Jani Päiväsaari

Synthesis, structure and properties of volatile lanthanide complexes containing amidinate ligands: application for Er2O3 thin film growth by atomic layer deposition

Treatment of anhydrous rare earth chlorides with three equivalents of lithium 1,3-di-tert-butylacetamidinate (prepared in situ from the di-tert-butylcarbodiimide and methyllithium) in tetrahydrofuran at ambient temperature afforded Ln(tBuNC(CH3)NtBu)3 (Ln = Y, La, Ce, Nd, Eu, Er, Lu) in 57–72% isolated yields. X-Ray crystal structures of these complexes demonstrated monomeric formulations with distorted octahedral geometry about the lanthanide(III) ions. These new complexes are thermally stable at >300 °C, and sublime without decomposition between 180–220 °C/0.05 Torr. The atomic layer deposition of Er2O3 films was demonstrated using Er(tBuNC(CH3)NtBu)3 and ozone with substrate temperatures between 225–300 °C. The growth rate increased linearly with substrate temperature from 0.37 A per cycle at 225 °C to 0.55 A per cycle at 300 °C. Substrate temperatures of >300 °C resulted in significant thickness gradients across the substrates, suggesting thermal decomposition of the precursor. The film growth rate increased slightly with an erbium precursor pulse length between 1.0 and 3.0 s, with growth rates of 0.39 and 0.51 A per cycle, respectively. In a series of films deposited at 250 °C, the growth rates varied linearly with the number of deposition cycles. Time of flight elastic recoil analyses demonstrated slightly oxygen-rich Er2O3 films, with carbon, hydrogen and fluorine levels of 1.0–1.9, 1.7–1.9 and 0.3–1.3 atom%, respectively, at substrate temperatures of 250 and 300 °C. Infrared spectroscopy showed the presence of carbonate, suggesting that the carbon and slight excess of oxygen in the films are due to this species. The as-deposited films were amorphous below 300 °C, but showed reflections due to cubic Er2O3 at 300 °C. Atomic force microscopy showed a root mean square surface roughness of 0.3 and 2.8 nm for films deposited at 250 and 300 °C, respectively.

Cerium dioxide buffer layers at low temperature by atomic layer deposition

Thin films of cerium dioxide were deposited by atomic layer deposition (ALD). Temperature ranges studied in detail were 175–375 °C and 225–350 °C for the Ce(thd)4 and Ce(thd)3phen (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate, phen = 1,10-phenanthroline) precursors, respectively. Ozone was used in both cases as oxygen source. Thickness, crystallinity and morphology of the CeO2 films were determined by UV-VIS spectroscopic, XRD and AFM measurements, respectively. Narrow ALD windows, i.e. temperature ranges with constant growth rate, were observed at temperatures 175–250 °C for Ce(thd)4 and 225–275 °C for Ce(thd)3phen. The growth rates of CeO2 inside the ALD windows were 0.32 A (cycle)−1 and 0.42 A (cycle)−1 for Ce(thd)4 and Ce(thd)3phen, respectively. CeO2 films grown on soda lime glass and Si(100) were polycrystalline and slightly oriented with the (200) and (111) peaks as the strongest reflections. TOF-ERD analysis of the Ce∶O ratio showed that the films were nearly stoichiometric but that they contained hydrogen (7–10 atom%), as well as some carbon and fluorine, as impurities.

Characterisation of thin films of ceria-based electrolytes for IntermediateTemperature — Solid oxide fuel cells (IT-SOFC)

At the present, a major technological challenge for the development of solid oxide fuel cells (SOFC) is the reduction of their operation temperature in order to reduce the costs and increase the fuel cell lifetime. Nevertheless, decrease in the operating temperature leads to losses in cell performance mainly due to the ohmic drop through the electrolyte. Therefore, several approaches are currently under investigation to overcome the electrolyte problem and the use of oxygen ion conductor thin films seems to be the most promising solution. In this respect, the well-known electrolyte CeO2-Gd2O3 (CGO) was investigated. Thin layers of less than 5 µm of CGO were deposited using two different techniques: RF magnetron sputtering and Atomic Layer Deposition (ALD). Physicochemical properties of the thin films obtained were characterised by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Furthermore, impedance measurements were carried out in order to determine the electrical properties of the CGO films, in particular their ionic conductivity.

The growth of ErxGa2−xO3 films by atomic layer deposition from two different precursor systems

The atomic layer deposition (ALD) growth of ErxGa2−xO3 (0 ≤ x ≤ 2) thin films was demonstrated using two precursor systems: Er(thd)3, Ga(acac)3, and ozone and Er(C5H4Me)3, Ga2(NMe2)6, and water at substrate temperatures of 350 and 250 °C, respectively. Both processes provided uniform films and exhibited surface-limited ALD growth. The value of x in ErxGa2−xO3 was easily varied by selecting a pulse sequence with an appropriate erbium to gallium precursor ratio. The Er(thd)3, Ga(acac)3, and ozone precursor system provided stoichiometric ErxGa2−xO3 films with carbon, hydrogen, nitrogen, and fluorine levels of <0.2, <0.2, <0.3, and 0.6–2.2 atomic percent, respectively, as determined by Rutherford backscattering spectrometry (RBS) and time of flight-elastic recoil detection analysis (TOF-ERDA). The film growth rate was between 0.25 and 0.28 A cycle−1. The effective permittivity of representative samples was between 10.8 and 11.3. The Er(C5H4Me)3, Ga2(NMe2)6, and water precursor system provided stoichiometric ErxGa2−xO3 films with carbon, hydrogen, nitrogen, and fluorine levels of 2.0–6.1, 5.0–10.3, <0.3–0.7, and ≤0.1 atom percent, respectively, as determined by RBS and TOF-ERDA. The film growth rate was between 1.0 and 1.5 A cycle−1 and varied as a function of the Er(C5H4Me)3 to Ga2(NMe2)6 pulse ratio. The effective permittivity of representative samples was between 9.2 and 10.4. The as-deposited films of both precursor systems were amorphous, but crystallized either to Er3Ga5O12 or to a mixture of β-Ga2O3 and Er3Ga5O12 upon annealing between 900 and 1000 °C under a nitrogen atmosphere. Atomic force microscopy showed root mean square surface roughnesses of <1.0 nm for typical films regardless of precursor system or film composition.

Scandium dipivaloyl methanate as a volatile precursor for thin film deposition

The coupling of a quadrupole mass spectrometer (QMS) via a heated capillary to a commercial thermogravimetric analyser is described. The amu and temperature ranges available were up to 1000 amu and 1500°C, respectively. The system was evaluated with test compounds, yielding gaseous species in the m/z range of 17-80, and then used for the study of thermal behaviour of scandium dipivaloyl methanate or Sc(thd)3 which is discussed in detail. Sc(thd)2 appears as the major Sc-containing species with m/z=411 in the gas phase at 200-300°C.