Photoluminescence-free photoreflectance spectra using dual frequency modulation

Abstract

Photoreflectance (PR) spectra are usually obtained by measuring the relative change on the reflectivity of a semiconducting sample induced by a chopped laser beam. The laser beam can also produce photoluminescence (PL) emission at the sample surface which, detected at its same frequency, could appear as an offset distorting the PR spectrum. This parasitic and intrinsically noisy PL signal, not easily discriminated electronically nor optically filtered, can become the dominant part of the PR spectrum at low sample temperatures, hiding spectrum features under its associated noise, or even avoiding data acquisition. An alternative method for PL signal discrimination is proposed in this work, using a different chopping frequency for each light beam: PL and reflected signals will appear each one at its own chopping frequency, while PR signal will be detected at its frequency sum, allowing signal separation by frequency. Both experimental setups are compared using a highly luminescent quantum well structure at low temperatures. While the standard setup suffers the PL limitation, the proposed method overcomes this constraint, allowing good quality spectra to be measured at temperatures as low as 12 K