Growth of high In content InGaN alloys

The growth of high In content InGaN alloys with sufficiently high crystal quality constitutes a puzzling situation where the complex interplay between surface morphologies, partial pressures and growth temperature plays a central role. We have investigated the bulk and surface thermodynamics of InxGa1-xN growth for the technologically relevant (0001) and (0001) growth planes by means of density functional theory calculations.

Our calculations reveal that chemical effects (i.e. stronger Ga-N as compared to In-N bonds) result in a strong driving force for In surface segregation in the (0001) growth. Furthermore, the thermal stability of excess indium is found to be remarkably higher on N-polar surfaces than on the Ga-polar surfaces, indicating that for a given level of In incorporation, higher temperatures can be used for N-polar growth as compared to Ga-polar growth.

We have furthermore investigated the thermodynamics of solid and gas phases in molecular beam epitaxy like growth conditions and we highlighted the effect that plasma discharges have on the critical temperatures for InGaN decomposition: The weaker binding of the active nitrogen species produced by plasma as compared to ground state molecular nitrogen shifts the equilibrium and thus the maximum allowed growth temperatures to ≈500 K higher values. However, the aforementioned temperatures have to be decreased by ≈100 K for every 25% InN added to the InGaN alloy.

Phase diagram of In covered (a) Ga-polar and (b) N-polar surfaces, with clean and In ad-, bi- and tri-layer surfaces considered. For various values of temperature and In effective partial pressures the lowest energy surface is shown. The solid (dotted) lines indicate the calculated boundaries where vibrational contributions to the surface free energy are (not) included, respectively. Within the lower gray shaded areas the aforementioned surfaces are unstable against bulk indium. The N-polar In-bilayer structure is not energetically favorable within the considered range of temperatures and In effective partial pressures and so is not shown in (b).

 

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