The scaling of complementary metal oxide semiconductor (CMOS) transistors has led to the silicon dioxide layer used as a gate dielectric becoming so thin that the gate leakage current becomes too large. This led to the replacement of SiO2 by a physically thicker layer of a higher dielectric constant or ‘high-K’ oxide such as hafnium oxide. Intensive research was carried out to develop these oxides into high quality electronic materials. In addition, the incorporation of Ge in the CMOS transistor structure has been employed to enable higher carrier mobility and performance. This review covers both scientific and technological issues related to the high-K gate stack – the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure, their electronic structure, band offsets, electronic defects, charge trapping and conduction mechanisms, reliability, mobility degradation and oxygen scavenging to achieve the thinnest oxide thicknesses. The high K oxides were implemented in conjunction with a replacement of polycrystalline Si gate electrodes with metal gates. The strong metallurgical interactions between the gate electrodes and the HfO2 which resulted an unstable gate threshold voltage resulted in the use of the lower temperature ‘gate last’ process flow, in addition to the standard ‘gate first’ approach. Work function control by metal gate electrodes and by oxide dipole layers is discussed. The problems associated with high K oxides on Ge channels are also discussed.

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High-K materials and metal gates for CMOS applications

This contribution is both a review of different aspects of X-ray photoelectron spectroscopy that can help one determine U oxidation states and a personal perspective on how to effectively model the X-ray photoelectron spectroscopy of complicated mixed-valence U phases. After a discussion of the valence band, the focus lingers on the U4f region, where the use of binding energies, satellite structures, and peak shapes is discussed in some detail. Binding energies were shown to be very dependent on composition/structure and consequently unreliable guides to oxidation state, particularly where assignment of composition is difficult. Likewise, the spin orbit split 4f7/2 and 4f5/2 peak shapes do not carry significant information on oxidation states. In contrast, both satellite-primary peak binding energy separations, as well as intensities to a lesser extent, are relatively insensitive to composition/structure within the oxide–hydroxide–hydrate system and can be used to both identify and help quantify U oxidation states in mixed valence phases. An example of the usefulness of the satellite structure in constraining the interpretation of a complex multivalence U compound is given. Copyright © 2011 John Wiley & Sons, Ltd.

XPS determination of uranium oxidation states

Due to its high intrinsic mobility, germanium (Ge) is a promising candidate as a channel material (offering a mobility gain of approximately??2 for electrons and??4 for holes when compared to conventional Si channels) However, many issues still need to be addressed before Ge can be implemented in high-performance field-effect-transistor (FET) devices One of the key issues is to provide a high-quality interfacial layer, which does not lead to substantial drive current degradation in both low equivalent oxide thickness and short channel regime In recent years, a wide range of materials and processes have been investigated to obtain proper interfacial properties, including different methods for Ge surface passivation, various high-k dielectrics and metal gate materials and deposition methods, and different post-deposition annealing treatments It is observed that each process step can significantly affect the overall metal?oxide?semiconductor (MOS)-FET device performance In this review, we describe and compare combinations of the most commonly used Ge surface passivation methods (eg epi-Si passivation, surface oxidation and/or nitridation, and S-passivation) with various high-k dielectrics In particular, plasma-based processes for surface passivation in combination with plasma-enhanced atomic layer deposition for high-k depositions are shown to result in high-quality MOS structures To further improve properties, the gate stack can be annealed after deposition The effects of annealing temperature and ambient on the electrical properties of the MOS structure are also discussed

https://iopscience.iop.org/article/10.1088/0268-1242/27/7/074012/pdf

Germanium surface passivation and atomic layer deposition of high-k dielectrics—a tutorial review on Ge-based MOS capacitors

Four metal-organic frameworks (MOF) with tetravalent uranium have been solvothermally synthesized by treating UCl4 with rigid dicarboxylate linkers in N,N-dimethylfomamide (DMF). The use of the ditopic ligands 4,4'-biphenyldicarboxylate (1), 2,6-naphthalenedicarboxylate (2), terephthalate (3), and fumarate (4) resulted in the formation of three-dimensional networks based on the hexanuclear uranium-centered motif [U6O4(OH)4(H2O)6]. This motif corresponds to an octahedral configuration of uranium nodes and is also known for thorium in crystalline solids. The atomic arrangement of this specific building unit with organic linkers is similar to that found in the zirconium-based porous compounds of the UiO-66/67 series. The structure of [U6O4(OH)4(H2O)6(L)6]⋅X (L = dicarboxylate ligand; X = DMF) shows the inorganic hexamers connected in a face-centered cubic manner through the ditopic linkers to build up a three-dimensional framework that delimits octahedral (from 5.4 A for 4 up to 14.0 A for 1) and tetrahedral cavities. The four compounds have been characterized by using single-crystal X-ray diffraction analysis (or powder diffraction analysis for 4). The tetravalent state of uranium has been examined by using XPS and solid-state UV/Vis analyses. The measurement of the Brunauer-Emmett-Teller surface area indicated very low values (Langmuir <300 m(2)  g(-1) for 1, <7 m(2)  g(-1) for 2-4) and showed that the structures are quite unstable upon removal of the encapsulated DMF solvent.

