High Dielectric Constant Oxide Research Articles

Impact of Q-Time on the Passivation of Al2O3/p-In0.53Ga0.47As Interfaces Using Various Surface Treatments

The downscaling of complementary metal-oxide-semiconductor (CMOS)mayneedtheintegrationofIII-Vsemiconductorsandrelated high-k materials, since Si-based devices have reached their physical limitations. Unfortunately, the inherent poor interfacial quality of IIIV substrates and high dielectric constant gate oxides lead to the large defect state densities (Dit) which results in Fermi level pinning and the loss of channel controlling. 1,2 This issue is still a major challenge that needs to be addressed in spite of many achievements in interface passivation treatments. 3‐12 Recently, either ex-situ chemical and in-situ ALD surface treatments of the In0.53Ga0.47As substrates exhibited the strong minority carrier response at inversion region, which means Fermi level can be freely swept through the bandgap of III/V semiconductor. 9‐11 Similarly, O’Connor et al. 12 achieved the strong inversion behavior for n, p-In0.53Ga0.47As substrates with sulfur treatment and ALD-Al2O3 deposition. They also emphasized the impact of the delay time between the ex-situ chemical surface treatment and the ALD chamber loading (Q-time) on the quality of the high-k/III-V interface. The Q-time should be limited as low as possible; otherwise, the interfacial quality of high-k/III-V will be degraded. In this study, p-In0.53Ga0.47As substrates were treated by several chemical solutions with different Q-times prior to ALD chamber loading. Yet the nice C-V response was still observed even after 24 hours of Q-time for the MOSCAP structures studied. In terms of Q-time property, our results have supported the conclusion which might stand in contrast with previous studies, 7,8,12 i.e., reduced transfer time (less than few minutes) is not necessarily a prerequisite for an efficient chemical passivation of Al2O3/In0.53Ga0.47As interfaces. The nature of interface trap states discussed in this work would facilitate further understanding and passivating of high-k/III-V interfaces.

High dielectric constant nickel-doped titanium oxide films prepared by liquid-phase deposition

The electrical characteristics of nickel-doped titanium oxide films prepared by liquid-phase deposition on p-type (100) silicon substrate were investigated. The aqueous solutions of ammonium hexafluorotitanate and boric acid were used as precursors for the growth of titanium oxide films and the dielectric constant is 29. The dielectric constant can be improved to 94 by nickel doping at the thermal annealing at 700 °C in nitrous oxide.

Band offsets, Schottky barrier heights, and their effects on electronic devices

The authors review the band line-ups and band offsets between semiconductors, dielectrics, and metals,including the theory, experimental data, and the chemical trends. Band offsets have been critical in the choice of high dielectric constant oxides to be used in advanced metal oxide semiconductor field effect transistors. It turns out that band offsets are also critical in the theory of doping limits, design of transparent conducting oxides, organic semiconductors, and electrodes to use in light emitting devices, photovoltaic cells, and photochemical cells. It is shown how band line-ups can be understood in terms of charge neutrality levels. These are also related to states due to interstitial hydrogen.

Selective Sodium Sensing with Gold-Coated Silicon Nanowire Field-Effect Transistors in a Differential Setup

Ion-sensitive field-effect transistors based on silicon nanowires with high dielectric constant gate oxide layers (e.g., Al2O3 or HfO2) display hydroxyl groups which are known to be sensitive to pH variations but also to other ions present in the electrolyte at high concentration. This intrinsically nonselective sensitivity of the oxide surface greatly complicates the selective sensing of ionic species other than protons. Here, we modify individual nanowires with thin gold films as a novel approach to surface functionalization for the detection of specific analytes. We demonstrate sodium ion (Na(+)) sensing by a self-assembled monolayer (SAM) of thiol-modified crown ethers in a differential measurement setup. A selective Na(+) response of ≈-44 mV per decade in a NaCl solution is achieved and tested in the presence of protons (H(+)), potassium (K(+)), and chloride (Cl(-)) ions, by measuring the difference between a nanowire with a gold surface functionalized by the SAM (active) and a nanowire with a bare gold surface (control). We find that the functional SAM does not affect the unspecific response of gold to pH and background ionic species. This represents a clear advantage of gold compared to oxide surfaces and makes it an ideal candidate for differential measurements.

First principles study on InP (001)-(2 × 4) surface oxidation

A theoretical study of the oxidation of InP(001)-(2 × 4) surface is performed using density functional theory methods. Our results on surface oxidation show that the oxygen adsorption does not produce any gap states in the bulk InP band gap, due to the saturation of surface In dangling bonds, whereas substitutional oxygen atoms produce gap states. This study also shows that the surface stability increases with the oxygen content, indicating a strong tendency for surface oxidation. Our results help to clarify the origin of surface gap states upon surface oxidation and provide an insight at the atomic level the mechanism of surface oxidation, which will assist in the understanding of the degradation of III-V devices upon oxygen exposure or interfacial oxidation with high dielectric constant oxides.

