The effect of different proximity caps on quantum well intermixing in InGaAsP/InP QW structures

A low-temperature InP-cap layer is used to enhance quantum well intermixing (QWI) following rapid thermal annealing (RTA) on an InGaAsP quantum well (QW) structure. The influence of different proximity caps (Si, InP and GaAs) used during the RTA step has been investigated. A combination of cross-sectional transmission electron microscopy and high-resolution TEM together with energy-dispersive x-ray analysis directly reveals compositional and morphological changes resulting after QWI. Room temperature photoluminescence is used to establish the changes in emission wavelength and intensity resulting from the QW modification. Use of a Si proximity cap leads to the formation of a sharply defined QW with the least amount of QW modification, while GaAs proximity capping results in the broadest QW, broadened QW/barrier interfaces and the highest concentration of phosphorous in the well. Use of an InP proximity cap results in intermediate QW modification. In all cases, after QWI the bottoms of the QWs are flat and the well is square-like with broadened sidewalls. It has also been found that group V rich precipitates are produced on the top two layers of the structure. The concentration and size of the precipitates are minimum with Si proximity capping, while the use of GaAs produces the highest concentration and largest precipitates. The concentration and size of the precipitates are intermediate for the InP capping. The choice of proximity cap material affects the intermixing through changes of the effective P diffusivity which has been calculated from obtained profiles.