Probing the transitions between 2D and 3D states in InGaN/GaN quantum wells via admittance spectroscopy

Carrier transport and recombination mechanisms in InGaN/GaN multiple quantum well (MQW) structures are fundamental to the performance of advanced optoelectronic devices. Thermally activated signals with activation energies of 60–100 meV are frequently observed, yet their physical origin remains a subject of debate between point defects and interface states. In this work, we provide a new perspective on these signals through systematic admittance spectroscopy analysis of MQW p-n junction diodes. Experimental results demonstrate that the observed electrical response originates from the intrinsic kinetic process of carrier emission from 2-dimentional (2D) bound states in the quantum wells to 3-dimentional (3D) free states. The signal intensity exhibits a pronounced bias dependence, which correlates directly with the number of quantum wells intersecting the Fermi level, a behavior fundamentally distinct from that of bulk defects. Furthermore, the activation energy remains nearly constant under varying bias, ruling out the interface-state hypothesis where energy levels typically shift with surface potential. By employing a 2D-to-3D emission model, we quantitatively extract the carrier capture velocity and associated kinetic parameters. This study not only helps clarify the long-standing controversy surrounding low-energy signals in InGaN/GaN systems but also provides essential insights into carrier accumulation and transport dynamics for the optimization of MQW-based optoelectronic devices.