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Epitaxial growth of (100)-oriented rare-earth nitrides (RENs) SmN, GdN, and DyN on (100)LaAlO3 (LAO) substrates is demonstrated using molecular beam epitaxy. RHEED and ϕ-scans confirm that the cubic RENs grow 45° rotated with respect to the pseudocubic (100)-surface of LAO, which leads to lattice mismatches between −5.8% and −8.7% for the selected RENs. Those lattice mismatches, despite being significant, are smaller than in previously reported epitaxial (100)RENs, with the exception of growth on (100)YSZ (yttria stabilized zirconia), which however causes the formation of an interface oxide layer. The SmN RHEED pattern shows intense streaks, indicating a high quality, flat surface, and the rocking curves are among the narrowest reported for (100)RENs. In contrast to growth on Si, the epitaxial RENs readily form over a wide range of substrate temperatures, without the need of special substrate treatment or intermediate layers. This robust, high quality growth paired with their clear magnetic switching behavior makes epitaxial RENs grown on LAO ideal candidates for future spintronic devices. [ABSTRACT FROM AUTHOR]

We report magnetoresistance measurements on epitaxial films of the intrinsic ferromagnetic semiconductor GdN electron doped with ∼ 10 20 cm − 3 to ∼ 10 21 cm − 3 . The magnetoresistance across the temperature range of 10–300 K is dominated by a reduction of spin-disorder scattering in the presence of a magnetic field, imposing a resistance reduction of 27% in a field of 8 T. We show that the magnetoresistance closely follows the magnetic disorder as signaled by the departure of the magnetization from its fully saturated value M s of 7 μ B / G d 3 + . [ABSTRACT FROM AUTHOR]

Combining elastic incoherent neutron scattering and differential scanning calorimetry, we investigate the occurrence of the volume phase transition (VPT) in very concentrated poly-(N-isopropyl-acrylamide) (PNIPAM) microgel suspensions, from a polymer weight fraction of 30 wt. % up to dry conditions. Although samples are arrested at the macroscopic scale, atomic degrees of freedom are equilibrated and can be probed in a reproducible way. A clear signature of the VPT is present as a sharp drop in the mean square displacement of PNIPAM hydrogen atoms obtained by neutron scattering. As a function of concentration, the VPT gets smoother as dry conditions are approached, whereas the VPT temperature shows a minimum at about 43 wt. %. This behavior is qualitatively confirmed by calorimetry measurements. Molecular dynamics simulations are employed to complement experimental results and gain further insights into the nature of the VPT, confirming that it involves the formation of an attractive gel state between the microgels. Overall, these results provide evidence that the VPT in PNIPAM-based systems can be detected at different time- and length-scales as well as under overcrowded conditions. [ABSTRACT FROM AUTHOR]

We report electrical transport and optical spectroscopy measurements on LuN thin films variously doped with nitrogen vacancies along with the computed band structures of stoichiometric and nitrogen vacancy doped LuN. LuN has been the subject of several recent computational studies; however, the most recent experimental studies regarding its electronic properties are already over four decades old. Here, we bridge the void between computation and experiment with a combined study of LuN focusing on its electronic properties. We find that LuN is a semiconductor with an optical bandgap of ∼1.7 eV. Its conductivity can be controlled by nitrogen vacancy doping, which results in defect states at the conduction band minimum and valence band maximum. These results not only provide information on LuN but also help underpin understanding of the electronic properties of the entire rare-earth nitride series. [ABSTRACT FROM AUTHOR]

Myelin is an insulating, multi-lamellar membrane structure wrapped around selected nerve axons. In-creasing the speed of nerve impulses, it is crucial for the proper functioning of the vertebrate nervous system. Human neurodegenerative diseases, such as multiple sclerosis, are linked to damage to the myelin sheath through demyelination. Myelin exhibits a well defined subset of myelin-specific pro-teins, whose influence on membrane dynamics, i.e., myelin flexibility and stability, has not yet been explored in detail. In a first paper [W. Knoll, J. Peters, P. Kursula, Y. Gerelli, J. Ollivier, B. Demé, M. Telling, E. Kemner, and F. Natali, Soft Matter 10, 519 (2014)] we were able to spotlight, through neutron scattering experiments, the role of peripheral nervous system myelin proteins on membrane stability at room temperature. In particular, the myelin basic protein and peripheral myelin protein 2 were found to synergistically influence the membrane structure while keeping almost unchanged the membrane mobility. Further insight is provided by this work, in which we particularly address the investigation of the membrane flexibility in the low temperature regime. We evidence a different behavior suggesting that the proton dynamics is reduced by the addition of the myelin basic protein accompanied by negligible membrane structural changes. Moreover, we address the importance of correct sample preparation and characterization for the success of the experiment and for the relia-bility of the obtained results. [ABSTRACT FROM AUTHOR]

