Simulation of the eddy current sensing of gallium arsenide Czochralski crystal growth

The feasibility of eddy current sensing of (1) the melt surface position and (2) the liquid-solid interface shape of 3-inch gallium arsenide crystals being grown by the Czochralski technique has been investigated using an axisymmetric finite element method. The results show clearly that differential sensor designs operating at high frequency ( ≈ 1 MHz) are very sensitive to the distance between the sensor and the surface of the melt providing the opportunity to precisely monitor and control this important variable of the growth process. The calculations also show a weaker effect of interface shape upon the imaginary impedance component at lower frequencies (1–10 kHz). Its physical basis is due to the different skin depths of solid and liquid GaAs. We show that a sensor's response to this interface effect can be enhanced by appropriate design of the differential sensor's pick-up coils.     

Growth of SiGe bulk crystals with uniform composition by utilizing feedback control system of the crystal–melt interface position for precise control of the growth temperature

We developed an automatic feedback control system of the crystal–melt interface position to keep the temperature at the interface constant during growth, and demonstrate its successful application to grow Ge-rich SiGe bulk crystals with uniform composition. In this system, the position of the crystal–melt interface was automatically detected by analyzing the images captured using in situ monitoring system based on charge-coupled-devices camera, and the pulling rate of the crucible was corrected at every 1 min. The system was found to be effective to keep the crystal–melt interface position during growth even when the variation of the growth rate is quite large. Especially, the interface position was kept for 470 h during growth of Ge-rich SiGe bulk crystal when we started with a long growth melt of 80 mm. As a result, a 23 mm-long Si_0.22Ge_0.78 bulk crystal with uniform composition was obtained thanks to the constancy of the growth temperature during growth through the control of the interface position. Our technique opens a possibility to put multicomponent bulk crystal in a practical use.     

Numerical investigation of heat transport and fluid flow during the seeding process of oxide Czochralski crystal growth Part 1: non‐rotating seed

For the seeding process of oxide Czochralski crystal growth, the flow and temperature field of the system as well as the seed‐melt interface shape have been studied numerically using the finite element method. The configuration usually used initially in a real Czochralski crystal growth process consists of a crucible, active afterheater, induction coil with two parts, insulation, melt, gas and non‐rotating seed crystal. At first the volumetric distribution of heat inside the metal crucible and afterheater inducted by the RF coil was calculated. Using this heat source the fluid flow and temperature field were determined in the whole system. We have considered two cases with respect to the seed position: (1) before and (2) after seed touch with the melt. It was observed that in the case of no seed rotation (ω_seed = 0), the flow pattern in the bulk melt consists of a single circulation of a slow moving fluid. In the gas domain, there are different types of flow motion related to different positions of the seed crystal. In the case of touched seed, the seed‐melt interface has a deep conic shape towards the melt. It was shown that an active afterheater and its location with respect to the crucible, influences markedly the temperature and flow field of the gas phase in the system and partly in the melt. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoelectron spectroscopy study of the effect of substrate doping on an HfO2/SiO2/n-Si gate stack

Core level photoelectron spectroscopy has been used to investigate the effect of substrate doping on the binding energies of 1 nm HfO₂/0.6 nm SiO₂/Si films. A characteristic 0.26–0.30 nm Hf_0.35Si_0.65O₂ silicate interface is formed between the gate oxide and the SiO₂ layer with an equivalent oxide thickness of 0.5 nm. High substrate doping shifts the Fermi level upwards by 0.5 eV. An interface dipole forms giving rise to a shift in the local work function. Screening from substrate electrons is confined to the SiO₂/Si interface. The principal contributions modifying the core level binding energies in the oxide are the doping dependant Fermi level position and the interface dipole strength.     

On the thermodynamic stability of disperse systems

Investigation has been carried out on the effect of the surface activity of capillary or surface active substances (impurities), called surfactants (i.e. the effect of the lowering (Δσ) of the specific surface energy after impurity adsorption at the solid/gas, solid/liquid, liquid/gas or liquid/liquid interface), on the thermodynamic equilibrium in a polydisperse system. Attention has been paid to the fact that the presence of a surfactant with an activity within the range 0 < Δσ < σ₀ (where σ₀ is the specific surface energy of an adsorption-free interface) reduces the thermodynamic driving force for the appearance of the Gibbs-Thomson effect and for the recondensation (recrystallization) processes in the heterogeneous polydisperse system. It is shown that with an impurity activity Δσ = σ₀, the supersaturation becomes zero and the condensed phase formations should be, irrespective of their size and shape, in equilibrium with one another because their vapour pressure becomes equal to that of an infinitely large phase. In this case the Gibbs-Thomson effect completely disappears, the system become thermodynamically stable and, regardless of its polydispersity, it can exist infinitely long. The behaviour of such a heterogeneous system, which is far from its critical point (the system temperature is T ≪ T_critical), is similar to the behaviour of a system in the region of its critical point: the surface tension becomes nearly equal to zero, and the fluctuations sharply increase. The case when the surfactant activity is so high (Δσ > σ₀) that σ_a = σ₀ − Δσ < 0 (σ_a, specific surface energy of the interface in the presence of a surfactant), is of special interest. It represents a paradox in the phase formation theory: as a result of the adsorption of an impurity with such a high activity, the condensed phase surface would be destabilized and the phase would be spontaneously dispersed.     

