Impurity-free intermixing in compressively strained InGaAsP multiple quantum well structures

We report on controlled band gap modification in a compressively strained InGaAsP multi-quantum well-laser structure using different encapsulating layers followed by rapid thermal processing (RTP). The structure used was designed as a 1.55μm laser with an active region consisting of three In0.76Ga0.24As0.85P0.15 quantum wells with In0.76Ga0.24As0.52P0.48 barriers grown by metal organic chemical vapor deposition. The heterostructure is capped with 100nm thick InGaAs layer. Prior to RTP, the samples were coated with various dielectric layers or a thin film of low temperature (300°C) grown InP. Using a SixNy film deposited by plasma-enhanced chemical vapor deposition with a refractive index of about 2.0, quantum well intermixing (QWI) was effectively suppressed. The suppression effect was independent of the SixNy film thickness for layers of 30–2400nm. With an e-beam-evaporated SiO2 film, QWI was enhanced and a net blue shift of about 100nm can be achieved between the samples covered with SiO2 and SixNy after RTP at 750°C for 100s. Furthermore, InP grown at a low temperature by gas-source molecular beam epitaxy was proved to be even more efficient in enhancing QWI. Group V interstitial diffusion is used to explain the enhanced QWI between the wells and adjacent barriers which have the same group III compositions. Two-section tunable laser operated around 1.55μm based on this laser structure was fabricated using this technique.