Structural characterization of nc-Si films grown by low-energy PECVD on different substrates
Introduction
Nanocrystalline silicon (nc-Si), a biphasic material consisting of a dispersion of silicon nanocrystals embedded in an amorphous silicon matrix, has attracted in the last decades considerable interest as a promising candidate for solar cells [1], [2], [3], optoelectronic devices [4] and thin film transistors [5]. Notwithstanding its potential, the industrial applications of nc-Si have been limited, primarily by the relatively low growth rate typical of most deposition techniques used for its growth [6]. Recently, however, it has been shown that in the case of both the Very High Frequency Chemical Vapour Deposition [7] and the Low Energy Plasma Enhanced Chemical Vapour Deposition [8] techniques growth rates of 3–5 nm/s can be obtained, compatible with industrial applications.
Furthermore, although much work has been done to study the structure and properties of nc-Si in the past few years, current scientific knowledge cannot provide precise correlations between growth parameters and optoelectronic properties, as it was observed that the material features are subtly related to the specific deposition technique [9]. As a consequence, the real challenge in the nc-Si based technology development is the realization of a proper tuning of the material nanomorphology [10], in view of its different applications.
It is widely known that several deposition parameters, such as silane dilution, growth temperature, plasma energy and density strongly affect the nc-Si structure. It was also already observed that the substrate has a relevant influence on the film structure [11], [12], [13]. In fact, depending on its nature, structure (glassy, amorphous or crystalline) and surface defectivity, the nucleation could give rise to very different microstructures [11].
Aim of the present work is to investigate the effect of substrate on the structural details (texture, crystallite environment and size) of nc-Si films grown by LEPECVD [14], since, as stated above, for most PECVD processes the nc-Si structural properties depend in a complex fashion on details of the deposition procedure. This particular variant of PECVD, based on a low-energy DC plasma, was, in fact, originally developed for Si homoepitaxy at low substrate temperatures and SiGe/Si heteroepitaxy [14] and only recently used for nc-Si deposition [8]. In this work, samples grown by LEPECVD [14] on two different kinds of substrates were submitted to a systematic characterization using Raman spectroscopy, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM).
Section snippets
Experimental details
The nc-Si samples were grown by the LEPECVD apparatus described in ref. [14] on Czochralski (CZ) silicon (S set) and Corning 7059 glass (G set). The CZ substrates were 4″ (1 0 0) wafers while the Corning glass ones were square shaped 5 cm × 5 cm. The growth temperature was 280 °C, while the silane dilution in the hydrogen–silane feed gas mixture was varied between 10 and 50%. The growth rate ranged between 1.3 and 3.5 nm/s and linearly increases with increasing silane flux, i.e. d%.
Results and discussion
Preliminary results on similar samples obtained by the present authors and reported in ref. [19] showed that, almost independently of the deposition conditions, the films present a columnar structure, with columns oriented normally to the substrate.
From a general point of view, the grains forming the columns could be distributed randomly or could have a preferential orientation or texture. The results of XRD measurements carried out both on the S and the G sets, reported in Fig. 1, showed that
Conclusions
In this work, two series of samples grown with high deposition rates by LEPECVD respectively on crystalline Si and glass substrates were investigated by Raman spectroscopy, XRD and HRTEM in order to study the influence of substrate on the film microstructure.
The results have clearly demonstrated that the structural properties of the nc-Si films are strictly related to the substrate nature, particularly for high silane dilutions. In this case, in fact, the nc-Si films deposited on crystalline
Acknowledgments
The authors would thank Dr. Lucia Galimberti for the technical assistance in the XRD measurements. The financial support of the European Commission through the project NANOPHOTO (contract number 013944) is also gratefully acknowledged.
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