A Detailed Chemical Model for the Diffusion of Phosphorus Into the Silicon Wafer During POCl3 Diffusion

• Published in: IEEE journal of photovoltaics Vol. 11; no. 1; pp. 50 - 57
• Main Authors: Jager, Philip, Mertens, Verena, Baumann, Ulrike, Dullweber, Thorsten
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 01.01.2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: <named-content xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="LaTeX"> in situ</tex-math> </inline-formula> </named-content> oxidation; Atmospheric measurements; Chemicals; Diffusion; Diffusion profile; emitter saturation current density; Nitrogen; Oxidation; P-n junctions; Phosphorus; phosphorus diffusion; Phosphorus pentoxide; phosphosilicate glass; Photovoltaic cells; Silicon; Silicon dioxide; Silicon wafers; SiO<named-content xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="LaTeX"> _{2}</tex-math> </inline-formula> </named-content> growth; Solar cells; Thickness
• DOI: 10.1109/JPHOTOV.2020.3038331
• ISSN: 2156-3381

The POCl 3 diffusion is the main technology to form the p-n junction of industrial silicon solar cells. However, the diffusion mechanism of phosphorus (P) into the silicon wafer is not fully understood. In this article, we study the P diffusion mechanism during drive-in by systematically varying the drive-in time in the oxygen (O 2 ) atmosphere and subsequently in nitrogen (N 2 ). When increasing the drive-in time in O 2 from 0 to 120 min, the sheet resistance R sheet stays constant at 485±30 Ω/sq. Hence, we demonstrate for the first time that the phosphorus diffusion can be completely suppressed in the O 2 atmosphere. When adding a drive-in in the N 2 atmosphere directly after the drive-in in O 2 , we find that the SiO 2 thickness d SiO2,O2 changes from initially 2 to 10 nm after O 2 drive-in to an equilibrium SiO 2 thickness d SiO2,eq of 4.7 nm after N 2 drive-in. We prove for the first time that if d SiO2,O2 > d SiO2,eq , no P diffuses into the silicon wafer even in the N 2 atmosphere. Only if d SiO2,O2 < d SiO2,eq , phosphorus diffuses into the silicon wafer in the N 2 atmosphere. We propose a detailed chemical model to explain our experimental results, which assumes that the diffusion of Si from the wafer through the SiO 2 interface toward the PSG plays a key role. In this model, P can only diffuse into the Si wafer if P 2 O 5 in the PSG is reduced by the Si from the wafer to P and SiO 2 .