Engineering of electric field distribution in GaN(cap)/AlGaN/GaN heterostructures: theoretical and experimental studies

Polarization engineering of GaN-based heterostructures opens a way to develop advanced transistor heterostructures, although measurement of the electric field in such heterostructures is not a simple task. In this work, contactless electroreflectance (CER) spectroscopy has been applied to measure the electric field in GaN-based heterostructures. For a set of GaN( d   =  0, 5, 15, and 30 nm)/AlGaN(20 nm)/GaN(buffer) heterostructures a decrease of electric field in the GaN(cap) layer from 0.66 MV cm⁻¹ to 0.27 MV cm⁻¹ and an increase of the electric field in the AlGaN layer from 0.57 MV cm⁻¹ to 0.99 MV cm⁻¹ have been observed with the increase in the GaN(cap) thickness from 5–30 nm. For a set of GaN(20 nm)/AlGaN( d   =  10, 20, 30, and 40 nm)/GaN(buffer) heterostructures a decrease of the electric field in the AlGaN layer from 1.77 MV cm⁻¹ to 0.64 MV cm⁻¹ and an increase of the electric field in the GaN layer from 0.57 MV cm⁻¹ to 0.99 MV cm⁻¹ were observed with the increase in the AlGaN thickness from 10–40 nm. To determine the distribution of the electric field in these heterostructures the Schrödinger and Poisson equations are solved in a self-consistent manner and matched with experimental data. It is shown that the built-in electric field in the GaN(cap) and AlGaN layers obtained from measurements does not reach values of electric field resulting only from polarization effects. The measured electric fields are smaller due to a screening of polarization effects by free carriers, which are inhomogeneously distributed across the heterostructure and accumulate at interfaces. The results clearly demonstrate that CER measurements supported by theoretical calculations are able to determine the electric field distribution in GaN-based heterostructures quantitatively, which is very important for polarization engineering in this material system. (paper)