Vertical Gradient Freeze Technique Research Articles

Controllable convex interface shape during vertical gradient freeze (VGF) growth of CdZnTe crystal

A convex growth interface shape is desired but is usually hard to form during the Bridgman process of CdZnTe crystal due to its low thermal conductivity. In this work, the convex interface shape was first analyzed and the mathematical criterion was determined. Theoretical analysis indicates that a convex interface can form when the derivative of axial temperature gradient (∂2T/∂y2) near the interface is negative. The temperature profiles were then proposed according to the theoretical results, and correlative temperature gradient characteristics were presented. The 90 mm diameter CdZnTe crystal has been grown by VGF technique using the proposed temperature profiles. The experimental result shows the interface shape is always convex throughout the entire growth run, which could make a better CdZnTe single-crystalline yield possible.

Numerical Simulation of CdTe Crystal Growth Using the Vertical Gradient Freeze Technique Assisted by Axial Low-Frequency Oscillations of the Melt

The problem of intensification of the melt crystal growth process has been analyzed using CdTe as an actual material. Numerical simulation of 100 mm diameter CdTe crystal growth using the VGF technique has been carried out. The heat–mass transfer was controlled by introducing low-frequency oscillating baffle into the melt, which is a so-called axial vibrational control (AVC) technique. The baffle configuration has been optimized to destroy solid “tails”, which were formed near the crucible walls at high cooling rates due to the low thermoconductivity and the corresponding latent heat. Analysis of CdTe homogeneity range showed that during fast crystal cooling, Te micro precipitations were formed, resulting from the decay of oversaturated Cd-rich nonstoichiometric solid solution during the Bridgman crystal growth technique. After full crystallization, a VGF-grown CdTe crystal stays inside the phase field of the high-temperature wurtzite polymorph. This makes it possible to go through the polymorph transition without Te micro-precipitating using the advantages of the VGF-specific feature of very slow cooling.

Evolution features and optimization of interface shape during CdZnTe crystal growth using Vertical Gradient Freeze (VGF) technique

The growth of CdZnTe crystal suffers from poor yield due to the low thermal conductivity, and a convex growth interface is usually essential to grow large-volume CdZnTe single crystal because it can lessen the adverse interactions between the growing crystal and crucible wall. The growth of 90 mm diameter CdZnTe crystal by VGF technique has been investigated using experimental and simulation methods. The influences of the growth parameters (such as temperature gradient and growth rate) and the heat transfer configurations on the growth interface shape have been inspected respectively by the unsteady simulation. The simulation results confirm that the interface shape exhibits the tendency to be “convex → flat → concave” due to the gradually deteriorated heat transfer environment with time during growth. In addition, a modified configuration has been designed and used in experiments based on the simulation results, with the aim of strengthening the conduction heat transfer of the center region of the configuration geometry. The modified configuration consists of the splicing tapered-contact quartz&graphite support with a shorter conical region and seed region, as well as the SiC holder. Experimental results demonstrate that the modified configuration would make the growth interface maintain a convex shape for a longer duration. The CdZnTe crystal grown by using the modified configuration possesses fewer grains compared with that using the initial configuration.

Production of detector grade CdZnTe crystal with VGF furnace by analyzing segregation of Zn and In

Production of CdZnTe crystals for radiation detector applications through Bridgman and Vertical Gradient Freeze (VGF) techniques is challenging in many aspects. Due to the structural characteristics of CdZnTe, it is not possible to obtain a large single crystalline ingots from the melt growth technique. In addition, the segregation of dopants and Zn decrease the yield significantly. In order to achieve detector-grade CdZnTe crystals, zinc segregation behavior should be fully understood, and optimum dopant concentration (In) should be determined. In this work, indium-doped Cd0.9Zn0.1Te single crystals grown via the Vertical Gradient Freeze (VGF) technique were compared in terms of dopant segregation, defect concentration, electron mobility, and electrical resistivity. It is observed that Zn segregates towards the tip of the ingot while In segregates towards the heel. Zn segregation coefficient is calculated as 1.35 while In segregation coefficient is found to be 0.4 for the VGF method. The variation of Zn throughout the ingot and excessive In dopant concentration degraded the detector performance. CdZnTe crystals having In doping concentrations of more than 5 ppm were unresponsive to gamma radiation. (µτ)e of VGF-grown crystals in METU Crystal Growth Laboratory is calculated as 4–5 × 10−4 cm2/V.

