Vertical Bridgman-Stockbarger Method Research Articles

Growth and characterization of physical properties of photovoltaic (K,Ba)(Ni,Nb)O3 single crystals

The substitution of fossil energy for green-energy sources is a world demand. Photovoltaic devices are a well-known safety solution to convert solar energy into electricity. Ferroelectric materials are one of the most promising candidates for substituting Si-based compounds in solar-energy conversion devices due to their relatively low cost and bulk photovoltaic effect instead of the classical p-n junction. In this work, (K,Ba)(Nb,Ni)O3 single crystal was grown by vertical Bridgman-Stockbarger method, and structural, chemical, and optical properties of the as-grown material were characterized. Growth parameters were chosen based on the thermal properties of the precursor oxide and on theoretical models.

Development of Nd-doped CsI single crystal scintillators emitting near-infrared light

0.05, 0.1, 0.5, and 1% Nd-doped CsI single crystals were synthesized by the vertical Bridgman-Stockbarger method, and the photoluminescence and scintillation properties were evaluated. Under X-ray irradiation, two emission peaks were observed at 310 and 430 nm ascribed to on- and off-center configuration self-trapped excitons, respectively. In near-infrared regions, sharp peaks were observed at 900, 1080, and 1350 nm due to 4F3/2-4I9/2, 4F3/2-4I11/2, and 4F3/2-4I13/2 transitions of Nd3+, respectively. Dose rate response functions had good linearity in the range of 0.06–6 Gy/h in all the samples, and the 0.5% Nd-doped sample showed the highest intensity among all the samples.

Correlation between thermal properties, growth parameters and low segregation coefficient in (K, Na)NbO3-based growth by Bridgman-Stockbarger route

In order to achieve optimized growth conditions to reduce the compositional segregation in (K,Na)NbO3-based crystals produced by the vertical Bridgman-Stockbarger (VB-S) method, both a complete previous analysis of the thermal properties of the precursor oxides and a suitable choice of the dimensions of the crucible were made based on theoretical models proposed in the literature. By following this approach, chemical analysis of the as-grown crystals revealed very small effective segregation coefficients in the radial and longitudinal directions of the as-grown crystals. The obtained results revealed that is possible to grow single crystals by VB-S even for incongruent compounds, through a suitable establishment of the growth parameters adjusted according to the thermal properties of the piezoelectric oxide and the relative dimensions of the crucible.

Optimizing the temperature gradient for CdZnTe crystal growth using the vertical Bridgman–Stockbarger method

CdZnTe (CZT) is considered an ideal material for the growth of HgCdTe (MCT) epitaxial layers and nuclear detection devices. The crystal quality of CZT is the key factor in the MCT-based infrared plane array and the performance of nuclear devices. Herein, we report the growth of CZT by optimizing the temperature gradient vertical Bridgman–Stockbarger method. The crystalline character of the obtained CZT has been examined using optical microscopy, IR transmission microscopy, high-resolution X-ray diffraction, X-ray tomography (XRT), Fourier-transform infrared spectroscopy and glow discharge mass spectrometry. The etch pit dislocation density of the CZT sliced wafers (111)-B was less than 4.0 × 103 cm−2, the triangular shape inclusions in the CZT volume of the double-grinding wafers have a sparse distribution, and the size can be controlled to be less than 5 μm, the typical full-width at half-maximum of the Bragg diffraction peak was 17.4″. The XRT image of polished CZT wafers (111)-B showed a uniform structure without strain and defects. About 100 % of the CZT wafers showed transmittance >50 % at 20 µm and the detected impurities in the crystal were low, Li 4.9 ppb and Na 2.9 ppb, indicating the formation of high-quality CZT crystals. This work provides a scheme to improve the crystal quality of CZT for high-performance device applications.

