Quantum well intermixing in InGaAsP laser structures using a low temperature grown InP cap layer

Quantum well intermixing (QWI) in a 1.55 µm InGaAsP laser-like structure has been enhanced using the defects incorporated in an InP capping layer grown at low temperature (below the congruent sublimation temperature) by molecular beam epitaxy and subsequently subjected to rapid thermal annealing. The structures used had quantum wells (QWs) and barrier layers with identical group III compositions so the inter-diffusion occurs only on the group V sub-lattice. This inter-diffusion is induced by the diffusion of P-interstitials that result from the dissociation of PIn anti-site defects that are present in large concentrations in the low temperature InP (LT-InP) layer. The magnitude of the QWI is determined by measuring the blueshift in the wavelength of room temperature photoluminescence emission from the QWs. It was found that the magnitude of the blueshift is dependent on the growth conditions of the LT-InP such that larger blueshifts are observed for LT-InP layers either grown at lower temperatures or with increasing P2 overpressures. These features correlate with the expected changes in the concentration of PIn defects with these changes in growth conditions. Also, there is a change in the rate of change in blueshift with the thickness of the LT-InP layer. For thin layers the rate of change of blueshift with thickness is rapid, but at a certain thickness a transition occurs to a lower rate of change with thickness. This transition thickness is temperature dependent such that the transition to the reduced rate occurs at larger thicknesses at higher anneal temperatures. This transition is interpreted as re-trapping of the P-interstitials in the LT-InP by the In-vacancies resulting from the PIn dissociation which leads to a reduced rate of supply of P-interstitials into the underlying laser structure.