We developed a technique for fabricating (100)-oriented NbN thin films on Si (100) wafers by dc magnetron sputtering using TiN as a buffer layer. The lattice constant of the TiN buffer layer measured using X-ray diffraction was 0.4242 nm, which is relatively close to the NbN lattice constant of 0.446 nm. Therefore, NbN on the TiN buffer layer was epitaxially grown along the (100) direction. The junctions consisted of epitaxial NbN/AlN/NbN tunnel junctions using a TiN buffer layer fabricated on single-crystal Si (100) substrates. The NbN/AlN/NbN trilayer with the TiN buffer layer exhibited a single-crystal structure with a (200) orientation without other orientations. The gap voltage and the ratio of R _sg /R _N were approximately 4.9 mV and 100, respectively, for a junctions size of 1.4 × 1.4 μm ² and J _C = 8 A/cm ² . Here, R _sg is the gap resistance at 4 mV, and R _N is the junction resistance at 10 mV. These results suggest that the NbN films using the TiN buffer technique offers significant promise for applications in several superconducting devices.