In this paper, we report for the first time on the use of IR reflectance spectroscopy to study the crystallinity of GaN films grown on Si substrate at various growth temperatures (i.e. from 50 degrees C to 1000 degrees C) by metalorganic chemical vapor deposition (MOCVD) and electron cyclotron resonance (ECR) plasma-assisted MOCVD. All results show that the structure type of the GaN deposited films is sensitive to the growth temperature. In addition, we also show that the crystalline structure of GaN films can be correlated to the variations of the IR reststrahlen band. Finally, we compared our results to the X-ray diffraction (XRD) results (i.e. taken from our earlier works); we found that both results are in good agreement. Hence, we suggest that IR reflectance spectroscopic can be used as an alternative technique to determine the crystallinity of the GaN deposited films as well as the IR optical properties. (c) 2004 Elsevier B.V. All rights reserved.