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 (J0 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-x Gex Si (SiGe) substrates with a TDD of 1× 106 cm-2. It is shown that the ratio of measured J0 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 Voc, 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 (Voc) for the n+ p GaAs solar cell grown on a SiGe substrate with a TDD of ∼1× 106 cm-2 was ∼880 mV which was significantly lower than the ∼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 III-V devices.
ASJC Scopus subject areas
- Physics and Astronomy(all)