Crystal growth of Si-Ge alloys [1] without contact to container walls by the so called “detached Bridgman” technique was found to have considerable improvements in crystalline quality being ascribed to a reduced radial temperature gradient during solidification under detached conditions which, in turn, is associated with a lower thermal shear stress at the interface. The progress under microgravity conditions encouraged our ground-based experiments to grow Si-Ge crystals in the absence of the crucible. Besides, Si-rich Si_xGe_1-x compounds (x>70-80%), like silicon itself for the purposes of electronics, seem to be practically impossible to grow in containers. However, it is very difficult to apply the detached technique on the ground especially for Si-Ge system because of instability of the process leading to breakage of the conditions of the crucibleless growth. Thus, the Floating Zone (FZ) technique was taken as a basis.

Crystal growth was carried out in a laboratory double-ellipsoid mirror FZ furnace in which two light beams of 10 mm in a height were focused at the centerline of the setup. The Si single crystal of 8 mm diameter was used as a seed; while a 20 mm high cylinder of a uniform Si-20%atGe compound was specially prepared as a feeding material. Solidification of the Si-Ge crystals was conducted at the pulling rates of 0.2-0.4 mm/min to prevent the occurrence of morphological instabilities due to the constitutional supercooling.

Along with the conventional process of the crystal growth, the modified FZ technique was applied. An additional heater enabling the realization of the Axial Heating Process (AHP) was used with the end purpose to control the both shape of the melt/crystal interface and thermal conditions over it during all the run. A crystal grows from the thin melt layer under conditions of weak melt flows similar to those established for conventional methods under microgravity conditions.

The graphite casing of the AHP heater of 15 mm in a diameter and 8 mm high was coated with SiC of a special nanostructure to protect it against the Si melt attack. The melt on both the top and bottom surfaces of the AHP heater was suspended by forces of surface tension. During the growth process, the melt layer at the top side of the heater was held within 2-3 mm, while the height of the melt layer under the heater was varied from 2 to 8 mm.

To examine the technique, a set of growth experiments with Si as a model material was conducted [2]. Crystals in a wide diameter range -  relative to the diameter of the AHP heater -  were grown: from 2-3 mm up to 20 mm, twice exceeding the diameter of the feed rod. Thus the complete procedure beginning from necking and then passing to the solidification of the large-scale crystal can be realized. Special experiments were conducted to examine the design of the AHP heater allowing to maintain the different temperatures at its top and bottom faces taking in account 1360°C and 1280^oC as liquidus and solidus temperatures, respectively, of the Si-20%at Ge compound.

A number of Si and its alloy with Ge crystals were obtained, sliced along the axis, polished and etched. The microscope images of the grown Si-Ge crystals are presented in Fig. 1. 

Figure 1. Microscope images of the grown Si-Ge crystals by conventional (a) and modified (b) FZ technique

The crystallinity of the grown silicon and silicon-germanium crystals, as well as interstitial defects in bulk of the material were investigated by means of microprobe/EDX measurements, SEM, NDIC and high resolution XRD are discussed. In order to determine the effects of different growth conditions on the main physico-chemical properties, to reveal the presence of SiC together with products of its decomposition and to get the nature of inclusions available, structural chemical, electrical and spectroscopic characterization were done.

The presence of the main deleterious impurities through electrical measurements and spectroscopic tools, as well as the density and distribution of dislocations and grain boundaries along the ingot are determined. Both the longitudinal and lateral distributions of the second component in Si-Ge crystals are presented.

1. Croell A., Mitric A., Senchenkov A. Abstracts in ICASP-2, Seggau, Austria, (2008).

2. M. Gonik, A.Croell. CrystEngComm 15(12) (2013) 2287-2293.

1. FULLTEXT: SiGe crystal growth in the absence of the crucible, JPEG image data, JFIF standard 1.02, comment: "ACD Systems Digital Imaging", 0.1MB

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1. Grain and Subgrain Boundaries in Multi-Crystalline Silicon
2. Behaviour of Particle Depots in Molten Silicon Under Terrestrial And Microgravity Conditions
3. Material development for directional solidification of multicrystalline silicon by AHP method
4. Interaction of Foreign phase particles with moving solid-liquid interface during directional solidification of silicon for photovoltaics