Interfaces between - and silicon

As the epitaxy of crystalline LaAlO₃ has not been realized yet, we investigated the use of a γ-Al₂O₃ buffer layer between the high-κ and the substrate. We firstly studied the structural matching of γ-Al₂O₃(0 0 1) with a Si(0 0 1)-p(2×1) reconstructed surface. According to experimental data and computations in the density functional theory framework, we found stable interfaces between γ-Al₂O₃ and Si which encounters surface reconstruction changes. These interfaces satisfy the criterion of an insulating buffer layer.     

Development of robust interfaces based on crystalline γ-Al2O3(001) for subsequent deposition of amorphous high-κ oxides

This work reports on the development of thin crystalline γ-Al₂O₃(001) interfacial layers for subsequent deposition of amorphous high-κ oxides on Si(001). High quality and single crystal of γ-Al₂O₃ have been grown by Molecular Beam Epitaxy (MBE) on silicon. This γ-Al₂O₃/Si system leads to the formation of sharp and robust interfaces. They could be used for subsequent growth of amorphous high-κ oxides like lanthanum aluminate (LaAlO₃) without forming interfacial reactions. Despite high temperatures and oxygen pressure conditions during deposition or post-annealing processes, the heterostructures are stable with respect to the silicon substrate up to 850°C.     

Microstructure and Characteristics of Thin-Film La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>Co<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>O<Subscript>3</Subscript>/ZnO:Al Heterocontact CO Sensors Prepared by RF Magnetron Sputtering

Highly sensitive CO gas sensors based on heterocontacts of ZnO:Al on La_0.8Sr_0.2Co_0.5Ni_0.5O₃ (LSCNO) were fabricated successfully. La_0.8Sr_0.2Co_0.5Ni_0.5O₃ thin films were coated on (100) silicon wafers by a sol-gel method including the Pechini process followed by a spin-coating procedure. Then, ZnO:Al films prepared by radiofrequency (RF) magnetron sputtering at various oxygen partial pressures and deposited on as-deposited La_0.8Sr_0.2Co_0.5Ni_0.5O₃ films were investigated. The results revealed that the CO sensing ability of the film prepared with the ratio of O₂/Ar = 5/5 (ratio of volume flow rate) was the worst, owing to the highest (002) plane orientation in the ZnO:Al film. In contrast, the ZnO:Al film prepared with O₂/Ar = 3/7 exhibited better CO sensitivity. Furthermore, all two-layer samples showed higher CO sensitivities than single-layer samples. The CO sensitivity of ZnO:Al/La_0.8Sr_0.2Co_0.5Ni_0.5O₃ thin film was 45% for 500 ppm CO at a sensing temperature of 200°C.     

Preparation of α-alumina/polymethacrylic acid composite abrasive and its CMP performance on glass substrate

Chemical mechanic polishing (CMP) has become a widely accepted global planarization technology. Abrasive is one of key elements during CMP process. α-Alumina particles, as a kind of widely used abrasive in CMP slurries, often cause surface defects due to its high hardness and agglomeration. In order to enhance the dispersion stability and prevent agglomeration of pure alumina abrasives in CMP, α-alumina grafted with polymethacrylic acid (α-Al₂O₃-g-PMAA) composite abrasives was prepared and characterized by means of Fourier transform infrared (FTIR), Mastersizer 2000 instrument and scanning electron microscope (SEM). The results indicated that the prepared α-Al₂O₃-g-PMAA composite abrasives have better dispersion stability than pure α-Al₂O₃ abrasives. The CMP performances of the α-Al₂O₃-g-PMAA composite abrasives on glass substrate were investigated by SPEEDFAM-16B-4M CMP equipment with different process parameters. By using the optimum process parameters, the prepared abrasives exhibit better glass substrate CMP performance than pure α-Al₂O₃ abrasives. Further, the CMP mechanism of glass substrate was deduced preliminarily.     

