APPLICATION OF DELTA-DOPING IN GAAS TUNNEL-JUNCTIONS

The technique of delta-doping has been employed to fabricate MBE grown GaAs tunnel junctions for application as intercell contacts in tandem solar cells. By applying delta-doping, the effective Si concentration can be increased by almost a factor 10. In this way an effective n-ty pe doping level of similar to 2 x 10(19) cm(-3) can be obtained. By gr owth at low temperature a p-type concentration of 2 x 10(19)cm(-3) can be achieved using Be, The as-grown delta-doped tunnel junction showed a tunnel current of 55A cm(-2). A tunnel diode with homogeneous Si do ping of 4 x 10(18) cm(-3) shows a much lower tunnel current of 1.3mAcm (-2). To simulate the growth of the top cell, the tunnel junction was annealed at 650 degrees C for 2h. After annealing the peak current of the delta-doped tunnel junction dropped to 183mAcm(-2). This is still sufficient for forming intercell contacts in GaAs-AlGaAs tandem cells.