ZrO<Subscript>2</Subscript> thin films on Si substrate

In the advancement of metal–oxide–semiconductor technology, Si-based semiconductor, with SiO₂ as outstanding dielectric, has been dominating microelectronic industry for decades. However, the drastic down-scaling in ultra-large-scale integrated circuitry has made ultrathin SiO₂ (~1.2 nm) unacceptable for many practical reasons. Introduction of ZrO₂ as high-κ dielectrics replacing SiO₂ is undeniably a potential yet formidable solution for the aforementioned problem. The objective of this review is to present the current knowledge of ZrO₂ thin film as gate dielectric on Si, in terms of its material and electrical properties produced by various deposition techniques. One of the techniques being focused is thermal oxidation of sputtered Zr and the mechanisms of transforming the metal into oxide has been extensively reviewed.     

X-ray diffraction studies of NbTe<Subscript>2</Subscript> single crystal

NbTe₂ is a member of transition metal dichalcogenide (TMDC) group. Single crystals of niobium ditelluride (NbTe₂) have been grown by a chemical vapour transport technique using iodine as transporting agent. The composition of the grown crystals was confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and remaining structural characterization was also accomplished by X-ray diffraction (XRD) studies. Lattice parameters, volume and X-ray density have been carried out for the grown crystals. The particle size for a number of reflections has been calculated using Scherrer’s formula.     

Characterization of ZrO<Subscript>2</Subscript> thin films deposited by MOCVD as ceramic coatings

Transparent cubic-, tetragonal-zirconia thin films were successfully deposited on glass and quartz substrates by using the metalorganic chemical deposition technique. The thin films were achieved by adjusting deposition parameters such as substrate temperature, oxygen partial pressure, and zirconium acetylacetonate (Zr(acac)₄) used as precursor. Structural and morphological characterizations of the as-deposited thin films were studied by XRD, Raman spectroscopy, SEM, and AFM techniques, while some optical properties such as transmittance and refractive index were determined by means of the UV–vis technique. The ZrO₂ films, grown at 700 °C and different P_{\textO 2} :P_{\textZr(acac)4} P_{{{\text{O}}_{ 2} }}{:}P_{{\text{Zr(acac)}_{4} }} ratios, displayed very variable particle sizes ranging from ~0.2 to 1.0 μm, and crystallite sizes within 10–30 nm forming a uniform film. Low mean roughness was obtained in the samples, which varied from 0.674 to 1.33 nm. These films grew with a columnar structure and apparently with low carbon content (<0.2%). All the synthesized thin films showed an adequate optical transmission, but the most transparent (>80%) was obtained with a P_{\textO 2} :P_{\textZr(acac)4} P_{{{\text{O}}_{ 2} }} {:}P_{{\text{Zr(acac)}_{4} }} ratio of (Pa) 107:0.2. The oxygen partial pressure influences the crystallinity of the as-deposited films, while the refractive index remains constant.     

Orientation and microstructure of chemoepitaxial ZrO<Subscript>2</Subscript> films

The orientation and microstructure of ZrO₂ films produced by oxidizing oriented thin Zr films have been studied by transmission electron microscopy and high-energy electron diffraction. The results demonstrate that biaxial textures of the oxide are governed by the textures of the parent zirconium film. We have established a set of orientation relationships between the Zr and ZrO₂ lattices. The nanocrystalline structure of the oxide is due to the fact that there are several equivalent orientations within one Zr grain (multiple-orientation chemoepitaxy). Using high-resolution transmission electron microscopy, we detected twin boundaries, stacking faults, and intragranular dislocations.     

