Electronic Structure And Energetics Of TiO₂-based Resistive Switching Materials

Resistive random access memory(RRAM)devices have been extensively applied to a wide range of fields closely related to the embedded and independent non-volatile memory,owing to the low operating voltage,good scalability and endurance.As a result,the exploration of resistive random access mechanism has become one of hot topics.With the deepening of the research on the resistance switching phenomenon of various materials at home and abroad,great progress has been made in the research of resistance switching mechanism and logic circuit.For example,the introduction of oxygen vacancies or element doping into the resistance switching material can be extremely improves its performance.In recent years,great efforts have been made to perform in-depth studies on resistance switching phenomena of various semiconducting materials Among such semiconductors,titanium dioxide(TiO₂)is one of the most classic resistive materials,In TiO₂-based resistive materials,oxygen vacancy(V_O)has been found to play a crucial role in improving the performance,and a wide range of conversion ratios can be achieved by controlling the concentration of oxygen vacancies.However,there is a notable lack in systematic investigations on electronic properties of TiO₂ with oxygen vacancy clusters,and its resistance-switching mechanism as well.Such investigations are important and necessary for fundamentally understanding the resistance switching and tailoring or optimizing the resistance-switching functionality of TiO₂-based materials.This work presents oxygen vacancies in rutile TiO₂,and their influences on the electronic properties and resistance-switching performance using the first-principles method.First,the GGA functional and its correction with the Hubbard U are utilized to address the pure TiO₂.Various parameters and properties of pure TiO₂ have been examined and compared with the experimental data.Second,TiO₂ with a single oxygen vacancy is considered using the GGA+U formalism through adjusting the selection of the Hubbard U parameter.Results show that the reasonable choice of the effective U would be 3 e V,which accounts well for the appearance of the characteristic energy level observed experimentally in the intrinsic TiO₂.Third,two,three and four oxygen vacancies are examined in various positions in TiO₂ using the GGA+U formalism with the effective U of 3 e V,and various oxygen vacancy accumulations and their influences on the electronic and resistive properties of TiO₂ have been systematically investigated.It is found that oxygen vacancies tend to accumulate in the form of V_O-Ti-V_O chain and produce multiple energy levels within the band gap of TiO₂.With the V_O-Ti-V_O chain TiO₂appears conducting,and its charge-density analysis exhibits a continuous charge distribution,thus forming a conductive path and explaining the"on"conduction mechanism.However,other V_O configurations cause the systems to be insulating with the isolated charge distributions,providing an accountable orgin of the“off”conduction mechanism.The above stated V_O configurations and associated conductivities are indicative of the resistance switching with the change of V_O concentration and distribution.The outcome of this work provides in-depth insights into resistance switching mechanism in TiO₂,having implications in broadening and optimizing the applications of TiO₂-based RRAM materials and devices.