Traveling Heater Method Research Articles

Effects of the Mushy Zone on the Temperature Field and the Flow Field During GaInSb Crystal Growth with the Traveling Heater Method

Effects of the Mushy Zone on the Temperature Field and the Flow Field During GaInSb Crystal Growth with the Traveling Heater Method

Effect of a traveling magnetic field on cadmium zinc telluride growth by the traveling heater method

The growth of high-quality cadmium zinc telluride (CZT) crystals using the traveling heater method (THM) is strongly influenced by the morphology of growth interface. Here, we investigate the effect of a traveling magnetic field (TMF) on the growth of CZT single crystals by the THM using the multi-physics numerical simulation method. When no magnetic field is applied, the concave morphology of the growth interface caused by the gravity-driven natural convection is confirmed using a multi-component mass transfer model based on the Maxwell-Stefan equations. In contrast, the application of an upward TMF accelerates the flow near the crucible wall and hence the lateral flow along the growth interface, finally leading to a relatively flat growth interface. Conversely, the increase of a downward TMF intensity significantly weakens natural convection in the central melt region; particularly, when the magnetic induction strength is increased to 300 mT at a frequency of 50 Hz, the desired convex-shaped growth interface can be achieved with the appearance of strong vortices at the dissolution interface. Therefore, we conclude that both the upward and downward TMF can improve the growth interface and thereby facilitates the growth of high-quality single crystals.

Development of travelling heater method for growth of detector grade CdZnTe single crystals

Development of travelling heater method for growth of detector grade CdZnTe single crystals

Defect structure of high-resistance CdTe:Cl single crystals and MoOx/CdTe:Cl/MoOx heterostructures according to the data of high-resolution X-ray diffractometry

Clorine doped CdTe single crystals (CdTe:Cl) were grown by the traveling heater method. MoO x /CdTe:Cl/MoO x films were deposited using the reactive magnetron sputtering technique. The defect structure of the obtained single crystals and heterostructures was investigated using high-resolution X-ray diffractometry. The optimized models of dislocation systems in the CdTe:Cl single crystals were constructed based on the Thompson tetrahedron. The distribution of the intensity of diffracted X-rays as a function of reciprocal space coordinates and rocking curves was analyzed using the kinematic theory of X-ray scattering in real crystals. The experimental and theoretically predicted values of the helical dislocation densities in the CdTe:Cl and MoO x /CdTe:Cl crystals with perfect and mosaic structures were compared. Two-fold increase in the dislocation concentration in the MoO x /CdTe:Cl heterostructures as a result of compression deformations of the CdTe:Cl crystal lattice was found. The ~0.1 μm thick transition deformed layer at the boundary between the MoO x film and CdTe:Cl single crystal significantly affects the electrical and spectroscopic properties of the obtained systems as the materials for γ-radiation detection.

Assessment of deep levels with selenium concentration in Cd1–xZnxTe1–ySey room temperature detector materials

Incorporation of Se into Cd1−xZnxTe (CZT) to form the quaternary compound semiconductor Cd1−xZnxTe1–ySey (CZTS) has proven to be an effective solution for compensating the major flaws associated with CZT, including poor homogeneity and high concentrations of electronically active deep levels that limit the performance of CZT detectors. In order to investigate how deep levels are affected by the Se concentration in CZTS, we performed photoinduced current transient spectroscopy (PICTS) measurements on CZTS crystals grown by the traveling heater method (THM) with 10% atomic Zn and varying atomic percentage of Se from 1.5% to 7.0%. The PICTS scans for up to 4% Se showed an exponential reduction in the capture cross section of deep levels associated with Te secondary phases in conjunction with an increase in a deep level positioned near the mid-gap, which initially increases the electron trapping time before degrading again at higher Se concentrations. The PICTS peaks present in 7% Se were anomalous relative to the other crystals and are expected to originate from transition metal impurities found in the lower-purity CdSe precursor material.

