Effect of the Growth Conditions on the Properties of Rare Earth Nitride Thin Films
Authors:   , Shaib Ali 
Published: November 2022

<p><b>The members of the rare earth nitride (REN) series are intrinsic ferromagnetic semiconductors that bring together attractive electronic and magnetic properties for fundamental investigations and spintronics applications. So far, most of the research on rare earth nitrides is focused on their electrical transport and magnetic properties, leaving a significant knowledge gap to determine and understand the effect of growth conditions on their structural properties. In this thesis, we propose a study on the effects of the growth conditions, with an emphasis on the influence of the ratio of N2 to RE (RN2/Gd) during the growth, on the structural, electrical transport, magnetic, and optical properties of a series of REN thin films.</b></p>
<p>We begin by investigating the effects of RN2/Gd during the catalytic growth of polycrystalline GdN thin films of their structural, electrical transport, magnetic, and optical properties. Electrical resistivity of the films increases by four orders-of-magnitude for a less than two orders increase of RN2/Gd. Hall resistivity measurements indicate a parallel decrease in the carrier concentration, suggesting decreased nitrogen vacancy (VN) levels in the films for higher RN2/Gd. 2θ-θ X-ray measurements indicate arising asymmetry in the dominant GdN 111 peak at higher VN concentrations, suggesting the presence of a second structural phase of GdN (GdN-II) of a smaller lattice constant that is associated with N-deficient regions of the films, alongside a stoichiometric phase (GdN-I). Magnetic data indicate enhanced Curie temperature at higher VN levels, possibly due to the formation of magnetic polarons at VN sites, and the coercive field data at low temperature suggests a soft ferromagnetic nature of GdN-II. Raman spectroscopy does not point to a clear link between the weak Raman signal observed in the RENs and the level of VN in the films.</p>
<p>We next explore the preferential growth nature of SmN and the effect of RN2/RE on the structural, electric transport, magnetic, and optical properties of polycrystalline SmN and DyN thin films. XRD diffractograms indicate SmN exhibits a competitive growth following an evolutionary model and grows preferentially along the (111)-crystal orientation. Both SmN and DyN experience a stark increase in film resistivity for a small increase in RN2/RE, and Hall effect measurements on SmN suggest high VN concentration in the most conductive films (~9%). Although both SmN and DyN experience lattice constant reduction at higher VN content, neither show evidence for a second structural phase. The Raman signal in SmN films is also weak; however, the broadness of the RN2/RE range used during film growth allows identifying strengthening of the Raman signal when more vacancies are present. The origin of the signal is still uncertain, yet 2nd order distortions such as tetragonal distortions associated with VN could likely be the culprit. Magnetic measurements on DyN show enhancement of TC for lower RN2/Dy in a similar fashion to GdN. DyN’s magnetic moment nears Hund’s rule moment in films with high VN, possibly indicating a role played by VN in suppressing the crystal field quenching effects.</p>
<p>Lastly, we report on growing epitaxial films of Gd and Sm on AlN templates that we nitride via flooding the growth chamber with pure N2 gas, and RHEED is used to monitor the real-time lattice evolution. The Gd thin film displays rapid incorporation of nitrogen at ambient temperature, yet experiences delay at high temperature. XRD diffractograms show evidence of a broad peak corresponding to GdN with a shrunk lattice constant, while magnetic data suggest the nitrided layer is not what to expect from a traditional diffusion model, but instead is formed of two separate layers of GdN and Gd. We carried a cycle of Sm growth followed by nitridation/denitridation, while increasing the nitrogen pressure with each new stack. RHEED patterns display lattice spacing corresponding to both divalent and trivalent Sm, in contrast with previous reports. Incorporation of nitrogen is fast for all cycles, and a SmN RHEED pattern corresponding to VN-rich SmN is produced. Although the first cycle of denitridation results in no change to the RHEED pattern, subsequent cycles achieve complete reversal of the pattern back into a pattern resembling Sm metal. This is of great importance for applications requiring the presence of atomic nitrogen like the production of ammonia.</p>