Mechanical Deformation Behavior of Nonpolar GaN Thick Films by Berkovich Nanoindentation

In this study, the deformation mechanisms of nonpolar GaN thick films grown on m-sapphire by hydride vapor phase epitaxy (HVPE) are investigated using nanoindentation with a Berkovich indenter, cathodoluminescence (CL), and Raman microscopy. Results show that nonpolar GaN is more susceptible to plastic deformation and has lower hardness than c-plane GaN. After indentation, lateral cracks emerge on the nonpolar GaN surface and preferentially propagate parallel to the orientation due to anisotropic defect-related stresses. Moreover, the quenching of CL luminescence can be observed to extend exclusively out from the center of the indentations along the orientation, a trend which is consistent with the evolution of cracks. The recrystallization process happens in the indented regions for the load of 500 mN. Raman area mapping indicates that the distribution of strain field coincides well with the profile of defect-expanded dark regions, while the enhanced compressive stress mainly concentrates in the facets of the indentation.     

Growth of GaN on nano-crystalline diamond substrates

In this study GaN has been grown on nano-crystalline diamond substrates utilizing metal-organic chemical vapour deposition (MOCVD). It is shown that the growth of closed GaN films onto synthetic diamond substrates is feasible, when applying the correct buffer layer and growth parameters. XRD measurements showed that the GaN formed is of wurzite structure and polycrystalline, but the high intensity of the (0002) diffraction peak indicates a preferential crystallite orientation. This preferred [0001] orientation was confirmed by SEM analysis. The optical quality of the deposited GaN layer was investigated using cathodoluminescence and showed a large yellow luminescence peak. This work comprises a first step in preparing heterogeneous layers and GaN devices with a diamond heat sink as a substrate, facilitating the thermal management of these devices.     

Effective reduction of AlN defect luminescence by fluorine implantation

We report on the effective reduction of AlN host lattice defect cathodoluminescence by high dose ion implantation of light elements such as fluorine as well as chlorine and neon with peak concentrations of 1 at.%. In order to distinguish between luminescence suppression in the visible to luminescence quenching due to radiation damage, all samples were additionally implanted with europium at fluences of 1 · 10¹³ ions/cm². After annealing the samples at 1373 K under vacuum conditions cathodoluminescence spectra were recorded at room temperature (300 K) and at cryogenic temperature (12 K). These investigations reveal that different light ion species have different influences on the defect luminescence of the AlN host lattice which is likely due to selective passivation of these defects. The best ratio of defect luminescence suppression to radiation damage induced luminescence quenching is achieved in the case of fluorine co-doping.     

The fabrication of white light-emitting diodes using the n-ZnO/NiO/p-GaN heterojunction with enhanced luminescence

Cheap and efficient white light-emitting diodes (LEDs) are of great interest due to the energy crisis all over the world. Herein, we have developed heterojunction LEDs based on the well-aligned ZnO nanorods and nanotubes on the p-type GaN with the insertion of the NiO buffer layer that showed enhancement in the light emission. Scanning electron microscopy have well demonstrated the arrays of the ZnO nanorods and the proper etching into the nanotubes. X-ray diffraction study describes the wurtzite crystal structure array of ZnO nanorods with the involvement of GaN at the (002) peak. The cathodoluminescence spectra represent strong and broad visible emission peaks compared to the UV emission and a weak peak at 425 nm which is originated from GaN. Electroluminescence study has shown highly improved luminescence response for the LEDs fabricated with NiO buffer layer compared to that without NiO layer. Introducing a sandwich-thin layer of NiO between the n-type ZnO and the p-type GaN will possibly block the injection of electrons from the ZnO to the GaN. Moreover, the presence of NiO buffer layer might create the confinement effect.     

Microstructure and luminescence properties of the high pressure high temperature sintered AlN–TiN ceramics

Composite ceramics of titanium nitride grains incorporated in an aluminium nitride matrix have been synthesized by high pressure high temperature treatment of a mechanical mixture of AlN–TiN (3 mol % TiN) powders. The microstructure of the samples analysed by means of electron beam microanalysis and Raman spectroscopy shows that the formation of cubic AlN in the composite begins near titanium nitride grains. The areas of mixed chemical composition, which can be assigned to the formation of the solid solutions Al_1?xTi_xN, have been observed at the phase interfaces. The luminescence properties of AlN–TiN ceramics have been considered focusing on the choice of high pressure and high temperature treatment conditions. Three main components at 2.0 eV, 2.4 eV and 3.1 eV are revealed in the cathodoluminescence spectra analysed quantitatively. The observed emission originates from the radiative transitions with participation of valence band states, oxygen-vacancy centres (V_Al–O_N), nitrogen vacancies V_N, and shallow donors which form a complex system of energy levels in the bandgap of the wurtzite-type AlN.     

