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.
Silicon based multi-junction solar cells are a promising option to overcome the theoretical efficiency limit of a silicon solar cell (29.4%). With III-V semiconductors, high bandgap materials applicable for top cells are available. For the application of such silicon based multi-junction devices, a full integration of all solar cell layers in one 2-terminal device is of great advantage. We realized a triple-junction device by wafer-bonding two III-V-based top cells onto the silicon bottom cell. However, in such a series connected solar cell system, the currents of all sub-cells need to be matched in order to achieve highest efficiencies. To fulfil the current matching condition and maximise the power output, photonic structures were investigated. The reference system without photonic structures, a triple-junction cell with identical GaInP/GaAs top cells, suffered from a current limitation by the weakly absorbing indirect semiconductor silicon bottom cell. Therefore rear side diffraction gratings manufactured by nanoimprint lithography were implemented to trap the infrared light and boost the solar cell current by more than 1 mA/cm2. Since planar passivated surfaces with an additional photonic structure (i.e. electrically planar but optically structured) were used, the optical gain could be realized without deterioration of the electrical cell properties, leading to a strong efficiency increase of 1.9% absolute. With this technology, an efficiency of 33.3% could be achieved.