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Electron backscatter diffraction is utilized to determine the distribution of coexisting structural domains in MnAs layers prepared on GaAs substrates. In a layer grown on a GaAs(111)B substrate using solid phase epitaxy, the structural domains roughly correspond to the morphological features of the surface. The domains are, in contrast, considerably larger in size than the surface roughness when the substrate is GaAs(113)A. We examine the role of the freedom in in-plane crystalline alignment for causing such a difference using a numerical model. In a layer grown at an extraordinary high temperature (600 °C) on a GaAs(111)B substrate, (0001) and (110l) orientations are interwoven. While the (0001) component is present mainly as thin flat films, a mixture of all the components forms thick elongated islands in the surrounding of the films. Consequences of such an inhomogeneity on the magnetic and electrical properties of the layer are discussed. [ABSTRACT FROM AUTHOR]

One of the major challenges in the growth of quantum well and quantum dot heterostructures is the realization of atomically sharp interfaces. Nanowires provide a new opportunity to engineer the band structure as they facilitate the controlled switching of the crystal structure between the zinc-blende (ZB) and wurtzite (WZ) phases. Such a crystal phase switching results in the formation of crystal phase quantum wells (CPQWs) and quantum dots (CPQDs). For GaP CPQWs, the inherent electric fields due to the discontinuity of the spontaneous polarization at the WZ/ZB junctions lead to the confinement of both types of charge carriers at the opposite interfaces of the WZ/ZB/WZ structure. This confinement leads to a novel type of transition across a ZB flat plate barrier. Here, we show digital tuning of the visible emission of WZ/ZB/WZ CPQWs in a GaP nanowire by changing the thickness of the ZB barrier. The energy spacing between the sharp emission lines is uniform and is defined by the addition of single ZB monolayers. The controlled growth of identical quantum wells with atomically flat interfaces at predefined positions featuring digitally tunable discrete emission energies may provide a new route to further advance entangled photons in solid state quantum systems. [ABSTRACT FROM AUTHOR]

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]

Abstract: The amount of copper excess provided during the Cu(In,Ga)Se2 (CIGSe) 3-stage co-evaporation process is among the most operator subjective. In the present paper the influence of this parameter on the properties of the CIGSe films as well as on the behaviour of the related solar cells is investigated. It is observed that both the In/Ga lateral intermixing and the grain size are enhanced when the excess of copper is increased. Contrary to what could be expected, these changes only weakly affect the performance of the solar cells. Increasing the copper excess also yields a rougher CIGSe morphology. This latter evolution is observed to be the most important factor influencing the device behaviour. Through accurate analysis of quantum efficiency, it is concluded that, in the case of the standard cell structure, there exists a threshold in copper excess, beyond which the cell performance is significantly reduced. [Copyright &y& Elsevier]

(In, Ga) alloy droplets are used to catalyse the growth of (In, Ga)As nanowires by molecular beam epitaxy on Si(111) substrates. The composition, morphology and optical properties of these nanowires can be tuned by the employed elemental fluxes. To incorporate more than 10% of In, a high In/(In+Ga) flux ratio above 0.7 is required. We report a maximum In content of almost 30% in bulk (In, Ga)As nanowires for an In/(In+Ga) flux ratio of 0.8. However, with increasing In/(In+Ga) flux ratio, the nanowire length and diameter are notably reduced. Using photoluminescence and cathodoluminescence spectroscopy on nanowires covered by a passivating (In, Al)As shell, two luminescence bands are observed. A significant segment of the nanowires shows homogeneous emission, with a wavelength corresponding to the In content in this segment, while the consumption of the catalyst droplet leads to a spectrally-shifted emission band at the top of the nanowires. The (In,Ga)As nanowires studied in this work provide a new approach for the integration of infrared emitters on Si platforms. [ABSTRACT FROM AUTHOR]

We analyse the morphological, structural and luminescence properties of self-assembled ZnO nanowires grown by chemical vapour transport on Si(001). The examination of nanowire ensembles by scanning electron microscopy reveals that a non-negligible fraction of nanowires merge together forming coalesced aggregates during growth. We show that the coalescence degree can be unambiguously quantified by a statistical analysis of the cross-sectional shape of the nanowires. The examination of the structural properties by x-ray diffraction evidences that the nanowires crystallize in the wurtzite phase, elongate along the c-axis, and are randomly oriented in plane. The luminescence of the ZnO nanowires, investigated by photoluminescence and cathodoluminescence spectroscopy, is characterized by two bands, the near-band-edge emission and the characteristic defect-related green luminescence of ZnO. The cross-correlation of scanning electron micrographs and monochromatic cathodoluminescence intensity maps reveals that: (i) coalescence joints act as a source of non-radiative recombination, and (ii) the luminescence of ZnO nanowires is inhomogeneously distributed at the single nanowire level. Specifically, the near-band-edge emission arises from the nanowire cores, while the defect-related green luminescence originates from the volume close to the nanowire sidewalls. Two-dimensional simulations of the optical guided modes supported by ZnO nanowires allow us to exclude waveguiding effects as the underlying reason for the luminescence inhomogeneities. We thus attribute this observation to the formation of a core–shell structure in which the shell is characterized by a high concentration of green-emitting radiative point defects as compared to the core. [ABSTRACT FROM AUTHOR]

