Influence of resonant and non-resonant vibrational excitation of ammonia molecules in gallium nitride synthesis

Attempts on the selective promotion of gallium nitride (GaN) growth were investigated by deploying laser-assisted vibrational excitation of reactant molecules, which deposits energy selectively into specific molecules and activate the molecules towards the selected reaction pathways. Laser-assisted metal organic chemical vapor deposition (LMOCVD) of GaN was studied using a wavelength-tunable CO₂ laser. The NH-wagging modes (υ₂) of ammonia (NH₃) precursor molecules are strongly infrared active and perfectly match the emission line of the CO₂ laser at 9.219, 10.350, and 10.719 µm. On-and off-resonance excitations of molecules were performed via tuning the incident laser wavelengths at on-resonant wavelength 9.219 µm and off-resonant wavelength of 9.201 µm. The on-resonant vibrational excitation allowed a largest fraction of the absorbed laser energy coupled directly into NH₃ molecules whereas energy coupling under off-resonant excitations is less efficient in energy coupling and influencing the GaN growth process. The GaN deposition rate was enhanced by a factor of 2.6 accompanied with an improvement of crystalline quality under the on-resonant excitation. Optical emission spectroscopic (OES) studies confirmed that the on-resonant vibrational excitation effectively promotes the dissociation of NH₃ molecules and creates N-containing species favoring the GaN growth. This study indicates that the resonant vibrational excitation is an efficient route coupling energy into the reactant molecules to surmount the chemical reaction barrier and steering the growth process.