Abstract
Contactless electroreflectance (CER) spectroscopy was applied to study (i) the type of surface band bending, (ii) the built-in electric field, and (iii) the Stokes shift in selected III-nitrides samples. The surface band bending was studied for truly bulk GaN crystals with various types of conductivity obtained by the ammonothermal method and GaN epilayers doped by Si and Mg. It has been shown that the shape of CER resonance, which is related to the band-to-band absorption, is sensitive to the type of surface band bending. This feature of semiconductor surface can be determined in contactless manner from CER measurements. The built-in electric field was studied for AlGaN/GaN heterojunction structures. From the period of AlGaN-related Franz-Keldysh oscillation the value of electric field has been determined to be 0.27 MV/cm. This value is much smaller that the polarization-related electric field in this system and results from the Fermi level pining at AlGaN surface and the Fermi level position at AlGaN/GaN interface. The Stokes shift was studied for InGaN/GaN multi quantum wells. This shift has been found to be ∼ 50 meV at room temperature.
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