Andrew N. MacInnes

From minerals to materials: synthesis of alumoxanes from the reaction of boehmite with carboxylic acids

Reaction of pseudo-boehmite, [Al(O)(OH)]n, with carboxylic acids (RCO2H) results in the formation of the carboxylatoalumoxanes, [Al(O)x(OH)y(O2CR)z]n where 2x + y + z = 3 and R = C1–C13. The physical properties of the alumoxanes are highly dependent on the identity of the alkyl substituents, R, and range from insoluble crystalline powders to powders which readily form solutions or gels in hydrocarbon solvents, from which films may be readily spin-coated. The physical and chemical changes that occur during the reaction of boehmite with carboxylic acids, and the resulting alumoxanes, have been characterized by scanning electron and transmission electron microscopy (SEM and TEM), IR and multinuclear NMR spectroscopy, and thermogravimetric/differential thermal analysis (TG/DTA). The carboxylatoalumoxanes reported herein are spectroscopically similar to analogues prepared from small molecule precursors. Based on the IR and NMR spectra of the alumoxanes as well as comparison with the aluminium carboxylate compounds [Me2Al(µ-O2CR)]2 and Al(O2CR)(salen)(R = CH3, n-C5H11), a model structure of the alumoxanes is proposed, consisting of a boehmite-like core with the carboxylate substituents bound in a bridging mode. Furthermore, the alumoxane particles appear as rod or sheet-like particles, not linear polymers. This is proposed to be due to the destruction of hydrogen bonding within the mineral as hydroxide groups are removed and replaced with acid functionalities. All of the alumoxanes decompose under mild thermolysis to yield alumina. Mass spectral studies indicate that upon thermolysis the volatile decomposition products are water and the carboxylic acid.

Gallium arsenide transistors: realization through a molecularly designed insulator.

A GaAs-based transistor, analogous to commercial silicon devices, has been fabricated with vapor-deposited cubic GaS as the insulator material. The n-channel, depletion mode, GaAs field-effect transistor shows, in addition to classical transistor characteristics, a channel mobility of 4665.6 square centimeters per volt per second, an interfacial trap density of 1011 per electron volt per square centimeter, and a transconductance of 7 millisiemens for a 5-micrometer gate length at a gate voltage of 8 volts. Furthermore, the GaAs transistor shows an on-to-off resistance ratio comparable to that of commercial devices.

Enhancement of photoluminescence intensity of GaAs with cubic GaS chemical vapor deposited using a structurally designed single-source precursor

A two order‐of‐magnitude enhancement of photoluminescence intensity relative to untreated GaAs has been observed for GaAs surfaces coated with chemical vapor‐deposited GaS. The increase in photoluminescence intensity can be viewed as an effective reduction in surface recombination velocity and/or band bending. The gallium cluster [(t‐Bu)GaS]4 was used as a single‐source precursor for the deposition of GaS thin films. The cubane core of the structurally characterized precursor is retained in the deposited film producing a cubic phase. Furthermore, a near‐epitaxial growth is observed for the GaS passivating layer. Films were characterized by transmission electron microscopy, x‐ray powder diffraction, and x‐ray photoelectron and Rutherford backscattering spectroscopies.

Electronic passivation of n‐ and p‐type GaAs using chemical vapor deposited GaS

We report on the electronic passivation of n‐ and p‐type GaAs using chemical vapor deposited cubic GaS. Au/GaS/GaAs fabricated metal‐insulator‐semiconductor (MIS) structures exhibit classical high‐frequency capacitor versus voltage (C‐V) behavior with well‐defined accumulation and inversion regions. Using high‐ and low‐frequency C‐V, the interface trap densities of ∼1011 eV−1 cm−2 on both n‐ and p‐type GaAs are determined. The electronic condition of GaS/GaAs interface did not show any deterioration after a six week time period.

Indium tert-butylthiolates as single source precursors for indium sulfide thin films: Is molecular design enough?

Abstract The dimeric indium thiolates [R2In(μ-StBu)]2 R  tBu (1), nBu (2), Me (3), and [(tBuS)MeIn(μ-St Bu)]2 (4) have been synthesized and used as single source precursors for the metal-organic chemical vapor deposition (MOCVD) of In/InS and InS thin films. In the case of the atmospheric pressure film grown from either 1 or 2, deposition at temperatures between 290 and 350°C results in the formation of indium rich films (In: S ∼ 2) consisting of indium metal and orthorhombic InS, while at 400°C a single phase; the tetragonal high pressure phase of InS, is the only product deposited. Use of compound 3 as the precursor results in amorphous indium rich films being deposited at 300°C. While films grown from 3 at 400°C have a In: S ratio of 1, they consist of an indium rich phase and In2S3. The dependence of the film composition i.e., indium rich versus stoichiometric InS and structure (orthorhombic versus tetrag onal InS) with the deposition temperature and molecular precursor is discussed with respect to the decomposition pathways available to the precursor molecules (1–3). Based on these results compound 4 was proposed to be a suitable precursor for the low temperature deposition of stoichiometric InS, indeed its solid state pyrolysis does yield InS. However, although low pressure MOCVD using 4 yields amorphous films of stoichiometry InS, upon annealing β-In2S3 is formed as the crystalline phase. The efficacy of molecular design of solid state materials is discussed. The indium thiolates were characterized by 1H and 13C NMR spectroscopy and mass spectrometry. Analysis of the deposited films has been obtained by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), with associated energy dispersive X-ray analysis (EDX).

Reaction bonded refractory metal carbide/carbon composites

Abstract A simple chemical process has been developed that allows the rapid conversion of carbon substrates to the refractory metal carbides (MC) of titanium, zirconium, hafnium, vanadium, niobium, tantalum and tungsten. As a demonstration of the technique, graphite rods are infiltrated with an ethanolic solution of various metal oxide-halides, and resistively fired (1350–3000°C) under an argon atmosphere. The final composite produced is found to contain a residual carbon core within a fully dense metal carbide-carbon matrix outer layer. The effects of both firing temperatures, and the number of infiltration-fire treatments, have been investigated in order to determine optimum firing conditions (i.e. maximum carbide formation) for each individual metal carbide. To demonstrate the applicability of the technique for a range of substrate materials, niobium and titanium carbides were formed on carbon fibre tows and non-porous substrates, respectively. The formation of mixed Nb-Ti carbide coatings of varying composition were investigated in order to determine the factors controlling the formation of ternary phases. The identity of the transition metal precursors and the possible pathway of their conversion to the appropriate carbide is discussed. All the composites have been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with associated energy-dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD).

Optical constants of thin-film gallium sulfide layers

Gallium sulfide (GaS) deposited by chemical vapor deposition (CVD) is known to passivate GaAs surfaces. In this paper we examine the thin film optical properties of GaS as they relate to the fabrication of optical waveguides. Spectroscopic ellipsometry was used to determine the index of refraction of GaS films deposited on various substrates. Results indicate that GaS has a high index of refraction suitable for waveguide structures. A gallium sulfide waveguide could provide both the optical interconnect and the passivating layer of GaAs integrated circuits. Progress toward fabricating GaS waveguides is also discussed.

A Novel Route to Silocon Based Ceramic Coatings on Carbon Substrates

Carbon fiber tows have been impregnated by ethanolic solutions of organo-silicon chlorides, and fired at temperatures up to 900°C to form silicon based coatings. Fired tows were subsequently examined by X-ray photoelectron spectroscopy and scanning electron microscopy to characterize the coated material. A uniform silicon oxycarbide is formed at temperatures upwards of 400°C, which provides an oxidation barrier in carbon fiber reinforced metals.