Synthesis and evaluation of new molecular precursors for the chemical vapour deposition of electronic materials

Hawken, Samantha Louise (2018) Synthesis and evaluation of new molecular precursors for the chemical vapour deposition of electronic materials. University of Southampton, Doctoral Thesis, 289pp.

Record type: Thesis (Doctoral)

Abstract

MCl₃ (M = Sb, Bi) react with EⁿBu₂ (E = Se, Te) to form [MCl₃(EⁿBu₂)₃], which are effective single source precursors for the low pressure chemical vapour deposition (LPCVD) of metal chalcogenide, M₂E₃, thin films. The precursors are extensively dissociated in solution. Flash evaporation of the precursor led to polycrystalline (by XRD), stoichiometric (EDX) M₂E₃ films, and their electrical properties have been established by Hall and van de Pauw measurements. It is possible to selectively deposit the p-type Sb₂Te₃ onto TiN over SiO₂ using [SbCl₃(TeⁿBu₂)₃], Combining [SbCl₃(TeⁿBu₂)₃] and [BiCl₃(TeⁿBu₂)₃] in various ratios allows the deposition of the ternary phase, (Sb_xBi_1-x)₂Te₃, where x varies mainly with deposition temperature.

The synthesis of the germanium tellurolate compound, [Ge(TeⁿBu)₄], has been developed by reacting GeCl₄ with ⁿBuTeLi and optimised by the alternative reaction of activated Ge with ⁿBu2Te₂. It has been demonstrated that using [Ge(TeⁿBu)₄] leads to the deposition of thin stoichiometric and polycrystalline GeTe films by LPCVD at 723 K at 0.02 mmHg. Deposition conditions were varied to reduce the co-deposition of crystalline tellurium in these films. These films were characterised by XRD, Raman spectroscopy, SEM and EDX analysis.

The distorted octahedral coordination complexes [SnCl₄{ⁿBuS(CH₂)₃SⁿBu}] and [SnCl₄(SⁿBu₂)₂] are effective precursors for the deposition of SⁿS₂ at 559 and 645 K respectively, and 0.02 mm Hg whilst SⁿSe₂ can be deposited via LPCVD between 598 K and 743 K and 0.02 mm Hg using the selenoether analogue, [SⁿCl₄(SeⁿBu₂)₂]. By optimising the conditions, it is also possible to produce polycrystalline SⁿS and SⁿSe thin films, as determined by XRD, Raman spectroscopy and EDX analysis, using the dichalcogenoether precursors, [SⁿCl₄{ⁿBuS(CH₂)₃SⁿBu}] and [SⁿCl₄{ⁿBuSe(CH₂)₃SeⁿBu}] at higher temperature (831 K and 861 K, respectively). The electrical properties of these films were also measured by Hall and van de Pauw measurements.

[SⁿCl₄{ⁿBuSe(CH₂)₃SeⁿBu}] has also been used to grow SⁿSe₂ nanowires (~100 nm) using a liquid injection CVD and vapour liquid solid (VLS) method and a silicon substrate with Au nanoparticles or a 3 nm Au film. The density and morphology of the nanowires can be controlled by the carrier gas flow rate and concentration of precursor. The nanowires produced were stoichiometric by EDX. Preliminary experiments using this method have also produced M₂Se₃ (M = Bi, Sb) nanowires from [BiCl₃(SeⁿBu₂)₃] and [SbCl₃(SeⁿBu²)₃], respectively, though further optimisation is required. Nanowires were also characterised by SEM, TEM, XRD and Raman spectroscopy.

Films of TiSe₂ can be deposited via LPCVD from [TiCl₄{ⁿBuSe(CH₂)_nSeⁿBu}] (n = 2, 3) between 723 and 873 K. XRD analysis of the films grown from ~ 5 mg of precursor showed a high degree of preferred orientation of the c axis perpendicular to the substrate. A series of hybrid amine-selenoether and telluroether ligands were prepared in an effort to tether the soft donor to the Ti(IV) ion more securely. However, this approach did not prove successful and was not pursued further.