Properties Of Gate Dielectrics Research Articles

Triacetate cellulose gate dielectric organic thin-film transistors

Here, we report on the performance and the characterization of all solution-processable top-contact organic thin-film transistors (OTFTs) consisting of a natural-resourced triacetate cellulose gate dielectric and a representative hole-transport poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) semiconductor layer on rigid or flexible substrates. The bio-based triacetate cellulose layer has an important role in the OTFT fabrication because it provides the pBTTT semiconducting polymer with highly suitable gate dielectric properties including a low surface roughness, hydrophobic surface, appropriate dielectric constant, and low leakage current. The triacetate cellulose gate dielectric-based pBTTT OTFTs exhibit an average filed-effect mobility of 0.031 cm2/Vs similar to that obtained from a SiO2 gate dielectric-based OTFT device in ambient conditions. Even after a bending stimulation of 100 times and in an outward bending state, the flexible triacetate cellulose gate pBTTT OTFT device still showed excellent electrical device performance without any hysteresis.

(Invited) Gate Stack Technology for Advanced AlGaN/GaN Mos-Hemt Power Devices

AlGaN/GaN high electron mobility transistor (HEMT) has gained much attention as next-generation high frequency and high power devices. Since AlGaN/GaN HEMT with Schottky gate is restricted in device application due to large gate leakage current and normally-on operation, metal-oxide-semiconductor (MOS) gate stack with deposited gate insulator has been widely investigated to overcome these limitations. Among various insulating materials, aluminum based oxide such as Al2O3 is one of the potential candidate because of its wide bandgap and high thermal stability. However, large amount of interface traps at MOS interfaces severely degrade both carrier mobility and threshold voltage (Vth) stability. Furthermore, significant electron injection into Al2O3 layer causing positive Vth shift was often observed for Al2O3/AlGaN/GaN MOS-HEMT. We have recently reported that N incorporation into Al2O3 (aluminum oxynitride) significantly reduces not only the negative fixed charges but also the electron traps in Si and SiC MOS devices. In this study, superior physical and electrical properties of AlON gate dielectrics on AlGaN/GaN substrates were demonstrated. We fabricated high-quality AlON/AlGaN/GaN MOS capacitors with improved stability against charge injection and a reduced interface state density. Moreover, the impact of nitrogen incorporation into the gate insulator will be discussed on the basis of results from physical characterizations.

Comparison of HfAlO, HfO2/Al2O3, and HfO2 on n-type GaAs using atomic layer deposition

Different high-permittivity (high-k) gate dielectric structures of HfO2, HfAlO, and HfO2/Al2O3 deposited on HF-etched n-GaAs using ALD have been investigated. It has been demonstrated that the stacked structure of HfO2/Al2O3 has the lowest interface state density of 8.12 × 1012eV−1 cm−2 due to the “self-cleaning” reaction process, but the sample of HfAlO shows much better frequency dispersion and much higher dielectric permittivity extracted from the C-V curves. The investigation reveals that the electrical properties of gate dielectrics are improved by introducing alumina into HfO2.

ZrO2 and HfO2 dielectrics on (001) n-InAs with atomic-layer-deposited in situ surface treatment

The electrical properties of ZrO2 and HfO2 gate dielectrics on n-InAs were evaluated. Particularly, an in situ surface treatment method including cyclic nitrogen plasma and trimethylaluminum pulses was used to improve the quality of the high-κ oxides. The quality of the InAs-oxide interface was evaluated with a full equivalent circuit model developed for narrow band gap metal-oxide-semiconductor (MOS) capacitors. Capacitance–voltage (C–V) measurements exhibit a total trap density profile with a minimum of 1 × 1012 cm−2 eV−1 and 4 × 1012 cm−2 eV−1 for ZrO2 and HfO2, respectively, both of which are comparable to the best values reported for high-κ/III-V devices. Our simulations showed that the measured capacitance is to a large extent affected by the border trap response suggesting a very low density of interface traps. Charge trapping in MOS structures was also investigated using the hysteresis in the C–V measurements. The experimental results demonstrated that the magnitude of the hysteresis increases with increase in accumulation voltage, indicating an increase in the charge trapping response.