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]

GaN nanowire ensembles with axial InxGa1−xN multi-quantum-wells (MQWs) were grown by molecular beam epitaxy. In a series of samples we varied the In content in the MQWs from almost zero to around 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range of 2.2 to 2.5 eV, depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures. [ABSTRACT FROM AUTHOR]

The oxidation-related issues in controlling Si doping from the Si source material in oxide molecular beam epitaxy (MBE) are addressed by using its solid suboxide, SiO, as an alternative source material in a conventional effusion cell. Line-of-sight quadrupole mass spectrometry of the direct SiO-flux ( Φ SiO ) from the source at different temperatures (TSiO) confirmed SiO molecules to sublime with an activation energy of 3.3 eV. The TSiO-dependent Φ SiO was measured in vacuum before and after subjecting the source material to an O2-background of 10 − 5 mbar (typical oxide MBE regime). The absence of a significant Φ SiO difference indicates negligible source oxidation in molecular O2. Mounted in an oxygen plasma-assisted MBE, Si-doped β-Ga2O3 layers were grown using this source. The Φ SiO at the substrate was evaluated [from 2.9 × 109 cm−2 s−1 (TSiO = 700 °C) to 5.5 × 1013 cm−2 s−1 (TSiO = 1000 °C)] and Si-concentration in the β-Ga2O3 layers measured by secondary ion mass spectrometry highlighting unprecedented control of continuous Si-doping for oxide MBE, i.e., N Si from 4 × 1017 cm−3 (TSiO = 700 °C) up to 1.7 × 1020 cm−3 (TSiO = 900 °C). For a homoepitaxial β-Ga2O3 layer, a Hall charge carrier concentration of 3 × 1019 cm−3 in line with the provided Φ SiO (TSiO = 800 °C) is demonstrated. No SiO-incorporation difference was found between β-Ga2O3(010) layers homoepitaxially grown at 750 °C and β-Ga2O3(−201) heteroepitaxial layers grown at 550 °C on c-plane sapphire. However, the presence of activated oxygen (plasma) resulted in partial source oxidation and related decrease in doping concentration (particularly at TSiO < 800 °C), which has been tentatively explained with a simple model. Degassing the source at 1100 °C reverted this oxidation. Concepts to reduce source oxidation during MBE-growth are referenced. [ABSTRACT FROM AUTHOR]

This paper assesses nonpolar m- and a-plane GaN/Al(Ga)N multi-quantum-wells grown on bulk GaN for intersubband optoelectronics in the short- and mid-wavelength infrared ranges. The characterization results are compared to those for reference samples grown on the polar c-plane, and are verified by self-consistent Schrödinger-Poisson calculations. The best results in terms of mosaicity, surface roughness, photoluminescence linewidth and intensity, as well as intersubband absorption are obtained from m-plane structures, which display room-temperature intersubband absorption in the range from 1.5 to 2.9 µm. Based on these results, a series of m-plane GaN/AlGaN multi-quantum-wells were designed to determine the accessible spectral range in the mid-infrared. These samples exhibit tunable room-temperature intersubband absorption from 4.0 to 5.8 µm, the long-wavelength limit being set by the absorption associated with the second order of the Reststrahlen band in the GaN substrates. [ABSTRACT FROM AUTHOR]

We investigate the optical switching process and the related structural properties of (GeTe)(Sb2Te3) epitaxial films close to Ge2Sb2Te5 composition on GaSb(001). While the amorphization process can take place in a single or in multiple steps, the re-crystallization process always takes place in multiple steps. Intermediate stages of the re-crystallization process are characterized by small crystalline islands within the amorphous area. The structural properties are investigated by optical microscopy and electron backscatter diffraction (EBSD) in a scanning electron microscope. The analysis of the EBSD pattern demonstrates that the crystalline islands at intermediate stages of the re-crystallization process exhibit different orientations. We conclude that the re-crystallization process is driven by nucleation without any orientation information from the substrate. [ABSTRACT FROM AUTHOR]

