P. Dold

Segregation in Si floating-zone crystals grown under microgravity and in a magnetic field

Abstract The effects of microgravity and of axial magnetic fields on the micro- and macrosegregation in Si floating-zone crystals are compared. Whereas in the absence of radio frequency (RF) heating thermocapillary and buoyancy convection contribute to macrosegregation, thermocapillary convection leads to dopant striations due to its time dependency. Consequently, floating-zone growth under microgravity (μg) is not capable of suppressing the formation of striations, but due to the reduced mixing of the melt the striation intensity is modestly decreased and the axial macrosegregation is changed. The application of an axial magnetic field up to 0.5 T under terrestrial growth conditions allows a suppression of striations together with a change in the axial macrosegregation. It is, however, detrimental to the radial uniformity of the dopant concentration by producing a non-facet “core” in the crystal, with the core diameter depending on the magnetic induction.

Measurement of Temperature Fluctuations and Microscopic Growth Rates in a Silicon Floating Zone Under Microgravity

Abstract A silicon crystal growth experiment has been accomplished using the floating-zone technique under microgravity on a sounding rocket (TEXUS 36). Measurements of temperature fluctuations in the silicon melt zone due to time-dependent thermocapillary convection (Marangoni convection) and an observation of the microscopic growth rate were simultaneously performed during the experiment. Temperature fluctuations of about 0.5–0.7°C with a frequency range

Detached growth of gallium doped germanium

Detached Bridgman growth of gallium-doped germanium itself as well as the transition from detached to attached growth was observed in-situ for the first time, using a quartz-glass ampoule in a mirror furnace. Crystal diameter was 9 mm, the growth length 41 mm, and the growth velocity 0.5 mm/min. Undoped 1 1 1-oriented germanium served as seed material; the melt was doped with gallium (C0=8.2×1018 at/cm3). Detachment took place after a growth length of 7 mm and continued for 27 mm; the remaining 7 mm grew with wall contact again. The wall-free growth could be observed around the entire circumference except for some small bridges (width: a few tens of micrometers, length: some hundreds of micrometers), where the crystal grew in contact with the wall. In the detached-grown part of the crystal, the 1 1 1-related growth lines are clearly visible. The transition from attached to detached growth and vice versa did not take place along a straight line but transitioned as islands over a length of about 1 mm. The gap between the growing crystal and the container wall varied between 10 and 80 μm, as measured by a profilometer. The etch pit density is greatly reduced in the part of the crystal that grew free of the wall. An increase in the EPD is seen in the area where the crystal had contact with the ampoule wall by the bridges described above.

The influence of axial magnetic fields on the growth of III–V semiconductors from metallic solutions

Abstract III–V semiconductor crystals are grown by the travelling heater method in a homogeneous axial magnetic field. First results demonstrate an influence not only on transport phenomena but also on growth kinetics and morphological stability. An interpretation of the growth results is given by an order of magnitude analysis using the dimensionless Rayleigh and Hartmann numbers.

Crystal growth under microgravity: present results and future prospects towards the International Space Station

Abstract The role which microgravity plays for the understanding of the growth of semiconducting materials is reviewed. Main emphasis is placed on the interaction of fluid flow and mass transport phenomena during different growth processes, the character and strength of surface tension driven flows, the limitations of diffusive transport regimes, and possibilities provided by magnetic fields with respect to support or replace microgravity conditions. Some of the most relevant topics to be investigated during the next decade on board the International Space Station are outlined. The first results of experiments carried out on the ISS are presented.

Solutocapillary convection in the float-zone process with a strong magnetic field

Abstract This paper treats the steady axisymmetric flow and mass transport in a cylindrical liquid bridge between the melting end of a feed rod and the solidifying end of an alloyed semiconductor crystal. There is a strong, uniform, steady, axial magnetic field. The surface tension depends on the temperature and the concentration of the species, while variations of the concentration occur because one species is rejected into the liquid during solidification. The thermocapillary and solutocapillary convections tend to cancel over part of the liquid bridge. For certain parameter ranges, there are two different stable solutions: one where the concentration gradient along the free surface leads to dominance by the solutocapillary convection and one where the mass transport due to the thermocapillary convection makes the concentration gradient along the free surface small, so that the thermocapillary convection is dominant.

