We investigate reactively sputtered films of zirconium nitride, ZrN, for use as highly reflecting back contacts in Cu(In,Ga)Se-2 (CIGS) devices with sub-micrometer absorbers. We identify the nitrogen flow and the sputter current as the decisive parameters for the composition, and demonstrate a method for determining the nitrogen flow at which the transition from metallic to compound sputtering mode occurs for a given current. Films prepared at this working point consist of stoichiometric ZrN with a low resistivity, a high reflectance for red and infrared light, and have a fairly high sputter rate. Calculations show that the reflectance at the ZrN/CIGS interface is significantly superior to that at the standard Mo/CIGS interface. (C) 2009 Elsevier B.V. All rights reserved.