Zinc oxide thin films grown by atomic layer deposition have been subject to great attention over the past few years. In this work, we study ZnO films grown on Si substrates by atomic layer deposition with di-ethyl zinc (DEZ) as metal precursor and H2O or D2O water vapour as oxidant. Film composition as a function of growth temperature is studied by Ion Beam Analysis (IBA) using Rutherford Backscattering Spectrometry (RBS) to determine Zn areal density; Nuclear Reaction Analysis (NRA) for C and O areal density, and Elastic Recoil Detection Analysis (ERDA) for hydrogen areal density. The Zn/O ratio is close to 1 within a broad ALD window consistent with previous studies, however the carbon content varies substantially within this window and is minimum only within a much narrower temperature range.