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A new approach to beta-Ga2O3 single crystal growth was studied, using the vertical Bridgman (VB) method in ambient air, while measuring the beta-Ga2O3 melting temperature and investigating the effects of crucible composition and shape. beta-Ga2O3 single crystals 25 mm in diameter were grown in platinum rhodium alloy crucibles in ambient air, with no adhesion of the crystals to the crucible wall. Single crystal growth without a crystal seed was realized by (100) faceted growth with a growth direction perpendicular to the (100) faceted plane. (C) 2016 Elsevier B.V. All rights reserved., Article, JOURNAL OF CRYSTAL GROWTH.447:36-41(2016)

α-Ga2O3 is a metastable phase of Ga2O3 of interest for wide bandgap engineering since it is isostructural with α-In2O3 and α-Al2O3. α-Ga2O3 is generally synthesised under high pressure (several GPa) or relatively high temperature (∼500 °C). In this study, we report the growth of α-Ga2O3 by low temperature atomic layer deposition (ALD) on sapphire substrate. The film was grown at a rate of 0.48 A/cycle, and predominantly consists of α-Ga2O3 in the form of ( 0001 ) -oriented columns originating from the interface with the substrate. Some inclusions were also present, typically at the tips of the α phase columns and most likely comprising e-Ga2O3. The remainder of the Ga2O3 film – i.e. nearer the surface and between the α-Ga2O3 columns, was amorphous. The film was found to be highly resistive, as is expected for undoped material. This study demonstrates that α-Ga2O3 films can be grown by low temperature ALD and suggests the possibility of a new range of ultraviolet optoelectronic and power devices grown by ALD. The study also shows that scanning electron diffraction is a powerful technique to identify the different polymorphs of Ga2O3 present in multiphase samples.

Cubic zincblende (zb-)GaN nucleation layers (NLs) grown by MOVPE on 3C-SiC/Si substrates were studied to determine their optimal thickness for subsequent zb-GaN epilayer growth. The layers were characterised by atomic force microscopy, X-ray diffraction and scanning transmission electron microscopy. The as-grown NLs, with nominal thicknesses varying from 3 nm to 44 nm, consist of small grains which are elongated in the [1 −1 0] direction, and cover the underlying SiC surface almost entirely. Thermal annealing of the NLs by heating in a H2/NH3 atmosphere to the elevated epilayer growth temperature reduces the substrate coverage of the films that are less than 22 nm thick, due to both material desorption and the ripening of islands. The compressive biaxial in-plane strain of the NLs reduces with increasing NL thickness to the value of relaxed GaN for a thickness of 44 nm. Both the as-grown and annealed NLs are crystalline and have high zincblende phase purity, but contain defects including misfit dislocations and stacking faults. The zb-GaN epilayers grown on the thinnest NLs show an enhanced fraction of the wurtzite phase, most likely formed by nucleation on the exposed substrate surface at elevated temperature, thus dictating the minimum NL thickness for phase-pure zb-GaN epilayer growth.

Here we report excitonic improvement in color-converting colloidal nanocrystal powders enabled by co-integrating nonpolar green-and red-emitting nanocrystal energy transfer pairs into a single LiCl salt matrix. This leads to nonradiative energy transfer (NRET) between the cointegrated nanocrystals in the host matrix. Here we systematically studied the resulting NRET process by varying donor and acceptor concentrations in the powders. We observed that NRET is a strong function of both of the nanocrystal concentrations and that NRET efficiency increases with increasing acceptor concentration. Nevertheless, with increasing donor concentration in the powders, NRET efficiency was found to first increase ( up to a maximum level of 53.9%) but then to decrease. As a device demonstrator, we employed these NRET-improved nanocrystal powders as color-converters on blue light-emitting diodes ( LEDs), with the resulting hybrid LED exhibiting a luminous efficiency > 70 lm/Welect. The proposed excitonic nanocrystal powders potentially hold great promise for quality lighting and color enrichment applications. (C) 2015 Optical Society of America

