Reflectance modulation produced by an intense secondary light beam (photoreflectance) has been studied for wavelengths near the fundamental edge in ultrapure epitaxial layers of GaAs as a function of doping, temperature, and intensity of the modulating light beam. As the doping increases, the built-in electric field at the surface increases and the photoreflectance line shape stretches, in qualitative agreement with the predictions of the Franz-Keldysh theory. These results show directly that the photoreflectance is due to the modulation of the electric field in the Schottky surface barrier by photoexcited carriers. Since the line shape is much narrower than earlier electroreflectance measurements, we arrive at a more accurate value for the band gap of 1.420 eV at 300 °K. As the temperature is lowered, the spacing in energy between adjacent peaks in the photoreflectance spectrum decreases, indicating that the electric field at the surface is decreasing. This temperature dependence is explained by a simple model for the surface consisting of a large density of electron traps located a fixed energy below the conduction-band edge. For all but the purest sample, the line shape is independent of the intensity of the modulating beam, consistent with the model that the secondary light beam is only slightly modifying the surface electric field. However, in the purest sample available ($n_e=1.65\mathrm{}\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$), a new effect is observed at the highest intensities of the modulating beam (∼ 0.1 ${\mathrm{W}/\mathrm{c}\mathrm{m}}^2$). The qualitative features of the new line shape agree with the predictions for both the Burstein effect and exciton screening by free carriers. Since the discrete exciton is thermally quenched at 300 °K, the new line shape may result from the Burstein effect.

• Received 29 December 1969

DOI:https://doi.org/10.1103/PhysRevB.2.803