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The development of efficient, low-cost, and stable solid-state materials for portable thermal neutron detection is highly expected in order to substitute the currently used 3He and BF3 tank detectors. A few Li-based glasses and halide compounds have emerged as candidates, however, all of them present critical drawbacks for their practical implementations. Ce:Li6Y(BO3)3 is a priori a very promising oxide candidate that however has been disregarded so far due to its negligibly low light yield, caused by a poor crystalline and optical quality. In this study, we demonstrate that a post-growth thermal treatment is the key parameter to drastically reduce the concentration of intrinsic defects and scattering centers that lead to severe non-radiative recombination of excited electrons. Even though this annealing step also involves the oxidation of activator Ce3+ ions to Ce4+, a drastic enhancement of the light yield by ∼600% is achieved independently of the Ce3+ concentration within the considered range. The obtained light yield of 4650 ph n−1 is already close to that of reference Li-glass (commercial GS20 with 6000 ph n−1). An additional improvement can be envisaged upon further optimization of the Ce3+ concentration and the annealing time, so that Ce:Li6Y(BO3)3 reaches a light yield comparable to the state-of-the-art one for thermal neutron detection.

A thermal actuator based on thin flexible lamellas of Ga_2O_3 was designed in order to demonstrate the potential of this semiconductor for micro-opto-electro-mechanical applications (MOEMs). The working principle of these devices is based on the thermal expansion that induces a vertical movement resultant of the lamella elongation due the self-heating/Joule effect. Upon excitation with photons, electrons or ions, intrinsic luminescence bands associated with self-trapped excitons and donor-acceptor pair recombination dominate the emission spectrum. When a current passes through the device, simultaneously with the thermal expansion, this luminescence is strongly quenched. Based on systematic Photoluminescence and Raman studies as a function of temperature and as a function of the applied power it is demonstrated that the observed luminescence quenching is directly related with the Joule heating effect. This work presents for the first time a thermal actuator based on Ga_2O_3 and it intends to be a stimulus for future works on MOEMs applications based on this semiconductor.

Tb 3Sc 2Al 3O 12 (TSAG) is a paramagnetic insulator which, in spite of its congruent nature and excellent magneto-optical properties as Faraday rotators for optical isolators (OIs), cannot be industrially produced. The reason is that its lattice structure is so stressed that single crystals easily crack upon cooling after growth or during the cutting process. By the incorporation of a small amount of Lu, it has been demonstrated that the stress in the structure can be effectively released, and, therefore, the crystal cracking can be avoided. This has enabled the substitution of standard Tb 3Ga 5O 12 (TGG), achieving better high-power performance and compacter OIs. In this study, the charge state and the lattice site location of Lu as well as Y dopants in TSAG are investigated by X-ray Absorption Near Edge Structure and Extended X-ray Absorption Fine Structure Analysis. Measurements on single crystal and powdered samples indicate that both dopants are incorporated in the 3+ charge state and substitute predominantly in the dodecahedral Tb-site ( >87.5%), while a minor fraction is found in the octahedral Sc-site.

Ce:LaB3O6 (LBO) glass, whose constituents are abundant elements and fabrication is easy and cheap, is found to be a promising thermoluminescence (TL) dosimeter. This is originally achieved by CeF3 doping and melting under a reducing atmosphere, with the optimum concentration of 0.1% (quantum efficiency = 66%). The corresponding Ce interatomic distance is ∼4 nm, below which concentration quenching occurs via Ce dipole–dipole interaction, as elucidated experimentally by Dexter’s theory. Ce:LBO exhibits a good dose resolution, with a linear dependence covering five orders of magnitude on both irradiation-dose and TL-response. Furthermore, it can be cyclically irradiated and read without degradation.

