Dot Density Effect by Quantity of Deposited Material in InP/AlGaInP Structures

Optical absorption spectra have been measured by the segmented contact method on InP quantum-dot (QD) laser structures for different quantities of deposited material, equivalent to 2, 2.5, and 3 mono-layers, and growth temperatures of 690°C and 730°C. The spectra suggest inhomogeneous distributions of “large” and “small” groups of dots and a group of “very large” dots in structures grown at 690°C. The absorption peak energies do not change significantly with the amount of deposited material so we interpret changes in the magnitude of absorption as being due to changes in the density of dots. Using calculated values for the optical cross sections, we have estimated the variation of the number of dots in each group with monolayers of deposited material. The structures grown at 690°C are unusual in that the density of small dots decreases with increasing material deposited whereas the density of very large dots increases superlinearly, suggesting the small dots agglomerate to form the “very large” dots, which may in fact be due to quantum mechanical coupling of closely spaced small dots.