Like the analogous tungsten complex, molybdenum hexacarbonyl, [Mo(CO)6] has been used as a single source precursor and in conjunction with a secondary source to deposit molybdenum oxide films using a number of CVD techniques. When used as an APCVD precursor in the absence of oxygen molybdenum metal contaminated with carbon known as ‘reflective molybdenum’ is formed whereas deposition in the presence of oxygen yields
black molybdenum , 72 The reflective molybdenum and black molybdenum films both
crystallised in the orthorhombic modification of M0O3 following annealing in air at 600°C,
with crystallites preferentially orientated along the < 1 1 0> and < 0 1 0> direction
respectively.
The optical properties of the molybdenum oxide films deposited from the APCVD reaction
■70
of [Mo(CC>6)] and oxygen at low temperatures were investigated by Gesheva et al. Films
and 220°C with the [Mo(C0 6 ) ] : 0 2 ratio ranging from 1:24 to 1:40. The as-deposited films
were amorphous in nature and after post deposition annealing for one hour at a minimum
temperature of 300°C crystallized in the orthorhombic modification of M0O3. The films
deposited on conductive glass showed high optical transmittance (60 - 70 CA ) in the visible
region, with the transmittance increasing with annealing temperature. The optical band gap
of molybdenum oxide films was dependant on the [Mo(C0 6 ) ] : 0 2 ratio: films deposited
from 1:24 and 1:40 [Mo(C0 6 ) ] : 0 2 ratios had band gaps of 2.7 and 3.4 eV respectively.
Molybdenum hexacarbonyl and molybdenum hexafluoride, have been used as PECVD precursors, in conjunction with hydrogen, methane or oxygen to deposit molybdenum oxide films on silicon substrates. 74 The as-deposited films were all amorphous in structure. X ray
photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) showed that the
PECVD reaction of [Mo(CO)6] with CHU afforded films composed of a mixture of MoCK
M0 2C. and elemental Mo and depositions using PE yielded films of molybdenum
oxycarbide. Depositions using MoF6 with hydrogen and CH4 both produced films
comprising of elemental Mo. however the latter also contained M0 2C. In both cases the
films were free of fluorine contamination. All films could be oxidised to MoCE following annealing in air. In contrast MoCE could be obtained directly via the PECVD reaction of [Mo(CO)6] and oxygen.
Molybdenum oxide films have been deposited via hot filament metal oxide deposition
(HFMOD). 5 The technique involves heating a molybdenum filament in a low pressure
oxygen environment to generate volatile molybdenum oxide species which form a film on a nearby substrate. The oxygen flow rate and molybdenum film current are the main deposition parameters. Amorphous, compact films which showed good adherence to the substrate were obtained under all deposition conditions. XPS revealed the presence of Mo(H and Mo' + cations with a high predominance of the former over the latter and the RBS data indicated an average atomic ratio of 2.9, confirming the MoCE x film stoichiometry. The optical band gap value of the films was found to be independent of the oxygen flow rate and molybdenum filament current.
Molybdenum oxide films have been deposited on Si wafers and Si coated float glass at
100°C by AACCVD using an aqueous solution of ammonium molybdate, [NH4M M0 7O2 4]
in deionised water.66 The films were amorphous and had stoichiometries close to MoCfl.
There are far fewer reports detailing the CVD of molybdenum oxide films compared with tungsten oxide. Furthermore there is only one report describing the single source approach and an AACVD based technique. This project aims to expand on this and hence demonstrate the versatility of polyoxometalates as AACVD precursors. The subsequent section describes the synthesis, structure and properties of polyoxometalates, which have been used as precursors to tungsten oxide and molybdenum oxide films.
1.7 Polyoxometalates
Polyoxometalates are large anionic metal oxygen clusters, which are formed by the early transition metals in their highest oxidation states, in particular by the elements tungsten, vanadium and molybdenum and to a lesser extent by niobium and tantalum.5 76 77 Clusters which contain one type of metal are known as isopolyoxometalates and those containing additional elements (heteroatoms) are known as heteropolyoxometalates.
Most polyoxometalates are formed by varying the pH of an aqueous solution of M O 42 (M
= W, Mo, Nb. V. Ta) ions. The elements which can form polyoxometalates are able to change their coordination with oxygen from 4 to 6 as they polymerize in solution upon acidification and can form double bonds with unshared oxygen atoms from their M 0 6
7X
octahedra. by p7t-d7t interactions. ' Polyoxometalates can accommodate a wide range ot heteroatoms, including most non-metals and transition metals. The formation of isopolyoxometalates involves the polymerization of the metal-oxygen polyhedra around a heteroatom as the solution is acidified 9 5(1 Polyoxometalates can be rendered soluble in most solvents including water and hydrocarbons by variation of the counter ions. In this study the polyoxometalates have been isolated as their tetrabutylammonium salts, rendering them soluble in aprotic solvents such as acetone and acetonitrile, which is preferable for AACVD reactions.
Although the existence of polyoxometalates has been known since the nineteenth century, it was only during the 1980’s that advances in characterization techniques such as O17 NMR
spectroscopy and X-ray diffraction enabled their structures to be resolved. 76
The following polyoxometalates have been synthesized for use as AACVD precursors in this study: ["Bu4N]2[W60 19], ["Bu4N]4[W,()0 32] . l"Bu4N]3lNbW50 , 9]. ["Bu4N l3|TaW30 , 9].
[CpTi(WsOix)H][;,Bu4N]2, ["Bu4N]4Ph[SiM oW ,,04l)]. ["Bu4N]4|Mox()2f,l.
