Photoreflectance study of changes in the QW profile of 1.55-micrometer laser structure induced by SiO2 cap layers

Quantum well (QW) band gap modification has been carried out in 1.55 μm InGaAsP-based laser structures using a silicon dioxide cap (SiO2) and rapid thermal annealing (RTA). Migration of the semiconductor atoms across the QW interface changes the profile of QW from a square well to a rounded well. The modification of the profile causes the shift of energy levels in the QW. In consequence, a blue shift of the emission peak is observed in photoluminescence (PL). In this paper, the contactless electromodulation technique called photoreflectance (PR), which is an absorption-like experiment, has been used. This method allows the detection of excited states transitions, as well as the fundamental one, in low-dimensional structures. In PR experiment, different energy shifts are observed for different confined state transitions reflecting the character of the changes in the well profile. The role of the microwave power of ECR-PECVD process is analysed and its impact on the blue shift of both ground and excited states is discussed in this paper. It is shown that the value of the blue shift can be controlled by the microwave power.