In this paper, we describe the design and characterization of 400 nm long (88 periods) AlxGa1−xN/AlN (0 ≤ x ≤ 0.1) quantum dot superlattices deposited on self-assembled GaN nanowires for application in electron-pumped ultraviolet sources. The optical performance of GaN/AlN superlattices on nanowires is compared with the emission of planar GaN/AlN superlattices with the same periodicity and thickness grown on bulk GaN substrates along the N-polar and metal-polar crystallographic axes. The nanowire samples are less sensitive to nonradiative recombination than planar layers, attaining internal quantum efficiencies (IQE) in excess of 60% at room temperature even under low injection conditions. The IQE remains stable for higher excitation power densities, up to 50 kW cm−2. We demonstrate that the nanowire superlattice is long enough to collect the electron–hole pairs generated by an electron beam with an acceleration voltage VA = 5 kV. At such VA, the light emitted from the nanowire ensemble does not show any sign of quenching under constant electron beam excitation (tested for an excitation power density around 8 kW cm−2 over the scale of minutes). Varying the dot/barrier thickness ratio and the Al content in the dots, the nanowire peak emission can be tuned in the range from 340 to 258 nm. [ABSTRACT FROM AUTHOR]

Films of transition-metal–chalcogen alloys are produced using hot wall epitaxy by means of sulphurization in the exposure of Cu, Ag, Ni and NiCu films to Sb 2 S 3 vapor. The properties of the resultant chalcogenide films change widely depending on the material combination and the synthesis conditions. We investigate the influences of the coexistence of Sb in the alloys on the electrical properties of the films. The Ni-containing films are highly conductive, whereas the resistivity of the films produced from Cu and Ag is large. The sulphurization occurs two-dimensionally as homogeneous films consisting of Cu-Sb-S and Ag-Sb-S ternary alloys, giving rise to weak and negligible dependencies of the resistivity on the synthesis temperature in the respective cases. The thermal activation energy for the electrical transport varies in correspondence to the resistivity values. The Cu- and Ag-based films exhibit a suppression of current when Ag electrodes are used in comparison to using Au electrodes. The transient of photocurrent is shown for these films to be remarkably slow with an order of magnitude change of the current. The spontaneous reaction with Al contacts to trigger interfacial resistance switching is weak for the Sb-containing materials. The structural properties of the films are furthermore presented together with the Raman spectra to identify the synthesized alloys. A catalytic effect of Ni in the sulphurization of Cu and the surfactant effect of Sb are thereby revealed. • Films produced by sulphurizing Cu and Ag using Sb2S3 are electrically resistive due to the generation of ternary alloys. • The variation of the resistivity of the alloy films is associated with the thermal activation energy. • Inclusion of Sb in the chalcogenide alloys weakens the spontaneous redox reactions for the interfacial resistance switching. • The ternary transition-metal–chalcogenides exhibit remarkably slow phototransients. [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 investigate the effect of the p-type top contact on the optoelectronic characteristics of light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. We compare devices fabricated with either Ni/Au or indium tin oxide (ITO) top contact. The NW-LEDs with ITO exhibit a number density of NWs emitting electroluminescence about ten times higher, significantly lower turn-on voltage and series resistance, and a relative external quantum efficiency more than one order of magnitude higher than the sample with Ni/Au. These results show that limitations in the performance of such devices reported so far can be overcome by improving the p-type top-contact. [ABSTRACT FROM AUTHOR]

Light emitting diodes (LEDs) have been fabricated using ensembles of free-standing (In, Ga)N/GaN nanowires (NWs) grown on Si substrates in the self-induced growth mode by molecular beam epitaxy. Electron-beam-induced current analysis, cathodoluminescence as well as biased μ-photoluminescence spectroscopy, transmission electron microscopy, and electrical measurements indicate that the electroluminescence of such LEDs is governed by the differences in the individual current densities of the single-NW LEDs operated in parallel, i.e. by the inhomogeneity of the current path in the ensemble LED. In addition, the optoelectronic characterization leads to the conclusion that these NWs exhibit N-polarity and that the (In, Ga)N quantum well states in the NWs are subject to a non-vanishing quantum confined Stark effect. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate the morphological properties of height, diameter and shape controlled N-face GaN nanorods (NRs) by adjusting conventional growth parameters of a standard metalorganic vapour phase epitaxy (MOVPE) growth process. Particularly the hydrogen fraction within the carrier gas was shown to be an important shaping tool for the grown nanostructures. Additionally, the aspect ratio of the NRs was successfully tuned by increasing the pitch of the nanoimprint lithography (NIL) pattern, while maintaining the hole-diameter constant. An optimum aspect ratio could be found at pitches between 400 and 800nm, whereas larger pitches are counter-productive. The major conclusion drawn from our experiments is that the whole amount of growth material available over the masked surface contributes to the growth of the NRs. [Copyright &y& Elsevier]