Pyrolytic Boron Nitride Crucibles Research Articles

Growth of 2-inch diameter Mg<sub>2</sub>Si crystal by the VGF method under Ar normal pressure

A 2-inch diameter Mg2Si crystal was synthesized via the vertical gradient freeze (VGF) method in an open-tube system using pyrolytic boron nitride crucibles coated with boron nitride. Using this open-tube system, Mg evaporation was significantly prevented under Ar gas flow. Many grain boundaries were observed at the crystal cross section. A 5-mm square wafer without grain boundaries was cut out from the crystal and characterized by X-ray rocking curve (XRC) analysis, exhibiting a sharp single peak with a full width at half maximum (FWHM) of 50.4 arcsec. This indicates good crystallinity of the single crystalline area of the synthesized crystal.

Single crystal growth of small-angle-grain-boundary-free Mg2Si via vertical Bridgman method

Mg2Si single crystal was grown via vertical Bridgman method using pyrolytic boron nitride (pBN) crucibles coated with boron nitride (BN). No stress was imposed from the liquid/ solid interface (molten Mg2Si/ BN-coated crucible surface) during the solidification leading to a small angle grain boundary free crystal. As a result of X-ray rocking curve measurement of the cut wafer, in-plane high crystallinity was confirmed and a sharp single peak with FWHM of 44–51 arcsec was measured. The typical electron concentration of grown crystal was 6.6 × 1017 cm−3 and Hall mobility was 280 cm2/Vs. The high electron concentration was attributed to the contamination of crystal by boron from BN coating source, confirmed by glow discharge mass spectroscopic (GDMS) analysis.

Centimetric CrSi2 crystal grown by the vertical gradient Freeze method

CrSi2 ingots were grown by the Vertical Gradient Freeze (VGF) method in silica (SiO2) crucibles with boron nitride (BN) coating and in pyrolytic boron nitride (pBN) crucibles in order to minimize the sticking and the reaction with the molten Cr-Si alloy. High quality chromium disilicide single crystal was obtained with a mosaicity of 1–2° using pBN crucible with a thermal gradient of 0.6 K/mm and a growth rate of 10 mm/h. To recycle the crucibles and so to reduce the production costs, pBN crucibles were cleaned with hydrofluoric acid solution. As a result of this treatment, it was not possible to obtain CrSi2 single crystals due to a modification of the crucible surface and the subsequent increase of boron compounds in the melt during the growth. The effect of the growth conditions on the microstructure of polycrystalline sample was also investigated using the texture analysis technique.

Nanopipe formation as a result of boron impurity segregation in gallium nitride grown by halogen-free vapor phase epitaxy

The work reported herein demonstrated that nanopipes can be formed via a surfactant effect, in which boron impurities preferentially migrate to semipolar and nonpolar facets. Approximately 3 μm-thick GaN layers were grown using halogen-free vapor phase epitaxy. All layers grown in pyrolytic boron nitride (pBN) crucibles were found to contain a high density of nanopipes in the range of 1010 to 1011 cm−2. The structural properties of these nanopipes were analyzed by X-ray rocking curve measurements, transmission electron microscopy, and three-dimensional atom probe (3DAP) tomography. The resulting 3DAP maps showed nanopipe-sized regions of boron segregation, and these nanopipes were not associated with the presence of dislocations. A mechanism for nanopipe formation was developed based on the role of boron as a surfactant and considering energy minima. A drastic reduction in the nanopipe density was achieved upon replacing the pBN crucibles with tantalum carbide-coated carbon crucibles. Consequently, we have confirmed that nanopipes can be formed solely due to surface energy changes induced by boron impurity surface segregation. For this reason, these results also indicate that nanopipes should be formed by other surfactant impurities such as Mg and Si.

