Impact of dislocation densities on n+∕p and p+∕n junction GaAs diodes and solar cells on SiGe virtual substrates

Abstract

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 $\mathrm{Ge}∕{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x∕\mathrm{Si}$ (SiGe) substrates with a TDD of $1\times {10}^6{\mathrm{cm}}^{-2}$ . 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 $V_{\mathrm{oc}}$ , 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 $\left(V_{\mathrm{oc}}\right)$ for the $n^{+}∕p$ GaAs solar cell grown on a SiGe substrate with a TDD of $\sim 1\times {10}^6{\mathrm{cm}}^{-2}$ was $\sim 880\mathrm{mV}$ which was significantly lower than the $\sim 980\mathrm{mV}$ 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.