Photo-Cross-Linkable Organic–Inorganic Hybrid Gate Dielectric for High Performance Organic Thin Film Transistors

In this study, we have demonstrated a novel organic–inorganic hybrid gate dielectric material, zirconium tetraacrylate (ZrTA). ZrTA gate dielectric, where inorganic Zr elements are embedded in organic acrylate matrix, takes advantage of the complementary properties of single organic or inorganic gate dielectrics. A simple spin-coating and UV-assisted cross-linking reaction of acrylate moieties allowed ZrTA film to be photopatterned. The cross-linked ZrTA film by UV and heat treatments (UV, 365 nm for 3 min; heat, 120 °C for 30 min) showed high dielectric strength (10–7 A/cm2 at 2 MV/cm), and dielectric constant (5.48). In addition, surface properties of the ZrTA film (surface energy, surface roughness) were favorable for the growth of overlying pentacene organic semiconductor. Consequently, the organic thin-film transistor composed of a pentacene semiconductor and a cross-linked ZrTA gate dielectric displayed a moderately high field-effect mobility of 0.50 cm2/(V·s) with a negligible hysteresis transfer characteristic.

Microstructure, optical and electrical properties of solution-derived peroxo-zirconium oxide gate dielectrics for CMOS application

Solution-processed high-k gate dielectrics for metal-oxide-semiconductor (MOS) capacitor have been widely investigated with the objective of achieving high performance field effect transistors (FETs) for next-generation microelectronic device. In current work, the effect of postdeposition annealing temperature on the microstructure, optical and electrical properties of peroxo-zirconium oxide (ZrO2) gate dielectrics fabricated by a simple spin-coating method has been studied systemically. By means of characterization of spectroscopy ellipsometry and ultraviolet–visible spectroscopy, the thicknesses and optical constants of ZrO2 gate dielectrics have been determined precisely. The annealing temperature dependent electrical properties of ZrO2 films were determined by capacitance–voltage (C–V) and leakage current density–voltage (J–V) measurements. Based on the implementation as gate insulator, the solution-processed ZrO2-based MOS capacitor showed good and stable performances during annealing at 700°C. As a result, the dominant leakage conduction mechanism for optimized 700°C-annealed sample has been discussed in detail. Our results show the possibility of the solution-processed ZrO2 dielectric layer as a gate insulator for application in MOSFETs.

Effects of boron incorporation on the structural, optical and electrical properties of sol–gel-derived ZrO2 gate dielectrics

The effect of boron (B) doping on the microstructure, band gap energy and electrical properties of ZrO2 gate dielectrics deposited by sol–gel method at low temperature were systemically investigated. By means of characterization of spectroscopy ellipsometry and ultraviolet–visible spectroscopy, the thicknesses and optical constants of ZrO2 gate dielectrics have been determined precisely. The B incorporation dependent electrical properties of ZrO2 films were determined by capacitance–voltage (C–V) and leakage current density–voltage (J–V) measurements. The dominant conduction mechanisms of Al/B:ZrO2/n-Si MOS structures have been discussed systematically in detail. As a result, the optimized B incorporation content has been obtained to achieve ZrO2 gate dielectrics with high quality.

Deposition-power-modulated optical and electrical properties of sputtering-derived HfTiOx gate dielectrics

High-k gate dielectric HfTiOx thin films have been deposited on Si and quartz substrate by radio frequency (RF) magnetron sputtering. The structural and optical properties of HfTiOx thin films related to deposition power are investigated by X-ray diffraction (XRD), ultraviolet–visible spectroscopy (UV–Vis), and spectroscopic ellipsometry (SE). Results indicate that the as-deposited HfTiOx thin films are amorphous state regardless of the deposition power. The increase of band gap of the samples is observed with the increase of deposition power. Moreover, the increase of the thickness, deposition rate, refractive index (n) and the decrease of the extinction coefficient with the increase of deposition power are also confirmed. Additionally, the electrical properties of films are analyzed by measurement of high frequency capacitance–voltage (C–V) and leakage current density-voltage (J-V) characteristics. And the leakage current conduction mechanisms are also discussed.

