Abstract
Pump polarization dependent carrier dynamics, particularly excitonic dynamics, of non-polar a-plane zinc oxide (ZnO) epifilms with two different thicknesses were investigated using time resolved measurements. Unlike the electron and hole dynamics through the above-bandgap excitation, transient differential reflectance (TDR) traces revealed similar trends under two orthogonal pump polarization conditions relative to the c-axis (Epu⊥c and Epu∥c) of a-ZnO around near-exciton-resonance excitation. By means of a band diagram, the bandgap renormalization (BGR) effect can be reasonably explained by the screening of the Coulomb potential energy due to the accumulation of relaxed free carriers that were initially excited through the absorption of two cascaded pump photons via the excitonic level, a process known as two photon absorption (TPA). Thus, the modulation depths of the TPA around zero time delay, due to simultaneous absorption of one pump and one probe photon via the excitonic level, increased linearly with the pump fluence, proportional to the modulation depth resulting from the BGR effects.
Highlights
To date, ZnO, a direct and wide bandgap semiconductor (∼3.37 eV) with relatively large exciton binding energy (∼60 meV),[1] has attracted considerable attention in the development of room temperature (RT) UV photonic devices, such as light-emitting diodes, laser diodes, etc.[2,3] Because of its wurtzite structure, the growth direction of ZnO along the [0001] direction (c-ZnO) possesses a net dipole along the c-axis
The polar field would cause the band to tilt in quantum wells, termed the quantum confinement Stark effect (QCSE), which compresses the wavefunctions of electrons and holes to the opposite sides of the quantum wells
In order to eliminate the QCSE, a non-polar direction, such as (112 ̄0) a-plane ZnO (a-ZnO) epifilm was grown in which the c-axis lies on the surface of the growth plane.[5,6,7]
Summary
ZnO, a direct and wide bandgap semiconductor (∼3.37 eV) with relatively large exciton binding energy (∼60 meV),[1] has attracted considerable attention in the development of room temperature (RT) UV photonic devices, such as light-emitting diodes, laser diodes, etc.[2,3] Because of its wurtzite structure, the growth direction of ZnO along the [0001] direction (c-ZnO) possesses a net dipole (or polar field) along the c-axis. Ou et al reported the thickness related ultrafast carrier dynamics of c-ZnO epifilm through the degenerate pump-probe technique using far-above-bandgap excitation.[15] For the thin (70 nm) epifilm, the time delay τm of minimal TDR trace (|∆R/R|BGR) caused by the bandgap renormalization (BGR) effect increased with the high excited photon energy.
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