Dedong Han

Characteristics and mechanism of conduction/set process in TiN∕ZnO∕Pt resistance switching random-access memories

The characteristics and mechanism of conduction/set process in TiN∕ZnO∕Pt-based resistance random access memory devices with stable and reproducible nanosecond bipolar switching behavior were studied. The dependencies of memory behavior on cell area, operating temperature, and frequency indicate that the conduction mechanism in low-resistance states is due to electrons hopping through filament paths. We also identify that the set process is essentially equivalent to a soft dielectric breakdown associated with a polarization effect caused by the migration of space charges under a low electric field stress. The generation/recovery of oxygen vacancies and nonlattice oxygen ions play a critical role in resistance switching.

Bipolar switching behavior in TiN/ZnO/Pt resistive nonvolatile memory with fast switching and long retention

Highly stable bipolar resistive switching behaviors of TiN/ZnO/Pt devices were demonstrated for the first time. The excellent memory characteristics including fast switching speed (<20 ns for set and <60 ns for reset), long retention (in the order of 105 s) and non-electroforming process were demonstrated. The bipolar switching behaviors can be explained by formation and rupture of the filamentary conductive path consisting of oxygen vacancies. The excellent bipolar switching behavior can be attributed to the significant amount of oxygen vacancies in ZnO film and the effect of TiN layer serving as an oxygen reservoir.

High-Performance Transparent AZO TFTs Fabricated on Glass Substrate

High-performance transparent low-temperature top-gate type aluminum doped zinc oxide (AZO) thin-film transistors (TFTs) (<i>W</i>/<i>L</i>=100 or 10 μm) are successfully fabricated on glass substrate. All the process temperature is below 100°C. For <i>VG</i>=-2 to 5 V, the TFTs using sputtering deposit AZO layer at room temperature as channel layer exhibits excellent properties, such as a saturation mobility μ<i>s</i> of 285 cm²/V·s, a linear field effect mobility μ<i>l</i> of 143 cm²/V·s, a threshold voltage <i>Vth</i> of 0.9 V, a steep subthreshold swing of 108 mV/decade, a low off-state current value <i>I</i>_off of 5×10^-13 A, a high ON/OFF ratio of 2×10⁹ and a high transmittance of 82.5%. The results highlight that excellent device performance can be realized in AZO TFTs. Note that this is the best performance of AZO TFT ever reported.

Reliability characteristics of high- K gate dielectrics HfO 2 in metal-oxide semiconductor capacitors

High-K gate dielectric will be needed when MOS devices are scaled down to the sub-100 nm level. HfO2 is a promising high-K gate dielectrics candidate to replace SiO2. In the paper, HfO2 gate dielectrics films were fabricated by ion beam sputtering a sintered HfO2 target. Reliability of gate dielectrics was studied. We have studied the SILC (stress-induced leakage current) effect and the TDDB (time-dependent dielectric breakdown) characteristic. Soft breakdown behavior was observed under constant current and voltage stress. Results indicate HfO2 holds good reliability for gate dielectrics.

Fully Transparent Al-Doped ZnO Thin-Film Transistors on Flexible Plastic Substrates

We have fabricated fully transparent Al-doped ZnO thin-film transistors (AZO TFTs) on a flexible plastic substrate at room temperature. A double-stacked channel structure composed of a high-density layer and a low-density layer is also investigated to improve the device performance. As-fabricated TFTs exhibit excellent electrical performance as well as optical performance, with a saturation mobility of 31.4 cm2 V-1 s-1, a drain current on/off ratio of about 108, a subthreshold swing of 330 mV/dec, and an average transmittance in the visible wavelength range of above 70%.

High-performance dual-layer channel indium gallium zinc oxide thin-film transistors fabricated in different oxygen contents at low temperature

In this work, fully transparent dual-layer channel amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs) are fabricated on glass substrates at low temperature. Dual-layer channel a-IGZO TFTs are studied by changing the partial pressure of oxygen while sputtering IGZO layers for comparison with single-layer channel TFTs which are fabricated with a constant oxygen content. All four types of dual-layer channel TFT sample demonstrate better performance, on-to-off ratios of ~108 and low subthreshold swing (SS) of less than 200 mV/decade, than the single-layer ones. TFTs with two layers, a low-oxygen layer and a high-oxygen layer formed using oxygen partial pressures of 0.01 and 0.05 Pa, respectively demonstrate relatively better performance with a mobility of more than 60 cm2 V−1 s−1. Among them, the TFTs with a channel layer thickness ratio of 3 : 1 show the best transfer characteristics with a high on-to-off current ratio (Ion/off) of 1.8 × 108 and a low SS of 135 mV/decade.

