Cubic GaN layers prepared by radio frequency plasma assisted molecular beam epitaxy on semiinsulating GaAs (100) substrate were investigated. Si doped n-type GaN films with electron concentrations varying between 10(14) and 10(19) cm(-3) were analyzed. The Si-doping was performed by varying the Si-effusion cell temperatures. In the DC-photocurrent spectra an incorporation of a Si-related shallow donor level of about E-C-35meV was found. The responsivity of the cubic GaN layers enhances with increasing Si-doping, which could be explained by a space charge model. In addition, a higher Si-doping causes an increase of both the time constant of the persistent photocurrent and the deep defect-to-band transitions in the optical admittance spectroscopy.