Identification of Nucleation Center Sites in Thermally Annealed Hydrogenated Amorphous Silicon

Abstract

Utilizing the concepts of a critical crystallite size and local film inhomogeneity, it is shown that nucleation in thermally annealed hydrogenated amorphous silicon occurs in the more well ordered spatial regions in the network, which are defined by the initial inhomogeneous H distributions in the as‐grown films. Although the film H evolves very early during annealing, the local film order is largely retained in the still amorphous films even after the vast majority of the H is evolved, and the more well ordered regions which are the nucleation center sites for crystallization are those spatial regions which do not initially contain clustered H, as probed by H NMR spectroscopy. The sizes of these better ordered regions relative to a critical crystallite size determine the film incubation times (the time before the onset of crystallization). Changes in film short range order upon H evolution, and the presence of microvoid type structures in the as grown films play no role in the crystallization process. While the creation of dangling bonds upon H evolution may play a role in the actual phase transformation itself, the film defect densities measured just prior to the onset of crystallization exhibit no trends which can be correlated with the film incubation times.

Can the crystallization rate be independent from the crystallization enthalpy? The case of amorphous silicon

“…This kind of explanation was recently given to interpret why the incubation period was shorter in HWCVD films with low H content than in conventional PECVD films [8]. According to that research, nucleation in both kinds of films would occur in H-free regions, which would be better ordered.…”

“…Our own experiments on EBE [39] and PECVD samples show that, just before any crystalline fraction appears in the XRD curves, the width of the first diffraction peak tends to a common minimum value (maximum MRO) that coincides with that of [37]. Furthermore, a nice correlation can be established between the initial MRO (before annealing) and the incubation time, t inc (figure 6): samples having a broader first diffraction peak crystallize after longer incubation periods (this correlation was not clear in [8] because the MRO approached its maximum value there). Since the interfacial energy, σ ac , has an important effect on the nucleation barrier (equation (3)) and we have verified [39] that the SRO does not change during the incubation period (i.e., g c remains constant), we propose that σ ac is related to the MRO and that it is lower for higher MRO.…”

In situ crystallization kinetics studies of plasma-deposited, hydrogenated amorphous silicon layers

“…1 Poly-Si as obtained by SPC of a-Si:H 2-4 is characterized by large grains ($2 lm) and good area uniformity. For example, CSG Solar has reported on a conversion efficiency of 10.4% based on the SPC of a-Si:H layers deposited by plasma-enhanced chemical vapor deposition (PECVD).…”

“…The information related to the MRO scale (3-6 Å to as high as 15-25 Å 20,27-30 ) were obtained by XRD linewidth analysis of the full-width-at-half-maximum (FWHM) of the lowest angle x ray scattering peak. 1,[31][32][33] The FWHM was computed by fitting the first diffraction peak of a-Si:H spectra. 1,29,32,33 The FWHM usually varied between 5 and 6 for all of a-Si:H samples.…”

“…The incubation time was chosen as the time needed to observe the c-Si XRD (111) peak rise to 1/10 of its final intensity. 1,11,34 Lift-out focused ion beam (FIB) sample preparation was performed to prepare samples for XTEM. The XTEM studies were carried out using an FEI Tecnai F30ST.…”