Crystalline silicon surface passivation by high-frequency plasma-enhanced chemical-vapor-deposited nanocomposite silicon suboxides for solar cell applications

A passivation scheme, featuring nanocomposite amorphous silicon suboxides (a-SiOx:H) is investigated and analyzed in this work. The a-SiOx:H films are deposited by high-frequency plasma-enhanced chemical-vapor deposition via decomposition of silane (SiH4), carbon dioxide (CO2), and hydrogen (H2) as source gases. The plasma deposition parameters of a-SiOx:H films are optimized in terms of effective lifetime, while the oxygen content and the resulting optical band gap EG of the a-SiOx:H films are controlled by varying the CO2 partial pressure χO=[CO2]/([CO2]+[SiH4]). Postannealing at low temperatures of those films shows a beneficial effect in form of a drastic increase of the effective lifetime. This improvement of the passivation quality by low temperature annealing for the a-SiOx:H likely originates from defect reduction of the film close to the interface. Raman spectra reveal the existence of Si–(OH)x and Si–O–Si bonds after thermal annealing of the layers, leading to a higher effective lifetime, as it reduces the defect absorption of the suboxides. The surface passivation quality of a-SiOx:H within both n-type and p-type silicon has been studied as a function of injection level. Record high effective lifetime values of 4.7 ms on 1 Ω cm n-type float zone (FZ) wafers and 14.2 ms on 130 Ω cm p-type FZ wafers prove the applicability for a surface passivation of silicon wafers applicable to any kind of silicon-based solar cells. The effective lifetime values achieved on a highly doped crystalline wafer (1 Ω cm resistivity) appears to be the highest value ever reported. Samples prepared in this way feature a high quality passivation yielding effective lifetime values exceeding those of record SiO2 and SiNx passivation schemes.