Interface Dynamics during Indium Antimonide Crystal Growth

Experiments with a stoichiometric InSb compound were first performed at small temperature gradient across the crystal/melt interface of 3 °C/cm and furnace translation velocity, V_frn, of 2 μ/sec. Known growth requirements for quality crystals were confirmed. They are, (1) the interface temperature must be close to the congruent melting temperature and, (2) the interface must be located within the adiabatic zone. These requirements can be obtained only through specific settings of the heater temperatures. An X-ray radioscopic system has been modified to accommodate real-time visualization of the crystal/melt interface during vertical Bridgman-Stockbarger growth of InSb. It is shown that asymmetric temperature settings of the heaters can be advantageously used to minimize defect formation. The interface temperature was assessed indirectly with calibrated outside thermocouples. Optical microscopy and electron microprobe analyses provided feedback on crystalline homogeneity.     

Numerical investigation of heat transport and fluid flow during the seeding process of oxide Czochralski crystal growth Part 2: rotating seed

In this paper, the role of seed rotation on the characteristics of the two‐dimensional temperature and flow field in the oxide Czochralski crystal growth system has been studied numerically for the seeding process. Based on the finite element method, a set of two‐dimensional quasi‐steady state numerical simulations were carried out to analyze the seed‐melt interface shape and heat transfer mechanism in a Czochralski furnace with different seed rotation rates: ω_seed = 5‐30 rpm. The results presented here demonstrate the important role played by the seed rotation for influencing the shape of the seed‐melt interface during the seeding process. The seed‐melt interface shape is quite sensitive to the convective heat transfer in the melt and gaseous domain. When the local flow close to the seed‐melt interface is formed mainly due to the natural convection and the Marangoni effect, the interface becomes convex towards the melt. When the local flow under the seed‐melt interface is of forced convection flow type (seed rotation), the interface becomes more concave towards the melt as the seed rotation rate (ω_seed) is increased. A linear variation of the interface deflection with respect to the seed rotation rate has been found, too. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Introduction and characterization of interfacial defects in SrRuO3/BaTiO3/SrRuO3 multilayer films

A Ruddlesden–Popper-type planar fault was introduced at the SrRuO₃/BaTiO₃ interface of a SrRuO₃/BaTiO₃/SrRuO₃ heterofilm system using different processing conditions for the individual film layer. This fault occurs continuously and homogeneously along the interface, forming an extra Sr-rich sub-nanometer layer. The structure of the fault and the lattice behavior in the interface area were characterized on an atomic scale by properly imaging all types of atomic columns, especially the pure oxygen columns, by means of spherical-aberration-corrected high-resolution transmission electron microscopy. Information on local interdiffusion and lattice strain at the interface was obtained by quantitative evaluation of the atomic resolution images.     

Characterization of ultrathin gate dielectrics by grazing X-ray reflectance and VUV spectroscopic ellipsometry on the same instrument

Precise characterization of high k gate dielectrics becomes a challenging task due to the very thin thickness (<3-4 nm), which will be needed in the next generation of integrated circuits. Conventional techniques such as spectroscopic ellipsometry (SE) in the UV-visible range becomes difficult to use alone because of the great correlation between thickness and optical indices. To overcome this problem a new versatile instrument integrating SE in the VUV spectral range and grazing X-ray reflectance (GXR) has been developed recently by SOPRA. Both kinds of measurements can be made at the same location on the sample and at the same time. The analysis is made with a common optical model adjusting the layer thickness and the surface and interface roughness on the GXR data and the optical indices and other parameters like surface or interface roughness or inter-diffusion on the SE data. The paper describes some experimental results obtained with this system on ZrO₂, HfO₂ and La₂O₃ films. Results are correlated with other experimental techniques in some cases.     