Growth interface study of CdTeSe crystals grown by the THM technique

The Traveling Heater Method (THM) is a widely accepted technique to grow high-quality detector grade CdTe and CdZnTe ingots, especially for X-ray and gamma-ray detector applications. Unlike melt-growth techniques, the growth interface for ingots produced by the THM technique consists of two different compounds, viz. the solidified region (just below the interface) consists of the compound to be grown (e.g., CdZnTe for CdZnTe growth), while the molten zone just above the growth interface consists of Te-rich CdZnTe. Thus, optimization of the growth parameters is critical to obtain a clean growth interface with the required shape and presence of minimal Te-rich inclusions. In this study CdTeSe ingots were grown employing the THM technique to investigate the growth interface. Both macroscopic and microscopic behavior of the interface were studied. The study revealed that a microscopically smooth growth interface can be achieved by optimizing the growth parameters, which is essential for obtaining high-quality, inclusion-free ingots.

Growth and characterization of Fe2+:ZnSe single crystals for tunable mid-infrared lasers

Iron-doped ZnSe (Fe:ZnSe) single crystals were grown by the traveling heater method with the accelerated crucible rotation technique (ACRT) using SnSe as a solvent. The as-grown Fe:ZnSe single crystals were characterized by XRD, Raman, XPS, IR transmission imaging microscope, and the X-ray rocking curve. The results show two kinds of inclusions existing in Fe:ZnSe crystals, i.e. solvent SnSe inclusions, and hexagonal ZnSe inclusions with a wurtzite structure. The inclusions with sizes ranging from 30 to 150 μm were homogeneous distributed with a total concentration of only 2.68 × 102 cm−2. The full width at half-maximum (FWHM) of X-ray rocking curve on (220) diffraction planes was 38 arcsec and the average infrared transmittance exceeded 70%. Furthermore, it was confirmed that the iron ions were successfully incorporated into ZnSe lattice without degrading the crystalline quality, and the bivalence was dominant for iron atoms in Fe:ZnSe crystals, meaning that they are suitable for developing mid-infrared (3–5 μm) gain medium applications.

Luminescence Properties of Fe2+:ZnSe Single Crystals Grown via a Traveling Heater Method

The luminescence properties of iron-doped ZnSe (Fe2+:ZnSe) single crystals grown via a traveling heater method have been studied via photoluminescence (PL). Nine emission bands were identified in the PL spectra of Fe2+:ZnSe single crystals and their origins were also discussed. The near-infrared emission bands seen at 820 nm and 978 nm can be attributed to the emission bands formed by the background Fe or other impurity-related defect complexes in Fe2+:ZnSe single crystals, rather than by doped transition-metal-related defects. With the increase in temperature, the PL intensity increased slightly and reached a maximum near room temperature for bound excitons (430–490 nm), but the PL intensity decreased significantly for impurity-defect emission bands (500–720 nm), indicating the occurrence of a thermal quenching effect. The excitation wavelength-dependent PL spectra showed that PL intensity first increased and then decreased with an increase in the excitation wavelengths, and the maximum PL intensity of the bound excitons was obtained at 364 nm. In addition, the X-ray photoelectron spectroscopy (XPS) results showed that both bivalent and trivalent iron ions were found, but bivalence was the dominant charge state for iron atoms in the iron-doped ZnSe single crystals, meaning that they are suitable for developing mid-infrared gain medium applications.

Effect of rotating magnetic field intensity on structure and electrical and optical properties of GaInSb crystal grown with travelling heater method

Ga1−x In x Sb(0 < x < 1) crystal is a very promising substrate material, which can be used to fabricate a variety of high-performance infrared detectors and lasers by epitaxial growth. Herein, we prepared high quality Ga0.86In0.14Sb crystal (25 mm diameter, 100 mm length) with travelling heater method applied rotating magnetic field (RMF), and revealed the effects of RMF intensity on structure and electrical and optical properties of the crystal. As the RMF intensity increased the crystal quality significantly improved, and the radial segregation of indium decreased from 0.258 to 0.031 mol% mm−1, in the center of the crystal. Similarly, the segregation of indium along the ingot between 20 to 80 mm, toward the traveling zone, decreased from 0.114 to 0.038 mol% mm−1. The dislocation density of GaInSb crystal ingot also decreased from 5.361 × 105 cm−2 to 5.295 × 103 cm−2. The electric properties of the crystal also improved, the carrier mobility increased to 1.902 × 103 from 1.358 × 103cm2 (V·s)−1, the resistivity decreased to 1.047 × 10−3 from 1.822 × 10−3 Ω·cm. Moreover, the infrared transmittance increased from 36% to 39%.