X-Ray and Morphological Analysis of InAs-CrAs Eutectic Nanocomposite Systems

InAs-CrAs systems are synthesized by the vertical Bridgman–Stockbarger method. XRD analysis and microstructural study of InAs-CrAs composites show that CrAs metallic inclusions are uniformly distributed in the InAs matrices. By investigating of microstructure of InAs-CrAs eutectic composite by electron microscope, it has been established that the interfacial zone between the semiconductor matrix and metallic inclusions is generated. In computations of effective electrical and thermal conductivity of the composite were taken into account the role of these interfacial zones.

Comparative pulse shape discrimination study for Ca(Br, I)[formula omitted] scintillators using machine learning and conventional methods

In particle physics experiments, pulse shape discrimination (PSD) is a powerful tool for eliminating the major background from signals. However, the analysis methods have been a bottleneck to improving PSD performance. In this study, two machine learning methods—multilayer perceptron and convolutional neural network—were applied to PSD, and their PSD performance was compared with that of conventional analysis methods. Three calcium-based halide scintillators were grown using the vertical Bridgman–Stockbarger method and used for the evaluation of PSD. Compared with conventional analysis methods, the machine learning methods achieved better PSD performance for all the scintillators. For scintillators with low light output, the machine learning methods were more effective for PSD accuracy than the conventional methods in the low-energy region.

Physical-chemical properties of InSb+Mg3Sb2 eutectic systems: synthesis, characterization, and applications

InSb+Mg3Sb2 systems are synthesized by the vertical Bridgman–Stockbarger method. InSb and Mg3Sb2, a form of lamellar eutectic. XRD analysis and microstructural study of InSb+Mg3Sb2 composites show that Mg3Sb2 lamellar are uniformly distributed in the InSb matrices. The initial and final melting temperatures for InSb+Mg3Sb2 eutectic alloys are 770K and 772K, respectively.

Scintillation property of undoped NaI transparent ceramics

In this study, NaI transparent ceramics were synthesized by the spark plasma sintering method, also optical and scintillation properties were investigated. To evaluate the potential of the transparent ceramics, a NaI single crystal was also prepared by the vertical Bridgman–Stockbarger method. The diffuse transmittance of the transparent ceramic fabricated at 470 °C was equivalent to that of the single crystal. All the samples showed scintillation peaking at 320 nm due to self-trapped exciton. In addition, scintillation peaks ascribed from lattice defects were detected at 450 nm for the transparent ceramics and 540 nm for the single crystal. The scintillation decay time constants of the transparent ceramic fabricated at 440 °C were 5.7, 18, and 189 ns. The light yield of the transparent ceramic turned out to be 910 photons MeV−1 and was two-thirds of the single crystal.

Growth of thallium-doped CsI/CsCl/KCl eutectics and their scintillation properties

In this study, Tl:CsI/CsCl/KCl ternary eutectic scintillators were grown using the vertical Bridgman–Stockbarger method by unidirectional solidification. The CsI and KCl phases were elongated along the growth direction, based on backscattered electron imaging (BEI) observations. The Tl:CsI scintillator phase grew as a matrix, which can be used to create optical waveguides for scintillation light. The expected Tl+ emission peak was observed at 550 nm under X-ray excitation. This emission was attributed to the self-trapped excitons perturbed by TI+ in Tl:CsI. The decay time of the Tl:CsI/CsCl/KCl eutectic was 285 ns, and its light output was estimated to be 10,800 photons/MeV.

Growth of 6Li-enriched LiCl/BaCl2 eutectic as a novel neutron scintillator

In this study, Eu:6LiCl/BaCl2 with a high Li concentration was developed as a novel thermal neutron scintillator. Eu ions were doped as activators for the BaCl2 phase, and Eu:6LiCl/BaCl2 eutectics were grown via the vertical Bridgman–Stockbarger method in quartz ampoules (inner diameter = 4 mm). The Eu:6LiCl/BaCl2 eutectic exhibited a lamellar eutectic structure and optical transparency. The 400 nm emission due to the Eu2+ 4f–5d transition was observed in the BaCl2 phase by a cathode luminescence measurement. The light yield under neutrons was estimated to be over 20 200 photons MeV−1. A PSD study was also performed using gamma and alpha-rays. The Eu:6LiCl/BaCl2 eutectic scintillator showed good potential for PSD.

