Impact of Growth Temperature, Pressure, and Strain on the Morphology of GaN Films

1996 ◽  
Vol 449 ◽  
Author(s):  
H. Fujii ◽  
C. Kisielowski ◽  
J. Krueger ◽  
M. S. H. Leung ◽  
R. Klockenbrink ◽  
...  
Keyword(s):  
Finite Size ◽  
Growth Mode ◽  
Temperature Dependent ◽  
Grain Sizes ◽  
Sapphire Substrates ◽  
Size Limitation ◽  
Different Temperatures ◽  
Bulk Gan ◽  
Gas Molecules ◽  
Background Gas

ABSTRACTGaN films grown on sapphire at different temperatures are investigated. A Volmer-Weber growth mode is observed at temperatures below 1000K that leads to thin films composed of oriented grains with finite size. Their size is temperature dependent and can actively be influenced by strain. Largest grains are observed in compressed films. It is argued that diffusing Ga ad-atoms dominate the observed effects with an activation energy of 2.3 ± 0.5 eV. Comparably large grain sizes are observed in films grown on off-axes sapphire substrates and on bulk GaN. This assures that the observed size limitation is a consequence of the 3D growth mode and not dependent on the choice of the substrate. In addition, the grain size and the surface roughness of the films depend on the nitrogen partial pressure in the molecular beam epitaxy (MBE) chamber,most likely due to collisions between the reactive species and the background gas molecules. This effect is utilized to grow improved nucleation layers on sapphire.

scholarly journals Light Emission by Nanoporous GaN Produced by a Top-Down, Nonlithographical Nanopatterning

2018 ◽  
Vol 2018 ◽  
pp. 1-6
Author(s):  
E. Kheirandish ◽  
N. A. Kouklin ◽  
J. Liang
Keyword(s):  
Light Emission ◽  
Electronic Devices ◽  
Emission Spectra ◽  
Temperature Dependent ◽  
Physical Mechanisms ◽  
Spectral Shifts ◽  
Nanopore Arrays ◽  
Bulk Gan ◽  
Pl Emission ◽  
Temperature Dependent Photoluminescence

Temperature-dependent photoluminescence (PL) spectroscopy is carried out to probe radiative recombination and related light emission processes in two-dimensional periodic close-packed nanopore arrays in gallium nitride (np-GaN). The arrays were produced by nonlithographic nanopatterning of wurtzite GaN followed by a dry etching. The results of Raman spectroscopy point to a small relaxation of the compressive stress of ~0.24 GPa in nanoporous vs. bulk GaN. At ~300 K, the PL emission is induced by excitons and not free-carrier interband radiative recombinations. An evolution of the emission spectra with T is confirmed to be mainly a result of a decay of nonexcitonic PL emission and less of spectral shifts of the underlying PL bands. A switching of excitonic PL regime observed experimentally was analyzed within the exciton recombination-generation framework. The study provides new insights into the behaviors and physical mechanisms regulating light emission processes in np-GaN, critical to the development of nano-opto-electronic devices based on mesoscopic GaN.

3D-[Pr(Im)3(ImH)]@ImH: Ein dreidimensionales Netzwerk mit vollständiger Stickstoffkoordination aus einer Imidazolschmelze / 3D-[Pr(Im)3(ImH)]@ImH: A Three-Dimensional Network with Complete Nitrogen Coordination Obtained from an Imidazole Melt

2006 ◽  
Vol 61 (7) ◽  
pp. 792-798 ◽  
Author(s):  
Klaus Müller-Buschbaum
Keyword(s):  
Crystal Engineering ◽  
Material Science ◽  
Three Dimensional ◽  
Temperature Dependent ◽  
Temperature Synthesis ◽  
3 Dimensional ◽  
High Temperature Synthesis ◽  
Dimensional Network ◽  
Bright Green ◽  
Gas Molecules

