Malte Hellwig

Precursor chemistry for TiO2: titanium complexes with a mixed nitrogen/oxygen ligand sphere

Novel mixed amido-malonato complexes of titanium are reported. The complexes were synthesized by partially replacing the amido groups from the complexes [Ti(NMe2)4] and [Ti(NEt2)4] via Brønstedt acid/base reactions, using the malonate-ligands di-isopropylmalonate (Hdpml) and di-tert-butylmalonate (Hdbml). Four representative complexes were synthesized and fully characterised by 1H NMR, 13C NMR, CHN analysis and mass spectrometry. The crystal structures of the six-coordinated complexes [Ti(NMe2)2(dbml)2] (3) and [Ti(NEt2)2(dbml)2] (4) are presented and discussed. The complexes are solids and the chemical and thermal characteristics of the complexes strongly depend on the substitution at the malonate ligand. While dpml containing complexes show a promising behaviour for classical MOCVD, dbml containing complexes seem to be more suitable for liquid injection-metal-organic chemical vapour deposition (LI-MOCVD). Based on its thermal characteristics, the most promising complex for thermal CVD, [Ti(NEt2)2(dpml)2] (2) was selected for preliminary MOCVD experiments, which indicate a good suitability for the deposition of TiO2 thin films.

MOCVD of Gallium Oxide Thin Films Using Homoleptic Gallium Complexes: Precursor Evaluation and Thin Film Characterisation

Gallium oxide (Ga2O3) is a wide band gap (~5 eV) material possessing good chemical and thermal stabilities and therefore hosting a range of potential applications, from gas sensors to optoelectronic devices. Among the various thin film deposition techniques, metalorganic chemical vapor deposition (MOCVD) has intrinsic advantages such as good step coverage and uniformity. However, compared to other oxide materials, only limited reports are available on the MOCVD growth of Ga2O3, mainly for the lack of suitable precursors. In fact, despite the use of chlorides, alkyls and β-diketonates has been reported, the design of novel gallium complexes with improved properties still represents an open challenge. The goal should be the coordinative saturation and stabilization of the Lewis acidic metal centre, which is essential to prevent oligomerisation and consequent volatility losses. In this context, we have employed different strategies to develop novel and improved precursors in terms of volatility and handling. For example, the use malonic diester anions as chelating ligands led to stable monomeric gallium complexes [1]. In this paper, the thermal characterization and the MOCVD of Ga2O3 thin films of the most promising homoleptic gallium malonate precursor candidate, Ga(dEtml)3 1 (dEtml =diethylmalonate) [1], are compared with those of the commercially available Ga β-diketonate complex Ga(acac)3 2 (acac = 2,4-pentanedionate) and the cyclic dimethylamide complex DMG [GaNMe2)3]2 3. Among the various β-diketonate ligands, acetylacetonate probably represents the most investigated one for the development of MOCVD oxide precursors. Nevertheless, there are only a few reports using 2 for the deposition of gallium oxide [2]. DMG, which is very sensitive to air and moisture, was primarily used as a staring material for the synthesis of gallium alkoxides by ligand exchange [3,4]. The volatility and decomposition characteristics of the three different precursors (1-3) were investigated by thermogravimetric (TG) studies. The obtained results revealed that in call cases the temperature window between sublimation and decomposition was sufficiently large for MOCVD applications. Compound 3 exhibited the lowest temperature for the volatilization onset (3>1>2), which was also consistent with the respective melting points measured in sealed capillaries {90°C (3) 180°C (1) > 125°C (3)}. Beside the investigation on the compound thermal properties, our primary target was to investigate the application of compounds 13 in the MOCVD of Ga2O3 thin films. The ultimate goal of this research is the comparative study of film crystallinity, surface morphology and composition as a function of the adopted CVD process parameters.