Ca3TaAl3Si2O14 (CTAS) single crystal is attracting much attention for high temperature sensor applications due to its high electrical resistivity and excellent piezoelectric properties. However, it is a big challenge to grow high quality CTAS single crystals. In this work, its growth conditions are optimized. Crack- and inclusion-free crystals are grown, achieving even a high quality single crystal with a width over 2 in. In addition, the temperature dependence of electro-elastic properties is systematically studied. This crystal presents a high thermal stability of structure and electro-elastic properties. The dielectric loss is less than 3% from room temperature to 650 °C. Therefore, CTAS single crystal is promising for high temperature sensor applications.

Substitutional disordered garnet crystal hosts such as (Y1-xGdx)3(Al1-yScy)5O12 (YGSAG) produce inhomogeneous broadening of the optical spectra of the dopant, making them good candidates for solar-pumped laser materials. In the case of codoping with Nd and Cr ions, the spectral overlapping between the Cr absorption bands and the solar spectrum and between the Cr emission lines/bands and the Nd absorption lines leads to the increase of the pumping efficiency for the Nd emission. As a consequence of energy transfer from Cr to Nd, the threshold of the Nd laser oscillation is expected to be reduced when compared with that of the single doped Nd crystal. The laser oscillation of the Nd stimulated emission for Nd:Cr:YGSAG has been evaluated using an Ar ion laser, giving evidence of the energy transfer excitation processes involved in the laser output.

Ce:Li6Y(BO3)3 (LYBO) is a promising thermal neutron scintillator that can be grown into bulk single crystals by the Czochralski method although it melts incongruently. This study achieves 1-inch Ce:LYBO, and demonstrates the collateral flux-growth of LiY6O5(BO3)3 (LYOB) single crystals at the bottom of the crucible during the pulling of LYBO from a stoichiometric melt. Both LYBO and LYOB belong to monoclinic crystal system with the same space group. Thermal analysis shows that LYOB itself is more incongruent than LYBO, decomposing mainly into Y16.86B8O38 phase when heated up to 1500 ​°C. In contrast to the typical bluish emission of Ce-doped scintillators, like the case of Ce:LYBO, Ce:LYOB crystals exhibit a broad emission peaking in the orange wavelength region, as a result from an extraordinarily large Stokes shift. The ultrafast photoluminescence (PL) decay of Ce:LYOB is due to dominant non-radiative processes that also cause obvious luminescence quenching. This study reports for the first time high-quality Ce:LYOB single crystals, which are characterized in terms of single crystal diffraction, thermal analysis, optical transmittance, excitation and PL at room temperature, PL decay and Raman spectroscopy.

The structural and luminescence properties of gallium oxide nanowires doped with chromium or manganese have been investigated. Undoped Ga₂O₃ nanostructures have been fabricated by a thermal evaporation method, while doping was subsequently achieved by ion implantation followed by thermal annealing. Scanning electron microscopy (SEM) analysis has shown that this doping process does not alter the morphology of the nanostructures. Ion implantation results in partial amorphization of the crystal lattice, as deduced from Raman spectroscopy studies. Thermal annealing at different temperatures was carried out in order to restore the crystallinity of the nanowires. Raman spectroscopy analysis demonstrates that recrystallization starts at about 700 °C and a complete recrystallization is achieved at. abOut 1000 °C. Cathodoluminescence (CL) analysis has been used to study the emissions in the 300-900 nm range. As-implanted nanowires virtually do not emit any light, which is related to their Poor crystal quality and the implantation induced defects. Thermal annealing results in effective CL emission. In particular, a clear correlation between crystallinity of the nanowires doped with Cr and the emission from the ²E-⁴A₂ and ⁴T₂-⁴A₂ intraionic transitions has been observed. On the other hand, emissions directly related to intraionic transitions of Mn have not been found in the nanowires implanted with this ion. The influence of the implantation process and annealing temperature on the observed changes in the donor -acceptor pairs (DAP) band of Ga₂O₃ is discussed.

