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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.

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.

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.

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.

β-Ga2O3 bulk single crystals were doped by ion implantation at temperatures from room temperature to 1000 °C, using a 300 keV Europium beam with a fluence of 1 × 1015 at cm−2. Rising the implantation temperature from room temperature to 400–600 °C resulted in a significant increase of the substitutional Eu fraction and of the number of Eu ions in the 3+ charge state as well as in a considerable decrease of implantation damage. Eu is found in both charge states 2+ and 3+ and their relative fractions are critically dependent on the implantation and annealing temperature, suggesting that defects play an important role in stabilizing one of the charge states. The damage recovery during post-implant annealing is a complex process and typically defect levels first increase for intermediate annealing temperatures and a significant recovery of the crystal only starts around 1000 °C. Cathodoluminescence spectra are dominated by the sharp Eu3+ related intra-ionic 4f transition lines in the red spectral region. They show a strong increase of the emission intensity with increasing annealing temperature, in particular for samples implanted at elevated temperature, indicating the optical activation of Eu3+ ions. However, no direct correlation of emission intensity and Eu3+ fraction was found, again pointing to the important role of defects on the physical properties of these luminescent materials.

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 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.

The halide vapor phase epitaxy of α-Ga2O3 is demonstrated for the first time. The films are twin-free and heteroepitaxially grown on sapphire (0001) substrates using gallium chloride and oxygen as precursors. X-ray ω–2θ and pole figure measurements reveal that the film is single-crystalline (0001) α-Ga2O3 with no detectable formation of β-Ga2O3. The optical bandgap is determined to be 5.16 eV based on the transmittance spectrum. The growth rate monotonically increases with the partial pressures of the raw material gases, reaching approximately 150 µm/h, which is over two orders of magnitude larger than those of conventional vapor phase epitaxial growth techniques, such as mist CVD or MBE.

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.

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).

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.

Thin film solar cell absorbers of nominal composition CuIn1-xGaxS2 for(0 < x < 0.5) were prepared by sequential evaporation of the metals and by subsequent annealing in either a sulfur or a H2S atmosphere. The film formation was investigated experimentally and elucidated by thermochemical calculations.