[21a-S322-1~13] 1.4 Energy conversion, storage, resources and environment

▲ [21a-S322-4] Excellent Surface Passivation of Crystalline Silicon by Atomic Layer Deposition Al_xMg_1-xO_y Thin Films

〇HYUNJU LEE¹, Takefumi Kamioka¹, Dongyan Zhang¹, Naotaka Iwata¹, Yoshio Ohshita¹ (1.Toyota Technol. Inst.)

Keywords:Silicon solar cell,Surface passivation,aluminum magnesium oxide

In the field of crystalline silicon (c-Si) photovoltaics, trends toward lighter dopant diffusions, thinner substrates, and higher lifetime substrates have placed an increased importance on surface passivation. Recently, AlO_x has been intensively studied and enhanced solar cell efficiencies largely by providing excellent passivation on highly and lowly doped p-type silicon surfaces. Meanwhile, to achieve high conversion efficiencies, advanced silicon solar cell architectures such as interdigitated back contact solar cells demand that both the n⁺ and p⁺ doped Si surfaces are passivated simultaneously by a single passivation scheme. However, on heavily doped n⁺ Si surfaces, the AlO_x passivation is compromised because the minority carrier (i.e., hole) concentration at the surface is increased by the significant negative fixed charges of the AlO_x passivation. Therefore, the absence of significant negative fixed charges and high chemical passivation are expected to be beneficial for the passivation of n⁺ surfaces. In this study, we focused on a ternary oxide, Al_xMg_1-xO_y (AMO) to make a solution for the passivation of both the n⁺ and p⁺ doped Si surfaces because MgO_x has not only excellent properties such as high permittivity (κ~9.8), large band gap (7.3 ~7.9 eV), and higher breakdown field (12 MV/cm) but also positive fixed charges on Si surfaces. In addition, magnesium aluminate fabricated by atomic layer deposition is hydrogen-rich and amorphous below 800 ºC. Hence, based on these facts, we have developed ternary Al_xMg_1-xO_y as a new class of passivation materials. From the experiment, we found that a level of surface passivation and interface properties of ternary Al_xMg_1-xO_y thin films can be controlled by varying the Al and Mg precursor cycle ratio during plasma-enhanced atomic layer deposition processes. In addition, we also found that Al_xMg_1-xO_y thin films are superior to pure AlO_x thin films for the passivation of c-Si under a given experimental condition. In particular, we can obtain very low S_max of ~5.5 cm/s using Al_xMg_1-xO_y thin films on a p-type Czochralski (Cz) (100) Si wafer after post deposition annealing at 500 ºC in N₂. Therefore, we can conclude that ALD with a super-cycle consisted of a number of sub-cycles of AlO_x (n_Al) and a cycle of MgO_x can be applied for effective engineering of physical properties of the ternary Al_xMg_1-xO_y thin films and provide excellent surface passivation on p-type c-Si. During our presentation, we will demonstrate detailed study on the ternary Al_xMg_1-xO_y passivation and discuss its passivation mechanism for c-Si.