Microvoids are well-known crystal defects, usually exhibiting a rounded and/or irregular shape, which act as undesired scattering centers in optical applications. Here, for the first time, residual microvoids in the shape of perfect regular hexagonal-prisms are observed in apparently scattering-free CeF3 bulk single crystals. These microvoids reflect exactly the natural crystal habit in “negative” form: the prisms stand along the c-axis and the hexagonal sides are parallel to the six equivalent planes perpendicular to the a-axis. They distribute randomly with sizes up to 13 µm and heights in the submicron order with a maximum around 500 nm. The microvoids are formed by the embedding of bubbles during the crystallization process. The bubbles’ gas is possibly caused by the high vapor pressure of CeF3 itself at the high melting point.

In this work the partial substitution of Ca by Sr in the known langasite single crystal Ca3TaAl3Si2O14 is investigated from the point of view of crystal growth in order to improve the piezoelectric properties. For it, a (SrxCa1-x)3TaAl3Si2O14 (SCTAS) single crystal with x = 0.25 was grown by the Czochralski method. The grown crystal was strongly faceted, highly transparent, and of a high crystalline quality, as indicated by a full width at half maximum of the X-ray rocking curve as small as 31 arcsec. On the other hand, the crystal exhibited a high resistivity, with a value over 1010 Ω cm at 400 °C. Temperature dependent piezoelectric measurements confirmed the effective enhancement of the d11 piezoelectric coefficient at any temperature, while maintaining the dielectric loss below 1% up to 600 °C. Therefore, SCTAS exhibits superior properties compared with other piezoelectric materials for high temperature applications.

High quality colorless Ca 3 TaGa 3−x Al x Si 2 O 14 (CTGAS) single crystals are successfully grown by the Czochralski technique within the full solid solution range. The influence of Al content on the electrical resistivity, dielectric and piezoelectric properties is systematically investigated for the first time at room temperature, as well as a function of temperature up to 700 °C. With the increase of Al content the resistivity, the electromechanical coupling factor k 12 and the piezoelectric coefficient d 11 increase, while the dielectric permittivity e 11 T / e 0 decreases. CTGAS single crystals exhibit a very low dielectric loss ( 10 Ω cm at 400 °C for all samples, i.e. more than two orders of magnitude higher than that of the considered material La 3 Ta 0.5 Ga 5.5−x Al x O 14 . Present results indicate that, among the langasite family, Ca 3 TaAl 3 Si 2 O 14 is at present the most promising crystal for high temperature sensor applications, since it exhibits the highest resistivity and a good piezoelectric response.

Among the langasite family, Ca3TaAl3Si2O14 (CTAS) is particularly promising for high temperature sensor applications due to its relatively high resistivity. In this work, CTAS single crystals have been grown by the Czochralski technique under various oxygen partial pressures using Ir and Pt crucibles. Colorless CTAS crystals were grown for the first time; however, the elimination of related defects did not influence the electrical properties. Furthermore, dielectric and piezoelectric properties were independent of growth conditions. Instead, the electrical resistivity was found to be remarkably higher the lower the oxygen partial pressure, reaching a value of ∼7 × 1010 Ω cm at 400 °C, which is 3 orders of magnitude higher than that of disordered LTGA. Therefore, grown CTAS crystals are promising for high temperature piezoelectric sensor applications.

Garnet crystals codoped with Nd3+ and Cr3+ ions are a candidate for solar-pumped laser materials. Substitutional disordered crystals Y3GaxAl5-xO12 were prepared to improve the pumping efficiency of Nd3+ luminescence and energy transfer from Cr3+ to Nd3+ ions. The substitutional disordered crystal host produced inhomogeneous broadening of the Nd3+ and Cr3+ optical spectra. Enhancement of overlapping between the Cr3+ absorption bands and the solar spectrum, and between the Cr3+ luminescence bands and the Nd3+ absorption lines led to the increases of the pumping efficiency and the energy transfer rates, respectively. The excitation spectrum of the Nd3+ luminescence, the nonexponential decay curves of the Cr3+ luminescence, and the quantum yields of the Cr3+ and Nd3+ luminescence have given evidence on the energy transfer from Cr3+ to Nd3+ ions.

