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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.

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.

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.

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.

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)

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.

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.

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.

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.

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.

Ferroelectric patterning is often used in advanced photonics and optoelectronic devices to increase their operational bandwidth and functionality, providing novel and unique performances. However, the extension of the ferroelectric structures to two-dimensional geometries is currently limited to very few oxides and phosphates systems, which constrains its current and future applications. Here, careful processing based on e-beam lithography and poling is employed to fabricate the first example of a two-dimensional nonlinear photonic crystal in Barium Magnesium Fluoride, BaMgF4, a ferroelectric fluoride crystal with an extraordinary transparency ranging from the deep ultraviolet (≈126 nm) to the mid infrared (≈13 μm). The optical characterization shows the possibility of obtaining simultaneously up to three different Cerenkov-type second harmonic generation processes distributed in a conical geometry via χ(2)-quasi-phase-matching technique. Additionally, the remarkably high χ(3) nonlinear response of BaMgF4 crystal in the UV spectral region is exploited to demonstrate what is believed to be the highest direct UV-third harmonic generation conversion efficiency in a solid state system via pure χ(3) nonlinear process. Together, the results highlight the outstanding opportunities offered by nonlinear photonic structures as innovative avenues to manipulate the light generation and control with reliable multifunctional optical character. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., This work has been supported by FP7 NoE Nanophotonics4Energy EU grant No. 248855; the Spanish MICINN CSD2007–0046 (Nanolight.es), MAT2010–17443, MAT2012–31659 (SAMAP) and the Comunidad de Madrid S2009/MAT-1756 (PHAMA) projects.

The ferroelectric domain wall thickness of a fluoride BaMgF4 single crystal was investigated by piezoresponse force microscopy. It was found that the domain wall thickness shows a strong spatial variation in the as-grown crystal and the polarization reversal process. The original wall thickness is greater (about two to seven times) than that switched by the tip fields of the atomic force microscope. A significantly narrower domain wall was obtained in the higher tip-field. The trapped defects at the domain wall play an important role in the spatial variation of the polarization width of 180° domain wall in the BaMgF4 single crystal. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanoscale structures and physical properties are of fundamental interest, and thus variations in scanning probe microscopy techniques have been developed to understand nanoscale physical phenomena in basic research and potential technological applications. 1) Very recently, scanning near-field ultrasound holography has been developed that provides nanoscale-resolution images of the internal substructures of microelectronic materials and malaria parasites in red blood cells. 2) In the field of ferroelectrics, nanoscale domain structures of ferroelectric thin films, ceramics and single crystals can be imaged and controlled directly using piezoresponse force microscopy. 3–10) Besides, acoustic mode scanning force microscopy including atomic force acoustic microscopy, 11,12) ultrasonic force microscopy, 13–16) and scanning probe acoustic microscopy 17–21) have the advantage of visualizing nanoscale domain structures, elastic properties or subsurface configurations. In the present study, a type of alternating-force-modulated atomic force microscope (AFM) in the acoustic mode was developed by exciting acoustic waves on the piezoelectric transducer, which is fixed on a cantilever in the commercial AFM. This acoustic mode AFM was used to visualize the ferroelectric domains in BaTiO3 single crystals. An alternating force-modulated AFM, functioning in the acoustic mode, was set up by modifying a commercial AFM (SPA 400, SPI3800N, Seiko, Japan). A schematic sketch of the modified AFM is shown in Fig. 1. An external sinusoidal signal provided by a function generator (33120A, Agilent) was applied to the piezoelectric transducer attached to the cantilever, thereby inducing a periodical vibration in the cantilever/tip system. This modulated-tip vibration driven by the function generator emits a local stress field underneath the tip, resulting in an acoustic vibration inside the sample, which gives rise to a remarkable acoustic response of the ferroelectric microstructure due to physical phenomenal effects. The resultant acoustic signal carrying ferroelectric information was converted into an electrical signal through a piezoelectric lead zirconium titanate (PZT) ceramic transducer bonded onto the sample, and then fed to a lock-in amplifier (Model 7280 DSP, Signal Recovery Instrumentation) for detection. For the experiment, a 2-mmthick 90-mm-long Ti/Pt-coated silicon cantilever (Micro Mash, NSC12-B) with a spring constant of 14 N/m and a resonance frequency of 315 kHz was used. For the present study, we chose a 0.3-mm-thick (001)-oriented BaTiO3 single crystal. Figures 2(a) and 2(b) are a topography image and the corresponding acoustic image of a BaTiO3 single crystal, respectively, at a modulation frequency of 133.42 kHz. It can be clearly seen that the acoustic features are nearly the same as those of the topography image, indicating only slight variations in the surface microstructure after sample polishing. Figure 2(c) shows an acoustic image taken at a frequency of 1.419 MHz in the same scanning area. Unlike Fig. 2(b), a different acoustic response is clearly seen in Fig. 2(c), exhibiting remarkable acoustic contrasts in parallel strips about 300 nm in width, which are obviously the 90 ferroelectric domains in the BaTiO3 single crystal. The striped patterns are more distinguished in another large scanning area of 10 10 mm 2 measured at a frequency of 1.419 kHz, as shown in Fig. 3. The intercrossing domain phenomena shown in Fig. 3(b) reflect the domain state distribution at different depths throughout the specimen. Such phenomena were observed in Han and Cao’s studies of the averaged domain configurations using a polarized optical microscope, in which the domain configurations at various depths were exhibited in an optical image by adjusting the focus. 22)

