A monitoring method based on ultraviolet absorption spectroscopy (UVAS) coupled with a multivariate analysis technique was developed to measure the partial pressure of several metalorganic reactant species and the hydride commonly used for III-V nitride growth. Trimethylaluminum, triethylgallium, dimethylethylamine alane and ammonia are all shown to absorb strongly in the UV with highly overlapping spectra. It was found that a single absorption spectrum can be decomposed to obtain accurate predictions of pure component partial pressures in a multicomponent stream. This method was used to monitor and control the gas-phase concentrations for the multicomponent mixture of triethylgallium and ammonia diluted in nitrogen, which is used to grow GaN thin films via MOCVD. There was a 5% average error for the predicted partial pressure of TEGa in the range 245-700 mTorr and a 2% average error for the predicted partial pressure of NH3in the range 15-35 Torr. A detection limit of 10 mTorr and 1 Torr were measured for TEGa and NH3, respectively. For simple feedback control and using this method as a measurement device, settling times of 1 min for TEGa and 30 s for NH3to reach within 2% of the desired partial pressure have been obtained. Control tests included both setpoint tracking and gas-phase disturbance rejection. These settling times were found to be no greater than 1/40 of the total growth time for a commonly grown GaN thin film.
Johnson, M. C., Poochinda, K., Ricker, N. L., Rogers, J. W., & Pearsall, T. P. (2000). In situ monitoring and control of multicomponent gas-phase streams for growth of GaN via MOCVD. Journal of Crystal Growth, 212(1), 11–20. https://doi.org/10.1016/S0022-0248(00)00240-2