La 3 Ta 0.5 Ga 5.5-x Al x O 14 (LTGA) is a piezoelectric crystal from the langasite family which presents advantageous properties for high temperature sensor applications. It possesses a high piezoelectric constant and its structure remains stable up to 1500 °C. In this work, a LTGA single crystal has been grown by using the Czochralski technique under N 2 atmosphere with 1% O 2 . The high crystalline quality of grown crystal is confirmed by the 26 arcsec FWHM of X-ray rocking curves. High temperature X-ray diffraction measurements indicate the absence of phase transitions up to 1300 °C. The thermal properties of the crystal, including thermal expansion, specific heat, thermal diffusivity and thermal conductivity, are investigated as a function of temperature. The average thermal expansion coefficients are α a = 8.48 × 10 −6 K −1 and α c = 6.32 × 10 −6 K −1 . The thermal conductivities along the a and c axes at 50 °C are 1.2 and 2.0 W m −1 K −1 , respectively. The resistivity of grown crystal decreases from 4 × 10 10 to 2 × 10 5 Ω cm between 200 and 900 °C. The measured piezoelectric coefficient d 11 , electromechanical coupling factor k 12 and corresponding mechanical quality factor Q at room temperature are 6.94 pC/N, 17% and 12,000, respectively.

High optical quality LiREF4(RE = Tb3+, Dy3+, Ho3+, Er3+and Yb3+), PrF3and CeF3single crystals have been grown by the Czochralski technique. Their magneto-optical properties have been measured and analyzed in detail in the ultraviolet-visible wavelength region, and their figures of merit as Faraday rotators have been determined. CeF3presents superior properties above 300 nm, showing a figure of merit higher than that of the reference material, terbium-gallium-garnet, which is nowadays used in the visible-near infrared. PrF3is the best rotator for the 220-300 nm range. Towards shorter wavelength and in the vacuum ultraviolet, it is shown that the LiREF4crystals are unique rotators.

Czochralski grown Ce and Gd-doped Y 3 Al 5 O 12 (Ce,Gd:YAG) single crystal demonstrated superior performance features in a new concept of white LED where it is used as thin single crystal phosphor plates (SCPPs). New SCPP-based white LED realizes epoxy resin free package, which allows to eliminate the photodegradation issue of the organic material. Optical, thermal and electrical properties of SCPPs and SCPP-based white LEDs are discussed with the powder phosphor and commercial LED reference. SCPPs showed significantly higher values and better stability under the high temperatures, irradiation and current. Quantum efficiency was found to be as high as 93%.

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.

β-Ga 2 O 3 crystals are one of the transparent conductive oxides (TCOs). The n-type conductivity of β-Ga 2 O 3 is attributed to an oxygen deficit. In order to examine a mechanism of the conductivity, electron-spin resonance (ESR) measurements were carried out for β-Ga 2 O 3 doped with various impurity ions (Mn, Cr, Si, Zr, Hf). A single narrow ESR line with an anisotropic g value (~ 1.96) was observed for Si, Zr, or Hf doped β-Ga 2 O 3 and ascribed to “ motional narrowing ” of electrons trapped around oxygen vacancies, namely, conduction electrons. Extinction of the narrow ESR line for Mn or Cr doped β-Ga 2 O 3 may be due to a decrease of the conduction electron density and/or a strong spin–spin interaction between the conduction electron and the paramagnetic ions. The fine structure of the ESR spectrum for Mn or Cr doped β-Ga 2 O 3 gives evidence that Mn 2 + and Cr 3 + ions substitute for octahedral Ga 3 + ions in the lattice without charge compensators.

Ferroelectric fluoride BaMgF 4 , β -Ga 2 O 3 and F-doped core-free Y 3 Al 5 O 12 single crystals for ultraviolet and vacuum ultraviolet (UV/VUV) applications, are introduced. 2-in. size BaMgF 4 single crystals have been grown. Subsequently, second harmonic generation (SHG) in different wavelengths was demonstrated by the quasi-phase matching (QPM) devices. 2-in. size β -Ga 2 O 3 single crystals, which have large bandgap, 4.8 eV, have been grown. Blue light emitting diodes (LEDs) on β -Ga 2 O 3 were demonstrated. F-doped core-free Y 3 Al 5 O 12 single crystals have been grown as a potential new lens material for UV/VUV.

