Homoleptic Gadolinium Amidinates as Precursors for MOCVD of Oriented Gadolinium Nitride (GdN) Thin Films

Five new homoleptic gadolinium tris-amidinate complexes
are reported,
which were synthesized via the salt-elimination reaction of GdCl3 with 3 equiv of lithiated symmetric and asymmetric amidinates
at ambient temperature. The Gd-tris-amidinates [Gd­{(NiPr)2­CR}3] [R
= Me (1), Et (2), tBu (3), nBu (4)] and [Gd­{(NEt)­(NtBu)­CMe}3] (5) are solids at room temperature
and sublime at temperatures of about 125 °C (6 × 10–2 mbar) with the exception of compound 4, which is a viscous liquid at room temperature. According to X-ray
diffraction analysis of 3 and 5 as representative
examples of the series, the complexes adopt a distorted octahedral
structure in the solid state. Mass spectrometric (MS) data confirmed
the monomeric structure in the gas phase, and high-resolution MS allowed
the identification of characteristic fragments, such as [{(NiPr)2CR}GdCH3]+ and [{(NiPr)2CR}GdNH]+. The alkyl
substitution patterns of the amidinate ligands clearly show an influence
on the thermal properties, and specifically, the introduction of the
asymmetric carbodiimide leads to a lowering of the onset of volatilization
and decomposition. Compound 5, which is the first Gd
complex with an asymmetric amidinate ligand system to be reported,
was, therefore, tested for the MOCVD of GdN thin films. The as-deposited
GdN films were capped with Cu in a subsequent MOCVD process to prevent
postdeposition oxidation of the films. Cubic GdN on Si(100) substrates
with a preferred orientation in the (200) direction were grown at
750 °C under an ammonia atmosphere and exhibited a columnar morphology
and low levels of C or O impurities according to scanning electron
microscopy, Rutherford backscattering, and nuclear reaction analysis.