As a result of the large band gap bowing caused by low N concentrations, dilute nitride materials (GaAsN, InAsN, InGaAsN) have attracted a considerable interest for applications including infrared/mid-infrared laser diodes and photovoltaic solar cells. A major challenge is the very low thermodynamic solubility of N in bulk GaAs and InAs. For example, theoretical studies predict N concentrations in bulk GaAs of ~10^-7% [1]. However, N concentrations up to few percents can be achieved in GaAs using molecular beam epitaxy growth [2].

Theoretical studies indicate that the solubility of impurities is enhanced near surfaces [3]. This enhanced surface solubility can be frozen-in during growth by employing surface kinetics, which allows to overcome the limited bulk solubility. For the reconstructed GaAs and InAs(001) surfaces the solubility of N has its highest value in the second anion layer (subsurface). However, not only the surface layer has an effect on the solubility but also the structure of the surface (the surface reconstruction) itself. It is therefore necessary to identify the growth conditions that should be applied to achieve the highest surface solubility. This is done by constructing the thermodynamic stability phase diagrams of the N substitutions at the different surface layers and for the various relevant surface reconstructions. Using these phase diagrams we calculate the achievable N surface concentrations as a function of growth conditions (temperature and partial pressures).

In addition to surface thermodynamics, an important issue that should be considered is surface kinetics. The knowledge of surface kinetics helps to identify the conditions that allow for the subsurface incorporation under the typical growth conditions (see Fig. 1). Therefore, we study the kinetic barriers for the N adatom to diffuse and incorporate in the subsurface layer. Using this we identify the possible N surface/subsurface diffusion paths and incorporation mechanisms. Our results indicate that the subsurface incorporation is kinetically prohibited under typical MBE conditions by a trapping mechanism in the topmost surface layer.

With our results we are able to revise previously proposed growth models for dilute nitride alloys. In addition, based on our computed surface thermodynamics and kinetics data we were able to provide a direct microscopic basis of the recently reported In-N compositional anti-correlation in quaternary InGaAsN quantum wells [4]. This insight provided a generic basis for an atomic scale understanding of the highly mismatched quaternary systems.