Impact of Corrosion Test Container Material in Molten Fluorides

The effects of crucible material choice on alloy corrosion rates in immersion tests in molten LiF–NaF–KF (46.5–11.5-42 mol. %) salt held at 850 °C for 500 hrs are described. Four crucible materials were studied. Molten salt exposures of Incoloy-800H in graphite, Ni, Incoloy-800H, and pyrolytic boron nitride (PyBN) crucibles all led to weight-loss in the Incoloy-800H coupons. Alloy weight loss was ∼30 times higher in the graphite and Ni crucibles in comparison to the Incoloy-800H and PyBN crucibles. It is hypothesized galvanic coupling between the alloy coupons and crucible materials contributed to the higher corrosion rates. Alloy salt immersion in graphite and Ni crucibles had similar weight-loss hypothesized to occur due to the rate limiting out diffusion of Cr in the alloys to the surface where it reacts with and dissolves into the molten salt, followed by the reduction of Cr from solution at the molten salt and graphite/Ni interfaces. Both the graphite and the Ni crucibles provided sinks for the Cr, in the formation of a Ni–Cr alloy in the case of the Ni crucible, and Cr carbide in the case of the graphite crucible.

On crucible effects during the growth of cadmium zinc telluride in an electrodynamic gradient freeze furnace

The CrysMAS code of the Crystal Growth Laboratory, Fraunhofer IISB, is applied to reveal conditions occurring in electrodynamic gradient freeze furnaces during the growth of cadmium zinc telluride crystals. Of particular interest are heat transfer and growth conditions associated with crucibles of different design, one constructed of graphite and the other of pyrolytic boron nitride (PBN). Under identical furnace set-point schedules, the two systems exhibit very different behaviors. Specifically, the temperature field through the cone region of the PBN crucible displays much steeper axial thermal profiles and promotes convex solid–liquid interface shapes (rather than the concave shapes computed for the graphite crucible). Both systems exhibit a concave interface during growth through the cylindrical part of the crucible. However, the axial thermal profile through the graphite-crucible charge is considerably more offset from the set-point profile of the furnace due to significant axial heat flows through the crucible walls. These factors argue in favor of the PBN crucible; however, comparatively larger radial gradients in the PBN system could lead to higher dislocation levels.

Production of multicharged iron ions by using pyrolytic boron nitride crucible and application to material processing

Multiply charged ions of iron are produced from solid material in a 2.45 GHz electron cyclotron resonance (ECR) ion source by using a crucible made from pyrolytic boron–nitride (pBN) with several shields suppressing radiation. The evaporator is set on the geometrical axis. The multicharged ions are extracted from the opposite side of mirror end against the evaporator. Extraction voltage is normally 10 kV. The optimum conditions for production of multicharged iron ions are investigated experimentally. The ion beams can be provided to a newly constructed beam line for the ion irradiation of the substrate installed on the beam line. Multicharged iron ion beams have been utilized to form iron silicides and to enhance light catalytic performance of titanium–dioxide (TiO2) thin films. Formation of iron disilicide (β-FeSi2) has been identified, as well as enhancement of photocatalytic performance of the TiO2 thin films in the visible light region without degradation in the UV light region.

Semi-insulating GaAs grown in outer space

A semi-insulating GaAs single crystal ingot was grown in a recoverable satellite, within a specially designed pyrolytic boron nitride crucible, in a power-traveling furnace under microgravity. The characteristics of a compound semiconductor single crystal depends fundamentally on its stoichiometry, i.e. the ration of two types of atoms in the crystal. A practical technique for nondestructive and quantitative measuring stoichiometry in GaAs single crystal was used to analyze the space-grown GaAs single crystal. The distribution of stoichiometry in a GaAs wafer was measured for the first time. The electrical, optical and structural properties of the space-grown GaAs crystal were studied systematically. Device fabricating experiments prove that the quality of field effect transistors fabricated from direct ion-implantation in semi-insulating GaAs wafers has a close correlation with the crystal’s stoichiometry.