High optical quality terbium calcium fluoride (Tb 0.81 Ca 0.19 F 2.81 ) and terbium strontium fluoride (Tb 0.76 Sr 0.24 F 2.76 ) single crystals have been grown by the Czochralski method under controlled atmosphere. While both compounds present similar optical and magneto-optical properties (determined by the high concentration of Tb 3+ ions), the lower absorption losses and high Faraday rotation exhibited by terbium calcium fluoride, compared to those of terbium strontium fluoride, lead to a notable figure of merit for Faraday isolators operating in the visible wavelength region. In particular at short wavelengths, below 500 nm, the figure of merit of terbium calcium fluoride is remarkably higher than that of the industrial standard terbium gallium garnet.

β-Ga2O3 single crystals have the band gap energy of 4.8 eV. Luminescence spectra in β-Ga2O3 crystals excited above the band gap energy consist of UV and blue/green broad bands. Polarization of the optical absorption and luminescence spectra and decay curves of the luminescence observed in the temperature range from 14 to 300 K can identify the luminescent centres. The UV luminescent centre with a lifetime of 1.5 μs is assigned as self-trapped exciton, consisting of self-trapped hole in the form of GaO4 tetrahedral complex and bound electron. As the decay curves of the blue/green luminescence bands fit a power function of t-n (n∼0.8), the luminescence occurs through recombination of donors and acceptors in the crystal. The donor is coincident with a conduction electron trapped at single or coupled oxygen vacancies in the crystal estimated from electron-spin resonance (ESR) (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Peculiarities of cascade photon emission (CPE) and energy storage in M1−xPrxF2+x (M=Ca, Sr, Ba, x≈0.35) crystals were studied. The investigation of lattice parameters revealed that these solid solutions belong to the fluorite structure type with the lattice constant noticeably different from that of MF2 crystals. Absorption, emission and excitation spectra of M0.65Pr0.35F2.35 were measured at LHeT and RT. As it turned out the typical for 4f2→4f2 transition in Pr3+ emission lines are broadened as compared with the PrF3 crystal. The analysis of the excitation spectra broadening does not allow bringing out the type of the superlattice, which is inherent to the material, but it indicates clearly the simultaneous presence of different types of the Pr centers in mixed crystals. Yet another specific feature is the higher radiation sensitivity of these fluorides relatively PrF3, MF2 and Pr-doped MF2 crystals. Coloration efficiency enhances in direction Ca→Sr→Ba, and the positions of induced absorption band depend on composition of the solid solution. Colorization, thermo-stimulated luminescence and afterglow of the M1−xPrxF2+x crystals denote high radiation sensitivity as compared with M1−xCexF2+x.

The spectral and kinetic parameters of LiCaAlF 6 :Pr and LiSrAlF 6 :Pr single crystals are studied at LHeT and room temperature (RT). Photon cascade emission (PCE), i.e., 1 S 0 → 1 I 6 and 3 P 0 → 3 H j , 3 F j transitions after Pr 3+ excitation via 4f 2 →4f 5d absorption results in the conversion of the vacuum ultraviolet photon to two visible photons. The excitation and photoemission spectra as well as decay times have been measured and compared with those for CaF 2 :Pr and SrF 2 :Pr crystals. X-ray luminescence was measured to study the emission origin from both 1 S 0 and 3 P 0 states of Pr 3+ . An intense phosphorescence of LiSrAlF 6 :Pr was observed in the visible range after X-irradiation at RT, contrary to LiCaAlF 6 :Pr.

