MOCVD growth of gallium sulfide using di-tert-butyl gallium dithiocarbamate precursors: Formation of a metastable phase of GaS

The reaction of [(Bu-t)(2)Ga(mu-Cl)](2) with 1 molar equiv of Na(S2CNR2) yi elds the di-tert-butyl gallium dithiocarbamate compounds (Bu-t)(2)Ga(S2CNR2 ), R = Me (1), Et (3), Pr-n (5). The tert-butyl gallium bis(dithiocarbamate ) compounds (Bu-t)Ga(S2CNR2)(2), R = Me (2), Et (4), Pr-n (6), are formed a s minor products. Separation of (Bu-t)(2)Ga(S2CNR2) from (Bu-t)Ga(S2CNR2)(2 ) may be readily accomplished by sublimation of the former. Compounds 1 and 3 are low melting point solids allowing their ready use as liquid precurso rs for MOCVD. The vaporization enthalpies (Delta H-v) have been determined, by thermogravimetric methods, for compounds 1, 3, 5, (Bu-n)(2)Ga(S2CNMe2) (7), and (Bu-sec)(2)Ga(S2CNMe2) (8), and are dependent on both the identity of the substituents on gallium and the dithiocarbamate ligand. An inverse relationship is observed between the Delta H-v and the extent of branching of the gallium alkyl. Compounds 1-6 are air stable, but compounds 7 and 8 d ecompose in humid air. The molecular structures of compounds 1 and 3 have b een determined by X-ray crystallography. Gallium sulfide (GaS) thin films h ave been grown at 375-425 degrees C by atmospheric pressure metal-organic c hemical vapor deposition (AP-MOCVD) using compound 1. Characterization of t he films by wavelength dispersive spectroscopy (WDS) microprobe analysis sh ows the films to have Ga:S compositions of 1:1 with a low degree of impurit ies (C < 3%; O < 1%). Gallium-rich films were grown from compound 3 using A P-MOCVD and from compound 1 under reduced pressure. XPS studies of the GaS films additionally show an abundance of N on the surface, which has been co nfirmed to be present throughout the thin film by SIMS measurements. From X -ray diffraction (XRD) and transmission electron microscopy (TEM), the GaS films were found to be a new distorted hexagonal wurtzite phase (a = 4.590 Angstrom, c = 6.195 Angstrom).