GdN and SmN are two of the lanthanide nitrides, most of which are intrinsic ferromagnetic semiconductors. Superlattices comprising the pair offer a unique opportunity to investigate heterojunctions that feature simultaneous conductivity and magnetic interface influences. Here we report an investigation of these influences, using magnetisation and X-ray magnetic circular dichroism for magnetic effects, and magnetoresistance and Hall effect studies of their electrical conductance. Magnetic data show clear signatures of a conflicting Zeeman vs. interfacial exchange and the magnetic disruption that results, while resistivity and Hall measurements show conduction in both GdN and SmN. [ABSTRACT FROM AUTHOR]

We report on the effect of the nitrogen to rare earth (N2/RE) flux ratio on the structural, transport, and magnetic properties of samarium nitride (SmN) and dysprosium nitride (DyN) thin films. Both materials display a reduced lattice constant when the N2/RE flux ratio decreases, i.e., with increased nitrogen vacancies (VN) concentration. The films show several orders of magnitude increase in the electrical resistivity with increased N2/RE flux ratio. Finally, magnetic measurements on DyN films display a deviation from the free ion moment at low temperature, which is eased in more conductive films. This was interpreted as a further reduction in the quenching of the orbital angular momentum caused by the increased VN concentration. The Curie temperature was also found to increase with VN. [ABSTRACT FROM AUTHOR]

Most members of the ferromagnetic rare-earth nitride series display doping control over electron transport, with nitrogen vacancies being the most common donor. This paper reports the control and characterization of vacancies in one of the fourteen in the series, DyN. Electrical transport and optical spectra in films with controlled concentrations of vacancies show a pair of in-gap impurity levels ∼ 0.4 eV below the conduction band minimum and a third impurity level that lies nearly coincident with the conduction band minimum. Electron transport is found to be activated for concentrations ≤ 1019 cm−3, with signatures of extended state conduction at the Fermi level for higher concentrations. [ABSTRACT FROM AUTHOR]

A blue light emitting diode (LED) is grown on top of a (Ga, In)N/GaN multiple quantum well (QW) acting as a light converter from blue to green-yellow wavelength. The blue light is produced by electrical injection, while the green-yellow emitting QWs are optically pumped by the blue photons. It is shown that the final color of the LED is strongly dependent on the blue pumping wavelength, the absorption and the internal quantum efficiency of the light converter. Depending on these parameters, blue to green LEDs or even white LEDs can be obtained. In addition, the injection current dependence of the LED electroluminescence is measured and analyzed. A very low blueshift is observed as a function of the injection current. It is explained by the fact that the carrier density per QW in the light converter stays relatively low compared to the case of classical current-injected green LEDs. [ABSTRACT FROM AUTHOR]

We show that the shape of GaN nanostructures grown by molecular beam epitaxy on AlxGa1-xN (0001) surfaces, for x≥0.4, can be controlled via the ammonia pressure. The nanostructures are obtained from a two dimensional to three dimensional transition of a GaN layer occurring upon a growth interruption. Atomic force microscopy measurements show that depending on the ammonia pressure during the growth interruption, dot or dash-shaped nanostructures can be obtained. Low temperature photoluminescence measurements reveal a large redshift in the emission energy of the quantum dashes, as compared to the quantum dots. By simply adjusting the GaN deposited thickness, it is shown that quantum dashes enable to strongly extend the emission range of GaN/Al0.5Ga0.5N nanostructures from the violet-blue (∼400–470 nm) to the green-orange range (∼500–600 nm). [ABSTRACT FROM AUTHOR]