Growth and optical properties of infrared nonlinear optical crystal BaGa4Se7

BaGa4Se7 is a new promising infrared nonlinear optical crystal. Recently, efforts are focused on the growth of high-quality single crystals. Single crystal with size of ϕ25 (diameter) × 35(length) mm3 was grown using vertical gradient freeze technique (VGF) for the first time. The yield of crack free crystals was greatly improved by selecting seed orientation for directional growth. Especially, the absorption coefficient of the BaGa4Se7 crystal is as low as 0.02 cm−1 at 2.05 µm, which means that the as-grown crystal can be used to fabricate optical elements directly and does not need post-thermal annealing like other growth methods. The as-grown crystals were also characterized by X-ray diffraction (XRD), transmission spectra, and optical homogeneity etc. The BGSe-OPO elements were fabricated for infrared nonlinear optical experiments.

In-situ observation and analysis of solid-state diffusion and liquid migration in a crystal growth system: A segregation-driven diffusion couple

Energy-resolved neutron imaging is employed for in-situ measurements of dopant transport in a simple experiment performed before the crystal growth of the scintillator BaBrCl:5%Eu via a vertical gradient freeze technique. During a stabilization period preceding growth, we observed the diffusion of Eu from the solid phase into the melt over a period of approximately 4 h. Comparing the measured centerline concentration profile with a mathematical model for the system, we estimate the solid-state diffusivity of Eu in BaBrCl as D1=1.9×10−10 m2/s and an upper limit for the liquid-phase diffusivity of Eu in the melt as D2*=2.5×10−10 m2/s, at temperatures near the melting point. We compare this experiment, where diffusion is driven by a concentration discontinuity arising from segregation, to the classical diffusion couple technique. Suggestions are offered on how this segregation-driven couple might be improved as a tool for measuring diffusion coefficients, and we draw attention to the great promise of neutron imaging for in-situ measurements of the distribution of elements, with sufficiently high neutron attenuation coefficients, in difficult environments.

Single crystal growth of ZnxCu1–x V2O7 (x = 0.05, 0.15) by the vertical gradient freezing technique

Single crystals of ZnxCu1−xV2O7 system with doping concentration of x = 0.05 and 0.15 were grown by the vertical gradient freezing technique. The crystal structures were confirmed by means of x-ray diffraction to be the beta phase of copper pyrovanadate, β-Cu2V2O7, when the Zn concentration was as low as 0.05, on the contrary to the previous studies on polycrystal samples. The Rietveld refinements on x-ray diffraction patterns showed that that lattice constants were slightly increased, whereas the angle β was slightly decreased, when the doping concentration was decreased from x = 0.15 to 0.05. θ-2θ scan confirmed that the natural cleaved facet is crystallographic a-axis with FWHM around (200) peak of lower than 0.2°, suggesting a high quality of the obtained single crystals.

Generation of 320 mW at 10.20 μm based on CdSe long-wave infrared crystal

CdSe single crystal, with the sizes of ∼54 mm in diameter and ∼25 mm in length, was grown by a high pressure vertical gradient freeze (HPVGF) technique using (0 0 1)-oriented seed. The CdSe crystal was characterized with transmission spectrophotometer. The transmission spectra showed that the infrared transmission was above 68% and the mean absorption coefficient was 0.041 cm−1 in the range of 2.5–20 μm. Using fabricated CdSe crystal with the dimensions of 6 mm × 10 mm × 44 mm, we demonstrated an optical parametric oscillator (OPO) pumped by a 2.05 μm Ho:YLF laser at a pulse repetition frequency of 5 kHz. Up to 320 mW output was obtained at the idler wavelength of 10.20 μm with a pump power of 18.06 W. 320 mW at 10.20 μm, to our knowledge, was the highest power obtained with a 2.05 μm laser-pumped CdSe OPO.

Growth and characterization of kesterite Cu2ZnSn(SxSe1 − x)4 crystals for photovoltaic applications

Cu2ZnSn(SxSe1−x)4 (CZTSSe) bulk crystals have been grown for the first time, in multi-gram quantities, using a novel vertical gradient freeze (VGF) technique. Large grain polycrystalline CZTSSe ingot with grain sizes ∼0.2–0.5mm was grown in a vacuum sealed quartz ampoule. Structural, compositional, and electronic properties of the ingot were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), and van der Pauw measurements. XRD analysis showed highly crystalline tetragonal structure corresponding to kesterite CZTSSe with lattice constants of a=5.563Å, and c=11.09Å (c/2a=0.997) and Raman spectra confirmed the formation of single phase CZTSSe. High-resolution XPS spectra confirmed the formation of metal-sulfoselenides. The ingot along the growth axis showed a homogeneous near stoichiometric elemental distribution corresponding to Cu/(Zn+Sn)=0.94 and Zn/Sn=0.98. EDS mapping on the ingot's outer surface revealed formation of islanded Cu-rich droplets at the grain boundaries. Grown crystals exhibited p-type conductivity with a measured bulk resistivity of ∼51Ωcm.

