Functional validation of novel Se and S alkyl precursors for the low temperature pyrolytic MOVPE growth of ZnSe, ZnS and ZnSSe

We report on the pyrolytic metalorganic vapour phase epitaxy of ZnSe, ZnS and ZnSSe by using diethyldisulphide [(C2H5)(2)S-2] and dimethyldiselenide [(CH3)(2)Se-2], these dialkyls being suitable for the low (<400°C) temperature growth of S- and Se-based compounds. (C2H5)(2)S-2 and (CH3)(2)Se-2 allow a substantial reduction of ZnSe and ZnS growth temperatures with respect to diethylsulphide and dimethylselenide. Mass spectrometry (MS) fragmentation products of (C2H5)(2)S-2 and (CH3)(2)Se-2 molecular ions are studied to investigate the relative strengths of specific bonds in the molecules and to identify their decomposition paths. The decomposition of (C2H5)(2)S-2 occurs mainly via the loss of ethylene molecules through beta-hydrogen elimination reactions. On the contrary, the sequential loss of methyl radicals seems the dominant path of (CH3)(2)Se-2, beta-like hydrogen elimination reactions being still possible, although less likely than for (C2H5)(2)S-2. The occurrence of a peculiar CH3. transposition reaction is also suggested for (CH3)(2)Se-2. Weak or negligible [for (C2H5)(2)S-2] contributions to the alkyl mass spectra are observed from Se-Se or S-S bond cleavage. This is ascribed to the strength of the Se-Se (or S-S) bond in the (CH3)(2)Se-2 [(C2H5)(2)S-2] molecule, which would destabilise the Se-C (S-C) bonds, leading to the alkyl low thermal stability. Low H concentrations, i.e. (1-3) x 10(17) cm(-3), are found by secondary ion MS in ZnSe samples and attributed to the low proclivity of (CH3)(2)Se-2 to decompose through beta-like hydrogen reactions.