Vertical Bridgman Research Articles

Cd0.90Mn0.10Te single crystal for γ – Ray detector application: A comprehensive analysis of structural and compositional homogeneity

This study provides a comprehensive examination of the Cd0.90Mn0.10Te (CMT) single crystal in its role as a radiation detector. Covering aspects from the growth process to detector performance, the primary emphasis is placed on ensuring the uniformity of composition along the growth axis. A cost-effective custom-built vertical Bridgman (VB) setup is developed for the growth of highly homogeneous Cd0.90Mn0.10Te (CMT) single crystal. CMT single crystals of 10 mm diameter and 130 mm length with near uniform stoichiometry along the growth axis were grown using VB method. Laue diffraction confirms the single crystallinity of the grown crystal. The measured lattice constant is comparable to the theoretical value calculated using Vegard’s law. The two mode vibrations of the CdTe and MnTe sublattices were confirmed by Raman spectroscopy. The observed high optical transmittance in the mid-infra-red (IR) region and a nearly constant bandgap of ∼1.6 eV for the wafers taken at different locations along the growth axis reveals the IR transparency and compositional uniformity of the grown crystal. The energy dispersive X-ray spectroscopy proved that the elemental distribution is uniform in the grown crystal. The IR microscopic imaging shows Te inclusions with a number density in the range of ∼2.6–4.5 × 104 cm−2. The resistivity of the CMT crystal for all wafers was found to be in the order of 109 Ωcm. The γ–ray spectral resolution of ∼22 % of 59.5 keV irradiation from 241Am source is achieved.

Improving the crystal quality and optoelectronic property of GaSb with Al doping

GaSb and Ga1-xAlxSb crystals were prepared with the vertical Bridgman (VB) method. The effects of aluminum atoms (Al) doping on the structure and photoelectric properties of GaSb crystal were investigated. The results show that the GaAlSb crystal maintains the zinc blende crystal structure of the GaSb crystal, and the crystal quality gradually improves with the increase of Al doping concentration, and the effective segregation coefficient keff of Al decreases from 3.69 to 2.83. Furthermore, Al doping significantly inhibits the formation of dislocations, of which density decreases from 3.634 × 103 cm−2 to 1.256 × 103 cm−2. As an element with the same equivalent electrons as Ga, Al doping makes GaSb crystal become into an n-type semiconductor with the free electrons as the main carriers, from a p-type semiconductor with the holes as the main carriers (8.867 × 1017 cm−3). And the carrier concentration, i.e., the free electrons concentration, increases with the doped Al concentration increase, up to the maximum of 1.929 × 1018 cm−3. And the mobility of electron carriers increases with the increase of Al concentration, up to 2.140 × 103 cm2/(V•s), far greater than that of the hole carriers 1.358 × 103 cm2/(V•s). And the resistivity decreases from 7.649 × 10−3 Ω•cm to 1.416 × 10−3 Ω•cm. Moreover, the infrared transmittance spectrum also shows a significant improvement of the crystal quality.

Crystal Growth and Structural Characterization of Cs2LiLaBr6:Ce Using Neutron Diffraction

AbstractCe‐doped Cs2LiLaBr6 (CLLB) scintillator crystals, known for their superior neutron/gamma dual‐mode detection capability and exceptional scintillation properties, have garnered significant attention for both fundamental science and practical applications. The role of Ce3+ cations as luminescent centers is pivotal, influencing the scintillation properties as their concentration varies. While the effects of Ce3+ concentration on scintillation performance are well‐documented, the ramifications for crystalline structure remain less explored. In this study, high‐quality CLLB:Ce single crystals are fabricated(atomic packing factor of maximum doped Ce is 0.04%) using the vertical Bridgman (VB) method and subsequently characterized their crystalline structure by neutron diffraction, X‐ray diffraction (XRD), and elemental analysis. The findings reveal a correlation between Ce3+ concentration and the crystal cell parameters, presenting intriguing deviations from Vegard's law. Such observations suggest the potential presence of alternative defects, potentially Li+ interstitials, in CLLB:Ce. This work offers critical insights for advancing the understanding and optimization of CLLB:Ce scintillators.

