Discussion

Prior to this work no reported use o f chromyl chloride as a precursor to chromium oxide, in CVD experiments had been reported. In this work it has been shown that it is possible to use chromyl chloride to produce Cr2 0 3 films, with a variety o f co-reactants, from water to ethyl acetate.

Chromium Oxide Films from APCVD 98

Previous CVD studies into the production o f chrom ium oxide thin films have concentrated on the use o f either C r(C O)6 or C r(acac)3 as the precursor. It is possible to produce Cr2 0] films from C r(acac)3 at atm ospheric pressure. H ow ever, the vapour pressure o f C r(acac)3 at these pressures leads to very long grow th tim es being necessary, w ith up to 32 hours being reported, com pared to grow th tim es o f as little as 30 seconds using chrom yl chloride. C r(acac)3 also requires an oxygen atm osphere to be m aintained during the deposition tim e to facilitate deposition.

Cr2 0 3 films have also been produced at low pressure from Cr(CO)6 in an oxygen atm osphere. A gain the use o f low pressure reduces the attainable grow th rates dram atically w ith deposition rates in the order o f 3 nm min'* being reported com pared to grow th rates o f 1.4 m icrons m in * being possible from the m ethods described in this work.

All film s grown in this study proved to be phase pure Cr2 0 3 from X RD analysis, w ith carbon contam ination levels o f less than 1% being seen b y XPS. This com pares well to literature w here contam ination levels o f 6 -1 0% have been reported for film s grow n at low pressure, over a m uch longer tim e period, from Cr(CO )6.

This is also the first report o f thin film CVD deposited Cr2 0 3 being analysed by Ram an m icroscopy and the results com pare w ell w ith literature spectra o f bulk Cr2Û3.

Chromium Oxide Films from APCVD___________________________________99

T he production o f Cr2 0] at all tem peratures shows that a reduction o f chrom ium from + 6 to +3 has taken place in all cases. T he exact nature o f the reaction is not know n, although it could proceed via a C1O3 interm ediary, w hich then goes on to decom pose at the elevated tem peratures o f the reaction, to give the final Cr2 0] film. This is possible as Cr2 0] is know n to be the only stable chrom ium oxide at tem peratures above 500 °C I f this w ere the case then a possible reaction schem e could be as in figure 4.12:

2Cr02Cl2

►

Cr20]

F ig u re 4 .1 2 p o s s ib le reaction s c h e m e for reaction o f Cr0 2 C l2 w ith w ater

A sim ilar reaction schem e can be draw n for m ethanol w ith h a lf o f the HCl that is produced being replaced w ith MeCl.

The film grow th m echanism does seem to be different, based on the m orphology o f the film s as seen by SEM . The green films that w ere produced by the reaction o f chrom yl chloride w ith w ater show crystallite grow th. This crystallite grow th suggests that a gas phase action has taken place to give a product sim ilar to that w hich w ould be produced from a traditional ceram ic synthesis. A nother possible cause for this w ould be that the reaction proceeds at such a rate that there is not enough tim e for atoms to m ove on the surface, and therefore react to form individual crystallites, rather than rearranging on the surface to form a film. This is in contrast

Chromium Oxide Films from APCVD___________________________________ 100

to the films that w ere produced form the reaction betw een chromyl chloride and organic precursors that can b e broken dow n into tw o groups, reactions involving ethanol or m ethanol and reactions involving ethyl acetate or acetic acid.

The first o f these tw o groups showed a deposition profile only over the first 2-3 cm o f the substrate. As the substrate is the same tem perature over the w hole surface this suggests that the deposition is being stopped for m ass transport reasons, i.e. the entire am ount o f precursor is being used up. I f this is the case then it m ust be the organic precursor (ethanol / m ethanol) that is being consum ed as under the same conditions it has been show n that w ith the other organic precursors a coating is achieved over the w hole o f the substrate. T he film s are also not thick enough for all o f the chrom yl chloride to have reacted to form chrom ium oxide by this point.

Titanium Dioxide