Motivated by the need for a passivation layer for Ge, thin (< 10 nm) films of amorphous boron carbonitride (BCN) were deposited on Ge(100) surfaces to study film continuity and interface bonding. Chemical vapor deposition on Ge(100) and Si(100) (studied for comparison) produced BC0.75N0.07 and BCN0.07 stoichiometry films, respectively. When BCN is nominally 2.5-5 nm thick, the film's coverage of Ge(100) is continuous. This range was determined by (i) 0.5 kV He+ ion scattering spectroscopy scans, in which the underlying Ge(100) substrate signal disappears upon continuous BCN coverage and (ii) the convergence to unity of the ratio of BCN thicknesses estimated by two angle resolved x-ray photoelectron spectroscopy (ARXPS) techniques. Only 1.5-2 nm BCN was required for continuous coverage of Si(100), possibly due to better nucleation on this surface. ARXPS analysis indicates that the BCN-Ge(100) interface is clean and abrupt, with no evidence of an interfacial layer due to elemental intermixing. Constituent bonding in the BCN bulk is maintained at the Ge(100) interface. Preservation of the Ge 3d and Ge 2p full width at half maximum values after BCN deposition suggests a lack of distortion of Ge surface bonds, a criterion necessary for passivation if low interface state densities are to be achieved. BCN-Si(100) interface characteristics were similar to BCN-Ge(100).