Low-temperature surface passivation of silicon solar cells by means of a TMCTS oxide
In this paper the possibility of low-temperature surface passivation by means of a Tetra-Methyl-Cyclo-Tetra-Siloxane (TMCTS) LPCVD oxide is investigated. The influence of the oxide deposition parameters on the surface passivation quality is studied. The lowest surface recombination velocity S is obtained for oxides grown at 600°C and 300 mTorr: S = 207 cm/s on 100 omega cm p-type silicon, and S = 667 cm/s on 1 omega cm p-type material. It is shown that these results are further improved by annealing at temperatures above 700°C. After a 30' anneal at 850°C in an Ar atmosphere, the surface recombination velocity is reduced to 11.4 cm/s on 100 omega cm samples and 24.9 cm/s on 1 omega cm samples. These values are lower than the ones measured for high-quality thermal oxides grown on the same material. Capacitance-voltage measurements show that this is related to the presence of a high positive charge in the TMCTS oxide (Q(ind f) = 1.1 10(exp 12) cm-2), and a relatively small interface state density (D(ind it) = 2.4 10(exp 10) eV(exp -1) cm-2). LPCVD oxides are also very useful to simplify the high-efficiency solar cell proces. Preliminary current-voltage characteristics are given for simple solar cells, in which a TMCTS oxide is also used as an antireflection coating, and in which P-doped and B-doped TMCTS oxides are used as doping sources for the emitter and BSF region.