Performance Study of Virtual Frisch Grid CdZnTeSe Detectors

Nuclear detectors for x-ray and gamma-ray spectroscopy and imaging are a vital tool in many homeland security, medical imaging, astrophysics and other applications. Most of these applications require room-temperature operation due to the operational constraints imposed by a cryogenic cooling system. CdZnTe (CZT) has been the main material with the desired detection properties, and CZT crystals have been used commercially for three decades. However, CdZnTe still suffers from long-standing issues of high densities of performance-limiting intrinsic defects such as Te inclusions and networks of dislocation walls (sub-grain boundaries). A recently invented new quaternary material CdZnTeSe showed excellent material properties for radiation detection. The material was found to be free from dislocation networks, possess reduced Te inclusions, and have better compositional homogeneity. Virtual Frisch grid detectors were fabricated from crystals taken from a CdZnTeSe ingot that was grown by the traveling heater method. The detectors were fabricated from an as-grown ingot, bypassing the post-growth annealing process commonly practiced for industrial-grade CZT. The performances of the detectors were studied with different Frisch grid lengths using an amplifier shaping time ranging from 1–6 µs. The detectors showed high-quality spectroscopic performance with an as-measured energy resolution of ~1.1% at 662 keV for an optimum Frisch grid length of 3 mm. The charge collection was observed to enhance for longer Frisch grids.

Vertical gradient freeze growth of detector grade CdZnTeSe single crystals

We report the growth of detector grade Cd0.9Zn0.1Te0.97Se0.03 (CZTS) single crystals, a recently discovered quaternary semiconductor for room temperature radiation detection, by a vertical gradient freeze (VGF) method. VGF is a comparatively low-temperature growth method and avoids relative motion between the heater and the ampoule containing the precursor materials which minimizes any thermal drift or temperature fluctuations. As a result, CZTS single crystals with superior charge transport properties has been obtained. Growth of detector-grade CZTS single crystals using VGF method has not been reported yet. X-ray spectroscopy based elemental analysis showed that the grown crystals demonstrated the desired stoichiometry required for high resolution radiation detection. Planar detectors fabricated by deposition of gold contacts (∼0.07 cm2) demonstrated high bulk resistivity ∼1010 Ω-cm and a very low leakage current density of 2.8 × 10−8 A/cm2 at a bias of 100 V when measured at room temperature. The detectors showed excellent radiation response with 100 % charge collection efficiency when exposed to 5486 keV alpha particles. The electron mobility-lifetime (μτ) product was measured to be 3 × 10−3 cm2/V using a single polarity Hecht analysis which is at par with the recently reported values measured in CZTS grown using conventional Bridgman or travelling heater method. The electron mobility has been calculated to be 964 cm2V−1s−1 using a time-of-flight (TOF) method, a substantial improvement over that obtained from conventionally grown CZTS single crystals.

Evaluation of crystalline quality of traveling heater method (THM) grown Cd0.9Zn0.1Te0.98Se0.02 crystals

Because of its excellent opto-electronic properties, CdZnTe (CZT) has been the material of choice for x- and gamma-ray detectors operable at room temperature. CZT is the leading commercially available room-temperature radiation detector material today. Although much progress has been made over the past three decades, today's CZT crystals still face certain challenges, especially the presence of the performance-limiting materials defects and the associated relatively high production cost. In this regard, CdxZn1−xTeySe1−y (CZTS) is emerging as a next-generation compound semiconductor, which overcomes some of the limitations of CZT technology for the stated applications. Here, we conducted a study to evaluate the crystalline quality of the traveling heater method grown CZTS with an optimized alloy composition, i.e., Cd0.9Zn0.1Te0.98Se0.02. The as-grown samples were evaluated by low-temperature photoluminescence (PL) spectroscopy and high-resolution x-ray diffraction using the synchrotron light source at Brookhaven National Laboratory. The full width at half maximum of both the PL and x-ray rocking curves was observed to be broadened due to the lattice disorder of the quaternary compound, eventually degrading the crystalline quality. This was consistent with density functional theory calculations.