We discuss the impact of kinetics, and in particular the effect of the Ehrlich-Schwobel barrier (ESB), on the growth and surface morphology of homoepitaxial GaN layers. The presence of an ESB can lead to various self-assembled surface features, which strongly affect the surface roughness. We present an indepth study of this phenomenon on GaN homoepitaxial layers grown by metal organic vapor phase epitaxy and molecular beam epitaxy. We show how a proper tuning of the growth parameters allows for the control of the surface morphology, independent of the growth technique. (C) 2015 Published by Elsevier By.

Growth of gallium oxide thin films was carried out by Metalorganic Chemical Vapor Deposition (MOCVD) at different temperatures. Pure epsilon-phase epilayers of Ga2O3, with good morphology and structural properties, were obtained, for the first time with this technique, on sapphire at the temperature of 650 degrees C. XRD analysis performed by high-resolution diffractometry confirmed the good crystallographic quality of the grown layers. At temperatures higher than 700 degrees C the usual stable beta-Ga2O3 phase was obtained. The epsilon-films were successfully deposited also on (0001)-oriented GaN and (111)- and (001)-oriented 3C-SiC templates, provided that the appropriate temperature was chosen. This indicates that the temperature, rather than substrate structure, is the growth parameter which decides what phase actually forms. The growth proceeds via coalescence of hexagonal islands and is favored when a substrate with an in-plane hexagonal arrangement of the atoms is employed. By applying Atomic Layer Deposition (ALD), epitaxial growth of the e-phase was achieved at lower temperature, while the overall uniformity resulted improved, even on large sapphire substrates. (C) 2016 Elsevier B.V. All rights reserved.

The structure and temperature dependent spectral photoconductivity of as-grown and N-and Si-implanted GaSe single crystals have been studied. It was observed that post-annealing results in a complete recovery of the crystalline nature that was moderately reduced upon implantation. The band edge is shifted in the implanted sample which is attributed to the structural modifications and continuous shallow levels introduced upon implantation and annealing. Our calculations showed that the trap density is increased upon implantation and annealing which confirms a possibility of explanation the phenomenon within a framework of continuous trap levels. Photocurrent measurements as a function of photo-excitation intensity also support continuous distribution of localized states in the band gap. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Eutectic alloys of GaSb-FeGa1.3 were prepared by the vertical Bridgman method. A microstructure with the needle-shaped metallic FeGa1.3 phase oriented in a specific direction and uniformly distributed within the GaSb matrix was obtained. In GaSb-FeGa1.3 eutectics, the electrical and thermal conductivity, thermal diffusivity, thermoelectric power and Hall coefficients were investigated in a wide temperature range. These properties were measured at different mutual directions of current, thermal flow, magnetic field and metal phase inclusions. The influence of metallic inclusions on these properties was revealed and the distinctive characteristics of electron and phonon processes were established., NATO under Collaborative Linkage Grant PST.CLGN 978434 and the Turkish Academy of Science in the framework of a Young Scientist Award (LO/TUBA-GEBIP/2002-1-17)

Nominally undoped p-GaS layered single crystals were grown using the Bridgman technique. Thermally stimulated current measurements in the temperature range 10-300 K were performed at a heating rate of 0.10 K/s. The analysis of the data revealed six trap levels at 0.05, 0.06, 0.12, 0.63, 0.71, and 0.75 eV. The calculations for these traps yielded 1.2 × 10-21, 2.9 × 10-23, 2.4 × 10-21, 8.0 × 10-9, 1.9 × 10-9 and 4.3 × 10-10 cm2 for the capture cross sections and 1.6 × 1013, 5.0 × 1012, 7.3 × 1012, 1.2 × 1014, 8.9 × 1013 and 2.6 × 1013 cm-3 for the concentrations, respectively.

