ICMCTF2014 Session C5-2: Thin Films for Active Devices

Resistive switching materials have been intensively studied for nonvolatile memory applications. Among them, SiO₂ stands out and is well studied due to ready availability of the material and minimum infrastructure modification. In this work we characterized the PECVD SiO_xN_1-x based resistive devices and discuss the important role of oxygen and oxygen vacancies in device performance.

Resistive Memory devices were fabricated on N⁺⁺ Si prime wafer cleaned with BOE for 1min to remove native oxide. A 50nm SiO_xN_1-x layer was PECVD deposited at 300C using NH₃, N₂O, and SiH₄ as reactive species. Refractive index of SiO_xN_1-x layer was measured using ellipsometry immediately after deposition. Using the SOPRA database for PECVD SiO_xN_1-x, the oxygen concentration x was determined. Different recipes with varying gas flow rate were used to achieve 5 different oxygen concentrations (10%, 32%, 49%, 66%, 80%) in the SiO_xN_1-x. Then 200nm tantalum nitride was deposited by sputtering followed by lithography and CF₄-based dry etch to define the top electrodes. After removing photoresist, all samples were wet etched in BOE for 3min to remove SiO_xN_1-x from field regions.

Devices were then tested in vacuum using substrate as ground and top electrode to apply bias. Ten devices with each oxygen concentration condition were measured and device yield was determined. It was found that device yield increases with increasing oxygen concentration. And HRS and LRS current level measured at 1V, set voltage and reset voltage were all extracted from DC IV data of each working device. Their respective cumulative distribution shows that devices with more oxygen have a steeper slope in HRS state, indicating a tighter distribution and better device stability; whereas for LRS state such trend is not clear. For set and reset voltage, all devices have a minimum of 0.5V variation due to its random nature of filamentary switching. SiO₁₀N₉₀ devices have the largest variation, up to 2V, showing the worst device stability.

Electron Spin Resonance (ESR) study on E’ center has shown that nitridation of SiO_xN_1-x would lower oxygen vacancy concentration in the film. Those works were used to explain our result that devices with higher oxygen concentration have higher oxygen vacancy concentration and more robust conductive filament, which results in better endurance characteristics and improved stability.

In conclusion, we demonstrated the switching behavior of PECVD SiO_xN_1-x resistive memory device, and show the impact of oxygen atom concentration in SiO_xN_1-x on device performance. The result suggests that reversible switching phenomenon may arise from the oxygen in SiO_xN_1-x.

The effects of oxygen ambiance on electrical characteristic degradation phenomena in a-InGaZnO thin film transistor with different biases and temperatures are investigated. It can be found that oxygen is substantially adsorbed on the backchannel and results in device instabilities during positive gate bias stress. However, visible light irradiation is found to desorb the adsorbed oxygen ions and this verifies that oxygen dominates the degradation behavior. Moreover, comparing with that in vacuum, hot-carrier stress in oxygen ambiance leads to an extra potential barrier height near the drain side due to oxygen adsorption and causes asymmetric degradation. Furthermore, the asymmetric degradation behavior after hot-carrier stress in oxygen ambiance is suppressed at high temperature due to temperature-induced oxygen desorption.

The Cu(InAl)S₂(CIAS)thin films, direct and band-gap adjustable chalcopyrite materials, have attracted much interest in recent years for optoelectronic, photovoltaic as well as bio-imaging applications. The widest band-gap CuAlS₂ film has potential to be used for building blue and green light emitting devices. Most of the previous researches were focus on material synthesis and/or characterizations. In this paper, we propose a novel approach to form a good quality wide band-gap CIASfilm and present its application to ultraviolet (UV) photodetector (PD).

