A. Cröll

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.

Ellipsometric characterization and density-functional theory analysis of anisotropic optical properties of single-crystal α-SnS

We report on the anisotropic optical properties of single-crystal tin monosulfide (SnS). The components ea, eb, and ec of the pseudodielectric-function tensor ⟨e⟩ = ⟨e1⟩ + i⟨e2⟩ spectra are taken from 0.73 to 6.45 eV by spectroscopic ellipsometry. The measured ⟨e⟩ spectra are in a good agreement with the results of the calculated dielectric response from hybrid density functional theory. The ⟨e⟩ spectra show the direct band-gap onset and a total of eight above-band-gap optical structures that are associated with the interband-transition critical points (CPs). We obtain accurate CP energies by fitting analytic CP expressions to second-energy-derivatives of the ⟨e⟩ data. Their probable electronic origins and implications for photovoltaic applications are discussed.

GaSb : surface tension and floating-zone growth

Abstract The dependence of the surface tension of Ga x Sb 1 − x melts on stoichiometry and temperature was measured using the sessile drop technique. A method of determining surface tension directly from pictures of the floating-zone growth process was used. The growth angle of GaSb was measured under 1 g and compared to μ g results. An estimation of the critical Marangoni number is given.

Floating-zone growth of GaAs

Abstract Te-doped and undoped GaAs single crystals of 6–7 mm diameter and up to 20 mm length have been grown by the floating-zone method. In addition, a Te-doped GaAs single crystal of 20 mm diameter and 20 mm length was successfully grown during the 2nd German Spacelab mission D2. It could be shown that growth rates up to 240 mm/h are possible without losing single crystallinity. Dopant striations due to Marangoni convection have been found, as well as a reduction of the etch pit density compared with that of the starting material.

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.

Silicon crystal growth by the modified FZ technique

An additional heater, similar to the axial heating process setup (AHP heater), was used in crystal growth of Si and its compound with 20% at Ge by the Floating Zone (FZ) method with the purpose of controlling both the shape of the melt–crystal interface and the thermal conditions at the interfaces during the run. The heater shapes the melt zone around itself by surface tension forces, being suspended between the growing crystal and feed rod. To protect the graphite casing of the heater against the aggressive action of molten silicon, the casing surface was coated with SiC having a special nano-crystalline structure. The range of the melt layer thickness, which one could establish as high as possible for the stability of the shaping process, was found to be up to 20 mm. Grown As- doped Si single crystals with a diameter of up to 15 mm was shown to have strong twinning directly caused by presence of the SiC inclusions revealed in the bulk of a crystal. The possibility of reducing the convexity and produce an interface close to a flat shape by means of the AHP heater was proved. Faceted growth of Si was found to be present in crystal growth on [111]-oriented seeds, with the faceted area occupying almost all of the cross section of the crystal under certain conditions.

A formula describing the temperature dependence of surface tension for some semiconductors melts

Abstract A simple semiempirical model is derived that provides estimates of the temperature coefficient of surface tension if the following properties are known at a certain temperature: density, temperature coefficient of density and surface tension. Adsorption effects are neglected. From a combination of microscopic and macroscopic arguments, a formula with a “geometry factor” λ results. The geometry factor can be estimated by the model to be of the order of unity. A choice of λ = 2.3 is in good agreement with measured values of semiconductor melts.

Temperature and growth rate fluctuations in silicon floating zones

Abstract Several earlier μg experiments have shown that time-dependent thermocapillary (Marangoni) convection is the major cause for the formation of dopant striations in floating-zone grown semiconductor crystals, at least in small-scale systems not employing RF heating. To quantify this correlation, a silicon floating-zone experiment was performed on the TEXUS36 flight (February 7, 1998) in a monoellipsoid mirror furnace to measure temperature fluctuations in the melt zone and the microscopic growth rate simultaneously. Fluctuations of 0.5K – 0.7K with main frequencies between 0.1Hz and 0.3Hz were detectable. The microscopic growth rate fluctuated considerably around the average growth rate of 1mm/min: rates from 4mm/min to negative values (backmelting) were observed. Dopant striations are clearly visible in the Sb-doped crystal. The frequencies associated with the dopant inhomogeneities correspond quite well with those of the temperature fluctuations and microscopic growth rates. 3D numerical simulations were performed to predict the optimum position of the temperature sensor, to evaluate characteristic temperature amplitudes and frequencies, and to give insight into the instability mechanisms of Marangoni convection in this configuration. The simulations were in good agreement with the experimental values, showing temperature fluctuations with frequencies f ≤ 0.25Hz and amplitudes up to 1.8K at a position equivalent to that of the sensor tip in the experiment. Future microgravity S-FZ experiments, currently scheduled for a MAXUS flight in 2001, will try to influence the formation of dopant striations by rotating magnetic fields or by vibrational convection.

К ВОЗМОЖНОСТИ ВЫРАЩИВАНИЯ ОБЪЕМНЫХ КРИСТАЛЛОВ Si—Ge МЕТОДОМ ОСЕВОГО ТЕПЛОВОГО ПОТОКА ВБЛИЗИ ФРОНТА КРИСТАЛЛИЗАЦИИ

A technique for crucibleless growth of single−crystal silicon and its alloys with germanium is developed. For this purpose, the setup of floating zone method was used, which was equipped with additional so−called AHP heater. The heater forms around itself a melt zone that is suspended between the growing crystal, the feeding rod and correspondingly the bottom and the top surfaces of the AHP heater by forces of surface tension. To protect the graphite casing of the heater against the aggressive action of molten silicon, the casing surface was coated with SiC having a special nano−crystalline structure. The system of automation control of the AHP crystallization mode is described. It allows controlling the thermal field near the growing crystal with an accuracy of about 0.05−0.1 K. Numerical computations of heat and mass transfer during the solidification of SixGe1−x alloy with a 2% Si content, as well as shaping of the free Si−Ge melt surface during the crystal pulling were performed. Uniform bulk crystals were obtained. The range of the highest melt layer at which the shaping process remains stable was found to be 10−20 mm. The grown As−doped Si single crystals showed to have strong twining directly caused by presence of the SiC inclusions revealed in the crystal bulk. The possibility to achieve a convex and nearly flat shape of the interface by means of the AHP heater was proved. The layered mechanism of Si crystallization was found to be present during crystal growth on a seed in the [111] direction, with the faceted area under certain conditions occupying almost the entire crystal cross section.

РАСПРЕДЕЛЕНИЕ Ge В СЛИТКЕ СПЛАВА Si0,9Ge0,1 ПРИ ВЫРАЩИВАНИИ КРИСТАЛЛА ИЗ ТОНКОГО СЛОЯ РАСПЛАВА

We studied experimentally and theoretically the possibility to obtain a uniform single crystal of SiGe alloy enriched at the Si side. The content of the second component in a crystal 15 mm in diameter and 40 mm in length grown by the modified floating zone technique from the charge of 79.8 at.% Si and 20 at.% Ge composition with 0.2% B admixture has been investigated using selected area X − ray analysis in different points and in line scanning mode along and across the crystal axis. The longitudinal changes in the germanium concentration of proved to be well described by the analytical equation previously derived for conditions of Sb (Ga) doped Ge growth from a thin melt layer in the presence of a heater submerged into the melt. For a more accurate description of the experimental data we made allowance for the change in the melt layer thickness between the growing crystal and the bottom of the submerged heater. The lateral distribution of the second component, not exceeding 5% over a diameter of the crystal, can be significantly improved by reducing the curvature of the phase interface during the growth.