D. Nelis

Thermal decomposition of the ammonium zinc acetate citrate precursor for aqueous chemical solution deposition of ZnO

The thermal decomposition of an aqueous chemical solution deposition Zn2+-precursor is studied by HT-DRIFT (high temperature diffuse reflectance infrared Fourier transform spectroscopy), on-line coupled TGA-EGA (thermogravimetric analysis - evolved gas analysis by Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS)), and HT-XRD (high temperature X-ray diffraction). Using these complementary techniques, it is found that the α-hydroxyl group of the citrate ligand plays a significant role in the decomposition pathway of the ammonium zinc acetate citrate precursor. TEM (transmission electron microscopy) shows that crystalline ZnO (zincite) is formed at 390°C, after dehydroxylation of the α-hydroxyl group and subsequent decarboxylation of the Zn2+-precursor complex. Before total calcination, ZnO particles are already formed and a residual organic backbone thereby remains. The results obtained by these complementary techniques clearly indicate the importance of thermal analysis in the preparation of ceramics through chemical solution deposition.

Aqueous Chemical Solution Deposition of Ferroelectric Thin Films

Thin films of various ferroelectric multimetal oxides such as (Bi 1 m x La x ) 4 Ti 3 O 12 (BLT), SrBi 2 Ta 2 O 9 (SBT) and PbZr 1 m x Ti x O 3 (PZT) have been prepared by an entirely aqueous chemical solution deposition (CSD) route. Two critical issues related with aqueous CSD have hereby been worked out: in spite of the high degree of hydrolysis of tetra- and pentavalent metal ions (Ti 4+ , Zr 4+ , Ta 5+ , ) we managed to prepare stable aqueous precursor solutions by chemical modification of these individual metals, avoiding phase segregation. Another problem related with aqueous CSD is the wetting of the substrate (both metallic and metal oxide) by the aqueous solution. The hydrophilicity of the substrates is optimized by a chemical treatment of the substrate surface. In this manner, the addition of wetting agents, hence possibly disturbing the gelation reactions, is avoided. In order to study the gelation, decomposition, crystalization and the morphology of the thin films, various characterization techniques ((cryo-)TEM, SEM, EDX, TGA-MS/FTIR, HT-DRIFT, HT-XRD, ) are used.

Aqueous Solution-Gel Synthesis of Strontium Bismuth Niobate (SrBi2Nb2O9)

The synthesis of multimetal oxides containing pentavalent elements like Nb and Ta out of an aqueous solution is very complicated due to the extremely high sensitivity of these metals towards hydrolysis. Moreover the only water-soluble starting compound is the oxalate which is not very suited for gel formation. Nevertheless, from Nb-oxalate it is possible to prepare a water-soluble Nb(V)-precursor by chemical modification. The synthesized precursor remains stable in the pH-range needed for gel formation. This Nb(V)-precursor is used for the synthesis of the ferroelectric material SrBi2Nb2O9 (SBN). An aqueous acetate-citrate solution-gel precursor for SBN was prepared. The chemical homogeneity of this precursor was investigated by means of TEM-experiments. All metal ions were found to be homogeneously dispersed in the precursor gel at least down to 10 nm. The SBN phase formation is followed by means of X-ray Diffraction. It has been shown that the perovskite phase already forms at 550°C.

H2S exposure of a (100)Ge surface : Evidences for a (2×1) electrically passivated surface

The experimental study of the bonding geometry of a (100)Ge surface exposed to H2S in the gas phase at 330°C shows that 1 ML S coverage with (2×1) surface reconstruction can be achieved. The amount of S on the Ge surface and the observed surface periodicity can be explained by the formation of disulfide bridges between Ge–Ge dimers on the surface. First-principles molecular dynamics simulations confirm the preserved (2×1) reconstruction after dissociative adsorption of H2S molecules on a (100)Ge (2×1) surface, and predict the formation of (S–H)–(S–H) inter-Ge dimer bridges, i.e., disulfide bridges interacting via hydrogen bonding. The computed energy band gap of this atomic configuration is shown to be free of surface states, a very important finding for the potential application of Ge in future high performance integrated circuits.

Synthesis of SrBi2Ta2O9 (SBT) by means of a soluble Ta(V) precursor

Abstract Aqueous sol-gel processing of pentavalent metals like Ta and Nb is very complicated since the only water soluble compound is the oxalate, which is not very suited for gel formation. Nevertheless, starting from Ta oxalate, it is possible to prepare a water soluble Ta(V) precursor that remains stable in the pH range needed for gel formation. During synthesis the oxalate is oxidized with H 2 O 2 and subsequently complexed with citric acid and as a result the peroxo–citrato tantalum(V) complex is formed. This Ta(V) precursor is afterwards used for the synthesis of the ferroelectric SrBi 2 Ta 2 O 9 (SBT). The formation of the SBT crystalline phases was investigated by means of high temperature X-ray diffraction (HT–XRD).

Synthesis of RuO2 and SrRuO3 powders by means of aqueous solution gel chemistry

Phase pure RUO 2 and SrRuO 3 powders were obtained by a water-based solution-gel route. By reacting ruthenium(III)acetylacetonate with H 2 O 2 in a citric acid solution, a stable aqueous Ru-precursor solution could be prepared. An aqueous Sr-precursor is synthesized by dissolving Sr acetate in a citric acid solution. After mixing the Ru-precursor with the Sr-precursor solution in stoichiometric quantities, a clear aqueous solution for the preparation of SrRuO 3 could be obtained. Gelation resulted in amorphous precursors that were homogeneous down to nanoscale as was confirmed by means of transmission electron microscopy. With the appropriate thermal treatment phase pure RUO 2 and SrRuO 3 powders were synthesised as verified by X-ray diffraction.

Aqueous CSD of Ferroelectric Bi3.5La0.5Ti3O12 (BLT) Thin Films

A new, environmentally friendly, aqueous chemical solution deposition (CSD) route for the fabrication of BLT thin films is presented. Well crystallized, preferentially c-axis oriented films were obtained at 650°C. The grain size and remanent polarization were increased by the addition of an excess of Bi3 + to the precursor solution. At 10% Bi excess a Pr of 6.2 μ C/cm2 and Ec of 105 kV/cm were obtained.

How trace analytical techniques contribute to the research and development of Ge and III/V semiconductor devices

The scaling of microelectronic transistors to ever-smaller dimensions has ultimately pushed the Si based materials to its physical limits. For the gate stack, dielectric materials with a higher dielectric constant κ than the conventional SiO2 are required. Further, high mobility channels are implemented (strained silicon), but are likely to be replaced on the longer term by the intrinsic higher mobility substrates such as Ge or III/V compound semiconductors (GaAs, GaN, InP,..). The introduction of these new substrates requires a reengineering of the standard IC processes with substantial efforts through research and development programs. In this paper, we demonstrate how the trace analytical techniques of total reflection X-ray fluorescence spectrometry (TXRF) and atomic absorption spectrometry (AAS) contribute to the realization of suitable processes at different stages of the process flow on Ge and GaAs substrates.