A Study On Resistance Switching Mechanism Of Alkali Metal And Alkali Earth Metal Doped Hafnium Oxide Memristor

In the information era,the massive amounts of data poses new challenges to the efficient storage,operation and transmission of data.At present,information storage technology is faced with two major challenges:high density integration and unbalanced development of data processing and transmission speed.As an emerging non-volatile memory device,memristor has many advantages,such as low power consumption,fast erasable speed,good scalability and three-dimensional integration.Thus,memristor has attracted much attention in the industry.In addition,the memristor can also realize neuromorphic computing and perform high-speed,parallel operations on data.Among the diverse materials used to memristors,hafnium oxide（HfO₂）possesses stable-controllable material properties,abundant physical effects and compatiblity with complementary metal-oxide-semiconductor process,which makes HfO₂ a hot spot.Moreover,HfO₂ based memristor is an excellent candidate for neuromorphic computation.However,the consistency,stability,reliability and conductance modulation of HfO₂ memristors cannot satisfy the requirements of large-scale integrated neuromorphic computing circuits.A variety of schemes have been proposed to optimize the performance of the HfO₂ based memristor,among which doping has been proved to be a simple and effective way to improve the performance of the memristor.However,due to the lack of understanding on the microscopic resistance switching mechanism in the memristor,there is no theoretical guidance on the selection of doping elements.On the other hand,the Li-doped memristor shows good conductance modulation characteristics,and the Li ion migration is highly reversible with low migration barrier,which can well simulate the neuronal information transmission process.However,the systematic researchs on the resistance switching mechanism of Li-doped devices and the doping characteristics of Li’s homolog elements are also lacking.Therefore,in order to deeply explore the microscopic resistance switching mechanism and the influence of alkali metal and alkali earth metal doping on the performance of the HfO₂based memristor,the in-depth theoretical and experimental reseachs have been carried out.The main research contents and innovative achievements are as follows:At first,through the first-principles calculation,the formation,migration and aggregation characteristics of oxygen vacancies（Vos）in HfO₂ were studied systematically.The results show that the low migration barrier of Vo at all directions leads to the poor stability and retention characteristics of the HfO₂ based device.Then,the crystal structures and topological relationship of HfO_x under different oxygen content are analyzed by establishing the structure models of HfO_x with full ratio range,and the conductivity characteristics of these structures are also analyzed.According to the results,monoclinic HfO_x and tetragonal HfO_x structures can achieve continuous conductivity modulation,but it is difficult to maintain their structures,and tends to collapse to hexagonal Hf₆O structure.Once the hexagonal Hf₆O structure is formed,the resistance switching（RS）of HfO₂ based memristor exhibits obvious binary characteristics.The theoretical basis is established for the abrupt and gradual resistance switching mechanisms in HfO₂ memristors.Then,the alkali metal and alkaline earth metal elements doping characteristics were studied though first-principles calculation.The results show that these elements tend to form interstitial doping and can migrate under applied voltages.But these elements don’t under go redox reaction to form metal CFs,the resistance switching behavior still depends on Vo migration.On this basis,this paper further studies that the effects of alkali metal and alkali earth metal elements on Vo migration and conductivity of CFs.The results show that there may be synergistic migration between doped elements and Vo,which increases the migration barriers of oxygen vacancies.It is beneficial to maintain the morphology of conductive filaments and improve the stability of memristors.At the same time,doping can inhibit the redox reaction and the aggregation of oxygen vacancies,which prevents the collapse of conductive filaments to h-Hf₆O,thus improving the conductivity modulation performance and linearity of the memristors.Finally,the Li-doped,Mg-doped and Ca-doped HfO₂ memristors were prepared by magnetron sputtering,and the resistance switching performances of the three memristors were tested.Due to low migration barrier of Li,it is easy to diffuse by thermal effect.And it has weak synergistic effect with Vo.Therefore,the improvement of the resistance switching performance in Li-doped HfO₂ memristor is not obvious.Ca has a high migration barrier and a strong binding effect on O atoms,which results in high operating voltage and mediocre resistance switching performances.The Mg-doped memristor shows excellent uniformity,durability,retention and conductive modulation.The reason is the strong synergistic effect between Mg and Vo.Mg can not only effectively constrain the Vo migration to improve the stability of CFs,but also adjust the conductance of CFs to obtain more intermediate resistance states.Thus Mg-doped improve the linearity and adjustable range of conductive modulation.