GaAs Crystal Growth Research Articles

Molecular dynamics simulations of GaAs crystal growth under different strains

The high-quality growth of GaAs crystals is extremely essential for the fabrication of high-performance high-frequency microwave electronic devices and light-emitting devices. In this work, the molecular dynamics (MD) simulation is used to simulate the induced crystallization of GaAs crystal along the [110] orientation. The effects of strain on the growth process and defect formation are analyzed by the largest standard cluster analysis, the pair distribution function, and visualization analysis. The results indicate that the crystallization process of GaAs crystal changes significantly under different strain conditions. At the initial stage, the crystal growth rate of the system decreases after a certain tensile strain and a large compressive strain have been applied, and the greater the strain, the lower the crystallization rate is. In addition, as the crystal grows, the system forms a zigzag interface bounded by the {111} facet, and the angle between the growth plane and the {111} facet affects the morphology of the solid-liquid interface and further affects the formation of twins. The larger the applied tensile strain and the smaller the angle, the more twin defects will form and the more irregular they will be. At the same time, a large proportion of the dislocations in the system is associated with twins. The application of strain can either inhibit or promote the nucleation of dislocations, and under an appropriate amount of strain size, crystals without dislocations can even grow. The study of the microstructural evolution of GaAs on an atomic scale provides a reference for crystal growth theory.

GaAs/Ge crystals grown on Si substrates patterned down to the micron scale

Monolithic integration of III-V compounds into high density Si integrated circuits is a key technological challenge for the next generation of optoelectronic devices. In this work, we report on the metal organic vapor phase epitaxy growth of strain-free GaAs crystals on Si substrates patterned down to the micron scale. The differences in thermal expansion coefficient and lattice parameter are adapted by a 2-μm-thick intermediate Ge layer grown by low-energy plasma enhanced chemical vapor deposition. The GaAs crystals evolve during growth towards a pyramidal shape, with lateral facets composed of {111} planes and an apex formed by {137} and (001) surfaces. The influence of the anisotropic GaAs growth kinetics on the final morphology is highlighted by means of scanning and transmission electron microscopy measurements. The effect of the Si pattern geometry, substrate orientation, and crystal aspect ratio on the GaAs structural properties was investigated by means of high resolution X-ray diffraction. The thermal strain relaxation process of GaAs crystals with different aspect ratio is discussed within the framework of linear elasticity theory by Finite Element Method simulations based on realistic geometries extracted from cross-sectional scanning electron microscopy images.

Selective Area Growth of GaAs in V-Grooved Trenches on Si(001) Substrates by Aspect-Ratio Trapping

A high quality of GaAs crystal growth in nanoscale V-shape trenches on Si(001) substrates is achieved by using the aspect-ratio trapping method. GaAs thin films are deposited via metal-organic chemical vapor deposition by using a two-step growth process. Threading dislocations arising from lattice mismatch are trapped by laterally confining sidewalls, and antiphase domains boundaries are completely restricted by V-groove trenches with Si {111} facets. Material quality is confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution X-ray diffraction. Low temperature photoluminescence (PL) measurement is used to analyze the thermal strain relaxation in GaAs layers. This approach shows great promise for the realization of high mobility devices or optoelectronic integrated circuits on Si substrates.

112 GaAs単結晶成長に及ぼす融液流動および温度分布の影響(熱工学II)

112 GaAs単結晶成長に及ぼす融液流動および温度分布の影響(熱工学II)

Boron‐ and stoichiometry‐related defect engineering during B2O3‐free GaAs crystal growth

AbstractGaAs crystals have been grown without B2O3 encapsulation by the vapour‐pressure controlled Czochralski (VCz) method to reduce undesired boron incorporation and to enable in situ control of the Ga/As ratio in the melt. With increasing Ga content of the melt the omnipresent As precipitate concentration decreases in as‐grown GaAs crystals. Semiinsulating (SI) GaAs crystals could been obtained even from melts with a composition of 46 at.% As. Comparative cathodoluminescence (CL) measurements on those crystals and on boron‐contaminated GaAs show that the luminescence bands at 1.316 eV and at 1.441 eV are related to BAs. Reduced concentrations of boron and EL2 result in significant lower near‐band edge absorption, mainly caused by the photoionization cross section of As antisites in SI GaAs. These VCz GaAs crystals possess improved optical properties. Semi‐conducting (SC) GaAs:Si crystals were grown from near‐stoichiometric melts. Positron annihilation lifetime spectroscopy measurements show that the low boron concentration in our SC VCz GaAs results always in 3 times higher Ga vacancy concentrations compared to standard SC GaAs. CL bands due to optical transitions at 0.95 eV and 1.15 eV are related to auto‐compensating (VGaSiGa)2‐ and (SiGaVGaSiGa)– complexes, respectively, and depend on the doping level. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Three-dimensional transient modeling of the melt flow in a TMF VCz system for GaAs crystal growth

