High-pressure sputtering deposition and in situ plasma oxidation of TiOx thin films as electron selective contact for photovoltaic applications

Abstract

In this article, we show the structural, optical, and electrical characterization of TiO_x deposited by the unconventional technique of High-Pressure Sputtering (HPS). This technique has the potential to reduce the plasma-induced damage of the samples. To fabricate the TiO_x, a 2-step process was used. Firstly, a thin Ti film was deposited in an Ar atmosphere. Secondly, O₂ was introduced into the HPS chamber to create an Ar/O₂ plasma that, along with low temperatures (150 °C or 200 °C), induces the oxidation of the deposited Ti film. With this approach, the Ti film is expected to behave as a capping layer that will reduce the oxidation of the Si substrate. This study aims to obtain a TiO_x layer with low specific contact resistivity (ρ_c) and high minority carrier lifetime. These are crucial characteristics for obtaining high-quality selective contact. It was found that the 2-step process can oxidize the Ti layer. These HPS TiO_x layers show a resistivity in the order of 0.3–10 Ωcm and a ratio Ti/O of ∼1.9. Moreover, the SiO_x regrowth is minimal since this is comparable to the native oxide. This was confirmed by transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The samples fabricated with a Ti layer (∼4 nm) plus an oxidation temperature of 200 °C (duration of less than 2 h) show a low ρ_c of 0.02 Ωcm², an excellent transmittance (>87 %) in the visible region and an optical bandgap of 2.8 eV. These TiO_x layers are amorphous, although some anatase phase crystalline clusters appear for the 200 °C processes. However, the minority carrier lifetime results of Si passivated by TiO_x were inadequate for fabricating efficient solar cells. We also found that using the RCA oxide improved lifetime. This indicates that introducing alternative low-temperature passivating layers can solve this issue.