The reaction of a melt of unsubstituted imidazole with praseodymium metal yields bright green crystals of 3D-[Pr(Im)3(ImH)]@ImH. Imidazolate ligands coordinate η1 via both N atoms their 1,3 positioning within the heterocycle being responsible for the connection of praseodymium atoms. A 3-dimensional network is formed with imidazole molecules from the melt intercalated in the crystal structure. The imidazole molecules can be released and temperature dependent reversibly be exchanged with gas molecules including argon. Thus the solvent free high temperature synthesis of rare earth elements with amine melts can also be utilized for “crystal engineering” and the synthesis of compounds with material science aspects. Furthermore 3D-[Pr(Im)3(ImH)]@ImH is the first unsubstituted imidazolate of the lanthanides.

scholarly journals Towards a better understanding of the dynamics of Aphis spiraecola Patch (Homoptera: Aphididae) populations in commercial alpine yarrow fields

2010 ◽  
Vol 42 (2) ◽  
pp. 103 ◽  
Author(s):  
Zulfaidah Penata Gama ◽  
Pablo Morlacchi ◽  
Giuseppe Carlo Lozzia ◽  
Johann Baumgärtner ◽  
Anna Giorgi
Keyword(s):  
Natural Enemy ◽  
Distributed Delay ◽  
Structured Populations ◽  
Temperature Dependent ◽  
Optimum Sample Size ◽  
Aphis Spiraecola ◽  
Different Temperatures ◽  
Mechanistic Basis ◽  
Age Structured ◽  
Optimum Sample

The spatial distribution of Aphis spiraecola Patch was studied in two commercial yarrow fields located in the Swiss and Italian Alps and represented by Taylor’s (1961) power law. The respective parameters indicate a highly aggregated distribution and lead to a high optimum sample size of 400-500 plants in the design of a sampling program. Opportunities for reducing the sampling efforts are discussed. The infestation patterns were studied on the basis of Vansickle’s (1977) time varying distributed delay adequate for modelling the dynamics of age-structured populations. Published literature data were used to parametrize the functions representing the temperature-dependent duration and survival of the nymphal and adult stage. Likewise, literature data were available to obtain reliable estimates for the parameters of the fecundity function comprising the reproductive profile and the number of nymphs produced at different temperatures. The field data were used to parametrize the functions for wing formation and a compound mortality compromising the effects of plant senescence, stem cutting and natural enemies. The model satisfactorily represented the observed infestation patterns. However, there are opportunities for improving parameter estimation and validation. Moreover, the separation of the compound mortality into host plant and natural enemy effects would improve the mechanistic basis of the model and lead towards a tool that could be used to study bottom-up and top-down effects in the yarrow-aphid-natural enemy system.

scholarly journals Effect of Sintering Temperature on Metal-Insulator Phase Transition in La1-xCaxMnO3 Perovskites

2020 ◽  
Vol 2 (1) ◽  
pp. 37-42
Author(s):  
Arunachalam M ◽  
Thamilmaran P ◽  
Sakthipandi K
Keyword(s):  
Phase Transition ◽  
Bulk Modulus ◽  
Sintering Temperature ◽  
Magnetic Sensors ◽  
Temperature Dependent ◽  
Metal Insulator ◽  
Wide Range ◽  
Different Temperatures ◽  
Insulator Phase Transition

Lanthanum calcium based perovskites are found to be advantageous for the possible applications in magnetic sensors/reading heads, cathodes in solid oxide fuel cells, and frequency switching devices. In the present investigation La0.3Ca0.7MnO3 perovskites were synthesised through solid state reaction and sintered at four different temperatures such as 900, 1000, 1100 and 1200˚ C. X-ray powder diffraction pattern confirms that the prepared La0.3Ca0.7MnO3 perovskites have orthorhombic structure with Pnma space group. Ultrasonic in-situ measurements have been carried out on the La0.3Ca0.7MnO3 perovskites over wide range of temperature and elastic constants such as bulk modulus of the prepared La0.3Ca0.7MnO3 perovskites was obtained as function of temperature. The temperature-dependent bulk modulus has shown an interesting anomaly at the metal-insulator phase transition. The metal insulator transition temperature derived from temperature-dependent bulk modulus increases from temperature 352˚ C to 367˚ C with the increase of sintering temperature from 900 to 1200˚ C.

