Measurement of Internal Polarization by QCSE Induced Level Shift in AlGaN Quantum Cascade Emitters

An indirect method to determine the internal polarization of a GaN/AlGaN-based quantum well is presented. The technique consists of carefully measuring the difference between the two lowest conduction subband levels under the application of an electrical bias across the epitaxial layers. Due to a quantum confined Stark effect, the bound energy levels of the well show a strong frequency shift, which depends on the size of the external field. Since this electric field was oriented oppositely to the internal polarization, partial screening of the latter occurred. By measuring these Stark shifts at three different fields, one was able to calculate the size of the internal polarization. Since the composition of the AlGaN barriers had previously been determined by X-ray diffraction, an extrapolated value of the internal polarization-as it would occur in a pure AlN/GaN interface-could be given. In agreement with the literature, a value of 720 MV/m was found.