Line-shaped defects in bulk β-Ga2O3 single crystals grown by the vertical Bridgman method

We conducted an investigation of line-shaped defects, extending in the 〈010〉 direction, in bulk β-Ga2O3 single crystals grown by the vertical Bridgman (VB) method. Parallelepiped cross-section samples with {010} polished surfaces and {100} cleavage planes were prepared and were then etched in phosphoric acid at 140 °C. Rhombic etch pits were observed on the (010) surface and they were similar in shape to those reported as nanometer-sized grooves or plate-like nanopipes in crystals grown by the edge-defined film-fed growth (EFG) method. Groove-like voids in the 〈010〉 direction were also observed on {100} cleavage planes below the etch pits observed on the {010} surface. Therefore, we concluded that line-shaped defects observed in VB-grown crystals are similar to defects observed in EFG-grown crystals. The size of these defects was considerably smaller than that observed in EFG-grown crystals, but with a density of 5 × 105 cm−2. Based on these results, possible formation mechanisms for such defects were discussed.

Two-inch Fe-doped β-Ga2O3 (010) substrates prepared using vertical Bridgman method

The growth of large-diameter high-resistivity β-Ga2O3 (010) substrates is important for the low-cost production of lateral Ga2O3 devices. We grew a 2 inch diameter Fe-doped high-resistivity β-Ga2O3 (010) single crystal by using the vertical Bridgman (VB) method, which is expected to grow large-diameter β-Ga2O3 crystals with various crystal orientations. Two-inch substrates were prepared from the obtained crystals, and their crystallinity, concentration of Fe dopants, and electrical properties were investigated. Consequently, a 2 inch β-Ga2O3 (010) substrate, which is comparable to the largest size of (010) substrate prepared using the Czochralski method, was successfully fabricated with the VB method. The in-plane distribution of the X-ray rocking curve from 020 diffraction of the fabricated 2 inch substrate showed that the full widths at half maximums were less than 35 arcsec at almost all measurement points, indicating high crystallinity and high in-plane uniformity. In addition, the crystals contain Fe concentrations in the range of 3.5 × 1018–1.9 × 1019 cm−3, indicating that impurity Si donors are sufficiently compensated by the Fe dopants. Therefore, substrates prepared using the VB method exhibited high resistivities of 6 × 1011–9 × 1012 Ω·cm at room temperature.

High crystal quality of vertical Bridgman and edge-defined film-fed growth β-Ga2O3 bulk crystals investigated using high-resolution X-ray diffraction and synchrotron X-ray topography

High crystal quality of vertical Bridgman (VB)-grown (001) β-Ga2O3 single-crystal was confirmed in comparison with edge-defined film-fed growth (EFG)-grown (001) β-Ga2O3 single-crystal. The FWHMs of VB were lower than FWHMs of EFG from X-ray diffraction rocking curve analysis. VB and EFG crystal curvatures were measured as 232.24 m and 45.83 m, respectively. By using transmission and reflection geometry in synchrotron X-ray topography, we unambiguously determined the Burgers vector ( b ) = [010] and [001]. For the VB crystal, screw dislocations with b = [010] and dislocation direction ( t ) = [010] were observed. For the EFG crystal, edge dislocations with b = [001] and t = [010] were observed. In addition, wandering dislocations on the (001) plane with b = [010] for EFG and b = [001] for both EFG and VB were observed. These phenomena indicate that the wandering dislocations were formed by a slip motion on the (001) slip plane.

Interface Shape under the Insulating Baffle in Vertical Bridgman Systems

AbstractIn the vertical Bridgman (VB) configuration, the shape of the solid–liquid (s/l) interface controls the yield and quality of single crystals. The two key effects for growing CdTe in VB configuration with the baffle is are considered: i) the thermal conductivity of the baffle and 2) baffle/crystal rotation. The baffle has a dominating effect on heat transfer to the s/l interface. The low thermal conductivity baffle (λ = 0.3 W m‐K−1) shields the s/l interface from the hot zones of the furnace above the baffle, so that the furnace temperature has to be increased, thus enforcing convex interface. The insulating baffle is expected to be particularly useful for growth of low conductivity crystals such as CdTe or CZT because heat conduction through the crystal is slow.The flow driven by the rotating crystal (i.e., ampoule) makes the interface less convex. By rotating the crystal relative to the baffle, the undesirable, unsteady natural convection can be made negligible compared to forced convection, thus providing steady laminar flow at the s/l interface.

Performance Comparison of CdTe:Na, CdTe:As, and CdTe:P Single Crystals for Solar Cell Applications.