Growth of Tb-doped BaCl2/NaCl/KCl ternary eutectic and its luminescence properties

In this study, Tb:BaCl2 and NaCl/KCl ternary eutectics were grown using quartz ampoules via the vertical Bridgman–Stockbarger method. Each crystal phase had a polygonal columnar shape with a length of 300 µm along the growth direction. The Tb:BaCl2 scintillator phase grew in the columnar phase. The BaCl2 scintillator phase is expected to make optical waveguides of the scintillation light. The expected Tb3+ emission peak was observed at approximately 360–630 nm under X-ray irradiation. This emission is mainly attributed to the Tb3+5D4→7F6,7F6, 7F5, 7F4, and 7F3 transitions and BaCl2 host emission transition. The photoluminescence decay times of the Tb:BaCl2/NaCl/KCl eutectics was 0.49 ms (7.5%) and 6.64 ms (92.5%).

Positron lifetime spectroscopy of defect structures in Cd1–x Zn x Te mixed crystals grown by vertical Bridgman–Stockbarger method

Positron annihilation lifetime spectroscopy was used to examine grown-in defects in Cd1–x Zn x Te mixed crystals as a function of Zn content (x = 0, 0.07, 0.11, 0.49, 0.9, 0.95, 1) and measuring temperature. All samples were prepared using the high-pressure modified vertical Bridgman–Stockbarger method. The crystal structure and material phase were characterized by X-ray diffraction. The positron lifetime spectra reveal the presence of both open volumes and shallow traps regardless of the sample composition. In particular, both average and bulk lifetimes are found to be much higher in ternary alloys (CdZnTe) than those in binary systems (CdTe and ZnTe). This originates from distinct differences in average electron densities and the nature of open-volume defects between binary and ternary samples. Competition in positron trapping with increasing Zn content is observed between defects characteristic for both structural systems. Moreover, a clear correlation is shown between defects and the lattice thermal conductivity of studied samples. The applicability of the positron trapping model to CdTe-based materials is discussed.

Influence of the degradation processes on luminescent and photoelectrical properties of CsPbBr3 single crystals

The degradation processes under the influences of atmosphere moisture, current passage, and ultraviolet radiation were studied in CsPbBr3 single crystals grown by the vertical Bridgman-Stockbarger method. According to the results of luminescent and photoelectric measurements the formation of CsPb2Br5 and Cs4PbBr6 phases in the result of CsPbBr3 crystal degradation was observed. Based on the spectral measurements of the photodiffusion current, the position of the energy levels of the intrinsic structural defects VBr and (VBr + e) in the forbidden bandgap of the CsPbBr3 crystal and the change of the defects concentration was determined. It is assumed that the darkening of the crystals in the degradation process during the passage of current is due to the formation of metallic phases of lead.

Characterization of optically-stimulated luminescence properties by NaCl:Eu2+ crystal and the thermal response

In this paper, we report optically-stimulated luminescence (OSL) properties of Eu2+-doped NaCl single crystal as a candidate phosphor for use in near real-time radiation measurement and dosimetry. The crystal was grown by the vertical Bridgman–Stockbarger method. The obtained crystal shows a strong photoluminescence (PL) band peaking at 430 nm due to the parity-allowed 5d-4f transitions of Eu2+. After irradiating with X-rays, it demonstrates OSL also at 430 nm due to the Eu2+ ion acting as an emission center while stimulating by light (500 ± 30 nm). Compared with a commercial BaFBr:Eu2+ OSL detector, the sensitivity to X-ray dose is lower by about one order of magnitude. In addition, while the crystal is reused multiple times, the OSL response signal increases despite the same given radiation dose. The OSL decay curve can be decomposed into three exponential decay functions with the lifetimes of 0.6, 6.4, and 120.5 s, and the decomposition analyses revealed that only the intensity of intermediate component increases with the number of reused cycles. Further analyses with thermally-stimulated luminescence (TSL) revealed that the intermediate OSL component is related to a TSL glow signal originated from trapping centers having the activation energies of ~0.93 eV and ~1.27 eV and the frequency factors of ~7.52 × 108 s−1 and ~6.29 × 1011 s−1, respectively.