Three-dimensional transient modeling of the melt flow in a TMF VCz system for GaAs crystal growth

LEC growth of semi-insulating GaAs crystals in traveling magnetic field generated in a heater–magnet module

For the first time semi-insulating (SI) GaAs single crystals were grown by the liquid encapsulated Czochralski (LEC) method using a traveling magnetic field (TMF) system generated in a heater–magnet module (HMM). The system was developed within the framework of the KRIST MAG ˜ ® project. The HMM, that generates heat and TMF simultaneously, was placed closely around the crucible inside the chamber of the industrial CI 358 puller. Before the growth experiments the induced vertical Lorentz force density F Lz was evaluated by the weight force response of a dummy. First growth experiments at various f/ ϕ ratios ( f—frequency and ϕ—phase shift) were carried out. Interface morphology and temperature fluctuation strengths were analyzed by a striation technique. Etch pit density, carbon, residual impurity and EL2° contents as well as electrical properties of the as-grown TMF–LEC GaAs crystals were measured. A preliminary correlation between crystal qualities and field frequency has been noticed.

Numerical simulation of Czochralski crystal growth under the influence of a traveling magnetic field generated by an internal heater-magnet module (HMM)

Abstract We present numerical simulations of vapor pressure controlled (VCz) and liquid encapsulated Czochralski (LEC) crystal growth of GaAs under the influence of a traveling magnetic field (TMF) with melt diameters of approximately 6 in and melt heights of approximately 1.8 in. The TMF is generated by an internal heater-magnet module (HMM) inside the pressure chamber in the growth arrangement and has been developed within the project KRIST MAG ˜ , see http://www.kristmag.com . For the global simulation, i.e. for the computation in the entire pressure chamber, the software WIAS-HiTNIHS is used. We validate this software by comparing measured and simulated Lorentz forces. Furthermore, we discuss how to account for effects of non-axisymmetric parts of the growth configuration during the axisymmetric computations performed by WIAS-HiTNIHS . Results computed by stationary global simulations are used as input data for transient local simulations of the melt. For the local simulations, we use the code NAVIER to solve the Navier–Stokes equations with the Boussinesq approximation together with the energy equation in an axisymmetric setting. Numerical simulations of the melt in an LEC configuration with an HMM are presented, showing that the Lorentz force generated by a TMF induced from the HMM can damp the temperature oscillations in the Taylor cell below the crystal.

The influence of the scatter of heat flux at the m/c interface on the frequency of appearance of poly body and twin defects during 6″ semi-insulating GaAs crystal growth by the VGF method

Abstract The challenge of increasing and maintaining a high yield for 6″ GaAs crystal growth is of utmost importance for meeting the price requirements dictated by today's requirements for semi-insulating GaAs substrates. For maintaining a low dislocation density in the grown ingots, the growth process time is typically long and, sometimes, the final ingots may exhibit twins and poly-crystalline formation. These defects may occur at the beginning of the cylindrical part of the ingot, or even at the conical part of pBN crucible so the whole ingot is rejected. On the other hand, these defects may appear further away from the seed and the location of the onset of these defects will determine the extent of the useful (production worthy) crystal length, also known as “yield”. The reasons for the onset of these defects are, however, not fully understood [M. Jurisch, F. Borner, Th. Bunger, St. Eichler, T. Flade, U. Kretser, A. Kohler, J. Stenzenberger, B. Weinert. J. Crystal Growth 275 (2005) 283]. In this study, we conducted numerical simulation using the transient two-dimensional mathematical model of the GaAs crystal growth by vertical gradient freeze method (VGF-method). We defined a new parameter “ A ” that is equal to the scatter of heat fluxes at m/c interface. Our study showed that some correlation exists between the defect appearance and A -value at m/c interface close to crucible wall. We have found that the frequency of a totally bad crystal length is higher if the A -value exceeds a certain value. Close to the crystal tail the scatter must be less than a defined A -value at the beginning of crystallization. Reduction in A -value was found to occur due to anomalies in the melt flow close to the m/c interface and crucible wall leading to the higher frequency of defects close to the crystal tail. Based on the correlation found, we developed a new technology regime that results in crystals grown with a lower frequency of defect occurrence at crucible wall.

