In this paper, we present an experimental study of hydrogen (H) and deuterium (D) in single crystal and polycrystalline b-Ga2O3. H or D was introduced into the specimens using ampoule passivation at elevated temperatures. The samples were characterized with infrared absorption and gas effusion measurements. As-grown single crystal b-Ga2O3 contains a residual H concentration of about 1019 cm-3 and exhibits a local vibrational mode at 3437.6 cm-1 that has been attributed to O – H. Polycrystalline b-Ga2O3 thin-films were grown by pulsed laser deposition. Hydrogen effusion measurements show that these samples contain H concentrations of up to about 1020 cm-3. From the hydrogen effusion spectra, the H chemical potential was determined as a function of the H concentration, which can be related to the H density-of-states distribution.
White light-emitting diodes (wLEDs) are the new environmental friendly sources for general lighting purposes. For applications requiring a high-brightness, current wLEDs present overheating problems, which drastically decrease their emission efficiency, color quality and lifetime. This work gives an overview of the recent investigations on single-crystal phosphors (SCPs), which are proposed as novel alternative to conventional ceramic powder phosphors (CPPs). This totally new approach takes advantage of the superior properties of single-crystals in comparison with ceramic materials. SCPs exhibit an outstanding conversion efficiency and thermal stability up to 300°C. Furthermore, compared with encapsulated CPPs, SCPs possess a superior thermal conductivity, so that generated heat can be released efficiently. The conjunction of all these characteristics results in a low temperature rise of SCPs even under high blue irradiances, where conventional CPPs are overheated or even burned. Therefore, SCPs represent the ideal, long-demanded all-inorganic phosphors for high-brightness white light sources, especially those involving the use of high-density laser-diode beams.
K. Lorenz, M. Peres, M. Felizardo, J. Correia, L. Alves, E. Alves, I. López, E. Nogales, B. Méndez, J. Piqueras, M. Barbosa, J. Araújo, J. Gonçalves, J. Rodrigues, L. Rino, T. Monteiro, E. Villora, K. Shimamura
KEYWORDS: Crystals, Europium, Annealing, Ions, Picture Archiving and Communication System, Backscatter, Ion implantation, Doping, Sensors, Chemical species
Ga2O3 bulk single crystals have been implanted with 300 keV Europium ions to fluences ranging from 1×1013 to 4×1015 at/cm2. The damage build-up and Eu-incorporation was assessed by Rutherford Backscattering Spectrometry in the channeling mode (RBS/C). RBS/C results suggest that implantation causes a mixture of defect clusters and extended defects such as dislocations. Amorphisation starts at the surface for fluences around 1×1015 at/cm2 and then proceeds to deeper regions of the sample with increasing fluence. Amorphous regions and defect clusters are efficiently removed during rapid thermal annealing at ~1100 °C; however, Eu diffuses towards the surface. Nevertheless, Eu ions are optically activated and show cathodoluminescence at room temperature. Results in bulk samples are compared to those in Eu-implanted Ga2O3 nanowires and despite strong similarities in the structural properties differences were found in the optical activation. Furthermore, damage and dopant incorporation studies were performed using the Perturbed Angular Correlation technique, which allows probing the immediate lattice surroundings of an implanted radioactive probe at the atomic level.
β-Ga2O3 is the most transparent conductive oxide, well known since several decades for its large bandgap of 4.8 eV. Its potential as semiconductor material, however, is just emerging in recent years. Present work shows the development of βGa2O3 for semiconductor applications and its current state-of-the-art. The discussion is focused on three different aspects: (1) Advantageous growth from melt of large-size β-Ga2O3single-crystals. High-crystalline quality and carrier control make possible the production of conductive and semi-insulating wafers. (2) β-Ga2O3as substrate for homoepitaxy as well as for heteroepitaxial deposition of GaN-based devices. High-brightness blue-LEDs with vertical current injection are demonstrated. (3) Potential of β-Ga2O3for high-power devices with higher breakdown voltage than GaN and SiC counterparts. The first Schottky barrier diode is shown, as well as first transistors (MESFET and MOSFET) are indicated.
Single-crystal phosphors are introduced as novel alternative to currently used powder phosphors. In connection with high-brightness white light-sources, based on LEDs or LDs plus phosphor converters, single-crystal phosphors possess advantageous features. These avoid the use of resins and exhibit a very high internal quantum efficiency, which remains stable with the temperature increase.
Large size single crystals of beta-Ga2O3 with 1 inch in diameter have been grown by the floating zone technique. The stable growth conditions have been determined by the examination of the crystal structure. Wafers have been cut and fine polished in the (100), (010) and (001) planes. These were highly transparent in the visible and near UV, as well as electrically conductive, indicating the potential use of beta-Ga2O3 as a substrate for optoelectic devices operating in the visible/near UV and with vertical current flow. Epitaxial growth of nitride compounds by the metalorganic vapor phase epitaxy (MOVPE) technique is demonstrated on beta-Ga2O3 single crystal substrates. High-quality (0001) GaN epi-layers with a narrow bandedge luminescence are obtained using a low temperature conductive buffer layer. InGaN multi-quantum well (MQW) structures were also successfully grown. The first blue light-emitting diode (LED) on beta-Ga2O3 with vertical current injection is demonstrated.
