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.本文报道了五种新型同配体三脒合钆（gadolinium tris-amidinate）配合物。它们以三氯化钆（GdCl₃）与3当量锂化对称及不对称脒基配体为原料，在室温下经盐消除反应合成得到。所合成的三脒合钆配合物包括[Gd{(iPrN)₂CR}₃] [R = 甲基（Me，1）、乙基（Et，2）、叔丁基（tBu，3）、正丁基（nBu，4）]以及[Gd{(EtN)(tBuN)CMe}₃]（5）：除化合物4在室温下为粘稠液体外，其余均为室温固体，且均可在约125 ℃、6×10⁻² mbar的真空条件下升华。以该系列中的代表性化合物3和5为对象开展X射线衍射分析，结果表明该类配合物在固态下采取畸变八面体配位构型。质谱（MS）数据证实其气相中为单分子结构，高分辨质谱可鉴定出特征碎片离子，如[{(iPrN)₂CR}GdCH₃]⁺和[{(iPrN)₂CR}GdNH]⁺。脒基配体的烷基取代模式对其热性质具有显著影响，具体而言，引入不对称脒基配体可降低挥发与分解的起始温度。化合物5是首例报道的含不对称脒基配体的钆配合物，因此将其用于氮化钆（GdN）薄膜的金属有机化学气相沉积（metal-organic chemical vapor deposition，MOCVD）研究。为防止沉积后薄膜发生氧化，在后续MOCVD工艺中采用铜对沉积态GdN薄膜进行包覆。在750 ℃、氨气气氛下于Si(100)衬底上生长出沿(200)晶面择优取向的立方相GdN薄膜；经扫描电子显微镜、卢瑟福背散射及核反应分析表征，该薄膜呈现柱状形貌且碳、氧杂质含量极低。