CHAPTER 3 RESULTS AND DISCUSSIONS
3.1 D EVICE S TRUCTURES AND C HARACTERISTICS
3.1.1 Various bottom electrode metals
First of all, the resistive switching phenomenon is studied in Al (400nm) / SiO2
(50nm) / Pt (50nm) structure and its depiction of process flows in Fig. 3-2 and Fig.
3-3. The initial state of as deposition Al / SiO
2 / Pt structure processes high resistance value before forming process (Fig. 3-4). It should be noted that, in the previous research, the Au/SiO2/Al structure had resistance switching behavior [34]. However,the Al/SiO2/Pt structure in my thesis does not possess resistance switching phenomena after the forming process (Fig. 3-5).
In order to study resistance switching behavior for SiO2 on alloy electrode, the Al (400nm) / SiO2 (50nm) / PtFe (50nm) is fabricated. Its structure and process flows are depicted in Fig. 3-6 and Fig. 3-7. Unlike Al / SiO2 / Pt structure, the Al / SiO2 / PtFe structure processes stably and repetitively resistance switching behaviors after the forming process (Fig. 3-8). In order to clarify the mechanisms of resistance switching behavior, seven parts will be discussed in this section.
3.1.1.1 Mechanism of Forming Process
The forming process means that the application of a large voltage a will soften the breakdown of a device. In order to avoid the device being permanent damage, the measurement parameter would set a compliance current which is controlled by a feedback system in apparatus, like Fig. 3-9. A large voltage would produce a high electrical field which induces impact ionization breakdown (Fig. 3-10) in SiO2 for Al / SiO2 / Pt structure [25]. Hence, there are many low resistance paths produced, like
Fig. 3-11.
For the Al / SiO2 / PtFe structure, it is at initial state and processes high resistance value before forming process (Fig. 3-12). During the forming process (Fig.
3-13), bottom PtFe electrode is applied in a large negative voltage which will produce
a high electrical field. Hence, the breakdown of impact ionization would happen in SiO2 and Fe2O3 which is formed on PtFe alloy electrode during the fabrication process (Fig. 3-14). Then, there are many low resistance paths produced in SiO2, and an“electric faucet” would form in Fe2O3, depicted as Fig. 3-15 [37]. Further discussions
would be continued and its related switching mechanisms are similar with the set process.
3.1.1.2 Mechanism of Phase Change in the Reset Process
After the set and forming process, reset process is the second step to switch resistance. The reset process is that the bottom electrode applies positive voltage, and the top electrode is grounding, like Fig. 3-1. Due to the electrical field direction (Fig.
3-16) and high current flowed through the “electric faucet” [37], the oxygen ion and
localization Joule heating (Fig. 3-17) [39] cause the phase change of Fe3O4 (Fig. 3-18) which is formed during the forming or set process. The chemical reaction possesses relationship as following [18]:2 Fe3O4 + O2- => 3 Fe2O3 + 2 e
-The resistance between Fe2O3 and Fe3O4 is different because the band gap of Fe2O3 and Fe3O4 are 2.6eV and 1.6eV, respectively [40, 41]. The total resistance of the insulating layer is the low resistance paths in SiO2 series connection with Fe2O3 thin film which is less conductive. Hence, the phase change from Fe3O4 to Fe2O3 would switch the current or resistance from LRS to HRS in the process of reset.
3.1.1.3 Mechanism of Oxygen Vacancies in the Reset Process
To affect resistance switching behavior is another factor, the amounts of oxygen vacancies. Reset process follows the step of the set or following process. The voltage sweep mode is also the same with Fig. 3-1. Due to the electrical field direction (Fig.
3-16) and high current flowed through the “electric faucet” [37], the oxygen ion and
localization Joule heating (Fig. 3-17)[39] would decrease the amounts of oxygen
vacancies which are increased at electric faucet region during the forming or setprocess. The chemical reaction possesses relationship as following [23]:
V’’ (donor) + O2- => O
Due to oxygen vacancies could be seen as donor type defects [23], they would supply electron concentrations in the intrinsic n-type Fe2O3 semiconductor (further discussion about band diagram would be continued in the next section) [38]. The less amounts of oxygen vacancies exist at the electric faucet, the less conductive paths would form. Therefore, after the reset process, the total resistance of the insulating layer is the low resistance paths in SiO2 series connection with less oxygen vacancies of Fe2O3 thin film which is less conductive. The amounts of oxygen vacancies that are decreased in the intrinsic n-type Fe2O3 semiconductor would switch the current or resistance from LRS to HRS during the reset process.
3.1.1.4 Band diagram in the Reset Process
During the reset process, the phase change of Fe2O3 and oxygen vacancies both play important roles in causing resistance switching. After reset process, the band diagram of intrinsic n-type Fe2O3 semiconductor shows in Fig. 3-19. Finally, the Frenkel-Poole emission is predicted by current fitting with different temperature (shows in section 3.3.4) (Fig. 3-20).
3.1.1.5 Mechanism of Phase Change in the Set Process
After the reset process, set process is the second step to switch resistance. The set process is that the bottom electrode applies negative voltage, and the top electrode is grounding, like Fig. 3-1. Due to the electrical field direction (Fig. 3-21) and the applied power by measurement system, the electric faucet would be opened [37]. That means the phase change of Fe O (Fig. 3-22) during the forming or set process. The
chemical reaction possesses relationship as following [18]:
2 Fe3O4 + O2- <= 3 Fe2O3 + 2 e
-Due to band gap differences between Fe2O3 and Fe3O4, the total resistance of the insulating layer is the low resistance paths in SiO2 series connection with Fe3O4 thin film which is conductive. Hence, the phase change from Fe2O3 to Fe3O4 would switch the current or resistance from HRS to LRS during the set process.
3.1.1.6 Mechanism of Oxygen Vacancies in the Set Process
The role of Oxygen Vacancies is also an important factor to cause resistance switching. First, the set process is the following step to switch resistance after reset process. The voltage sweep mode is also the same with Fig. 3-1. Due to the electrical field direction (Fig. 3-21) and the applied power by measurement system, the electric faucet would be opened in Fe2O3 after forming or set process [37]. The reason is that the amounts of oxygen vacancies would increase in some regions, and these regions represent highly conductive paths in Fe2O3 thin film. The chemical reaction possesses relationship as following [23]:
V’’ (donor) + O2- <= O
From the previous sections, oxygen vacancies could be seen as donor type defects, which could supply electron (donor) concentrations in the intrinsic n-type Fe2O3 semiconductor (further discussion about band diagram would be continued in the following section) [38]. The more amounts of oxygen vacancies exist at the electric faucet, the more conductive paths would form. Hence, after the set process, the total resistance of the insulating layer is the low resistance paths in SiO2 series
connection with many oxygen vacancies of Fe2O3 thin film which is highly conductive. The amounts of oxygen vacancies increased in the intrinsic n-type Fe2O3
semiconductor would switch the current or resistance from HRS to LRS during reset process.
3.1.1.7 Band diagram in the Set Process
During the set process, the phase change of Fe2O3 and oxygen vacancies both play important roles to cause resistance switching. After set process, the band diagram of enhanced n-type Fe2O3 semiconductor shows in Fig. 3-23. Finally, the tunneling is predicted by current fitting with different temperature, where the current is less temperature sensitive at LRS (shows in section 3.3.4) (Fig. 3-24).