Application of Spectral Analysis for Crystal Shape Improvement during Czochralski Growth of Oxide Crystals

The automatic diameter control system using the results of spectral analysis of weight sensor data as a criterion for the amount of feedback tuning was applied successfully for the growth of La₃Ga₅SiO₁₄ single crystals. The developed method enabled to determine timely the appearance of oscillations in the control system and to adapt the amplification factor automatically for the maintenance of stable crystal shape. Furthermore, the developed system made it possible to estimate the formation of a second phase at a very early stage.     

Experimental Study of Interface Inversion of Tb3ScxAl5‐xO12 Single Crystals Grown by the Czochralski Method

Thermal conditions and rotation rate were examined experimentally for obtaining a flat interface growth of high melting‐point oxide (Tb₃Sc_xAl_5‐xO₁₂ ‐ TSAG) by the Czochralski method. The critical crystal rotation rate can be significantly reduced, of about twice at low and very low temperature gradients comparing to medium temperature gradients in the melt and surroundings of the crystal. The interface shape of TSAG crystals is not very sensitive on crystal rotation rate at small rotations and becomes very sensitive at higher rotations, when the interface transition takes place. The range of crystal rotation rates during the interface transition from convex to concave decreases with a decrease of temperature gradients. At low temperature gradients interface inversion crystals takes place in very narrow range of rotation rates, which does not allow one to growth such crystals with the flat interface. Even changing crystal rotation rate during the growth process in a suitable manner did not prevent the interface inversion from convex to concave and thus did not allow to obtain and maintain the flat interface.     

Well parameters of two‐dimensional electron gas in Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by MOCVD

Resistivity and Hall effect measurements were carried out as a function of magnetic field (0‐1.5 T) and temperature (30‐300 K) for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two‐dimensional electron gas (2DEG) channel located at the Al_0.88In_0.12N/GaN interface with an AlN interlayer and a two‐dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al_0.88In_0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In situ observations in the melt growth of Pb5Ge3O11 crystals

The interface motion during the growth process of Pb₅Ge₃O₁₁ from the melt is visualized in an in situ observation system. The formation of the striation aggregation is observed, and the rates of the interface motion R_im are measured. It turns out that there are sharp changes in the growth rate during the formation of the striation aggregation. It has been found that the typical fluctuation in the rates can be associated with the kinetics and transport mixed control during the crystal growth. The non-uniform composition normal to the interface gives rise to the striation aggregation. Of particular importance in the investigation are the typical interfacial melt flows from the corners to the center of the interface observed in the experiments. The relation between the rates of this interfacial melt flow R_if and the time t is also provided showing that the change of supersaturation adjacent to the interface gives rise to the interfacial melt flow.     

Numerical simulation of the solidification processes of copper during vacuum continuous casting

A numerical simulation method is used to analyze the microstructure evolution of 8-mm-diameter copper rods during the vacuum continuous casting (VCC) process. The macro–microscopic coupling method is adopted to develop a temperature field model and a microstructure prediction model. The effects of casting parameters, including casting speed, pouring temperature, cooling rate, and casting dimension on the location and shape of the solid–liquid (S/L) interface and solidified microstructure are considered. Simulation results show that the casting speed has a large effect on the position and shape of the S/L interface and grain morphology. With an increase of casting speed, the shape of the S/L interface changes from a planar shape into an elliptical shape or a narrow, pear shape, and the grain morphology indicates a change from axial growth to axial–radial growth or completely radial growth. The simulation predictions agree well with the microstructure observations of cast specimens. Further analysis of the effects of other casting parameters on the position and shape of the S/L interface reveals that the casting dimension has more influence on the position and shape of the S/L interface and grain morphology than do pouring temperature and cooling rate. The simulation results can be summarized to obtain a discriminant of shape factor (η), which defines the shape of the S/L interface and grain morphology.     

Ionic impurity transport and partitioning at the solid–liquid interface during growth of lithium niobate under an external electric field by electric current injection

Transport of ionic species in the melt and their partitioning at the solid–liquid interface during growth of lithium niobate was studied under the influence of intrinsic and external electric fields. A Mn-doped lithium niobate (Mn:LiNbO₃) single crystal was grown via the micro-pulling-down (μ-PD) method with electric current injection at the interface. Mn ions were accumulated or depleted at the interface, depending on the sign of the injected current. The electric current injection induced an interface electric field as well as a Coulomb force between the interface and Mn ions. The electric field modified the transportation of Mn ions and their partitioning into the crystal, while the Coulomb force led to adsorption or rejection of Mn ions at the interface in addition to Mn concentration change due to the electric field. Effect of the Coulomb force was often observed to be larger on Mn concentration at the interface than that of the induced electric field, and dominated the redistribution of Mn in the solid. It has been experimentally and analytically shown that Mn concentration partitioned into the crystal can be obtained by multiplying Mn concentration at the interface by a field-modified partition coefficient, k_E0, instead of the conventional equilibrium partition coefficient, k₀.     

