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Polycrystalline higher manganese silicides Mn1–xCrxSi1.74 (x = 0, 0.10, 0.20) with Cr single doping and Mn1–2yCryFeySi1.74 (y = 0.10, 0.20) with Cr–Fe double doping have been prepared by arc meltin...

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.

In this study, high purity n-type Mg2Ge single crystals are grown in graphite crucibles under nitrogen atmosphere using a facile growth process. X-ray diffraction proves that the grown crystal consists of only Mg2Ge single phase without any other additional impurity peaks. The clear symmetrical Laue diffraction pattern of the cleaved substrate demonstrates the single crystalline nature of the grown Mg2Ge crystal ingot. The Hall Effect measurement of the polished Mg2Ge wafer shows that the latter had a moderate carrier density (5.66 * 1016 cm−3), mobility (253 cm2/V) and electrical resistivity (0.5 Ω. cm) at room temperature. Detailed analysis of the temperature dependence of the carrier density demonstrates that the shallow donor level, which gives rise to the n-type conduction of grown Mg2Ge crystals is due to the unintentional Al impurities from Mg source material. Mg2Ge pn-junction photodiodes are also manufactured for the first time by thermal diffusion of silver thin layer into n-Mg2Ge at 550 °C for 5 min in an argon atmosphere. The results reveal that the fabricated p-n photodiodes had a clear rectifying behavior and a remarkable spectral response in the wavelength range from 0.8 to 1.7 μm with a maximum room temperature zero-biased photoresponse of 7 mA/W at 1.2 μm. These outcomes indicate that the fabricated Mg2Ge photodiodes are promising to detect short-wavelength infrared (SWIR) light in the above-mentioned wavelength domain.

N-type Mg2Si single crystal ingot has been successfully grown by the Vertical Bridgman technique. The Hall Effect measurement at RT revealed a moderate Hall mobility (~ 446 cm2/Vs), electrical resistivity (~ 1.4 Ω. cm) and carrier density (~1016 cm−3) for the grown crystal. The J-V characteristics of the fabricated Au/n-Mg2Si Schottky junction and Au/Ag-doped-p-type-Mg2Si/n-type-Mg2Si/Ag pn-junction diode showed better diode behavior with higher rectifying ratio in comparison with the same reported junctions. Analysis of the experimental forward J-V characteristics of our fabricated diodes based on the thermionic emission (TE) model demonstrated that our Schottky and pn-junction Mg2Si diode exhibited much lower series resistance in comparison with the survey data, confirming that precise polishing of Mg2Si surfaces with an oil-based diamond greatly improved the contact with the evaporated metal, enabling lower interfacial resistance between the evaporated metal and Mg2Si wafer. In addition, the manufactured Mg2Si pn-junction photodiode showed a clear photoresponsivity in the wavelength range from 0.95 to 1.8 µm with a maximum zero-biased photoresponse of 14 mA/W at 1.4 µm and a photon energy threshold of approximately 0.66 eV. These results indicate that our diode is promising to detect the SWIR light in the wavelength range from 0.95 to 1.8 µm.

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.

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.

Recently, monolayer SnS, a two-dimensional group IV monochalcogenide, was grown on a mica substrate at the micrometer-size scale by the simple physical vapor deposition (PVD), resulting in the successful demonstration of its in-plane room temperature ferroelectricity. However, the reason behind the monolayer growth remains unclear because it had been considered that the SnS growth inevitably results in a multilayer thickness due to the strong interlayer interaction arising from lone pair electrons. Here, we investigate the PVD growth of monolayer SnS from two different feed powders, highly purified SnS and commercial phase-impure SnS. Contrary to expectations, it is suggested that the mica substrate surface is modified by sulfur evaporated from the Sn2S3 contaminant in the as-purchased powder and the lateral growth of monolayer SnS is facilitated due to the enhanced surface diffusion of SnS precursor molecules, unlike the growth from the highly purified powder. This insight provides a guide to identify further controllable growth conditions.

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.

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.

Bulk gallium monosulfide (GaS) single crystals are increasingly required for research on optical devices used in the mid-infrared (IR) region and for 2D GaS. In this work, a GaS single crystal was grown in an evacuated quartz ampoule and characterized by the X-ray Laue backscattering method, differential thermal analysis (DTA), X-ray rocking curve, Raman spectroscopy, optical transmission spectroscopy, and scanning electron microscope-energy dispersive X-ray microanalysis (SEM-EDX). First, polycrystalline GaS was synthesized using our previously improved process, on a common horizontal furnace. Before the crystal growth of GaS, its melting point was measured via DTA of the synthesized polycrystalline GaS and was determined to be 965 °C. Based on the data obtained, GaS single crystals were grown in evacuated quartz ampoules using the vertical Bridgman method without a seed crystal. The X-ray Laue backscattering pattern and Raman spectra revealed that the grown GaS crystal was indeed a single crystal and that the growth direction was parallel to the (0 0 1) face. The optical transmittance range was found to be 0.48–14 μm.

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.

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.

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.

Remarkable optical/electrical features are expected in two-dimensional group-IV monochalcogenides (MXs; M = Sn/Ge and X = S/Se) with a uniquely distorted layered structure. The lone pair electrons in the group-IV atoms are the origin of this structural distortion, while they also cause a strong interlayer force and high chemical reactivity. The fabrication of chemically stable few-to-monolayer MX has been a significant challenge. We have observed that, once the SnS surface is oxidized, the SnOx top layer works as a passivation layer for the SnS layer underneath. In this work, the SnOx/SnS hetero-structure is studied structurally, optically, and electrically. When tape-exfoliated bulk SnS is oxygen-annealed under a reduced pressure at 10 Pa, surface oxidation and SnS sublimation proceed simultaneously, resulting in a monolayer-thick SnS layer with the SnOx passivation layer. The field-effect transistor of nine-layer SnS prepared via mechanical exfoliation exhibits a p-type characteristic because of intrinsic Sn vacancies, whereas ambipolar behavior is observed for the monolayer-thick SnS obtained via oxygen annealing probably owing to the additional n-type doping by S vacancies. This work on monolayer-thick SnS fabrication can be applied to other unstable lone pair analogues and can facilitate future research on MXs.