Cataloged from PDF version of article. The temperature dependencies (15-300 K) of seven Raman-active mode frequencies and linewidths in layered gallium selenide have been measured in the frequency range from 10 to 320 cm(-1). We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison between the experimental data and theories of the shift and broadening of the intralayer phonon lines during heating of the crystal showed that the experimental dependencies can be explained by the contributions from thermal expansion., lattice anharmonicity and crystal disorder. The pure-temperature contribution (phonon-phonon coupling) is due to three-phonon processes. Moreover, it was established that the effect of crystal disorder on the linewidth broadening of TO mode is stronger than that of LO mode. (C) 2002 Elsevier Science Ltd. All rights reserved.

We have fabricated a vacancy-ordered Ga2Se3 epitaxial film by molecular beam epitaxy on a vicinal Si(001) surface tilted toward the [110] direction by 4degrees. It was found by reflection high-energy electron diffraction observations that Ga2Se3 films grown around 470degreesC with Se/Ga flux ratios higher than 100 show excellent ordering of Ga vacancies, The vacancy ordering was not observed when the substrate temperature was higher than 490degreesC. In addition, polycrystalline film growth was dominant when the substrate temperature was lower than 450degreesC, and no vacancy ordering was observed when the Se/Ga ratio was 10. Only a poor crystallinity Ga2Se3 film was obtained on a nominal Si(001) substrate, which suggests that the single-domain 2 x I reconstruction on the vicinal Si(001) surface is essential to the growth of the vacancy-ordered Ga2Se3 film. (C) 2002 Elsevier Science B.V. All rights reserved.

Silicon is until now the most promising dopant for n-type GaN. Besides the near-band-gap emission centred at 3.461 eV, which becomes broader with increasing Si concentration also transitions at lower energies are observed. These emissions decrease strongly when the silicon concentration increases. While the near-band-gap emission is due to fast transitions (with decays shorter than 10 μs) the lower energy emissions have longer components whose behaviour is discussed.

We provide following important clues for resolving the reverse gate leakage mechanism in AlGaN/GaN high-electron mobility transistors (HEMTs), based on two-dimensional (2-D) simulation and analysis. First, measurement of the gate current-voltage, I/sub G/-V/sub G/, characteristics on devices having different gate structures, passivation layers and interface charges, can reveal the field sensitivity of this mechanism. Second, of the different mechanisms proposed so far, namely - direct tunneling (DT), direct tunneling through a thin surface barrier (DTTSB) and trap-assisted tunneling (TT), DT/DTTSB is sensitive to the 2-D field, while the TT is not. Finally, the DT/DTTSB mechanism appears unlikely, since its 2-D calculations fit the measured I/sub G/-V/sub G/ shape, only if we assume a physically unrealistic voltage-variable charge at the interface and/or the TSB layer.

A systematic first-principles study of atomic strings made by group IV, III-V, and II-VI elements has revealed interesting mechanical, electronic, and transport properties. The double bond structure underlies their unusual properties. We found that linear chain of C, Si, Ge, SiGe, GaAs, InSb, and CdTe are stable and good conductor, although their parent diamond (zincblende) crystals are covalent (polar) semiconductors but, compounds SiC, BN, AlP, and ZnSe are semiconductors. First row elements do not form zigzag structures.

In this paper, we present experimental results from site-selected single quantum dots that have undergone a number of intermixing process steps via rapid thermal annealing. We show that the intermixing process blueshifts the dot's emission spectrum without affecting the linewidth as well as decreasing its biexciton binding energy and s-p shell spacing. The anisotropic exchange splitting is shown to have undergone a sign inversion implying that the splitting had gone through zero. Intermixing provides another nanoengineering tool for the design of scalable solid-state photon and entangled photon pair sources. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)., Quantum Dots and Nanostructures: Synthesis, Characterization, and Modeling VIII, January 25-26, 2011, San Francisco, CA, USA, Series: Proceedings of SPIE; no. 7947

