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We show that the degree of linear polarization of the photoluminescence of tris-(8-hydroxyquinoline)aluminium (Alq3) thin film can be increased by about 10 times, using a method of oblique-angle deposition. This is due to greater ordering of molecular alignment in this thin film. [ABSTRACT FROM AUTHOR]

Highlights: • "proof–of–concept" experiments probing Fe-Ga-N system for bulk growth of GaN are discussed. • melting of Fe and Fe-Ga alloys in contact with nitrogen is studied by DTA. • new possibility of GaN crystallization from Fe-Ga-N solution at high pressure and high temperature is demonstrated. • physical properties of newly grown GaN single crystals are investigated. Abstract The available data indicate that solubility of nitrogen in the liquid transition metals at high temperatures attains very interesting, high values even for relatively low pressure of N 2 gas. For instance, at p N2 = 10 MPa (app. 100 bar) and 1600 °C, the N solubility in Fe-35%Cr solution is as high as 10 at%, what is significantly above N-solubility in liquid gallium at the typical conditions of high pressure solution growth of GaN (p N2 = 1.0 GPa, T = 1500 °C). Therefore, for pressures required for GaN stability at T > 1300 °C (much higher than 100 bar) one can expect N solubility in Fe and its mixtures with Co and Cr on the level of 10–20 wt% which is extremely promising for growth of GaN from liquid solution. The results of some "proof – of – concept" experiments exploring Fe-Ga-N system in both high pressure gas systems of high volume (1.0 GPa, 1600 °C, 1–5 dcm3) used previously for crystal growth of GaN at the Institute of High Pressure Physics PAS and in the toroid high pressure (8.0 GPa, 2500 °C) systems for bulk growth of diamond at the Institute of Superhard Materials in Kyiv are shown and discussed. The results of studies of: melting of Fe and Fe-Ga alloys in contact with nitrogen by DTA, dissolution of nitrogen from gas and solid (GaN) sources as well as physical properties of GaN crystals grown in Fe-Ga-N system are reported. [ABSTRACT FROM AUTHOR]

Highlights • Melting of tetrahedrally bonded semiconductors is explained in terms of universal p-T diagrams. • Stable low-density liquid phase with semiconducting properties is theoretically possible. • Liquid phase with a coordination number close to 4 may be observable in case of GaN. Abstract In this paper we initiate further discussion on phase diagrams and structural transitions in tetrahedrally bonded semiconductors with particular emphasis on GaN. Comprehensive experimental data and the most important theoretical predictions are comprised in an effort to bring up a generalized universal scheme that explain melting that may or may not be accompanied by the change of the coordination number. The scheme considers the "anomalous" melting temperature dependence on pressure for GaN. Principal possibility of GaN melting into low density semiconducting liquid is demonstrated and challenges on the way to experimental observation of such liquid phase are disputed. Prospects of future experiments that can provide reliable information on the nature of atomic bonding for different GaN phases are laid out. [ABSTRACT FROM AUTHOR]

A photovoltaic cell based on p-GaN film/n-ZnO microrods quasi-array heterojunction was fabricated and investigated for the first time for harvesting energy from a near-ultraviolet source (395-400 nm). The source was a commercially available indoor light-emitting diode. According to the scanning electron microscopy data, the ZnO array consisted of tightly packed vertical microrods with a diameter of approximately 2-3 µm. The turn-on voltage of the heterojunction of ZnO/GaN (rods/film) was around 0.6 V. The diode-ideality factor was estimated to be of around 4. The current-voltage characteristic of the photovoltaic cell under near-ultraviolet illumination showed an open-circuit voltage of 0.26 V, a short-circuit current of 0.124 nA, and a fill factor of 39%, resulting in an overall efficiency of 1.4 × 10-5%. These results may be useful in the engineering of electronic devices based on the materials with optical transparency. [ABSTRACT FROM AUTHOR]

Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by Halide Vapor Phase Epitaxy (HVPE) are reported. Oxygen concentration profiles were studied in as-grown GaN crystals and also after annealing of crystals at temperatures up to 3400 K and pressures up to 9 GPa. Investigated crystals contained large conical defects i.e. pinholes of significantly higher oxygen concentration ( N O =(2–4)×1019 cm −3 ) than that in the bulk matrix ( N O <1×1017 cm −3 ). The pinholes were revealed by a photo-etching method in as-grown and annealed GaN samples. Confocal micro-Raman spectroscopy was applied to measure the profiles of free electron concentration, which directly corresponds to the concentration of oxygen impurity. Lateral scanning across the interfaces between pinholes and matrix in the as-grown HVPE GaN crystals showed sharp step-like carrier concentration profiles. Annealing at high temperature and high pressure resulted in the diffusion blurring of the profiles. Analysis of obtained data allowed for the first time for estimation of oxygen diffusion coefficients D O ( T , P ). The obtained values of D O ( T , P ) are anomalously small similarly to the values obtained by Harafuji et al. by molecular dynamic calculations for self-diffusion of nitrogen. Whereas oxygen and nitrogen are on the same sublattice it could explain the similarity of their diffusion coefficients. [ABSTRACT FROM AUTHOR]

