Charles L. Dezelah

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.

Advanced cyclopentadienyl precursors for atomic layer deposition of ZrO2 thin films

ZrO2 thin films were grown onto silicon (100) substrates by atomic layer deposition (ALD) using novel cyclopentadienyl-type precursors, namely (CpMe)2ZrMe2 and (CpMe)2Zr(OMe)Me (Cp = cyclopentadienyl, C5H5) together with ozone as the oxygen source. Growth characteristics were studied in the temperature range of 250 to 500 °C. An ALD-type self-limiting growth mode was verified for both processes at 350 °C where highly conformal films were deposited onto high aspect ratio trenches. Signs of thermal decomposition were not observed at or below 400 °C, a temperature considerably exceeding the thermal decomposition temperature of the Zr-alkylamides. Processing parameters were optimised at 350 °C, where deposition rates of 0.55 and 0.65 A cycle−1 were obtained for (CpMe)2ZrMe2/O3 and (CpMe)2Zr(OMe)Me/O3, respectively. The films grown from both precursors were stoichiometric and polycrystalline with an increasing contribution from the metastable cubic phase with decreasing film thickness. In the films grown from (CpMe)2ZrMe2, the breakdown field did not essentially depend on the film thickness, whereas in the films grown from (CpMe)2Zr(OMe)Me the structural homogeneity and breakdown field increased with decreasing film thickness. The films exhibited good capacitive properties that were characteristic of insulating oxides and did not essentially depend on the precursor chemistry.

Preparation and characterization of molybdenum and tungsten nitride nanoparticles obtained by thermolysis of molecular precursors

Treatment of Mo(NtBu)2Cl2 with [K(Ph2pz)(THF)]6 (pz = pyrazolyl) in tetrahydrofuran afforded Mo(NtBu)2(Ph2pz)2 (85%), while treatment of W(NtBu)2(NHtBu)2 with 3,5-diphenylpyrazole afforded W(NtBu)2(Ph2pz)2 (97%). The complexes M(NtBu)2(Ph2pz)2 were characterized completely by spectral and analytical data, and by an X-ray crystal structure determination for Mo(NtBu)2(Ph2pz)2. Thermolysis of M(NtBu)2(Ph2pz)2 at 800 °C under nitrogen afforded 2–3 nm metal nitride nanoparticles that were embedded in an amorphous carbon–oxygen matrix, as determined by X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. X-Ray photoelectron spectroscopy also revealed the presence of metal oxide phases, which were amorphous by X-ray powder diffraction. The nanoparticles prepared at 800 °C were insoluble. Thermolysis of M(NtBu)2(Ph2pz)2 at 425 °C afforded amorphous 2–3 nm nanoparticles, as determined by X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. X-Ray photoelectron spectroscopy suggested a molybdenum(IV) nitride and W2N/WN, as well as MoO3 and an oxidized tungsten nitride. Materials prepared at 425 °C were not embedded in a matrix, and were soluble in tetrahydrofuran. Infrared and NMR spectroscopy suggested the presence of surface organic fragments that contain alkyl-substituted phenyl groups. Such surface groups are most likely derived from decomposition of the heterocyclic ligands. Simultaneous differential thermal analysis/thermogravimetric analysis indicated that the amorphous nitride materials convert to crystalline nanoparticles consistent with M2N phases between 600–700 °C, along with a weight loss that may correspond to dinitrogen evolution. The amorphous carbon–oxygen matrix also forms upon heating from 425 to 800 °C. This work provides the first description of tungsten nitride nanoparticles, as well as the first description of soluble group 4–6 nanoparticles.

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.

Substrate selectivity in the low temperature atomic layer deposition of cobalt metal films from bis(1,4-di- tert -butyl-1,3-diazadienyl)cobalt and formic acid

The initial stages of cobalt metal growth by atomic layer deposition are described using the precursors bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and formic acid. Ruthenium, platinum, copper, Si(100), Si-H, SiO2, and carbon-doped oxide substrates were used with a growth temperature of 180 °C. On platinum and copper, plots of thickness versus number of growth cycles were linear between 25 and 250 cycles, with growth rates of 0.98 Å/cycle. By contrast, growth on ruthenium showed a delay of up to 250 cycles before a normal growth rate was obtained. No films were observed after 25 and 50 cycles. Between 100 and 150 cycles, a rapid growth rate of ∼1.6 Å/cycle was observed, which suggests that a chemical vapor deposition-like growth occurs until the ruthenium surface is covered with ∼10 nm of cobalt metal. Atomic force microscopy showed smooth, continuous cobalt metal films on platinum after 150 cycles, with an rms surface roughness of 0.6 nm. Films grown on copper gave rms surface roughnesses of 1.1-2.4 nm after 150 cycles. Films grown on ruthenium, platinum, and copper showed resistivities of <20 μΩ cm after 250 cycles and had values close to those of the uncoated substrates at ≤150 cycles. X-ray photoelectron spectroscopy of films grown with 150 cycles on a platinum substrate showed surface oxidation of the cobalt, with cobalt metal underneath. Analogous analysis of a film grown with 150 cycles on a copper substrate showed cobalt oxide throughout the film. No film growth was observed after 1000 cycles on Si(100), Si-H, and carbon-doped oxide substrates. Growth on thermal SiO2 substrates gave ∼35 nm thick layers of cobalt(ii) formate after ≥500 cycles. Inherently selective deposition of cobalt on metallic substrates over Si(100), Si-H, and carbon-doped oxide was observed from 160 °C to 200 °C. Particle deposition occurred on carbon-doped oxide substrates at 220 °C.

Reaction Mechanisms of the Atomic Layer Deposition of Tin Oxide Thin Films Using Tributyltin Ethoxide and Ozone

Uniform and conformal deposition of tin oxide thin films is important for several applications in electronics, gas sensing, and transparent conducting electrodes. Thermal atomic layer deposition (ALD) is often best suited for these applications, but its implementation requires a mechanistic understanding of the initial nucleation and subsequent ALD processes. To this end, in situ FTIR and ex situ XPS have been used to explore the ALD of tin oxide films using tributyltin ethoxide and ozone on an OH-terminated, SiO2-passivated Si(111) substrate. Direct chemisorption of tributyltin ethoxide on surface OH groups and clear evidence that subsequent ligand exchange are obtained, providing mechanistic insight. Upon ozone pulse, the butyl groups react with ozone, forming surface carbonate and formate. The subsequent tributyltin ethoxide pulse removes the carbonate and formate features with the appearance of the bands for CH stretching and bending modes of the precursor butyl ligands. This ligand-exchange behavior is repeated for subsequent cycles, as is characteristic of ALD processes, and is clearly observed for deposition temperatures of 200 and 300 °C. On the basis of the in situ vibrational data, a reaction mechanism for the ALD process of tributyltin ethoxide and ozone is presented, whereby ligands are fully eliminated. Complementary ex situ XPS depth profiles confirm that the bulk of the films is carbon-free, that is, formate and carbonate are not incorporated into the film during the deposition process, and that good-quality SnOx films are produced. Furthermore, the process was scaled up in a cross-flow reactor at 225 °C, which allowed the determination of the growth rate (0.62 Å/cycle) and confirmed a self-limiting ALD growth at 225 and 268 °C. An analysis of the temperature-dependence data reveals that growth rate increases linearly between 200 and 300 °C.