Nitrogen passivation at GaAs:Al2O3 interfaces

The effects of nitrogen passivation at interfaces between III–V semiconductors and high dielectric constant oxides are calculated using the GaAs:Al2O3 interface as a model. Nitrogen replaces As from interface sites. The N-N dimer is found to be unstable and breaks into N dangling bonds. The N dangling bond has states lying well below the bulk valence band edge, away from the band gap, due to the electronegativity of nitrogen. Thus nitrogen is an effective passivant because it removes the states of two candidate species away from the gap.

High‐density remote plasma sputtering of high‐dielectric‐constant amorphous hafnium oxide films

Abstractmagnified imageHafnium oxide (HfOx) is a high dielectric constant (k) oxide which has been identified as being suitable for use as the gate dielectric in thin film transistors (TFTs). Amorphous materials are preferred for a gate dielectric, but it has been an ongoing challenge to produce amorphous HfOx while maintaining a high dielectric constant. A technique called high target utilization sputtering (HiTUS) is demonstrated to be capable of depositing high‐k amorphous HfOx thin films at room temperature. The plasma is generated in a remote chamber, allowing higher rate deposition of films with minimal ion damage. Compared to a conventional sputtering system, the HiTUS technique allows finer control of the thin film microstructure. Using a conventional reactive rf magnetron sputtering technique, monoclinic nanocrystalline HfOx thin films have been deposited at a rate of ∼1.6 nm min−1 at room temperature, with a resistivity of 1013 Ω cm, a breakdown strength of 3.5 MV cm−1 and a dielectric constant of ∼18.2. By comparison, using the HiTUS process, amorphous HfOx (x = 2.1) thin films which appear to have a cubic‐like short‐range order have been deposited at a high deposition rate of ∼25 nm min−1 with a high resistivity of 1014 Ω cm, a breakdown strength of 3 MV cm−1 and a high dielectric constant of ∼30. Two key conditions must be satisfied in the HiTUS system for high‐k HfOx to be produced. Firstly, the correct oxygen flow rate is required for a given sputtering rate from the metallic target. Secondly, there must be an absence of energetic oxygen ion bombardment to maintain an amorphous microstructure and a high flux of medium energy species emitted from the metallic sputtering target to induce a cubic‐like short range order. This HfOx is very attractive as a dielectric material for large‐area electronic applications on flexible substrates.

Thermally deposited Ag-doped CdS thin film transistors with high-k rare-earth oxide Nd2O3 as gate dielectric

The performance of thermally deposited CdS thin film transistors doped with Ag has been reported. Ag-doped CdS thin films have been prepared using chemical method. High dielectric constant rare earth oxide Nd2O3 has been used as gate insulator. The thin film trasistors are fabricated in coplanar electrode structure on ultrasonically cleaned glass substrates with a channel length of 50 μm. The thin film transistors exhibit a high mobility of 4.3 cm2 V−1 s−1 and low threshold voltage of 1 V. The ON-OFF ratio of the thin film transistors is found as 105. The TFTs also exhibit good transconductance and gain band-width product of 1.15 × 10−3 mho and 71 kHz respectively.

Solution-deposited Zr-doped AlOx gate dielectrics enabling high-performance flexible transparent thin film transistors

Although high dielectric constant (k) oxide thin film has been considered as a key element for high performance and low-voltage driven thin-film transistors (TFTs), there are no solution processable high-k oxide dielectrics that satisfy the stringent requirements of low-temperature processability, mechanical flexibility, and good TFT performance. Here, we demonstrate that the incorporation of a zirconium component that has strong bonding to oxygen enables a significant reduction in the processing temperature for soluble alumina dielectrics to as low as 250 °C. Based on these Zr-AlOx films, high performance, low operational voltage, flexible TFTs are achieved. Flexible TFTs operate well under a gate bias of 5 V, exhibiting a high saturation mobility of 51 cm2 V−1 s−1, an on/off current ratio of 104, and a low threshold voltage of 1.2 V with good mechanical flexibility. This is the first study demonstrating the mechanical flexibility of all-oxide soluble high-k dielectric–semiconductor-based TFTs with an emphasis on the influence of annealing temperature on the solution-deposited high-k oxide dielectric characteristics.

