Role of Metal Vacancies in the Mechanism of Thermal Degradation of InGaN Quantum Wells

In this work, we study the thermal degradation of In-rich In_xGa_1-xN quantum wells (QWs) and propose explanation of its origin based on the diffusion of metal vacancies. The structural transformation of the In_xGa_1-xN QWs is initiated by the formation of small initial voids created due to agglomeration of metal vacancies diffusing from the layers beneath the QW. The presence of voids in the QW relaxes the mismatch stress in the vicinity of the void and drives In atoms to diffuse to the relaxed void surroundings. The void walls enriched in In atoms are prone for thermal decomposition, what leads to a subsequent disintegration of the surrounding lattice. The phases observed in the degraded areas of QWs contain voids partly filled with crystalline In and amorphous material, surrounded by the rim of high In-content In_xGa_1-xN or pure InN; the remaining QW between the voids contains residual amount of In. In the case of the In_xGa_1-xN QWs deposited on the GaN layer doped to n-type or on unintentionally doped GaN, we observe a preferential degradation of the first grown QW, while doping of the underlying GaN layer with Mg prevents the degradation of the closest In_xGa_1-xN QW. The reduction in the metal vacancy concentration in the In_xGa_1-xN QWs and their surroundings is crucial for making them more resistant to thermal degradation.