We compared thermal stability, open-circuit voltage, short-circuit current, and fill factor values of single-crystal Cadmium telluride (CdTe) grown using the vertical Bridgman (VB) technique and doped with group V elements (phosphorus and arsenic), and group Ⅰ element (sodium), followed by an annealing process. The sodium-doped CdTe maintained a hole density of 1016 cm−3 or higher; after annealing for a long time, this decreased to 1015 cm−3 or less. The arsenic-doped CdTe maintained a hole density of approximately 1016 cm−3 even after the annealing process; however its bulk minority carrier lifetime decreased by approximately 10%. The phosphorus-doped CdTe maintained its properties after the annealing process, ultimately achieving a hole density of ~1016 cm−3 and a minority carrier lifetime of ~40 ns. The characteristics of a single-crystal solar cell were evaluated using a solar cell device that contained single-crystal CdTe with various dopants. The sodium-doped sample exhibited poor interfacial properties, and its performance decreased rapidly during annealing. The samples doped with group V elements exhibited stable characteristics even during long-term annealing. We concluded, therefore, that group V elements dopants are more suitable for CdTe single-crystal-based solar cell applications involving thermal stress conditions, such as space missions or extreme fabrication temperature environments.

Re-evaluation of the electronic structure and thermoelectric properties of narrow-gap semiconducting α-SrSi2: A complementary experimental and first-principles hybrid-functional approach

Thermoelectric power generation in the low temperature region has attracted considerable attention as a means of the effective use of distributed energy and for sensor applications. However, it is difficult to theoretically predict the exact thermoelectric transport properties owing to the relatively narrow bandgap of low temperature thermoelectric materials. In this study, a high-purity α-SrSi2 crystal was synthesized by the vertical Bridgman (VB) method. The carrier density of the VB-grown α-SrSi2 was investigated, and, from the temperature dependence of the carrier density, it was estimated that the bandgap was 13.1 meV. First-principles calculations using the Heyd–Scuseria–Ernzerhof screened hybrid functional for α-SrSi2 predicted the bandgap to be very close to this value (13.27 meV) when assuming the mixing parameter of the Hartree–Fock contribution to the exact exchange is 18.7%. Using the calculated electronic structure and the measured carrier concentration, the predicted temperature dependence of the Seebeck coefficient was in good agreement with the experimental results.

“Liquinert” Process for High-Quality Bulk Single Crystal Growth

Bulk crystal growth technologies originate from the Czochralski (CZ) and Vertical Bridgman (VB) methods developed almost one century ago. Both methods were applied to prepare single crystals of many kind of inorganic materials, for example, semiconductors, halides and many oxides. In the VB process, molten raw materials are wetting the crucible wall easily. This phenomenon causes the sticking of grown crystals with crucibles and often leads to the cracking of the crystal and crucible. These issues prohibit us from obtaining high quality single crystals. Therefore, practical application of VB method is limited only on several materials such as CaF₂ and GaAs single crystals. The issue of crucible’s wetting is present in the CZ method as well. For example, the purity of silicon single crystals is degraded from 11N raw material to 5~6N level by the oxygen and carbon contamination caused by the wetting between silicon melt and quartz crucible. These issues are yet to be solved in VB and CZ methods. Many molten materials reach the spherical shape driven by the surface tension when a residual moisture (H₂O) is completely removed from the raw material, the crucible, and atmosphere. We denote this condition as the “Liquinert” state meaning “liquid being in an inert state”, non-wetting and non-reactive with the crucible at high temperature. The author has prepared many high-quality bulk crystals of mainly metallic halides and semiconductors, except oxides, by VB method when applying the “Liquinert” process. This technology is applicable to high quality bulk crystal growth of silicon as well as other inorganic materials of huge industrial interest. In this review, the “Liquinert” process, its background, methodology, examples of applications in fundamental research, and practical development are exposed. In addition, we also discuss the future of this industrial process on bulk silicon crystals for semiconductors and solar cells.

Growth of (1 0 0), (0 1 0) and (0 0 1) β-Ga2O3 single crystals by vertical Bridgman method

Growth of β-Ga2O3 single crystals with three growth orientations perpendicular to the (100), (010) and (001) planes was investigated by the vertical Bridgman (VB) method. Three kinds of seed crystals with different orientations were prepared, and crystals with a diameter of 1 in. were obtained under the same growth conditions. Examination by crossed polarizer and X-ray topography shows that the three ingots were twin-free single crystals. For (100), (010) and (001) plane wafers, selective etching using H3PO4 was conducted, then different etching rates due to the anisotropy of β-Ga2O3 were determined and specifically shaped etch pits were also revealed. Line-shaped defects extending in the [010] direction were observed in the (100), (010) and (001) grown crystals.