A1. Atomic force microscopy A1. Crystal structure A1. X-ray diffraction A3. Metalorganic vapor phase epitaxy B1. Nitrides B2. Semiconducting gallium compounds abstract Non-polar a-plane GaN film with crystalline quality and anisotropy improvement is grown by use of high temperature AlN/AlGaN buffer, which is directly deposited on r-plane sapphire by pulse flows. Compared to the a-plane GaN grown on AlN buffer, X-ray rocking curve analysis reveals a remarkable reduction in the full width at half maximum, both on-axis and off-axis. Atomic force microscopy image exhibits a fully coalesced pit-free surface morphology with low root-mean-square roughness ( � 1.5 nm). Photoluminescence is carried out on the a-plane GaN grown on r-plane sapphire. It is found that, at low temperature, the dominant emission at � 3.42 eV is composed of two separate peaks with different characteristics, which provide explanations for the controversial attributions of this peak in previous studies.

Date of Conference: 4-8 Oct. 2009 Conference name: 2009 IEEE LEOS Annual Meeting Conference Proceedings Photodetectors on semi-insulating GaN templates were demonstrated. They exhibit lower dark current compared to photodetectors fabricated on regular GaN templates. Similar behavior observed in photodetectors fabricated on epitaxially thick SiC templates. © 2009 IEEE.

We demonstrate the tunability of a GaAsAlGaAs quantum ring showing coherent Aharonov-Bohm oscillations up to a gate voltage regime where the ring splits into three quantum dots. This is explained based on the structure's special confinement potential. We characterized the dots by analyzing the corresponding sets of Coulomb blockade lines. Additionally, the spatial configuration of the dots was confirmed by charge measurements using an adjacent quantum point contact. © 2008 American Institute of Physics.

This work presents the selective growth of three-dimensional metallic gallium nitride structures on silicon substrates by chemical beam epitaxy (CBE) with a subsequent plasma nitridation process. The use of titanium pre-deposited stripes over silicon (100) is shown to provide high selectivity where spherical and semi-spherical structures are obtained only over the metal. These structures have diameters ranging from 100 nm to 3 mu m depending on the growth conditions. The nitridation process was performed on an electron cyclotron resonance (ECR) plasma system. Raman micro-spectroscopy results showed GaN formation with zinc blend crystal structure and photoluminescence emission in the visible spectrum in a range between 350 and 650 nm FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP Fechado

Cataloged from PDF version of article. In this paper, we present four GaN based polar quantum structures grown on c-plane embedded in p-i-n diode architecture as a part of high-speed electroabsorption modulators for use in optical communication (free-space non-line-of-sight optical links) in the ultraviolet (UV): the first modulator incorporates similar to 4-6 nm thick GaN/AlGaN quantum structures for operation in the deep-UV spectral region and the other three incorporate similar to 2-3 nm thick InGaN/GaN quantum structures tuned for operation in violet to near-UV spectral region. Here, we report on the design, epitaxial growth, fabrication, and characterization of these quantum electroabsorption modulators. In reverse bias, these devices exhibit a strong electroabsorption (optical absorption coefficient change in the range of 5500-13 000 cm(-1) with electric field swings of 40-75 V/mu m) at their specific operating wavelengths. In this work, we show that these quantum electroabsorption structures hold great promise for future applications in ultraviolet optoelectronics technology such as external modulation and data coding in secure non-line-of-sight communication systems. (C) 2007 American Institute of Physics.

Cataloged from PDF version of article. Semi-insulating character ( sheet resistivity of 3.26 x 10(11) ohm/sq ) of thick GaN layers was developed for AlGaN/GaN high electron mobility transistor ( HEMT ) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN/GaN HEMT structures on sapphire substrates. (c) 2006 American Institute of Physics.