Effect of V<Subscript>2</Subscript>O<Subscript>5</Subscript> content on the optical,structural and electrochromic properties of TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript> thin films

Vanadium oxide (V₂O₅) mixed titanium oxide (TiO₂) and zirconium oxide (ZrO₂) thin films were fabricated on glass substrates (corning 2947) and on indium tin oxide (ITO) coated glass substrates by sol gel spin coating process. Their optical, structural and electrochromic properties were investigated. The results were compared with pure TiO₂ and ZrO₂ thin films. Mixture of V₂O₅ with both types of film reduces the transmittance at the higher wavelengths. The refractive index of the V₂O₅ mixed TiO₂ and ZrO₂ films increases when compared with pure TiO₂ and ZrO₂ films. AFM images demonstrate no significant topographical changes for V₂O₅ mixed TiO₂ whereas for V₂O₅ mixed ZrO₂ films a topographical change is observed. V₂O₅ mixed TiO₂ showed slight increase in their charge capacity.     

Structure of multilayer ZrO<Subscript>2</Subscript>/SrTiO<Subscript>3</Subscript>

Multilayered oxide heteroepitaxial systems, including that of a 1-nm-thick Y₂O₃-stabilised ZrO₂ (YSZ) sandwiched between layers of SrTiO₃ (STO) [1], have been a subject of much interest lately due to their significantly enhanced ionic conductivities as compared to the bulk materials. We aim to provide the foundation for understanding this increase in conductivity by considering the atomic configurations at the interfaces of such systems, specifically a ZrO₂/STO multilayer system. Possible stable lattice structures of pure ZrO₂ in the system are explored using a genetic algorithm in which the interatomic interactions are modelled by simple pair potentials. The energies of several of the more stable of these structures are then evaluated more accurately within density functional theory (DFT). We find that the fluorite ZrO₂ phase is unstable as a coherently strained epitaxial layer in the multilayer system. Instead, anatase-, columbite-, rutile-, and pyrite-like ZrO₂ epitaxies are found to be more stable, with the anatase-like epitaxy being the most stable structure over a wide range of chemical potential of the components. We also find a high energy metastable structure resembling the tetragonal fluorite structure which is predicted by DFT to be stabilised by SrO-terminated STO but not by TiO₂-terminated STO.     

The ZrO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> phase diagram

The ZrO₂-TiO₂ phase diagram was determined experimentally between 800 and 1200°C, 1 atm, extending our knowledge of this system to temperatures previously inaccessible for equilibrium experiments due to sluggish kinetics. The crystallization of the ordered (Zr,Ti)₂O₄ phase from the oxides was facilitated by the addition of flux (CuO or Li₂MoO₄/MoO₃), and seeds. Two ordered (Zr,Ti)₂O₄ phases with different compositions were identified, and their phase relationships with TiO₂ and ZrO₂ solid solutions investigated. Structure data, superstructure reflections and composition were used to locate the ordering phase transition of (Zr,Ti)₂O₄ in equilibrium with ZrO₂ and TiO₂. At the onset of ordering between 1130 and 1080°C, (Zr,Ti)₂O₄ is of composition X_Ti = 0.495 ± 0.02, and displays a dramatic change in b-dimension. At 1060°C and below, the composition of (Zr,Ti)₂O₄ is significantly more Ti-rich and dependent on temperature, ranging from X_Ti = 0.576 at 1060°C to 0.658 at 800°C. This variability in composition of the ordered phase contrasts with previous studies that suggested the composition to be constant at either X_Ti = 0.667 [ZrTi₂O₆] or 0.583 [Zr₅Ti₇O₂₄]. When grown at low temperatures and with lithium molybdate, the crystals of ordered (Zr,Ti)₂O₄ are acicular to needle shape, and develop distinct square cross-sections and end facets.     

Preparation and Properties of Thin HfO<Subscript>2</Subscript> Films

HfO₂ layers were grown on silicon by metalorganic chemical vapor deposition using (C₅H₅)₂Hf(CH₃)₂, (C₅H₅)₂Hf(N(C₂H₅)₂)₂, and Hf(dpm)₄ as volatile precursors and were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, and IR spectroscopy. The films were shown to consist of monoclinic HfO₂ and to contain hafnium silicide and silicate at the HfO₂/Si interface. The presence of hafnium silicide was attributed to oxygen deficiency produced by argon ion milling. Hafnium silicate was formed as a result of the reaction between hafnium and silicon oxides during annealing. Current-voltage and capacitance-voltage measurements on Al/HfO₂/Si test structures were used to determine the dielectric permittivity and electrical resistivity of the films: ? = 15–20, ρ ～ 10¹⁵ cm.     

