Modulation spectroscopy of AlGaN/GaN heterostructures: The influence of electron–hole interaction



A detailed analysis of electroreflectance (ER) spectra for AlGaN/GaN heterostructures containing a two-dimensional electron gas (2DEG) is presented. The properties of two samples with different Al-content in the barrier layer are compared. Calculated field-dependent dielectric functions (DF) for GaN and AlGaN which take into account electron–hole interaction (exciton effects) provide the basis for the modeling of the spectra. It is demonstrated that the Seraphin coefficients exhibit a strong spectral dependence around the gap which cannot be neglected in a fit of ER spectra. The use of the calculated DFs in a multi-layer model allows to reproduce all peculiarities of experimental ER spectra for both, the GaN channel layer and the AlGaN barrier layer, despite the inhomogeneous field distribution. The determined AlGaN barrier field strengths are very close to the values obtained by applying the asymptotic formulation of the Franz–Keldysh effect. Finally, the polarization gradient at the AlGaN/GaN boundary is estimated from these field strengths. The gradient is slightly lower than theoretically predicted which confirms previous studies. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)