The luminescence spectra in β-Ga2O3 (4N) and β-Ga2O3:Si single crystals excited with the band-edge energy consist of UV/blue and blue/green broad bands. The luminescence with decay times (>1 s) is called persistent phosphorescence (PP). The PP spectra observed in both β-Ga2O3 single crystals at 15 K are in agreement with the blue/green band. The PP intensities decrease gradually by increasing the temperature from 15 K and disappear above ~150 K. The decay curves of the luminescence in wide time range between 4 and 100 μs and between 1 and 103 s at 15 K fit a power function of t-n with n~1.1 and 0.9, respectively. The decay occurs through the same mechanism as the recombination of donors and acceptors in semiconductors. Thermal excitation of shallowly trapped electrons in β-Ga2O3 into the conduction band leads to the decrease of the PP intensity and the increase of the electrical conductivity and the photocurrent.

CeF3 and PrF3 single crystals are investigated as Faraday rotators for the UV-visible region. Their properties are compared with those of the industrial standard reference terbium-gallium-garnet (TGG) single crystal. In contrast to TGG, CeF3 exhibits a higher transparency in the whole near UV-visible-IR, and a remarkable higher figure of merit which rapidly increases towards the cutoff. In the case of PrF3, the transparency extends to even shorter wavelengths, and analogously to CeF3 its figure of merit increases notably in the UV. These results indicate the potential of CeF3 and PrF3 as optical isolators in the UV-visible, where at present there are no alternative candidates.

International audience; Cd1−xMnxTe (x=0.1, 0.3, 0.5, 0.7 and 0.9) (CMT) single crystals were grown by the vertical Bridgman method. The optical studies reveal that with the increase in Mn concentration, the band gap values increase, which is attributed to s, p-d exchange interaction between the band carriers and Mn ions. Faraday rotation angle of the grown CMT (x=0.5) crystals were measured at the following wavelengths: 825, 1060 and 1575 nm. It was inferred that CMT exhibit larger Faraday effect (3-6 times larger than terbium-gallium garnet (TGG) currently used for optical isolators) making it as an efficient material for optical isolator at longer wavelengths. Field-cooled and zero field-cooled magnetizations of CMT were measured as a function of temperature and magnetic field. The spin-glass like behavior of CMT and their tendency to decrease in magnitude with increasing Mn concentration have been analyzed. The metal contacts on the Cd1−xMnxTe (x=0.1, 0.5, 0.7 and 0.9) crystals have been made with various metals and metal alloys to establish the ohmic contact. The detector characteristics of CMT have been tested using γ-rays with 511 keV (22 Na) and 59.5 keV (241 Am).

International audience; {Tb3}[Sc2−xLux](Al3)O12 crystals have been grown and investigated for the first time for magneto-optical applications. Tb3Al5O12 exhibits the best magneto-optical features; however, its incongruent melting nature has led to the industrial use of Tb3Ga5O12. Tb3Sc2Al3O12 had been proposed as an alternative to Tb3Al5O12, but unfortunately its growth also presents several drawbacks. The present investigation shows that by the isovalent substitution of Sc3+ by Lu3+ in the octahedral site of the garnet structure, it is possible to improve the growth characteristics while preserving the superior magneto-optical properties. We demonstrate that {Tb3}[Sc2−xLux](Al3)O12 crystals show a higher visible transparency and a larger Faraday rotation than Tb3Ga5O12 crystals. {Tb3}[Sc2−xLux](Al3)O12 is therefore a very promising material in particular for new magneto-optical applications in the visible-near IR wavelength region.

