Effect of Nickel Diffusion on Trap States and Interface Quality in Polycrystalline Silicon Structures

Abstract

This work presents a comprehensive Deep-Level Transient Spectroscopy (DLTS) investigation into the influence of nickel (Ni) diffusion on the defect landscape and electronic properties of polycrystalline silicon (poly-Si) structures. The study aims to clarify how Ni incorporation modifies electrically active traps, alters charge carrier dynamics, and affects interface quality in Schottky diodes formed on poly-Si substrates. Two types of samples—undoped and Ni-diffused—were prepared via controlled thermal processing at 1000 °C, followed by surface passivation and gold/aluminum metallization to form Au/Poly-Si/Al Schottky diodes. DLTS measurements over the temperature range 20–300 K revealed distinct differences in the deep-level trap behavior of the two sample types. In undoped samples, only weak and broad trap signals were observed, primarily associated with intrinsic grain boundary defects and residual impurities. In contrast, Ni-diffused samples exhibited sharp, intense DLTS peaks, with a dominant trap level observed between 200 and 220 K. The corresponding activation energy was estimated to be approximately 0.492 eV, and the capture cross-section was in the range of 10⁻¹⁴–10⁻¹³ cm². These parameters indicate the formation of nickel-related complex defects, such as Ni–V or Ni–O clusters, primarily located at grain boundaries. C–V profiling further confirmed the influence of Ni incorporation, showing reduced capacitance, a smoother transition in the depletion region, and improved interface uniformity, suggesting partial passivation of native and boundary-related traps. The Ni-diffused sample displayed a smoother capacitance–voltage transition, reduced junction capacitance, and improved interface uniformity, suggesting partial passivation of native defects. Complementary Gp–V measurements showed a significant decrease in parallel conductance for Ni-doped structures, indicating a reduction in interface trap density and recombination centers. These results suggest a dual role of nickel—both as a source of deep-level traps and as a passivating agent, depending on local atomic environment and thermal treatment conditions. Surface morphology analysis using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) confirmed the formation of Ni-rich precipitates, particularly at grain boundaries. The spatial correlation between Ni and oxygen suggests the formation of Ni–O-based complexes, which likely contribute to the electrical passivation effects observed in DLTS and G_p–V data. Overall, this study demonstrates that controlled Ni diffusion offers a promising approach for defect engineering in polycrystalline semiconductors. By selectively introducing and passivating defect states, Ni doping can enhance the electronic quality and thermal stability of poly-Si, thereby improving its suitability for high-efficiency solar cells, radiation detectors, and other advanced electronic and optoelectronic devices.