Methods for characterizing the growth regime of AlN are presented to deal with the unresolved critical issue of in situ growth regime determination during molecular beam epitaxy. Previously, there were no known reports of in situ measurements to determine the growth regime of AIN, making reproducibility difficult. Reflection high energy electron diffraction (RHEED) intensity transitions were observed upon opening and closing Al effusion cells, and three key signatures were identified. First, the time constant of the falling RHEED specular spot intensity upon Al shutter opening was found to vary inversely with the Al flux, providing a means of in situ calibration of Al flux. Second, a RHEED intensity spike or oscillation feature was observed at the onset of Al flux. Third, the behavior of the RHEED intensity rise upon Al shutter closing can be used to identify the Al-rich intermediate and Al-rich droplet regimes. These techniques and observations provide in situ methods of determining the growth regime of AIN, improving reproducibility and control. (c) 2006 American Vacuum Society.