Synthesis and Calculations of Wurtzite Al1- xGdxN Heterostructural Alloys

Abstract

Al1-xGdxN is one of a series of novel heterostructural alloys involving rare earth cations with potentially interesting properties for (opto)electronic, magnetic, and neutron detector applications. Using alloy models in conjunction with density functional theory, we explored the full composition range for Al1-xGdxN and found that wurtzite is the ground-state structure up to a critical composition of xc = 0.82. The calculated temperature-composition phase diagram reveals a large miscibility gap inducing spinodal decomposition at equilibrium conditions, with higher Gd substitution (meta)stabilized at higher temperatures. By depositing combinatorial thin films at high effective temperatures using radio-frequency cosputtering, we have achieved the highest Gd3+ incorporation into the wurtzite phase reported to date, with single-phase compositions at least up to x ≈ 0.25 confirmed by high-resolution synchrotron grazing incidence wide-angle X-ray scattering. High-resolution transmission electron microscopy on material with x ≈ 0.13 and x ≈ 0.24 confirmed a uniform composition polycrystalline film with uniform columnar grains having the wurtzite structure. Spectroscopic ellipsometry and cathodoluminescence spectroscopy measurements are employed to probe the optoelectronic properties, showing that the band gap decreases with increasing Gd content x and that this effect causes the ideal Gd substitution level for cathodoluminescence applications to be low. Expanding our calculations to other rare earth cations (Pr3+ and Tb3+) reveals similar thermodynamic stability and solubility behavior to Gd. From this and previous studies on Al1-xScxN, we elucidate that both smaller ionic radius and higher bond ionicity promote increased incorporation of group IIIB cations into wurtzite AlN. This work furthers the development of design rules for new alloys in this material family.