Optical characterisation of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix for third generation photovoltaics
© Di et al; licensee Springer. 2011
Received: 12 September 2011
Accepted: 3 December 2011
Published: 3 December 2011
Silicon nanocrystals with an average size of approximately 4 nm dispersed in SiO2/Si3N4 hybrid matrix have been synthesised by magnetron sputtering followed by a high-temperature anneal. To gain understanding of the photon absorption and emission mechanisms of this material, several samples are characterised optically via spectroscopy and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption and luminescence spectra. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Possible mechanisms of absorption and emission as well as an estimation of exciton binding energy are also discussed.
Keywordssilicon nanocrystals third generation photovoltaics absorption coefficient photoluminescence band gap extraction
Self-assembled silicon nanocrystals [Si NCs] embedded in a dielectric matrix are believed to be a promising material for applications in optoelectronics [1–3] and photovoltaic solar cells [4–10]. One major advantage of Si nanocrystals over bulk Si is the freedom to engineer the material's effective band gap by varying the size of the Si NCs or by modifying the properties of the matrix material. A simple method of fabricating 'SiO/SiO2 superlattice' or 'Si NCs in SiO2 matrix' was described by Zacharias et al. . The optical absorption properties of this kind of superlattices were investigated by a number of groups [12–14]. Photovoltaic diodes fabricated using similar approaches have been demonstrated by some of the present authors [5, 6]. Their limitations include high device resistivity and the lower-than-expected output voltages.
To overcome these problems, an improved nanostructure, 'Si NCs in SiO2/Si3N4 hybrid matrix', has been recently proposed by us for the application of 'Si quantum dot photovoltaics' . Experimental investigations have shown that the material possesses better nanocrystal growth and carrier transport properties . However, few studies have been conducted to comprehensively examine the new material's optical characteristics, which are essential in the understanding of device operation. In this paper, we report some initial experimental observations on the optical properties of Si NCs embedded in a SiO2/Si3N4 hybrid matrix.
Analysis and discussion
Region I is a region in which the absorption curves generally exhibit square dependence. By applying the Tauc analysis (in its generalised form: (αhν) γ versus hν) on region I and take γ = 1/2, the resultant graph is shown in Figure 4b. The intercepts of the quasi-linear sections on the energy axis represent the band gaps extracted from the optical absorption measurements. The band gaps are of indirect nature, as γ = 1/2 is used to obtain the linearised spectra [17, 18]. The estimated first indirect gaps are 1.90 eV, 1.95 eV and 1.84 eV for undoped, B-doped and P2O5-doped samples, respectively. This transition, although about 0.78 eV higher in energy due to quantum confinement, can be related to the first indirect transition (Г25' - X1) in Si.
The absorption curves in region III are mostly linear. Therefore, Tauc plots with γ = 1 are best suited for the analysis (Figure 4c). The linear extrapolations cross the energy axis at around 3.4 eV. Since γ = 1, and thus 1/2 < γ < 2, the photon absorption that occurs in this region is a 'quasi-direct' transition. We assign this to the joint contribution of the indirect (Г25' - L1) and the direct (Г25' - Г15) transitions.
In region V, the lower density of data acquisition and the instrument's measurement limit lead to some uncertainty in the analysis. However, the absorption curves in this region generally follow a square-root dependence. Thus by taking γ = 2 in the generalised Tauc analysis (Figure 4d), we obtain x-intercepts in the photon energy region of 4.1 to 4.3 eV. These absorption bands resemble direct transitions (γ = 2) . The average value of the energy gaps (4.2 eV) is comparable with the direct transition (Г25' - Г2') in unconfined Si. However, it should be noted that the Tauc analysis may not be strictly applicable because it assumes parabolic energy bands. This is not necessarily the case for NCs and is the reason for the mixed direct/indirect nature of the analysis presented here.
The absorption peaks in regions II, IV and V have not been clearly understood. Since they appear at certain energies regardless of the kind of dopant introduced, they are likely due to measurement errors or defect states. The measurement error of our spectrometer is within 2%, as specified by the manufacturer. The main sources of experimental error include different sample placements in reflection and transmission modes as well as the change of detector/source during measurement. However, the influence of these factors on the accuracy of the optical band gap estimation is very small because of the following reasons: (1) the analysis method we presented in this paper calculates absorption coefficient versus wavelength data on a point-by-point basis, which means each data point is analysed separately so that errors or noises present in particular points do not affect the analysis of their neighbouring points; and (2) to further eliminate the effects of instrumental errors and noises, we examine only the non-abrupt and relatively smooth regions (e.g., I, II and V) of the absorption curves.
In conclusion, we have synthesised approximately 4-nm Si NCs of different dopant inclusions (B, P2O5 and undoped) dispersed in SiO2/Si3N4 hybrid matrix by magnetron sputtering followed by a high temperature anneal. Analyses of the interference-free optical absorption and photoluminescence spectra reveal that the direct/indirect character of the Si NCs is mixed. Based on the absorption spectra, the materials appear to have an indirect band gap at about 1.90 eV, a quasi-direct band gap at 3.4 eV and a direct gap at around 4.2 eV. The PL emission of these NCs occurs at around 1.57 eV, suggesting sub-band gap radiative transitions. A possible estimate of the exciton binding energy is around 0.33 eV. Future works could include the following: (1) improvement of material properties by defect passivation techniques, (2) fabrication of working devices based on these materials and (3) investigation on photocarrier lifetime and charge distribution in the devices.
- Si NC:
This work was supported by the Global Climate and Energy Project (GCEP) administrated by Stanford University as well as by the Australian Research Council (ARC) via its Centers of Excellence scheme.
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