Thin films of KCl, CsCl and KBr deposited on Si(111) were investigated by means of target current spectroscopy (TCS). From the TCS initial peak position the surface potential changes during deposition were obtained. From the surface potential dependence on the alkali halide film thickness the critical island size in the initial stage of the film growth was estimated : for CsCl, 10 Angstrom; for KCl, 18 Angstrom; for KBr, 25 Angstrom. In the target current spectra of the alkali halides an intensive structure in the energy region corresponding with the bandgap was discovered. One group of peaks was induced by electron irradiation and vanished at high temperatures. This fine structure was considered to be due to an electronic transition related to the defect states in the bandgap. From the evolution of the TCS maximum intensities the defect concentration changes caused by electron irradiation were estimated.