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 1.55 μm laser with an active region consisting of three In_0.76Ga_0.24As_0.85P_0.15 quantum wells with In_0.76Ga_0.24As_0.52P_0.48 barriers grown by metal organic chemical vapor deposition. The heterostructure is capped with 100 nm 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 Si_xN_y 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 Si_xN_y film thickness for layers of 30–2400 nm. With an e-beam-evaporated SiO₂ film, QWI was enhanced and a net blue shift of about 100 nm can be achieved between the samples covered with SiO₂ and Si_xN_y after RTP at 750°C for 100 s. 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 1.55 μm based on this laser structure was fabricated using this technique.