Hywel O. Davies

Novel Mononuclear Alkoxide Precursors for the MOCVD of ZrO2 and HfO2 Thin Films

Thin films of ZrO 2 and HfO 2 have been deposited by liquid injection metal-organic (MO)CVD using a range of new alkoxide precursors, [Zr(O t Bu) 2 (mmp) 2 ] (1), [Hf(O t Bu) 2 (mmp) 2 ] (2), [Zr(mmp) 4 ] (3), and [Hf(mmp) 4 ] (4): (mmp = 1-methoxy-2-methyl-2-propanolate, OCMe 2 CH 2 OMe). The complexes are mononuclear and volatile, and are significantly less reactive to air and moisture than existing Zr and Hf alkoxide precursors such as [Zr(O t Bu) 4 ] and [Hf(O t Bu) 4 ]. The ZrO 2 and HfO 2 films were grown over a wide range of substrate temperatures (350-650 °C), and analysis by laser Raman spectroscopy shows that the films were deposited in the thermodynamically stable α- or monoclinic phase.

Nickel oxide sol–gel films from nickel diacetate for electrochromic applications

Abstract Nickel diacetate tetrahydrate, [Ni(acetate)2·4H2O] and nickel diacetate dimethylaminoethanol, [Ni(acetate)2(dmaeH)2] were successfully used to deposit NiOx thin films on conductive glass substrates by sol–gel techniques for large area electrochromic applications. Homogeneous one layer films 100 nm thick were deposited by spin coating 0.5 M [Ni(acetate)2·4H2O] in dmaeH at 1000 rpm and by dip coating methods. The NiOx films were characterised by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The thin film electrochromic performances were characterised by means of optical (transmittance) and electrochemical (cyclic voltammetry) methods. On early cycling NiOx thin films present an activation period, related to an increase in capacity. The electro-optical data show an increase in the electrochromic response (i.e. an increase in contrast and colouration efficiency) upon cycling. Following this initial activation period a steady state is reached in which the thin films reversibly switch from transparent to brown. The anodically coloured NiOx thin films are therefore suitable for use in a complete electrochromic cell with tungsten oxide as the cathodic colouring layer. However, the films are not fully stable with long cycling.

Investigation of TiO2-doped HfO2 thin films deposited by photo-CVD

Abstract TiO 2 -doped HfO 2 thin films, as potential replacements for SiO 2 as high- k gate dielectric material, have been grown by photo-induced CVD using 222 nm excimer lamps at a temperature of 400 °C. Vaporised titanium isopropoxide and hafnium (IV) tetra- t -butoxide were used as the precursors. Films from approximately 45–70 nm in thickness with refractive indices from 1.850 to 2.424 were grown with various Ti:Hf ratios. The as-deposited films were found to be amorphous by X-ray diffraction when the Ti/(Ti+Hf) value was up to 33%, while the crystalline TiO 2 anatase phase formed when the Ti/(Ti+Hf) was 41%. We also found that the refractive index increased sharply when the Ti/(Ti+Hf) was over 25%. Fourier transform infrared spectroscopy, XPS and TEM were also used to monitor as well as the presence of Ti, interface and microstructure of the films on Si-substrate. The effect of UV-annealing on the electrical properties of these films will also be discussed.

Crystal Structure of Bi(OCMe2CH2OMe)3 and Its Use in the MOCVD of Bi2O3

The crystal structure of the volatile bismuth alkoxide tris(-methoxy-2-methyl-2-propanolato)bismuth(III), Bi(OCMe 2 CH 2 OMe) 3 , 1, has been determined by single crystal X-ray diffraction (XRD). The complex is mononuclear, containing a six-coordinate Bi III atom in a distorted octahedral environment. Using a solution of 1 in heptane, thin films of bismuth oxide have been deposited by liquid injection metal-organic (MO) CVD. Deposition was carried out over a wide range of substrate temperatures (250-550°C) and, using Auger electron spectroscopy (AES), films were shown to be pure bismuth oxide (Bi 2 O 3 ) with no detectable carbon.