Crystal growth, electronic and optical properties of Tl2CdSnSe4, a recently discovered prospective semiconductor for application in thin film solar cells and optoelectronics

Abstract A single crystal of the non-centrosymmetric low-temperature modification of Tl2CdSnSe4 (space group I 4 ‾ 2m) was grown, for the first time, and its electronic and optical properties were studied from both experimental and theoretical viewpoints. The Tl2CdSnSe4 crystal was grown from the solution-melt by vertical Bridgman-Stockbarger method. Centimeter dimensions of the crystal allow its application in practice. The bandgap energy, Eg = 1.32 eV, estimated from the optical absorption coefficient agrees well with the value of Eg = 1.29 eV calculated from the photosensitivity measurements. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure and charge states of atoms composing the crystal under consideration. To detect the best agreement of the curve of total densities of states (DOS) with the experimental XPS spectrum of valence electrons of Tl2CdSnSe4, we performed theoretical calculations within a density functional theory (DFT) framework treating different models for exchange correlation (XC) potential. Our findings yield that the best agreement of the experimental and theoretical distributions of the valence electronic states is derived when using in the computation procedure for XC potential the modified Becke-Johnson potential in the form of Tran-Blaha (TB-mBJ), which involves also the Hubbard parameter U for strongly correlated d electrons and spin-orbit coupling (SOC) effect (TB-mBJ + U + SOC model). Based on this approach, we calculated partial densities of states, energy band dispersion, and main optical constants of Tl2CdSnSe4. Importantly, the TB-mBJ + U + SOC model reveals the Eg value which is close to those determined experimentally; therefore, scissors correction adjustment is not required when performing DFT calculations of the optical constants in such a case. The present experimental XPS measurements of the treated with middle-energy Ar+-ions Tl2CdSnSe4 crystal surface and the theoretical DFT calculations data indicate, in spite of its rather hazardous chemical elements, thallium and cadmium, the crystal surface is rather stable and, coupled with suitable energy band gap, makes quaternary selenide under discussion a very promising material for using in thin film solar cells and optoelectronics as well as in highly efficient photocatalytic devices.

Photoluminescence and scintillation properties of undoped and Tl-doped Cs2BaBr4 crystals

Photoluminescence and scintillation properties of undoped and Tl-doped Cs2BaBr4 crystals grown by the vertical Bridgman-Stockbarger method were investigated. The undoped sample showed scintillation with a broad peak from 300 to 600 nm and exhibited an emission peak similar to those observed in several alkali and alkaline earth halides. Meanwhile, the Tl-doped samples showed scintillation with two emission peaks at around 380 and 510 nm, and the origins of the emissions were considered to be 3P1→1S0 transitions of Tl+ ion and off-center self-trapped excitons localized near the Tl+ ion, respectively. The light yields of the undoped and 0.5% Tl-doped samples were estimated to be 2700 and 1700 ph/MeV with a typical error of ±10%, respectively. The improvement of Cs2BaBr4 crystal for light yield was not observed by doping of Tl. In the X-ray-induced afterglow properties, the 1% Tl-doped sample possessed the lowest afterglow level among the present samples, suggesting that Tl-doping improved the afterglow properties of Cs2BaBr4.

Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method

Cadmium zinc telluride is a tunable band gap II–VI compound semiconductor, which has received considerable attention due to its applications such as nuclear radiation detectors and industrial process monitoring. We have studied the optical properties of the ternary compound semiconductors Cd1−xZnxTe grown by vertical Bridgman-Stockbarger method in the whole range of zinc content 0 < x < 1. The crystal structure and material phase were characterized by X-ray diffraction (XRD) and Raman spectra. We measured the band gap energy for each of the samples by absorption and photoluminescence (PL) spectroscopy. From the experimental results we found that the band gap energy of Cd1−xZnxTe was located at around 1.457 eV with x = 0, and gradually shift to 2.174 eV for x equal to 1. The temperature-dependent absorption measurements to study the temperature dependence of band gap energy were also performed. Based on these analyses, the relationship between the band gap energies and the zinc content are verified and discussed. Furthermore, we will study the defect recombination mechanism by using temperature-dependent PL experiments.

