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Direct wafer-bonding after argon-beam surface activation is a low temperature process, which allows for the monolithic integration of various materials including Si, Ge, III-V compound semiconductors, SiC or Al2O3 etc.
The process requires smooth wafer surfaces with RMS roughnesses < 1 nm and minimal particle contaminations, which is usually achieved by chemical-mechanical polishing. These wafers are sputtered with Ar in ultra-high vacuum (< 3 x 10-6 Pa) to remove few nanometers of oxides and contaminants. The process results in a thin amorphous surface layer with dangling bonds. Subsequently, the wafers are pressed together so that covalent bonds are formed, permanently joining the materials.
As no intermediate layers are applied, the approach enables a high optical transparency together with mechanical stability as well as highest electrical and thermal conductivity. The process parameters are optimized for various material to obtain electrical bond resistances < 5 mΩcm2. Even in multi-junction cells operated at a few hundred suns with current densities of ~5 A/cm2, these resistances do not significantly limit the cell efficiencies. These unique characteristics of the resulting wafer-bonds make the technique promising for a wide range of innovations in photonics or power electronics.
We apply direct wafer-bonding in the fabrication of various concepts for III-V based multi-junction solar cells reaching highest efficiencies. Examples are a wafer-bonded GaInP/GaAs//GaInAsP/GaInAs solar cell that exhibits an efficiency of 46.1 % at 312 suns as well as a GaInP/GaAs/GaInAs//GaSb solar cell with 43.8 % efficiency at 796 suns. Further, the process enables the monolithic integration of III-V materials on Si, at which a record efficiency of 34.1 % at 1 sun could be recently achieved with a GaInP/AlGaAs//Si solar cell.
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