肖特基势垒和氧空位在钙钛矿类材料阻变中的作用

Si-based electronic devices are approaching the ultimate physical limit, therefore, one has to develop memories based on new principles and materials；in this aspect, ionic devices based on ion transport is a good alternative. Resistance random access memory (RRAM), on the basis of resistive switching of ion-conducting oxides, is one of the very promising candidates for the next generation memories. Compared with electronic memories, ionic ones have following features: nonvolatile, high storage density, low writing/erasing energy, fast programming speed, and good compatibilty with modern CMOS technology. In various oxides, the phenomenon of resistive switching has been extensively studied, and quite diverse switching mechanisms have been proposed, based on thermal, electronic, and/or electrochemical effects.It is quite often that the switching behavior strongly depends on the material system. Therefore, only the SrTiO3 system is addressed in this work. Among perovskite oxides, SrTiO3 of the entire doping spectrum, i.e. from acceptor to donor doped SrTiO3, has been investigated.In doped SrTiO3,oxygen vacancies are the predominant defects. And the commonly used electrode materials, e.g. Pt, Au and Ag, usually form the Schottky barrier on SrTiO3. Previous works (including my own) indicate that the Schottky barrier and oxygen vacancies are the two key issues in the resistive switching of SrTiO3. Most works on switching mechanism were done on thin films. However, thin films have two disadvantages: (1) the properties are not homogeneous over the entire area of a film; it is quite often that different areas of a film show different switching behaviors. (2) The defect physics and chemistry of films with thickness on nano-scale is rather unknown. Thin films with nano-meter thicknesses have quite different defect structures, which significantly complicates the situation. On the contrary, the defect physics and chemistry of single crystalline SrTiO3 have been well- established, and the roles of the Schottky barrier and oxygen vacancies in various physical and chemical processes have been thoroughly investigated. To take these advantages, SrTiO3 single crystals are investigated in this work. Although the previous works on SrTiO3 were mostly done at room temperature, the sample temperature during electroforming was actually discovered to be high,because the electrical power dissipation can lead to a pronounced thermal effect in the sample. Therefore, the electroforming in this work will be done at 200 C (or higher), but the resistive switching will still be realized at room temperature. Since SrTiO3 is used as a model system here, the knowledge about SrTiO3 helps understanding the switching behavior of perovskites in general.

由于硅基半导体器件即将达到其物理极限，现在急需开发基于全新原理和材料的信息存储器；在这一方面，以离子传导为基础的离子器件是一个很好的选择。基于氧化物阻变效应的离子器件极有可能成为下一代电阻式随机存储器（RRAM）的主流，这主要是由于这类离子型存储器的以下特点：非易失性、极高的存储密度、极低擦写能耗、极快的擦写速度，而且与现有的CMOS技术有极好的相容性。但是，缺乏对于氧化物阻变机理的透彻理解已成为发展RRAM的一大障碍。钙钛矿类材料是氧化物中最重要的体系之一，在本项目中，SrTiO3单晶被用作钙钛矿类材料的模型体系，来分析肖特基势垒和氧空位在电预处理和阻变中的作用。与通常的薄膜样品相比，SrTiO3单晶的优势在于其极佳的性质均一性，而且其缺陷结构和相关的电化学过程已被彻底研究，这极大地便利了阻变机理的探索。关于SrTiO3的研究对于理解钙钛矿类氧化物的阻变行为具有普遍意义。

钙钛矿类材料是最重要的氧化物体系之一，在本项目中，SrTiO3被用作钙钛矿类材料的模型体系，来分析肖特基势垒和氧空位在电预处理和阻变中的作用。我们分别研究了具有两种不同缺陷结构的SrTiO3忆阻器件。使用施主铌（Nb）掺杂的SrTiO3 （Nb-SrTiO3），构建了Ni/Nb-SrTiO3/Ti忆阻器，细致地研究了电极/氧化物界面处肖特基势垒的变化对阻变的作用，结合实验现象与模型计算证明肖特基势垒的变化可归因于界面处与氧空位相关的电子捕获/去捕获过程。利用脉冲激光沉积技术制备了受主掺杂的SrTiO3薄膜，研究了沉积参数和衬底材料对薄膜晶体结构与形貌的影响；研究了忆阻器电极材料，电极组成以及电预处理过程对阻变特性的影响规律。关于SrTiO3的研究对于理解钙钛矿类氧化物的阻变行为具有普遍意义。更进一步，该研究也有助于了解其它氧化物的阻变机理。.同时，我们利用SrTiO3忆阻器件还实现了对多种神经学和心理学功能的模拟。通过设计电脉冲信号，模拟了生物突触的活动时序依赖可塑性（STDP）；利用SrTiO3忆阻器件电阻态所具有的独特的部分易失特性，很好地再现了艾宾浩斯学习-遗忘曲线；器件对外加电脉冲刺激的响应表现出心理学上的学习、遗忘、记忆规律，同时器件的电学响应很好地模拟了心理学上的内隐记忆的时间节省特征；通过设计简单神经形态电路，系统地模拟了巴甫洛夫条件反射，成功实现了巴甫洛夫条件反射的几个重要特征，如获取，消退和恢复。.更进一步，我们还研究了TiO2，WO3， MoO3，TiO2和BiFeO3等氧化物的阻变特性，并利用这些氧化物实现了突触的再可塑性以及对行为可塑性的模拟。.本项目从阻变机理到类脑功能模拟等方面对基于氧化物的忆阻器进行了系统研究，对脑启发智能硬件及神经形态功能的硬件开发具有重要意义。