Effect of supercooling on the microstructural development and optimization of physical properties of melt grown SnSe crystals

The microstructural development of stoichiometric tin monoselenide (SnSe) crystals grown by vertical Bridgman–Stockbarger method using an indigenously fabricated furnace has been investigated under high vacuum (~ 10−6 mbar). The ampoule translation rate (tr) and supercooling, ΔT (= Tm − T, where Tm is the melting point and T is the crystallization temperature) were varied in the range, 12–2 mm/h and 20–100 °C respectively. Enhancement of ΔT and tr led to constitutional supercooling, inducing compositional changes and non-stoichiometry. Low ΔT (20–40 °C) and high tr (12–10 mm/h) resulted in globules, flakes and cavities. When ΔT = 60 °C and tr = 9 to 7 mm/h, mounds were formed with closed contours and ripples, due to atomically rough liquid–solid (l-β) interface. Fine tuning of ΔT (60 °C) and tr (2 mm/h) enabled smooth planar interface, so as to yield good quality crystalline structures with periodic atomic deposition promoting crystal growth, layer-by-layer. Energy dispersive analysis by X-rays and powder X-ray diffraction studies revealed appreciable crystallinity, chemical homogeneity and phase purity. The density of crystals estimated from crystallographic data (6.183 g/cm3) corroborates with that obtained utilizing Archimedes principle. Thermogravimetric and microindentation analyses established thermal and mechanical stability. The low etch pit density (~ 102 cm−2) manifests nearly perfect growth of crystals than their melt counterparts. UV–Vis–NIR and PL spectra reflected direct transition with an energy gap of 1.32 eV, validating immense potential of the grown crystals for photovoltaic applications.

Optically-stimulated luminescence properties of BaBr2:Eu translucent ceramic and single crystal

BaBr2:Eu (0.5 mol%) translucent ceramic has been synthesized using spark plasma sintering, and then the optical, scintillation and optically-stimulated luminescence (OSL) properties have been investigated. To evaluate the potential of translucent ceramics, BaBr2:Eu (0.5 mol%) single crystal has been grown by the vertical Bridgman–Stockbarger method for the comparison. When the thickness of the crystal and ceramic samples were fixed to approximately 0.5 mm, both the samples showed translucency in the visible light range. In the scintillation spectra, both the samples showed two emission peaks at around 400 and 500 nm, and the origin of the emissions were the 5d–4f transitions of Eu2+ and the F-VK center recombination, respectively. Regarding the OSL properties after x-ray irradiation, the OSL spectra of the samples showed an emission band peaking at around 400 nm, and the detection limit of the crystal and ceramic samples were 1 and 10 mGy, respectively.

Growth and Luminescent Properties of Cs2HfCl6 Scintillators Doped With Alkaline Earth Metals

Nondoped Cs2HfCl6 (CHC) and alkaline earth metal-doped CHC (AE:CHC) crystals were grown by the vertical Bridgman–Stockbarger method. The spectra of the AE:CHC under X-ray excitation were similar to those for the AE-free crystal; thus, AE doping did not change emission wavelength of CHC. We determined the light outputs and energy resolutions of nondoped CHC and AE:CHC by measuring pulse-height spectra under 662-keV gamma-ray excitation of a 137Cs source. Both light output and energy resolution were deteriorated with AE doping. For example, nondoped CHC had a light output of ~44 000 photons/MeV and energy resolution of ~5.2%; on the other hand, Sr 0.5 at.%-doped CHC had a light output of only ~28 000 photons/MeV and energy resolution of ~10%. Scintillation decay also did not change. The nonproportional response values of light output versus gamma-ray energy did not improve in AE:CHC either. Therefore, doped AEs can be responsible for creation of defect trap levels which would hamper the carrier transport in CHC structure.