Chemical single-source precursors

Later on, this concept was extended to precursors containing both elements of the desired material already connected by a chemical bond in a single molecule. Such precursors are mainly referred to as single source precursors. Their potential application for the deposition of thin films of the corresponding binary materials by MOCVD processes could be demonstrated. In particular Lewis acid-base adducts R3M—ER3 and four- and six-membered heterocycles [R2MER x (Fig- 1) have been in the focus of research groups both in industry and university. Consequently, the development of powerful synthetic pathways for the preparation of such precursors has been forced. 

Metal dithiophosphinates focused recent interest due to their potential use as single source precursors for low-pressure, metal-organic chemical vapor deposition (LP-MOCVD) of metal sulfides. Thus, zinc and cadmium dithiophosphinates and their phenanthroline and bipyridine adducts were characterized and used for deposition of ZnS and CdS.85,86... 

Jin, M. H. -G Banger, K. K. Harris, J. D. Hepp, A. F. 2005. CuInS2 films deposited by aerosol-assisted chemical vapor deposition using ternary single-source precursors. Mater. Sci. Eng. B 116 395-401. 

Cowen, J. E. Riga, A. T. Hepp, A. F. Duraj, S. A. Banger, K. K. McClarnon, R. 2004. Synthesis and characterization of CuInS2 single source precursors for chemical vapor deposition. J. Therm. Analysis and Color. 75 929-936. 

Recently we published a short review of the single source precursor concept in chemical vapor deposition (CVD) and in the sol/gel process [1]. hi this article we described in which way several constituent elements of a targeted material can be assembled in a precursor molecule and how this assembly has an effect on the properties of the hnal material. Three types of precursors have been distinguished (1) Precursors which contain the correct ratio of metallic elements (SSP-I), (2) precursors which besides the correct ratio of metallic elements also have ligands which interact with one another to form only few side-products (SSP-II), and (3) precursors with a surplus of one metallic element compared to a thermodynamically stable phase and which form biphasic mixed-materials on a nanometer scale (SSP-III) [1],... 

An alternative concept for the deposition of III-V material films, the so-called single-source precursor concept, was introduced by Cowley, Jones and others almost 15 years ago [10]. Typical group 13/15 single-source precursors such as Lewis acid-base adducts R3M - ER 3 or heterocycles [R2MER 2]% as shown in Scheme 1 contain the specific elements of the desired material connected by a stable chemical bond in a single molecule. 

Single-source precursors for the chemical vapor deposition of titanium and vanadium carbide and nitride... 

Figure 19 TEMs showing the variety of shapes obtained from the PbS system grown from the single-source precursor, Pb(S2CNEt2)2, at several temperatures, (a) Multipods prepared at 140 °C. (b) Tadpole-shaped monopod (140 °C). (c) I-shaped bipod (140 °C). (d) L-shaped bipod (140 °C). (e) T-shaped tripod (140 °C). (f) Cross-shaped tetrapod (140 °C). (g) Pentapod (140 °C). (h) Star-shaped nanocrystals prepared at 180 °C. (i)Roimded star-shaped nanocrystals prepared at 230 °C. (j) Truncated octahedra prepared at 250 °C. (Reprinted with permission from Ref 47. 2002 American Chemical Society)...

Figure 4.44. Synthetic pathway for single-source precursor design. Reproduced with permission from Inorg. Chem. 2003, 42, 7713. Copyright 2003 American Chemical Society.

The bis-Cp titanium bis(/-butanethiolato) and bis(ethanethiolato) complexes have been used as a single-source precursors for the preparation of thin films of titanium sulfides by metal-organic chemical vapour deposition (MOCVD). The crystal and molecular structures of the precursor complexes have been determined for comparison with homologous complexes of the general formula Cp2Ti(SR)2.1618... 

In spite of the advantages of CVD techniques, such as versatility, adaptability, compatibility, quality, simplicity, reproducibility, productivity, and cheapness, some drawbacks are observed, mainly those concerning chemical hazards due to toxicity and instability of precursors. Other disadvantages relate to difficulties concerning the deposition of multicomponent materials using more than one precursor with different vapor rates. However, in a small number of experiments, this difficulty has been overcome by using single-source precursors. In addition, some sophisticated variants of CVD can increase the fabrication cost of materials. ... 

Chemical vapor deposition130 of a single source precursor offers a safer alternative (4.4) 131... 

From the chemical point of view, there are two main processes to obtain thin films of TiN by CVD, namely (1) via a chemical reaction of a titanium source and a nitrogen source, or (2) via the decomposition of a nitrogen containing titanium precursor (single source precursor). Each of them needs a special development of precursors. 

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