SWEEPING PHOTOREFLECTANCE SPECTROSCOPY OF SEMICONDUCTORS

We report a new type of photoreflectance (PR) by means of sweeping the pump laser beam. The modulation is achieved by moving the position of the pump beam with respect to the probe beam. In the conventional photoreflectance (using a mechanical chopper) the problem is caused by photoluminescence (PL) produced by the pump light. This is particularly acute at low temperatures where the PL is large. In our novel sweeping photoreflectance (SPR) technique the pump intensity is constant. Hence the problem associated with the luminescence is eliminated. Therefore SPR spectra can be obtained at much lower temperatures than conventional PR. The signal to noise ratio in SPR is usually better than that in conventional PR. The sources of noise in both conventional and sweeping photoreflectance is also discussed.