Phase analysis of the ZrO<Subscript>2</Subscript>-GeO<Subscript>2</Subscript> system

This paper presents a physicochemical study of poorly explored compounds in the zirconia-germania system. Reactions between these oxides were investigated by differential scanning calorimetry and thermogravimetry. The morphology of the reaction products was studied by scanning electron microscopy. The reaction products of ZrO₂ and GeO₂ powders were characterized by quantitative phase analysis. X-ray diffraction and Raman spectroscopy data indicate the formation of the germanates Zr₃GeO₈ and ZrGeO₄. These compounds are shown to be substitutional solid solutions, and their homogeneity ranges are determined. GeO₂ dissolution in ZrO₂ stabilizes its tetragonal structure.     

The effect of HfO<Subscript>2</Subscript> second phase in Fe films upon ion irradiation

Ferrum of BCC crystal structure is a typical kind of matrix in structural alloy steels which could be strengthened by introducing some second phase. In the present study, BCC Fe thin films with hafnium oxide (HfO₂) second phase have been synthesized in an electron beam evaporation system. Multi-layered and glancing angle deposition (GLAD) techniques were taken to form some HfO₂ second phase in Fe films. Ion irradiation was conducted to investigate the irradiation resistance of the obtained samples with and without HfO₂ second phase.     

Optical properties of CeO<Subscript>2</Subscript> thin films

Cerium oxide (CeO₂) thin films have been prepared by electron beam evaporation technique onto glass substrate at a pressure of about 6 × 10⁻⁶ Torr. The thickness of CeO₂ films ranges from 140–180 nm. The optical properties of cerium oxide films are studied in the wavelength range of 200–850 nm. The film is highly transparent in the visible region. It is also observed that the film has low reflectance in the ultra-violet region. The optical band gap of the film is determined and is found to decrease with the increase of film thickness. The values of absorption coefficient, extinction coefficient, refractive index, dielectric constant, phase angle and loss angle have been calculated from the optical measurements. The X-ray diffraction of the film showed that the film is crystalline in nature. The crystallite size of CeO₂ films have been evaluated and found to be small. The experimental d-values of the film agreed closely with the standard values.     

Nanocoating of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> additive on ZrO<Subscript>2</Subscript> powder and its effect on the sintering behaviour in ZrO<Subscript>2</Subscript> ceramic

ZrO₂ powder was coated with Al₂O₃ precursor generated by a polymeric precursor method in aqueous solution. The system of nanocoated particles formed a core shell-like structure in which the particle is the core and the nanocoating (additive) is the shell. A new approach is reported in order to control the superficial mass transport and the exaggerated grain growth during the sintering of zirconia powder. Transmission electron microscopy (TEM) observations clearly showed the formation of an alumina layer on the surface of the zirconia particles. This layer modifies the sintering process and retards the maximum shrinkage temperature of the pure zirconia.     

Synthesis and x-ray diffraction study of solid solutions in the Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript>-Zn<Subscript>2</Subscript>TiO<Subscript>4</Subscript>-Zn<Subscript>2</Subscript>ZrO<Subscript>4</Subscript> system

The multicomponent refractory oxide system Zn₂(Ti_aSn_b)_{1 ? x} Zr_xO₄ (a + b = 1; a: b = 1: 5, 1: 4, 1: 3, 1: 2, 1: 1, 1: 0, 2: 1, 3: 1, 4: 1; x = 0?1.0; Δx = 0.05) has been studied by x-ray diffraction, using samples prepared by melting appropriate oxide mixtures in a low-temperature hydrogen-oxygen plasma. Two phases, both with wide homogeneity ranges, have been identified: α-phase, with a cubic inverse spinel structure, and β-phase, with a tetragonal spinel structure. The phase boundaries in the system have been determined. Structural data are presented for about 100 solid solutions of different compositions.     

