On the blistering of atomic layer deposited Al<inf>2</inf>O<inf>3</inf> as Si surface passivation

This work proves that blistering is the partial delamination of a thick enough Al ₂ O ₃ layer caused by gaseous desorption in the Al ₂ O ₃ layer upon thermal treatments above a critical temperature: the Al ₂ O ₃ layer acts as a gas barrier and bubble formation occurs. First, using an atmospheric pressure rapid thermal processor with an atmospheric pressure ionization mass spectrometry, desorbing species upon heating of Si/Al ₂ O ₃ samples are identified: evident desorption peaks are observed around 400 °C for all spectra. The spectrum for m/e = 18, an indication of H ₂ O, illustrates that gaseous desorption from Al ₂ O ₃ and from the Si substrate itself continues up to 600 °C and 700 °C, respectively. Also, it is shown that in the case of a 30 nm Al ₂ O ₃ layer, blistering starts at same annealing temperatures as gaseous desorption begins. In the case of a thin enough (<; 10 nm) Al ₂ O ₃ film, blistering does not show. To complete the proof, elastic recoil detection measurements clearly show that after annealing a thick Al ₂ O ₃ film above 400 °C the H content is higher near the c-Si interface as compared to the near surface. Fortunately, effective lifetime and capacitance voltage measurements show that 5 to 10 nm Al ₂ O ₃ layers can still be adequate passivation layers after being annealed in N ₂ environment at temperatures up to 500-700 °C: (i) interface trap densities (D _it ) can remain below 1×10 ¹¹ cm ^-2 and (ii) fixed charge densities (Q _f ) stay negative and in the order of -3×10 ¹² cm ^-2 Random local Al back surface field (BSF) solar cells, fabricated using a blistered film as rear surface passivation and no additional contact opening step, clearly show that random local BSFs are created upon firing of a blistered rear passivation layer covered by metal. Therefore, it is clear that blistering should be avoided, since it will reduce the overall rear surface passivation. The key to avoid blistering is using 5 to 10 nm Al ₂ O ₃ passivation layers and performing an annealing step prior to capping and co-firing. Al ₂ O ₃ /SiN _x passivated local Al BSF p-type Si solar cells are made using an out-gassing step with temperatures up to 700 °C. For these cells, the reduction in blistering and hence improvement in rear surface passivation is clearly reflected in the gain in average Voc as a function of out-gassing temperature.