Investigation on the growth and characterization of 4-aminobenzophenone single crystal by the vertical dynamic gradient freeze technique

Growth of bulk single crystal of 4-Aminobenzophenone (4-ABP) from the vertical dynamic gradient freeze (VDGF) setup designed with eight zone furnace was investigated. The experimental parameters for the growth of 4-ABP single crystal with respect to the design of VDGF setup are discussed. The eight zones were used to generate multiple temperature gradients over the furnace, and video imaging system helped to capture the real time growth and solid–liquid interface. 4-ABP single crystal with the size of 18mm diameter and 40mm length was grown from this investigation. Structural and optical quality of grown crystal was examined by high resolution X-ray diffraction and UV–visible spectral analysis, respectively and the blue emission was also confirmed from the photoluminescence spectrum. Microhardness number of the crystal was estimated at different loads using Vicker's microhardness tester. The size and quality of single crystal grown from the present investigation are compared with the vertical Bridgman grown 4-ABP.

Cu2ZnSnSe4 Photovoltaic Absorber Grown by Vertical Gradient Freeze Technique

High quality large grain single phase Cu2ZnSnSe4 (CZTSe) photovoltaic absorber material was grown by vertical gradient freeze (VGF) technique for the first time. Polycrystalline CZTSe ingot was grown in a vacuum sealed quartz ampoule inside a modified three-zone vertical Bridgman furnace employing a directional cooling. Structural and compositional analyses of the grown crystals were performed by X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The grown crystals exhibited highly crystalline tetragonal structure corresponding to kesterite Cu2ZnSnSe4 with lattice parameters of a = 5.696 Å and c = 11.338 Å as evidenced from XRD pattern. Raman spectra showed three characteristic peaks at 171.5, 194.6, and 231.1 cm-1 attributed to kesterite phase CZTSe. No other secondary phases were detected in the grown crystals. Thermoelectric probe measurements showed p-type conductivity of the grown crystals and energy dispersive X-ray spectroscopy (EDS) along the crystal growth direction showed uniform and stoichiometric elemental distribution. Our results show that VGF technique can be used to grow high quality kesterite compounds for photovoltaic application.

Vertical gradient freeze growth of ZnGeP2 crystals for nonlinear optical applications

ZnGeP2 crystal is the leader among nonlinear crystals for middle IR. Large sized Φ20×90mm3 ZGP single crystals have been grown successfully by the vertical gradient freeze technique. The crystallinity and optical properties of as-grown crystals were measured by XRD and transmission spectra respectively. The thermal conductivity was tested using PPMS. The absorption coefficient can be reduced to 0.08cm−1 at 2μm by post-annealing. The fabricated devices were also utilized to demonstrate SHG and OPO experiments for nonlinear optical applications.

Influence of carbon coated pBN crucible on crystal growth of Cd0.9Zn0.1Te for radiation detector applications

In the Bridgman and Vertical Gradient Freeze (VGF) techniques, it is common to use a high purity crucible for crystal growth. One disadvantage of pBN crucibles is associated with the crucible material interacting with the melt during the growth cycle. Using pBN crucibles for the growth of CZT, we have observed that the crucible does in fact interact with the CZT melt. To prevent this interaction, a vacuum carbon coating system for applying a carbon coating to pBN crucibles has been developed. The effects of crucible use with time are studied, as are the compositional and device properties of the bulk material. GDMS measurements are used to investigate on broron and nitrogen content, among other trace impurities found in the crystal. Devices harvested from ingots grown using cc-pBN exhibit high resistivity and good functionality as room temperature gamma ray detectors.

Scintillation kinetics and thermoluminescence of SrI2:Eu2+ single crystals

Single crystal of strontium iodide doped with 1% europium (SrI2:1% Eu2+) was grown by Vertical Gradient Freeze technique. UV excited emission spectra were studied as a function of temperature. Results indicate the thermal quenching of Eu2+ emission starts from ∼400K with a thermal activation energy of 0.39eV. Gamma and UV excited decay measurements indicate that the scintillation decay time of SrI2:Eu2+ is longer than the lifetime of Eu2+ luminescence center in the SrI2 host. The thermoluminescence glow curve revealed a highly concentrated charge carrier trap at 50K. Elimination of this trap is expected to enhance the energy migration of charge carriers and result in faster scintillation decay.

