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GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs(EgZB). This result is consistent with the assumption that the band gap energy of wurtzite GaAs (EgWZ) is larger than EgZB and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the neighboring WZ segments. Thus, the corresponding transition energy depends on the degree of confinement of the electrons, and transition energies exceeding EgZB are possible for very thin ZB segments. At low temperatures, the incorporation of carbon acceptors plays a major role in determining the spectral profile as these can effectively bind holes in the ZB segments. From cathodoluminescence measurements of single GaAs NWs performed at room temperature, we deduce a lower bound of 55 meV for the difference EgWZ − EgZB. [ABSTRACT FROM AUTHOR]

We investigate the nature of excitons bound to I, basal-plane stacking faults [(I1.Y)] in GaN nanowire ensembles by continuous-wave and time-resolved photoluminescence spectroscopy. Based on the linear increase of the radiative lifetime of these excitons with temperature, they are demonstrated to exhibit a two-dimensional density of states, i.e., a basal-plane stacking fault acts as a quantum well. From the slope of the linear increase, we determine the oscillator strength of the (I1, X) and show that the value obtained reflects the presence of large internal electrostatic fields across the stacking fault. While the recombination of donor-bound and free excitons in the GaN nanowire ensemble is dominated by nonradiative phenonema already at 10 K, we observe that the (I1,X) recombines purely radiatively up to 60 K. This finding provides important insight into the nonradiative recombination processes in GaN nanowires. First, the radiative lifetime of about 6 ns measured at 60 K sets an upper limit for the surface recombination velocity of 210 cms-1 considering the nanowires mean diameter of 50 nm. Second, the density of nonradiative centers responsible for the fast decay of donor-bound and free excitons cannot be higher than 6 × 1016 cm-3. As a consequence, the nonradiative decay of donor-bound excitons in these GaN nanowire ensembles has to occur indirectly via the free exciton state. [ABSTRACT FROM AUTHOR]

We observe that the growth rate of Ga_{2}O_{3} in plasma-assisted molecular beam epitaxy can be drastically enhanced by an additional In supply. This enhancement is shown to result from a catalytic effect, namely, the rapid formation of In_{2}O_{3}, immediately followed by a transformation of In_{2}O_{3} to Ga_{2}O_{3} due to an In-Ga interatomic exchange. We derive a simple model that quantitatively describes this process as well as its consequences on the formation rate of Ga_{2}O_{3}. Moreover, we demonstrate that the catalytic action of In_{2}O_{3} allows the synthesis of the metastable hexagonal phase of Ga_{2}O_{3}. Since the Ga_{2}O_{3}(0001)/In_{2}O_{3}(111) interface is closely lattice matched, this novel growth mode opens a new path for the fabrication of sesquioxide heterostructures.

We report an unexpected mechanism by which an epitaxial interface can form between materials having strongly mismatched lattice constants. A simple model is proposed in which one material tilts out of the interface plane to create a coincidence-site lattice that balances two competing geometrical criteria--low residual strain and short coincidence-lattice period. We apply this model, along with complementary first-principles total-energy calculations, to the interface formed by molecular-beam epitaxy of cubic Fe on hexagonal GaN and find excellent agreement between theory and experiment.