We studied the incorporation of Mg-related impurity centers in GaN crystals depending on the direction of the crystallization front. Two series of GaN crystals – (i) undoped and (ii) Mg-doped – were grown by High Nitrogen Pressure Solution (HNPS) method under otherwise identical conditions. Each series contained four samples with ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) , ( 20 2 ¯ 1 ¯ ) and ( 20 2 ¯ 1 ) orientations. The low-temperature photoluminescence (PL) spectroscopy was used for characterization of the obtained crystals. The observed differences in the PL spectra of GaN:Mg crystals suggested that Mg incorporation in GaN grown by HNPS method depends considerably on the orientation of crystallization front. The concentration of Mg impurity incorporated into the GaN crystals subsequently increases for the following sequence of planes: ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) , ( 20 2 ¯ 1 ¯ ) and ( 20 2 ¯ 1 ) . For ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) and ( 20 2 ¯ 1 ¯ ) planes the blue band is related only to O N – Mg Ga donor-acceptor pair (DAP) transitions, while for ( 20 2 ¯ 1 ) plane the incorporation of Mg-H complexes occurs additionally to the formation O N – Mg Ga DAP. [ABSTRACT FROM AUTHOR]

Gallium nitride (GaN) is considered to be one of the most important semiconductors nowadays. In this report a solution of the long standing puzzle regarding GaN decomposition and melting under high pressure and high temperature is presented. This includes the discussion of results obtained so far. The possibility of a consistent parameterisation of pressure ( P ) evolution of the melting temperature ( T m ) in basic semiconductors (GaN, germanium, silicon…), independently from signs of d T m / d P is also presented. [ABSTRACT FROM AUTHOR]

Light emitting diodes (LEDs) structures based on p-GaN film/n-ZnO nanorods quasiarray heterojunction were fabricated. ZnO nanostructures were grown on the p-type GaN templates using two different methods. The turn-on voltages of ZnO/GaN heterojunctions based on ZnO nanorods grown using the gas-transport reaction and hydrothermal methods were equal to 3.2 V and 6.5 V, respectively. The diode-ideality factors were estimated to be of around 45 and 36 for the samples with ZnO nanorods grown using the method of the gas-transport reaction and the hydrothermal method, respectively. The large values of the ideality factors can be explained by a high density of trap states and quality of the contacts with the p - n junctions. The electroluminescence (EL) spectra of LEDs with ZnO nanorods grown by the gas-transport reaction and hydrothermal methods were approximated by four and three Gaussians, respectively. On the basis of the X-ray diffraction (XRD), electrical and optical studies data, one can conclude that the emission peaks at 389-391, 410-412, 436-438 and 502 nm correspond to the near-band-edge (NBE) recombination in ZnO, interface carriers recombination in ZnO/GaN junction, the electrons transition from GaN conduction band to Mg2+ doping level, and to the emission from the defect levels in ZnO, respectively. The LED based on ZnO nanorods synthesized using the hydrothermal method emitted a more pure ultraviolet (UV) light. [ABSTRACT FROM AUTHOR]

The paper is devoted to study of the magnetic and ferroelectric properties of [N(C2H5)4]2CoCl2Br2 (TEACCB-2) solid solution. Investigations of the temperature dependence of the spontaneous polarization confirmed existence of the improper ferroelectric phase in this crystal. The anomalies corresponding to the ferroelectric phase transition were also observed in the temperature dependences of the inverse magnetic susceptibility -- respectively at 227.7 K and 226.4 K, depending on the orientation of the magnetic field. These anomalies confirm both ferromagnetic interactions in a very small temperature range around this point and the spontaneous magnetoelectric effect arising in the phase, which simultaneously is improper ferroelectric and paramagnetic. Observation of such an effect testifies that the ferroelectric ordering is closely connected with magnetic interactions between Co2+ ions. The analyzed Co(II) complexes are monomeric and are characterized by antiferromagnetic interaction (θ < 0 K) around the central ion. [ABSTRACT FROM AUTHOR]

The light emitting diode structure based on p-GaN film/n-ZnO microdisks quasiarray heterojunction was fabricated. It is shown that the epitaxial quality of p-GaN films upon growth from the vapor phase can lead to growth of the hexagonal microdisk ZnO rather than the vertical nanorods. The density of the microdisks changed across the substrate surface. ZnO hexagonal microdisks are characterized by the average height of about 5 μm with diameters ranging from 25 μm up to 60 μm. The turn-on voltage of the heterojunction of ZnO/GaN (disks/film) is around 5 V. The diode-ideality factor was estimated to be of around 30. The large values of the ideality factors indicate a high density of trap states and also may be connected with the quality of the contacts to the p–n junction. The electroluminescence (EL) spectrum acquired from the p-GaN film/n-ZnO microdisks junction exhibited the bands with maxima at 366, 394 and 495 nm. On the basis of the data of X-ray diffraction, electrical and optical studies these peaks were associated with GaN near-band-edge (NBE) emission, ZnO NBE emission, and emission from the defect levels in ZnO, respectively. [ABSTRACT FROM AUTHOR]

