The authors use real-time spectroscopic polarimetric observations of growth, and a chemical model derived therefrom, to develop a method of growing dense, two-dimensional zinc oxide epitaxially on sapphire by metal organic chemical vapor deposition. Particulate zinc oxide formed in the gas phase is used to advantage as the deposition source. Their real-time data provide unequivocal evidence that a seed layer is required, unwanted fractions of ZnO are deposited, but these fractions can be removed by cycling between brief periods of net deposition and etching. The transition between deposition and etching occurs with zinc precursor concentrations that only differ by 13%. These processes are understood by considering the chemistry involved. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3305814]