The phosphides InP and GaP with a zinc blende structure are examined as anode materials for lithium-ion batteries. During discharge, X-ray diffraction phase analysis reveals the formation of Li-In/Li-Ga alloy and amorphous Li3P. On charge, lithium is extracted from both LixM (M = In, Ga) alloy and Li3P. InP shows a reversible capacity of ?475 mAh g-1 in the voltage range between 0.2 and 1.5 V, whereas GaP exhibits poor capacity retention compared with that of InP. � 2005 Elsevier B.V. All rights reserved.

We have investigated polarization-resolved photoluminescence in epitaxially grown CuGaSe2/GaAs(001) films. Spin-polarized excitons are optically excited both below and above the characteristic crystal field splitting of the chalcopyrite. At low temperatures, a large exciton spin polarization of 35% is measured under resonant pumping but this is reduced by an order of magnitude and reverses its sign for nonresonant excitation. The measurements suggest that optical pumping within a small energy window just above the band gap results in the preferential generation of light holes and electrons that exhibit a long spin relaxation time, comparable to the recombination time in CuGaSe 2. © 2006 American Institute of Physics. 89 3 Cited By :4

Structural, optical and electrical properties of Ge implanted GaSe single crystal have been studied by means of X-Ray Diffraction (XRD), temperature dependent conductivity and photoconductivity (PC) measurements for different annealing temperatures. It was observed that upon implanting GaSe with Ge and applying annealing process, the resistivity is reduced from 2.1 × 109 to 6.5 × 105 Ω-cm. From the temperature dependent conductivities, the activation energies have been found to be 4, 34, and 314 meV for as-grown, 36 and 472 meV for as-implanted and 39 and 647 meV for implanted and annealed GaSe single crystals at 500°C. Calculated activation energies from the conductivity measurements indicated that the transport mechanisms are dominated by thermal excitation at different temperature intervals in the implanted and unimplanted samples. By measuring photoconductivity (PC) measurement as a function of temperature and illumination intensity, the relation between photocurrent (IPC) and illumination intensity (Φ) was studied and it was observed that the relation obeys the power law, IPC αΦn with n between 1 and 2, which is indication of behaving as a supralinear character and existing continuous distribution of localized states in the band gap. As a result of transmission measurements, it was observed that there is almost no considerable change in optical band gap of samples with increasing annealing temperatures for as-grown GaSe; however, a slight shift of optical band gap toward higher energies for Ge-implanted sample was observed with increasing annealing temperatures. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two monolithic matrix amplifiers for ECM and ESM military equipment have been designed and realized using 0.25 m GaAs PHEMT technology from UMS. Design trade-offs and performances are discussed detail. The effects of the biasing network, termination load and total gate periphery on the performance of the amplifiers were considered for optimum design. The first amplifier , designed for gain flatness and noise figure yields gain of 19 1 dB and a typical noise figure of 4 dB. The second unit exhibits a positive linear gain slope from 16 dB to 20 dB. Output power at 1 dB compression point is +12 dBm. The die size and bandwidth of each MMIC is 7 mm2 and 0.5-20 GHz respectively.

Aytekin, İsmail (Balikesir Author), Thermally stimulated current (TSC) measurements are carried out on nominally undoped Ga4SeS3 layered semiconductor samples with the cur-rent flowing along the c-axis in the temperature range of 10 to 150 K. The results are analyzed according to various methods, such as curve fitting, initial rise and Chen's methods, which seem to be in good agreement with each other. Experimental evidence is found for the presence of three trapping centers in Ga4SeS3 with activation energies of 70, 210 and 357 meV. The calculation yielded 7.9 x 10(-21), 7.0 x 10(-19) and 1.5 x 10(-13) cm(2) for the capture cross section, and 1.6 x 10(10), 6.5 x 10(10) and 1.2 x 10(11) cm(-3) for the concentration of the traps studied.

Cataloged from PDF version of article. Trap levels in nominally undoped Ga2SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga2SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7 x 10(-23), 1.8 x 10(-23) and 2.8 x 10(-22) cm(2) with concentrations of 1.3 x 10(12), 5.4 x 10(12) and 4.2 x 10(12) cm(-3), respectively. (C) 2004 Elsevier Ltd. All rights reserved.