[,,Bu4N]:[Mo6O ly]. ["Bu4N]2[Mo2 0 7] and ["Bu4N]3[PMoi20 4()]. These have been chosen to
study the effect of cluster size and the introduction of dopants on the resulting film stoichiometry and properties.
Polyoxometalates are composed of comer and edge-sharing MC>6 octehedra (M = W. Mo).
The polyoxometalates ["Bu4N]2[W60 19]. ["Bu4N]2[Mo6 0 19] and [''Bu4N]4[W,uO d have
structures comprising of a neutral Wn0 7n cage encapsulating one or more formally anionic sub-units. The structure of [''Bu4N]2[W6O i9] and ["Bu4N]2[Mo6 0 i9], can be described as a
M60 !s cage encapsulating an O2 anion. 8 ' 7 6 The M6Oix cage is comprised of six [MO]4+
units occupying the vertices of an octahedron. Each hexavalent tungsten or molybdenum centre forms single bonds to four adjacent doubly bridging oxygen atoms, a double bond to
its terminal oxo ligand and a very long weak bond to the central bridging OM6 atom
(Figure 1.6). The structure of [MftOiy]"" is shown in Figure 1.7. The structures of ["BuiNfdNbWsOisd and ["Bu4N ]dTaW5 0 i9] are similar except that one tungsten atom is
substituted by a niobium or a tantalum atom respectively. 81 Similarly the polyoxometalate
[CpTi(W5 0 ls)H]["Bu4N]2 is generated by replacing a [OWvl]4+ unit with [CpTiIV ] 3 + . s2
O
W "
Figure 1.6 Representation of the three types of metal-oxygen bonds at each hexavalent tungsten centre in [W6 0i9]“\
a b
Figure 1.7 Structural representation of the anion, [IV^O^]2* (M = W, Mo), (a) Shows M atoms in pink and oxygen atoms in red; (b) depicts the edge-shared MC>6 octehedra.
The [WioC^]4* ion has a similar structure, where each tungsten centre has the local environment just described for the tungsten atoms in [W6Oi9]2', however is part of a larger
W10O30 cage which encapsulates two O2' ions (Figure 1.8) . 83 The [M0 8O2 6 ] 4 anion is
comprised of a ring of six M o06 octehedra which are linked to two distorted M0O4
tetrahedra, one above and one below the plane of the ring (Figure 1.9) . 84 The structure of
[M0 2O7]2' consists of two M0O4 units sharing a bridging oxygen atom. 85
a b
Figure 1.8 Structural representation of the anion, fWioC^]4’ (a) Shows tungsten atoms in pink and oxygen atoms in red; (b) depicts the corner and edge-shared W 0 6 octehedra.
a b
Figure 1.9 Structural representation of the anion, [Mo^t^]4* (a) Shows molybdenum atoms in pink and oxygen atoms in red; (b) depicts the edge-shared M o06 octehedra.
The structure of ["BU4NMPM0 1 2O4 0] is derived from the Keggin anion [PM o^O^ ] 3' . 5 ,7 6
The Keggin anion is composed of a central tetrahedron PO4 surrounded by 12 edge and
comer sharing MoC>6 octahedra, which are arranged in four M0 3O13 groups. Each M0 3O13
group consists of three edge sharing octahedra which have a common oxygen atom which
is also shared with the central PO4 tetrahedron. Figure 1.10 shows the structure of
[ P M 0 1 2 O 4 0 ] 3 ’ .
a b
Figure 1.10 Structural representation of the Keggin anion, [PMonO^]3" (a) Shows molybdenum atoms in pink, oxygen atoms in red and the phosphorous atom in blue; (b) depicts the corner and edge-shared M o06 octehedra.
The structure of [SiM oW n04o]4’ is derived from that of the dodecatungstosilicic ion
[SiW1204o] \ which exists in three isomeric forms, a , (3 and y. 86 The a-isomer has the
Keggin structure and is composed of four edge sharing W3O13 octehdera around a central
S i04 tetrahedron. The p and y isomers are obtained by rotating one or two of the W3O13
isomers and each can be isolated by varying the acidification conditions of the preparation. The structure of [SiM oW nO^]4" is isomorphous with that of p - [ S iW i20 4o]4'.87 The p-
[SiWi2 0 4o]4" ion has W atoms in three environments; three which are situated opposite the
rotated W3O13 group, six in the adjacent octahedral, and three tungsten atoms in the rotated
W3O13 group. In [SiMoW iiO^]4' the Mo atom occupies the site of a tungsten atom which
lies opposite the rotated W3O13 group. Figure 1.11 shows the structure of [SiMoWj 1O40]4’ .
a b
Figure 1.11 Structural representation of the anion, [SiMoWn04o]4' (a) Shows tungsten and molybdenum atoms in pink, oxygen atoms in red and the silicon atom in grey; (b) depicts the corner and edge-shared W(>6 octehedra.
Films have also been deposited from the tetrabutlyammonium salt of the tetrahedral
[W O 4]3' ion for comparison.
A number of excellent reviews have been published on polyoxometalate chemistry. 5 7 6X0
88.89.9o.9i These provide compressive guides to the synthesis and structure of a wide range of
polyoxometalates as well as describing their potential applications which include catalysis, photocatalysis, and antiviral and anti-HIV activity.
With their large caged structures and electronic charges polyoxometalates are the antithesis of CVD precursors. This project aims to illustrate that the use of an aerosol to transport the precursor broadens the range of materials that are suitable for use as CVD precursors as the volatility requirement is eliminated.