On the Design of Double‐Ellipsoid Mirror Furnace and its Thermal Characteristics for Floating‐Zone Growth of SrxBa1‐xTiO3 Single Crystals

We have designed a double ellipsoid mirror furnace for floating‐zone crystal growth using lamps with rectangular filaments. Its thermal characteristics were studied using an alumina tube for several system configurations. A simple comparison with a commercial furnace that used cylinder lamps for the heating profile was also conducted. By adjusting lamp orientation and positions, one could modify heating profiles easily. In general, the thermal characteristics of the furnace were consistent with the model's prediction [J. Crystal Growth 173 (1997) 561]. The effects of growth chamber and heat pipe were further illustrated. Furthermore, a suitable system configuration leading to better heating uniformity and lower thermal gradients near the growth interface was found for the floating‐zone growth of Sr_xBa_1‐xTiO₃ single crystals.     

Interface and material characterization of thin Al2O3 layers deposited by ALD using TMA/H2O

Thin Al₂O₃ layers were grown by atomic layer deposition using trimethylaluminum (TMA) and water as precursors on 1.2 nm thermal SiO₂ and HF cleaned Si surfaces. The stoichiometry and the contamination (H, OH and C) of as-deposited and N₂ annealed thick Al₂O₃ layers were characterized by secondary ion mass spectrometry (SIMS), elastic recoil detection analysis (ERDA) and X-ray photoelectron spectroscopy (XPS). We show a perturbed region (≈5 nm thick) at the Al₂O₃/Si interface by XPS and Auger electron spectrometry (AES). Post-deposition annealings induced important interface oxidation, Si atoms injection and SiO₂/Al₂O₃ mixture whereas the initial interface was abrupt. Silicon oxidation before Al₂O₃ growth highly limits interfacial oxidation and improves interfacial quality. We proposed that OH groups may play a key role to explain silicon oxidation during post-deposition annealings in inert ambience with low oxygen contamination levels.     

Experimental study of bubble generation during β‐BaB2O4 single crystal growth

The generation of bubble‐inclusions during BaB₂O₄ (BBO) crystal growth from high temperature solution has been optically observed by an in situ observation technique. It was found that bubbles are formed from the peripheries of some hexagonal defects in the (0001) plane of the growing crystal, which may be caused by the evaporation of the air‐opened interface at the high temperature. In addition, atomic force microscope (AFM) was used to investigate the distribution of bubbles. Results revealed that the bubble generation and distribution depend strongly on the microscopic structure of the interface: on a rough interface, bubbles are easily formed and grow rapidly; however, they are greatly suppressed by step trains on a vicinal interface. In the latter case, the height value of a bubble is close to that of the step, which is in the order of several tens of nanometers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precipitation of nanostructured calcite in a controlled multiphase process

The way of conducting the precipitation process is crucial for the final product properties and its further applications. In the presented experiments, CaCO₃ powders were produced by controlled fast precipitation through gaseous CO₂ absorption in Ca(OH)₂ slurry. This multiphase reaction was conducted in a new rotating disc reactor unit, which enables one to control inter- and intra-face mass and energy transfer as well as the macro- and micro-mixing effects in the reacting system. The effect of calcium hydroxide concentration, mixing conditions in the reactor and gas–liquid interface development at discs surface on precipitated CaCO₃ was investigated.     

3D simulation of the effects of growth parameters on the growth of sapphire crystals using heat exchanger method

3D simulations using the commercial CFDRC and FIDAP code, which are based on finite element techniques, were performed to investigate the effects of anisotropic conductivity on the convexity of the melt–crystal interface and the hot spots of sapphire crystal in a heat‐exchanger‐method crystal growth system. The convection boundary conditions of both the energy input to the crucible by the radiation as well as convection inside the furnace and the energy output through the heat exchanger are modeled. The cross‐sectional flow pattern and the shape of the melt–crystal interface are confirmed by comparing the 3‐D modeling results with previous 2D simulation results. In the 3D model, the “hot spots” in the corners of the crucible are donut shaped, and the shape changes with the value of the conductivity of anisotropic crystal. The outline of the crystal becomes more convex as the conductivity in the z direction (k_sz) increases. The outline of melt–crystal interface is elliptical when the anisotropic conductivity is moving in the radial direction (k_sx and k_sy). The portion at the outline touching the bottom of the crucible is smaller than the maximum outline of the crystal, meaning that the shape at the “hot spot”, changes with the value of the conductivities of anisotropic crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)