Real-time in situ optical monitoring of growth rate, refractive index, and layer thickness has been achieved for molecular beam epitaxy growth of AIN/ GaN on GaN templates on Si(111) using laser interferometry at normal incidence. The reflectance data were analyzed using the proprietary (ORS Ltd.) software package R-FIT V2.0, which is capable of real-time fitting of the data to solutions of the equations of thin-film interference, hence yielding optimized values for optical constants (refractive index n, absorption coefficient k), growth rate (and hence film thickness) and surface roughness where appropriate. Changes in optimal fitting during growth are thus able to reveal changes in parameters such as growth rate. The problem of intermittent optical access due to the necessity of using a shutter to minimize view-port deposits was solved in two ways. First data were acquired continuously, only when the shutter was open, (1 min in three), leading to widely spaced clusters of several data points. Subsequently the data acquisition was synchronized with the opening of the shutter, resulting in a shutter cycle of 1 s in six and yielding one data point per rotation. Both methods were found to result in successful fitting, with stable refractive index values in excellent agreement with the literature. Furthermore, both methods were able to detect changes in growth rate during deposition, whether such changes were due to initial establishment of steady growth or to deliberate changes in the temperature of the Ga Knudsen cell. This demonstrates the value of this technique for process monitoring as it can potentially yield system diagnostic information (e.g., evidence of flux drift) as well as data on materials and growth parameters. (c) 2007 American Vacuum Society.