Some considerations of MOCVD for the preparation of high Tc thin films

The deposition of inorganic compounds is important for preparation of high Tc superconducting films. Currently the best quality high Tc films are prepared by laser ablation. As the demand for large area samples, high deposition throughput, good intra- and inter-sample uniformity of layers on patterned structures increases then CVD has the potential to become a powerful technique for prepairing thin films of superconducters. However, although a significant number of papers have been published on the CVD of high Tc layers there are still problems associated with the reproducibility of growth and control of the stoichiometry of layers. These problems often arise because of an inadequate understanding of the chemistry and kinetics of deposition. Therefore if materials of improved quality are to be produced then it is clearly important that a better understanding of the chemistry of layer growth is obtained. In this presentation some results for the CVD of high Tc materials will be discussed and, in particular, we shall give some interpetations and modelling based on analyses of the kinetics of the deposition processes. Some aspects of precursor chemistry will also be discussed and some suggestions will be given for strategies for control of layer growth and improving layer quality

A consideration of adsorption processes in the CVD of polysilicon

Although the growth mechanism of the CVD of polysilicon from silane has been extensively studied for some years, the chemistry of the gas-solid interactions is still not fully understood. In particular, there has been very little consideration of possible bonding modes of homogeneous silane species to silicon surfaces. In fact, most models of silicon growth from silane assume a simple, single site adsorption. In this paper we consider some models based on the interaction of SiH4 and SiH2 with dangling bonds on silicon surfaces. Initially, possible structures of silicon surfaces are described and then the adsorption and subsequent heterogeneous decomposition of silane species are examined. From this analysis it is concluded that it is rather unlikely that only a single site will be involved in any adsorption step, and that the most likely number of sites for adsorption of silicon containing species will be two. We then show that after the adsorption step, SiH2 probably plays an important role in forming Si-Si bonds by rotational or translational steps with the loss of hydrogen. Finally, a more quantitative analysis is given to allow a comparison to be made between theory and experiment. The analysis confirms that the most likely route for interaction of gaseous species with the surface involves two adsorption sites and that subsequent surface dissociation of the adsorbed species leads to incorporation of silicon atoms into the crystal lattice

The LPCVD of rutile at low temperatures

The crystalline structures of titanium dioxide (TiO2) thin films deposited on various substrates by LPCVD for temperatures <322°C using titanium tetrabutoxide (TTB) have been studied. It has been found that the structures are dependent on the deposition temperature. XRD and Raman scattering show that amorphous TiO2 is formed below 215°C, anatase between 236°C~257°C, a mixture of anatase and rutile at ca 270°C, mainly rutile between 279°C-290°C, and pure rutile above 300°C

A study of remote plasma enhanced CVD of silicon nitride films

The growth of silicon nitride (SiNxHy) films from SiH4 and N2 by capacitively coupled remote PECVD is described for the first time. The influence of process parameters on the growth rate, concentration of bonded hydrogen, and properties of deposited films is discussed. The most probable mechanism of film formation is proposed on the basis of the experimental results obtained

The reduction of l-cystine hydrochloride at stationary and rotating disc mercury electrodes

The kinetics of l-cystine hydrochloride reduction have been studied at a mercury-plated copper rotating disc electrode (RDE) and at a stationary mercury disc electrode (SMDE) in 0.1 mol dm?3 HCl at 298 K. The reduction of the disulphide is irreversible and hydrogen evolution is the major side reaction. In contrast to steady state electrode kinetic studies at a mercury drop electrode (which shows a well-defined limiting current), the mercury-plated Cu RDE shows overlap between disulphide reduction and hydrogen evolution. These effects are attributable to strong reactant adsorption with a calculated surface coverage close to 100%. A Tafel slope of ?185 mV per decade is found with a cathodic transfer coefficient of 0.32 and a formal rate constant of 6.7 × 10?9 m s?1. The relative merits of steady state voltammetry at a mercury-plated copper RDE and linear sweep voltammetry at the SMDE are discussed, as is the mechanism of l-cysteine hydrochloride formation

CVD of BaF2_2 with new stable and volatile complexes of barium(II)

The new $\beta$-diketone, 1,1,1,2,2,3,3,7,7,8,8,9,9,9-tetradecafluorononane-4,6-dione (HTDFND) has been prepared and employed in the formation of a complex of Ba. A complex of the same metal derived from the $\beta$-diketone, 1,1,1,2,2,3,3,7,7,7,-decafluoroheptane-2,4-dione (HDFHD) has also been prepared. The compounds have the formula [ M(TDFND)$_2$H$_2$O] or [ M(DHFD)$_2$H$_2$O] . The complexes have been characterised by analytical and spectroscopic means. Simultaneous thermal analysis at one atmosphere reveals that the compounds lose water but then sublime, and in the case of [ Ba(TDFND)$_2$H$_2$O] without any decomposition. [ Ba(TDFND)$_2$H$_2$O] is the first barium complex for which this is the case and it can be dehydrated to give [ Ba(TDFND)$_2$] which is also volatile. This suggests that [ Ba(TDFND)$_2$H$_2$O] could be a suitable precursor for the CVD growth of BaF$_2$. This has been investigated. On silicon substrates complete orientation in the (111) direction is observed. Changes in the film growth rate with time are attributed to slow reorganisation of the crystal structure of [ Ba(TDFND)$_2$]

A study by GC-MS of the decomposition of precursors for the MOCVD of high temperature superconductors

This paper describes a study by gas chromatography/mass spectrometry (GC-MS) of the gas phase thermal decomposition of HTMHD (where HTMHD is 2,2,6,6-tetramethylheptane-3,5-dione), [Y(TMHD)3] and [Y(TMHD)3-hydrate] in the presence and absence of oxygen over the temperature range 200-600 °C. The gas phase at the outlet of the reactor was condensed in a liquid nitrogen trap and analysed by GC-MS to try to elucidate the composition of the gas mixture in the reactor. On the basis of the identified decomposition products, decomposition processes of the various compounds are suggested. The results show that the compounds are, not unexpectedly, less thermally stable in oxygen and the mechanism of their thermal decomposition is changed. It is also shown that the residence time of the precursors in the reactor has significant effect of decomposition chemistry.

Organic chemistry by CVD

While CVD has been extensively used for the preparation of thin films of inorganic materials, apart from its application to plasma polymerisation, very little has been reported on the formation of organic layers. In this paper we report the first CVD preparation of an organic pigment, quinacridone. A two-step CVD method and the effects of various deposition conditions on the deposition process are described. The characterisation of deposits by FTIR, Raman, XRD and chemical microanalysis analysis shows that they have very similar compositions and structures to commercial samples of the pigment

A study by In Situ FTIR Spectroscopy of the Decomposition of Precursors for the MOCVD of High Temperature Superconductors

In this paper we report on a study by FTIR spectroscopy of the gas phase thermal decomposition of [Y(TMHD)3], [Cu(TMHD)2] and [Ba(TDFND)2,tetraglyme] under a total pressure of 10 Torr, for a temperature range 200 - 650°C, and in the absence and presence of oxygen. The variation of the decomposition characteristics for each of the complexes as a function of temperature, and as shown by changes in the absorption coefficients, are related to the molecular structures