Although bulk crystalline structure is preserved in nanowires (NWs), the large surface-to-volume ratio and the possible occurrence of quantum confinement for narrow diameters may lead to size dependent carrier dynamics that would influence the operation of active and passive optoelectronic devices.
Here we present a study on ultrafast carrier dynamics in ZnSe and Si NWs performed by optical pump-probe femtosecond transient absorption spectroscopy (FTAS).
The measurements have been performed at room temperature on as-grown samples exploiting the growth of the NWs on transparent, glassy substrates. The use of glassy substrates has a high technological relevance because one of the main advantages of nanowires is that they can be grown on any substrate that withstands the growth conditions, hence including low-cost materials like glasses or plastics. ZnSe NWs have been grown by molecular beam epitaxy at 300 °C, while In- and Au-seeded Si NWs have been grown by plasma-enhanced chemical vapor deposition at 400 °C. The unusually low growth temperature for ZnSe NWs is crucial to obtain material with good optical properties.
In the transient absorption of ZnSe NWs two main features are observed: a narrow dip at energies larger than 2.6 eV and a broad signal with a maximum at about 2.5 eV. The maximum absorption variation was achieved within 600 fs after excitation. It subsequently decays in about 20 ps, with the dynamics of the broad band (due to point defects) being faster than that of the narrow signal (due to band gap states). The fast rising represents the thermalization process to reach a quasi-equilibrium distribution of hot carriers after pump excitation. The subsequent decay gives information about carrier relaxation from probed states to lower energy states. An important feature is the band gap red shift of about 34 meV occurring during the rise time and its recovery when photo-bleaching disappeared. The red shift is attributed to band gap renormalization and is a result of sudden increase of carrier density when many-body effects modify the electronic band-structure.Using the measured red shift we have estimated the photo-excited carrier density to be 6×1017 cm-3.
Band gap renormalization (47 meV) is also observed for the direct band-gap of Si NWs at about 3.3 eV. In this case the red shift takes place much faster than in ZnSe NWs, with times comparable with our resolution (50 fs) while the blue shift following carrier relaxation takes place in about 10 ps.