Effects of rapid thermal annealing on interfacial and electrical properties of Gd-doped HfO2 high-k gate dielectrics

In this work, we systematically investigated the effects of rapid thermal annealing on the interfacial and electrical properties, as well as band offsets of sputtering-derived Gd-doped HfO2 film on Si substrate. The results indicate that annealing temperature has little effect on the interfacial properties of Gd-doped HfO2 thin films, and the main component of interfacial layer is silicate without any silicide. Moreover, heat treatment can effectively improve the C–V characteristics of the samples, such as the increase of effective permittivity, reduction in ΔVfb and no obvious frequency dispersion. However, the increase of annealing temperature leads to the increase of leakage current density of Al/Gd-doped HfO2/Si MOS capacitor due to the decrease of the conduction band offset (ΔEc) and further crystallization of the samples. These investigation results might provide the guidance for the performance optimization of high k gated dielectrics to meet the requirements of future complementary-metal-oxide-semiconductor (CMOS) devices.

Deposition temperature dependent optical and electrical properties of ALD HfO2 gate dielectrics pretreated with tetrakisethylmethylamino hafnium

The effect of deposition temperature on the growth rate, band gap energy and electrical properties of HfO2 thin film deposited by atomic layer deposition (ALD) has been investigated. By means of characterization of spectroscopy ellipsometry and ultraviolet–visible spectroscopy, the growth rate and optical constant of ALD-derived HfO2 gate dielectrics are determined precisely. The deposition temperature dependent electrical properties of HfO2 films were determined by capacitance–voltage (C–V) and leakage current density–voltage (J–V) measurements. The leakage current mechanism for different deposition temperature has been discussed systematically. As a result, the optimized deposition temperature has been obtained to achieve HfO2 thin film with high quality.

Determination of optical constant and electrical properties of sputtering-derived HfTiON gate dielectrics

Nitrogen content dependent optical constant and electrical properties of sputtering-derived HfTiON gate dielectrics on Si substrate have been investigated. By means of characterization of ultraviolet–visible spectroscopy, reduction in band gap of HfTiON film has been detected with the increase in nitrogen concentration. Electrical measurements of MOS capacitor based on Al/HfTiON/Si gate stacks have indicated that a low interface-state density with almost ignorable frequency dispersion at applied gate voltage of 1V have been obtained for all samples. With increasing the nitrogen concentration, the improved electrical performance has been observed. For the sample with Ar/N2 flow ratio of 20/5, rapid thermal annealing (RTA) treatment at 200°C achieves relatively favorable excellent electrical properties in terms of low gate leakage current density and oxide charge density (Qox). The conduction mechanism for 200°C-annealed sample has been analyzed systematically.

Engineering gate dielectric surface properties for enhanced polymer field-effect transistor performance

A high-performance “hybrid” dual-silane SAM enables the attainment of both a high mobility and on/off ratio, together with other desirable FET properties.

Effect of annealing temperature on structural and electrical properties of high-κ YbTixOy gate dielectrics for InGaZnO thin film transistors

This paper describes the effect of annealing temperature on the structural properties and electrical characteristics of high–κ YbTixOy gate dielectrics for indium–gallium–zinc–oxide (IGZO) thin-film transistors (TFTs). X-ray diffraction, x-ray photoelectron spectroscopy and atomic force microscopy were used to study the structural, chemical and morphological features, respectively, of these dielectric films annealed at 200, 300 and 400 °C. The YbTixOy IGZO TFT that had been annealed at 400 °C exhibited better electrical characteristics, such as a small threshold voltage of 0.53 V, a large field-effect mobility of 19.1 cm2 V−1 s−1, a high Ion/Ioff ratio of 2.8 × 107, and a low subthreshold swing of 176 mV dec.−1, relative to those of the systems that had been subjected to other annealing conditions. This result suggests that YbTixOy dielectric possesses a higher dielectric constant as well as lower oxygen vacancies (or defects) in the film. In addition, the instability of YbTixOy IGZO TFT was studied under positive gate-bias stress and negative gate-bias stress conditions.