The influence of periodic changes of carrier gas composition (H 2 and N 2 +10%H 2 ) on structural and physical properties of GaN crystals grown by Hydride Vapor Phase Epitaxy (HVPE) was studied using photo-etching, defect-selective etching (DSE), secondary ions mass spectroscopy (SIMS), low temperature photo-luminescence (LTPL) and Raman spectroscopy. The studies were performed on (1–100) cleavage planes of thick crystals grown with short (a few minutes) and long (one hour) periods of the carrier gas exchange. Markedly large differences between the level of impurities (Si, C, O 2 , H 2 ) were measured by SIMS in the GaN grown in [0001] direction and inside the V-shape defects, that well correlates with corresponding different carrier concentration levels revealed by Raman and photo-etching. Small differences in carrier concentration across the striations in the [0001] direction were clearly revealed by photo-etching, while PL and Raman appeared to be not sensitive to them. [ABSTRACT FROM AUTHOR]

The results of the Hydride Vapor Phase Epitaxy (HVPE) on ammonothermal GaN seeds (Am-GaN) with various misorientations: 0.3, 0.5 and 1 degree to the [1 0 −1 0] and [1 1 −2 0] directions are presented. Growth rate and structural quality of the HVPE-GaN layers are analyzed. Morphology of the crystal growing surface at the beginning of the crystallization process (after one and two hours of growth) and at the end of it (after eight hours) is presented as a function of the Am-GaN seed misorientation. Based on these results a rough growth model is proposed and discussed in detail. [ABSTRACT FROM AUTHOR]

In this work homoepitaxial HVPE-GaN growth on non-polar and semi-polar GaN seeds was described. Two crystallization processes, in the same experimental conditions but using different carrier gases: N 2 and H 2 , were performed. An influence of growth directions and growth conditions on the growth rate and properties (morphology, structural quality and oxygen and silicon contaminations) of obtained crystals were investigated and discussed. It was shown that the growth rate strongly depends on the growth direction and the carrier gas. It was demonstrated that for the semi-polar [20–21] direction it was possible to obtain high quality and highly conductive (without intentional doping) gallium nitride layers. [ABSTRACT FROM AUTHOR]

Abstract: An influence of a nitridation process on a self lift-off phenomenon during the HVPE crystallization of GaN on a MOCVD-GaN/sapphire template with photolitographically patterned Ti mask was investigated. Duration of the nitridation process and flows of reagents in this process were modified. A correlation between degradation degree of a GaN template surface and the conditions of the nitridation process was observed. It was also observed that the degree of degradation had a direct influence on a moment of the self lift-off phenomenon (separation of a HVPE-GaN crystal from the template). It was shown that the best free-standing HVPE-GaN crystals, in sense of their structural quality, were obtained when the separation process occurred during cooling down step. [Copyright &y& Elsevier]

Abstract: Crystallization of GaN by High Nitrogen Pressure Solution method in multi-feed–seed (MFS) configuration without intentional doping results in: (1) Growth of strongly n-type crystals with free electron concentration increasing with growth temperature in ranges of 2–6×1019 cm−3 and 1350–1430°C, (2) stable growth on Ga-polar surface and unstable growth on N-polar surface, crystals slightly brown, (3) improvement of (0001) crystallographic planes curvature (flattening) with respect to bowing of these planes in the seed crystals. The addition of magnesium into the growth solution causes strong compensation of free electrons in the crystals. Therefore, highly resistive GaN crystals can be grown. In this work, the crystallization of Mg doped GaN on flat ∼1in. seeds (substrates) grown by HVPE in MFS configuration has been studied. It is shown that: (1) Highly resistive GaN:Mg crystals with resistivity higher than 107 Ωcm were grown, (2) the growth is stable on N-polar surfaces of the seeds whereas it is unstable on the Ga-polar surfaces, which is opposite to the HNPS growth of the n-type crystals. The GaN:Mg crystals are fully transparent with no visible color, (3) shape of (0001) crystallographic planes improves (flattens) with respect to bowing of these planes in the seed crystals (HVPE substrates). [Copyright &y& Elsevier]

Abstract: The growth and physical properties of GaN crystallized in a multi feed-seed (MFS) configuration by High Nitrogen Pressure Solution (HNPS) growth method are presented in detail. The conversion of free standing HVPE-GaN crystals to free standing HNPS-GaN is the basis of the MFS configuration. The influence of the experimental conditions (i.e. growth temperature, temperature gradient, etc.), the c-plane bowing of the initial substrate, the electrical properties of HNPS-GaN, and the rate and mode of growth from solution are analyzed. We show that the HNPS-GaN crystals have better structural quality than their HVPE-GaN seeds. The defect density decreases with increasing growth temperature, reaching 5×105 cm−2 for crystals grown at 1420°C or higher. In contrast, the free carrier concentration in HNPS-GaN increases with increasing growth temperature, reaching 7×1019 cm−3 for samples crystallized at 1440°C. Thus the possibility to obtain good quality plasmonic GaN substrates for laser diodes can be realized. [Copyright &y& Elsevier]