CdZnTe substrate producibility and its impact on IRFPA yield

The need for cost effective production of HgCdTe infrared detectors and focal plane assemblies has led to increased attention to the availability of high quality large-area CdZnTe substrates. Reasonable yield of large-area substrates (≥4 cm×6 cm format) is necessary for fabrication of focal plane assemblies (FPAs) now in production, and for future infrared (IR) detectors which are growing in size and complexity. Raytheon’s infrared materials producibility (IRMP) program has addressed this issue, after identifying critical drivers of FPA yield coming from substrates, and targeted certain improvements in substrate process steps for highest impact on large-area substrate yield. Three specific areas of improvements in the substrate process were addressed: (1) compounding of a large 6 kg charge of CdTe; (2) vertical Bridgman growth of 92 mm diameter CdZnTe boules in both quartz and pyrolytic boron nitride (PBN) crucibles; and (3) optimized Cd overpressure control during growth and cool-down of the boule. It was shown that the Cd overpressure and the cooling schedule had the strongest effects on defect populations. The resulting improvements include a 33% increase in wafer yield per unit starting weight, an estimated 50% reduction in substrate cost per cm2, better morphology of epitaxial HgCdTe layers, and improved yield of satisfactory IR detectors. The criteria for selecting substrates have also improved as a result of this work. In addition, photovoltaic detectors were fabricated on wafers from a variety of sources, and tested. Results compare favorably with those on baseline (earlier process) substrates.

Improvements in production of CdZnTe crystals grown by the Bridgman method

Vertical Bridgman growth of CdZnTe using in situ compounding, Cd vapor-pressure control and pyrolytic boron nitride crucibles yields crystals having low Cu impurity concentration, low dislocation density, low precipitate density and high infrared transmission. Cu often is undetectable by glow discharge mass spectroscopy near the first-to-freeze end of the boule and averages 38 ppba near the last-to-freeze end. Dislocation density, precipitate density and transmission requirements are easily met for applications as infrared detector substrates.

Control of Defects and Impurities in Production of CdZnTe Crystals by the Bridgman Method

AbstractCadmium zinc telluride crystals were grown by vertical Bridgman processes using in situ compdunding from high purity elements into pyrolytic boron nitride crucibles within sealed fused quartz ampoules containing cadmium vapor at a pressure of roughly one atmosphere. These conditions produce material having the low etch pit density, low precipitate density, high infrared transmission and high purity required for use as substrates for infrared focal plane detector arrays fabricated in epitaxial mercury cadmium telluride. Similar processes should be satisfactory for producing cadmium zinc telluride for gamma ray detectors.

Control of Defects and Impurities in Production of CdZnTe Crystals by the Bridgman Method

AbstractCadmium zinc telluride crystals were grown by vertical Bridgman processes using in situ compounding from high purity elements into pyrolytic boron nitride crucibles within sealed fused quartz ampoules containing cadmium vapor at a pressure of roughly one atmosphere. These conditions produce material having the low etch pit density, low precipitate density, high infrared transmission and high purity required for use as substrates for infrared focal plane detector arrays fabricated in epitaxial mercury cadmium telluride. Similar processes should be satisfactory for producing cadmium zinc telluride for gamma ray detectors.

Influence of pyrolytic boron nitride crucibles on GaAs crystal growth process and crystal properties

The influence of pyrolytic boron nitride (pBN) crucibles on the GaAs liquid encapsulated Crochralski (LEC) growth process and on the crystal properties was investigated. An increased boron concentration in the crystals is found to be caused by oxidation processes between B 2O 3 · xH 2O, GaAs and impurity elements in GaAs as a function of water content and not by dissolution of boron from the crucible. Variations of pBN deposition parameters influence the texture and properties of pBN crucibles with regard to their suitability for the GaAs LEC crystal growth application. Correlations between pBN morphologies and thermal conditions of GaAs growth process and the crystal perfection were found. The best results are obtained by using so-called substrate nucleated pBN crucibles.

Reactions between molten aluminum and pyrolytic boron nitride crucibles

Pyrolytic boron nitride crucibles, used for the containment of aluminum in molecular-beam epitaxy systems, are attacked by the molten metal. The reaction appears to be self-limiting, with the formation of a coating of aluminum nitride on the crucible walls. The boron is held in solution, in the aluminum, until the concentration is sufficient to cause crystallization of a mixture of aluminum borides. When this point in the cell’s life cycle is reached, the flux from the cell no longer follows a simple exponential temperature dependence.

