UV-Raman scattering of thin film Si with ultrathin silicon oxide tunnel contact for high efficiency crystal silicon solar cells

A precise characterization of thin film silicon in devices is a great challenge for device optimization because the material properties depend on the substrate and change with film thickness. A specific situation is thin poly-Si layer in tunnel oxide passivated contact (TOPCon) structure in high-efficiency solar cells. In this paper, we present a systematic study using a wide range UV-Raman spectroscopy to measure the crystalline structure and chemical bonding configurations in the TOPCon structure. Because of the high absorption coefficient of 325-nm laser in silicon films, the Raman probes only the top surface region and minimizes the contribution from c-Si substrate. However, the comparison with the Raman spectra collected with a green laser of 532 nm, the UV-Raman spectrum of a-Si:H shows a very different spectral shape with the disappearance of the LA mode of Si-Si vibration and an appearance of additional signal at 325 cm−1. In addition, the UV-Raman spectrum of the pre-crystallized P doped a-Si:H layer shows most of the vibration modes of Si–Si, Si–H, Si–O–Si and Si–O–H bonds, where the Si–O–H bonds are in the top surface oxide layer. After the crystallization, the Si-Si TO mode shows a high degree of crystallization, the Si-H vibration modes disappear, and an additional Si-O-Si mode is observed which is believed from the ultrathin SiOx passivation layer. In addition, we provide a set of UV-Raman measurements on the samples made with the same layer structure but at different annealing temperatures to show the correlation between the UV-Raman spectra and the passivation quality of the TOPCon structure on the n-type c-Si wafers. These observations prove that UV-Raman spectroscopy is a useful characterization tool for high efficiency c-Si solar cells with TOPCon structure. In addition, the impacts of the material properties on the passivation quality are discussed.