Raman and photoluminescence spectra of indented cubic boron nitride and polycrystalline cubic boron nitride

The use of Raman spectroscopy, and in particular Raman line shifts, to measure stress in diamond and nitrides such as gallium nitride (GaN), is well known. In both diamond and GaN the application is principally to study stresses in thin films and at the substrate–thin film interface. Stresses in polycrystalline diamond composites have also been measured by this method. Typically stresses of the order of GPa can be determined with a spatial resolution of a few micrometers. In this paper, Raman spectra of indentations on cubic boron nitride (cBN) crystals and polycrystalline cubic boron nitride (PcBN) composites are presented. Shifts of the cBN Raman lines from their unstressed positions quantify the residual stresses in the boron nitride due to the deformation brought about by the indentation. Making use of the measured coefficient of shift of 3.39 cm⁻¹/GPa for the transverse optical Raman peak, these are of the order of 1 GPa. These measurements illustrate, for the first time, the use of Raman spectroscopy to study residual stresses in boron nitride. Plastic deformation is usually associated with the creation of vacancies. To investigate the possible presence of vacancy defects and vacancy-related defects, the indented boron nitride samples were also studied with photoluminescence spectroscopy.     

Effect of annealing atmosphere on the structure and dielectric properties of unfilled tungsten bronze ceramics Ba4PrFe0.5Nb9.5O30

Unfilled tungsten bronze ceramics with the nominal formula Ba₄PrFe_0.5Nb_9.5O₃₀ were synthesized via the standard solid-state sintering route, and the effects of oxygen vacancies on the dielectric and electrical properties were investigated in addition to the structure. Room-temperature X-ray diffraction showed that the N₂-annealed sample had the largest cell volume. Low-temperature spectrum showed that N₂ annealing rendered the dielectric constant and dielectric loss more frequency dispersive, whereas O₂ annealing inhibited the frequency dispersion. The dc conductivity of all the samples originated from the electrons produced in the second ionization of oxygen vacancies and was most likely controlled by a mixed conduction mechanism of the electron and oxygen-vacancy ions. The N₂-annealed sample has the highest dc conductivity owing to its high concentration of oxygen vacancies. The broadening of the Raman lines and the decrease of Raman intensity for the N₂-annealed sample originated from a significant structural disorder. X-ray photoelectron spectra demonstrated that the increased oxygen vacancies caused by the change of valences of Fe and Pr ions contributed to the structural disorder.     

High-pressure and high-temperature annealing effects on CVD homoepitaxial diamond films

High-pressure and high-temperature (HPHT) annealing effects on the chemical vapor-deposited (CVD) homoepitaxial diamond films were investigated. By the HPHT annealing, the intensity of free-exciton (FE)-related emission was increased by  2 times and the luminescence bands from 270 to 320 nm, which originate from 5RL and 2BD bands, were almost completely eliminated in the cathodoluminescence (CL) spectrum. The CL intensity of band-A emission, which is related to crystal defects in diamond, was also decreased. The hole mobility at room temperature was increased from 826 to 1030 cm²/Vs by HPHT annealing. These results suggest that HPHT annealing decreases the crystalline defects and improves the optical and electronic properties of homoepitaxial diamond films.     

Spectroscopic studies of glasses based on rare-earth borates

Raman light scattering and Nd³⁺ luminescence are investigated in glasses based on rare-earth borates with a high content of the rare-earth oxide and subjected to annealing. It is established that, during annealing, structural relaxation brings about the formation of phases with a structure similar to the structure of rare-earth orthoborates and oxoborates. This leads to considerable changes in the characteristics of Nd³⁺ luminescence.     

Optical and structural properties of Diamond/GaN films

We report on the successful deposition of continuous boron-doped diamond films on p-GaN substrate in a hot-filament chemical vapor deposition (HFCVD) system. The Raman spectrum observed at 1331 cm^− 1 is a distinctive, asymmetric Fano line shape, which shows that the films under the present growth condition are heavily doped. The photoluminescence spectra for GaN (the diamond is taken away from the Diamond/GaN sample) are characterized by significant spectral peak located at 3.358 eV. After annealing at 700 °C, a sharp PL line located at 358 nm (3.468 eV) is then observed. Additionally, the zero phonon line is located at 3.27 eV. However, this line has a blueshift of 0.25 eV in comparison with PL spectra for p-type GaN:Mg. This blueshift, which may be accounted for a by large lattice relaxation is associated with the dissociation of C diffusion into GaN.     

