Structural variations of Si1−xC x and their light absorption controllability
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The emergence of third-generation photovoltaics based on Si relies on tunable bandgap materials with embedded nanocrystalline Si. One of the most promising approaches is based on the mixed-phase Si1 − xC x . We have investigated the light absorption controllability of nanocrystalline Si-embedded Si1 − xC x produced by thermal annealing of the Si-rich Si1 − xC x and composition-modulated superlattice structure. In addition, stoichiometric SiC was also investigated to comparatively analyze the characteristic differences. As a result, it was found that stoichiometric changes of the matrix material and incorporation of oxygen play key roles in light absorption controllability. Based on the results of this work and literature, a design strategy of nanocrystalline Si-embedded absorber materials for third-generation photovoltaics is discussed.
KeywordsNanocrystalline Si Solar cell Silicon carbide Light absorption Superlattice PACS 78.20. + e 78.30.Ly 78.40.Fy.
Amorphous materials with embedded nanocrystals enable a design method for specific optical and electrical properties of thin film materials. This design enablement of this mixed-phase material originates from the well-known physical principle called quantum confinement. The size-dependent bandgap tuning of nanocrystals embedded in a material with a larger bandgap has been experimentally demonstrated by several groups [1–3], and its application has been also successfully demonstrated in the fields of single-electron devices , memories , light-emitting devices , and solar cells . In solar cells, nanocrystals and their quantum confinement serve a route to the third-generation photovoltaics . For example, intermediate band solar cells  and multi-exciton collection [7, 10] have been demonstrated, which were expected to provide groundbreaking enhancement of solar cell efficiency. However, those demonstrations for third-generation photovoltaics are based on III-V epitaxial thin films or lead chalcogenide-based colloidal nanocrystals, which might not be cost-effective or environmentally friendly. On the other hand, an aggressive consideration called all-Si tandem solar cells is under research in some research groups . They have suggested multi-junction solar cells composed of silicon nanocrystals whose bandgaps are controlled by their sizes. Some fundamental works such as size-dependent photoluminescence wavelength [1, 2, 11], window layer application of heterojunction Si solar cells , and primitive absorber layer application in thin film Si solar cells  have been reported.
Previous works on the Si nanocrystal in Si1 − xC x demonstrated that thermal annealing can be used to control the bandgap of this mixed-phase film within the range between 1.4 and 2.2 eV, which renders the optimal combination of triple-junction all-Si tandem solar cells. This bandgap variation was mainly attributed to the bandgap increase of the Si1 − xC x matrix due to the limited effect of the quantum confinement of Si nanocrystals . According to this argument, the Si nanocrystal is not necessary in the formation of all-Si tandem solar cells, which is well supported by earlier findings on the bandgap tunability of amorphous SiC . Nevertheless, the inclusion of silicon nanocrystal would provide additional opportunities for breaking the Shockley-Queisser limit  via intermediate band solar cells  or multiple exciton generation . Therefore, the research on the Si nanocrystal is still meaningful for the potential third-generation photovoltaics. In this work, we have performed structural and optical characterization of thermally annealed SiC thin films with structural variations. As a result of systematic analysis, we have found that nanocrystalline Si (nc-Si) formation significantly affects the optical properties due to the stoichiometric changes of the matrix material, which also seems to be related to the oxygen incorporation. This coupled effect of stoichiometric change and oxygen incorporation will be discussed in detail, and a novel strategy on the tunable absorber design of solar cells will be also presented.
Si1 − xC x thin films were deposited on Si wafers and quartz substrates simultaneously by radio frequency (RF) magnetron co-sputtering at 200°C. High-purity (4N) Si and C targets (diameter, 4 in.) were used. After cleaning the substrates, the Si wafers were dipped in 5% HF solution for 1 min just before loading into the chamber to remove native oxides. The composition of Si1 − xC x films was controlled by adjusting RF powers to each target material. We have chosen two kinds of composition for the annealing experiment of Si1 − xC x : stoichiometric SiC (SSC) with x = 0.56 and Si-rich SiC (SRSC) with x = 0.08, where the composition was characterized by Rutherford backscattering spectroscopy. SSC and SRSC samples were prepared to have a film thickness of 150 nm for all the experiment and characterization. In addition, superlattice structures (SL) were also prepared by alternative deposition of 36 periods of SSC layers (approximately 1 nm) and SRSC layers (approximately 4 nm), which has a total film thickness around 180 nm. Thermal annealing experiments for SSC, SRSC, and SL samples were performed in a quartz tube furnace at 800°C, 900°C, and 1,000°C for 20 min in nitrogen atmosphere.
The structural and crystallographic characterization of the nanocrystals in the SL was performed by high-resolution transmission electron microscopy (HRTEM), transmission electron diffraction (TED), and grazing incidence X-ray diffraction (GIXRD). Raman spectroscopy was used to analyze the crystal volume fractions, and chemical bonding configurations were studied with Fourier transform infrared (FTIR) spectroscopy and X-ray photoemission spectroscopy (XPS). Photoluminescence (PL) characteristics were studied with an Ar+ laser (λ = 488 nm) excitation source within the temperature range from 5 K to room temperature. Optical transmission and reflection measurements within the wavelength range between 300 and 1,800 nm were performed with an ultraviolet-visible-near infrared spectrophotometer, and optical bandgaps were determined from the Tauc plot.
Results and discussion
Light absorption in Si1 − xC x is basically controlled by its stoichiometry  and bonding configurations . This may enable the all-Si1 − xC x tandem solar cell structure, but employing the mixed-phase structure, i.e., nc-Si with various Si1 − xC x , would provide more opportunities in high-efficiency strategies such as intermediate bands or multiple exciton generation. It is evident that tuning the sizes of nc-Si is not a very efficient method to cover a broad range of absorption band, but tuning the stoichiometry of the matrix material would be highly viable. In addition, both the stoichiometry of the matrix material and the oxygen incorporation can be applied to tune the absorption property of the material. In this work and several previous reports [14–16], thermal annealing methods have been presented to demonstrate bandgap tuning properties of mixed-phase Si1 − xC x thin films; however, direct forming methods using low-temperature deposition tools are highly necessary to attain progresses towards device demonstration. There have been several reports regarding the mixed-phase Si1 − xC x thin film using low-temperature processes [33, 34], polymorphous Si thin films in fast deposition regime [35–37], and formation of nc-Si using atomic hydrogen treatment which have been known to be feasible for photovoltaic thin film production . Using these pre-existing technologies, further investigation on nc-Si-embedded mixed-phase Si1 − x − yC x O y seems to provide a promising route for Si-based third-generation photovoltaics.
In summary, we have performed thermal annealing experiments on Si1 − xC x with various film structures and compositions. As a result, we have found that stoichiometric changes and oxygen incorporation of the matrix Si1 − xC x significantly affects the light absorption properties of mixed-phase Si1 − xC x thin films. This clarifies the strategy towards implementing a light absorber of third-generation photovoltaics: nc-Si-embedded mixed-phase Si1 − -x − yC x O y with pre-existing low-temperature deposition technologies.
This work was supported by the Korea Institute of Energy Research (No. GP2012-0002) and by the IT R&D program of MKE/KEIT [10039200, Development of High Performance Phase Change Materials].
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