ICMCTF2014 Session C5-2: Thin Films for Active Devices

Thursday, May 1, 2014 8:00 AM in Room Sunset

Thursday Morning

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8:00 AM C5-2-1 Characteristics of PECVD SiOxN1-x for Resistive Memory Application
Fei Zhou, Yaofeng Chang (University of Texas at at Austin, US); Burt Fowler (Privatran LLC, US); Jack Lee (University of Texas at at Austin, US)

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 SiOxN1-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 SiOxN1-x layer was PECVD deposited at 300C using NH3, N2O, and SiH4 as reactive species. Refractive index of SiOxN1-x layer was measured using ellipsometry immediately after deposition. Using the SOPRA database for PECVD SiOxN1-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 SiOxN1-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 SiOxN1-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 SiOxN1-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 SiOxN1-x resistive memory device, and show the impact of oxygen atom concentration in SiOxN1-x on device performance. The result suggests that reversible switching phenomenon may arise from the oxygen in SiOxN1-x.

8:20 AM C5-2-2 Investigation of Temperature-Dependent Asymmetric Degradation Behavior Induced by Hot Carrier Effect in Oxygen Ambiance in In-Ga-Zn-O Thin Film Transistor
Bo-Wei Chen, Ting-Chang Chang (National Sun Yat-Sen University, Taiwan)

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.

8:40 AM C5-2-3 Investigating Characteristics and Reliabilities of Dual Gate a-InGaZnO Thin Film Transistor with an Etch Stop Layer
Po-Yung Liao, Ting-Chang Chang (National Sun Yat-Sen University, Taiwan)
Dual gate amorphous-InGaZnO thin-film transistors (TFTs) with a bottom-gate that covers the whole channel and a top-gate that covers only a portion of the channel are investigated. The degree of degradation between top- and bottom-gate structured amorphous InGaZnO TFT is compared with each other. Under hot-carrier stress (HCS), the degradation of dual gate TFT is more significant than that of bottom-gate. This HCS-induced degradation phenomenon is dominated by electron-trapping in the etch stop layer (ESL). In addition, under negative gate bias illumination stress (NBIS), the threshold voltage of bottom-gate TFT monotonically shifts in the negative direction, whereas top-gate TFT exhibits on-state current increases without VT shift. Such anomalous degradation behavior under top-gate NBIS is due to hole-trapping in the ESL above the central portion of channel. These phenomena can be ascribed to the screening of electric field by redundant source/drain electrodes.

9:00 AM C5-2-4 Wide Band-gap CuInAlS2 Thin Film and Its Application to UV Detectors
Dung-Ching Perng (National Cheng Kung University, Taiwan); Tzung-Ta Kao (National Kaohsiung First University of Science and Technology, Taiwan); Ruo-Ping Chang (National Cheng Kung University, Taiwan)

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 700oC. A rapid thermal annealing process was adopted to anneal the Cu/In/Al precursors at 600oC for one hour to form a single phased Cu9Al4 compound. Good quality CIAS film can be successfully formed by sulfurizing the Cu9Al4 film at 700oC 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.

CuIn1-xAlxS2 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.

10:00 AM C5-2-7 Surface Decoration using Pd Nanoislands for YBCO Superconducting Thin Film using Pulsed Laser Deposition
Mehmet Ertugrul, Demet Tatar, Erdal Sonmez, MustafaTolga Yurtcan (Ataturk University, Turkey)

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.

10:20 AM C5-2-8 Abnormal Temperature-dependent Floating-body Effect on Hot-carrier Degradation in PDSOI n-MOSFET
Kuan-Ju Liu, Ting-Chang Chang (National Sun Yat-Sen University, Taiwan)
This letter investigates the abnormal degradation behavior after hot-carrier stress (HCS) in partially depleted silicon-on-insulator n-channel metal–oxide–semiconductor field effect transistors. HCS (Ib,max) shows that the floating-body (FB) device degrades more than body-contact (BC) device at room temperature. However, the degree of degradation has no significant difference between BC and FB devices at high temperature. Furthermore, unlike the degraded drain current measured by conventional characterization method after HCS, ultra-fast measurement shows that transient rising drain current becomes unobvious at elevated stress temperature on FB device. This behavior can be attributed to faster recombination rate of accumulated holes at the body at high temperature. Hence, the floating body effect is obviously reduced at high temperature.
10:40 AM C5-2-9 Anomalous Vt Shifts after PBTI Stress by Fast I-V Measurement in Input/Output High-k/Metal Gate Stack
Szu-Han Ho (National Chiao Tung University, Taiwan); Ting-Chang Chang (National Sun Yat-Sen University, Taiwan)

This letter investigates anomalous Vt 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 Vt 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 ΔVt 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 Vt shift after PBTI due to electrons discharging from pre-existing high-k bulk traps.

11:00 AM C5-2-10 Investigation of Carrier Transport Behavior in Amorphous In-Ga-Zn-O Thin Film Transistors
Tien-Yu Hsieh, Ting-Chang Chang, Po-Yung Liao (National Sun Yat-Sen University, Taiwan)
Behaviors of carrier transport in amorphous In-Ga-Zn-O thin film transistor are investigated. It is found that the electron mobility is higher at elevated temperature, which is contrary to that in crystalline Si devices. Drain current enhancement with regard to temperature at corresponding gate voltage follows Arrhenius equation. This implies that carrier transport is limited by the potential barrier heights induced by trap states within In-Ga-Zn-O, and therefore current conduction is heat-activated to overcome those barriers. In addition, the extracted activation energy decreases with increasing gate voltage, indicating the effective potential barrier height is lowered when abundant electrons are injected into the channel. Furthermore, the relationship between carrier mobility and carrier concentration is also investigated, with the carrier mobility monotonically increasing with carrier concentration. Such behavior can be ascribed to lowered effective barrier above conduction band when the Fermi-level rises.
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