A method is presented to achieve thick high quality crack-free AlGaN layers on GaN. This method uses jointly plastic relaxation and lateral growth. In a first step, plastic relaxation by cracking and misfit dislocation introduction is realized. Then the cracks are overgrown to obtain a smooth surface. By this reproducible technique, we grew smooth metal-organic chemical vapor deposition Al[sub 0.2]Ga[sub 0.8]N films with a threading dislocation density as low as 5×10[sup 8] cm[sup -2]. This result is the best ever reported for crack-free AlGaN growth over a large area. The control of the complete plastic relaxation opens up perspectives for the realization of high performance devices. In order to explain the mechanisms involved in the full relaxation of the AlGaN/GaN heterostructure, we propose a relaxation scheme and discuss its different steps. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The refractive indices of several Al[sub x]Ga[sub 1-x]N alloys deposited on silicon are determined by ellipsometry and reflectivity experiments at room temperature. The AlGaN layers are grown on (111)Si substrate by molecular-beam epitaxy on top of an AlN/GaN/AlN buffer in order to reduce the strain of the alloy. The Al composition is deduced from energy dispersive x-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k are determined in the 300–600 nm range. For the transparent region of Al[sub x]Ga[sub 1-x]N, the refractive index is given in form of a Sellmeier law. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report persistent photoconductivity in Mg-doped GdN thin films grown by molecular beam epitaxy. Temperature-dependent measurements were carried out in the time and frequency domains to probe the nature of Mg impurities in GdN. The results reveal an initial fast decay followed by a slow persistent photoconductivity. The magnitude of the photoconductivity as well as the characteristic fast- and slow-decay times was found to decrease systematically with increasing the Mg-doping level. Our experimental results suggest that Mg impurities in epitaxial GdN thin films act as acceptor-like centres. Interestingly they also show that the incorporation of Mg result in a significant decrease in the concentration of nitrogen vacancies, as is demonstrated also to be in agreement with an ab initio calculation. [ABSTRACT FROM AUTHOR]

We report the growth by molecular beam epitaxy of highly resistive GdN, using intentional doping with magnesium. Mg-doped GdN layers with resistivities of 10³ Ω cm and carrier concentrations of 1016cm-3 are obtained for films with Mg concentrations up to 5 x 1019 atoms/cm³. X-ray diffraction rocking curves indicate that Mg-doped GdN films have crystalline quality very similar to undoped GdN films, showing that the Mg doping did not affect the structural properties of the films. A decrease of the Curie temperature with decreasing the electron density is observed, supporting a recently suggested magnetic polaron scenario [F. Natali, B. J. Ruck, H. J. Trodahl, D. L. Binh, S. Vezian, B. Damilano, Y. Cordier, F. Semond, and C. Meyer, Phys. Rev. B 87, 035202 (2013)]. [ABSTRACT FROM AUTHOR]

Polycrystalline GdN thin films have been grown at room temperature with varying N2 pressure. By varying the nitrogen pressure during growth we alter the carrier concentrations of the films. Films grown at low nitrogen pressures display onset of magnetization at temperatures as high as 200 K and a resistivity of 0.3 mΩ cm, whereas films grown at high nitrogen pressures all show a Curie temperature very close to 70 K and resistivity ranges over 1-1000 Ω cm are observed. For all GdN films a peak in the resistivity occurs at TC. [ABSTRACT FROM AUTHOR]

Electron spin dynamics in n-type c-oriented wurtzite GaN epilayers is studied by time-resolved Kerr-rotation measurements at T=80 K. The electron spin lifetime shows a sudden increase if an external magnetic field is applied in the sample plane. This enhancement is explained by anisotropic Dyakonov–Perel spin relaxation in bulk GaN as a direct consequence of the anisotropy of spin-orbit coupling in semiconductors with wurtzite structure. [ABSTRACT FROM AUTHOR]

A monolithic white light emitting diode using a (Ga,In)N/GaN multiple quantum well (MQW) light converter is demonstrated. Blue photons emitted under electrical injection by (Ga,In)N/GaN QWs located inside a GaN p-n junction are partly absorbed by another (Ga,In)N/GaN MQW situated outside the junction which emits yellow-green light. The combination of the blue and yellow-green components results in white light emission. [ABSTRACT FROM AUTHOR]

