Identifying the influence of GaN/InxGa1-xN type last quantum barrier on internal quantum efficiency for III-nitride based light-emitting diode

GaN/InxGa1-xN-type last quantum barrier (LQB) proves to be useful for Ⅲ-nitride based light-emitting diode (LED) in enhancing the internal quantum efficiency (IQE) and suppressing the efficiency droop level that often takes place especially when the injection current is high. In this work, GaN/InxGa1-xN-type LQB reported by the scientific community to enhance the IQE is first reviewed and summarized. Then, the influences of indium composition and thickness of the InxGa1-xN layer on the performance of LED incorporated with the GaN/InxGa1-xN-type LQB are studied. Through analyzing energy band diagrams calculated with APSYS, we find that the[0001] oriented LQB features an electron depletion due to the polarization induced negative charges at the GaN/InxGa1-xN interface. The electron depletion enhances the electron blocking effect and reduces the electron accumulation at the InxGa1-xN/AlGaN interface, leading to an improved IQE for the LED. In addition, increasing the indium composition of the InxGa1-xN layer will generate more negative interface charges, which result in further increased conduction band barrier height for the electrons and reduced electron leakage. On the other hand, for the GaN/InxGa1-xN-type LQB with a fixed indium composition, there exists an optimum thickness for the InxGa1-xN layer in maximizing the improvement of IQE for the LED, mainly because the interaction between two mechanisms co-exists when varying the thickness of the InxGa1-xN layer, i.e., the initial increase in the InxGa1-xN layer thickness will lead to an increased conduction band barrier height, which prevents electrons from leaking into the InxGa1-xN layer. However, further increasing the InxGa1-xN layer thickness to a certain value, tunneling effect will kick in as a result of the simultaneously reduced GaN thickness-the electrons will tunnel through the thin GaN layer in the LQB from the quantum wells to the InxGa1-xN layer. This will cause electrons to increase in the InxGa1-xN layer. Therefore, as a result of the interaction between the above-mentioned two mechanisms, there is an optimum thickness for the InxGa1-xN layer such that the electrons in the InxGa1-xN layer will reach a minimal value, which in turn will lead to a maximized conduction band barrier height for the AlGaN electron blocking layer and facilitate the performance of LEDs.