August 13th, 2020
Multi-junctions solar cells can be processed on germanium substrates. However, these substrates are very expensive and the possibility of replacing them with germanium layers bonded on silicon substrates has been studied . The process used consists in implanting H+ ions into germanium, to bond the implanted germanium wafer to a virgin silicon wafer and splitting the germanium layer. In this experiment, direct bonding of germanium and silicon was used, in order to obtain an ohmic contact without altering the transmission by a metallic layer. A specific contact resistance of 400 ^ cm2 was obtained after annealing at 350 °C. This value of resistance is too high for solar cells applications but it could be improved by annealing at higher temperature. Triple-junction solar cell structures with photoluminescence intensity and decay lifetimes comparable to those of solar cells grown on bulk germanium were grown on these Ge/Si heterostructures. The transferred surfaces are limited to 1 cm2, due to the difficulty of
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direct bonding: while bonding via an oxide occurs spontaneously when the two wafers are put in contact, direct bonding must be done under pressure.
Wafer-bonding can be used to fabricate multi-junction cells. The main advantages are that the stacking is not limited by lattice parameter adaptation and the cells can be electrically independent, giving more freedom in the choice of gaps. Three bonding techniques can be used: direct bonding, bonding via a metallic layer and bonding via an oxide. Direct bonding is attractive since there is no light absorption at the interfaces. However, this technique is difficult and requires very good surface preparation (very low roughness and very clean surfaces, free of oxides) and must be done under pressure. Bonding with metallic layers is easier but the bonding layer must be transparent and so very thin, making the bonding difficult. However, bonding via an oxide is a very well controlled technique, used in the semiconductor industry to make SOI structures on 300 mm wafers and the oxide used for the bonding has very good optical transmission. Since the layers are isolated from each other, the technique can be used to make four (or more) terminal multi-junction cells, which offer more flexibility in the choice of the gaps.