Effect of surface potential pinning on strain behavior of AlGaN/GaN device structures

Understanding the varied strain effects in AlGaN/GaN devices is crucial for realizing optimized flexible electronics systems and strain sensors. Here, we report on the effects of surface potential pinning, altered by the deposition of device-relevant SiNx passivation and Ni gate layers, on the strain-dependent carrier density, ns, of AlGaN/GaN two-dimensional electron gas structures. Flexible van der Pauw samples were made by separating AlGaN/GaN layers from the sapphire growth substrate using a two-dimensional boron nitride van der Waals release layer and transferring them to flexible substrates. For bare surface samples, we observed relatively large decreases in ns with tensile strain (Δns of −2 × 1011 cm−2 at 0.1% uniaxial strain), indicating an unpinned AlGaN surface potential. For the SiNx and Ni covered samples, the ns-strain trends were nearly flat, indicating a more pinned surface potential. Additionally, sub-bandgap 400 nm light is shown to effectively pin the surface potential as evidenced by flattening the ns-strain trend, the mechanism of which we explain in terms of the persistent photoconductivity effect. These observations could have important implications in tuning strain sensors and minimizing device variability in flexible electronics.