Application of δ-doping in GaAs tunnel junctions

The technique of δ-doping has been employed to fabricate MBE grown GaAs tunnel junctions for application as intercell contacts in tandem solar cells. By applying δ-doping, the effective Si concentration can be increased by almost a factor 10. In this way an effective n-type doping level of ~2 × 1019 cm-3 can be obtained. By growth at low temperature a p-type concentration of 2 × 1019 cm-3 can be achieved using Be. The as-grown δ-doped tunnel junction showed a tunnel current of 55 A cm-2. A tunnel diode with homogeneous Si doping of 4 × 1018 cm-3 shows a much lower tunnel current of 1.3 mA cm-2. To simulate the growth of the top cell, the tunnel junction was annealed at 650 °C for 2 h. After annealing the peak current of the δ-doped tunnel junction dropped to 183 mA cm-2. This is still sufficient for forming intercell contacts in GaAs-AlGaAs tandem cells.

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