Atomic layer deposition of ZrO2 thin films using a new alkoxide precursor

Abstract Zirconium oxide thin films were grown by atomic layer deposition using a new type of Zr alkoxide: [Zr(O t Bu) 2 (dmae) 2 ] 2 (dmae is dimethylaminoethoxide). Water was used as the oxygen source. The films grown at 190–240 °C were amorphous, and the films grown at 290–340 °C were nanocrystalline. The highest refractive index of the films was 2.08 at a wavelength of 580 nm. The permittivity of a film grown at 240 °C was 25.

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.

Novel mononuclear zirconium and hafnium alkoxides; improved precursors for the MOCVD of ZrO2 and HfO2

The novel complexes [Zr(OBut)2(OCMe2CH2OMe)2] (1) and [Hf(OBut)2(OCMe2CH2OMe)2] (2) are mononuclear and volatile, and are highly promising precursors for the deposition of zirconium dioxide and hafnium dioxide thin films by metalorganic chemical vapour deposition (MOCVD).

Synthesis and crystal structures of dimethylaminoethanol adducts of Ni(II) acetate and Ni(II) acetylacetonate. Precursors for the sol–gel deposition of electrochromic nickel oxide thin films

The reaction between N,N-dimethylaminoethanol (dmaeH) and nickel(II) acetylacetonate, [Ni(acac)2]3 or Ni(II) acetate tetrahydrate, [Ni(CH3CO2)2(H2O)4] yields the new complexes [Ni(acac)2(dmaeH)] (1) and [Ni(CH3CO2)2(dmaeH)2] (2). Complexes (1) and (2) are mononuclear, containing six-coordinate Ni(II) atoms in slightly distorted octahedral environments. Complex (1) contains two bidentate and chelating acetylacetonate groups with a chelating dmaeH ligand. Complex (2) contains two monodentate acetate groups in a cis configuration and two chelating dmaeH ligands. Both complexes dissolve readily in dmaeH to form stable solutions, and are good precursors for the deposition of NiO thin films by sol–gel techniques. (1) and (2) were successfully used to grow electrochromic NiO thin films on conductive glass substrates. The surface morphology of the films was characterised by scanning electron microscopy (SEM) and atom force microscopy (AFM). The thin film performances were characterised by means of optical (transmittance) and electrochemical (cyclic voltammetry) methods. Upon cycling, the NiO thin films switch from brown to transparent in a reversible way exhibiting anodic electrochromic performance.

Liquid Injection MOCVD of Zirconium Dioxide Using a Novel Mixed Ligand Zirconium Precursor

Thin films of ZrO2 have been deposited by liquid injection MOCVD using tetrahydrofuran solutions of the novel mixed ligand precursor Zr2(OiPr)6(thd)2 (thd = 2,2,6,6,-tetramethyl-3,5-heptanedionate). Oxide growth was observed over an unusually wide range of temperatures from 250°C to at least 600°C. Optimized growth of ZrO2 occurred between 350°C and 550°C, which is considerably lower than the optimum temperature for deposition from the conventional Zr(thd)4 precursor. Analysis of the films by Auger electron spectroscopy showed that carbon was present at levels of between ∼2 at.-% and 11 at.-%, depending on substrate temperature and oxygen concentration in the gas phase.

Atomic Layer Deposition of Strontium Tantalate Thin Films from Bimetallic Precursors and Water

Strontium tantalate thin films were grown with atomic layer deposition at 200-350°C using bimetallic donor-functionalized alkoxides and water as precursors. SrTa 2 (OEt) 10 (dmae) 2 (dmae = dimethylaminoethoxide) was found to be a suitable atomic layer deposition precursor for depositing SrTa 2 O 6 films with compositions very close to the 1:2 metal ratio found in the precursor. Film compositions and impurity levels were studied with Rutherford backscattering spectrometry and time-of-flight elastic recoil detection analysis. The as-deposited films were amorphous, but after annealing in air at 800°C orthorhombic SrTa 2 O 6 was observed. Dielectric permittivities were 16 and 50 for as-deposited films and for films annealed in air at 800°C, respectively.