The Effect of Current Compliance on the Resistive Switching Behaviors in TiN/ZrO2/Pt Memory Device

In this paper, TiN/ZrO2/Pt sandwiched resistive switching memory devices were fabricated. The effect of set current compliance on the resistive switching behaviors in TiN/ZrO2/Pt memory device was studied. The different dependence of low resistance state on the set current compliance were observed under the different magnitudes of set current compliance: 1) the average read current was linearly dependent on the set current compliance in the magnitude of low set current compliance; 2) then a weaker dependence of the average read current on the set current compliance was observed in the magnitude of higher set current compliance; 3) when the current compliance is high enough, the unipolar resistive switching behaviors instead of the bipolar resistive switching was shown. A physical model based on oxygen vacancy conducting filamentary paths is proposed to explain the effect of set current compliance on the resistive switching behaviors in TiN/ZrO2/Pt memory device.

Electrical properties of Al 2 O 3 gate dielectrics

Al2O3 gate dielectric thin films were deposited by reactive dc magnetron sputtering. The electrical properties and the carrier transport mechanisms were studied. The results indicate that higher te...Al2O3 gate dielectric thin films were deposited by reactive dc magnetron sputtering. The electrical properties and the carrier transport mechanisms were studied. The results indicate that higher temperature annealing in oxygen ambient is helpful to improve the electrical properties of Al2O3 gate dielectric films with few interfacial traps formed at the Al2O3/Si interface and the carrier transport mechanisms are dominate by thermionic emission.

Unipolar resistive switching and mechanism in Gd-doped-TiO2-based resistive switching memory devices

Gd-doped-TiO2-based resistive switching random-access memory (RRAM) devices with MOM structure of top metal electrode (W, Ti)/Gd-doped-TiO2/Pt/Ti/SiO2/Si were fabricated, and resistive switching characteristics were investigated. The W/Gd-doped-TiO2/Pt RRAM device showed typical unipolar resistive switching behavior irrespective of the bias polarity on the W top electrode. However, no unipolar switching was observed in the Ti/Gd-doped-TiO2/Pt devices when positive bias was applied on the Ti top electrode. A new resistive switching model was proposed to explain the unipolar switching behavior in the W/Gd-doped-TiO2/Pt RRAM device, based on the formation and destruction of conducting filaments which result from pinning and unpinning effects of surface trap states of oxide film on the Fermi level of metal electrode. The model clearly elucidates the mechanism of the low resistance state (LRS) and the high resistance state (HRS) as well as the switching behavior between LRS and HRS in the W/Gd-doped-TiO2/Pt RRAM device. The model also indicates the important role of the metal electrode material in unipolar resistive switching of oxide-based RRAM devices.

Fully transparent flexible tin-doped zinc oxide thin film transistors fabricated on plastic substrate

In this work, we have successfully fabricated bottom gate fully transparent tin-doped zinc oxide thin film transistors (TZO TFTs) fabricated on flexible plastic substrate at low temperature by RF magnetron sputtering. The effect of O2/Ar gas flow ratio during channel deposition on the electrical properties of TZO TFTs was investigated, and we found that the O2/Ar gas flow ratio have a great influence on the electrical properties. TZO TFTs on flexible substrate has very nice electrical characteristics with a low off-state current (Ioff) of 3 pA, a high on/off current ratio of 2 × 107, a high saturation mobility (μsat) of 66.7 cm2/V•s, a steep subthreshold slope (SS) of 333 mV/decade and a threshold voltage (Vth) of 1.2 V. Root-Mean-Square (RMS) roughness of TZO thin film is about 0.52 nm. The transmittance of TZO thin film is about 98%. These results highlight that the excellent device performance can be realized in TZO film and TZO TFT can be a promising candidate for flexible displays.