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. 

Malik, M. A. Revaprasadu, N. and O Brien, P. (2001). Air-stable single-source precursors for the synthesis of chalcoeenide semiconductor nanoparticles. Chem. Mater., 13, 913-920. 

Xinglong Wu, Cao M, Changwen Hu, He X (2006) Sonochemical synthesis of prussian blue nanocubes from a single-source precursor. Cryst Growth Design 6(l) 26-28... 

In attempts to reduce toxic hazards, eliminate homogeneous prereactions, lower deposition temperatures, and use cheaper precursors there have been considerable efforts directed toward the development of alternative precursors for use in MOCVD.10 Another approach to using intrinsically less reactive compounds is to use a single-molecule (or single-source ) precursor, in which all the elements required in the film are present in the one molecular species. The compounds potentially exhibit many or all of the following advantages over conventional MOCVD precursors ... 

Initially, the main disadvantage of using single-source precursors was their lack of volatility. However, the advent of ultralow-pressure and vacuum CVD techniques, along with the use of alternative delivery methods which do not rely primarily on precursor volatility, means that many compounds that are essentially nonvolatile can now be considered as potential CVD precursors. A major problem that remains, however, is that of stoichiometry control a molecular precursor containing two (or more) elements in a specific ratio, as desired within the thin film to be grown, will not necessarily deposit those elements in the same proportions. 

Despite the interest in single-source precursors, there are very few commercial applications that use them however, it seems likely that such compounds will eventually be used for large-scale coverage applications, or in special cases where low-temperature growth is needed and where large areas need to be covered at relatively low cost. 

The most extensively studied of these complexes for use in CVD are the diethyldichalcogeno-carbamates [M(E2CNEt2)2] (M = Zn, Cd E = Se, S). These complexes are air-stable solids, and are dimeric in the solid state175,176 but monomeric in the vapor phase, as judged from mass spectra. Thin films of CdS, CdSe, ZnS, and ZnSe have been grown by a number of different MOCVD techniques using these complexes as single-source precursors they are listed in Table 11. 

These compounds were studied as potential single-source precursors. However, although they gave clean deposition routes to powders of II VI materials, their nonvolatility means that thin-film growth by conventional CVD techniques was hampered, although it could be improved using new delivery techniques. 

Table 13 Some compounds used as single-source precursors for III-V materials.

Although many complexes of this type have appeared in the literature, with many publications emphasizing their potential use as single-source precursors, this section will only address compounds that have actually been used in the deposition of III-V thin films. 

AIN, GaN, and InN are attractive materials for applications such as blue lasers and field emitters single-source precursors for these of formula [R2MNR 2]2 (R = alkyl, R = alkyl or H) have been reported.236 The reaction of alkylamines with group 13 trialkyl metal compounds affords oligomeric or polymeric ring and cage structures of metal amides and imides (see section on nitrides). 

The single-source precursors [Me2Al(/u-NHR)]2 (R = Pr , Bu Figure 37c) have been used to deposit AIN films by vacuum CVD at substrate temperatures between 377 °C and 677 °C.273 The films were shown by X-ray photoelectron spectroscopy (XPS) to contain extremely high levels of carbon (40-50%), which was attributed to the decomposition of methyl groups attached to the Al atom. [Me2Al(NHBu )]2 as precursor was shown to deposit AIN at lower temperatures than... 

The reaction of CdCl2 with tV,7V -bis(thiocarbamoyl)hydrazine gave [Cd SC( = NH)NH2 2Cl2] which has been used as a single-source precursor. Upon thermolysis in TOPO at 230 °C it produced CdS particles which showed an absorption edge blue shift of 37 nm in relation to that of the bulk material no chlorine contamination was reported.385... 

The trimeric indium antimonide single-source precursor [Et2InSb(SiMe3)2]3, prepared from Et2InCl and Sb(SiMe3)3, was shown in a preliminary study to form InSb nanocrystals (average particle size 10 nm) by solid-state thermolysis at 400 °C under a static vacuum. However, the particles were contaminated with elemental indium.399... 

Table 15 Single-source precursors used in the preparation of quantum dots.

The work in this group has focussed mainly in antimony and bismuth because of the thermoelectric properties of the chalcogenides186 and as low temperature single-source precursors to related semiconductor materials.187 The use of bismuth compounds in the treatment of gastrointestinal disorders has lead to the study of several thiolate compounds as models to understand the bioactivity. 

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... 

The anionic species ROCS2 resulted from O-alkyl(aryl) esters of the hypothetical dithiocarbonic acids, ROC(S)SH, better known as xanthates, are versatile ligands and they generate an extensive coordination chemistry. The interest for metal xanthates is stimulated by their potential use as single source precursors for nanoscopic metal sulfides in photochemical or thermal vapor deposition systems under mild conditions,218 221 e.g. for Zn,222 Cd,223 In,224... 

B. Deposition of AlSb Films by Use of Single Source Precursors. 301... 

Syntheses, structures and properties of heterocyclic group 13/15 compounds of the type [R2M LR 2]V have been the subject of intense studies for many years. In particular, aminoalanes and -gallanes [IGIVINR/ (M = Al, Ga) have received considerable attention. The interest in these particular compounds has increased within the last two decades due to their potential application as single source precursors for the deposition of AIN and GaN... 

---