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Current memory technologies, such as DRAM, SRAM, and NAND Flash, which are approaching very difficult issues related to the continuous scaling to and beyond the 16 nm generation, has led research over the past two decades to the discovery of several new memory technologies. In recent years, new emerging nonvolatile memories (NVMs), such as phase-change random access memory (PCRAM), ferroelectric random access memory (FRAM), magnetic random access memory (MRAM), and resistive random access memory (RRAM), have been intensively studied. Among these candidates, RRAM is a very promising and worldwide studied candidate for alternative NVM and a high potential successor for Flash in terms of energy consumption (write current in the μA range compared to mA) and simplicity of process integration.
A fully CMOS compatible TiN/Ti/HfO2/TiN RRAM module was successfully integrated with a select transistor (1T1R memory) in IHP’s technology. Nonetheless, reliability and insufficient understanding of the resistive switching mechanism are the two main issues limiting this memory technology development for e.g. wireless sensor network (WSN) applications. The still unclear atomic-scale mechanism of HfO2-based resistive switches and the identification of the material changes within the insulator must be addressed to suggest a knowledge-based improvement of device performance. In this frame, the Ti/HfO2 interface is thoroughly investigated in this Thesis by complementary materials science techniques.
First, the investigation of the as-deposited Ti/HfO2/TiN cells revealed that: (1) the Ti layer scavenges oxygen atoms stronger from amorphous (a-HfO2) than from monoclinic (m-HfO2) HfO2 films; (2) not only oxygen vacancies but also other impurities in the atomic vapor deposited (AVD) a-HfO2 film, such like nitrogen and carbon (probably resulting from the used AVD precursor chemistry) are present in the HfO2 insulator.
Next, the electrical characterization of Ti/AVD a-HfO2/TiN cells (with voltage applied to the Ti top electrode while TiN bottom electrode was grounded) revealed a clockwise bipolar resistive switching behavior after an electroforming process at positive voltage polarity. Besides, the chemical and electronic changes observed by hard X-ray photoelectron spectroscopy (HAXPES), indicate the creation of n-type dopants in the a-HfO2 film during the electroforming process, probably related to the formation of positively charged oxygen vacancies in a-HfO2 by the electrochemically induced Ti/a-HfO2 interface oxidation.
In order to directly compare electrical with electronic and chemical changes of one and the same RRAM cell, an in-operando HAXPES technique was developed. These unique studies have revealed the following characteristics of the Ti/AVD a-HfO2/TiN cells: (1) the as-deposited cells are able to switch at low electrical power; (2) However, this resistive switching is not stable and an electroforming process with a slightly increased power is required to stabilize the switching event; (3) Electrical changes correlated with HAXPES results and literature indicate that (i) the forming/set electrical power defines the oxygen vacancies concentration in the a-HfO2 and thus the stability of the resistive switching properties and (ii) the stable resistive switching can be described by a push-pull model of oxygen vacancies migration under the influence of an electrical field; (4) Besides, carbon segregation at the Ti/a-HfO2 interface – while increasing the electrical power or cycling the device – shows that the defects physics is not limited only to oxygen vacancies; other defects may thus contribute under electrical stress to the resistive switching phenomenon and need to be included in theoretical models to correctly describe the switching characteristics.
Finally, according to the presented HAXPES results, the Ti/AVD a-HfO2/TiN RRAM cells are classified to the valence change mechanism. The resistive switching mechanism is attributed to the creation and rupture of oxygen vacancies-based conducting filaments and the Ti/HfO2 interface oxidation is of central importance for the defect balance of the RRAM cell. Most importantly, a reduction of carbon content in the AVD-deposited HfO2 improved the reliability of these memory cells.