Effect of Sintering Temperature on Dielectric Properties of CaCu3Ti4O12 Ceramics Prepared by Mechanochemical Process

2014 ◽  
Vol 974 ◽  
pp. 157-161
Author(s):  
Masturah Mohamed ◽  
Mahesh Talari ◽  
Mohd Salleh Mohd Deni ◽  
Azlan Zakaria
Keyword(s):  
Dielectric Constant ◽  
Dielectric Properties ◽  
Sintering Temperature ◽  
Processing Conditions ◽  
Dielectric Measurements ◽  
Frequency Range ◽  
Grain Sizes ◽  
Barrier Capacitance ◽  
Different Temperatures ◽  
Increasing Temperature

CaCu3Ti4O12(CCTO) is well known to have colossal dielectric constant in the range of 105.It is widely accepted that this phenomenon may be attributed to internal layer barrier capacitance (IBLC) model. The dielectric properties of CCTO were reported to be strongly dependent on the processing conditions and grain size. In this work, CCTO samples with different grain sizes were produced by varying sintering temperature in order to investigate IBLC effect on dielectric properties of CCTO. The samples were sintered at four different temperatures, (T=1100°C, 1050°C, 1000°C and 950°C). Dielectric measurements were carried out for the samples in the frequency range of 102– 106Hz using impedance spectrometer. Electron micrographs showed that increasing temperature promoted the grain growth of CCTO while sintering. The internal crystalline defects are seen to play major role by increasing the grain conductivity in dipole formation and increased the dielectric constant of the samples.

Free and Bound Excitons in GaN Epitaxial Films

1996 ◽  
Vol 449 ◽  
Author(s):  
B.K. Meyer
Keyword(s):  
Thermal Activation ◽  
Chemical Nature ◽  
Epitaxial Films ◽  
Binding Energies ◽  
Temperature Dependent ◽  
Neutral Donor ◽  
Exciton Localization ◽  
Sapphire Substrates ◽  
Shallow Donors ◽  
Shallow Impurities

ABSTRACTWe report on photoluminescence experiments on hexagonal GaN epitaxial films grown on 6H-SiC and sapphire substrates by organo-metallic and hydide vapor phase epitaxy. At low temperatures we observe free and neutral donor bound exciton transitions which allow to establish properties of the free excitons and localisation energies of the bound excitons involving different shallow donors. From temperature dependent luminescence experiments thermal activation energies are determined which measure the exciton localization and donor binding energies. The localization energies of the excitons scale with the respective donor binding energies (Haynes rule). The inter-impurity transitions of neutral donors are observed in fourier transform infra-red absorption. Three shallow donors with binding energies of 34.7 meV, 55 meV and 58 meV can be seen. We present evidence for the chemical nature of the shallow impurities.

Study Of Al Surface Segregation In Icosahedral A162Cu25.5Fe12.5

1998 ◽  
Vol 553 ◽  
Author(s):  
M. GIL-GAVATZ ◽  
D. Rouxel ◽  
P. Pigeat ◽  
B. Weber ◽  
J.-M. Dubois
Keyword(s):  
Surface Segregation ◽  
Vacuum Chamber ◽  
High Vacuum ◽  
Ultra High Vacuum ◽  
Electron Spectrometer ◽  
Temperature Dependent ◽  
Self Diffusion ◽  
Different Temperatures ◽  
High Vacuum Chamber

AbstractSurface segregation of aluminium was observed during oxidation experiments of icosahedral A162Cu25.5 Fel12.5, performed in-situ and at different temperatures in the ultra-high vacuum chamber of a scanning Auger electron spectrometer. Two regimes, below and above 770K, were observed in relation with severe segregation of Al atoms at the surface for T > 770K. We postulate that this temperature dependent segregation rate is representative of the aluminium transport towards the surface of the quasicrystal. By analogy with classical diffusion experiments, we can thus determine reasonable estimates of the activation energy for Al self-diffusion in this quasicrystal. The results are consistent with the existence of phason flips below 770K and thermal vacancies above this temperature.

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