The control and evaluation of blue shift in GaInAs/GaInAsP multiple quantum well structures for integrated lasers and Stark-effect modulators
The authors report on the structural and optical characterization of nominally lattice-matched GaInAs/GaInAsP multiple quantum well (MQW) structures grown on (100) InP substrates by metalorganic chemical vapour deposition (MOCVD) which undergo a 'blue shift' in photoluminescence upon thermal annealing. Electron microscope and magneto-optical analyses show that the shifts are principally due to layer interdiffusion, which results in a change in composition of the well centres. These compositional variations are quantitatively measured by high-resolution analytical electron microscopy. This analysis demonstrates that the diffusion of the group V elements in the undoped MQWS is faster than that of the group III elements, resulting in the incorporation of excess coherency strain in the material. Analysis of a number of samples grown on a variety of substrates shows that the wavelength shift is particularly large when the substrates are S doped, although the substrate dopant does not participate directly in the diffusion mechanism. The authors attribute this behavior to the typically low dislocation density of S-doped substrates. They report, for the first time, a direct measurement of the correlation between the spatial variation in the magnitude of the blue shift and the presence of dislocations in the MQWS.