In summary, a number of depositions were attempted from [R3SnS2CN R’2] where
R=Me, Bu; R ’=Me, Et. [Me3SnS2CNMeBu] was also used as a precursor.
No films were deposited from [R3SnS2C N R’2] (R=Me, Bu; R ’=Me, Et) using the
bubbler delivery method.
Using [Me3SnS2CNM e2] without hydrogen sulfide em ploying the aerosol method of
delivery also produced unexpected results. No film could be deposited at 450 °C, while at 550 °C tin(II) sulfide was deposited. At 500 °C the film deposited was shown not to
be tin sulfide or tin oxide by Ram an m icroscopy. In all cases, som e sulfur was detected by ED A X , although a large am ount of tin was seen in the underlying glass.
The related precursor, [M e3SnS2CN Et2], was used in a num ber of aerosol assisted
reactions w ith hydrogen sulfide. It was found that tin(II) sulfide was the m ajor com ponent in all films deposited at 450 °C and above, although a sm all am ount of m ixed valent Sn2S3 was seen at 450 °C.
[M e3SnS2CN M eBu] was used w ith the bubbler delivery m ethod, as asym m etric
dithiocarbam ates are m ore volatile than their sym m etric counterparts. Reactions were carried out w ith 0.2 dm^min"^ hydrogen sulfide (total gas flow 12.3 dm^min'^). It was found that, at 450 °C, tin(II) tin(IV ) trisulfide was the m ajor phase, w ith a small am ount of tin(IV ) sulfide. At higher tem peratures tin(II) sulfide was the only phase formed. No film could be deposited at 600 °C.
W ith the exception o f films deposited from [M e3SnS2C N M e2], all reactions showed the
fam iliar trend found in previous sections. Reactions w ere not carried out at tem peratures below 450 °C. M ixed valent tin sulfide was generally observed at the low est deposition tem perature of 450 °C, som etim es w ith tin(II) sulfide. A t higher tem peratures tin(II) sulfide was the only phase observed. A lso, thinner films w ere deposited at the highest tem perature of 600 °C, as has been observed with other systems.
In the case of reactions o f [M e3SnS2C N M e2], results w ere not as expected. The first
result that is inconsistent with previous observations is the spectrum of the film deposited at 500 °C from the reaction w ithout hydrogen sulfide using the aerosol delivery method. The Ram an spectrum o f this film is not indicative o f a tin sulfide or tin oxide. As yet, this spectrum has not been identified.
5.6 Summary and conclusions
5.6.1 Sum m ary
In this Chapter, a variety of precursors was investigated for their potential as single source precursors to tin sulfides. The precursors investigated fell into one of the follow ing categories
• H om oleptic tin(IV ) thiolates • H om oleptic tin(II) thiolates
• H om oleptic tin(IV) dithiocarbam ates • Alkyl tin(IV ) dithiocarbam ates
It was found that tin(II) thiolates, due to the incom plete coordination sphere around the tin atom, tended to polym erise, thus rendering them insoluble and in volatile. The only exception was [Sn(SCPh3)2]. The bulky ligand prohibited polym eristation, so the
com pound was soluble. However, investigation by aerosol assisted CVD proved that tin sulfide film s could not be deposited.
Tin(IV ) thiolates w ere altogether m ore successful. Three precursors w ere extensively investigated, all giving some positive results.
In sum m ary, it was found that tin(IV) thiolates could form film s by CVD. In the case of [Sn(SR)4] (R=Ph, CH2CF3) if a film could be produced by using the precursor alone, it
was tin oxide. W ith hydrogen sulfide in the system, this was sulfidised to tin sulfide. This was due to loss o f ligands (either SR or RSSR) in the initial stages of the reaction. In the case of chelating [!sn(-SCH2C H2!s)2] loss o f a ligand w as less favourable, as the
other end was anchored (the chelate effect). The equivalent o f losing an RSSR m oiety in this case w ould be to lose an unstable 4-m em bered ring. TG A indicated that
CH3CHS is lost, follow ing rearrangem ent and leaving a tin-sulfur bond. All these factors led to the form ation o f tin(II) sulfide film s when no hydrogen sulfide was present in the system.
Tin(IV ) dithiocarbam ates w ere also investigated. O nly a few reactions were carried out, w ithout hydrogen sulfide. Tin(II) sulfide film s could be produced. A gain, this result is probably due to the chelate effect, prohibiting loss of ligands.
Alkyl tin(IV ) dithiocarbam ates investigated follow ed the established trend of low er tem peratures leading to higher oxidation states of tin sulfide form ed. U sing asymm etric dithiocarbam ate ligand allow ed bubbler delivery to be used, as m elting and boiling points w ere lower. U sing this class of precursor led to one unexpected result. At 500 °C a film was deposited w ithout hydrogen sulfide; this could not be identified as a tin oxide or sulfide by Ram an m icroscopy. A t low er tem peratures, no film was deposited, w hich is unusual. N orm ally, tin(IV) sulfide or tin sesquisulfide could be formed. This unknow n phase has yet to be identified.
5.6.1 Conclusions
W hen com pounds are designed with a view to form ing single source CVD precursors, a tin-sulfur bond is the first prerequisite. If m onodentate ligands are used, these are easily lost, either singly or as dimers. This leaves an active tin species, w hich reacts with the small am ount o f w ater present in nitrogen gas cylinders. B identate ligands are altogether m ore effective. This is due to the chelate effect. Ligands cannot be lost easily, so the m olecule reaches the surface o f the glass intact, before undergoing decom position.
O f the precursors investigated, [Sn(-SCH2C H2^ )2], [M e3SnS2CN M e2] and
[M e3SnS2CNM eBu] all produced films w ithout hydrogen sulfide present in the system.
O nly [M e3SnS2CNM eBu] could be used with the bubbler delivery method. O ther
sim ilar precursors, with asym m etric dithiocarbam ate groups, m ay also be suitable for form ation of tin sulfide films.