Crystal growth of mixed AlN–SiC bulk crystals

Bulk AlN–SiC mixed single crystals are prepared by sublimation growth employing pure AlN or mixed AlN–SiC sources and 6H-SiC seed crystals. As the growth temperature is increased from 1900 to 2050 °C, using seeds with different off-axis orientations, inclined up to 42° from the basal plane toward the (0 1 –1 0)-plane, or using different source materials, crystals with different Si/C contents are obtained. Dependent on the Si and/or C content, crystal coloration changes from yellowish to greenish to blackish. Modification in crystals’ coloration and corresponding changes in below band-gap optical absorption and cathodoluminescence spectra are discussed.     

Evolution and structure of low-angle grain boundaries in 6H–SiC single crystals grown by sublimation method

KOH etching and high-resolution X-ray diffractometry (HRXRD) were used to study the evolution and structure of low-angle grain boundaries (LAGBs), which extended along 〈1 -1 0 0〉 in 6H–SiC bulk crystals grown by the sublimation method. It was found that LAGBs formed in the growth process consisted of an array of threading dislocations and took different configurations under different radial temperature gradients (RTGs). HRXRD results proved that the domain at one side of LAGBs formed under a low radial temperature gradient has only tilts around the c-axis with respect to the other domain at another side of LAGBs.     

Electrical properties and deep levels spectra of bulk SiC crystals grown by hybrid physical–chemical vapor transport method

Concentrations of nitrogen shallow donors, boron shallow acceptors, charge carriers, and electron traps were measured as a function of position along the growth axis in a series of undoped 6H–SiC boules grown by sublimation method with and without addition of hydrogen to the growth atmosphere. Elemental analysis by secondary ion mass spectrometry and measurements of electrical properties indicate that the addition of hydrogen suppresses nitrogen incorporation and formation of all electron traps. Concentration of boron is not affected by hydrogen presence. The addition of hydrogen to the growth ambient improves the uniformity of nitrogen incorporation and deep trap distribution along the growth axis. The results are interpreted as due to increased carbon transport and corresponding shift of crystal stoichiometry toward carbon-rich side of the SiC existence range.     

Optimisation of the VGF growth process by inverse modelling

Numerical and experimental results on the thermal optimisation of vertical gradient freeze crystal growth are presented. An inverse modelling approach is described aimed at solidification with a constant growth rate and planar solid–liquid interface. As a result of modelling an optimised growth process characterised by a modified ampoule configuration and thermal regime was established. For experimental confirmation Ga-doped germanium single crystals were grown with the optimised process. In good agreement with the numerical results, solidification with an almost constant growth rate was achieved with the interface deflection being significantly lower than in conventionally grown crystals.     

Direct observation of sublimation process on a CdTe(1 1 1) surface using an ultrafine particle

A particle with a CdTe core and carbon mantle was produced by the advanced carbon-coating method which enables direct coverage with a carbon layer using an electron microscope. The coagulated particles containing approximately 30–200 CdTe particles produced by the gas evaporation method were covered with a carbon layer of about 7 nm thickness at 300°C. By heating these particles above 500°C, the sublimation process of a part of the CdTe particle can be directly captured by high-resolution transmission electron microscopy and recorded in real time on videotape. Sublimation on the CdTe(1 1 1) surfaces occurred in the step flow mode of two (1 1 1) layers. It was observed that two (1 1 1) zinc-blende layers changed to the (0 0 0 2) würtzite configuration unit just before sublimation. The condensation of CdTe on the sublimated particle surface and growth of CdTe in the carbon layers were also captured in the video image. These sublimation processes were discussed in terms of the existence of the polarity of II–VI compounds.     

Total pressure‐controlled PVT SiC growth for polytype stability during using 2D nucleation theory

A total pressure‐controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si₂C and SiC₂ transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype‐stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H‐SiC prepared using this pressure‐controlled method was more stable than that of 4H‐SiC formed using the conventional constant‐pressure method.     

