The migration of Zn and Si from a tunnel junction during epitaxial growth, and the effects of impurity diffusion upon InGaP/GaAs tandem cell properties, were investigated. The Zn diffusion from the tunnel junction was found to degrade the properties of both parts of the cell. The Zn diffusion was enhanced around threading dislocations from the GaAs substrate, and created shunt paths only in the top cell. Diffusion of Si, which degraded the properties of the GaAs bottom cell, was also observed when a different substrate with a high etch-pit density was used. The anomalous diffusion of Zn could be suppressed by using a double-hetero structure InGaP tunnel junction which was sandwiched between AlInP layers. It was found that Zn diffusion occurred while a highly Si-doped layer formed nearby, and that Zn diffused in the opposite direction to the Si-doped layer. The Zn diffusion was thought to be caused by group-III self-diffusion which originated in the highly-doped n-type layer. The direction of the Zn diffusion was thought to be due to Coulombic repulsion between substitutional Zn on Ga sites and substitutional Si on As sites. The large activation energies for the formation and migration of group-III vacancies in AlInP barrier layers InGaP tunnel junction layers, were expected to suppress Zn diffusion from the tunnel junction.

Mechanism of Zn and Si diffusion from a highly-doped tunnel junction for InGaP/GaAs tandem solar cells T.Takamoto, M.Yumaguchi, E.Ikeda, T.Agui, H.Kurita, M.Al-Jassim: Journal of Applied Physics, 1999, 85[3], 1481-6