Three-dimensional MOF-type architectures with tetravalent uranium hexanuclear motifs (U6O8).

In order to achieve improved properties and performance with WC-based cemented carbides, research efforts have been directed towards the development of nanostructured cemented carbides. With the recent development of ‘spray conversion process’ for synthesizing nanosized powders and the advent of spark plasma sintering technique, it has been possible to successfully develop bulk nanostructured cemented carbides, possessing improved hardness and wear resistance. On a different note, realisation of the fact that the presence of metallic binder phase is deleterious towards certain applications of WC-based cermets has led to a recent surge of interest towards the development of novel ‘binderless’ WC-based ceramics by replacing the metallic binder phase with ceramic sinter-additives. More recently, it has been possible to develop dense WC-based ceramic composites without considerable deterioration of fracture toughness in the absence of the metallic binder phase. In the above perspective, the present review focuses mainly on the recent research results concerned with the processing and characterisation of nanostructured WC-based cermets and binderless WC-based ceramic composites.

Recent developments on WC-based bulk composites

XPS spectra of the U5+ compounds KUO3, NaUO3 and Ba2U2O7

The interrelationships between the microstructure and electrical discharge machining (EDM) behaviour of B 4 C–TiB 2 composites with respectively 30, 40 and 60 vol.% TiB 2 are investigated. Special attention was given to the influence of the grain size on the EDM behaviour by producing composites with an ultrafine TiB 2 phase using in situ synthesis during PECS. The experimental work revealed that 40 vol.% of TiB 2 results in an optimal material removal rate while the surface roughness for rough cut EDM decreases with increasing TiB 2 content. The finer microstructure of the ultrafine composite shows higher MRR's and lower R a values than the commercial powder based composites. The major material removal mechanism for the PECS based composites was melting. The 3 point bending strength of all composites after grinding, EDM rough cut and EDM finish cut was not statistically different and about 800 MPa. The EDM recast layer was analysed by X-ray photoelectron spectroscopy.

Electrical discharge machining of B4C–TiB2 composites

In situ NH3 plasma surface-nitridation treatments at 250 °C on both p- and n-type Ge(100) wafers were investigated. An ultrathin high quality GeOxNy interlayer was formed and exhibited dielectric breakdown for electric fields greater than 15 MV/cm. Well behaved capacitance-voltage characteristics were obtained for the complementary metal-oxide-semiconductor capacitors (CMOSCAPs) with HfO2(3 nm)/GeOxNy(1 nm) gate stacks. Gate leakage current density was below 5×10−7 A/cm2 at VFB±1 V for both MOSCAPs with an equivalent oxide thickness of 1.1 nm. Promising electrical properties of the CMOSCAPs indicate effective passivation of the Ge interface with GeOxNy interlayer formed by in situ NH3 plasma treatment.

Ultrathin GeOxNy interlayer formed by in situ NH3 plasma pretreatment for passivation of germanium metal-oxide-semiconductor devices

Electrical discharge machining of ZrO2 toughened WC composites

The interrelationships between the dispersion of the ZrO2 phase and the electrical discharge machining (EDM) behaviour of WC based composites with 0, 5 or 10 vol% of ZrO2 are investigated. Special attention was given to the homogeneity of the ultrafine WC – nanometric ZrO2 powder based composites which were fully densified by means of pulsed electric current sintering (PECS). X-ray photoelectron spectroscopy (XPS) measurements revealed the presence of a nanometric WO3 layer on the EDM surface, confirming oxidation as the major material removal mechanism (MRM). The surface roughness after the final EDM finishing regime was strongly influenced by the composite homogeneity and could be reduced down to 0.15 m( Ra) on agglomerate-free composites. Residual stress measurements indicated that EDM did not introduce a significant amount of surface stresses, especially not after the final finishing regime. XRD measurements of EDM surfaces however indicated surface depletion of ZrO2 by the formation of ZrC and W2C during spark erosion.

Junhu Liu

Papers

XPS spectra of the U5+ compounds KUO3, NaUO3 and Ba2U2O7

Electrical discharge machining of B4C–TiB2 composites

Ultrathin GeOxNy interlayer formed by in situ NH3 plasma pretreatment for passivation of germanium metal-oxide-semiconductor devices

Electrical discharge machining of ZrO2 toughened WC composites

Electrical Discharge Machining of ZrO2 Toughened WC Composites