Integration of high-dielectric constant Ta2O5 oxides on diamond for power devices

The authors report on the direct integration of high-dielectric constant (high-k) Ta2O5 films on p-type single crystal diamond for high-power electronic devices. Crystallized hexagonal phase δ-Ta2O5 film is achieved on diamond by annealing the amorphous Ta2O5 film deposited by a sputter-deposition technique. The electrical properties of the Ta2O5 thin films are investigated by fabricating metal-insulator-semiconductor (MIS) diodes. The leakage current of the MIS diode is as low as 10−8 A/cm2 for the as-deposited amorphous Ta2O5 film and 10−2 A/cm2 for the crystallized film, which is 108 and 102 times lower than that of the Schottky diode at a forward bias of −3 V, respectively. The dielectric constant of the amorphous Ta2O5 films is measured to be 16 and increases to 29 after annealing at 800 °C. Different current leakage mechanisms and charge trapping behaviors are proposed for the amorphous and crystallized Ta2O5 thin films.

Electronic structure of epitaxial germanium—Metal germanate interfaces

We have calculated the electronic structure of various Ge-based high dielectric constant oxides LaGeOx and HfGeOx using hybrid functional methods. The band gaps of bulk La2Ge2O7 and La2GeO5 are found to be both close to 5.0 eV and the band gap of HfGeO4 is about 5.5 eV. We have also built fully bonded epitaxial interface models of LaGeOx, HfGeOx, and Al2O3 on Ge(100) with no dangling bonds. The calculated valence band offsets for LaGeOx, HfGeOx, and Al2O3 on Ge(100) are 3.0 eV, 3.0 eV, and 3.0 eV, respectively.

Thin film high dielectric constant metal oxides prepared by reactive sputtering

High dielectric constant, low loss dielectric thin film materials produced by reactive RF sputtering have been investigated for use as capacitor dielectrics in integrated circuits, using oxides of niobium, tantalum, titanium, hafnium, and zirconium and mixtures of these with aluminum oxide. High breakdown fields and low leakage currents are found for the best materials and a reduction in capacitor area of a factor of >3 compared with Si3N4 capacitors of the same value, using a simple production process compatible with semiconductor device manufacturing.

Defect compensation in LaAlO3 perovskite-based high dielectric constant oxides

We show that the gap states of the oxygen vacancy in LaAlO3 and related high dielectric constant (high K) perovskite oxides can be passivated by fluorine, or by substitutional nitrogen or by substitutional alkaline earth metal atoms at adjacent sites. The mechanism works by the substitutions completing an electronic closed shell, plus a repulsion of the now empty vacancy gap state into the conduction band by a relaxation of the adjacent Al and La ions away from the vacancy because it is locally +2 charged.

Thermal stability of high‐κ oxides on SiO2/Si or SixNy/SiO2/Si for charge‐trapping nonvolatile memories

High dielectric constant (high‐κ) oxides are foreseen replacement materials in innovative metal–oxide–semiconductor devices and memory capacitors. In particular, when considering nonvolatile memories, the charge‐trapping concept appears to be a promising solution for flash‐type floating gate replacement. Among the high‐κ oxide properties to be considered, it is essential to study the compatibility towards the integration of these materials in a complementary metal–oxide–semiconductor process, in particular to control the stack integrity and any onset of diffusion phenomena upon thermal treatments at temperatures higher than 1000 °C. Here, we report on the results obtained from time‐of‐flight secondary ion mass spectrometry depth profiling of stacks on the basis of high‐κ/SiO2/Si, integrating HfO2, ZrO2, or DyScOx as charge‐trapping layer or high‐κ/SixNy/SiO2/Si integrating DyScOx as control (blocking) oxide. The high‐κ oxides are all grown by atomic layer deposition. We will discuss the role of the different substrate/oxide coupling in preserving the stack and propose the better combinations in terms of thermal stability. Copyright © 2012 John Wiley & Sons, Ltd.

Recent progress in ab initio simulations of hafnia-based gate stacks

The continuous size downscaling of complementary metal–oxide–semiconductor (CMOS) transistors has led to the replacement of SiO2 with a HfO2-based high dielectric constant (or high-k) oxide, and the polysilicon electrode with a metal gate. The approach to this technological evolution has spurred a plethora of fundamental research to address several pressing issues. This review focusses on the large body of first principles (or ab initio) computational work employing conventional density functional theory (DFT) and beyond-DFT calculations pertaining to HfO2-based dielectric stacks. Specifically, structural, thermodynamic, electronic, and point-defect properties of bulk HfO2, Si/HfO2 interfaces, and metal/HfO2 interfaces are covered in detail. Interfaces between HfO2 and substrates with high mobility such as Ge and GaAs are also briefly reviewed. In sum, first principles studies have provided important insights and guidances to the CMOS research community and are expected to play an even more important role in the future with the further optimization and “scaling down” of transistors.