Effects of excess Te on flux inclusion formation in the growth of cadmium zinc telluride when forced melt convection is applied

The presence of second phase defects, particularly flux inclusions of tellurium rich composition, are of great concern for charge collection efficiency in cadmium zinc telluride (CZT) and cadmium telluride (CT) material intended for applications such as radiation detection. These inclusions can distort applied electric field lines within the detector as well as act as trapping centers for charge carriers. Reduction and/or elimination of these inclusions is required to achieve appropriate charge collection efficiencies, especially in detectors of thicknesses greater than 5 mm. These so-called flux inclusions are understood to form as a consequence of constitutional undercooling at the crystal growth interface. In this study, a forced melt convection technique was applied in Vertical Bridgman (VB) melt growth of CZT without reducing imposed growth rates of ∼2 mm/hr. Several rotation profiles were tested while adjusting the melt composition from 51.62 to 61.75 atomic percent (at%) Te where the Te concentration was initially increased to improve overall material purity. With forced melt convection, the best inclusion distributions were achieved with highly Te rich melt compositions, far beyond the stoichiometric composition range for the CZT system. Average inclusion diameters were reduced to 2 µm while inclusions greater than 5 µm were essentially eliminated. Composition analyses of these ingots revealed near equilibrium concentrations of Te, even with ingots grown from Te concentrations as high as 61.75 at% Te. In this paper, a recipe for the reduction of inclusions in CZT melt growth is put forward and the implications of this method on our understanding of inclusion formation are discussed.

Vertical Bridgman Growth of 42°RY‐Oriented LiTaO3 Single Crystal

AbstractCrack‐free single crystals of 42°RY‐oriented lithium tantalate, 42°RY‐LiTaO3 (42°RY‐LT), are grown by the vertical Bridgman (VB) technique, using pure platinum bottomless crucibles to avoid cracking the seed crystal. The fundamental technical issues for 42°RY‐LT growth by the VB technique are clarified.

LiTaO3 Single Crystal Growth by the Vertical Bridgman Technique

AbstractLithium tantalate, LiTaO3 (LT), single crystal growth by the vertical Bridgman (VB) technique is attempt for the first time. In the present VB‐LT technique, pure platinum crucibles are used in a furnace with a graphite heater and an argon atmosphere. One‐hundred percent solidified crack‐free VB‐LT crystals are successfully grown by composing the initial melts with exactly congruent compositions. The fundamental technical issues for VB‐LT growth are also clarified.

Growth and Scintillation Properties of Two-Inch-Diameter SrI2(Eu) Single Crystals

The sealed vertical Bridgman (VB) method is the main technique used to grow SrI2(Eu) crystals because SrI2(Eu) is very hygroscopic. In this study, we grew SrI2(Eu) single crystals with two-inch diameters and four-inch lengths using the VB method equipped with a removable chamber. The SrI2(Eu) single crystals were produced by the Liquinert process, which requires careful consideration of the crucible material and growth atmosphere. After a review of available materials, carbon crucibles were selected due to their several advantages over traditional quartz ampules: (1) they are suitable for large size crystal growth due to which they do not require any process for sealing of quartz ampule before the growth; (2) ease in controlling the temperature gradient because carbon crucibles can be heated directly via a high frequency induction heating system; and (3) carbon crucibles can assist in removing oxygen and moisture from the melt. The starting material, seed, and hot zone were prepared inside the glovebox in...

Advanced crystal growth techniques for thallium bromide semiconductor radiation detectors

Thallium Bromide (TlBr) is a promising room-temperature radiation detector candidate with excellent charge transport properties. Currently, Travelling Molten Zone (TMZ) technique is widely used for growth of semiconductor-grade TlBr crystals. However, there are several challenges associated with this type of crystal growth process including lower yield, high thermal stress, and low crystal uniformity. To overcome these shortcomings of the current technique, several different crystal growth techniques have been implemented in this study. These include: Vertical Bridgman (VB), Physical Vapor Transport (PVT), Edge-defined Film-fed Growth (EFG), and Czochralski Growth (Cz). Techniques based on melt pulling (EFG and Cz) were demonstrated for the first time for semiconductor grade TlBr material. The viability of each process along with the associated challenges for TlBr growth has been discussed. The purity of the TlBr crystals along with its crystalline and electronic properties were analyzed and correlated with the growth techniques. Uncorrected 662 keV energy resolutions around 2% were obtained from 5 mm x 5 mm x 10 mm TlBr devices with virtual Frisch-grid configuration.

