Contactless electroreflectance studies of surface potential barrier in AlGaN/n‐AlGaN structures with various Al concentrations

Abstract

Contactless electroreflectance spectroscopy has been applied to study the surface potential barrier in AlxGa1−xN/n‐AlxGa1−xN structures with 0 ≤ x ≤ 0.25 grown by metaloorganic vapor phase epitaxy. A strong band‐to‐band transition followed by Franz–Keldysh oscillation (FKO) was clearly observed for all samples. The value of built‐in electric field was determined from the period of FKO, and the surface potential barrier was calculated with knowledge of the thickness of undoped AlxGa1−xN layer. For a set of GaN/n‐GaN structures with different thickness of undoped GaN layer (30–120 nm) an analysis of built‐in electric field gives the surface potential barrier of 0.59 ± 0.03 eV. For AlxGa1−xN(60 nm)/n‐AlxGa1−xN structures with 0.09 ≤ x ≤ 0.25 the surface potential barrier does not vary with Al concentration within the experimental accuracy. In this case the position of the Fermi level at the surface has been estimated to be ∼1.4 eV. The increase of surface potential barrier from 0.59 eV for GaN to 1.4 eV for AlGaN with 0.09 ≤ x ≤ 0.25 is attributed to low concentration of n‐type non‐intentional residual dopants in the undoped AlxGa1−xN layer as well as the presence of Al atoms at the surface and their higher sensitivity to oxidation and/or creation of defects states which are not present at GaN surfaces.