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Atomic intermixing during the growth of self-assembled InAs quantum dots ͑QDs͒ in InP͑001͒ by chemical beam epitaxy ͑CBE͒ can result in the formation of graded interfaces or even alloyed InAs 1−x P x QDs. Taking advantage of a series of samples in which photoluminescence ͑PL͒ spectra are characterized by the presence of a large number of distinct peaks corresponding to the emission from QD families having the same thicknesses ͑h QD ͒ in terms of an integer number of monolayers ͑MLs͒, we investigate intermixing by matching the experimentally observed transition energies to those calculated from tight-binding simulations. Two calculation frameworks are considered: ͑i͒ structures in which QDs are alloys with uniform P concentration ͓P͔ and ͑ii͒ nominally pure InAs QDs bordered by graded interfaces characterized by a diffusion length L D . Excellent agreement between theory and experiment is achieved with either framework. The analysis reveals that the two frameworks yield similar composition profiles as the composition at the center of the QD in the case of graded interfaces is modified to a point where it becomes essentially equal to that calculated for a QD of uniform composition. The calculations also indicate that in both frameworks, two substantially different solution sets are compatible with the experimental results. The first one is characterized by all QDs having the same P composition independent of their size, while the second solution shows a decreasing degree of intermixing with increasing h QD . To discriminate between the two solutions, we use them as input data in a Bloch-wave simulation of transmission electron microscopy ͑TEM͒ image contrast, providing a sequence of contrast versus h QD values. Only the scheme in which the amount of ͓P͔ is the same for all QD ensembles is compatible with the TEM observations. From the analysis of an extensive array of CBE-deposited QD samples, it is concluded that the observed PL transitions can be attributed to a 3 ML thick wetting layer and 4-14 ML thick QDs, with ͓P͔ ranging from 6% to 10% depending on the growth conditions. The determined values of ͓P͔ and L D suggest that surface As/ P exchange and strain-driven alloying are the most probable mechanisms for substantial P incorporation.

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Based on experimental observations for the InAs/InP(001) system and atomistic strain calculations using Keating's valence force field method, we propose a pseudophase diagram describing the regimes of 3D self-organization in quantum dot (QD) multilayers. The combined experimental and theoretical analyses--varying the spacer thickness (H), QD height (h), base (b), and lateral spacing (D)--indicate that the vertically aligned to antialigned transition occurs for a critical value of H/D which increases weakly with b/D, while varying h has virtually no effect on the transition point.

This work investigates the interdiffusion dynamics in self-assembled InAs/ InP͑001͒ quantum dots ͑QDs͒ subjected to rapid thermal annealing in the 600-775°C temperature range. We compare two QD samples capped with InP grown at either optimal or reduced temperature to induce grown-in defects. Atomic interdiffusion is assessed by using photoluminescence measurements in conjunction with tight-binding calculations. By assuming Fickian diffusion, the interdiffusion lengths L I are determined as a function of annealing conditions from the comparison of the measured optical transition energies with those calculated for InP / InAs 1−x P x / InP quantum wells with graded interfaces. L I values are then analyzed using a one-dimensional interdiffusion model that accounts for both the transport of nonequilibrium concentrations of P interstitials from the InP capping layer to the InAs active region and the P-As substitution in the QD vicinity. It is demonstrated that each process is characterized by a diffusion coefficient D ͑i͒ given by D ͑i͒ = D 0 ͑i͒ exp͑−E a ͑i͒ / k B T a ͒. The activation energy and pre-exponential factor for P interstitial diffusion in the InP matrix are E a ͑P-InP͒ = 2.7Ϯ 0.3 eV and D 0 ͑P-InP͒ =10 3.6Ϯ0.9 cm 2 s −1 , which are independent of the InP growth conditions. For the P-As substitution process, E a ͑P-As͒ = 2.3Ϯ 0.2 eV and ͑c o / n o ͒D 0 ͑P-As͒ ϳ 10 −5 −10 −4 cm 2 s −1 , which depend on the QD height and concentration of grown-in defects ͑c o / n o ͒.

We have studied the optical properties of ultrathin InAs/InP quantum wells and Stranski-Krastanov nanostructures using photoluminescence and photoluminescence excitation experiments. For InAs epilayers thinner than 2.4 monolayers, the emission spectrum consists of a single peak and the ground-state exciton energy is in good agreement with predictions based on the tight-binding method for ultrathin quantum wells. Beyond this thickness, the photoluminescence spectra evolve to a multimodal emission indicative of the presence of families of quantum dots with small heights. The emission of these quantum dots is blue-shifted significantly (∼100 meV) from the predicted values. The discrepancy is explained by As/P intermixing that occurs during quantum dot formation.

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