Numerical Simulation of Solid–Liquid Interface of GaInSb Crystal Growth with Travelling Heater Method

The heat transfer and liquid phase convection during GaInSb crystal growth via the traveling heater method (with a seed) were investigated using numerical simulation to optimize the process parameters and shorten the experimental period in order to produce a high-quality crystal widely used to make various optoelectronic devices. There will be a phenomenon of “thermal impermeability” with an increase in crystal radii for the same furnace temperature profile. The maximum furnace temperature should display an increase of at least 1030 K to 1060 K in order to ensure the successful introduction of the seed with an increase of the crystal radius from 0.01 m to 0.03 m. The interface bending of the solid–liquid interface significantly increases with an increase of the crystal radius from 0.01 m to 0.02 m by about 50%, 67%, and 140%, corresponding to the maximum furnace temperatures 1030 K, 1040 K, and 1050 K, respectively. However, it decreases significantly when the maximum temperature increases from 1030 K to 1050 K, from 0.16 to 0.05, 0.2 to 0.105, and 0.24 to 0.12, corresponding to the crystal radii 0.01 m, 0.015 m, and 0.02 m, respectively. The maximum flow velocity of melt increases slightly with the furnace maximum temperature for the same radius, less than about 6%. However, it increases significantly with the increase of the radius from 0.01 m to 0.02 m, more than 68%.

The crystal growth of ZnSe by the traveling heater method with the accelerated crucible rotation technique

The accelerated crucible rotation technique (ACRT) was employed in the traveling heater method (THM) growth of ZnSe crystals to manage the convection in the melt. Considering the large solubility, the PbCl2 was chosen as a solvent. During the growth, the growth interface changed from concave to slightly convex depending on the ACRT parameters. The ZnSe single crystals were obtained, when the ACRT with the maximum rotation of 15 rpm at the suitable growth rate (2.4 mm/day) was used. Furthermore, the chemical mappings of the energy-dispersive X-ray spectroscopy (EDS) show that there are two kinds of the solvent inclusions existed in the grown ZnSe crystals, i.e. PbCl2 and ZnCl2. And the PbCl2 inclusions are homogeneous with a total concentration of 7.03 × 102 cm−2. In addition, the FWHM on X-ray rocking curve of the (220) diffraction planes is 22 arcsec. The chemical etch pit density on the (110) cleavage plane is (1–2) × 105 cm−2 and the average infrared transmittance is over 68% in the range from 650 cm−1 to 4000 cm−1. All of these indicate that the grown ZnSe single crystals have good crystalline quality.

Interface stability problem towards the end of THM growth of cadmium zinc telluride crystals

Growth of CdZnTe crystals by Travelling Heater Method (THM) is widely investigated by different groups. Interface shape and stability are critical parameters for the successful growth of high-quality CZT crystals. Even though interface stability could be achieved for a significant portion of the CZT ingot, the situation towards the end of the growth is not well studied and understood. In this study, interface stability and the change of crystal quality towards the end of the growth are examined. The change of the interface shape and formation of large Te inclusions are observed. Moreover, towards the end of the growth, phase separation and formation of a new zinc-based phase are detected with a needle-like structure. The decrease of crystal quality and evolution of the CZT ingot towards the end of the growth are summarized.

Solution growth of chalcopyrite Cu(In1−x Ga x )Se2 single crystals for high open-circuit voltage photovoltaic device

Abstract I–III–VI2 Chalcopyrite Cu(In1−x Ga x )Se2 (CIGS) has attracted attention as absorbing layer in photovoltaic (PV) device. In this study, we investigated the fundamental properties of CIGS single crystals, and fabricated single crystal-based PV device. CIGS single crystals without secondary phase were successfully grown by In-solvent traveling heater method (THM). The conversion of conduction type from n- to p-type can be observed above 0.3 of Ga ratio x because of high acceptor defect concentration. PV device based on high-quality CIGS bulk single crystal demonstrates high open-circuit voltage of 0.765 V with the efficiency of 12.6%.

