Origin of SiC Impurities in Silicon Crystals Grown from the Melt in Vacuum

Abstract

A main source of high carbon levels in silicon crystals grown from melt under reduced pressures and contained in silica crucibles supported by graphite retainer/susceptor has been identified by thermodynamic analysis. The calculations have been verified by experimental results and the carbon level can be reduced by approximately 50% with the use of molybdenum retainers.The heat exchanger method (HEM) developed to grow large sapphire crystals (1-3) has been extended to the growth of silicon crystals (4). In this method the seed is placed at the bottom of the crucible and the temperature in the melt increases upwards. This suppresses convection that causes temperature and concentration fluctuations at the solid-liquid interface (5).Early experiments indicated that SiC particles were found in crystals solidified by the HEM (6). However, even with the presence of SiC particles, large grains have been grown with limited interface breakdown during solidification. This observation was contradistinct to Czochralski (CZ) growth where interface breakdown due to SiC is followed by twin/polycrystalline growth (7).The basic elements of the HEM and the CZ growth furnaces and the processes are quite similar (heaters, crucibles, insulation, etc.). A silica crucible loaded with the charge and set in a graphite retainer is placed in the furnace. The chamber is evacuated, and after melting the charge crystal growth is achieved. In the HEM process, the chamber is typically evacuated during growth to 0.1 Torr. For CZ growth an argon blanket is used and the chamber pressure can vary from 10 Torr to 1 atm (8). In early experiments with HEM growth it was quite surprising that high carbon concentrations were found in the silicon ingots. The purpose of this paper is to explain the origin of this carbon concentration and to present thermodynamic and experimental results that support the conclusions. Reactions between Graphite and SilicaSilicon, oxygen, and carbon are used in the HEM in the form of silicon melt stock, silica crucibles and graphite furnace parts, and retainers. A thermodynamic evaluation has considered possible reactions between these three reactants and the salient reactions are reported.Carbon has been observed in silicon crystals as silicon carbide, Silicon melt stock and graphite parts are not in direct contact; therefore, the following reactions between graphite and silica crucible were studied 2C + SiO2--> SiC + CO2[1]3C + 3SIO2 --> SiC + 2CO2 + 2SiO[2]4C + 2Si02--> SiC + 3C0 + SiO [3] * Electrochemical Society Active Member.

Growth of carbon and nitrogen containing precipitates in crystalline solar silicon and their influence on solar cells

“…SiC precipitates in Si solar cells (in this case in edge‐defined film‐fed grown (EFG) ribbon solar cells) have been found already in 1976 . The sources of carbon detected in the Si material were found to stem from graphite parts of the crystallization furnaces some years later. An idea of the mechanism for generation of such so‐called silicon carbide microdefects has been published in 1986 .…”

The effect of graphite components and crucible coating on the behaviour of carbon and oxygen in multicrystalline silicon

“…The equilibrium is determined by the interaction of gaseous species and by several heterogeneous reactions at the interfaces graphite-atmosphere, melt-atmosphere, and meltcrucible. The contamination of the melt with carbon and oxygen is influenced by CO, SiO, and SiC, which are the dominant species in the system [16]. CO can be formed due to the following reactions between carbon and the oxygen-containing species:…”

“…Another SiO source is the reaction between liquid silicon and the surrounding atmosphere according to Ref. [16] CO+ 2Si2SiC +SiO (4) Apart from reactions (1) and (2) SiO in the system is also consumed by a disproportion according to 2SiO2Si +SiO 2 (5) Eqs. (1), (2), and (4) indicate the strong correlation between SiO and CO if carbon is present in the system.…”

Influence of reaction between silica crucible and graphite susceptor on impurities of multicrystalline silicon in a unidirectional solidification furnace

“…The standard free energy change for reaction (2) in the temperature range from 1300 to 1800 K is expressed as [14] DG 0ð2Þ ¼ 600,691À335TðJ=molÞ ð 8Þ Substitution of Eq. (8) in Eq.…”