Spectral-kinetic study of Pr3+ luminescence has been performed for LiLuF4:Pr(0.1 mol%) single crystal upon the excitation within 5–12 eV range at T=8 K. The fine-structure of Pr3+ 4f 2→4f 5d excitation spectra is shown for LiLuF4:Pr(0.1 mol%) to be affected by the efficient absorption transitions of Pr3+ ions into 4f 5d involving 4f 1 core in the ground state. Favourable conditions have been revealed in LiLuF4:Pr(0.1 mol%) for the transformation of UV–VUV excitation quanta into the visible range. Lightly doped LiLuF4:Pr crystals are considered as the promising luminescent materials possessing the efficient Pr3+ 3P0 visible emission upon UV–VUV excitation. The mechanism of energy transfer between Lu3+ host ion and Pr3+ impurity is discussed.

We report the acoustic imaging frequency dynamics of ferroelectric domains by low-frequency acoustic probe microscopy based on the commercial atomic force microscopy It is found that ferroelectric domain could be firstly visualized at lower frequency down to 0.5 kHz by AFM-based acoustic microscopy The frequency-dependent acoustic signal revealed a strong acoustic response in the frequency range from 7kHz to 10kHz, and reached maximum at 8.1kHz. The acoustic contrast mechanism can be ascribed to the different elastic response of ferroelectric microstructures to local elastic stress fields, which is induced by the acoustic wave transmitting in the sample when the piezoelectric transducer is vibrating and exciting acoustic wave under ac electric fields due to normal piezoelectric effects.

Static and dynamic domain behavior of ferroelectric fluoride BaMgF4 single crystal was investigated using high-resolution piezoresponse force microscopy. High domain wall anisotropy was found in the as-grown BaMgF4 crystal. Antiparallel domain wall strain phenomena due to defects across the domain boundary were directly observed. The lateral domain growth in the inhomogeneous AFM tip field revealed the activation mechanism of the domain wall motion.

Piezoresponse mode atomic force microscopy was used to induce a dynamic polarization reversal process and to characterize domain configuration behavior in BaMgF4 single crystal. Triangular reversed domains observed on the polar and non-polar surface demonstrate a sideways domain wall motion mechanism prevailing in the switching process of the bulk BaMgF4 crystal. The domain wall bowing phenomenon due to the strong pinning of the crystal defects was first visualized in the piezoresponse image of the engineering domain pattern in BaMgF4 crystal.

The optical properties of as-grown and X-ray irradiated Ce, Eu and Pr-doped LiLuF 4 crystals, prospective for lasers, display panels, fluorescent lamps and scintillation application, are presented. The luminescence of Ce 3 + -doped crystal exhibits the fast radiative spin-allowed df transitions in the UV region, whereas Pr 3 + containing sample shows the UV df and visible ff emissions. LiLuF 4 : Eu reveals the line emission spectrum of Eu 3 + ions and the band peculiar to Eu 2 + ion as well. It was found that the X-ray induced absorption bands and the TSL peak positions are independent of the type of RE 3 + impurity due to intrinsic nature of radiation defects. The total radiation damage and TSL spectra are specific for each dopant. The distinctive features of the energy storage in RE-doped LiLuF 4 crystals are discussed with respect to redox reaction and level positions of RE ion within the gap.

The oxide garnet {Tb3}[Sc2-xLux](Al3)O12 (TSLAG) is developed as alternative to Tb3Ga5O12 (TGG). TSLAG exhibits better transmittance, Verdet constant, and radiation resistance. Further, PrF3 and CeF3 fluoride crystals are investigated as unique Faraday rotators for the UV-visible region.

The isolated, six-side polygonal domain structures are observed in the orthorhombic as-grown BaMgF4 single crystal by optical microscope and atomic force microscope. The domain configurations demonstrate highly domain wall anisotropy along one principal crystal axis and polar–surface inhomogeneity. The anisotropic behaviour is due to the anisotropy of domain wall energy density in the thermodynamic equilibrium process, and the asymmetric domain surface phenomena is ascribed to the combined contribution from the local electric fields and structure defects-induced mechanical stress during the domain evolution process.

