Excellent Passivation of n‐Type Silicon Surfaces Enabled by Pulsed‐Flow Plasma‐Enhanced Chemical Vapor Deposition of Phosphorus Oxide Capped by Aluminum Oxide

Abstract

Phosphorus oxide (POx) capped by aluminum oxide (Al2O3), prepared by atomic layer deposition (ALD), has recently been introduced as a surface passivation scheme for planar n‐type FZ silicon. In this work, a fast pulsed‐flow plasma‐enhanced chemical vapor deposition (PECVD) process for the POx layer is introduced, making it possible to increase the POx deposition rate significantly while maintaining the POx/Al2O3 passivation quality. An excellent surface passivation is realized on n‐type planar FZ and Cz substrates (J0 = 3.0 fA cm−2). Furthermore, it is demonstrated that the POx/Al2O3 stack can passivate textured surfaces and that the application of an additional PECVD SiNx capping layer renders the stack stable to a firing treatment that is typically used in fire‐through contact formation (J0 = 12 fA cm−2). The excellent surface passivation is enabled by a high positive fixed charge density (Qf ≈ 4 × 1012 cm−2) and an ultralow interface defect density (Dit ≈ 5 × 1010 eV−1 cm−2). Finally, outstanding passivation is demonstrated on textured silicon with a heavy n+ surface doping, as is used in solar cells, on par with alnealed SiO2. These findings indicate that POx/Al2O3 is a highly suited passivation scheme for n‐type silicon surfaces in typical industrial solar cells.

POx/Al2O3 Stacks for c-Si Surface Passivation: Material and Interface Properties

“…Al 2 O 3 , (red squares) for example can reach a combination of low D it and high negative Q f , which makes Al 2 O 3 the preferred material for the passivation of p -type silicon surfaces. PO x /Al 2 O 3 − (blue circles) can reach a combination of low D it and high positive fixed charge (data from the current work also plotted in Figure ). It therefore appears to be the positively charged counterpart to Al 2 O 3 for c-Si surface passivation, which makes it highly suited for passivating n -type silicon surfaces.…”

“…Al 2 O 3 was deposited as a capping layer directly following the PO x deposition in the same reactor at the same substrate temperature using trimethylaluminum (TMA; Al­(CH 3 ) 3 ) as precursor and an O 2 plasma as reactant in an ALD process. Further details of these two processes are described elsewhere …”

“…The relative permittivity of the PO x /Al 2 O 3 stack is determined from the effective oxide thickness obtained from CV characterization and was found to range between 6.4 and 7.4 for the different deposition temperatures. More details of the used methodology are described elsewhere …”

“…An overview of c-Si surface passivation materials and stacks in terms of interface defect density ( D it ) and fixed charge density ( Q f ). D it with corresponding Q f are given for Al 2 O 3 , ,,− PO x /Al 2 O 3 , SiN x , − SiO x , − SiO x /SiN x , , Ga 2 O 3 , , AlN, ZrO 2 , and HfO 2. − The data are divided into materials with negative (left) and positive (right) fixed charge on silicon.…”

“…It has been shown that self-aligned contact openings with local n + doping can be achieved from the PO x /Al 2 O 3 stack using nanosecond laser pulses . The passivation on textured Si and n + -doped Si surfaces was studied using a pulsed-flow plasma-enhanced chemical vapor deposition process . These investigations have been very useful to show the potential of PO x /Al 2 O 3 stacks as a versatile passivation scheme.…”