Dislocation clustering and grain nucleation in THM growth of CdTe single crystals

We report a volume defect behavior in CdTe crystals grown by the travelling heater method (THM), involving dislocation clustering and grain nucleation. Spurious grain nucleation events occur within a specific radial zone in the crystal where the growth interface is planar. These grains were characterized by Laue x-ray diffraction and were found to have the same angle of misorientation to the bulk, with exception of an in-plane rotation. The grains were connected to columnar dislocation clusters that were aligned parallel to the growth axis of the crystal and were evidently linked to the planar region of the growth interface. Simulation of the growth process was used to investigate the effects of solvent zone convection and interface curvature with different rotational rates. The observed defect behavior was attributed to the effect of melt convection on the interface shape and instability. Evidence of faceting on the growth interface and its correlation with spurious grain nucleation is shown. These findings highlight the importance of interface engineering in THM growth of high quality CdTe single crystals.

Stability-based optimization of ACRT for the growth of CZT by the traveling heater method

Numerical modeling is employed to study the effects of several accelerated crucible rotation technique (ACRT) rotation schedules on the growth of cadmium zinc telluride (CZT) by the traveling heater method (THM). In conjunction with these analyses, a measure is developed to estimate the amplitude of disturbances associated with cellular interface growth morphologies that develop over time and across the melt-solid interface. This disturbance amplitude measure is used to assess the ability of ACRT to stabilize the growth interface and thereby minimize inclusion formation processes. Notably, our analyses reveal that interfacial instability is not well correlated with melt undercooling, explaining why classical ACRT approaches to reduce undercooling by mixing may not stabilize growth. Computations show that interface stability is best achieved via rotation schedules that accentuate outward Ekman flows during spin-up and, during spin-down, eliminate Taylor-Görtler flows and minimize inward Ekman flows.

Characterization of Structural Defects in (Cd,Zn)Te Crystals Grown by the Travelling Heater Method

Structural defects and compositional uniformity remain the major problems affecting the performance of (Cd, Zn)Te (CZT) based detector devices. Understanding the mechanism of growth and defect formation is therefore fundamental to improving the crystal quality. In this frame, space experiments for the growth of CZT by the Travelling Heater Method (THM) under microgravity are scheduled. A detailed ground-based program was performed to determine experimental parameters and three CZT crystals were grown by the THM. The structural defects, compositional homogeneity and resistivity of these ground-based crystals were investigated. A ZnTe content variation was observed at the growth interface and a high degree of stress associated with extensive dislocation networks was induced, which propagated into the grown crystal region according to the birefringence and X-ray White Beam Topography (XWBT) results. By adjusting the growth parameters, the ZnTe variations and the resulting stress were efficiently reduced. In addition, it was revealed that large inclusions and grain boundaries can generate a high degree of stress, leading to the formation of dislocation slip bands and subgrain boundaries. The dominant defects, including grain boundaries, dislocation networks and cracks in the interior of crystals, led to the resistivity variation in the crystals. The bulk resistivity of the as-grown crystals ranged from 109 Ωcm to 1010 Ωcm.

The effects of ACRT on melt undercooling during the growth of CZT via the traveling heater method: Ekman versus Taylor-Görtler flows

For the first time, a comprehensive model for the traveling heater method, including detailed species transport and phase-change thermodynamics, is applied to understand the effects of accelerated crucible rotation (ACRT) on the growth of cadmium zinc telluride. A rotation schedule that is designed from classical considerations does not completely mix the melt, rather undercooled regions of localized melt are swept periodically across the growth interface. Even without melt homogenization, these ACRT-driven flows disrupt the persistent undercooling that occurs during THM growth without rotation. An ACRT schedule designed to promote Ekman flows results in lower levels of melt undercooling than a schedule designed to promote Taylor-Görtler flows and greater mixing.