Comparison of structural and electrical properties of Lu2O3 and Lu2TiO5 gate dielectrics for α-InGaZnO thin-film transistors

We compared the structural properties and electrical characteristics of high-κ Lu2O3 and Lu2TiO5 gate dielectrics for amorphous indium-gallium-zinc oxide (α-InGaZnO) thin-film transistor (TFT) applications. The Lu2O3 film has a strong Lu2O3 (400) peak in the X-ray diffraction pattern, while the Lu2TiO5 sample shows a relatively weak Lu2TiO5 (102) peak. Atomic force microscopy reveals that the Lu2O3 dielectric exhibits a rougher surface (about three times) than Lu2TiO5 one. In X-ray photoelectron spectroscopy analysis, we found that the intensity of the O 1s peak corresponding to Lu(OH)x for Lu2O3 film was higher than that of Lu2TiO5 film. Furthermore, compared with the Lu2O3 dielectric, the α-InGaZnO TFT using the Lu2TiO5 gate dielectric exhibited a lower threshold voltage (from 0.43 to 0.25 V), a higher Ion/Ioff current ratio (from 3.5 × 106 to 1.3 × 108), a smaller subthreshold swing (from 276 to 130 mV/decade), and a larger field-effect mobility (from 14.5 to 24.4 cm2/V s). These results are probably due to the incorporation of TiOx into the Lu2O3 film to form a Lu2TiO5 structure featuring a smooth surface, a low moisture absorption, a high dielectric constant, and a low interface state density at the oxide/channel interface. Furthermore, the stability of Lu2O3 and Lu2TiO5 α-InGaZnO TFTs was investigated under positive gate-bias stress (PGBS) and negative gate-bias stress (NGBS). The threshold voltage of the TFT performed under NGBS is more degradation than that under PGBS. This behavior may be attributed to the electron charge trapping at the dielectric–channel interface under PGBS, whereas the oxygen vacancies occurred in the InGaZnO under NGBS.

Impact of nitrogen depth profiles on the electrical properties of crystalline high-K gate dielectrics

The electrical characteristics of crystalline ZrO2 gate dielectrics with different nitrogen depth profiles were investigated, which were treated by the in-situ atomic layer doping of nitrogen and post-deposition nitridation processes, respectively, using remote NH3 plasma at a low treatment temperature of 250°C. The crystalline ZrO2 gate dielectric of the tetragonal/cubic phase was formed by post-metallization annealing (PMA) at a low temperature of 450°C, resulting in an increase of the dielectric constant. As compared with the in-situ atomic layer doping of nitrogen, the post-deposition nitrogen process leads to a lower capacitance equivalent thickness of 1.13nm with a low leakage current density of 1.35×10−5A/cm2. The enhanced capacitance density caused by the post-deposition nitrogen treatment may be ascribed to the high nitrogen concentration at the top surface of gate dielectric, giving rise to the suppression of oxygen diffusion from the ambient toward the interface and so a thinner interfacial layer. The result reveals that the nitrogen incorporation at the top surface of gate oxide is favorable to the scaling of crystalline high-K gate dielectrics.

RF Power Effect of Post-Deposition Oxygen Plasma Treatment on HfO2 Gate Dielectrics

The RF power effect of post-deposition O2 plasma treatment on the physical, electrical characteristics, and reliability performance of high-k HfO2 dielectric films were investigated in this study. Increasing RF power in post-deposition O2 plasma treatment results in a stoichiometric HfO2 film, but leading to a thinner interfacial layer with a new Hf-Si bond. Additionally, the electrical performance and reliability of high-k HfO2 dielectric films were related to the RF power in post-deposition O2 plasma treatment. As RF power is less than 50 W, the leakage current density and time-to-breakdown were improved as compared to those of un-treated samples. However, these properties were continuously degraded with an increase of RF power due to plasma damage induced by oxygen active spices in a O2 plasma environment. Therefore, the post-deposition O2 plasma treatment is benefit in the improvement of HfO2 gate dielectric properties, but the plasma power is needed to be well controlled.

