Growth undercooling in multi-crystalline pure silicon and in silicon containing light impurities (C and O)

Abstract

Undercooling during the solidification of silicon is an essential parameter that plays a major role in grain nucleation and growth. In this study, the undercooling of the solid-liquid interface during growth of multi-crystalline silicon samples is measured for two types of silicon: pure, and containing light elements (carbon and oxygen) to assess and compare their impact on crystal growth. The solid-liquid interface undercooling is measured using in situ and real time X-ray synchrotron imaging during solidification. As a subsequent step, ex situ Electron Backscattered Diffraction (EBSD) is performed to obtain information about the crystalline structure, the grain orientation and the grain boundary character. Two main conclusions arise: i) the undercooling of the global solid-liquid front increases linearly with the growth rate which indicates uniform attachment, i.e. all atoms are equivalent, ii) the same trend is observed for pure silicon and silicon containing carbon and oxygen. Indeed, the growth law obtained is comparable in both cases, which suggests that the solutal effect is negligible as concern the undercooling in the case of a contamination with carbon (C) and oxygen (O). However, there is a clear effect of the impurity presence on the crystalline structure and grain boundary type distribution. Many grains nucleate during growth in samples containing C and O, which suggests the presence of precipitates on which grain nucleation is favored.

{1 1 1} facet growth laws and grain competition during silicon crystallization

“…Recently, we showed that there exists undercooling of the global solid-liquid interface of a few kelvins for the processing parameters used in our experiments [21]. Thus, the measured undercooling inside the grain boundary groove or at the level of the edge facets is an additional component to the undercooling.…”

Role of Impurities in Silicon Solidification and Electrical Properties Studied by Complementary In Situ and Ex Situ Methods

“…Indeed, the presence of more grain boundary grooves at the solid-liquid interface reveals the presence of more grain boundaries as grain boundary grooves are formed at the encounter of a grain boundary with the interface. [43] Additionally, it is expected that impurities or precipitates accumulate in grain boundary grooves as proposed by Fujiwara et al [48] Inside grain boundary grooves the undercooling is higher as measured in our previous work [41] so that there is a higher probability of grain nucleation especially if some impurities / precipitates favorable for nucleation are present, enhancing the effect of the presence of nucleation sites. As nucleation takes place on precipitates/impurities, the new grain does not have to be necessarily in twin relationship at the level of the {111} facets that can exist in the grain boundary groove.…”

X-ray Based in Situ Investigation of Silicon Growth Mechanism Dynamics—Application to Grain and Defect Formation

“…The measured maximum undercooling was, thus, calculated inside grain boundary grooves for several experiments with seeds oriented along <100>, <111> and <110> directions. In all cases, the maximum undercooling inside the grain boundary groove is found to be always lower than 1 K ranging from 1 × 10 −1 to 4 × 10 −1 K and adds to the solid-liquid interface undercooling [47]. Eventually, the mean facet velocity evolution as a function of the additional undercooling inside the grain boundary grooves can be obtained.…”

“…When comparing the growth rate of the global solid-liquid interface to the {111} facet growth rates, it appears first that the growth rates of the {111} facets both inside the grooves and at the edges are smaller than the one of the global solid-liquid interface. This is expected because of the slower kinetics of the {111} planes compared to the other crystallographic orientations so that they are lagging behind other growing orientations and generally behind the global solid-liquid interface [47]. A major consequence is that the undercooling is higher in the groove and at the level of the edge facets compared to the undercooling at the level of the global solid-liquid interface.…”

“…The GaTSBI (Growth at high Temperature observed by Synchrotron Beam Imaging) tool was developed to fulfil this objective. This present paper is a review paper of our major results concerning the formation of grains, twinning, and competition [44][45][46][47][48][49][50][51][52][53] using advanced in situ and complementary ex situ characterization methods. In situ X-ray imaging and methods are described in detail.…”