Bulk and epitaxial growth of silicon carbide

Silicon carbide (SiC) is a wide bandgap semiconductor having high critical electric field strength, making it especially attractive for high-power and high-temperature devices. Recent development of SiC devices relies on rapid progress in bulk and epitaxial growth technology of high-quality SiC crystals. At present, the standard technique for SiC bulk growth is the seeded sublimation method. In spite of difficulties in the growth at very high temperature above 2300 °C, 150-mm-diameter SiC wafers are currently produced. Through extensive growth simulation studies and minimizing thermal stress during sublimation growth, the dislocation density of SiC wafers has been reduced to 3000–5000 cm⁻² or lower. Homoepitaxial growth of SiC by chemical vapor deposition has shown remarkable progress, with polytype replication and wide range control of doping densities (10¹⁴–10¹⁹ cm⁻³) in both n- and p-type materials, which was achieved using step-flow growth and controlling the C/Si ratio, respectively. Types and structures of major extended and point defects in SiC epitaxial layers have been investigated, and basic phenomena of defect generation and reduction during SiC epitaxy have been clarified. In this paper, the fundamental aspects and technological developments involved in SiC bulk and homoepitaxial growth are reviewed.     

The growth of mixed alkaline-earth fluorides for laser host applications

A comprehensive analysis is implemented concerning the growth, properties, and applications of doped-co-doped single and mixed alkali earth fluoride systems. Calcium-strontium fluoride solid solutions with a Sr content proportion varying widely between 0.007 and 0.675 mol.% are obtained as a batch of axis-symmetrical boules grown by a Bridgman-Stockbarger (BS) method. The crystallization front (CF) can be controlled to retain a convex CF-shape that is favourable for normal growth of single crystals. This achieved using a broad adiabatic furnace zone (AdZ) independently of the boules’ composition. The influence of the thermal field distribution on the CF and the real crystallization rate (CR), which are both critically decisive in controlling crystal quality, were originally assessed using empirically derived formulas. The optical characteristics of the grown boules were monitored by measuring the external transmittance t and calculating the total losses following light irradiation of optical windows that were prepared from sections of the boules that had been cut parallel to one another. The t-measurements were performed by two different techniques and the comparative analysis of the results reliably indicates any inhomogeneity in the grown boules. A simple supercooling criterion proved to closely relate the morphological stability of the CF enabling one to set up the optimum growth conditions. Thus the normal growth criterion outlines the concentration bounds where the isotropic growth mechanism is replaced by cellular anisotropic growth. A procedure has been established for provisioning researchers with optical quality calcium-strontium fluoride crystals with widely varying composition grown under practically identical conditions. As a consequence one can explore possible reasons that can affect the growth mechanism for this or any other systems with a fluoride structure and so provide scope aimed at the future improvement of the crystal quality thereby enlarging the field of mixed fluoride systems’ applications.     

Compositional variation in Si-rich SiGe single crystals grown by multi-component zone melting method using Si seed and source crystals

The effect of the supply of depleted Si solute elements on the compositional variation in the Si-rich SiGe bulk crystals was studied using the method which was used to grow Ge-rich SiGe single crystals with a uniform composition. By selecting the proper pulling rate, we can obtain Si-rich Si_1−xGe_x bulk crystals with uniform composition of x=0.1 without using the supply mechanism of depleted Si solute elements. When the supply mechanism of Si solute elements was used, the initial composition in Si-rich SiGe crystals can be much more easily determined by controlling the growth temperature than that in Ge-rich crystals because the Si seed crystal is not melted down. The supply of Si solute elements is very effective to change the compositional variation even for Si-rich SiGe crystals.     

A simple synthesis of large-scale SiC–SiO2 nanocables by using thermal decomposition of methanol: Structure, FTIR, Raman and PL characterization

Large-scale SiC nanocables were synthesized on a Ni(NO₃)₂-catalyzed Si substrate by using a simple and cheap method based on thermal decomposition of methanol. Based on X-ray diffraction and high-magnification transmission electron microscopy, the as-grown nanocables consisted of crystalline SiC cores and amorphous SiO₂ shells. The diameters of SiC cores were 5.7–10 nm and the thicknesses of SiO₂ shells were 9–20 nm. Dividing of nanocables was observed and its origin was investigated. An asymmetric feature of SiC TO band with a shoulder at the high-frequency side was attributed to the contribution of SiC TO mode. The nanocables displayed strong violet–blue emission. A possible growth mechanism was proposed.     

