Effect of well profile on optical and structural properties in InGaN/GaN quantum wells and light emitting diodes

Effects of the quantum well (QW) shape on structural and optical properties of QWs fabricated by gradating In composition during the well layer growth and characteristics of light emitting diodes (LEDs) composed of those QW structures are investigated. Photoluminescence (PL) spectra of trapezoidal-shape QWs show the best light emission efficiency. PL spectra exhibit a main peak associated with transition in In-rich quantum dot-like clusters at 468 nm and a relatively small peak originated from QWs at 430 nm, and their phonon replicas. Temperature dependent PL spectra indicated that excitons in trapezoidal QW structure are more strongly localized than in rectangular QWs. Under external electric fields, the magnitude of the variation in PL peak and intensity of trapezoidal QWs at room temperature is almost constant in contrast to those of rectangular ones. In a plan-view transmission electron microscopic image, it was observed that quantum dots (QDs) formed inside the dislocation loop in trapezoidal QWs. The size and density of QDs depend on the QW shape. Furthermore, the interactions of QDs with dislocations such as dislocation pinning by QDs and the preferential formation of QDs on dislocations lying on the growth plane are observed. LEDs fabricated using trapezoidal QWs have superior device properties such as high light output power, low forward operation voltage, good thermal stability, and stability under external electric fields. It suggests that the shape of QWs plays an important role in dislocation density control and QD engineering, and, in turn, the light emission efficiency.