Low temperature annealed Pd/Ge/Ti metal systems for concentrator inverted metamorphic solar cells

Obtaining low front contact resistances in inverted metamorphic multijunction (IMM) solar cell is essential for reducing resistance losses at high concentration. However, these devices present some particularities which make this issue specially challenging. On one hand, the solar cell fabrication requires a flip-chip process in order to transfer the semiconductor structure to a handling substrate. This implies that the front contact deposition must not compromise the integrity of the bonding material, which is usually temperature sensitive. On the other hand, the heavily doped front contact layer is grown firstly, therefore is subjected to an important thermal load caused by the epitaxy growth, which is suspected to damage its electronic properties. In this work, the specific front contact of several solar cells, grown in inverted and non- inverted configurations and deposited by electroplating and evaporation, is studied. In addition, we present a low temperature annealing for an evaporated Pd/Ge/Ti metal system on n-GaAs which achieves very low resistance values ρc ≈ 10-7 Ω·cm2 in all inverted solar cells tested regardless of the thermal load applied to the front contact layer.