Scatterings of Shallow Threshold Voltage on Si-Implanted WN Self-Alignment Gate GaAs Metal-Semiconductor Field-Effect Transistors on Different Composition 2-Inch Substrates by Growing in Three Kinds of Furnaces

Two-inch undoped GaAs crystal boules were grown from different composition melts in a pyrolytic boron nitride crucible in three kinds of furnaces in order to survey composition effect through a consecutive thermal cooling cycle after crystallization on Vth scattering of Si-implanted metal-semiconductor field-effect transistors (MESFETs). On the substrates (dislocation density: 2×103-105 cm-2 order) obtained from the crystal boules, a tungsten nitride self-alignment gate MESFET array of shallow Vth (3×1012 cm-2, 50 keV) was fabricated, and Vth scatterings were observed on them. The Vth scattering for the most As-rich melt-grown crystal boule in the furnace with the smallest ratio of bottom heating was the smallest as compared with those for the slightly Ga-rich or As-rich melt-grown crystal boules in the furnaces with the greatest ratio of bottom heating. There was an anticorrelation between the Vth scattering amplitude and carbon content in the substrate crystals.

A two-zone molecular-beam epitaxy furnace for evaporation of II–VI materials

The design and operation of a two-zone furnace and crucible for the evaporation of II–VI semiconducting materials is described. The furnace contains separate heating elements and temperature sensors for each of the zones. The second zone is located at the open end of the furnace. The use of two independently-controlled temperature zones allows for the elimination of clogging problems that often occur with the evaporation of materials such as tellurium. The pyrolytic boron nitride (PBN) crucible contains a cap with a 2 mm inside diameter and 16 mm long orifice, which is designed to yield higher operating temperatures and better flux stability. A special PBN crucible retainer has also been designed to minimize contact of evaporated material with the tantalum structure of the furnace.

A new shutter design with a modified effusion cell for use in an ultrahigh vacuum system

A new, inexpensive, reliable, computer-controlled shutter has been designed with a modified effusion cell on the same flange for thermal evaporation in an ultrahigh vacuum system. Only 5 W are required to control the shutter open/close states with a response time of less than 0.1 s. The evaporation rate can be modulated by varying the speed of rotation of the shutter. Rotary action is obtained through a coupled magnetic field. Doping levels between maximum and minimum flux can be achieved. About 100 W are needed for 1000 °C operation. A functional modified effusion cell was constructed using an available standard pyrolytic boron nitride crucible(10 cc capacity). As an example of the capabilities of this shutter-controller system, a linear ramped profile was demonstrated.

Crystal growth of CdTe in a multi-zone vertical bridgman furnace with a PBN crucible

Single crystal CdTe growth in a multi-zone vertical Bridgman furnace is reported. The furnace could produce any temperature profile between 26 and 0°C/cm on a programmed time schedule. The sticking mechanism of the grown ingot to the quartz ampoule was studied using XPS. It was found that TeO 2 formed on the source material caused the adhesion. A treatment of the source material to avoid adhesion has been found. Crystal growth in a pyrolytic boron nitride crucible has also been examined and promising results for the growth of high quality crystal were obtained.

Si/GexSi1−x/Si resonant tunneling diode doped by thermal boron source

A study of resonant tunneling of holes in a Si/GexSi1−x/Si double barrier structure doped by a thermal boron doping source is presented. The source consists of a pyrolytic boron nitride crucible and uses filament heating. Sharp and constant doping levels between 1×1017 and 4×1019 cm−3 are obtained with a maximum K-cell temperature of ∼1560 °C. The double barrier tunneling devices realized by this source shows 2.1/1 peak-to-valley ratio at 4.2 K in current–voltage characteristics. Magnetotunneling measurements confirm that both the light and heavy holes participate in the resonant tunneling.

A single-filament effusion cell with reduced thermal gradient for molecular-beam epitaxy

A molecular-beam source utilizing single-filament construction to generate a reproducible and uniform beam of elemental molecules is presented. The filament design is constructed to conform to the shape of pyrolytic boron nitride crucibles utilized in molecular-beam epitaxy (MBE) systems, and to minimize the thermal gradient between the crucible opening and the crucible bottom which is inherent in Knudsen cell technology. The design also improves heat transfer, efficiency, and flux response of the cell. We show that this cell exhibits a better flux response than typical commercially available effusion cells and successfully prevents source material redistribution, thus providing for long-term flux stability and reproducibility in certain materials used in MBE. No increase in background impurity concentration in epitaxial AlGaAs films was observed, when compared to films grown using a standard Varian effusion cell. This cell is currently installed and being used as an aluminum source in a Varian GEN II system.