Room temperature fabricated transparent amorphous indium zinc oxide based thin film transistor using high-κ HfO2 as gate insulator

We report here the performance of amorphous oxide transparent thin film transistors (TFTs) fabricated using DC magnetron sputter techniques at room temperature. Amorphous indium zinc oxide (a-IZO, In2O3:ZnO, 90:10wt.%) and high dielectric constant hafnium oxide (HfO2) films were deposited for the semiconducting channel and gate insulator under a mixture of ambient argon and oxygen gas, respectively. The reported transparent TFT structure was a bottom-gate type, consisting of indium-tin-oxide and aluminium as gate electrode and source/drain electrodes, respectively. The optical transmittance of a-IZO films in the visible region was greater than 84%. The a-IZO TFTs showed that the field effect saturation mobility, Ion/Ioff ratio, sub-threshold swing, and threshold voltage were extracted to give 13.09cm2/V-s, 4×105, 0.42V/decade, and 3.54V, respectively. The reported a-IZO TFT with high dielectric constant HfO2 gate dielectric sputtered at room temperature presents high quality characteristics, which can be used as good candidate for the application of low-temperature fabricated optoelectronic devices on organic flexible substrates.

Optical properties and band gap characterization of high dielectric constant oxides

The optical characterization of amorphous and crystalline HfO2, SrHfO3, BaZrO3, BaHfO3, BaHf0.5Ti0.5O3, SrTiO3, Sr2Ta2O7 and Ta3Ti2Ox high dielectric constant thin (15–50nm) films was performed using a combination of spectroscopic ellipsometry and reflectometry in the photon-energy range 1.5–6.8eV. The optical dielectric function, absorption coefficient, optical band gap energies Eg, Urbach energy Eu and 3D interband critical point energies were obtained from these studies. The Eg as well as the Eu of the dielectric materials are important for device performance. Indirect Eg was obtained for all materials investigated. Lower Eg tends to result in higher value of permittivity κ. The results obtained deliver the necessary information for the selection of alternative high-κ dielectrics with adequate Eu, Eg and κ values and additionally provide optical metrology for such films.

Electrical and Structural Properties of Ternary Rare-Earth Oxides on Si and Higher Mobility Substrates and their Integration as High-k Gate Dielectrics in MOSFET Devices

The continuous downscaling in metal-oxide-semiconductor field effect transistors is approaching fundamental limits. Allied to new device architectures, novel materials are needed in order to continue the evolution of complementary metal-oxide-semiconductor technologies. The combination of high dielectric constant (k) oxides with silicon and other semiconductors having a higher charge carrier mobility (ex.: germanium) is currently a fundamental technologic issue that requires extensive investigation on materials science. The search for high-k oxides (with k > 20) that can offer stable interfaces combined with a low density of electrically active defects is a topic of major interest. In this contribution, we will review some of our results on the structural and electrical properties of REScO3 (RE = La, Gd, Tb, Sm) and LaLuO3 amorphous films on Si as well as on high mobility substrates, showing their potential as high-k dielectrics for future CMOS applications.

Thermal tunability in terahertz metamaterials fabricated on strontium titanate single-crystal substrates

We report an experimental demonstration of thermal tuning of resonance frequency in a planar terahertz metamaterial consisting of a gold split-ring resonator array fabricated on a bulk single-crystal strontium titanate (SrTiO₃) substrate. Cooling the metamaterial starting from 409 K down to 150 K causes about a 43% shift in resonance frequency, and there is very little variation in resonance strength. The resonance shift is due to the temperature-dependent dielectric constant of the strontium titanate. The experiment opens up avenues for designing tunable terahertz devices by exploiting the temperature-sensitive characteristic of high dielectric constant substrates and complex metal oxide materials.

The influence of high dielectric constant aluminum oxide sputter deposition on thestructure and properties of multilayer epitaxial graphene

High dielectric constant aluminum oxide (Al2O3) is frequently used as the gate oxide in high electron mobility transistors andthe impact of its deposition by radio frequency (RF) magnetron sputtering onthe structural and electrical properties of multilayer epitaxial graphene (MLG)grown by graphitization of silicon carbide (SiC) is reported. Micro-Ramanspectroscopy and temperature dependent Hall mobility measurements reveal thatthe processing induced changes to the structural and electrical propertiesof the MLG can be minimal when the oxide deposition conditions are optimal.High-resolution transmission electron microscopy (HRTEM) analysis confirms that theAl2O3/MLG interface is relatively sharp and that our thickness approximation of the MLG using angleresolved x-ray photoelectron spectroscopy (ARXPS) is accurate. An interface trap density of5.1 × 1010 eV − 1 cm − 2 wasdetermined using capacitance–voltage techniques. The totality of our results indicates that ARXPScan be used as a nondestructive tool to measure the thickness of MLG, and that RF sputteredAl2O3 can be used as a highdielectric (high-k) constant gate oxide in multilayer graphene based transistor applications.