Design, fabrication, and characterization of high-performance AI xGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky. p-i-n, and metal-semicondnctor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA, leakage currents at 6-V reverse bias were measured on p-i-n samples. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9×1014 cmHz1/2W -1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.09 and 0.11 A/W peak responsivity values at 267 and 261 nm, respectively. A visible rejection of 2×104 was achieved with Schottky samples. High-speed measurements at 267 nm resulted in fast pulse responses with greater than gigahertz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.

The temperature dependencies (10–300 K) of the eight Raman-active mode frequencies and linewidths in GaSe0.5S0.5 layered crystal have been measured in the frequency range from 10 to 320 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison of the experimental data with the theories of the shift and broadening of the interlayer and intralayer phonon lines showed that the temperature dependencies can be explained by the contributions from thermal expansion, lattice anharmonicity and crystal disorder. The purely anharmonic contribution (phonon-phonon coupling) is found to be due to three-phonon processes. It was established that the effect of crystal disorder on the broadening of phonon lines is greater for GaSe0.5S0.5 than for binary compounds GaSe and GaS.

The temperature dependencies (10-300 K) of seven Raman-active mode frequencies in layered semiconductor gallium telluride have been measured in the frequency range from 25 to 300 cm -1 . Softening and broadening of the optical phonon lines are observed with increasing temperature. Comparison between the experimental data and theories of the shift of the phonon lines during heating of the crystal showed that the experimental dependencies can be explained by contributions from thermal expansion and lattice anharmonicity. Lattice anharmonicity is determined to be due to three-phonon processes.

A number of semiconductors such as GaS, GaSe, GaSSe show layered structure where intralayer bonding is strong and interlayer bonding quite weak. With bandgaps mostly in the visible and the near infrared and high crystal structure anisotropy, such semiconductors offer interesting possibilities for optoelectronic applications. In this review, we will summarize the recent developments on the photoluminescent properties of these materials such as luminescence due to donor-acceptor pair recombination. As these materials are undoped, the observed photoluminescence is attributed mostly to defect states present in these materials.

Photoluminescence (PL) spectra of undoped single crystals of the layered semiconductor GaSe have been measured in the temperature range from 10 K to room temperature and in the wavelength range from 635 to 750 nm. Two wide bands centred at 644 and 695 nm have been observed at T = 10 K. A detailed analysis of the spectra obtained by varying the excitation intensity and temperature resulted in the identification of the levels involved. A simple model is proposed to account for the observed data.

Cataloged from PDF version of article. Photoluminescence (PL) spectra of GaS0.5Se0.5 layered single crystals have been studied in the wavelength range of 565-860 nm and in the temperature range of 15-170 K. Two PL bands centered at 585 nm (2.120 eV) and 640 nm (1.938 eV) were observed at T = 15 K. Variations of both bands were studied as a function of excitation laser intensity in the range from 10(-3) to 15.9 W cm(-2). These bands are attributed to recombination of charge carriers through donor-acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located at 0.029 and 0.040 eV below the bottom of the conduction band to deep acceptor levels located 0.185 and 0.356 eV above the top of the valence band are suggested to be responsible for the observed A- and B-bands in the PL spectra, respectively. A simple energy level diagram explaining the recombination process is proposed. (C) 2001 Elsevier Science Ltd. All rights reserved.

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Undoped p-GaSe layered single crystals were grown using Bridgman technique. Thermally stimulated current measurements in the temperature range of 10-300 K were performed at a heating rate of 0.18 K/s. The analysis of the data revealed three trap levels at 0.02, 0.10 and 0.26 eV. The calculation for these traps yielded 8.8 × 10-27, 1.9 × 10-25, and 3.2 × 10-21 cm2 for capture cross sections and 3.2 × 1014, 1.1 × 1016, and 1.2 × 1016 cm-3 for the concentrations, respectively.