The first ultraviolet (UV) and visible optical isolators based on CeF3 are demonstrated. CeF3 possesses unique properties as Faraday rotator for the UV-visible wavelength region: a wide transparency range (wavelength of >300 nm) and an outstanding Verdet constant. In contrast, currently used terbium-gallium garnets and magneto-optical glasses possess a low transparency in the visible and a small Verdet constant in the UV, respectively. The optical isolator prototypes consist of a CeF3 rod, a single ring magnet, and a couple of beam splitters. The ring magnets have been designed to guarantee a homogeneous magnetic field; for it, numerical simulations have been carried out. The two prototypes are very compact and operate in the UV at 355 and in the visible at 405 nm, respectively. The performance of these devices indicates the high potential of CeF3 as a new UV-visible Faraday rotator, specially for shorter wavelengths where at present there are no optical isolators available.

The growth of Ce:Y3Al5O12(Ce:YAG) single-crystal phosphors (SCPs) by the Czochralski technique is analyzed in terms of segregation coefficient, solubility and absorption cross-section. The emission characteristics of these SCPs are investigated in a wide temperature range, from liquid He temperature up to 500 °C. The internal quantum efficiency of SCPs achieves its maximum at about 250 °C. Thermal quenching of SCPs at high temperature is attributed to the Mott–Seitz mechanism. In the case of ceramic powder phosphors, a continuous droop is observed with the temperature due to defect-related non-radiative recombination paths. It is shown that (Ce:YAG SCPs + blue LEDs/LDs) can cover a wide range of color temperatures 5500–7000 K, with color rendering indices around 70. In conclusion, it is shown that Ce:YAG SCPs are the most efficient and temperature stable converters to fabricate high-brightness white light sources with high-power blue LEDs and LDs.

Large size single crystals of beta-Ga 2 O 3 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-Ga 2 O 3 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-Ga 2 O 3 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-Ga 2 O 3 with vertical current injection is demonstrated.

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Ce:(YLu)AlO single-crystals are investigated in detail as phosphors for high-brightness LEDs and LDs. It is demonstrated that single-crystal phosphor plates (SCPPs) present very advantageous characteristics in comparison with standard powder phosphors. On the one side, the very high quantum efficiency of the SCPPs does not decrease with the temperature increase, and on the other side, the SCPPs possess a high thermal conductivity, so that these can be efficiently cooled to avoid device overheating. The optical properties of these SCPPs are investigated from room temperature to . Ce:YAlO SCPPs are adequate for high-brightness cool white light sources. Instead, Ce:(YLu)AlO SCPPs, whose emission shifts to the green with the Lu concentration, are very promising for neutral and warm white light sources in conjunction with a red-phosphor. It is shown that these white sources exhibit a color rendering over 90 for a wide range of excitation and SCPP emission wavelengths.

High optical quality LiREF(4) (RE = Tb(3+), Dy(3+), Ho(3+), Er(3+) and Yb(3+)), PrF(3) and CeF(3) single 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. CeF(3) presents 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. PrF(3) is the best rotator for the 220-300 nm range. Towards shorter wavelength and in the vacuum ultraviolet, it is shown that the LiREF(4) crystals are unique rotators. Overall, the rare-earth fluoride single crystals studied here exhibit better properties than other materials considered so far, and therefore they have potential to cover the increasing demand for new and improved Faraday rotators in the ultraviolet-visible wavelength region.

Lateral Schottky rectifiers were fabricated on bulk single-crystal free-standing AlN substrates. The unintentionally doped substrates display n-type conductivity. The diode shows a low reverse leakage current of ∼0.1 nA at -40 V at room temperature. The ideality factor for forward characteristics is 11.7 at room temperature and shows temperature dependence, suggesting the lateral nonuniformities at the metal/semiconductor interface. The fabricated devices are stably operated even at 573 K, owing to the wide band gap (6.2 eV) of AlN. The reverse leakage current of the device is explained by either a trap-assisted tunneling process or one-dimensional variable-range-hopping conduction along the dislocations.

