Gallium trihydride dimer (digallane) Ga2H6 and gallane adducts GaH3(X)
Digallane Ga2H6 was first synthesized in 1989 by reduction of (H2Ga(μ-Cl))2 with solvent-free LiGaH4 at -23°C; digallane is volatile and condenses at −50 °C to give a white solid.
Pure digallane decomposes at ambient temperatures to gallium and hydrogen, therefore it isunsuitable for CVD applications. However, it can be stabilized by adduct formation with amines, phosphines and other ligands. With trimethylamine it forms both 1:1 and 2:1 adducts, i.e. Me3N·GaH3 and (Me3N)2·GaH3, respectively, however 2:1 adduct dissociates to 1:1 adduct and amine at -63°C/1Torr or 0°C/378Torr.[[i],[ii]] The adducts do not contain gallium-carbon bonds and therefore, Ga-containing layers (f.e GaAs) with low carbon backgrounds can be grown.
[i] N. N. Greenwood, A. Storr, M. G. H. Wallbridge (1963). Inorg. Chem. 2 (5): 1036–1039.
[ii] D. F. Shriver, C. E. Nordman, Inorg. Chem., 1963, 2 (6): 1298–1300
Trimethylamine gallane (TMAG) GaH3·NMe3 has been used for the hot-wall atmospheric pressure chemical vapor deposition (CVD) of polycrystalline GaAs thin films (with arsine as co-reactant, growth rates 1-4 µm/h for smooth films) as well as cubic GaN (with ammonia as co-reactant). In a low pressure CVD reactor, elemental arsenic vapor was also found to react with the TMAG to give GaAs thin films. [420] However, it is not lastingly stable at ambient temperatures: dissociative adsorption prevents it from being a useful source of gallium bearing films [[i]]
The structure of the gallane adduct Me3N·GaH3 has been investigated by ab initio quantum chemical calculations, single-crystal XRD (one of the first gallium hydrides to be characterised structurally), electron diffraction and microwave spectroscopy in the gas phase. The results of gas-phase electron-diffraction (GED) measurements, together with earlier microwave measurements, have been reanalysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rao structure) were found to be: r(Ga–H) 151.1(13), r(Ga–N) 213.4(4), r(N–C) 147.6(3), r(C–H) 108.4(4) pm; H–Ga–N 99.3(8) and Ga–N–C 108.8(2)8. Unlike the corresponding alane derivative, the adduct is monomeric in the crystalline phase with dimensions very close to those of the gaseous molecule, as confirmed by a redetermination of the structure of a single crystal at 150 K. [415]
[i] A. A. Melas, US Pat., 4 740 606, 1988 (Chem.Abstr., 1988, 109, 121598d); K. W. Butz. F. M. Elms, C. L. Raston,R. N. Lamb and P. J. Pigram, Inorg. Chem., 1993, 32, 3985; J. S.Foord, T. J. Whitaker, E. N. Downing, D. O’Hare and A. C. Jones,Appl. Phys. Lett., 1993, 63, 1270; J. S. Foord, A. T. S. Wee, N. K. Singh, T. J. Whitaker and D. O’Hare, Mater. Res. Soc. Symp. Proc., 1993, 282, 27; J. S. Foord, T. J. Whitaker, D. O’Hare and A. C. Jones, J. Cryst. Growth, 1994, 136, 127; H. Yokoyama and M. Shinohara,
Jpn. Kokai Tokkyo Koho, JP 07 226 380, 1995 (Chem. Abstr., 1995, 123, 356749g).
Dimethylamine gallane (DMAG) GaH3·HNMe2 was found to be promising precursor for low-temperature growth of high-purity (low-carbon incorporation) GaAs AlGaAs layers. According to surface photo-absorption (SPA), DMAG starts to decompose at about 150°C on an As-stabilized surface: much lower than the decomposition onsets of trialkyl Al and Ga compounds. Low temperature PL spectra exhibit dominant excitonic emissions for GaAs layers grown by DMAG at substrate temperatures above 400°C, indicating that carbon incorporation and the crystal quality deterioration due to incomplete decomposition on surface is much suppressed by using DMAG.[348]
High purity, high quality, epitaxial n-type GaAs films were grown with DMAG by by low-pressure MOCVD. GaAs layers had background donor concentrations of 5x1014 cm-3, room temperature mobilities of 7840 cm2/V·s, and liquid nitrogen mobilities of 60,000 cm2/V·s. Photoluminiscence spectra at 4.2 K showed strong narrow exciton peaks. The results were compared with those obtained using other gallium sources (GaMe3 and GaiBu3). [424]