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Ultrafast carrier dynamics in GaN epilayers studied by femtosecond pump-probe spectroscopy
Abstract
Femtosecond pump-probe measurements were performed in GaN epilayers to study carrier dynamics in the band edge region. Excitonic absorption was found to begin saturating at a pump fluence of 20 (mu) J/cm2 which corresponds to an estimated carrier density of 1 X 1018 cm-3. At zero delay between pump and probe, induced absorption is observed below the unpumped band gap due to ultrafast bandgap renormalization. After 375 fs, a large induced transparency is observed just below the excitonic resonance which is due to a transient electron-hole plasma. After 1 ps, the absorption has partially recovered to a level associated with excitonic phase-space filling. The absorption then recovers with a characteristic time of approximately 20 ps, a value which increases with increasing excitation density.
... Experimental, in cm −3 Reference 2.2 × 10 19 Hess [11] 6 × 10 18 -5 × 10 19 Khym [12] 1 × 10 18 Fischer [13] 1.8-3.8 × 10 18 Binet [14] 6 × 10 16 Present work that, from a general point of view, may involve polaritonic relaxation stimulated by the final state occupation [5], P-band exciton interactions scattering processes [6], or standard degenerate electron-hole plasma [7]. ...
... At the critical Mott density, excitons are no longer present in the material. Table 1 summarizes the experimental values of the Mott densities reported in the literature for GaN with lattice temperatures lower than 10 K [11][12][13][14]. The reported data for GaN Mott transitions varies from 1 × 10 18 to 5 × 10 19 cm −3 . ...
... Fisher's work reports another detailed analysis of the screening of excitons in a GaN slab through femtosecond pump-probe absorption [13]. Absorption spectra are measured at 5 K for various excitations intensities. ...
The research on GaN lasers aims for a continuous reduction of the lasing threshold. An approach to achieve it consists in exploiting stimulated polariton scattering. This mechanism, and the associated polariton lasers, requires an in-depth knowledge of the GaN excitonic properties, as polaritons result from the coupling of excitons with photons. Under high excitation intensities, exciton states no longer exist due to the Coulomb screening by free carriers; this phenomenon occurs at the so-called Mott density. The aim of this work is to study the bleaching of excitons under a quasi-continuous optical excitation in a bulk GaN sample of high quality through power dependent micro-photoluminescence and time-resolved experiments at 5K. Time-resolved photoluminescence allows to measure the carrier lifetime as a function of excitation intensity, which is required for a reliable evaluation of the injected carrier density. The vanishing of excitonic lines together with the red-shift of the main emission evidences the occurrence of the Mott transition for a carrier concentration of (6 ± 3)x10 ¹⁶ cm ⁻³ .This value is more than an order of magnitude smaller than previous determinations published in the literature and is in accordance with many-body calculations.
... At the critical Mott density, excitons are no longer present in the material. [109,66,110,111]. The reported data for GaN Mott transitions varies from 1 × 10 18 to 5 × 10 19 cm −3 . ...
... At low temperature the main emission peak corresponds to D 0 X. Fisher's work reports another detailed analysis of the screening of excitons in a GaN slab through femtoscond pumpprobe absorption [110]. Absorption spectra are measured at 5K for various excitations intensities. ...
The aim of this thesis was to study the bleaching of excitons under a quasi-continuous optical excitation in a bulk GaN sample of high quality and single nanowires through power dependent micro-photoluminescence and time-resolved micro-photoluminescence experiments. Time-resolved photoluminescence allows to measure the carrier lifetime as a function of excitation intensity, which is required for a reliable evaluation of the injected carrier density. The experimental value of excitonic Mott density in bulk GaN found in this work is more than an order of magnitude smaller than previous determinations published in the literature and is in accordance with many-body calculations. An in-depth analysis of experimental results was carried out with the help of numerical simulations in order to estimate the evolution of the carrier distribution along the depth of the samples.
We report the results of nondegenerate optical pump-probe absorption experiments performed on GaN and InGaN thin films and quantum wells under the conditions of strong optical band to band excitation. The evolution of the band edge in these materials was monitored as the number of photoexcited free carriers was increased beyond that required to achieve population inversion and observe stimulated emission. The band edge of InGaN is shown to exhibit markedly different high excitation behavior than that of GaN, explaining in part the reduction in stimulated emission threshold that typically accompanies the incorporation of indium into GaN to form InGaN. A comparison of the band edge absorption changes observed in pump-probe experiments to the gain spectra measured in variable-stripe gain experiments is also given.
We present the results of a detailed study of the band edge absorption changes in GaN and InGaN thin films induced by the presence of excess photo-generated free carriers. The 1s A and B free excitonic resonances in GaN are shown to decrease with increasing above-gap nanosecond optical excitation due to screening by free carriers and exciton–exciton scattering. The decrease in excitonic absorption with increasing above-gap excitation is accompanied by a significant increase in the below-gap absorption coefficient. To further explore this behavior, femtosecond nondegenerate optical pump–probe experiments were also performed using an amplified Ti : sapphire laser. Exciton saturation due to screening by free carriers and excitonic phase space filling was again observed, along with a modest amount of below-gap induced absorption attributed to band gap renormalization. Similar experiments were performed on InGaN thin films. The band edge absorption changes observed in InGaN films were found to be significantly different than those observed in GaN.
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