Crystal growth of ErN and ScN via physical vapor transport: synthesis, properties, characterization, and process simulation

Abstract

Recently, the rare earth nitrides have received a considerable attention from theorists and experimentalists due to their potential applications in spintronic, piezoelectric, and thermoelectric devices. In this work, erbium nitride (ErN) and scandium nitride (ScN) crystals were grown and characterized experimentally, and the growth process was modeled and simulated. Erbium nitride (ErN) is a rare earth nitride notable for its magnetic and optical properties. Here we report on its growth on a non-native substrate, tungsten foil, via physical vapor transport, and its characterization. The source material was erbrium metal that was converted to ErN by heating in nitrogen. Subsequently, it was sublimed to form the ErN crystals. The operating conditions were 1620-1770 ⁰C and 150-330 Torr in pure nitrogen. The growth rate increased exponentially with temperature with an activation energy of 508 kJ/mol, and inversely with pressure. X-ray diffraction revealed the ErN preferentially adopted a (100) orientation, the same as the dominant orientation of the tungsten sheet. The lattice constant was 4.853 Å. The crystal shapes and sizes were dependent on the temperature, as revealed by SEM and optical microscopy. The ErN crystals were highly faceted, bound by (100) and (111) crystal planes. The ErN compound deviated from stoichiometry: the Er:N atomic ratio ranged from 1:1.15 to 1:1.2 according to EDX and XPS elemental analysis. Raman spectra was in good agreement with theoretical predictions. Scandium nitride single crystals (14–90 µm thick) were grown on tungsten (100) single crystal substrate by physical vapor transport in the temperature range of 1850-2000 ⁰C and pressure of 15-35 Torr. Epitaxial growth was confirmed using in-plane ɸ scan and out-of-plane x-ray diffraction techniques which revealed that ScN exhibited cube-on-cube growth with a plane relationship ScN (001) || W (001) and normal direction ScN [100] || W [110]. Atomic force microscopy revealed the surface roughness decreased from 83 nm to 18 nm as the growth temperature was increased. X-ray diffraction (XRD) rocking curves widths decreased indicating the crystal quality improved with increasing growth temperature. The lowest XRC FWHM was 821 arcsec, which is so far the lowest value reported for ScN. Scanning electron microscopy (SEM) exhibited the formation of macrosteps and cracks on the crystal surface with latter due to the mismatch of ScN’s and tungsten’s coefficients of thermal expansion . In general for crystal growth, material should deposit on the seed crystal and not on any adjacent supporting structures. This efficiently uses the source material and avoids the possibility of spurious polycrystals encroaching on, and interfering with the single crystal growth. To achieve this goal, a new crucible design with a cooling fin in contact with the seed was simulated and experimentally demonstrated on the physical vapor transport (PVT) crystal growth of scandium nitride. The heat transfer of the growth cavity for a conventional crucible and a modified crucible with the cooling fin were modeled theoretically via computational fluid dynamics (CFD) with FLUENT. The CFD results showed that the seed in the modified crucible was approximately 10 °C cooler than the crucible lid, while in the conventional crucible the temperature of the seed and lid were uniform. The experimental results showed that increasing the temperature gradient between the source and the seed by employing the cooling fin led to a dramatic increase in the growth rate of ScN on the seed and reduced growth on the lid. The relative growth rates were 80 % and 20 % on the seed and lid respectively, in the modified crucible, compared to 25% and 75% with the conventional crucible. Thus, the modified crucible improved the process by increasing the species transporting to the seed by sublimation.