Detailed Micro Raman Spectroscopy Analysis of Doped Silicon Thin Film Layers and Its Feasibility for Heterojunction Silicon Wafer Solar Cells

Abstract

Hydrogenated doped silicon thin films deposited using RF (13.56 MHz) PECVD were studied in detail using micro Raman spectroscopy to investigate the impact of doping gas flow, film thickness, and substrate type on the film characteristics. In particular, by deconvoluting the micro Raman spectra into amorphous and crystalline components, qualitative and quantitative information such as bond angle disorder, bond length, film stress, and film crystallinity can be determined. By selecting the optimum doped silicon thin film deposition conditions, and combining our p-doped and n-doped silicon thin films in different heterojunction structures, we demonstrate both (i) an efficient field effect passivation and (ii) further improvement to c-Si/a-Si:H(i) interface defect density with observed improvement in implied open-circuit voltage V OC and minority carrier lifetimes across all injections levels of interest. In particular, the heterojunction structure (a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(p)) demonstrates a minority carrier lifetime of 2.4 ms at an injection level of 10 15 cm −3 , and a high implied open-circuit voltage of 725 mV. Simulation studies reveal a strong dependence of the interface defect density D it on the heterojunction silicon wafer solar cell performance, affected by the deposition conditions of the overlying doped silicon thin film layers. Using our films, and a fitted D it of 5 × 10 10 cm −2 •eV −1 , we demonstrate that a solar cell efficiency of ~22.5% can be potentially achievable.

Depth-Sensitive Raman Investigation of Metal-Oxide-Semiconductor Structures: Absorption as a Tool for Variation of Exciting Light Penetration Depth

“…The position of crystalline component can also change in the range of about 2 cm −1 . For example, as reported in [46] maximum of Raman line observed for c-Si changes from 520.91 cm −1 to 519.17 cm −1 if concentration of defects in crystalline structure increases. Authors of [46] determined the position of one-phonon Raman line for polycrystalline Si as equal to about 510 cm −1 and, as mentioned above, for a-Si as equal to about 480 cm −1 .…”

“…Detailed study of the dependence between the type of silicon (crystalline, polycrystalline, or amorphous) and position of one-phonon line was done for Si layer deposited on different substrates. Presence of all types of material, in particular c-Si, polycrystalline Si, and a-Si, results in complicated shape of Raman line, because the line is composed of three components: crystalline, polycrystalline, and amorphous [46]. The position of crystalline component can also change in the range of about 2 cm −1 .…”

“…The other factor that influences the position of maximum of one-phonon line is the admixture gas [46]. Generally, amorphous components are shifted towards smaller values of Raman shift, crystalline to larger.…”

“…Large concentration produces an opposite effect. Authors of [46] also calculated the stress induced by doping agent. They used the formula developed for a-Si by Anastassakis.…”

Crystallinity of silicon films grown on carbon fibers by very high frequency plasma enhanced chemical vapor deposition

“…7(a). However, the amorphous silicon peak position was shifted from 467 cm -1 to 479 cm -1 after annealing at 400 °C for 30 second, possibly due to a structural relaxation with thermal annealing [24]. For the thermal annealing above 500 °C, Raman spectra show the drastic reduction of peak intensities of amorphous phases and sudden appearance of the crystalline peaks, which implies the phase transformation of amorphous Si to crystalline Si phase.…”