Gallium-nitride-based structures have become more and more important in recent years. Especially InGaN/GaN multi-quantum well (MQW) structures are used for optoelectronic devices such as light emitting diodes and diode lasers in the blue and green spectral region and for detectors and power amplifiers. AlGaN/GaN-based structures have the potential to extend optoelectronics towards the ultraviolet spectral region. Thus, carrier dynamics in MQW structures and superlattices containing aluminum are of strong interest.
The ultrafast processes of nonequilibrium carriers in such semiconductor superlattices are not yet fully understood. Therefore, we have investigated the carrier dynamics in Al0.18Ga0.82N/GaN superlattice samples by pump-probe measurements. The samples consist of 60 periods with 2 nm barriers and 3 nm quantum wells. A SiN coating prevents degradation effects during excitation . In addition, GaN bulk material was measured.
For the measurements, we used an Yb-based oscillator amplifier system (repetition rate 1 MHz) pumping an optical parametric amplifier, allowing second-harmonic wavelengths between 325 nm and 460 nm with a pulse length of 40 fs. Time-dependent pump-probe measurements at room temperature were performed in reflection because absorption in the GaN template between the superlattice and the substrate prevents transmission measurements. After interaction with the sample, the probe beam was spectrally resolved to determine transient spectra.
In the measurement, carriers are excited by the pump laser pulse above the superlattice band gap energy. Two processes are involved in the ensuing intra-band relaxation: the first leads to the thermalization of carriers by carrier-carrier scattering, the second is the cooling of carriers by phonon scattering . Due to the polar properties of GaN-based superlattices, one expects a much stronger coupling between electrons and optical phonons compared to GaAs-based systems. This should result in a much faster cooling process. The photo-excited carriers lead to band gap renormalization and therefore to an increase of the refractive index at energies below the band gap and to a decrease of the refractive index at energies above the band gap. These changes manifest themselves in the transient reflectivity measured in our pump-probe experiments.
Exciting 240 meV above the superlattice band gap, we see a decrease in reflectivity of up to 4 percent at an excitation density of 580 µJ/cm2 per pulse, decaying with a time constant of 1.7 ps. Furthermore, carrier cooling rates in superlattices and in bulk materials are compared .
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