Conclusion

This thesis examines the electrical and optoelectronic properties of the transition metal oxides including the HfOx, NiO, and IGZO thin films with the focus on the fabrication, characterization, and device applications of these metal oxide thin films. The potential applications of these transition metal oxide thin films in non-volatile memory, p-n junction diode, UV photodetector, and artificial synapse have been studied. The results in this thesis have enhanced the understanding of the electrical and optoelectronic properties of the transition metal oxide thin films. This section briefly summarizes the overall research presented in this thesis.

7.1.1 Resistive switching in HfO

-based RRAM device

The HfOx-based RRAM device with MIM structure has been fabricated. Stable bipolar resistive switching behavior has been observed at both the room and elevated temperatures.

The current conduction mechanisms of both LRS and HRS are examined with temperature dependent I-V characteristics. For LRS, ohmic conduction with little temperature dependence has been observed, which could be attributed to the very small activation energies of the carrier conduction in the oxygen vacancy based conductive filament. For HRS, when the applied electric field is low, the current conduction follows the ohmic conduction; when the applied electric field is high, the current conduction follows the Poole-Frenkel emission model. Multibit storage is realized in one RRAM cell by controlling the switching operation

conditions. Impedance spectroscopy has been used to study the multilevel high resistance states in the HfOx-based RRAM device. The high resistance states can be described with an equivalent circuit consisting of three components Rs, R, and C corresponding to the series resistance of the TiON interfacial layer, the equivalent parallel resistance and capacitance of the leakage gap between the TiON layer and the residual conductive filament, respectively.

These components show a strong dependence on the reset stop voltage, which can be explained in the framework of oxygen vacancy model and conductive filament concept. Both the CVS and RVS methods have been used to study the speed-disturb time dilemma.

Compared with CVS, RVS is proved to be an effective method with faster speed and low cost.

The HfOx-based RRAM device was also successfully fabricated with the 180 nm Cu BEOL process platform. The RRAM device in this Cu interconnection technology shows the similar bipolar resistive switching performance as in the conventional Al interconnection technology.

7.1.2 Resistive switching in p-NiO/n-IGZO thin film heterojunction structure

The as-fabricated p-NiO/n-IGZO heterojunction structure shows the typical rectifying property with the rectification ratio of 10³ at the bias voltage of ±1.5 V. After applying a large enough negative forming voltage, stable bipolar resistive switching can be achieved with tight distributions for both the resistance states and transition voltages. The transition between the HRS and LRS can be attributed to the connection and rupture of the oxygen vacancy based conductive filament. The current conduction mechanisms of both LRS and HRS are examined with temperature dependent I-V characteristics. For LRS, ohmic conduction with little temperature dependence has been observed, which could be attributed to the very small activation energies of the carrier conduction in the oxygen vacancy based

conductive filament. For HRS, Schottky emission model with the Schottky barrier height of ~ 0.31 eV can be used to describe the current conduction. Multibit storage can be realized by controlling either the compliance current in the set process or the reset stop voltage in the reset process.

7.1.3 The diode and ultraviolet photodetector applications of p-NiO/n-IGZO thin film heterojunction structure

The p-NiO/n-IGZO thin film heterojunction structure has been fabricated for both diode and UV light photodetector applications. The p-NiO thin films with different conductivity can be achieved by introducing different amount of O2 gas during the sputtering deposition.

The n-IGZO thin films with different conductivity can be achieved by O2 plasma treatment with different durations. For diode application, both the conductivity of the NiO and IGZO thin films has a strong influence on the rectifying property of the heterojunction diode. The best rectifying performance can be achieved for the diode that consists of both high conductive NiO and IGZO thin films. For the UV photodetector application, a high spectrum selectivity can be achieved due to the filter function of the top p-NiO layer. The overall performance of the p-NiO/n-IGZO photodetector is highly dependent of the conductivity of the NiO layer. The best performance can be achieved with NiO thin film with the highest conductivity. The photodetector in our work shows good repeatability and fast response.

7.1.4 A light-stimulated synaptic transistor with synaptic plasticity and memory functions based on IGZO

-Al

O

thin film structure

The IGZO-based thin film transistor with Al2O3 as the gate oxide has been fabricated for

plasticity like the paired-pulse facilitation has been emulated in this synaptic transistor. The memory behaviors including the transition from the STM to LTM and the Ebbinghaus forgetting curve have also been mimicked with the UV light stimulus. The synapse-like behaviors and memory behaviors of the synaptic transistor are due to the trapping and detrapping of the photo-generated holes at the IGZO/Al2O3 interface and/or in the Al2O3

layer.