MOCVD Growth of Gallium Sulfide Using Di-<i>tert</i>-butyl Gallium Dithiocarbamate Precursors:  Formation of a Metastable Phase of GaS

Abstract

The reaction of [(tBu)2Ga(μ-Cl)]2 with 1 molar equiv of Na(S2CNR2) yields the di-tert-butyl gallium dithiocarbamate compounds (tBu)2Ga(S2CNR2), R = Me (1), Et (3), nPr (5). The tert-butyl gallium bis(dithiocarbamate) compounds (tBu)Ga(S2CNR2)2, R = Me (2), Et (4), nPr (6), are formed as minor products. Separation of (tBu)2Ga(S2CNR2) from (tBu)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 precursors for MOCVD. The vaporization enthalpies (ΔHv) have been determined, by thermogravimetric methods, for compounds 1, 3, 5, (nBu)2Ga(S2CNMe2) (7), and (secBu)2Ga(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 ΔHv and the extent of branching of the gallium alkyl. Compounds 1−6 are air stable, but compounds 7 and 8 decompose in humid air. The molecular structures of compounds 1 and 3 have been determined by X-ray crystallography. Gallium sulfide (GaS) thin films have been grown at 375−425 °C by atmospheric pressure metal−organic chemical vapor deposition (AP−MOCVD) using compound 1. Characterization of the films by wavelength dispersive spectroscopy (WDS) microprobe analysis shows the films to have Ga:S compositions of 1:1 with a low degree of impurities (C < 3%; O < 1%). Gallium-rich films were grown from compound 3 using AP−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 confirmed 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 Å, c = 6.195 Å).