Byung-Woo Kang

Improvement in the bias stability of amorphous indium gallium zinc oxide thin-film transistors using an O2 plasma-treated insulator

The effects of an O2 plasma-treated SiNX-based insulator on the interfacial property and the device performances of amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs) were investigated. We tried to improve the interfacial characteristics by reducing the trap density between the SiNX gate insulator and a-IGZO channel by the O2 plasma treatment. The plasma treated-device performances were remarkably improved. The drastic improvements obtained for the O2 plasma-treated a-IGZO TFTs included excellent bias stability as well as a high field effect mobility (μFE) of 19.4 cm2/V s, an on/off current (ION/IOFF) of 108, and a subthreshold value (S) of 0.5 V/decade.

Interface-modified random circuit breaker network model applicable to both bipolar and unipolar resistance switching

We observed reversible-type changes between bipolar (BRS) and unipolar resistance switching (URS) in one Pt/SrTiOx/Pt capacitor. To explain both BRS and URS in a unified scheme, we introduce the “interface-modified random circuit breaker network model,” in which the bulk medium is represented by a percolating network of circuit breakers. To consider interface effects in BRS, we introduce circuit breakers to investigate resistance states near the interface. This percolation model explains the reversible-type changes in terms of connectivity changes in the circuit breakers and provides insights into many experimental observations of BRS which are under debate by earlier theoretical models.

Reduction in high reset currents in unipolar resistance switching Pt/SrTiOx/Pt capacitors using acceptor doping

The high reset current, IR, in unipolar resistance switching is an important issue which should be resolved for practical applications in nonvolatile memories. We showed that, during the forming and set processes, the compliance current, Icomp, can work as a crucial parameter to reduce IR. Doping with Co or Mn can significantly reduce the leakage current in capacitors made using SrTiOx film, opening a larger operation window for Icomp. By decreasing Icomp with acceptor doping, we could reduce IR in SrTiOx films by a factor of approximately 20. Our work suggests that the decrease in Icomp by carrier doping could be a viable alternative for reducing IR in unipolar resistance switching.

Time-dependent current-voltage curves during the forming process in unipolar resistance switching

We investigated the time-dependent current-voltage curves of the forming process in unipolar resistance switching. We applied triggered-voltage triangular-waveform (pulse-waveform) signals with varied sweep rate (amplitude) to Pt/SrTiOx/Pt capacitors. By investigating their temperature dependences, we found that the forming process was driven by two different mechanisms, depending on the sweep rate (amplitude): a purely electrical dielectric breakdown and a thermally assisted dielectric breakdown. For the latter process, we observed precursory changes in the current I(t) before the forming process. By fitting the time-dependent precursory changes with I(t)=Io−A exp(−t/τ), we suggest that the thermally activated migration of oxygen vacancies/ions could help the thermally assisted dielectric breakdown.

Avoiding fatal damage to the top electrodes when forming unipolar resistance switching in nano-thick material systems

When forming unipolar resistance switching in material systems, fatal damage often occurs to the top electrodes of Pt/SrTiOx/Pt, Pt/TiOy/Pt and Pt/NiOz/Pt cells. To develop a means of overcoming this problem, we systematically investigated the forming process by applying triangular- and pulse-waveform voltage signals to the cells. By investigating the dependence on sweep rate of the triangular-waveform voltage signals and amplitude of the pulse-waveform voltage signals, we discovered that the forming process occurred by two different mechanisms, irrespective of the material: either a thermally assisted dielectric breakdown or a purely electrical dielectric breakdown. During the former process, the top electrodes remained clean, even though oxygen bubbles formed on them. We observed that the top electrodes were blown off only for the latter (electrical) breakdown as a result of the formation of many conducting channels. We were able to overcome the fatal damage to the top electrodes by modifying the forming process into the thermally assisted dielectric breakdown.

Amorphous Indium Gallium Zinc Oxide Semiconductor Thin Film Transistors Using O2 Plasma Treatment on the SiNx Gate Insulator

In this study, we investigated the role of processing parameters on the electrical characteristics of amorphous In–Ga–Zn–O (a-IGZO) thin film transistors (TFTs) fabricated using DC magnetron sputtering at room temperature. Processing parameters including the oxygen partial pressure, annealing temperature, and channel thickness have a great influence on TFT performance and better devices are obtained at a low oxygen partial pressure, annealing at 200 °C, and a low channel thickness. We attempted to improve the a-IGZO TFT performance and stability under a gate bias stress using O2 plasma treatment. With an O2 plasma treated gate insulator, remarkable properties including excellent bias stability as well as a field effect mobility (µFE) of 11.5 cm2 V-1 s-1, a subthreshold swing (S) of 0.59 V/decade, a turn-on voltage (VON) of -1.3 V, and an on/off current ratio (ION/IOFF) of 105 were achieved.

Characteristics of lanthanum hafnium oxide deposited by electron cyclotron resonance atomic layer deposition

Lanthanum hafnium oxide (LHO) thin films were grown using an electron cyclotron resonance atomic layer deposition technique. Tetrakis(ethylmethylamino)hafnium (TEMAHf) and tris(ethylcyclopentadienyl) lanthanum (III) [La(EtCp)3] were utilized as the hafnium and lanthanum precursors, respectively. O2 plasma was used as a reactant gas. Transmission electron microscopy analyses revealed that the as-deposited LHO film had a crystalline structure at a deposition temperature of 400°C. Rapid thermal annealing of the LHO films induced dramatic changes in the electrical properties. The VFB for the films shifted toward the ideal VFB and the amount of positive fixed charge disappeared in the LHO films. The leakage current density of the film deposited at 400°C was estimated to be 4.6×10−7A∕cm2 at −1V. The leakage characteristic of the LHO films was improved with annealing at temperatures above 900°C.

Titanium Oxide Thin Films Prepared by Plasma Enhanced Atomic Layer Deposition Using Remote Electron Cyclotron Resonance Plasma for Organic Devices Passivation

We examined titanium oxide films and their barrier characteristics. Titanium oxide films were deposited at room temperature with electron cyclotron resonance plasma power of 300 W. We measured the growth rate of the titanium oxide film to be 1.8 A/cycle. Barrier layers on poly(ether sulfon) (PES) substrates were observed to provide activation energy for water permeation. Activation energy of 12.4 kJ/mol was added by applying a titanium oxide coating with a thickness of 100 nm. The passivation performance of the titanium oxide film was also investigated using organic light-emitting diodes (OLEDs). The relative luminance of a 400-nm-thick-coated OLED device was diminished by 11.6% for 20 h.

Copper seed layer using atomic layer deposition for Cu interconnect

Cu has replaced Al for interconnection material in ultra large integrated circuit (ULSI), resulting in reducing the RC delay and achieving higher electro-migration reliability. However, it becomes more difficult to form reliable Cu wire as the feature size more decreases. For successful electrochemical plating (ECP) filling, continuous Cu seed layer with low resistivity and conformallity is essentially required. We have studied a Cu seed layer deposited using plasma enhanced atomic layer deposition (PE-ALD). The electrical property of sub-10 nm thickness Cu thin film was determined by the continuity and morphology of thin film. Based on these results, Cu seed layers were deposited on the 32 nm sized Ta/SiO2 trench substrate, and ECP was performed under the conventional conditions. It was observed that the continuous seed layer was deposited using PE-ALD and the trench was perfectly filled.