Timothy J. Leedham

Low-temperature, high-performance solution-processed metal oxide thin-film transistors formed by a ‘sol–gel on chip’ process

At present there is no ‘ideal’ thin-film transistor technology for demanding display applications, such as organic light-emitting diode displays, that allows combining the low-temperature, solution-processability offered by organic semiconductors with the high level of performance achievable with microcrystalline silicon1. N-type amorphous mixed metal oxide semiconductors, such as ternary oxides Mx1My2Oz, where M1 and M2 are metals such as In, Ga, Sn, or Zn, have recently gained momentum because of their high carrier mobility and stability2, 3 and good optical transparency, but they are mostly deposited by sputtering. So far no route is available for forming high-performance mixed oxide materials from solution at low process temperatures <250 °C. Ionic mixed metal oxides should in principle be ideal candidates for solution-processable materials because the conduction band states derived from metal s-orbitals are relatively insensitive to the presence of structural disorder and high charge carrier mobilities are achievable in amorphous structures2. Here we report the formation of amorphous metal oxide semiconducting thin-films using a ‘sol–gel on chip’ hydrolysis approach from soluble metal alkoxide precursors, which affords unprecedented high field-effect mobilities of 10 cm2 V−1 s−1, reproducible and stable turn-on voltages Von≈0 V and high operational stability at maximum process temperatures as low as 230 °C.

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.

Synthesis and characterisation of two novel titanium isopropoxides stabilised with a chelating alkoxide: their use in the liquid injection MOCVD of titanium dioxide thin films

The new precursors [Ti(OPri)3(OCH2CH2NMe2)] 1 and [Ti(OPri)2(OCH2CH2NMe2)2] 2 have been prepared and characterised. Compound 1 exists in solution at room temperature as a mixture of monomer and dimer(s), however at elevated temperatures the monomer predominates. Compound 2 exists in solution predominantly as a monomeric complex presumably with pseudo-octahedral coordination at the metal centre. Thin films of TiO2 have been deposited at 300-450C by liquid injection MOCVD using both 1 and 2. Analysis of the films by Auger electron spectroscopy failed to detect nitrogen although trace carbon was detected at levels of between 2.9 and 7.7. The potential utility of these modified compounds as precursors to TiO2 is 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.

High Performance, Low Temperature Solution-Processed Barium and Strontium Doped Oxide Thin Film Transistors.

Amorphous mixed metal oxides are emerging as high performance semiconductors for thin film transistor (TFT) applications, with indium gallium zinc oxide, InGaZnO (IGZO), being one of the most widely studied and best performing systems. Here, we investigate alkaline earth (barium or strontium) doped InBa(Sr)ZnO as alternative, semiconducting channel layers and compare their performance of the electrical stress stability with IGZO. In films fabricated by solution-processing from metal alkoxide precursors and annealed to 450 °C we achieve high field-effect electron mobility up to 26 cm2 V–1 s–1. We show that it is possible to solution-process these materials at low process temperature (225–200 °C yielding mobilities up to 4.4 cm2 V–1 s–1) and demonstrate a facile “ink-on-demand” process for these materials which utilizes the alcoholysis reaction of alkyl metal precursors to negate the need for complex synthesis and purification protocols. Electrical bias stress measurements which can serve as a figure of merit for performance stability for a TFT device reveal Sr- and Ba-doped semiconductors to exhibit enhanced electrical stability and reduced threshold voltage shift compared to IGZO irrespective of the process temperature and preparation method. This enhancement in stability can be attributed to the higher Gibbs energy of oxidation of barium and strontium compared to gallium.

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 metal organic chemical vapour deposition of nickel zinc ferrite thin films

Abstract Liquid injection metal organic chemical vapour deposition has been used to grow thin films of the single metal oxides of nickel, zinc and iron, the binary ferrites of nickel ferrite and zinc ferrite and the ternary nickel zinc ferrite. The precursor chemicals used for the deposition of the metal oxide layers were solutions of the metal thd compounds (thd=2,2,6,6-tetramethyl-3,5-heptanedionato) dissolved in tetrahydrofuran. The growth rates of the single metal oxide layers have been systematically determined as a function of substrate temperature in the temperature range 300–650°C and the ferrite layers were deposited at a substrate temperature of 500°C. The ferrite layers were polycrystalline with well-defined spinel crystal structures.