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Previously, we found that N-polar (In,Ga)N/GaN quantum wells prepared on freestanding GaN substrates by plasma-assisted molecular beam epitaxy at conventional growth temperatures of about 650 °C do not exhibit any detectable luminescence even at 10 K. In the present work, we investigate (In,Ga)N/GaN quantum wells grown on Ga- and N-polar GaN substrates at a constant temperature of 730 °C. This exceptionally high temperature results in a vanishing In incorporation for the Ga-polar sample. In contrast, quantum wells with an In content of 20% and abrupt interfaces are formed on N-polar GaN. Moreover, these quantum wells exhibit a spatially uniform green luminescence band up to room temperature, but the intensity of this band is observed to strongly quench with temperature. Temperature-dependent photoluminescence transients show that this thermal quenching is related to a high density of nonradiative Shockley-Read-Hall centers with large capture coefficients for electrons and holes. [ABSTRACT FROM AUTHOR]

We explore an alternative way to fabricate (In, Ga)N/GaN short-period superlattices on GaN(0001) by plasma-assisted molecular beam epitaxy. We exploit the existence of an In adsorbate structure manifesting itself by a (√3×√3)R30° surface reconstruction observed in-situ by reflection highenergy electron diffraction. This In adlayer accommodates a maximum of 1/3 monolayer of In on the GaN surface and, under suitable conditions, can be embedded into GaN to form an In0.33Ga0.67N quantum sheet whose width is naturally limited to a single monolayer. Periodically inserting these quantum sheets, we synthesize (In,Ga)N/GaN short-period superlattices with abrupt interfaces and high periodicity as demonstrated by x-ray diffractometry and scanning transmission electron microscopy. The embedded quantum sheets are found to consist of single monolayers with an In content of 0.25-0.29. For a barrier thickness of 6 monolayers, the superlattice gives rise to a photoluminescence band at 3.16 eV, close to the theoretically predicted values for these structures. [ABSTRACT FROM AUTHOR]

Abstract: The growth of multiquantum well (MQW) AlGaN/GaN structures was carried out by plasma assisted molecular beam epitaxy simultaneously on semipolar , m-plane and c-plane (0001) GaN substrates under nitrogen-rich (N-rich) conditions in order to investigate the growth mechanisms on different crystal surfaces. A smooth surface with atomic steps was found only for the semipolar structure as opposite to m-plane and c-plane surfaces which were rough and covered by islands. Transmission electron microscopy (TEM) studies of the semipolar MQW structure showed uniform AlGaN layers with sharp interfaces and no extended defect formation, which proves good structural quality. At higher resolution, on the surface and at the interfaces of the semipolar MQWs, {10-11} and {10-10} nano-facets were found. In contrast to the semipolar case, the TEM studies of the m-plane structure showed no clear periodicity and very uneven distribution of Al. The AlN was formed on island top surfaces and AlGaN grew only on island slopes that were tilted from the m-plane. The growth rate was slightly smaller for the semipolar structure and much smaller for the m-plane one, as compared to the c-plane MQW structure. This can be attributed to Ga losses from semipolar and m-plane surfaces, in contrast to c-plane where no Ga losses were observed. The mechanisms for the Ga losses under N-rich conditions are discussed. [Copyright &y& Elsevier]

Multi-quantum well (MQW) structures and light emitting diodes (LEDs) were grown on semipolar [formula] and polar (0001) GaN substrates by plasma-assisted molecular beam epitaxy. The In incorporation efficiency was found to be significantly lower for the semipolar plane as compared to the polar one. The semipolar MQWs exhibit a smooth surface morphology, abrupt interfaces, and a high photoluminescence intensity. The electroluminescence of semipolar [formula] and polar (0001) LEDs fabricated in the same growth run peaks at 387 and 462 nm, respectively. Semipolar LEDs with additional (Al,Ga)N cladding layers exhibit a higher optical output power but simultaneously a higher turn-on voltage. [ABSTRACT FROM AUTHOR]

The catalyst- and self-induced pathways of GaN nanowire growth by molecular beam epitaxy are compared. The catalyst-induced nanowires elongate faster than the self-induced ones and their growth rate is fully determined by the impinging N rate. The self-induced nanowire growth rate is identical on both Si(111) and Si(001) and approaches the impinging N rate only for the few longest nanowires. This difference is attributed to the presence of the Ni-catalyst which enhances the incorporation of Ga at the nanowire tip while for the self-induced nanowires, growth is limited by the different incorporation rates on the nanowire tip and sidewall facets. [ABSTRACT FROM AUTHOR]