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.

In this study, we developed a novel growth method named “Flux-Film-Coated (FFC) sputtering.” In this method, nitrogen radicals were supplied to the Al–Sn flux at around 600 °C followed by the deposition of an Al–Sn metal film on a sapphire substrate as flux, which resulted in the growth of high-quality AlN films. The nitrogen radical, which is the source of nitrogen for the growth of AlN, was generated by the radio frequency plasma in a nitrogen atmosphere. Using a zirconia target in the plasma generation process, nitrogen radicals were easily generated because the zirconia was not etched in a pure nitrogen atmosphere, which enabled us to fabricate the AlN film using the flux method in the sputtering chamber in a single step. The crystallinity of the synthesized AlN determined using the FFC-sputtering method was remarkably improved when compared with that of the AlN film deposited using the reactive sputtering method.

To drive the research and development of two-dimensional (2-D) germanium monosulfide (GeS) applications, a more facile method for obtaining (GeS) single crystals is required. To that end, we first improved the method for synthesizing polycrystalline GeS to reduce the reaction time and prevent the explosion of the quartz ampoule, and this step was carried out inside a horizontal furnace. We also successfully grew a GeS single crystal without a seed crystal by melting this polycrystalline GeS inside a closed, evacuated quartz ampoule using a two-zone horizontal furnace and then employing a vapor transport method. The growth ampoule was specifically designed to prevent the molten GeS from flowing from the evaporation side to the growth side, where the crystal grew through an evaporation–recondensation transition in the temperature range of 654 to 660 ℃. Single-crystal X-ray diffraction was carried out to precisely determine the crystal structure, which revealed that the grown crystal was a GeS single crystal with an orthorhombic structure of the GeS-type (space group Pnma) and lattice parameters of a = 10.5284 (10), b = 3.6584 (3), and c = 4.3166 (4) A. A differential thermal analysis confirmed that the melting point of the grown GeS crystal was 664 ℃.

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.

(Y1−xGdx)3Al5O12:Ce ((Y,Gd)3Al5O12:Ce) single crystal phosphors (SCPs) were successfully grown using the Czochralski method. The Gd segregation coefficient was determined. The lattice spacing, absorption and emission properties were systematically investigated as a function of Gd concentration in a wide temperature range. Gd incorporation red-shifts the SCP emission so that neutral white with correlated color temperatures in the range of 4098–6594 K can be achieved in conjunction with a blue light-emitting diode (LED). SCPs demonstrate high conversion efficiency (>90% above 150 °C) and better thermal stability compared to analogous ceramic powders. Additionally, SCPs are compatible with resin-free packaging, overcoming drawbacks associated with powder encapsulants. Therefore, (Y,Gd)3Al5O12:Ce SCPs are promising candidates for high-brightness neutral white sources based on intense blue LEDs and laser diodes.

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.

For the full benefit of the silicon chip industry and to further extend the photoresponse cut-off wavelength of the current Si photodetectors beyond 1100 nm, high-performance silicon-compatible Mg2Si/Si n-p photodiodes are constructed on the bulk silicon wafer by magnetron sputtering and post-annealing sequential processes. The results show that the annealing period appreciably affects microstructure, crystallite size, and roughness of the Mg2Si film sputtered on glass substrates and silicon wafers. Meanwhile, the fabricated Mg2Si/Si n-p photodiodes demonstrate clear rectifying behavior and evident spectral response characteristics in the wavelength domain from 0.8 to 1.35 μm, with a maximum room temperature zero-bias response of 0.1, 0.15, 0.86, and 0.47 A/W at 1.05 μm for the annealing periods of 10, 20, 40, and 60 min, respectively. The photoresponse cut-off wavelength of the fabricated Mg2Si/Si n-p photodiodes is 1.35 μm; however, the photosensitive cut-off of the commercially available Hamamatsu Si photodiode is 1.25 μm. The fabricated Mg2Si/Si n-p photodiodes at RTA period of 40 min reveal spectral response of longer cut-off wavelength and higher intensity near the photon energy threshold compared to the Hamamatsu Si photodiode. In this context, the fabricated photodiodes in this study offer huge potentials to develop high-performance silicon-compatible photodiodes for efficiently sensing the infrared light in the wavelength domain from 0.8 to 1.35 μm.

We demonstrate the high-speed growth of β-Ga2O3 quasi-heteroepilayers on off-angled sapphire (0 0 0 1) substrates by halide vapor phase epitaxy (HVPE). (2¯01) oriented β-Ga2O3 layers were successfully grown using GaCl and O2 as source gases. The growth rate monotonically increased with increasing the partial pressures of the source gases, reaching over 250 μm/h. This rate is over two orders of magnitude larger than those of conventional vapor phase epitaxial growth techniques such as molecular beam epitaxy or metalorganic vapor phase epitaxy. X-ray pole figure measurements indicated the presence of six in-plane rotational domains, in accordance with the substrate symmetry, plus some minor (3 1 0) domains. By the use of off-angled substrates and thick layer overgrowth, one of the in-plane orientations was strongly favored and the (3 1 0) residuals effectively suppressed, so that quasi-heteroepitaxial growth was achieved. Therefore, these results indicate the high-potential of the HVPE technique for the growth of thick and thin β-Ga2O3 layers for the cost-effective production of β-Ga2O3 based devices.