N-type Ga2O3 homoepitaxial thick films were grown on β-Ga2O3(010) substrates by ozone molecular beam epitaxy. The epitaxial growth rate was increased by more than ten times by changing from the (100) plane to the (010) plane. The carrier concentration of the epitaxial layers could be varied within the range of 1016–1019 cm-3 by changing the Sn doping concentration. Platinum Schottky barrier diodes (SBDs) on 1.4-µm-thick β-Ga2O3 homoepitaxial layers were demonstrated for the first time. The SBDs exhibited a reverse breakdown voltage of 100 V, an on-resistance of 2 mΩ cm2, and a forward voltage of 1.7 V (at 200 A/cm2).

The optical and magneto-optical properties of the {Tb3}[Sc1.95Lu0.05](Al3)O12 garnet (TSLAG) crystal are analyzed in detailed and compared with those of the reference compound Tb3Ga5O12 (TGG). TSLAG presents a very high transmittance, particularly higher than that of TGG in the visible region, and its Verdet constant is 20% larger at any wavelength. The figure of merit and extinction features point out the superior characteristics of TSLAG respect to TGG. A prototype, polarization independent optical isolator based on a TSLAG rotator is demonstrated at 1080 nm. Its performance confirms the high optical quality of this crystal, indicating its mature development for Faraday rotator applications.

BaMgF(4) is a ferroelectric fluoride which shows a very wide transparency range extending from 125 nm to 13 microm. The conjunction of these properties confers to BaMgF(4) a unique chance for optical applications in the UV and mid-IR wavelength regions, where other nonlinear materials cannot be used. In particular its application as frequency converter in all solid-state lasers is considered. The wavelength dispersion of the refractive indices along the three optical principal axes is measured in the transparent region, and the Sellmeier coefficients for the three refractive indices are determined. The conditions for nonlinear optical processes are calculated for birefringent-matching and quasi phase-matching, with special emphasis in the UV and IR wavelength regions. Quasi phase-matching can be achieved in the whole transparent wavelength region, in contrast to birefringent-matching, which can be obtained in a limited range 573-5634 nm. First demonstration of second harmonic generation by quasi phase-matching with a ferroelectric fluoride is shown by frequency-doubling the emissions of a 1064 nm Nd:YAG laser and a tunable Ti:sapphire laser. The shortest emission is obtained in the UV at 368 nm, indicating the potential of BaMgF(4) as nonlinear medium for the fabrication of all solid-state lasers in the vacuum-UV/UV and mid-IR wavelength regions.

Excitation and photoluminescence spectra of high purity and Si-doped β-Ga2O3 single crystals are investigated. Emission peaks in the UV and blue wavelength regions are observed. These are excited at two bands, which can be differentiated by polarized light. The features of the spectra are independent of excitation and emission wavelengths in the case of an undoped sample. On the contrary, after Si doping, the blue emission becomes strongly dependent. A model to interpret the characteristics of blue luminescence is proposed.

The ferroelectric properties of bulk BaMgF4 are determined by polarization hysteresis measurements. The spontaneous polarization Ps is found to be 6.6μC∕cm2, while the coercive field Ec varies between 4 and 24kV∕cm, depending exponentially on the frequency. Periodical poling of domains, with a 24μm period, is demonstrated and evidenced by a selective domain etching.

Epitaxial growth of β-Ga2O3 thin films by the rf-plasma-assisted molecular-beam epitaxy technique is demonstrated. Growth on (1 0 0) β-Ga2O3 substrates leads to very smooth epilayers, while (2 0 1¯) and (1 0 0) oriented β-Ga2O3 films are obtained on (0 0 1) sapphire and (1 0 0) MgO substrates, respectively. Internal transmittance, refractive index and direct bandgaps are determined.

Epitaxial growth of nitride compounds by the metalorganic vapor phase epitaxy (MOVPE) technique is demonstrated for the first time on β-Ga2O3 single crystal substrates, which are near-UV transparent and n-type conductive. High-quality (0 0 0 1) GaN epi-layer with a narrow bandedge luminescence was obtained using a low temperature conductive buffer layer. InGaN multi-quantum well (MQW) structure was also successfully grown. The first blue light-emitting diode (LED) on β-Ga2O3 with vertical current injection is demonstrated.