Positron lifetime investigations of diamond films

Positron lifetime investigations on B-doped and undoped chemical vapor deposition diamond films have shown that B-doping at 500 ppm almost completely removes vacancies from the film. The 1.68 eV photo luminescence (PL) line is also removed. For undoped films, large concentrations (≈10¹⁸ cm⁻³) of vacancy clusters (approximately 6 vacancies) and mono- or divacancies are observed. Small observable effects arising from annealing up to 1100°C could be found in both B-doped or undoped films. There is no correlation between the width of the 1.68 eV PL line and the vacancy concentration.     

Cathodoluminescence from Implanted and Anodized Polycrystalline Silicon Films

Luminescence emission of LPCVD polycrystalline silicon films has been studied by cathodoluminescence (CL) in the scanning electron microscope. As-deposited films show visible luminescence with dominant blue band. The relative intensity of blue emission is enhanced by implantation and by slight anodization treatments. Our investigations are consistent with previous PL results and indicate that the origin of blue emission is related to quantum confinement effects. On the other hand, the effect of annealing in these samples is a reduction of the CL signal that could be related to the increase of the nanocrystals size.     

Cathodoluminescence spectra of gallium nitride nanorods

ABSTRACT: Gallium nitride [GaN] nanorods grown on a Si(111) substrate at 720°C via plasma-assisted molecular beam epitaxy were studied by field-emission electron microscopy and cathodoluminescence [CL]. The surface topography and optical properties of the GaN nanorod cluster and single GaN nanorod were measured and discussed. The defect-related CL spectra of GaN nanorods and their dependence on temperature were investigated. The CL spectra along the length of the individual GaN nanorod were also studied. The results reveal that the 3.2-eV peak comes from the structural defect at the interface between the GaN nanorod and Si substrate. The surface state emission of the single GaN nanorod is stronger as the diameter of the GaN nanorod becomes smaller due to an increased surface-to-volume ratio.     

Highly Conductive p‐Type Zinc blende SiC Thin Films Fabricated on Silicon Substrates by Magnetron Sputtering

Polycrystalline silicon carbide (SiC) thin films were fabricated on Si(100) substrates using radio‐frequency magnetron sputtering followed by annealing at 1300°C in an Ar atmosphere. The SiC films exhibited a zinc blende structure with planar and point defects as detected by X‐ray diffraction and Raman spectroscopy. The SiC films were p‐type conductive with electrical resistivity as low as 2.8 × 10^?3 Ω·cm at room temperature. The p‐type character of the SiC films can be explained in terms of the Si vacancies in the C‐rich environment as evidenced by Raman spectroscopy.     

Preparation of YAG:Europium Red Phosphors by Spray Pyrolysis Using a Filter-Expansion Aerosol Generator

Europium-doped yttrium aluminum garnet (YAG) phosphor particles were prepared from mixed nitrate solutions by the FEAG (filter expansion aerosol generator) process. The crystallinity, morphology, and luminescence of the YAG:Eu particles were investigated. The prepared particles had an amorphous phase, which turned into phase-pure YAG particles after annealing above 1000°C. A cubic-structure YAG phase was formed by the reaction of crystalline Y₂O₃ and the aluminum component. The prepared particles had spherical morphology. The mean size of the YAG:Eu particles increased from 0.45 to 1.0 µm when the overall solution concentrations were increased from 0.02 to 1.2 mol/L. The optimum doping concentration of europium for the maximum brightness of phosphor particles was 1.3 at.%. The cathodoluminescence (CL) intensity was strongly affected by the annealing temperatures. The maximum CL value of the particles was 55 cd/m².     

Electroluminescence from electron injection junctions created by carbon and phosphorus ion implantation

A p-type (type IIb) natural diamond was implanted with either carbon or phosphorus ions using the cold implantation rapid annealing (CIRA) process. In each case, the energies and doses were chosen such that upon annealing, the implanted layer would act as an n-type electrode. The electroluminescence (EL) emitted from these carbon and phosphorus junctions, when biased in the forward direction, was compared as a function of annealing and diode temperatures. Typical luminescence bands such as those observed in cathodoluminescence (CL), in particular a blue band A (≈2.9 eV) and a green band (≈2.4 eV), were observed. Two bands centred around ≈2.1 and 4 eV were also observed for both the carbon and phosphorus junctions, whereas a band at ≈4.45 eV appeared only in the phosphorus-implanted junction.     