The growth characteristics and properties of large size SrAlF5 single crystals are described and compared with those of BaMgF4. Transmission spectra in the vacuum ultraviolet wavelength region indicate a high transparency of SrAlF5 (about 90% without considering surface reflection loses) down to 150 nm, on contrast to the optical loses observed for BaMgF4. The ferroelectric character of SrAlF5 is evidenced by the reversal of the spontaneous polarization in a hysteresis loop. The higher potential of SrAlF5 in comparison with BaMgF4 for the realization of all-solid-state lasers in the ultraviolet wavelength region by the quasi-phase matching (QPM) technique is pointed out. SrAlF5, besides a higher grade of transparency, shows a nonlinear effective coefficient similar to that of quartz and uniaxial nature, on contrast to the one order smaller nonlinear coefficient and biaxial character of BaMgF4. The refractive index of SrAlF5 from the ultraviolet to the near-infrared wavelength region is measured by the minimum deviation method. The Sellmeier and Cauchy coefficients are obtained from the fits to the curves of the ordinary and extraordinary refractive indices, and the grating period for the first order QPM is estimated as a function of the wavelength. The poling periodicity for 193 nm SHG from 386 nm is 4 micron-m.
Absorption and photoluminescence spectra of Tm3+ and Ho3+ ions in LiYF4 crystals have been investigated at various temperatures between 10 and 320 K. The photoluminescence is investigated under excitation with Xe-lamp and laser diode. In addition to blue up-converted emission of Tm3+ and Ho3+ and green up-converted Ho3+ emission, anti-Stokes emission bands are observed at 687 and 703 nm under excitation in the 3H4 state of Tm3+ with 785 nm laser diode. These bands are observed above 200 K, and their intensities increase exponentially with increasing temperature. They are attributed to endothermic Tm3+ emission due to the transition to the 3H6 ground state from the upper 3F3 state which is thermally populated from the 3H4 state. Discussion is given on the optical process of green up-converted Ho3+ emission which is generated by the 785 nm laser diode excitation.
High quality fluoride and oxide single crystals for optical, piezoelectric and other applications have been grown by advanced crystal growth techniques. Corquitiite- and Perovskite-type fluoride single crystals - LiCaAlF6, LiSrAlF6, KmgF3 and BaLiF3 - have been grown for solid state UV laser applications, and as window materials for next generation optical lithography. La3Nb0.5Ga5.5O14 and La3Ta0.5Ga5.5O14 piezoelectric single crystal of size and quality comparable to La3Ga5SiO14, have been produced. The piezoelectric and deice properties of the crystal were investigated. A search for new langasite-type materials was also performed. Promising new structural materials. Undoped and doped eutectic fibers, have been grown by the micro- pulling-down technique. Undoped and doped(beta) -Ga2O3 single crystals have been grown by the floating zone technique as promising transparent conductive oxides.
Ce-doped LiCAF (Ce:LiCAF), LiSrAlF6 (Ce:LiSAF) and LiSr0.8Ca0.2AlF6 (Ce:LiSCAF) single crystals were grown by the Czochralski technique as ultraviolet, tunable, solid- state laser media. Optical characterization and Laser activity were investigated. Crystal growth was performed in Czochralski system (CZ) under an atmosphere of CF4 with previous treatment in high vacuum (approximately equals 10-2 Pa). Under such conditions, we succeeded to grow Ce:LiCAF, Ce:LiSAF and Ce:LiSCAlF (18 mm in diameter and 60 mm in length) single crystals. Laser rods were cut from the above mentioned crystal boules. The laser resonator was established by a flat high reflector and a flat output coupler. The forth harmonic of a Q-switched Nd:YAG laser was used as the pumping source. Laser oscillations at around 290 nm were measured. 60 mJ pulses were generated directly from the Ce:LiCAF crystal. Gain has been observed for the Ce:LiSCAlF crystal and laser oscillation was confirmed.
Several Tm,Ho-doped LuLF and YLF crystals were grown using the Czochralski method. Crystals were pulled in a furnace with atmospheric control system. Pulling and rotation rates were 1 mm/hr and 15 rpm, respectively. High purity (>= 99.99%) fluorides were used as starting materials. High vacuum (approximately equals 10-5 torr) prior to the growth and CF4 gas during the growth were applied. Single crystals of up to 20 mm in diameter and 80 mm in length were successfully grown. Various laser rods of sizes 4 X 3 X 2.7 mm3 were prepared from the grown crystal boules. The crystal ends were Brewster- cut to minimize the reflection losses inside the cavity. Two quasi-CW LD-arrays of 6 X 60 W each side-pump the crystals. The pump beam is focused using two lens ducts of length 64 mm to a waist of 2.5 X 2.5 mm2. The nearly hemispherical laser cavity is formed by a flat high reflector and a 300 mm radius of curvature 5% transmission output coupler. At room temperature, up to 13.5 mJ (9.9 mJ) with a slope efficiency of 10.5% (7.5%) has been demonstrated at 1 Hz for 5% Tm, 0.5% Ho:LuLF (5% Tm, 0.5% Ho:YLF).
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