This study investigates defect states in GaN/InP multilayer structures fabricated using plasma-enhanced atomic layer deposition (PEALD) for potential applications in high-efficiency multijunction solar cells. Deep-level transient spectroscopy (DLTS) and admittance spectroscopy were employed to characterize defects in the heterostructures. The DLTS spectra revealed a distinct peak in the temperature range of 230−300 K, corresponding to defect states with activation energies of 0.46−0.58 eV under various bias voltages (from 0 to +2 V and from −1 to 0 V). Admittance spectroscopy confirmed the presence of similar defects, demonstrating voltage-dependent activation energies in the range of 0.36−0.57 eV, which is likely associated with interface states at the GaN/InP interface.