Vertical gradient freeze of 4 inch Ge crystals in a heater-magnet module

For the first time 4-in. Ge single crystals were grown using the vertical gradient freeze technique (VGF) in a traveling magnetic field (TMF) generated in a heater-magnet module (HMM). The HMM was placed closely around the growth container inside the chamber of the industrial Bridgman equipment “Kronos”. The HMM generates heat and a TMF together. It has a coil-shaped design and replaces the standard meander-type heater. Direct current (DC) for heat production and out-of-phase-accelerated currents (AC) for TMF generation were simultaneously delivered to three equally spaced coil segments connected by star-type wiring. In order to achieve a nearly flat and slightly convex growing interface the AC amplitude, frequency and phase shift have been optimized numerically by using the 3D CrysMAS code and validated by striation analysis on as-grown crystals. Low-field frequencies in the range f=20–50 Hz proved to be of most suitable condition. TMF programming is required to obtain constant interface morphology over the whole growth run. First Ge single crystals grown under nearly optimal conditions show reduced macro- and micro-inhomogeneities, relatively low dislocation density of (3–10)×10 2 cm −2, and high carrier mobility of μ p=2800 cm 2 V −1 s −1.

Radial segregation in VGF-RMF grown germanium

In this paper experimental results of the radial dopant segregation in Ge:Ga single crystals grown by the vertical gradient freeze technique with a rotating magnetic field are presented. The segregation is analysed on the basis of the carrier concentration measured by means of Hall effect and Fourier transform infrared (FTIR) spectroscopy. In growth without the field the carrier concentration increases towards the axis, whereas much more uniform radial concentration profiles are found in crystals grown under the influence of the rotating field indicating a pronounced impact of the melt flow on the dopant segregation. Apparently, the accumulation of the Ga solute near the centre of the melt during growth under natural buoyancy is reduced by the electromagnetically induced flow. This phenomenon is discussed with respect to analytical and numerical results published recently.

Deep level defects in CdTe materials studied by thermoelectric effect spectroscopy and photo-induced current transient spectroscopy

The deep levels in semi-insulating CdTe crystals grown by vertical gradient freezing technique were studied using thermoelectric effect spectroscopy (TEES) and photo-induced current transient spectroscopy (PICTS). The measurement of TEES spectra in the temperature range 90–400 K was performed at different heating rates. The positions of the levels and their capture cross-sections were obtained by using the heating rate method. It was found that near midgap levels in samples doped with shallow donors (Cl, In) have a low value of capture cross section (≈10−15 − 10−17 cm2) and are hole traps. Samples doped with a deep donor (Ge, Sn) have a much higher capture cross section of the midgap level (≈10−13 cm2), which acts as an electron trap. Comparison of the TEES results with the PICTS method has shown that while the evaluated values of ionization energies are comparable using both methods, the PICTS technique gives systematically approximately two orders of magnitude higher capture cross sections than TEES.

LEC- and VGF-growth of SI GaAs single crystals—recent developments and current issues

The paper reviews the progress made in crystal growth of semi-insulating GaAs by liquid encapsulation Czochralski and vertical gradient freeze techniques during the last few years under the continuous need for cost reduction of the production process.

Bridgman growth of paratellurite single crystals

The growth of paratellurite single crystals by the vertical-gradient freezing technique is reported for the first time. Boules of 120mm long and 25mm in diameter were obtained under a temperature gradient of 10°Ccm−1 and translation rates lower than 0.6mmh−1. The spatial distribution of defects along the growth axis reveals a continuous evolution of the free convective fluid-flow regime as growth proceeds. Gas bubbles and dark inclusions rejected to the periphery in the upper part of the crystal are observed to lay preferentially in (100), (001), (11¯0) and (112) crystallographic planes. Among them, SEM and microprobe analyses evidenced the presence of metallic platinum, while micro-Raman experiments allowed to assess oxygen as being actually the gas content of occluded bubbles, originated at the crucible wall through Pt-induced dissociation of Te–O bonds.

New Developments in Vertical Gradient Freeze Growth

The Vertical Gradient Freeze (VGF) technique is an important method for growing high quality compound semiconductors such as GaAs. Results obtained with a novel VGF set-up developed for the growth under influence of a rotating magnetic field (RMF) and under vapour pressure control are presented in this paper. The RMF is shown to be a powerful tool to affect the heat and mass transport within the melt in a definite way. In GaAs:Si growth, RMF induced flow results in a decreased curvature of a nominally concave-shaped interface, i.e., it contributes to an axial heat transfer at the solid-liquid interface. The axial dopant segregation of Ga in Ge is found to be improved under continuous RMF action due to better mixing of the melt. The set-up also allowed to determine the influence of carbon and the arsenic vapour pressure on the dopant incorporation and crystal quality.