A.F. Witt

On control of the crystal-melt interface shape during growth in a vertical bridgman configuration

Abstract Using a finite-element thermal model of vertical Bridgman growth, the effectiveness of several experimental approaches aimed at compensating for the interface effect are investigated. Growth of Ge and CdTe representing semiconductor materials of high and low thermal conductivity respectively, are used as illustrative cases. Results indicate that attainment of a flat growth interface is not possible except in highly idealized cases. The curvature of the interface can, however, be reversed in certain circumstances.

Dopant segregation during vertical Bridgman-Stockbarger growth with melt stabilization by strong axial magnetic fields

Ga-doped germanium was grown in a vertical Bridgman-Stockbarger system with melt stabilization by axial magnetic fields of 30 kG. It was found that radial segregation of gallium is negligible and that the initial transient of axial macro-segregation is in apparent compliance with the theory of diffusion controlled plane front solidification.

Liquid‐phase electroepitaxy: Growth kinetics

On the basis of mass‐transport principles, a theoretical model of electroepitaxial growth—current‐controlled liquid‐phase epitaxy—was developed which defines the contribution of the Peltier effect (at the solid‐solution interface) and that of solute electromigration to the overall growth process. According to the model, the contribution of electromigration to growth is dominant in the absence of convection in the solution, whereas the contribution of the Peltier effect can be dominant in the presence of convection. On the basis of the model, expressions were derived which relate quantitatively the growth velocity to growth parameters. The model was found to be in excellent agreement with extensive experimental data on the electroepitaxial growth of GaAs from a Ga‐As solution.

Heat transfer analysis of the Bridgman-Stockbarger configuration for crystal growth: I. Analytical treatment of the axial temperature profile

Abstract The parametric behavior of radial temperature variations within the charge during crystal growth in a Bridgman-Stockbarger configuration is derived by a two-dimensional heat transfer model. By treating the charge and the crucible as a series of concentric fins which exchange heat radially with each other, useful solutions in analytical form are obtained. The model considers the effects of the length of an insulating zone between hot and cold regions of the furnace, thickness and thermal conductivity of a charge confining crucible, thermal coupling between the charge and the furnace, change in the thermal conductivity at the crystal-melt interface, and the generation of latent heat. Results show that the presence of a crucible diminishes the ability to control the shape of the growth interface through its position within the insulating zone and, for semiconductors, leads to a growth interface shape for which the crystal is concave.

Thermoelastic analysis of GaAs in LEC growth configuration

Abstract Numerical solutions of the complete set of thermoelastic equations for GaAs in conventional LEC configuration show that the prevailing thermal stresses exceed by close to one order of magnitude the CRSS. The stresses are found to be strongly affected by the thermal transparency and thickness of the liquid encapsulant and by the thermal characteristics of the growth environment.

Analysis of crystal growth characteristics in a conventional vertical Bridgman configuration

Abstract The growth behavior in a conventional vertical Bridgman system was studied experimentally using gallium-doped germanium. The effects of varying modes of heat extraction from the seed end, the charge lowering rate, and the temperature distribution in the furnace on growth were investigated. It was found that the microscopic growth rate is transient at all times and that the growth interface remains concave into the solid. Initial transients follow a first order exponential rate law in most thermal configurations; extended configurational rate transients could be attributed to thermal end effects. The location of the control thermocouple was found to critically affect the growth behavior.

Infrared absorbance of B2O3 at temperatures to 1250 °C

Infrared absorbance spectra and corresponding absorption coefficients of B2O3 at temperatures up to 1250 °C were measured in an external high temperature system using modulated radiation from a Fourier transform IR spectrometer.

Oscillatory Interface Instability During Czochralski Growth of Heavily Doped Germanium

Oscillatory interface instability associated with constitutional supercooling was established during growth of germanium single crystals from gallium‐doped melts by the Czochralski technique. The wavelength and phase velocity of the instability were determined and found to be consistent with theory. The effects of pulling and rotation rates on oscillatory instability were quantitatively correlated with the interface stability theory. It was shown that during rotational pulling under conditions leading to constitutional supercooling, the destabilizing effects of rotation dominate its stabilizing effects for moderate rates of rotation in the presence of thermal asymmetry.

Microsegregation in conventional Si-doped LEC GaAs

Abstract The dopant distribution in 〈100〉 Si-doped GaAs was quantitatively analyzed using NIR transmittance measurements with a spatial resolution of better than 2 μm. Microsegregation inhomogeneities, in the form of striations, are found to be random and discontinuous; they are identified predominantly as abrupt dopant concentration decreases resulting from temporal, localized back melting associated with turbulent convection in the melt. Variations in free charge carrier density associated with the striations approach in many instances two orders of magnitude and are thus by a factor of ten larger than anticipated and reported in the open literature for Si and Ge. Periodic striations normally associated with rotational crystal pulling are absent.

Computational approach to equilibrium relaxation in central force systems

Abstract The equilibrium configurations of low index surfaces in materials subject to central force laws and the behavior of adatoms on such surfaces were investigated using pairwise interaction theory. A direct search optimization code utilizing trial solutions with both a one-variable-at-a-time search and a vector search to scan the variables for energy optimization was applied. It was found that the interplanar spacings adjacent to a planar free surface are larger than equivalent bulk spacings and that the degree of interplanar expansion diminishes rapidly with increasing depth below the surface. The magnitude of surface expansion depends strongly on the characteristics of the potential chosen to represent the crystal forces. Surfaces which terminate with a monatomic step show non-linear structures at the step edge. Adsorption energies for adatoms on such surfaces are large on (111) surfaces and smaller on (110) and (100) surfaces. They decrease with decreasing adatom mismatch and increase with less convergent binding potentials. The activation energy for adatom surface diffusion was found to be largest on the (110) surface and smaller on the (100) and (111) surfaces; it decreases with increasing adatom mismatch and increases with more convergent binding potentials. The clustering characteristics of adatoms on (100) surfaces were investigated and applied to the system silver on nickel. The results show that both (111) and (100) oriented silver films could grow simultaneously on (100) nickel surfaces. No special attempt is made to compare the computed results with other published data. The primary purpose is to indicate a novel computational approach to surface investigations.