A theoretical model of semi-shell structures formed by metal deposition is constructed for accurate prediction of plasmonic properties. Our model takes account of the influence of metal migration after deposition and the granular nature of the metal. Validity of our model was confirmed by the fact that the plasmonic resonances simulated by our model agreed well with those obtained by experiment. Simulations using the discrete dipole approximation revealed that the semi-shells formed by metal deposition show a larger absorption cross-section than semi-shells fabricated by reshaping of fully-covered core-shells. The deposition type semi-shell is suitable for the photothermal therapy because a larger temperature rise is expected due to greater absorption.