The roles of charged and neutral oxidising species in silicon oxidation from ab initio calculations

We examine the roles of charged oxidising species based on extensive ab initio density functional theory calculations. Six species are considered: interstitial atomic O, O⁻, O²⁻ and molecular species: O₂, O₂⁻, O₂²⁻. We calculate their incorporation energies into bulk silicon dioxide, vertical electron affinities and diffusion barriers. In our calculations, we assume that the electrons responsible for the change of charge state come from the silicon conduction band; however, the generalisation to any other source of electrons is possible, and hence, our results are also relevant to electron-beam assisted oxidation and plasma oxidation. The calculations yield information about the relative stability of oxidising species, and the possible transformations between them and their charging patterns. We discuss the ability to exchange O atoms between the mobile species and the host lattice during diffusion, since this determines whether or not isotope exchange is expected. Our results show very clear trends: (1) the molecular species are energetically preferable over atomic ones, (2) the charged species are energetically more favourable than neutral ones, (3) diffusion of atomic species (O, O⁻, O²⁻) will result in oxygen exchange, whereas the diffusion of molecular species (O₂, O₂⁻, O₂²⁻) is not likely to lead to a significant exchange with the lattice. On the basis of our calculation, we predict that charging of oxidising species may play a key role in silicon oxidation process.     

A chromium-free etchant for delineation of defects in heavily doped n-type silicon wafers

Delineation of defects in the heavily doped n-type Czochralski silicon wafers by preferential etching is an issue not having been essentially solved. Herein, a chromium-free etchant based on HNO₃–HF–H₂O system, with an optimum volume ratio of V_HNO3%:V_HF%:V_H2O%=20%:45%:35%, has been developed. It can reveal well the defects such as dislocation and oxygen precipitation-induced bulk microdefects (BMDs) in the heavily doped n-type silicon wafers with resistivities even lower than 1 mΩ cm. Moreover, this etchant is appropriate to delineate the defects on (1 1 1), (1 1 0) or (1 0 0) surface of silicon crystal. Furthermore, the density of oxygen precipitation-induced BMDs in the heavily doped n-type silicon wafers derived from the preferential etching using this newly developed etchant correlates well with that derived from scanning infrared microscopy (SIRM) within its detection limit.     

Dielectric tunable properties of (Ba1−xSrx)TiO3 thin films on LaAlO3 substrate

(Ba_1−xSr_x)TiO₃ (1−x=0.8, 0.7, 0.6 and 0.5) thin films were prepared on (0 0 1) LaAlO₃ substrates by sol–gel method. The films were found to be crystallized in preferential (0 0 1) orientation after post-deposition annealing at 750°C for 1.5 h and 1100°C for 2 h in air, respectively. We investigated the dependence of tunability and dissipation factor on annealing temperature and different Ba/Sr ratios. It was found that the tunability increased dramatically and dissipation factor decreased obviously with increasing annealing temperature, and Ba_0.6Sr_0.4TiO₃ thin films annealed at 1100°C for 2 h have a tunability of 46.9% at 80 kV/cm bias filed and a dissipation factor of 0.008 at 1 MHz.     

Modeling the silicon–hafnia interface

We report first-principles calculations of the structure and electronic properties of several different silicon–hafnia interfaces. The structures have been obtained by growing HfO_x layers of different stoichiometry on Si(1 0 0) and by repeated annealing of the system using molecular dynamics. The interfaces are characterised via their electronic and geometric properties. Moreover, electronic transport through the interfaces has been calculated using finite-element-based Green's function methods. We find that oxygen always diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, saturated by either Hf diffusion or formation of Hf–Si bonds. The generally poor performance of the interfaces suggests that it is important to stabilise the system with respect to oxygen lattice diffusion.     

