Effectiveness of UV-Ozone Treatment and Alneal Process for Si Surface Passivation with Sol-Gel Spray-Coated SiO2 Film

Abstract

Passivated Emitter and Rear Contact (PERC) cell is dominating solar market over conventional Aluminum-Back Surface Field cells [1]. PERC structure uses dielectric passivation layer at the rear to overcome optical and electrical losses, enhancing the cell performance. Silicon dioxide (SiO2) film passivates both n- and p-type silicon (Si). Thermal oxidation provides excellent Si passivation but adds to the thermal budget. An alternative, spray-coating, is a low-temperature deposition technique which is industrially viable and has high throughput and therefore, substantiates its potential for implementation in solar industry.We achieved low surface recombination velocity (Seff) of 8.07 cmsec-1 with spray-coated SiO2 film on p-type Si and anticipated that good passivation is due to UV-Ozone treatment and alneal [2]. In this work, a thorough investigation is performed in terms of interface state density (Dit) and fixed oxide charge density (Qf), extracted from capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of metal-oxide-semiconductor (MOS) capacitor.In this work, p-type Si wafers were RCA cleaned without final 2% HF dip. For MOS capacitor fabrication, samples were bifurcated into two sets. One set was processed as it is and another set was dipped in 2% HF and was UV-Ozone treated for 5 min. Sol-gel prepared using tetraethyl orthosilicate, iso-propyl alcohol, deionized water, and nitric acid was sprayed over these wafers using custom-designed spray-coater tool. Subsequently, wafers were baked at 150oC and subjected to Post Deposition Annealing (PDA) in O2 ambient for 30 min. Front and rear contacts were created by depositing Aluminum (Al) dots over the film and Al blanket at the back of Si, respectively. These were further divided into two. One set of each was not treated further and the other was subjected to Post Metallization Annealing (PMA) in Forming gas annealing (FGA; 4% H2 and 96% N2) ambient for 30 min. Lifetime structures were fabricated for RCA oxide and UV-Ozone treated samples with SiO2 deposited on both sides of wafer following the sequence discussed above and were subjected to PDA. Al was blanket deposited on both sides of the wafer. These structures were subjected to PMA in FG ambient for 30 min. Later, Al was etched.Effectiveness of UV-Ozone treatment is demonstrated by comparing Dit and Qf with RCA-oxide. Thickness of spray-coated SiO2 film is ∼17 nm and of RCA oxide and UV-Ozone oxide is ∼1.5 nm. Dit of 1.4-1.6×1012 cm-2eV-1 is obtained for RCA oxide and 3.0-4.1×1011 cm-2eV-1 for UV-Ozone oxide after PDA without PMA (Fig. 1(a)). Dit is an order of magnitude less for UV-Ozone oxide, providing better chemical passivation in comparison to the RCA oxide. UV-Ozone oxide gives lower Qf in comparison to the RCA oxide (Fig 1(b)). Qf values are 2.3-2.8×1012 and 0.92-1.5×1012 cm-2 for RCA oxide and UV-Ozone oxide, respectively. Therefore, UV-Ozone treatment prior to film deposition gives better interface quality than RCA oxide. This can be attributed to surface cleaning in addition to oxide formation during UV-Ozone treatment [3].Efficacy of alneal was studied by comparing PMA effect on films over RCA oxide and UV-Ozone oxide. Dit for film on RCA oxide and UV-Ozone oxide are 0.84-1.1×1011 cm-2eV-1 and 2.0-6.0×1010 cm-2eV-1, respectively after PDA and PMA. Dit has reduced by an order of magnitude after PMA. This is attributed to hydrogen passivation provided by PMA in FG ambient [4]. Process of annealing after Al deposition is termed as alneal. Qf has also reduced after alneal. Qf values are 6.2-7.0 × 1011 cm-2 and 3.8-6.5 × 1011 cm-2 for RCA oxide and UV-Ozone oxide, respectively. The trends in Qf and Dit can be correlated to the shift in C-V characteristics and G-V peak value, respectively (Fig. 2 (a) and (b)).Lifetime of 118.66 µsec and 459.52 µsec was measured using Sinton lifetime tester and corresponding Seff was calculated as 95.59 cms-1 and 8.07 cms-1 for film over RCA oxide and UV-Ozone oxide, respectively (Fig. 3). UV-Ozone oxide provides better passivation in comparison to the RCA oxide. Low Seff in case of UV-Ozone oxide is attributed to low Dit, as both these oxides show similar Qf.Therefore, it can be concluded that good passivation with low Seff for spray-coated SiO2 film is attributed to UV-Ozone treatment as well to the alneal process.