This paper presents an experimental study of the first stages of the growth of a semiconductor (Ge) deposited at room temperature on an oriented metallic surface (Ag(001)). It is achieved by using scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and photoelectron spectroscopy (PES). This work shows particular stable entities of four Ge atoms (tetramers) which are observed as early as the first stages of growth (0.1 ML) in coexistence with isolated Ge atoms inserted in substitutional sites of the Ag top layer. These tetramers order in a p(2 root 2 x 4 root 2)R45 degrees superstructure at half monolayer (0.5 Ge ML). Beyond this coverage, the adlayer disorders. The PES study reveals the metallic character of the Ge adlayer at least up to 0.5 ML.