The characterization of retention properties of metal–ferroelectric (PbZr0.53Ti0.47O3)–insulator (Dy2O3, Y2O3)–semiconductor devices

Metal–ferroelectric–insulator–semiconductor (MFIS) capacitors and field effect transistors with the structures of Al/Pb (Zr_0.53, Ti_0.47) O₃ (PZT)/Dy₂O₃/Si and Al/PZT/Y₂O₃/Si were fabricated. The memory windows of Al/PZT/Dy₂O₃/Si and Al/PZT/Y₂O₃/Si capacitors with sweep voltage of 10 V are 1.03 V and 1.48 V, respectively. The effect of band offset on the memory window was discussed. The retention times of Al/PZT/Y₂O₃/Si and Al/PZT/Dy₂O₃/Si MFISFETs are 11.5 days and 11.1 h, respectively. The longer retention time of Al/PZT/Y₂O₃/Si field effect transistors is attributed to the larger conduction band offset at the Y₂O₃/Si interface (2.3 eV) compared to that of Dy₂O₃/Si (0.79 eV).     

Electrochemically Induced Crystalline-to-Amorphous Transition of Dinuclear Polyoxovanadate for High-Rate Lithium-Ion Batteries

Polyoxometalates are intriguing high-capacity anode materials for alkali-metal-ion storage due to their multi-electron redox capabilities and flexible structure. However, their poor electrical conductivity and high working voltage severely restrict their practical application. Herein, the dinuclear polyoxovanadate Sr₂V₂O₇·H₂O with unusually high electrical conductivity is reported as a promising anode material for lithium-ion batteries. During the initial lithiation process, the Sr₂V₂O₇·H₂O anode experiences an electrochemically induced crystalline-to-amorphous transition. The resulting amorphous structure provides high redox activity and fast reaction kinetics via reversible V^4.9+/V^2.8+ redox couple through the intercalation mechanism. Furthermore, when coupled with the LiFePO₄ cathode, the strong V O bonds of the amorphous anode provide excellent structural stability, with the full-cell capable of performing >12 000 cycles with a capacity retention of 72%. Another advantage of Sr_2xV₂O_7-δ·yH₂O (0.5 ≤ x ≤ 1.0) is its composition adjustability, which enables delicately regulating the Sr vacancy content without destroying the structure. The defect Sr_2xV₂O_7-δ·yH₂O (x = 0.5) electrodes show significantly improved specific capacity and rate capability without sacrificing other key properties, delivering a high specific capacity of 479 mAh g^-1 at 0.1 mA cm^-2 and 41.9% of its capacity in 2 min. Overall, the preliminary study points the way forward for the facile preparation of high-quality polyoxometalates for advanced energy storage applications and beyond.     

Properties of low temperature Sn–Ag–Bi–In solder systems

The metallurgical and mechanical properties of Sn–3.5 wt%Ag–0.5 wt%Bi–xwt%In (x = 0–16) alloys and of their joints during 85 °C/85% relative humidity (RH) exposure and heat cycle test (−40–125 °C) were evaluated by microstructure observation, high temperature X-ray diffraction analysis, shear and peeling tests. The exposure of Sn–Ag–Bi–In joints to 85 °C/85%RH for up to 1000 h promotes In–O formation along the free surfaces of the solder fillets. The 85°C/85%RH exposure, however, does not influence the joint strength for 1000 h. Comparing with Sn–Zn–Bi solders, Sn–Ag–Bi–In solders are much stable against moisture, i.e. even at 85 °C/85%RH. Sn–Ag–Bi–In alloys with middle In content show severe deformation under a heat cycles between −40 °C and 125 °C after 2500 cycles, due to the phase transformation from β-Sn to β-Sn + γ-InSn₄ or γ-InSn₄ at 125 °C. Even though such deformation, high joint strength can be maintained for 1000 heat cycles.     

Electrical characterization of metal-oxide-high-k dielectric-oxide-semiconductor (MOHOS) structures for memory applications

Conventional SONOS (polysilicon-oxide-nitride-oxide-silicon) non-volatile memory devices use silicon nitride as the charge storage layer. In this work, metal-oxide-high-k dielectric-oxide-silicon (MOHOS) structures are fabricated using HfO₂ and Dy₂O₃ high-k dielectrics as the charge storage layer. The Al/SiO₂/Dy₂O₃/SiO₂/Si capacitors have a C–V memory window of 1.88 V and a leakage current density of 10⁻⁸ A/cm². This leakage current is lower than those of Al/SiO₂/HfO₂/SiO₂/Si capacitors and other similar capacitors reported in the literature. A minimum detection window of 0.5 V for MOHOS capacitors can be maintained up to 2 × 10⁸ s using as-deposited Dy₂O₃. The better performance of the Al/SiO₂/Dy₂O₃/SiO₂/Si structure over Al/SiO₂/HfO₂/SiO₂/Si is attributed to the larger conduction band offset at the Dy₂O₃/SiO₂ interface (2.3 eV) versus 1.6 eV at the HfO₂/SiO₂ interface.     

