Systematic molecular engineering of Zn-ketoiminates for application as precursors in atomic layer depositions of zinc oxide

Abstract

Molecular engineering of seven closely related zinc ketoiminates, namely, [Zn(dapki)], [Zn(daeki)], [Zn(epki)], [Zn(eeki)], [Zn(mpki)], [Zn(meki)], and [Zn(pki)], leads to the optimisation of precursor thermal properties in terms of volatilisation rate, onset of volatilisation, reactivity and thermal stability. The influence of functional groups at the imine side chain of the ligands on the precursor properties is studied with regard to their viability as precursors for atomic layer deposition (ALD) of ZnO. The synthesis of [Zn(eeki)], [Zn(epki)] and [Zn(dapki)] and the crystal structures of [Zn(mpki)], [Zn(eeki)], [Zn(dapki)] and [Zn(pki)] are presented. From the investigation of the physico-chemical characteristics, it was inferred that all compounds are monomeric, volatile and exhibit high thermal stability, all of which make them promising ALD precursors. Compound [Zn(eeki)] is in terms of thermal properties the most promising Zn-ketoiminate. It is reactive towards water, possesses a melting point of 39 °C, is stable up to 24 days at 220 °C and has an extended volatilisation rate compared to the literature known Zn-ketoiminates. It demonstrated self-saturated, water assisted growth of zinc oxide (ZnO) with growth rates in the order of 1.3 Å per cycle. Moreover, it displayed a broad temperature window from T = 175-300 °C and is the first report of a stable high temperature (>200 °C) ALD process for ZnO returning highly promising growth rates.

Tailored β‐Ketoiminato Complexes of Iron: Synthesis, Characterization, and Evaluation towards Solution‐Based Deposition of Iron Oxide Thin Films

“…The zinc analogue [Zn(eeki) 2 ] of 2 was recently reported by us, and interestingly does not show side‐chain interactions between the zinc center and the etheric oxygen atoms. One explanation for this could be the electronic nature of the respective metal centers.…”

“…In 5 , the Fe1–O1 and Fe1–N1 bond lengths are 1.939(3) and 2.027(3) Å, respectively. These bonds are considerably longer in 1 and 2 , and the bite angles (O1–Fe1–N1) of the solid‐state structures (Table ) are substantially smaller than those of most other divalent metal β‐ketoiminates, most likely due to the extra side‐chain coordination , , . Notably, the side‐chain coordination distances (Fe1–O2) in 1 and 2 [2.3718(15) and 2.419(3) Å] are not in the range of an Fe–O single‐bond, but this can be regarded as coordination saturating the ligand sphere of the two complexes and presumably shields the metal centers more effectively compared to 5 .…”

“…FeCl 2 (Alfa Aesar) was used as received. The lithium ketoiminates [Li(meki)], [Li(eeki)], [Li(mpki)], [Li(epki)], and [Li( i ppki)] were synthesized by literature procedures …”

“…In the search for suitable precursors for CSD of iron oxide thin films under ambient conditions, the class of metal β‐ketoiminates is a potential candidate. This ligand class, which has already been successfully applied in metal oxide and nitride CVD and atomic layer deposition (ALD) processes involving metals such as Fe, Zr, Pd, Ga, Ir, Ti, Nb, Ta, and Zn, features both oxygen and nitrogen coordination, which is a compromise between the highly reactive and thermally unstable nitrogen‐bonded metal β‐diketiminates and the highly stable oxygen‐coordinated metal β‐diketonates. As recently demonstrated by us, iron(II) β‐ketoiminates are highly promising for gas‐phase deposition of iron oxide nanostructures via both ALD and CVD routes at low temperatures …”

From Precursor Chemistry to Gas Sensors: Plasma‐Enhanced Atomic Layer Deposition Process Engineering for Zinc Oxide Layers from a Nonpyrophoric Zinc Precursor for Gas Barrier and Sensor Applications

“…[ 28,31–33 ] Recent efforts to lower the deposition temperatures was achieved using the bis[4‐((2‐ethoxyethyl)imino)‐pent‐2‐en‐2‐olato]zinc(II) ([Zn(eeki) 2 ]) using the β‐ketoiminates class of precursors with H 2 O at temperatures as low as 175 °C. [ 34 ] However, further reduction in deposition temperature was possible till date by using the extremely reactive and pyrophoric zinc alkyls namely dimethyl zinc ([ZnMe 2 ], DMZ) and diethyl zinc ([ZnEt 2 ], DEZ) among which, DEZ is by far the most commonly used precursor. [ 35 ] Owing to its high reactivity, it is the most explored Zn precursor and has been used in combination with H 2 O, [ 36 ] O 2 , [ 37 ] O 3 , [ 38 ] or N 2 O [ 39 ] as coreactants.…”