Paul A. Marshall

Ce(IV) complexes with donor-functionalized alkoxide ligands: improved precursors for chemical vapor deposition of CeO2.

Thin films of ceria (CeO(2)) have many applications, and their synthesis by liquid-injection MOCVD (metal-organic chemical vapor deposition) or ALD (atomic layer deposition) requires volatile precursor compounds. Here we report the synthesis of a series of homoleptic and heteroleptic Ce(IV) complexes with donor-functionalized alkoxide ligands mmp (1-methoxy-2-methylpropan-2-olate), dmap (1-(dimethylamino)propan-2-olate), and dmop (2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)propan-2-olate) and their potential as precursors for MOCVD and ALD of CeO(2). New complexes were synthesized by alcohol exchange reactions with [Ce(OBu(t))(4)]. [Ce(mmp)(4)] and [Ce(dmap)(4)] were both found to be excellent precursors for liquid-injection MOCVD of CeO(2), depositing high purity thin films with very low carbon contamination, and both have a large temperature window for diffusion controlled growth (350-600 °C for [Ce(mmp)(4)]; 300-600 °C for [Ce(dmap)(4)]). [Ce(mmp)(4)] is also an excellent precursor for liquid-injection ALD of CeO(2) using H(2)O as oxygen source and demonstrates self-limiting growth from 150 to 350 °C. [Ce(dmap)(4)] has lower thermal stability than [Ce(mmp)(4)] and does not show self-limiting growth in ALD. Heteroleptic complexes show a tendency to undergo ligand redistribution reactions to form mixtures in solution and are unsuitable as precursors for liquid-injection CVD.

Liquid injection atomic layer deposition of silver nanoparticles

Silver nanoparticles are being developed for applications in plasmonics, catalysts and analytical methods, amongst others. Herein, we demonstrate the growth of silver nanoparticles using an atomic layer deposition (ALD) process for the first time. The silver was deposited from pulses of the organometallic precursor (hfac)Ag(1,5-COD) ((hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene)) dissolved in a 0.1 M toluene solution. Catalytic oxidative dehydrogenation of the silver was achieved using intermittent pulses of propanol. The effect of substrate temperature on the size and distribution of nanoparticles has been investigated over the temperature range 110-150 degrees C. Transmission electron microscopy reveals that the nanoparticles consist of face centred cubic, facetted silver crystallites. The localized surface plasmon modes of the nanoparticles have been investigated using electron energy loss spectroscopy mapping. The distributions of plasmons within the ALD nanoparticles are comparable to those grown by solution methods. Both dipolar and quadrupolar resonant modes are observed, which is consistent with previous discrete dipole approximation models. Energy loss mapping of a loss feature at 8.1 eV reveals that it correlates with the bulk or volume region of the silver nanoparticles investigated here.

Atomic layer deposition of Ga-doped ZnO transparent conducting oxide substrates for CdTe-based photovoltaics

The atomic layer deposition of gallium doped zinc oxide films is investigated as a method of fabricating transparent conducting oxide substrates for cadmium telluride based photovoltaic cells. The growth parameters and properties of gallium-doped ZnO were established for a range of dopant concentrations. 1 at. % gallium-doped films exhibited the lowest electrical sheet resistances and were selected as substrates to deposit Cd1−xZnxS/CdTe photovoltaic cells. The average current density–voltage characteristics of 16 cells under AM1.5 illumination yielded a conversion efficiency of 10.8% and a fill-factor of 65%.

Characterization of hafnium aluminate gate dielectrics deposited by liquid injection metalorganic chemical vapor deposition

Thin films of hafnium aluminate, with varying aluminum content, have been deposited by liquid injection metalorganic chemical vapor deposition using the metal alkoxide precursors hafnium methyl-methoxy-propanolate and aluminum iso-propoxide. X-ray diffraction analysis showed that the films were amorphous at aluminum concentrations above 7 at. %. Postdeposition annealing indicated that the oxide-transition temperature from amorphous to crystalline increased with aluminum content. Medium-energy ion scattering showed that up to 900 °C, internal oxidation of the silicon substrate had been inhibited. The capacitance–voltage characteristics of the films significantly improved following annealing in dry air.

Deposition of hafnium silicate films by liquid injection MOCVD using a single source or dual source approach

Two approaches have been investigated for the MOCVD of high-κ hafnium silicate (HfSixOy) films for gate dielectric applications. The first approach using the novel “single source” precursor [Hf(OSiButMe2)4(Et2NH)], gave a level of Si (∼10 at%) which showed little variation over a range of growth temperatures and oxygen partial pressures. The second, more flexible approach, uses two separate Hf and Si sources, Hf(NMe2)4 and ButMe2SiOH, and allows good control of the Si concentration in HfSixOy, depending on the relative precursor ratios, up to a maximum level of 18 at%. The dielectric properties of the HfSixOy films deposited by this method were shown to be very good. In the dual source process it is proposed that an intermediate containing direct Hf–OSiButMe2 bonds is formed in situ in the gas phase. The crystal structure of [Hf(OSiButMe2)4(Et2NH)] is also reported.