Growth and characterization of GaAs crystals produced by the VCz method without boric oxide encapsulation

Unintentionally doped GaAs crystals grown from Ga-rich melts without B 2O 3 encapsulation by the modified vapour pressure controlled Czochralski (VCz) method are analysed. The influence of this growth technique on dislocation density and distribution is presented. The concentration of As precipitates in dependence on the melt composition is shown. Finally, the contents of residual impurities and their influence on some characteristic electrical parameters are discussed. The attention is focused on the feasibility of in situ controlled near-stoichiometric semi-insulating (SI) GaAs crystals with minimized content of As precipitates.

Investigation of residual dislocations in VGF-grown Si-doped GaAs

The VGF-method was applied for the growth of Si-doped GaAs crystals of 3 and 4 inch diameter ( n>10 18 cm −3). When the average dislocation density in these crystals falls below 200 cm −2 the stress-induced 60°-dislocations disappear and other dislocation types dominate the crystal. These remaining dislocations are classified and the conditions for their occurence are analyzed. Close to the crystal axis, dislocation walls are found which extend along [1 1 0]- and [ 1 1 ¯ 0 ] -directions and grow through the whole crystal. These walls are formed by edge dislocations belonging to the (1 1 0) 1 2 [ 1 ¯ 1 0 ] - or the ( 1 ¯ 1 0 ) 1 2 [1 1 0]-glide system. Apart from the dislocation walls, a dislocation arrangement in the form of a cross is observed in Si-doped GaAs. The orientation of the Burgers vector relative to the dislocation line of this dislocation type was determined. They include an angle close to 45°. Dislocations of this type belong to the (0 1 0) 1 2 [1 0 1] or the (0 1 0) 1 2 [ 1 0 1 ¯ ] -glide system. The examination of the growth in the seed well shows that the dislocation structures found in the crystal are partly already formed in the seed well. The growth in the seed well leads to a reduction of the EPD although the solid–liquid interface is concave. A reduction of the EPD in the seed well is accomplished by annihilation of dislocations.

2D simulation of carbon transport at the growth of GaAS crystals by liquid encapsulated Czochralski techniques

We present a 2D model for the unsteady carbon transport and incorporation into the crystal during the vapor pressure-controlled Czochralski growth of semi-insulating GaAs. The model is based on a conjugated analysis of global heat transfer, turbulent melt convection, and the flow in the encapsulant within a quasi steady-state approximation. Carbon transport in the melt is coupled to the transport of carbon monoxide in B 2O 3 encapsulant, assuming CO dissolution at the melt/encapsulant interface. CO concentration in the gas near the encapsulant surface, estimated from experimental observations, is used to formulate an external boundary condition. The model is validated using measurements of carbon concentration in grown 3-in GaAs crystals. The results obtained help to reveal basic mechanisms controlling carbon incorporation into the crystal.

Growth of GaAs crystals from Ga-rich melts by the VCz method without liquid encapsulation

The growth experiments and properties of undoped GaAs crystals grown by the vapour pressure controlled Czochralski (VCz) method without boric oxide encapsulant from Ga-rich melts are presented. Morphological and structural features were investigated by sensitive etching procedures. The precipitate content was analysed by laser scattering tomography. The mean scattering intensity of As precipitates decreases with increasing Ga excess in the melt. An enhanced probability of Ga-rich inclusion incorporation has been observed. In 3-in crystals, the mean etch pit density was decreased down to 5×10 3 cm −2 with improved radial homogeneity. The etch pits did not show cell patterning. Markedly reduced contents of boron and hydrogen have been detected. No oxygen defects and related complexes independently on the carbon concentration were ascertained by local vibrational mode. For the first time, the carbon concentration in the grown crystals was reduced by carefully controlling the CO content of the growth atmosphere in a VCz arrangement without boric oxide encapsulant. Such experiments without B 2O 3 encapsulant are of fundamental character and offer numerous information about the point defect situation in as-grown crystals.

Time-dependent magnetic field influence on GaAs crystal growth by vertical Bridgman method

The paper deals with the numerical investigation of time-dependent magnetic field influence on GaAs crystal growth by vertical Bridgman method. Fully unsteady numerical simulation is performed for three types of dynamic magnetic fields: rotating magnetic field (RMF), axial alternating magnetic field (AMF) and travelling magnetic field (TMF). The fields of stream function in the melt, temperature and dopant distribution in the melt and in the grown crystal with and without magnetic field are obtained and analyzed. RMF is found to pose significant influence on both azimuthal and meridional melt flows towards their intensification, and to substantially reduce the interface deflection. Higher magnetic field intensity results in the crystal–melt interface shape change from a concave interface to W-shaped one. At certain value of magnetic field intensity the steady-state flow regime becomes unstable and the transition to time-dependent regime is observed. The influence of AMF on GaAs crystal growth is found to be similar to that of static magnetic field. Application of travelling magnetic field significantly changes the flow structure, the interface shape and the dopant distribution in a melt and grown crystal. Depending on the propagation direction it can make either positive or negative effect on the resulting flow structure.