Spectroscopic characterization of thin SiC films

The electrical, structural and optical properties of thin SiC films were investigated. A new approach based on high temperature annealing of layered carbon–silicon structures was used for the formation of the films. The SiC films were prepared by deposition of 30 nm thick carbon films on crystalline silicon (c-Si) and on porous silicon layers grown on c-Si. The layers were annealed to temperatures between 800 and 1400°C for different annealing times ranging between 15 and 180 s. The structure of the resulting SiC films was analyzed by Raman spectroscopy. The Raman spectra of as-deposited films consist of two broad bands at 1350 and 1580 cm⁻¹ characteristic of the presence of amorphous carbon. These bands were shifted to lower frequencies in the spectra of annealed layers and were assigned to the hexagonal and cubic SiC phases. The photoluminescence spectra of the studied layers show a broad band at 550 nm. The most intense photoluminescence was observed from non-annealed porous silicon layers covered with thin carbon films. A degradation of the luminescence and a simultaneous increase of the conductivity of the layers with increasing annealing temperature and/or duration of annealing was observed. This behavior strongly suggests the creation of defect states which determine the conductivity of the layers and at the same time act as non-radiative centers. The increase of defect states was explained as originating from the dehydrogenation of the silicon carbide layers by annealing.     

Irradiation damage in xenon-irradiated α-Al2O3 before and after annealing

The irradiation damage build-up of α-Al₂O₃ under Xe²⁰⁺ ion irradiation has been investigated by a combination of Raman spectroscopy and transmission electron microscopy. α-Al₂O₃ crystalline was irradiated with 5 MeV Xe²⁰⁺ ions to fluences of 1 × 10¹⁴ cm⁻², 5 × 10¹⁴ cm⁻², 1 × 10¹⁵ cm⁻² and 5 × 10¹⁵ cm⁻² at room temperature. No amorphous phase was formed under the experimental condition. The Raman intensities of feature peaks of Al₂O₃ decrease after Xe ion irradiation. The interstitial-type dislocation loops with Burgers vectors of b = 1/3 < 10-11> on the {10-10} and (0001) habit planes were found. The formation of basal and prism dislocation loops is related to the lattice damage and position. After annealing, the Raman intensities of feature peaks of Al₂O₃ increases with annealing temperature. With annealing at 1500℃ for 30 min, lattice defects were completely annealed out in the near surface region. Meanwhile, long dislocations and facet cavities on long dislocations were found in the Xe deposition region. Some lattice defects beyond the projected region were found due to the diffusion toward deep region during thermal annealing.     

Nanoscale anisotropic plastic deformation in single crystal GaN

Elasto-plastic mechanical deformation behaviors of c-plane (0001) and nonpolar GaN single crystals are studied using nanoindentation, cathodoluminescence, and transmission electron microscopy. Nanoindentation tests show that c-plane GaN is less susceptible to plastic deformation and has higher hardness and Young''s modulus than the nonpolar GaN. Cathodoluminescence and transmission electron microscopy characterizations of indent-induced plastic deformation reveal that there are two primary slip systems for the c-plane GaN, while there is only one most favorable slip system for the nonplane GaN. We suggest that the anisotropic elasto-plastic mechanical properties of GaN are relative to its anisotropic plastic deformation behavior.PACS: 62.20.fq; 81.05.Ea; 61.72.Lk.     

Synthesis of Eu-doped gahnite in water and water–ammoniac fluids

The effect of ammonia contained in water fluid on the synthesis and properties of gahnite (ZnAl₂O₄) doped with (0.4–4 mol%) europium was investigated. Gahnite synthesized in water–ammoniac fluid has a smaller crystal size than does the one synthesized in water fluid, and bound nitrogen is detected in its structure. However, europium ions more effectively incorporate into the structure of nascent gahnite during synthesis in water fluid and generate formation there of complexes with oxygen vacancies. The excitation efficiency of luminescence of Eu³⁺ ions in the absorption band of oxygen vacancies in gahnite increases with the concentration of Eu³⁺. After annealing in air of gahnite samples synthesized in fluids of various compositions, the changes of Eu³⁺ ions luminescence differ. We conclude that ammonia not only participates in the processes of gahnite formation, causing an increased nucleation rate but it also remains in the gahnite structure.