Carrier recombination processes in Fe-doped GaN studied by optical pump–probe techniques
Patrik Ščajev, K. Jarašiūnas, Jacob H. Leach
Abstract
We applied time-resolved free carrier absorption and light-induced transient grating techniques for monitoring carrier dynamics in lightly Fe-doped GaN bulk crystals. Comparison of measured decay times at single- and two-photon carrier photoexcitation with those reported for higher Fe doping revealed two different branches of lifetime dependence on iron density. A deeper insight was reached by using different wavelengths for probing carrier dynamics, which enabled discrimination of electron and hole capture processes by Fe ions. The capture cross section for holes, σh = (4 ± 1) × 10−15 cm2 for the Fe2+* state (5E → 4T2 transition) was determined to be ten times larger than that for electrons, σe = (4 ± 1) × 10−16 cm2 for the Fe3+* state (4T1 → 3T1 transition). Thermal activation of the [Fe2+(5T2),hVB] complex, where hVB is a free hole, required an energy of 315 meV. Light-induced transient grating decay revealed decreasing diffusivity of electrons with Fe doping due to initial carrier capture to Fe3+ states. Simultaneous measurements of carrier diffusion coefficient and lifetime at low excitation conditions (3 × 1017 cm−3) revealed a decrease of bipolar diffusion length from 2 μm in the undoped GaN down to 0.08 μm in the Fe-doped samples at 300 K.