Effects of a cap layer on built-in electric fields of Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N/GaN heterostructures non-destructively probed by Franz-Keldysh oscillations

We have investigated electric field strengths in the Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N layer, F<Subscript>AlGaN</Subscript>'s, of Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N/GaN heterostructures with and without a GaN cap layer using photoreflectance (PR) spectroscopy. Franz-Keldysh oscillations (FKOs) from the Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N layer are clearly observed in the PR spectra. It is found from analysis of the FKOs that stacking of the cap layer causes a remarkable enhancement of F<Subscript>AlGaN</Subscript>. This fact demonstrates that the FKO profile is a non-destructive probe for a change of built-in electric field strength induced by a cap layer. Numerical calculations of F<Subscript>AlGaN</Subscript> based on a Schrödinger-Poisson equation clarify that the magnitude of the enhancement of F<Subscript>AlGaN</Subscript> is dominated by the cap-layer thickness. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2006