Torbjörn Carlberg

Lateral solute segregation during floating-zone crystal growth under different gravity conditions

Abstract A series of floating zone experiments with 5 mm diameter Ge crystal was performed. Experiments were made both at 1 g conditions on the ground and at about 10 -4 g in sounding rockets. Some of the samples had their surfaces covered by pyrex glass to eliminate surface tension forces, others were processed with free surfaces. Radial concentration profiles were studied and it was found that interface shape and convection both had a strong impact on radial homogeneity.

On the mechanism of lamellar spacing adjustment in eutectic alloys

Abstract Well controlled undirectional solidification experiments with continous increased rate were performed for eutectic alloys in the systems Ag-Cu, Al-Zn, Al-Cu and Al-Ni. In the Ag-Cu and Al-Zn alloys it was observed that discontinous changes in the lamellar spacing occured at low solidification rates. At high solidification rates such discontinuities could not be observed. Anyway the lamellar spacing always larger than observed at constant rate experiments. In Al-Cu alloys discontinous changes in the lamellar spacing with increasing solidification rate were also observed. However, after each change the lamellar spacing increased gradually up to a lamellar distance of 20% lower than before the abrupt decrease. All-Ni eutectic alloys gives a rod like eutectic structure. In this alloy no discontinous changes could be observed. A theoretical analyses of the observation based on well known theoretical models are discussed.

Some aspects on the formation of striations during crystal growth from the melt

Abstract In grown crystals periodic or random microscopic concentration variations often appear as striations in etched sections. The striations are results of growth rate fluctuations, caused by either the rotation of a growing crystal in an asymmetric temperature field or by thermal fluctuations in the melt. In the present paper a relation between thermal fluctuations in the melt and the growth rate is derived. The model, which is based on observations of thermal fluctuations in front of a growing crystal, results in a simple analytic expression and can be used for all types of thermal disturbances that affect the growth rate.

Weak flows in a floating zone configuration as a source of radial segregation

Abstract Recent experiments of floating-zone configurations performed in microgravity showed a different radial segregation than expected. These experiments had partially coated melt surfaces consisting of many small evenly distributed spots of free surfaces. The mixed boundary conditions on this composite surface is showed to be modeled with an equivalent Dirichlet boundary condition. This boundary condition is then used to calculate the flow and dopant field in the interior of the floating-zone. The radial segregation calculated with this model is then compared to the experimental results.

Czochralski growth of tin crystals under constant pull rate and IR diameter control

Abstract Czochralski crystal growth experiments with auxiliary IR heating of the melt surface were carried out to investigate the feasibility of thermal diameter control. Tin crystals were grown under constant pull rate using IR control and with pull rate control. Temperature measurements were made in the melt during growth with IR diameter control and with growth rate diameter control. IR diameter control decreased the vertical and horizontal temperature gradients in the melt.

Floating zone experiments with germanium crystals in sounding rockets

Abstract A floating zone experiment with a coated surface was performed in a sounding rocket. Analysis of the segregation showed that elimination of free surfaces decreased the convection in the liquid zone. Analysis of radial segregation during floating zone crystal growth under different experimental conditions demonstrated high sensitivity to interface shape and to convection. Under conditions of very weak convection, microgravity and coated surfaces, dopant concentration fields giving severe radial segregation were built up.

The Influence of Dopants on the Reaction Between Liquid Silicon and Silica

Oxygen incorporation into Czochralski silicon crystals during crystal growth is influenced by reactions at the crucible interface. In the present study it was found that different dopants have different effects on these reactions, both in sealed ampuls and in Czochralski crucibles. Different reaction products and morphologies are formed along the interface depending on dopant types and levels. It was concluded that some of these reactions can affect the oxygen concentration in the melt. The dopants investigated were B, Al, Ga, In, Ge, Sn, P, As, Sb, and Bi

Semi-confined Bridgman growth of germanium crystals in microgravity

To obtain quantitative data on the dopant distribution from different levels of Marangoni flows two Ga doped Ge samples of 6 mm diameter were processed in a modified Bridgman configuration with partially free surfaces. The experiments were performed during the flight of the rocket MASER 2. The results show significantly decreased mixing in the space experiments compared to ground based reference runs. A thin oxide film on all surfaces could explain why the effect of Marangoni forces was less than expected. The radial segregation was strongly dependent on the flow conditions.

The effect of convection upon off-eutectic composite growth of Al-Cu alloys

Abstract Different convection conditions were achieved during directional solidification of off-eutectic Al-Cu alloys by different arrangements of the thermal and solutal gradients versus the gravity vector. The macrosegregation and the stability of a planar solidification front were studied. Increasing convection was clearly shown to increase the macrosegregation and the difficulties in achieving a stable planar front. The convection, followed by thermal fluctuations in the melt, was studied, and the onset of oscillatory instabilities was found to coincide with the critical Rayleigh number. The theoretical analysis of the results was focused on conditions at fluctuating growth rates, for which an anomalous segregation behaviour was revealed.

A Model for Dopant Concentration in Czochralski Silicon Melts

By assuming that diffusion of the dopant molecules in argon gas within the crucible is the limiting step of the evaporation, a model for dopant concentration in Czochralski silicon melt is derived. The dopant concentration in the melt during crystal growth can be expressed as C(g)/C(o)=E(1-g) k-1 where E is an evaporation parameter defined here as 0<E≤1; other parameters have the usual meanings. The dopant concentration change before pulling is modeled for the first time based on the same assumptions. Both equations applied to an antimony-doped silicon system are discussed