Evidence and control of unintentional As-rich shells in GaAs1-x P x nanowires

Growth dynamics and compositional structure in periodic InAsSb nanowire arrays on Si (111) grown by selective area molecular beam epitaxy

We report a comprehensive study of the growth dynamics in highly periodic, composition tunable InAsSb nanowire (NW) arrays using catalyst-free selective area molecular beam epitaxy. Employing periodically patterned SiO₂-masks on Si (111) with various mask opening sizes (20-150 nm) and pitches (0.25-2 μm), high NW yield of >90% (irrespective of the InAsSb alloy composition) is realized by the creation of an As-terminated 1 × 1-Si(111) surface prior to NW nucleation. While the NW aspect ratio decreases continually with increasing Sb content (x _Sb from 0% to 30%), we find a remarkable dependence of the aspect ratio on the mask opening size yielding up to ∼8-fold increase for openings decreasing from 150 to 20 nm. The effects of the interwire separation (pitch) on the NW aspect ratio are strongest for pure InAs NWs and gradually vanish for increasing Sb content, suggesting that growth of InAsSb NW arrays is governed by an In surface diffusion limited regime even for the smallest investigated pitches. Compositional analysis using high-resolution x-ray diffraction reveals a substantial impact of the pitch on the alloy composition in homogeneous InAsSb NW arrays, leading to much larger x _Sb as the pitch increases due to decreasing competition for Sb adatoms. Scanning transmission electron microscopy and associated energy-dispersive x-ray spectroscopy performed on the cross-sections of individual NWs reveal an interesting growth-axis dependent core-shell like structure with a discontinuous few-nm thick Sb-deficient coaxial boundary layer and six Sb-deficient corner bands. Further analysis evidences the presence of a nanoscale facet at the truncation of the (111)B growth front and {1-10} sidewall surfaces that is found responsible for the formation of the characteristic core-shell structure.

Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications

Axial p-n and p-i-n junctions in GaAs_0.7P_0.3 nanowires are demonstrated and analyzed using electron beam induced current microscopy. Organized self-catalyzed nanowire arrays are grown by molecular beam epitaxy on nanopatterned Si substrates. The nanowires are doped using Be and Si impurities to obtain p- and n-type conductivity, respectively. A method to determine the doping type by analyzing the induced current in the vicinity of a Schottky contact is proposed. It is demonstrated that for the applied growth conditions using Ga as a catalyst, Si doping induces an n-type conductivity contrary to the GaAs self-catalyzed nanowire case, where Si was reported to yield a p-type doping. Active axial nanowire p-n junctions having a homogeneous composition along the axis are synthesized and the carrier concentration and minority carrier diffusion lengths are measured. To the best of our knowledge, this is the first report of axial p-n junctions in self-catalyzed GaAsP nanowires.

Computational Study of the Ene/Rearrangement Reaction between (F3C)2B═NMe2, , and Acetonitrile: Reactant-Catalyzed Mechanism of the Ketenimine-Nitrile-Like Rearrangement

The reaction between (F₃C)₂B═NMe₂, 1, and acetonitrile at low temperature in pentane yields a bora-acetonitrile rather than the expected coordination complex. This appears to arise from the two undergoing an ene reaction followed by a rearrangement analogous to a ketenimine-nitrile rearrangement. Computational studies indicate that mechanistic steps suggested for the latter require energies too large for the reaction to take place under the experimental conditions. Instead, a mechanism in which the ene reaction product is attacked by a second molecule of 1, followed by hydrogen transfer and decomposition, exhibits barriers lower than that for the ene reaction. The mechanism implies that the fragment of 1 in the observed product is not the one that underwent the ene reaction. The ene reaction barrier is rate-determining, and it is low enough to conform to the experimental conditions.