Analysis of SiC crystal sublimation growth by fully coupled compressible multi-phase flow simulation

A fully coupled compressible multi-phase flow solver was developed to effectively design a large furnace for producing large-size SiC crystals. Compressible effect, convection and buoyancy effects, flow coupling between argon gas and species, and the Stefan effect are included. A small and experimental furnace is used to validate the solver. First, the essentiality of 2D flow calculation and the significance of incorporating buoyancy effect and gas convection, the Stefan effect, and flow interaction between argon gas and species were investigated by numerical results. Then the effects of argon gas on deposition rate, growth rate, graphitization on the powder source, and supersaturation and stoichiometry on the seed were analyzed. Finally, the advantages of an extra chamber design were explained, and improvement of growth rate was validated by the present solver.     

Chemically assisted vapour transport for bulk ZnO crystal growth

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10⁻³ to 1 atm) and temperatures (800–1200 °C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182 cm²/(V s) and n=7 10¹⁵ cm⁻³) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10⁺¹⁵ cm⁻³). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.     

Gel growth of α and γ glycine and their characterization

The microbial free single crystals of α and γ glycine were grown from gel at room temperature in a new chemical route. These crystals showed a superior quality than the solution grown crystals. The metastable α-form and the stable γ-form of glycine were crystallized in silica gel by solubility reduction method. The form of crystallization is confirmed by single crystal and powder X-ray diffraction analyses. The crystals of α and γ glycine were found to crystallize in monoclinic and hexagonal crystal systems, respectively. For analyzing the functional group and thermal stability of α and γ glycine crystals, spectroscopic and thermal analyses have been carried out. The dielectric studies were performed to find the dielectric constant of the grown crystals and the results are discussed. Second harmonic generation efficiency of the crystal was measured by Kurtz’s powder method using Nd:YAG laser and it was found to be 2.68 times that of potassium dihydrogen phosphate crystals.     

Sublimation crystal growth of yttrium nitride

The sublimation–recombination crystal growth of bulk yttrium nitride crystals is reported. The YN source material was prepared by reacting yttrium metal with nitrogen at 1200 °C and 800 Torr total pressure. Crystals were produced by subliming this YN from the source zone, and recondensing it from the vapor as crystals at a lower temperature (by 50 °C). Crystals were grown from 2000 to 2100 °C and with a nitrogen pressure from 125 to 960 Torr. The highest rate was 9.64×10⁻⁵ mol/h (9.92 mg/h). The YN sublimation rate activation energy was 467.1±21.7 kJ/mol. Individual crystals up to 200 μm in dimension were prepared. X-ray diffraction confirmed that the crystals were rock salt YN, with a lattice constant of 4.88 Å. The YN crystals were unstable in air; they spontaneously converted to yttria (Y₂O₃) in 2–4 h. A small fraction of cubic yttria was detected in the XRD of a sample exposed to air for a limited time, while non-cubic yttria was detected in the Raman spectra for a sample exposed to air for more than 1 h.     

Vapour phase growth of epitaxial silicon carbide layers

After a brief overview of different epitaxial layer growth techniques, the homoepitaxial chemical vapour deposition (CVD) of SiC with a focus on hot-wall CVD is reviewed. Step-controlled epitaxy and site competition epitaxy have been utilized to grow polytype stable layers more than 50 μm in thickness and of high purity and crystalline perfection for power devices. The influence of growth parameters including gas flow, C/Si ratio, growth temperature and pressure on growth rate and layer uniformity in thickness and doping are discussed. Background doping levels as low as 10¹⁴ cm⁻³ have been achieved as well as layers doped over a wide n-type (nitrogen) and p-type (aluminium) range.

Furthermore the status of numerical process simulation is mentioned and SiC substrate preparation is described. In order to get flat and damage free epi-ready surfaces, they are prepared by different methods and characterised by atomic force microscopy and by scanning electron microscope using channelling patterns. For the investigation of defects in SiC high purity CVD layers are grown. The improvement of the quality of bulk crystal substrates by micropipe healing and so-called dislocation stop layers can further decrease the defect density and thus increase the yield and performance of devices. Due to its high growth rate functionality and scope for the use of multi-wafer equipment hot-wall CVD has become a well-established method in SiC-technology and has therefore great industrial potential.     

