Electrical Properties of BiFeO₃/Si Heterojunctions with SiO_x Interface Layer

As electronic technology and integrated circuits advance rapidly, the functional demands of devices are increasingly elevated. Ferroelectric/semiconductor heterostructures are significant in applications including memory, sensing, and optoelectronic devices because they combine the best qualities of both ferroelectric and semiconductor materials. In recent years, the variety and refinement of the preparation process have led to the successful direct integration of ferroelectric materials with semiconductor silicon. Nevertheless, constructing ferroelectric/semiconductor heterostructures devoid of any interface layers, particularly on silicon substrates, is exceedingly challenging. Due to thermal diffusion, there is usually a SiO_x layer at the interface, which subsequently affects device performance. However, systematic researches are urgently need regarding the effect of the interface layer on the potential barriers, ferroelectric polarization, and carrier transport. This work develops SiO_x layers of varying thicknesses and densities on the n-Si surface, followed by the fabrication of bismuth ferrite (BiFeO₃) films on the pretreated substrate. When the substrate pretreatment temperature rises, the SiO_x layer progressively thickens, thereby altering the interface barrier, polarization switching, and the transport of photogenerated carriers. This study presents a novel concept for the creation of ferroelectric/semiconductor heterostructures featuring several interface contact types and electrical transport pathways.

Oxygen was introduced into the tube furnace, which was later heated to 400–1000 ℃. Once the temperature stabilizes, the (100) oriented n-Si substrate (resistivity of approximately 10 Ω·cm) is put into the furnace. After heating for 1 h, the substrate is directly removed from the tube furnace and allowed to cool in a room temperature environment.

The raw materials Bi(NO₃)₃·5H₂O and Fe(NO₃)₃·9H₂O were dissolved in acetic acid and ethylene glycol methyl ether, respectively. Subsequently, these two solutions were mixed, stirred for 8 h, and filtered to obtain a BiFeO₃ precursor solution at a concentration of 0.1 mol/L. The precursors were spin-coated onto the pretreated n-Si (100) substrate at 3500 r/min for 30 s and heated at 180 ℃ for 5 min. The spin-coating process was done three times to attain a film thickness of approximately 60 nm, afterwards, the samples were annealed in a nitrogen environment at 380 ℃ for 15 min, followed by 550 ℃ for 50 min, with a heating rate of 10 ℃/min.

The n-Si substrate was pretreated at various temperatures in an oxygen environment. The cross-sectional high-resolution transmission electron microscopy indicates an increase in the thickness of the SiO_x layer with increasing substrate pretreatment temperature. The dielectric measurements show that when the pretreatment temperature is above 600 ℃, the activation energy of the dielectric relaxation begins to decline, signifying a decrease in the oxygen vacancy content while the transition of the oxygen vacancies from divalent to monovalent. Subsequently, BiFeO₃ films were fabricated on the pretreated substrate. X-ray diffraction studies demonstrated that all the BiFeO₃ films exhibit R3c polycrystalline structure.

After substrate pretreatment, the clamping phenomenon of the BiFeO₃/n-Si hysteresis loop disappears. As the pretreatment temperature increases, the loop becomes slimmer, and the coercive voltage decreases. The reason is that the increased thickness of the SiO_x layer hinders the carrier migration between BiFeO₃ and the n-Si substrate. At temperatures below and above 700 ℃, the SiO_x can be considered as a tunneling layer and a resistive layer, respectively. I–V curves show that the BiFeO₃/n-Si samples still exhibit rectification effects, but the cut–off voltage under negative bias and the conduction current under positive bias both decrease, which is consistent with the weakening of the interface barrier and the decrease in conductivity. Although the photocurrent and on–off ratio generally decline with the increasing pretreatment temperature, the on–off ratio of the sample on 700 ℃ pretreated substrate increases abnormally, attributed to the increased conductivity associated with the aggregation of monovalent oxygen vacancies.

BiFeO₃ films are grown on n-Si substrates pretreated at different temperatures. As the pretreatment temperature increases, the SiO_x layer on the n-Si surface became thicker and denser, accompanied by a reduction in oxygen vacancies and a transition from divalent to monovalent. The hysteresis loops are no longer clamped, with the coercive voltage deceased, and both the cut-off voltage of the I–V curves and the on–off ratio of the photocurrent reduce. The primary explanation is that the SiO_x layer reduces the pn junction barrier and the built-in electric field at the BiFeO₃/n-Si interface, facilitating the polarization switching of BiFeO₃ and the transport of photogenerated carriers. The accumulation of oxygen vacancies at a particular temperature can result in an anomalous rise in photocurrent.