Structure and Optical Properties of Silicon Nanocrystals Embedded in Amorphous Silicon Thin Films Obtained by PECVD

Abstract

Silicon nanocrystals embedded in amorphous silicon matrix were obtained by plasma enhanced chemical vapor deposition using dichlorosilane as silicon precursor. The RF power and dichlorosilane to hydrogen flow rate ratio were varied to obtain different crystalline fractions and average sizes of silicon nanocrystals. High-resolution transmission electron microscopy images and RAMAN measurements confirmed the existence of nanocrystals embedded in the amorphous matrix with average sizes between 2 and 6 nm. Different crystalline fractions (from 12% to 54%) can be achieved in these films by regulating the selected growth parameters. The global optical constants of the films were obtained by UV-visible transmittance measurements. Effective band gap variations from 1.78 to 2.3 eV were confirmed by Tauc plot method. Absorption coefficients higher than standard amorphous silicon were obtained in these thin films for specific growth parameters. The relationship between the optical properties is discussed in terms of the different internal nanostructures of the samples.

Crystalline Silicon under Acoustic Cavitation: From Mechanoluminescence to Amorphization

“…At atmospheric pressure, the untreated Si wafer exhibits a cubic diamond structure (space group Fd 3 m ) characterized by a narrow Raman peak at ∼521.5 cm –1 and corresponding to the transversal optical (TO) phonon of the crystalline c-Si (phase conventionally labeled Si–I). − This peak was found to be predominant in all of our Raman spectra and can be observed in Figure (depicted in red for all spectra); it corresponds to the triply degenerated optical phonon in the center of the Brillouin zone. − , After sonication, some spectra were found to present a broadened TO c-Si band with a measured full width at half-maximum (FWHM) up to Γ c ≈7.1 cm –1 (against ∼3.4 cm –1 for the nontreated c-Si, Figure a). TO peak broadening can be attributed not only to an increase in the density of defects within the crystals but also to a phonon confinement effect resulting from the presence of polycrystalline Si (poly-Si). ,− ,,, Other sonicated areas were found to present a high-frequency shift and showed the apparition of a shoulder on the TO c-Si peak (black arrow in Figure b). These features are representative for an anisotropic compressive stress going along with a lattice deformation and leading to removal of the degeneracy of the Raman modes. , The observed frequency shift is known to be influenced by the mechanical stress generated in the crystalline structure and reflects a discrepancy in the distribution of the bond length of the sonicated Si. , …”

“…TO peak broadening can be attributed not only to an increase in the density of defects within the crystals but also to a phonon confinement effect resulting from the polycrystalline Si (poly-Si). 23,[27][28][29]32,33,47 Other sonicated areas were found to present a high-frequency shift and showed the apparition of a shoulder on the TO c-Si peak (black arrow in Figure 5b). These features are representative for an anisotropic compressive stress going along with a lattice deformation and leading to removal of the degeneracy of the Raman modes.…”

Molecular hydrogen‐induced nucleation of hydrogenated silicon nanocrystals at low temperature

“…These NCs can either be free standing or embedded in a host matrix such as a‐Si:H, the result of which is usually called polymorphous silicon . In this case, the optical properties of the films will depend mainly on the amorphous matrix, while the electronic transport will be governed by the size and density of the NCs, which improve carrier mobility and insure stability of the films against radiation…”

Ultra‐thin electron collectors based on nc‐Si:H for high‐efficiency silicon heterojunction solar cells

“…To reduce the parasitic absorption at the illuminated side, doped a‐Si:H is often substituted by doped hydrogenated nanocrystalline silicon (nc‐Si:H) and/or its oxygen alloy (nc‐SiO x :H) that concurrently enable better transparency and higher conductivity 2–9 . These advantageous opto‐electrical properties of nc‐Si:H‐based thin‐films mainly originate from their mixed‐phase compositions, which consist of nanocrystals embedded in the amorphous matrix 10,11 …”

“…[2][3][4][5][6][7][8][9] These advantageous opto-electrical properties of nc-Si:H-based thin-films mainly originate from their mixed-phase compositions, which consist of nanocrystals embedded in the amorphous matrix. 10,11 However, nc-Si:H-based thin-films feature thickness-and substrate-dependent growth characteristics. 12,13 Thus, efforts have been devoted to minimizing the thickness of the initial amorphous incubation layer regardless of the influence from the substrate.…”