We investigate the interface properties of ZnSSe/GaAs heterovalent heterostructures, using pn heterojunction diodes. It is found that only when Ga-S bonds are formed at the interface, the current-voltage I-V characteristic exhibits a negative differential resistance (NDR). The photoresponse of the I-V characteristic suggests that the NDR originates from electron capture to an interface states band related to the Ga-S bonds. The presence of the interface states is also proved by a deep-level transient spectroscopy.