Influence of passivating interlayer on Ge/HfO2 and Ge/Al2O3 interface band diagrams

The energy band alignment between Ge, HfO₂ and Al₂O₃ was analyzed as influenced by passivating interlayers (ILs) of different composition (GeO₂, Ge₃N₄, Si/SiO_x). From internal photoemission and photoconductivity experiments we found no IL-sensitive dipoles at the Ge/HfO₂ interfaces, the latter being universally characterized by conduction and valence band offsets of 2.1 and 3.0 eV, respectively. However, in the case of HfO₂ growth using H₂O-based atomic layer deposition, the Ge oxide IL appears to have a narrower bandgap, 4.3 eV, than the 5.4–5.9 eV gap of bulk germania. Accordingly, formation of this IL yields significantly reduced barriers for hole and, particularly, electron injection from Ge into the insulator. Changing to a H-free process for HfO₂ and Al₂O₃ deposition suppresses the formation of the narrow-gap Ge oxide.     

Transport and Thermoelectric Properties of the Ca1?xSrxRu1?yMnyO3 System

The magnetic, transport, and thermoelectric properties of Ca_1−x Sr _x Ru_1−y Mn _y O₃ have been investigated. Ferromagnetism with relatively high T _C (>200 K) was introduced by Mn doping. In particular, ferromagnetism appeared in the Ca_0.5Sr_0.5Ru_1−y Mn _y O₃ system at y > 0.2. The maximum T _C (=270 K) was recorded for a specimen of Ca_0.5Sr_0.5Ru_0.4Mn_0.6O₃. The ferromagnetism seems to be due to the mixed-valence states of Mn³⁺, Mn⁴⁺, Ru⁴⁺, and Ru⁵⁺ ions. The metallic character of Ru-rich specimens was suppressed by Mn substitution, and the system was transformed into a semiconductor at relatively low Mn content near y = 0.1. Specimens with higher Mn content (y > 0.8) had large thermoelectric power (50 μV K⁻¹ to 130 μV K⁻¹ at 280 K) accompanied by relatively low resistivity (0.03 Ω cm to 1 Ω cm). The Ca_0.5Sr_0.5Ru_1−y Mn _y O₃ system seems to have good potential as a thermoelectric material for use above 300 K.     

Preparation and characterization of the brownmillerite Sr2Co2O5 as novel photocatalyst in the hydrogen generation

Sr₂Co₂O₅ is a semiconductor belonging to the brownmillerite family; it is prepared by nitrate route and the photo-electrochemical properties are assessed for the first time for the photocatalytic hydrogen production. Thermal analysis indicates the formation of the semiconductor phase at 750 °C. An optical transition at 1.10 eV, directly allowed is obtained from the diffuse reflectance spectrum, due to the internal Co³⁺: d-d transition in octahedral coordination. A flat band potential of 0.037 V_SCE is determined in KOH solution (0.1 M) from the Mott-Schottky characteristic and the results are relevant for the water reduction. The conduction band of Sr₂Co₂O₅ (−0.85 V_SCE), deriving from Co³⁺: 3d orbital is more cathodic than the potential of H₂O/H₂ couple and hydrogen is successfully evolved under visible light. A rate evolution of 68 µmol (g catalyst)⁻¹ min⁻¹ at pH ∼ 12 and a light-to-chemical energy efficiency of 0.82% are determined.     

