Intrinsically carbon-doped AlGaAs layers were grown on semi-insulating (001) GaAs (on axis) and Ge (6 degrees misoriented) substrates using tertiarybutylarsine (TBAs). Growth temperatures from 512 to 630 degrees C were studied and V/III ratios were varied in the range from 9.3 to 0.8. Carrier concentrations saturate at a value of 6 x 10(19) cm(-3). Multi-junction solar cell tunnel diodes were grown, using carbon doping, in different configurations. The highest obtained peak current density was 17.3 A/cm(2) with the tunnel junction embedded in GaAs layers. Using this junction InGaP/GaAs tandem solar cells were grown on 4" Ge which yield a maximum efficiency of 24.3% under the AM1.5 g spectrum. (c) 2006 Elsevier B.V. All rights reserved.