Silicon wafer with 100nm-thick SiO₂ film was used as substrate. A 400nm-thick Mo film was deposited on top of the SiO₂ film followed by co-sputter deposition of 500nm-thick Cu/In/Al metallic precursors (Small amount of In was added to minor adjust the band-gap). The Mo film was used to avoid any reaction of Cu, In or Al with the substrate during the subsequent high temperature process and also serve as an electrode when the device is fabricated.

Sulfurization of Cu/In/Al precursors directly to form wide band-gap CIAS film usually leads to a small grain-sized film with minor cracks. XRD results indicate that many Cu-S phases exist when the sulfurization temperature was lower than 700^oC. A rapid thermal annealing process was adopted to anneal the Cu/In/Al precursors at 600^oC for one hour to form a single phased Cu₉Al₄ compound. Good quality CIAS film can be successfully formed by sulfurizing the Cu₉Al₄ film at 700^oC for 100 min. The XRD patterns show that the post-sulfurized film is a single-phased CIAS film with (112)-preferred orientation, the medium grain size is approximately 400-500 nm and no cracks was found under scanning electron microscopy observation.

CuIn_1-xAl_xS₂ samples with x >0.85 were prepared to fabricate UV PDs. Two sensing schemes, metal-semiconductor-metal (MSM) and ZnS/CIAS pn junction, were tested for the PDs. The 5-μm finger spacing MSM scheme obtained a 12x magnification in photocurrent with UV illumination. The photo response data suggests that the UV PDs have cut-off frequency near 380 nm. However, there is a clear 423nm peak under photoluminescence measurement. The luminescence peak shifted to a longer wavelength is related to the defect structure of the CIAS film. The UV testing using pn junction scheme is more sensitive than MSM when the grain size of the film is small. Wide band-gap CIAS film is a good candidate as a UV sensing material.

Due to their outstanding features, superconductors give the opportunity to develop technology available in many areas and to improve the performance of the devices. Superconducting materials are used in many fields particularly in industry, transport, electronics, generation and storage of electricity and transport of electrical current. They are used in industry in the construction of powerful superconducting magnet and superconducting motor and transformer, in transportation, in levitation trains and superconducting motor boats, and in electronics, in the construction of the SQUID, superconducting transistors, particle accelerators, microchips, sensors, detectors, resonators and filters. Rapid developments in the field of electronics make it necessary to increase performances of electronic devices. Because of features such as high speed, low loss, and high resolution, superconductors are preferred in electronics more than the conventional devices. However, studies on superconducting devices have not been completed yet and the physics behind these devices has not been fully understood.

Micro-wave resonator improved in terms of quality and performance is used in many areas such as aerospace, communication, television technology, radar, medicine, industry and military. While superconducting devices are made of low-temperature super-conductors such as Nb and NbSn3, high-temperature superconductors are preferred in recent years because of their working capacity at liquid nitrogen temperature and their high critical current and field values. Among high-temperature superconductors, YBCO stands out for microwave applications.

Pd nano-dots have been formed on the substrates and, in YBCO, non-superconducting BZO nano-columns have been formed spontaneously on these nano-dots. These nano-columns in YBCO strip act as artificial pinning center. On the studies of use of the Pd to form nano-columns in YBCO, we have shown that artificial pinning centers significantly enhance the performance of the superconducting strip in magnetic field.

This letter investigates anomalous V_t shifts after PBTI stress by fast I-V measurement in Input/output high-k/metal gate stack. With fast I-V measurement, it shows that V_t shift in negative direction after PBTI. In addition, gate current is insignificant under initial. Hence, this phenomenon indicates that electrons discharge from high-k bulk traps to metal rather than that charge to high-k bulk traps from channel. Through a change in high level and low voltages by double sweep fast I-V measurement, it indicates that ΔV_t increases with a rise in high level voltage and a decrease in low level voltage. Furthermore, in comparison of SiO2, high-k/1nm and high-k/3nm devices in double sweep fast I-V measurement, it evidences that anomalous V_t shift after PBTI due to electrons discharging from pre-existing high-k bulk traps.