The nucleation of GaN nanowires grown by molecular beam epitaxy on bare Si(111) and Si(001) has been investigated in situ by reflection high-energy electron diffraction (RHEED) and line-of-sight quadrupole mass spectrometry. On either substrate, the incorporation rate of Ga increases in two steps to steady-state conditions, and the RHEED transmission pattern of GaN appears only in the second stage. Ex situ transmission electron microscopy on samples from both stages grown on Si(001) revealed that the nanowire nucleation is strongly affected by the simultaneous nitridation of the Si substrate. [ABSTRACT FROM AUTHOR]

GaN nanowires (NWs) were grown on sapphire by molecular beam epitaxy. NWs form only in the presence of Ni seed particles and only under N-rich conditions. Their length increases linearly with growth time up to about 7.5 μm while their diameter remains almost constant. In contrast, a switch to Ga-rich conditions after NW formation results in radial growth, i.e., the NW diameter increases while lengthening is negligible. These results corroborate the fact that the growth of III-V NWs is governed by the accumulation of group-III atoms in the seeds, while group-V species are not preferentially incorporated at the seeds. [ABSTRACT FROM AUTHOR]

GaN layers were grown on N-polar GaN substrates by plasma-assisted molecular beam epitaxy under different III/V ratios. Ga-rich conditions assure step-flow growth with atomically flat surface covered by doubly-bunched steps, as for Ga-polar GaN. Growth under N-excess however leads to an unstable step-flow morphology. Particularly, for substrates slightly miscut towards [formula], interlacing fingers are covered by atomic steps pinned on both sides by small hexagonal pits. In contrast, a three-dimensional island morphology is observed on the Ga-polar equivalent sample. We attribute this result to lower diffusion barriers on N-polar compared to Ga-polar GaN under N-rich conditions. [ABSTRACT FROM AUTHOR]

We investigate sub-monolayer InN quantum sheets embedded in GaN(0001) by temperature-dependent photoluminescence spectroscopy under both continuous-wave and pulsed excitation. Both the peak energy and the linewidth of the emission band associated with the quantum sheets exhibit an anomalous dependence on temperature indicative of carrier localization. Photoluminescence transients reveal a power law decay at low temperatures reflecting that the recombining electrons and holes occupy spatially separate, individual potential minima reminiscent of conventional (In,Ga)N(0001) quantum wells exhibiting the characteristic disorder of a random alloy. At elevated temperatures, carrier delocalization sets in and is accompanied by a thermally activated quenching of the emission. We ascribe the strong nonradiative recombination to extended states in the GaN barriers and confirm our assumption by a simple rate-equation model. [ABSTRACT FROM AUTHOR]

Based on the evaluation of lattice parameter maps in aberration corrected high resolution transmission electron microscopy images, we propose a simple method that allows quantifying the composition and disorder of a semiconductor alloy at the unit cell scale with high accuracy. This is realized by considering, next to the out-of-plane, also the in-plane lattice parameter component allowing to separate the chemical composition from the strain field. Considering only the out-of-plane lattice parameter component not only yields large deviations from the true local alloy content but also carries the risk of identifying false ordering phenomena like formations of chains or platelets. Our method is demonstrated on image simulations of relaxed supercells, as well as on experimental images of an In0.20Ga0.80N quantum well. Principally, our approach is applicable to all epitaxially strained compounds in the form of quantum wells, free standing islands, quantum dots, or wires. [ABSTRACT FROM AUTHOR]

Two series of N- and Ga-face GaN Van Hoof structures were grown by plasma-assisted molecular beam epitaxy to study the surface potential barrier by contactless electroreflectance (CER). A clear CER resonance followed by strong Franz-Keldysh oscillation of period varying with the thickness of undoped GaN layer was observed for these structures. This period was much shorter for N-polar structures that means smaller surface potential barrier in these structures than in Ga-polar structures. From the analysis of built-in electric field it was determined that the Fermi-level is located 0.27 ± 0.05 and 0.60 ± 0.05 eV below the conduction band for N- and Ga-face GaN surface, respectively. [ABSTRACT FROM AUTHOR]

We demonstrate ultra-violet laser diodes emitting at 388 nm grown by plasma-assisted molecular beam epitaxy on semipolar [formula]GaN substrates under metal-rich conditions. The threshold current density and voltage of 13.2 kA/cm2 and 10.8 V were measured at room temperature for devices with the laser ridge waveguide oriented along the [formula] direction. We show smooth, atomically flat surface morphology after growth. The excellent structural quality of the laser heterostructure was corroborated by transmission electron microscopy. [ABSTRACT FROM AUTHOR]