大尺寸电阻加热式碳化硅晶体生长热场设计与优化

大尺寸低缺陷碳化硅(SiC)单晶体是功率器件和射频(RF)器件的重要基础材料,物理气相传输(physical vapor transport, PVT)法是目前生长大尺寸SiC单晶体的主要方法。获得大尺寸高品质晶体的核心是通过调节组分、温度、压力实现气相组分在晶体生长界面均匀定向结晶,同时尽可能减小晶体的热应力。本文对电阻加热式8英寸(1英寸=2.54 cm)碳化硅大尺寸晶体生长系统展开热场设计研究。首先建立描述碳化硅原料受热分解热质输运及其多孔结构演变、系统热输运的物理和数学模型,进而使用数值模拟方法研究加热器位置、加热器功率和辐射孔径对温度分布的影响及其规律,并优化热场结构。数值模拟结果显示,通过优化散热孔形状、保温棉的结构等设计参数,电阻加热式大尺寸晶体生长系统在晶锭厚度变化、多孔介质原料消耗的情况下均能达到较低的晶体横向温度梯度和较高的纵向温度梯度。     

Solid mechanics and material strength studies on the melt growth of bulk single crystals

Microdefects such as dislocations and macrocracking should be controlled during the crystal growth process to obtain high-quality bulk single crystals. Solid mechanics and material strength studies on the single crystals are of importance to solve the problems related to the generation and multiplication of dislocations and the cracking of single crystals. The present paper reviews such research activities that comprise the thermal stress analysis during crystal growth process, the dislocation density estimation during crystal growth process, and the cracking of single crystal due to thermal stress.     

Efficient solution of a batch crystallization model with fines dissolution

In this paper, an efficient and accurate numerical method is proposed for solving a batch crystallization model with fines dissolution. The dissolution of small crystals (fines dissolution) is useful for improving the quality of a product. This effectively shifts the crystal size distribution (CSD) towards larger crystal sizes and often makes the distribution narrower. The growth rate can be size-dependent and a time-delay in the dissolution unit is also incorporated in the model. The proposed method has two parts. In the first part, a coupled system of ordinary differential equations (ODEs) for moments and solute mass is numerically solved in the time domain of interest. These discrete values are used to get growth and nucleation rates in the same time domain. In the second part, the discrete growth and nucleation rates along with the initial CSD are used to construct the final CSD. The analytical expression for CSD is obtained by applying the method of characteristics and Duhamel's principle on the given population balance model (PBM). A Gaussian quadrature method, based on orthogonal polynomials, is used for approximating integrals in the ODE-system of moments and solute mass. The efficiency and accuracy of the proposed numerical method is validated by a numerical test problem.     

Structural, thermal and dielectric studies on a new solution grown 4-dimethylaminopyridinium dihydrogen phosphate crystal

Single crystals of 4-dimethylaminopyridinium dihydrogen phosphate (DMAPDP) (C₇H₁₃N₂PO₄) were grown by the solvent evaporation method. The three-dimensional structure was solved by the single-crystal X-ray diffraction method which belongs to triclinic crystal system and the molecular arrangements in the crystal were studied. The thermal behaviour was investigated using differential scanning calorimetry (DSC) and no phase transition was identified in the temperature region −150 to 230 °C. The thermal parameters—thermal diffusivity (), thermal effusivity (e), thermal conductivity (K) and heat capacity (C_p) of DMAPDP were measured by an improved photopyroelectric technique at room temperature. Dielectric constant and dielectric loss of the grown crystal were evaluated for the frequency range 1–200 KHz in the temperature region 28–135 °C. The Vicker's hardness was measured as 42.2 for a load of 98.07 mN. The laser induced surface damage threshold of DMAPDP crystal was found to be 4.8 GW/cm² with nanosecond Nd:YAG laser.     

Ambient-condition growth of superconducting YBa2Cu4O8 single crystals using KOH flux

YBa₂Cu₄O₈ is a stoichiometric oxide superconductor of T_c80 K. Unlike YBa₂Cu₃O_7−δ, this compound is free from oxygen vacancy or twin formation and does not have any microscopic disorder in the crystal. Doping with Ca raises its T_c to 90 K. The compound is a promising superconductor for technological application. Up to now, single crystals have not been grown without using specialized apparatus with extremely high oxygen pressure up to 3000 bar and at over 1100 °C due to the limited range of reaction kinetics of the compound. This fact has delayed the progress in the study of its physical properties and potential applications. We present here a simple growth method using KOH as flux that acts effectively for obtaining high-quality single crystals in air/oxygen at the temperature as low as 550 °C. As-grown crystals can readily be separated from the flux and exhibit a perfect orthorhombic morphology with sizes up to 0.7×0.4×0.2 mm³. Our results are reproducible and suggest that the crystals can be grown using a conventional flux method under ambient condition.