Annealing temperature dependence on the structural and optical properties of sputtering-grown high-k HfO2 gate dielectrics

High-k gate dielectric HfO2 thin films have been deposited on Si and quartz substrates by radio frequency magnetron sputtering. The structural characteristics, surface morphology, and optical properties of the HfO2/Si gate stacks at various post-annealing temperatures were examined by X-ray diffraction (XRD), atomic force microscopy (AFM), fourier transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–Vis spectroscopy), and spectroscopic ellipsometry (SE). XRD measurement indicates that the 80 W-deposited HfO2 films demonstrate a polycrystalline structure. AFM measurements illustrate that the root mean square of the HfO2 thin films demonstrates an apparent increase with increasing the annealing temperature. Analysis from FTIR indicates that the Si–O–Si bonds vibration peak position shift toward lower wave numbers with increasing the annealing temperature. Combined with UV–Vis spectroscopy and SE measurements, it can be noted reduction in band gap with an increase in annealing temperature has been confirmed. Additionally, increase in refractive index (n) has been confirmed by SE.

Atomically flat La-silicate/Si interface using tungsten carbide gate electrode with nano-sized grain

Interface properties of La-silicate gate dielectrics on Si substrates with W or nano-sized grain W2C gate electrodes have been investigated. A low interface state density of 2.5 × 1011 cm−2/eV has been achieved with W2C gate electrodes, which is one third of those with W gate electrode. An interface roughness of 0.33 nm with spatial frequency comparable to the grain size of W gate electrode has been observed. Besides, an atomically flat interface of 0.12 nm has been obtained with W2C gate electrode. The origin of flat interface may be attributed to the elimination of inhomogeneous stress by grains in metal electrode.

Effects of Annealing Conditions on the Dielectric Properties of Solution-Processed Al<SUB>2</SUB>O<SUB>3</SUB> Layers for Indium-Zinc-Tin-Oxide Thin-Film Transistors

In this paper, the effects of annealing conditions on the dielectric properties of solution-processed aluminum oxide (Al2O3) layers for indium-zinc-tin-oxide (IZTO) thin-film transistors (TFTs) have been investigated. The dielectric properties of Al2O3 layers such as leakage current density and dielectric strength were largely affected by their annealing conditions. In particular, oxygen partial pressure in rapid thermal annealing, and the temperature profile of hot plate annealing had profound effects on the dielectric properties. From a refractive index analysis, the enhanced dielectric properties of Al2O3 gate dielectrics can be attributed to higher film density depending on the annealing conditions. With the low-temperature-annealed Al2O3 gate dielectric at 350 degrees C, solution-processed IZTO TFTs with a field-effect mobility of approximately 2.2 cm2/Vs were successfully fabricated.

Comparative study of arylene bisimides substituted with imidazole side group for different dielectrics on the OFET application

We analyzed the effect of polymeric dielectric with hydroxyl and hydroxyl-free group on current–voltage characteristics of organic thin film transistor by the use of benzimidazole-derived arylene bisimide derivatives. Polyvinyl alcohol (PVA) with polar group and benzocyclobutene (BCB) with non-polar group were used as solution processed dielectric materials to compare with each other in thin film transistor application. The hydroxyl group has a significant effect on turn-on voltage and turn-off current in depletion regime due to hydroxyl group. It is observed that the surface morphology is influenced by the chemical structure of polymeric dielectric concerning surface energy. The electron field effect mobility of both arylene bisimides is enhanced by decreased surface energy of dielectric. The highest mobility was obtained by employing naphthalene bis-benzimidazole as an active layer on both dielectrics compared with perylene bis-benzimidazole semiconductor. The electrical behaviors of these semiconductors are discussed in relation to gate dielectric surface properties.