ORGANOMETALLIC PRECURSORS TO THE FORMATION OF GAN BY MOCVD - STRUCTURAL CHARACTERIZATION OF ME3GA.NH3 BY GAS-PHASE ELECTRON-DIFFRACTION

The molecular structure of Me3Ga.NH3 [I] has been studied by gas-phase electron diffraction at 25-degrees-C. The experimental data are fitted by a model in which the C3-Ga-N core of the molecule has C3v symmetry. The molecule was defined in terms of four bond distances, three valence angles and two torsion angles. Of the bond distances three were refined (r(g)(Ga-N) = 2.161(22) angstrom, r(g)(Ga-C) = 1.979(3) angstrom, r(g)(C-H) = 1.109(7) angstrom). It was necessary to hold the fourth bond distance at an assumed value [r(g)(N-H)= 1.045 angstrom]. Two of the valence angles were refined (N-Ga-C = 101.8(62)-degrees, Ga-C-H = 111.3(16)-degrees) with the third (Ga-N-H) being held at 109.0-degrees. The torsion angle H-N/Ga-C was held at 60.0-degrees while the remaining torsion angle H-C/Ga-N was refined to 37.5(224)-degrees. The dependent angle C-Ga-C was 115.9(42)-degrees, so the C3Ga fragment is not far from planar, which is in accord with the lone pair from the nitrogen atom being donated into the p(z) orbital on the gallium atom. This suggestion is supported by the gas-phase and low temperature infra-red spectroscopic data that are reported. Evidence is also presented suggesting the Ga-N bond is weak and thus it is not surprising that when NH3 and Me3Ga are used to grow GaN it is necessary to use NH3/Me3Ga ratios greater than one.