Growth of AlGaN under the conditions of significant gallium evaporation: phase separation and enhanced lateral growth

TL;DR

This study investigates AlGaN growth under high gallium evaporation conditions, revealing phase separation and enhanced lateral growth due to Ga accumulation at surface defects, supported by experimental data and theoretical modeling.

Contribution

It introduces a kinetic model for AlGaN growth with significant Ga desorption and combines experimental and first-principles calculations to analyze Ga atom binding energies.

Findings

Gallium addition improves film morphology compared to pure AlN.

Larger Ga amounts are retained at surface defect boundaries during desorption.

The binding energy difference of Ga atoms at defects vs. terraces is approximately 0.3 eV.

Abstract

Growth kinetics of AlGaN in NH3 MBE under significant Ga desorption was studied. It was found that the addition of gallium stimulates 2D growth and provides better morphology of films compared to pure AlN. The effect was experimentally observed at up to 98% desorption of the impinging gallium. We found that, under the conditions of significant thermal desorption, larger amounts of gallium were retained at lateral boundaries of 3D surface features than at flat terraces because of the higher binding energy of Ga atoms at specific surface defects. The selective accumulation of gallium resulted in an increase in lateral growth component through the formation of the Ga-enriched AlGaN phase at boundaries of 3D surface features. We studied temperature dependence of AlGaN growth rate and developed a kinetic model analytically describing this dependence. As the model was in good agreement with the experimental data, we used it to estimate the increase in the binding energy of Ga atoms at surface defects compared to terrace surface cites using data on the Ga content in different AlGaN phases. We also applied first-principles calculations to the thermodynamic analysis of stable configurations on the AlN surface and then used these surface configurations to compare the binding energy of Ga atom at terraces and steps. Both first-principles calculations and analytical estimations of the experimental results gave similar values of difference in binding energies; this value is 0.3 eV.