Non-stoichiometric growth of GaAs by the vapour pressure controlled Czochralski (VCz) method without B 2O 3 encapsulation

Results of the growth of semi-insulating GaAs crystals by the VCz method without B 2O 3 encapsulant are presented. Crystals have been grown from Ga-rich and near-stoichiometric melts controlled by As partial pressure in the range from 590°C to 630°C. The CO content in the gas atmosphere has been controlled. We investigated the influence of melt composition, reduced boron incorporation and adjusted carbon concentration. Native and impurity defect concentrations are compared with those of conventional VCz samples.

3D unsteady numerical analysis of conjugate heat transport and turbulent/laminar flows in LEC growth of GaAs crystals

We present the conjugated 3D unsteady numerical analysis of industrial-scale LEC GaAs crystal growth, including the calculation of heat transfer in the crystal and crucibles, melt convection, and the encapsulant flow. The analysis of unsteady turbulent melt convection is performed in terms of the large eddy simulation approach. A special procedure was introduced into the calculations to predict the geometry of the crystallization front. The results of the 3D unsteady calculations are compared to the results obtained in terms of the conventional steady-state Reynolds averaged approach with respect to the calculation of the geometry of the crystallization front. The effect of convective heat transfer in the encapsulant is specially studied using the 3D unsteady analysis. To investigate details of dynamic interaction between two immiscible liquids having a plane interface, preliminary computational tests were performed in a model setup.

Global heat and mass transfer in vapor pressure controlled Czochralski growth of GaAs crystals

We present a numerical study of vapor pressure controlled Czochralski growth of GaAs bulk crystals, using a global two-dimensional (2D) model of conjugated heat transfer and flows in the GaAs melt, argon gas, and boron oxide encapsulant. At first, the model is examined with respect to the prediction of melt/crystal interface shape, whose accuracy is closely related to a correct simulation of the heat transfer near the crystallization front. A qualitative agreement between the computed and measured interface geometries exhibits, nevertheless, some overestimation of the interface convexity, provided by the 2D model. Then, an analysis of three-dimensional unsteady melt convection has been performed to clarify whether the 2D model is sufficient to describe correctly convective heat transfer. It has been shown that in spite of strong spatial and time variations of the melt flow, the 2D approximation is capable of providing reliable information on averaged melt characteristics, but does not reproduce properly the convection intensity in the under-crystal region. Finally, the 2D model is applied to study the specificities of the melt turbulent convection and transport mechanisms for various growth parameters.

Optimization of VGF-growth of GaAs crystals by the aid of numerical modelling

The VGF growth of Si-doped GaAs crystals is improved considerably by optimizing the design of the crucible support and the temperature profile during the growth run. Inverse simulation with the software program CrysVUN++ was used for this procedure. The criteria for the optimized conditions are flat phase boundaries and low thermal stress during the whole growth run. The crystals which were grown according to the simulated conditions indeed showed flat phase boundaries and a very low EPD (<100 cm −2) within the whole crystal. It is shown that the growth conditions in the seed well and conical part of the crystal have a major influence on the dislocation density in the whole crystal.

Convective interaction and instabilities in GaAs Czochralski model

Instability and temperature oscillations in the melt for idealized Czochralski model for typical parameters of the semiisolating GaAs crystal growth configuration are studied. Critical Gr and Re numbers are determined using linear stability analysis and direct numerical simulation. The impact of the height of the melt on convection structure and critical Gr number are studied. Microgravity alternatives for damping of the temperature oscillations are discussed.

Analysis of crystal-meniscus system behaviour under Czochralski crystal growth

On the basis of a simple model of Czochralski crystal growth method, a theoretical analysis of crystal-meniscus system behaviour has been carried out. It has been shown that in a finite radius crucible, mutual influence of crystal radius and meniscus height leads to the possibility of their autooscillation. The dependence of autooscillation characteristics on crystal growth model parameters has been analysed. It has been shown that the amplitude of autooscillations decreases with the increase of crystal radius/crucible radius ratio. Numerical estimations for LEC GaAs crystal growth have been made. The problems of crystal radius control in this connection are discussed.