MOCVD applications

A large number of all citations to organozinc compounds deal with the use of these compounds as precursors for solid-state materials, particularly for MOCVD applications. The latter area has become so interdisciplinary and grown to such an extent that the editors of this edition considered it prudent to devote an entire volume (Volume 12) to the use of organometallic compounds in materials chemistry. While the syntheses and structures of potential organozinc precursors for such applications will be treated in this chapter, the detailed aspects of the conversion of organozinc compounds to solid-state materials are the topic of Volume 12. 

The properties of Ge(OR)4 allow them to be considered more likely to be the esters of an inorganic acid than metal alkoxides these are colorless volatile liquids, containing monomeric tetrahedral molecules. The solid crystalline form is known only for R = Bu, OC6Hnc, and also 2,6-substituted phenoxides. All the members of the Ge(OR)4 homologous series are characterized by thoroughly determined physical characteristics — density, refraction index, surface tension, viscosity (and calculated parachor values), dipole moments in different solvents [222, 857, 1537] (Table 12.9). The results of the investigation of vapor pressure, density, viscosity polytherms, and so on. permitted rectification for the preparation of samples of high purity for sol-gel and MOCVD applications [682, 884]. 

As the interest in finding alkaline-earth compounds suitable for MOCVD applications has increased, investigation of ligands that are both thermally robust and able to encapsulate large metal centers has intensified. Some of the previously discussed cyclopentadienides meet these criteria, and the related poly(pyrazolyl)borates have recently received a great deal more attention. 

The most important applications of organolanthanides are as Ziegler-Nutta catalysts in polymerization of olefins and as reagents in stoichiometric organic synthesis. Organolan-thanide compounds are also useful as volatile precursors for MOCVD applications in the... 

Figure 2.33 ORTEP view of the two independent molecules in the asymmetric unit of Nd(L )3-monoglyme-HiO (ellipsoid probability 30%). Fluorine and hydrogen atoms have been omitted for clarity [72]. (Reprinted from Inorganica ChimicaActa, 362, R.L. Nigro, R.G. Toro, M.E. Fragala, P. Rossi, P. Dapporto and G. Malandrino, Neodymium -diketonate glyme complexes Synthesis and characterization of volatile precursors for MOCVD applications, 4623 629, 2009, with permission from Elsevier.)...

Nigro, R.L., Toro, R.G., Fragala, M.E., etal (2009) Neodymium -diketonate glyme complexes synthesis and characterization of volatile precursors for MOCVD applications. Inorganica Chimica Acta, 362, 4623 629. 

Although metal alkoxides proved to be excellent precursors for the preparation of ceramics by the sol-gel process, their applications in the MOCVD technique are not as widespread, in spite of the attractive features of their volatility and tendency to decompose to the metal oxides. Both of these desirable prerequisites appear to be enhanced, as discussed earlier, by choosing more ramified alkoxide groups. Despite these attractive features, the MOCVD applications of metal alkoxides appear to be limited by (a) their commercial nonavailability, (b) handling difficulties arising from their ready hydrolyzability, and (c) the lack of a clear understanding of their decomposition pathways, which could lead to improvement(s) in the purity of the deposited material. 

Derivatives Containing Transition Metals.-This Section can be conveniently divided into species with a direct bond to a transition metal (TM) and heterobimetallic species which lack this interaction. There seems a noticable shift in emphasis away from the latter grouping, for so long driven by the relevance of such species to homogeneous catalysis, towards MOCVD applications and the former class. 

Two maj or contributors to this rapid growth are plasma CVD and metallo-organic CVD (MOCVD). Both are extensively reviewed in this new edition. Likewise, the growing importance of CVD in the production of semiconductor and related applications is emphasized with a systematic and detailed analysis of the role of CVD in this field. 

Metallo-organic CVD (MOCVD) is major area of CVD which is rapidly growing, particularly in semiconductor and optoelectronic applications. It is treated separately in Ch. 4. 

Metallo-organic CVD (MOCVD) is a specialized area of CVD, which is a relatively newcomer, as its first reported use was in the 1960s for the deposition of indium phosphide and indium anti-monide. These early experiments demonstrated that deposition of critical semiconductor materials could be obtained at lower temperature than conventional thermal CVD and that epitaxial growth could be successfully achieved. The quality and complexity of the equipment and the diversity and purity of the precursor chemicals have steadily improved since then and MOCVD is now used on a large scale, particularly in semiconductor and opto-electronic applications.91P1... 

The equipment and chemicals used in MOCVD are all available commercially but are expensive and production cost is high. For these reasons, MOCVD is considered in applications where high quality is essential. 

Table 4.3. is a summary of the major applications of MOCVD. More details are found in Chs. 13-16.Pi... 

Strontium titanate (SrTi03) has a large dielectric constant of 12, and a high refractive index with potential opto-electronic applications. It is deposited by MOCVD from titanium isopropoxide and a strontium beta-diketonate complex at 600-850°C and 5 Torr.t" " ... 

The many possible combinations of II-V and II-VI compounds allow the tailoring of electronic and opto-electronic properties to suit specific applications. Of particular importance is the control of the stoichiometry of the element involved. This is achieved by the proper handling of the MOCVD reactions. Being able to tailor the bandgap imparts great flexibility in the design of transistors and optoelectronic devices. 

Wolfson, R. G., Application Specific Electronic Materials by Ion Implantation and MOCVD, Proc. of Conf. on High Performance Inorganic Coatings, Monterey, CA, G.A.M.I., Gorham, ME 04038 (1988)... 

Optoelectronics is a relatively new and fast-growing industry with many applications. Thin-film processes, such as reactive sputtering, molecular-beam epitaxy (MBE), and particularly MOCVD, play a major part in their production. Equipment and materials are similar to those used in the semiconductor industry and many companies manufacture both types of products. In fact the distinction between the two areas is often blurred. Statistics generally do not single out optoelectronics as such and, for that reason, it is difficult to define the scope of the industry accurately. 

LED materials include gallium arsenic phosphide, gallium aluminum arsenide, gallium phosphide, gallium indium phosphide, and gallium aluminum phosphide. The preferred deposition process is MOCVD, which permits very exacting control of the epitaxial growth and purity. Typical applications of LED s are watches, clocks, scales, calculators, computers, optical transmission devices, and many others. 

Transparent semiconductor oxide films, such as tin oxide (Sn02) and zinc oxide (ZnO), produced by MOCVD are also being considered for photovoltaic applications. ]... 

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. 

---