Gallium monochloride GaCl is not existing as stable compound, but is formed in situ from GaMe3 vapor and HCl. GaCl is volatile and easily transported to the the CVD reaction zone (or formed directly in this zone)
GaCl for GaN ALD
Growth mechanism in GaCl-based atomic layer epitaxy (ALE) was is studied using the temperature programmed desorption method (TPD), the surface photo-absorption (SPA) method and ab initio molecular orbital calculations. From the TPD measurements, the GaCl adsorption energy was calculated to be 32 kcal mol-1 for Ga-terminated and 38 kcal mol-1 for As-terminated surface. In the absence of H2, the Ga-Cl bond appears to be very stable in the adsorption state. Process simulation using a simple cluster model of GaCl(AsH2)2 indicated that adsorption was completed by forming σ and π covalent bonds, and that there is practically no self-dissociation of the Ga-Cl bond. From SPA measurements in the ALE system with ambient H2, a strong similarity was found between the spectra of the GaCl-supplying surface and the Ga-covered surface. Based on these results, a model of the chemical process in chloride ALE was proposed where adsorbed GaCl molecules can react immediately with hydrogen and Cl is released from the surface as HCl. A self-limiting mechanism results from the very short residence time of GaCl (~10-3 s at 450 °C) on the Ga-terminated surface.[398]
Gallium monochloride GaCl, formed directly in the CVD reaction zone from GaMe3 vapor and HCl, was used for LT-MOCVD/ LT-HVPE growth of single crystal GaN films at 560-950°C on Si(111) substrates coated with ultra-thin (less than 2 nm) oxynitride layer. Use of thicker oxynitride layer was detrimental to GaN epitaxy due to loss of the epitaxial relationship. [397]
GaCl formed in situ is used as GaN precursor in the H-MOVPE which is a growth method combining hydride vapor phase epitaxy with MOCVD, where MO precursors (f.e. GaMe3) reacts with HCl to supply volatile GaCl, which then reacts with NH3 downstream to form GaN).[[i]]
[i] O. Kryliouk, M. Reed, T. Dann, T. J. Anderson, and B. Chai, Mater. Sci. Eng. B 66, 26 (1999).
Gallium trichloride GaCl3 has been widely used for CVD and ALD of various gallium containing layers, especially GaN (in combination with NH3); the growth requires high temperatures (>800°C) to decompose NH3. [[i]]
[i] H.P. Maruska, J.J. Tietjen, J. Solid State Commun., 1969, 15, p.327
GaCl3 for GaN CVD
The chemical vapor deposition of GaN from the GaCl3 + NH3 system has been studied theoretically by quantum chemical calculations at the B3P86/6-311G(d,p) level The investigated molecules included the monomer (ClxGaNHx, x = 1−3) and oligomer species (Cl2GaNH2)n with n = 1−3 and (ClGaNH)n with n = 1−4 as well as the respective chain dimers and trimers. The calculations revealed the importance of intramolecular Cl···H hydrogen bonding and dipole−dipole interactions in determining the conformational properties of the larger species. Except for the ClGaNH monomer, the Ga−N bonding has a single bond character with a strong ionic contribution. The thermodynamic study of the composition of the gaseous phase supported the predominance of the Cl3GaNH3 complex under equilibrium conditions. The calculated Gibbs free energies of various GaCl3 + NH3 reactions imply the favored formation of “saturated” chain and cyclic oligomers below 1000 K.[[i]]
[i] Attila Kovács, Inorg. Chem., 2002, 41 (12), pp 3067–3075, DOI: 10.1021/ic011140s
GaCl3 for GaN ALD
GaN has been grown by ALD with GaCl3 and NH3 in wide temperature window 500–750°C on Si(100) substrates with GR 2.0 Å/ cycle, consistent with self-limiting adsorption. Both 0002 and 10¯11 orientations of GaN were evident as a result of competition between vertical and lateral growths. Long GaCl3 exposure times resulted in Cl incorporation according to XPS.[403]
GaCl3 has been applied for preaparation of highly purity and high crystalline quality Ga-doped epitaxial Si layers. By CVD. In contrast to trimethyl gallium, metal halide doping source does not cause carbon contamination of the epitaxial layer. Abrupt layer-to-layer transition regions have been demonstrated. Electrical and crystalline characteristics the layers was studied; achieved doping limit of 4·1017 cm-3 was well below the solubility limit, indicating unusual surface chemistry involved in doping. [406]
Gallium tribromide GaBr3 has been proposed as a viable CVD source for preparation of Ga – containing layers. [406]
Gallium tribromide tetrakis(ammonia) adduct GaBr3(NH3)4 has been used as CVD precursor for preparation of polycrystalline GaN layers on Si(111). Growth rate dependence on the deposition temperature, source evaporation temperature, gas flow rate was studied. The complex undergoes pyrolysis on the substrate at temperatures below 500°C.
Gallium (I) iodide GaI formed in situ by reaction of Ga with I2 vapor, has been used as precursor (with NH3 as co-reactant) for preparation of GaN films by CVD at 1000-1100°C with high growth rate of 5-40 µm/h on sapphire with MOCVD template GaN layer substrates. Films with mobility reaching 660 cm2/Vs with carrier concentrations in the range of n = 5 × 1016 to 3 × 1017 cm-3 were grown. [408]
Gallium triiodide GaI3 has been applied as precursor for the growth of metallic Ga thin films by low-pressure photochemical vapour deposition of gallium thin films; influence of growth temperature, precursor partial pressure and illumination intensity was studied. Deposition took place through the photodissociation of adsorbed GaI molecules derived from the thermal decomposition of the triiodide. Negligible dark deposition rates at 400-600°C indicate that the reactor was oxygen-free. The resulting gallium films, which oxidized ex situ on exposure to the atmosphere, were characterised by ellipsometry, profilometry measurements of thickness, and EDX analysis for film composition.
The attempted growth of GaAs by introducing molecular As or AsI3 into the system indicated negligible incorporation of arsenic into the films, even at high arsenic partial pressures, consistent with the presence of a stable GaI monolayer covering the gallium surface: which inhibits the formation of GaAs. [409]