Investigation Of The Realization And Physical Mechanism For The A-SiN_x:H-based Ultra-low Power And Forming-free Resistive Random Access Memory

With the down-scaling of device size and the increasing device density,traditional semiconductor memories including DRAM and Flash are expected to run into their physical limits in the near future.As a novel new storage technology,the resistive random access memories(RRAMs)are attracting more and more attention due to many natural advantages,such as simple structure,excellent scalability,fast speed(<5 ns),low power(?pJ),high on/off ratio(?109),good endurance(?1012)and long retention time(>10 years).The principle of RRAM is based on switching between different device resistance states,and it will not face the charge leakage problem which exsits in Flash and DRAM.Currently,the scope of resistive switching material was mainly limited to transition metal oxide.However,from the long-term point of view,silicon-based RRAMs have more great application potential due to their mature fabrication process,low-cost and full compatibility with traditional CMOS technology.As a result,it is very significant and valuable to investigate and develop the Si-based RRAM devices.In spite of these advantages,there are still some problems for RRAM application.Firstly,the physical mechanism of resistive switching is not very clear until now,which may bring some troubles when trying to design and improve the device structure.Secondarily,the normal programming currents of RRAMs(>10?A)are still much higher than those required for Flash memory(<10nA),and thus reduction of the device current and power consumption is an important issue for RRAM application.It is also pointed out that the metal wire resistance will reach several K? in sub-10 nm design rule.As a result,if the memory cell resistance is not sufficiently higher than this,most of the applied voltage just dissipates along the metal wires and resistive switching is not possible.From this point of view,ultra-low power RRAM device with high resistance of both on and off states is also necessary.In addition,another bottleneck for RRAM application is their high variability and insufficient level of uniformity.Due to the randomness of conduction paths,fluctuation of the device current between different cycles and different cells is prominent.Therefore,reducing the variability and improving the uniformity of RRAM devices is also very important.Considering all these issues in RRAM research field,this article aims to obtain an ultra-low power Si-based RRAM device with high performance,and make deep investigation on its internal mechanism.In this article,hydrogenated silicon nitride(a-SiNx:H)films were introduced to realize the ultra-low power Si-based RRAM device.The resistive switching characteristics,performance improvement and switching mechanism were systematically researched and discussed.The content of this article can be divided into four parts as follows:In the first part,we show that a high performance and ultra-low power RRAM device based on an Al/a-SiNx:H/p+-Si structure can be achieved by tuning the Si dangling bond conduction paths.We reveal the intrinsic relationship between the Si dangling bonds and the N/Si ratio x for the a-SiNx:H films,which ensures that the programming current can be reduced to less than 1 ?A by increasing the value of x to 1.17.Meanwhile the low set voltage(-2.5 V)and reset voltage(?-1.6 V)confirm that an ultra-low power resistive switching is realized in the Al/a-SiN1.17:H/p+-Si structure.These parameters here are obviously lower than those of other RRAMs reported.We found that the field-enhanced thermal breakage of weak Si-H bonds and migration of the H+ions result in the generation and re-passivation of Si dangling bonds in a-SiNx:H films.Consequently,tunable Si dangling bond conduction paths can be obtained by varying the N/Si ratio.In the second part,the improvement of uniformity for the ultra-low power Al/a-SiN1.17:H/p+-Si device is achieved through dehydrogenation treatment.The device composed of pristine a-SiN1.17:H film shows obvious current fluctuation between different switching cycles.Nevertheless,when the device was treated by 400? annealing for 10 minutes in vacuum,it exhibits extremely high uniformity and reliability,and the current fluctuation is decreased obviously.The switching operation can be successfully repeated for more than 1000 cycles and the on/off ratio increases to 100.It is supposed that the dehydrogenation treatment produce some pre-existing Si dangling bonds,where the electric field and current is enhanced.Therefore,the conduction paths are more likely to be formed around these regions,which decreases the randomness of conduction paths.In the third part,for the first time we reveal the intrinsic relationship between the dangling bond evolution and the transient current formed by discharging of trap centers in a-SiNx:H-based RRAM devices in nanoscale.We found that the in-situ transient current intensity variation at room temperature is consistent with the evolution of the intrinsic trap center and Si dangling bond in a-SiN1.17:H-based RRAM devices.It is discovered that both the intrinsic trap centers and Si dangling bonds make contribution to the transient current at room temperature.The transition of the transient current curve from e-t to 1/t dependence further reveals the dangling bond evolution when the device switches from IRS to LRS at low temperature.Our introduction of transient current provides a high effective way to insight the dynamic evolution of dangling bond during resistive switching operation,which illuminates the conduction path formation in a-SiN1.17:H-based RRAM.In the fourth part,It is demonstrated that an electroforming-free unipolar resistive switching effects can be realized in the annealed Si-rich SixN/SiyN multilayers with high on/off ratio of 104.The microstructure and nc-Si formation were firstly investigated.We found that,during annealing Si atoms migrate from SiyN sublayers with low Si concentration to the SixN sublayer with high Si concentration.The Si atom migration is ascribed to the Oswald ripening between SixN and SiyN sublayers during nc-Si clusters growth.Finally,nanocrystalline silicon(nc-Si)clusters with different sizes are formed in every SixN and SiyN sublayer.We demonstrate that the segregated multilayered nc-Si structure can increase the conductivity of intial device and facilitate the forming of final continuous conduction path,which eventually results in the electroforming-free unipolar resistive switching effects.