High‐Quality GaSb and GaInSb Crystals Prepared by Vertical Bridgman Method

AbstractHigh‐quality GaSb and Ga1‐xInxSb (x = 0.14) crystals were grown via the vertical Bridgman (VB) method. The lattice structure, morphology, optical, and transport properties of the GaSb and GaInSb crystals were characterized. The influence of the indium doping on the physical properties of the GaSb crystal was also investigated. The results demonstrate that the indium doping maintains the zinc‐blende lattice structure of the GaSb crystal. The segregation of indium is very low, with the radial and axial concentration variation of 2.32% and 4.84%, respectively. The indium doping significantly reduces the dislocation density down to 1.275 × 103 cm−2. The band gap of the indium‐doped GaSb is reduced to 0.549 eV, consistent with the decreased infrared transmission measured by FTIR. Surprisingly, the indium doping changes the majority carrier type of the crystal, from p‐type in the pristine GaSb crystal to n‐type in the GaInSb crystal. Compared to the GaSb crystal, the GaInSb crystal shows enhanced transport properties, with carrier mobility increased to 2.512 × 103 cm2/(V•s) and resistivity reduced to 0.521 × 10−3 ohm•cm.

Crystal growth of Mg2Si for IR-detector

Semiconducting Mg2Si is known as a thermoelectric material and recently attracts increasing attention as a Si-based, low-cost and environmental friendly material for an infrared (IR) sensor. With the aim of producing cost-effective Mg2Si single crystal substrates for the IR sensor, we have investigated single crystal growth of Mg2Si using the vertical Bridgman (VB) and micro-pulling-down (μ-PD) methods in open-system. Since the evaporation of Mg was not suppressed during the μ-PD, the composition of the Mg2Si single crystal grown by this method was not stoichiometric. On the other hand, single crystalline Mg2Si was produced by the VB method using the Si-treated inner carbon seat and BN-coated carbon crucible. The Mg2Si crystals with a diameter of 30mm were grown at a growth rate of 0.5mm/min and a temperature gradient of 5°C/cm. The electron density and mobility of the crystal so obtained were 1.8×1017cm−3 and 283cm2V−1s−1 at 300K, respectively. The high electron density was due to contamination from the impurities presented in the crucible.

Numerical Simulations and Experimental Investigations on Lead Iodide Single Crystals Growth

Lead iodide (PbI2) single crystal for room temperature radiation detectors are mainly produced by Vertical Bridgman (VB) method. Computer simulations of the VB method could be an effective tool to optimize the growth process and reduce the production costs. In the present study, we applied the general Computational Fluid Dynamics (CFD) code to solute the quasi-static two-dimensional (2D) axisymmetric model of the VB ampoule filled with PbI2 melt and direct the growth of high quality PbI2 single crystals with a diameter of 15 mm. The thermal conductivity and latent heat generation during the crystallization is considered in the numerical model. We focused on the crystal-melt interface deflection. The results demonstrated that the ampoule with 15 mm diameter is better than that of 30 mm. The experimental results showed that the single crystal with 15 mm diameter exhibited high electrical resistivity (109 Ωcm).

Growth interface of CdMnTe crystal by traveling heater method

The growth interfaces of CdMnTe (CMT) crystals grown by traveling heater method (THM) were studied. Two types of polycrystalline CMT feed ingots synthesized in a traditional rocking furnace and vertical Bridgman (VB) furnace were adopted in THM growth, and the effects of the polycrystalline feed on the growth interface were revealed. The morphology of the growth interface of CMT crystal (CMT2) grown from the feed by vertical Bridgman was smoother with lower curvature compared with that of CMT crystal (CMT1) from the feed by rocking furnace. The radial Mn composition and Te inclusion distribution of the CMT wafers were analyzed and correlated to the growth interface. The Mn segregation along the radial direction and Te inclusion density of CMT2 were lower than those of CMT1. The VB method synthesized polycrystalline feed could improve the growth interface morphology, which is beneficial for decreasing the Te inclusions and Mn segregation in CMT wafers.