Impact of dislocation densities on n+/p and p+/n junction GaAs diodes and solar cells on SiGe virtual substrates -: art. no. 014502

Recent experimental measurements have shown that in GaAs with elevated threading dislocation densities (TDDs) the electron lifetime is much lower than the hole lifetime [C. L. Andre, J. J. Boeckl, D. M. Wilt, A. J. Pitera, M. L. Lee, E. A. Fitzgerald, B. M. Keyes, and S. A. Ringel, Appl. Phys. Lett. 84, 3884 (2004)]. This lower electron lifetime suggests an increase in depletion region recombination and thus in the reverse saturation current (J(0) for an n(+)/p diode compared with a p(+)/n diode at a given TDD. To confirm this, GaAs diodes of both polarities were grown on compositionally graded Ge/Si1-xGex/Si (SiGe) substrates with a TDD of 1 X 10(6) cm(-2). It is shown that the ratio of measured J(0) values is consistent with the inverse ratio of the expected lifetimes. Using a TDD-dependent lifetime in solar cell current-voltage models we found that the V-oc, for a given short-circuit current, also exhibits a poorer TDD tolerance for GaAs n(+)/p solar cells compared with GaAs p(+)/n solar cells. Experimentally, the open-circuit voltage (V-oc) for the n(+)/p GaAs solar cell grown on a SiGe substrate with a TDD of similar to 1 X 10(6) cm(-2) was similar to 880 mV which was significantly lower than the similar to 980 mV measured for a p(+)/n GaAs solar cell grown on SiGe at the same TDD and was consistent with the solar cell modeling results reported in this paper. We conclude that p(+)/n polarity GaAs junctions demonstrate superior dislocation tolerance than n(+)/p configured GaAs junctions, which is important for optimization of lattice-mismatched Ill-V devices. (c) 2005 American Institute of Physics.