We investigate individual and ensembles of free-standing GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy using time-resolved photoluminescence (TRPL) spectroscopy. The PL transients of individual NWs always show a single exponential decay, independent of the excitation density. However, different NWs exhibit different decay times ranging from about 50 to 400 ps. In contrast, the PL transients of the NW ensemble always exhibit a nonexponential decay even at low excitation densities. We conclude that this nonexponential PL decay of the NW ensemble is a superposition of the different decay times of the different individual NWs in the ensemble. The origin of these different decay times is attributed to the impact of surface recombination. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance (CER) has been applied to study the Fermi-level position on c-plane GaN surface in Van Hoof structures grown by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE). A clear CER resonance followed by strong Franz-Keldysh oscillation (FKO) of various periods was clearly observed for the series of samples of different thicknesses (30, 50, and 70 nm) of undoped GaN layer. The built-in electric field in this layer has been determined from the period of GaN-related FKO. A good agreement between the calculated and measured electric fields has been found for the Fermi-level located ∼0.4 and ∼0.3 eV below the conduction band for the MBE and MOVPE samples, respectively. [ABSTRACT FROM AUTHOR]

Abstract: A study of GaN–AlGaN nanowire heterostructures grown by molecular beam epitaxy on sapphire substrates is presented. Nanowire growth was promoted using nickel seeds formed by in-situ annealing of a Ni thin film. Deposition sequences were designed to form two nanowire samples with embedded alternating multilayers of GaN and Al0.2Ga0.8N of variable thicknesses. Subsequent analyses showed the formation of two types of nanostructures in both samples, namely long and tapered nanowires and short columnar structures with uniform diameters along the whole length. Ni seeds were found at the end of the long nanowires only, indicating a catalyst-induced growth mechanism. No Ni seeds were found at the top of columnar structures indicating a catalyst-free growth mechanism. Analytical electron microscopy techniques were applied to the study of the Al distribution along both types of nanostructures. The layered heterostructure was found only for the columnar structures, whereas in nanowires the mediation of the Ni seed led to an incorporation of Al varying at monolayer level unrelated to the deposition sequence. The Al to Ga concentration ratio was found to decrease from the central segment of the nanowires towards the tip. Residual radial growth was also found to interfere with the effective deposition of the multilayer NW heterostructures. [Copyright &y& Elsevier]

Abstract: Scanning transmission electron microscopy has been used to investigate the growth mechanism of gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy. The composition of nickel seeds used to promote nanowire-type growth has been studied. These seeds remain at the end of the nanowires after growth as Ni-based metallic crystallites. The concentration of gallium, nickel, oxygen and nitrogen in the seeds were quantified by analysing electron energy-loss spectra. The seed crystallites at the ends of the GaN nanowires are found to comprise of a metallic core of mainly nickel and gallium, with a surrounding oxide shell consisting of Ni, Ga, and O, attributed to the post-growth atmospheric exposure of the nanowires. Only a background concentration of nitrogen was found in the metallic nickel seed cores. [Copyright &y& Elsevier]

As other xenobiotics, polyphenols are metabolized both by the endogenous detoxication system and the gut microflora. We hypothesized that the presence of a gut microflora may account for the effect of catechins on phase I and II xenobiotic-metabolizing enzymes and that the human bacterial metabolites may be different from those of a rodent gut microflora. Therefore, the effects of 2% (+)-catechin or 2% (−)-epicatechin were studied in germ free (GF) rats and rats inoculated with the flora of a human volunteer (HFA). In addition, the catechins were administered in ethanol as a vehicle. In the liver, (+)-catechin or (−)-epicatechin decreased the total amount of CYP450 in both GF and HFA rats while the isoenzyme CYP2E1 decreased. In GF rats only, CYP2C11 increased when compared to the rats treated with the vehicle alone. (+)-catechin increased the specific activity of UGT-chloramphenicol in GF rats only and that of cytosolic glutathion-S-transferase (GST) in HFA rats only. In the intestine, (+)-catechin and (−)-epicatechin increased the specific activity of UGT-4-methylumbelliferone in both GF and HFA rats and that of UGT- chloramphenicol in HFA rats only. In conclusion, the presence of a human flora in rats is able to modify the inducing effect of catechins on the UGT and GST activities suggesting the involvement of bacterial metabolites. The alterations on CYP 450 are independent of the presence of a human gut flora. [Copyright &y& Elsevier]

We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as "s-shape". We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions. [ABSTRACT FROM AUTHOR]