G. H. Schwuttke

Structural and electrical charaterization of crystallographic defects in silicon ribbons

Abstract Silicon ribbons grown as wide as 50 mm by the Capillary Action Shaping Technique (CAST) are classified according to perfection and electrical performance. The classification is obtained through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and minority carrier lifetime and solar cell efficiency measurements. First the electrical activity of the defects is measured through EBIC contrast in the SEM. Subsequently the defects are analyzed in the TEM. The defect state in the crystal thus obtained is correlated with minority carrier lifetime and related to solar cell efficiency. Four crystal quality groups are established for CAST ribbons. In these groups solar cell efficiency decrease from 5–8% in class I to less that 1% in class IV. Such degradation in solar cell efficiency is caused by dislocations (I) high order twin boundaries (II), low angle grain boundaries (III), and, silicon carbide dendrites (IV). The electrical activity of such defects varies from low activity to strong activity. Solar cell efficiency depends on the number of “electrically active defects” and not on the total defect content of the ribbon.

Directionally solidified solar-grade silicon using carbon crucibles

Abstract Directional solidification of silicon, in carbon crucibles, was achieved by two variations of the Bridgman method. One is a static technique, wherein liquid silicon in a 5 cm diameter × 5 cm high carbon crucible was positioned in a temperature gradient of about 35°C/cm, with the temperature at the crucible top being hottest. Solidification was achieved by lowering the system temperature at a rate of 4–5°C/min. The second technique entailed lowering a 5 cm × 5 cm × 12 cm high carbon crucible, loaded with silicon, through a fixed RF-coil at a rate of 5.5 mm/min. Crack-free silicon was produced by both methods. The equilibrium grain structure was initiated by nucleation at the crucible walls, with surviving grains tending to grow in alignment with the temperature gradient to produce an axially columnar grain structure, of mainly 〈110〉 orientation. This behavior dominated attempts to seed the growth at the crucible bottom. The average grain diameter was 1.08 mm and the typical length was 7 mm. Transmission electron microscopy was used to assess crystal perfection. A solar cell efficiency of 11.5% (AM1) was achieved using this material.

Factors influencing surface quality and impurity distribution in silicon ribbons grown by the capillary action shaping technique (CAST)

Abstract Parameters such as die top geometry, die channel geometry, and die composition are found to have significant effects upon surface quality, crystallographic perfection, and impurity characteristics of CAST grown silicon ribbons. Die designs which allow a high melt meniscus (0.5–0.8 mm) produce optimum ribbon properties. Such designs are presented. The natures of ribbon surface particles, surface films, surface striations and surface crystallographic characteristics are described and related to growth parameters. Ribbons have been grown in widths up to 100 mm and have been used to make solar cells with efficiencies up to 12%.

Resistivity and annealing properties of implanted Si:H+

Abstract N- or p-type 1 ohm-cm resistivity silicon crystals are bombarded with 1 MeV protons to doses greater than 1015 H+/cm2. The crystals are characterized through X-ray topography and surface and in-depth resistivity measurement techniques. Post-bombardment measurements indicate a high resistance layer (104 ohm-cm range on the silicon surface, extending to 16μm depth after 1 MeV H+ bombardment. Post-anneal results show that this insulating layer is still stable after 300 °C anneal cycles. At higher anneal temperatures the radiation damage starts to anneal out and at 800 °C the crystal returns to its original resistivity.

Electrical and structural characterization of silicon ribbons produced through capillary action shaping

Abstract Silicon ribbons grown as wide as 50 mm by the Capillary Action Shaping Technique (CAST) are classified according to perfection and electrical performance. The defect state in the crystal is correlated with minority carrier lifetime and related to solar cell efficiency. Four crystal quality groups are established for CAST ribbons. In these groups solar cell efficiency decreases from 5–8% in class I to less than 1% in class IV. Such degradation in solar cell efficiency is caused by dislocations (I) high order twin boundaries (II), low angle grain boundaries (III) and, silicon carbide dendrites (IV). The electrical activity of such defects varies from low activity to strong activity. Solar cell efficiency depends on the number of “electrically active defects” and not on the total defect content of the ribbon.

Feedback Control Scheme for Scanning X‐Ray Topography

Feedback control of scanning x‐ray topography is described. A control scheme is developed that maintains the x‐ray topography system at all times at the peak of the rocking curve I(β). The mathematical control scheme is implemented through an angular derivative generator capable of generating the signal dI/dβ on a continuous basis. Circuit requirements to measure dI/dβ continuously are established. Actual measurement of the command signal dI/dβ is accomplished through a modulation technique. Circuit design details are given and the operation of the system is discussed.