiquantum disc (MQDisc) structures is characterized by polarization-resolved microphotoluminescence (mu PL). Single nanowires exhibit at T=4.2 K two main luminescence contributions: one is peaked at E=3.45-3.48 eV related to near-band-edge GaN bulk excitonic transitions and is polarized parallel to the nanowire axis (pi polarization) at moderate excitation-power density; the other, lying at higher energy, is related to excitonic transitions confined in the MQDisc and is polarized perpendicularly to the nanowire axis (sigma polarization). The results are interpreted in terms of the selection rules for excitonic transitions in wurtzite semiconductor crystals and of the polarization anisotropy arising from the elongated nanowire shape. Finally, the analysis of photoluminescence at T=300 K shows that the thermal population of light-hole states in the MQDisc produces a blueshift of the PL peak when polarization is rotated from sigma to pi