Double AC photoreflectance spectroscopy of semiconductors

We report a new way of making photoreflectance (PR) measurements to overcome the problem of photoluminescence (PL) that is often encountered in low temperature PR measurements. In conventional PR, the probe beam is dc while the pump beam is chopped and in phase detection is done at that chopping frequency. At low temperatures a large PL signal arises at the chopping frequency of the pump beam (a laser) and swamps the PR signal. We overcome this problem by chopping both the pump beam as well as the probe beam at two different frequencies and detecting the PR signal at the sum frequency. This way we avoid the PL signal which now comes at a frequency which is not the frequency of detection. In this paper we discuss the details of this technique and present some low temperature PR data on epitaxial GaAs and GaAs-In/sub x/Ga/sub 1-x/As-GaAs strained layer quantum wells along with fitted lineshapes to show the feasibility of this technique.