Modelling of the temperature distribution in a three-zone resistance furnace: influence of furnace configuration and ampoule position

Abstract A numerical modelling of the heat transport in a three-zone resistance furnace has been performed.The results of the calculations agree very well with experimentally obtained temperature profiles. A strong dependence of the calculated temperature field on the thermal conductivity of the furnace material is observed and can be explained by nonisotropic effects like inhomogeneities in the insulation.The simulations can also predict the influence of modifications in the furnace set-up like an additional electromagnet instead of an outer insulation layer. Further, the computer model is used to reveal the influence of different positions of a growth ampoule. The temperature field in the ampoule shows a decrease of the temperature gradients at the phase boundary at the end of the growth process and a change of the interface shape from concave to convex.

The Influence of Static and Rotating Magnetic Fields on Heat and Mass Transfer in Silicon Floating Zones

Heat and mass transfer in float-zone processing are strongly influenced by convective flows in the zone. They are caused by buoyancy convection, thermocapillary (Marangoni) convection, or artificial sources such as rotation and radio-frequency heating. Flows in conducting melts can be controlled by the use of magnetic fields, either by damping fluid motion with static fields or by generating a defined flow with rotating fields. The possibilities of using static and rotating magnetic fields in silicon floating-zone growth have been investigated by experiments in axial static fields up to 5 T and in transverse rotating magnetic fields up to 7.5 mT. Static fields of a few 100 mT already suppress most striations but are detrimental to the radial segregation by introducing a coring effect. A complete suppression of dopant striations caused by time-dependent thermocapillary convection and a reduction of the coring to insignificant values, combined with a shift of the axial segregation profile toward a more diffusion-limited case, is possible with static fields greater than or equal to 1 T. However, under certain conditions the use of high axial magnetic fields can lead to the appearance of a new type of pronounced dopant striations, caused by thermoelectromagnetic convection. The use of a transverse rotating magnetic field influences the microscopic segregation at quite low inductions, of the order of a few millitesla. The field shifts time- dependent flows and the resulting striation patterns from a broad range of low frequencies at high amplitudes to a few high frequencies at low amplitudes.

Radiative Heat Transfer in a Resistance Heated Floating Zone Furnace: A Numerical Study with FIDAPTM

This paper presents a numerical study of radiative heat transfer in a floating zone (FZ) furnace which was performed by using the commercial finite element program FIDAP . This resistance furnace should provide a temperature higher than the melting temperature of silicon (i.e. T 1500 °C) and a variable temperature gradient at the liquid/solid interface (≥ 25 K/cm). Due to the high working temperatures, heat radiation plays the dominant role for the heat transfer in the furnace. For this reason, the quality of view factors used in the wall-to-wall model was carefully inspected with energy-balance checks. A numerical model with two control parameters is applied to study the influence of material and geometrical parameters on the temperature field. In addition, this model allows us to estimate the internal thermal conditions which were used as thermal boundary conditions for partial 3D simulations. The influences of an optical lens system on the radial symmetry of the temperature field were examined with these partial 3D simulations. Furthermore, we used the inverse modeling method to achieve maximum possible temperature gradients at the liquid/solid interface according to the limitation of maximum available power and the maximum stable height of a melt zone.

Vibration controlled convection — preparation and perspective of the Maxus 4 experiment

Abstract A comparatively new possibility to influence convection in crystal growth melts is the application of controlled interface vibration, especially in systems with free melt surfaces like the float-zone process. The present paper concerns the development and testing of a vibrational device which will be integrated into a microgravity crystal growth facility for the growth of silicon crystals. In the case of silicon grown by the float-zone technique, time-dependent thermocapillary convection is present even under mirogravity and leads to unfavourable variations of the crystal composition profile. The developed setup can operate in the range of approximately 0.1 to 50kHz producing maximum amplitudes of 0.25μm (non resonance case) and 3.5μm (resonance case) respectively. The power consumption is below 5W, the maximum operation temperature of the device is restricted to 200°C, limited by an epoxy-based connection between vibrator and sample. The first microgravity application will be during the European Maxus 4 campaign in April 2001.