The growth by molecular beam epitaxy and the optical properties of GaN/Al0.5Ga0.5N quantum dots on (0001) sapphire substrates are reported. The quantum dots are spontaneously formed via a two dimensional to three dimensional transition upon growth interruption. Photoluminescence over the blue range (435–470 nm) is obtained at room temperature by varying the GaN nominal thickness. A weak temperature dependence of the integrated photoluminescence intensity between low temperature and room temperature is observed indicating strong carrier localization in the quantum dots. [ABSTRACT FROM AUTHOR]

An experimental study of Bi, Sb, and Cu clusters incident at velocities >=50 m/s on SiO2, Si3N4, polymethylmethacrylate, and photoresist surfaces shows that the clusters adhere much more strongly to SiO2 and Si3N4 than to the polymer materials. The differences in adhesion properties allow assembly of a range of nanowire-based electronic devices from cluster building blocks using lithographically patterned polymer layers. Clusters adhere to the substrate but not to the surface of the polymer template, eliminating parasitic conduction. Molecular dynamics simulations show that differing cluster-surface interactions affect adhesion most strongly when high incident velocities cause significant plastic deformation of the clusters. [ABSTRACT FROM AUTHOR]

The reflectance spectra of simple design GaN-based microcavities have been studied in the 5 K–300 K range. The epitaxial structure consists of the silicon substrate and the stack of buffer layers as the back mirror, a GaN active layer, and a 100 Å thick aluminium layer as the top mirror. Active layer thicknesses of λ/2, λ, or 3λ/2 were investigated. The samples with GaN thicknesses λ/2 and λ display an anticrossing behavior between the cavity and exciton modes, with measured Rabi splittings of 47 and 60 meV, respectively, both at 5 K and room temperature. [ABSTRACT FROM AUTHOR]

We report on the influence of the structural properties on the refractive index of AlN films grown on Si(111) substrates by molecular-beam epitaxy using ammonia. The structural properties are assessed by reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and x-ray diffraction. Refractive index values are deduced from room-temperature spectroscopic ellipsometry. Optical data analysis is performed using the Kramers-Krönig relation in the transparent spectral region, from 1.6 to 3.2 eV. Evidence is presented showing the influence of strain and dislocation density on the AlN layer refractive index. [ABSTRACT FROM AUTHOR]

We report on the growth by molecular-beam epitaxy on 2 in. sapphire substrates of crack-free Al[sub x]Ga[sub 1-x]N/GaN distributed Bragg reflectors (DBRs) with high-Al composition (x=0.5). This is achieved by introducing a thick AlN interlayer and strain mediating Al[sub y]Ga[sub 1-y]N layer between the substrate and DBR. The relatively larger refractive index ratio between Al[sub 0.5]Ga[sub 0.5]N and GaN permits one to obtain a quite large spectral stopband width (49 nm) and a high reflectance value (69%) for only eight mirror periods. [ABSTRACT FROM AUTHOR]

Epitaxial growth of (100)-oriented rare-earth nitrides (RENs) SmN, GdN, and DyN on (100)LaAlO3 (LAO) substrates is demonstrated using molecular beam epitaxy. RHEED and ϕ-scans confirm that the cubic RENs grow 45° rotated with respect to the pseudocubic (100)-surface of LAO, which leads to lattice mismatches between −5.8% and −8.7% for the selected RENs. Those lattice mismatches, despite being significant, are smaller than in previously reported epitaxial (100)RENs, with the exception of growth on (100)YSZ (yttria stabilized zirconia), which however causes the formation of an interface oxide layer. The SmN RHEED pattern shows intense streaks, indicating a high quality, flat surface, and the rocking curves are among the narrowest reported for (100)RENs. In contrast to growth on Si, the epitaxial RENs readily form over a wide range of substrate temperatures, without the need of special substrate treatment or intermediate layers. This robust, high quality growth paired with their clear magnetic switching behavior makes epitaxial RENs grown on LAO ideal candidates for future spintronic devices. [ABSTRACT FROM AUTHOR]

We present both electric-field and photo-excited control of the carrier concentration in GdN. There is no evidence in the results of a carrier-mediated contribution to the Gd-Gd exchange interaction that has been suggested to explain a measured Curie temperature that is much higher than obtained within theoretical treatments. Persistent carrier concentrations seen in both the field-effect and photo-induced conductivities point to a distribution of long-lived trap states below the conduction band, very likely centered at nitrogen vacancies. [ABSTRACT FROM AUTHOR]