A method of passivating germanium that comprises providing a germanium material and carburizing the germanium material to form a germanium carbide layer. The germanium carbide layer may be formed by microwave plasma-enhanced chemical vapor deposition by exposing the germanium material to a microwave-generated plasma that is formed from a carbon-containing source gas and hydrogen. The source gas may be a carbon-containing gas selected from the group consisting of ethylene, acetylene, ethanol, a hydrocarbon gas having from one to ten carbon atoms per molecule, and mixtures thereof. The resulting germanium carbide layer may be amorphous and hydrogenated. The germanium material may be carburized without forming a distinct boundary at an interface between the germanium material and the germanium carbide layer. An intermediate semiconductor device structure and a semiconductor device structure, each of which comprises the germanium carbide layer, are also disclosed.