Modulated photoellipsometry: application to the measurement of GaAs internal field

A new optical system allowing photoellipsometric measurements is presented. Photoellipsometry (PE) is a modulation spectroscopy technique based on ellipsometry in presence of a chopped external light excitation. PE measurements are obtained using a double modulation system, combining spectroscopic phase-modulated ellipsometry (SPME) with a laser pump beam. The experimental system described here takes advantage of the high frequency polarisation of SPME (≈ 50 kHz). As a consequence the frequency of the pump beam can be varied up to 5 kHz. The field-induced changes in the real and imaginary parts of the bulk dielectric function can be directly measured and analysed in terms of the pump beam power and/or the probe beam photon energy. Demonstration of this method is made with measurements, recorded in the band-gap E₀ region (≈ 1.4 eV), on n-type GaAs sample. In particular, Franz-Keldysh oscillations are observed with good sensitivity. PE measurements are successfully compared with a theoretical model. From this preliminary study, it can be inferred that PE appears as a promising technique for semiconductor characterisation.