Ferroelectric domain phenomena and periodic domain patterning of orthorhombic as-grown BaMgF4 single crystal were investigated in the present study. An isolated ferroelectric domain shows a hexagonal form and exhibits a high domain-wall anisotropy. Periodic domain patterning was demonstrated on the polar surface by a periodic macropoling technique. In-plane polarization switching-induced microdomain patterning was tailored by the lateral component of the inhomogeneous electric field through a biased atomic force microscope tip. Such in-plane domain switching behavior exhibits potential promise for periodic domain engineered phonon devices.

Epitaxial growth of GaN on β-Ga 2 O 3 single crystal substrates by the molecular beam epitaxy technique is demonstrated for the first time. Standard and in-plane X-ray diffraction evidence the growth of c -plane wurtzite GaN on a -plane β-Ga 2 O 3 , the epitaxial relationship being 〈0 1 0〉 Ga 2 O 3 ‖ 〈1 1 2¯ 0〉 GaN and 〈0 0 1〉 Ga 2 O 3 ‖〈1¯ 1 0 0〉 GaN . Epitaxial growth without any buffer layer is achieved after an effective surface nitridation under NH 3 gas.

SrAlF 5 has recently become one of the most promising candidates for the realization of all-solid-state lasers in the ultraviolet wavelength region by the quasi-phase-matching technique. It has been shown that it is highly transparent down to the cutoff wavelength at about 150 nm, optically uniaxial and ferroelectric. The refractive index of SrAlF 5 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 quasi-phase-matching is estimated as a function of the wavelength. The effective nonlinear coefficient is similar to that of quartz, d eff ≈ 0.3 –0.4 pm/V, and the poling periodicity for 193 nm SHG from 386 nm is 4.1 μ m .

The growth characteristics of large size SrAlF 5 and BaMgF 4 single crystals by the Czochralski technique are described and compared. Transmission spectra in the vacuum ultraviolet wavelength region indicate a high transparency of SrAlF 5 (about 90% without considering surface reflection loses) down to 150 nm, in contrast to the optical loses observed for BaMgF 4 . The ferroelectric character of SrAlF 5 is evidenced by the reversal of the spontaneous polarization in a hysteresis loop. The higher potential of SrAlF 5 in comparison with BaMgF 4 for the realization of all-solid-state lasers in the ultraviolet wavelength region by the quasi-phase matching technique is pointed out. SrAlF 5 , besides a higher grade of transparency, shows a nonlinear effective coefficient similar to that of quartz and uniaxial nature, in contrast to the one-order smaller nonlinear coefficient and biaxial character of BaMgF 4 .

CeF3, PrF3 and NdF3 single crystals have been grown under CF4 atmosphere by the modified Czochralski technique. PrF3 exhibited a dominant sharp emission at about 400 nm (1S0→1I6), with a decay time of 510 ns. Photon cascade process as well as energy migration to RE-dopants were not observed. Light emission from CeF3 remarkably increased by Lu3+-doping, yielding to efficiencies over that of BGO.

Large size single crystals of β-Ga2O3 with 1 inch in diameter have been grown by the floating zone technique. 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 β-Ga2O3 as a substrate for optoelectric 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 β-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 β-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. SrAlF5 shows a higher grade of transparency and a larger nonlinear effective coefficient as well as uniaxial and ferroelectric nature, on contrast to the one order smaller nonlinear coefficient and biaxial character of BaMgF4. The Sellmeier and Cauchy coefficients are obtained fitting the measured ordinary and extraordinary refractive indices, and the grating period for the first order quasi-phase matching (QPM) is estimated as a function of the wavelength. The higher potential of SrAlF5 in comparison with BaMgF4 for the realization of all-solid-state lasers in the ultraviolet wavelength region by the QPM technique is pointed out.

Large-size single crystals of β-Ga 2 O 3 with 1 inc 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 (1 0 0), (0 1 0) and (0 0 1) planes. These were highly transparent in the visible and near UV, as well as electrically conductive, indicating the potential use of β-Ga 2 O 3 as a substrate for optoelectic devices operating in the visible/near UV and with vertical current flow.