Optical interference effect of a thick absorbing LT-GaAs layer on a Bragg reflector

Near-infrared reflectance spectra of 5 μm thick low-temperature (LT) GaAs films on GaAs/AlAs Bragg reflectors (BRs) have been studied by model calculations as a function of the linear absorption coefficient of the films α_f. With increasing α_f, the reflectance of the stop band decrease monotonously. In contrast, the interference modulation due to the LT-GaAs layer on the BR inside of the stop band increases with increasing α_f until 1×10³ cm⁻¹ and decreases for larger α_f. This unusual behavior of the Fabry–Perot features in the stop band is explained by the attenuation of the light in the film as well as by the interference in the case of destructive superposition between the partial waves, which are strongly reflected at the film/BR interface, and those partial waves, which are much more weakly reflected at the air/film interface.     

Growth and structural properties of crystalline LaAlO3 on Si (0 0 1)

Crystalline LaAlO₃ was grown by oxide molecular beam epitaxy (MBE) on Si (0 0 1) surfaces utilizing a 2 ML SrTiO₃ buffer layer. This SrTiO₃ buffer layer, also grown by oxide MBE, formed an abrupt interface with the silicon. No SiO₂ layer was detectable at the oxide-silicon interface when studied by cross-sectional transmission electron microscopy. The crystalline quality of the LaAlO₃ was assessed during and after growth by reflection high energy electron diffraction, indicating epitaxial growth with the LaAlO₃ unit cell rotated 45° relative to the silicon unit cell. X-ray diffraction indicates a (0 0 1) oriented single-crystalline LaAlO₃ film with a rocking curve of 0.15° and no secondary phases. The use of SrTiO₃ buffer layers on silicon allows perovskite oxides which otherwise would be incompatible with silicon to be integrated onto a silicon platform.     

Growth of chromium-doped alumina thin films by metal-organic chemical vapour deposition

Amorphous thin films of chromium-doped alumina were grown from Al(OⁱPr)₃ (ⁱPr, Isopropyl) and Cr(CO)₆ on silicon and quartz substrates by chemical vapour deposition at 673 K. The films were annealed at 1223 and 1473 K to form chromium-doped γ-Al₂O₃ and γ-Al₂O₃respectively. The lattice constant a of the γ-Al₂O₃thin films enlarged with increasing Cr Concentration. The lattice constant a (=b) of the α-Al₂O₃ thin films increased while the lattice constant c decreased with increasing Cr concentration. UV-visible spectra of the annealed films showed maximum absorptions near 380, 500 and 690 nm.     

Effect of Na2O content on properties of beta alumina solid electrolytes

Beta alumina ceramics are well-known fast-ionic conductors of Na-ion. The effect of Na₂O content (8.00–13.19%) on properties of beta alumina solid electrolytes was studied in the paper by XRD, SEM and EIS. The results showed that in the Na₂O–Al₂O₃ system, the β″-Al₂O₃ phase was formatted at the low temperature of 1250 °C, however the noticeable transformation from β″-Al₂O₃ to β-Al₂O₃ occurred in the higher temperature range. The β″-phase content of the precursor powders was independent of the Na₂O content, but β″-phase content, the volume density and the bending strength of beta alumina sinters grew with the increase of the Na₂O content. Samples with 10.84% Na₂O possessed the optimum ionic conductivity and the most uniform microstructure. The ionic conductivity depends not only on the phase content, but also on the phase microstructure.     

Investigation of high-K gate stacks with epitaxial Gd2O3 and FUSI NiSi metal gates down to CET=0.86 nm

Novel gate stacks with epitaxial gadolinium oxide (Gd₂O₃) high-k dielectrics and fully silicided (FUSI) nickel silicide (NiSi) gate electrodes are investigated. Ultra-low leakage current densities down to 10^–7 A cm^–2 are observed at a capacitance equivalent oxide thickness of CET=1.8 nm. The influence of a titanium nitride (TiN) capping layer during silicidation is studied. Furthermore, films with an ultra-thin CET of 0.86 nm at a Gd₂O₃ thickness of 3.1 nm yield current densities down to 0.5 A cm⁻² at V_g=+1 V. The extracted dielectric constant for these gate stacks ranges from k=13 to 14. These results emphasize the potential of NiSi/Gd₂O₃ gate stacks for future material-based scaling of CMOS technology.