Atomic layer deposition of germanium-doped zinc oxide films with tuneable ultraviolet emission

Thin films of germanium-doped zinc oxide have been deposited by atomic layer deposition. The zinc oxide matrix was grown from cyclic pulses of diethylzinc and water vapour over the temperature range of 100–350 °C substrate temperature. Tetramethoxygermanium(IV) was employed as a novel germanium-doping source, which could be incorporated up to 17 at%. At 2.1 at% germanium doping at a deposition temperature of 250 °C, the maximum carrier concentration of 2.14 × 1020 cm−3 coincides with a carrier mobility of approximately 5 cm2 V−1 s−1. No evidence for the formation of nanometre-scale germanium clustering or segregation was observed in the X-ray diffraction patterns or high-resolution transmission electron micrographs of these films. The near band edge photoluminescence shifts to higher energy with increasing germanium incorporation either by the Burstein–Moss mechanism or by alloy formation.

Vacuum ultraviolet photochemical selective area atomic layer deposition of Al2O3 dielectrics

We report the photochemical atomic layer deposition of Al2O3 thin films and the use of this process to achieve area-selective film deposition. A shuttered vacuum ultraviolet (VUV) light source is used to excite molecular oxygen and trimethyl aluminum to deposit films at 60°C. In-situ QCM and post-deposition ellipsometric measurements both show that the deposition rate is saturative as a function of irradiation time. Selective area deposition was achieved by projecting the VUV light through a metalized magnesium fluoride photolithographic mask and the selectivity of deposition on the illuminated and masked regions of the substrate is a logarithmic function of the UV exposure time. The Al2O3 films exhibit dielectric constants of 8 – 10 at 1 MHz after forming gas annealing, similar to films deposited by conventional thermal ALD.

Double-gated microtip emitters for brighter field-emission displays

Reducing the aperture of the beams emitted by microtips is important to increase the luminance of field-emission displays. Double-gated microtip emitters have been fabricated where the second gate is used to focus the beam emitted by the tip under the action of the attracting field produced by the first gate. The first gate is designed in a volcano shape to shield the apex of the tip from the second gate that is recessed under the level of the first one. This is rendered possible by using a thin rf sputtered SiO2 layer with very good dielectric properties between the two gates. The beam aperture is reduced from 25° to 7° when the second gate is biased at a voltage 15 V lower than the tip. No reduction in emitted current is observed when the second gate is biased at that voltage, which results in an eightfold increase in current density. Reducing the aperture of the beams emitted by microtips is important to increase the luminance of field-emission displays. Double-gated microtip emitters have been fabricated where the second gate is used to focus the beam emitted by the tip under the action of the attracting field produced by the first gate. The first gate is designed in a volcano shape to shield the apex of the tip from the second gate that is recessed under the level of the first one. This is rendered possible by using a thin rf sputtered SiO2 layer with very good dielectric properties between the two gates. The beam aperture is reduced from 25° to 7° when the second gate is biased at a voltage 15 V lower than the tip. No reduction in emitted current is observed when the second gate is biased at that voltage, which results in an eightfold increase in current density.

Liquid Injection MOCVD of Rare-Earth Oxides Using New Alkoxide Precursors

High purity lanthanum oxide and praseodymium oxide thin films (C 3 ] and [Pr(mmp) 3 ] in toluene-solution (mmp = OCMe 2 CH 2 OMe). 1 H NMR solution studies have shown that the addition of donor species, such as tetraglyme (CH 3 O(CH 2 CH 2 O) 4 CH 3 ) or mmpH prevent molecular aggregation and help stabilise the precursors.

Liquid injection MOCVD and ALD studies of “single source” Sr-Nb and Sr-Ta precursors

A range of “single source” Sr-Nb and Sr-Ta heterometal alkoxides precursors are investigated as potential sources for liquid injection MOCVD (metalorganic chemical vapour deposition) and ALD (atomic layer deposition) of SrBi 2 Ta 2 O 9 (SBT) and SrBi 2 (T x Nb 1-x ) 2 O 9 (SBTN). These “single source” precursors are designed to alleviate the mis-match between conventional Sr and Ta or Sr and Nb sources. Strontium-tantalate and strontium-niobate thin films were deposited on silicon using the “single source” alkoxide precursors [Sr{Ta(OEt) 5 (dmae} 2 ] and [Sr{Nb(OEt) 5 (dmae)} 2 ] (dmae = OCH 2 CH 2 NMe 2 ), and the optimum temperatures for deposition of stoichiometric SrTa 2 O 6 and SrNb 2 O 6 were determined. Separate ALD studies of [Sr{Ta(OEt) 5 (dmae)} 2 ] and [Sr{Ta(OEt) 5 (mee)} 2 ] (mee = OCH 2 CH 2 OMe) for the growth of strontium-tantalate were carried out to assess precursor suitability for this technique. Liquid injection MOCVD of Bi-oxide films using Bi(mmp) 3 indicates similar decomposition behaviour to the Sr-Ta and Sr-Nb alkoxides, demonstrating its suitability as a complementary source of Bi for SBT, SBN and SBTN.