Microstructure and electrical properties of thin HfO<Subscript>2</Subscript> deposited by plasma-enhanced atomic layer deposition

The influence of annealing in the temperature range of 150–300 °C during plasma-enhanced atomic layer deposition of HfO₂ and the conditions of the following thermal processing on microstructure and electrical properties have been studied. The microstructure was examined by transmission electron microscopy and electron diffraction. The as-deposited HfO₂ film consists of monoclinic crystallites embedded in an amorphous matrix. It was found that the crystallite density grows with the increase in the deposition temperature; however, the size of the crystallites does not change. Subsequent annealing at 425 °C for 30 min or at 950 °C for 4 s led to complete crystallization through the lateral growth of the crystallites of samples formed at 250 and 300 °C. However, for the sample formed at 150 °C, subsequent annealing at 425 °C resulted in the formation of dendritic-like crystalline clusters embedded in an amorphous matrix. The leakage currents in polycrystalline and even amorphous HfO₂ films after the annealing were drastically increased. That could be explained by crystallization after the annealing. However, the C impurity redistribution and the growth of an interfacial layer could also affect the leakage.     

Structural transformations in thin Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films

Structural transformations in thin Ge₂Sb₂Te₅ films for phase-change memory applications have been studied by differential scanning calorimetry. As-grown, amorphous films have been shown to undergo structural transitions to a cubic and then to a hexagonal phase. A reproducible endothermic peak has been detected, which had not been reported earlier. A mechanism for the underlying process has been proposed.     

Y<Subscript>2</Subscript>O<Subscript>3</Subscript> and Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> co-stabilized ZrO<Subscript>2</Subscript>-WC composites

Y₂O₃ + Nd₂O₃ co-stabilized ZrO₂-based composites with 40 vol% WC were fully densified by pulsed electric current sintering (PECS) at 1350 °C and 1450 °C. The influence of the PECS temperature and Nd₂O₃ co-stabilizer content on the densification, hardness, fracture toughness and bending strength of the composites was investigated. The best combination of properties was obtained for a 1 mol% Y₂O₃ and 0.75 mol% Nd₂O₃ co-stabilized composite densified for 2 min at 1450 °C under a pressure of 62 MPa, resulting in a hardness of 15.5 ± 0.2 GPa, an excellent toughness of 9.6 ± 0.4 MPa.m^0.5 and an impressive 3-point bending strength of 2.04 ± 0.08 GPa. The hydrothermal stability of the 1 mol% Y₂O₃ + 1 mol% Nd₂O₃ co-stabilized ZrO₂-WC (60/40) composites was compared with that of the equivalent 2 mol% Y₂O₃ stabilized ceramic. The double stabilized composite did not degrade in 1.5 MPa steam at 200 °C after 4000 min, whereas the yttria stabilized composite degraded after less than 2000 min. Moreover, the (1Y,1Nd) ZrO₂-WC composites have a substantially higher toughness (~9 MPa.m^0.5) than their 2Y stabilized equivalents (~7 MPa.m^0.5).     

Dielectric properties and electrical conductivity of ZrO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript> ceramics

ZrO₂-CeO₂ (10, 18, and 23 mol % CeO₂) solid solutions have been synthesized via coprecipitation. The powders have been sintered at 1875 K, and the electrical conductivity and dielectric properties of the resultant ceramics have been studied. The dielectric relaxation observed in the ceramics can be understood in terms of ionic transport accompanied by the formation of dipoles relaxing in an ac electric field.     

ZrO<Subscript>2</Subscript> coatings on SiC fibers

Zirconia coatings have been produced on Hi-Nicalon fibers via pyrolysis of zirconium oxalates extracted into amine-benzene solutions. X-ray diffraction and electron probe x-ray microanalysis results demonstrate that the pyrolysis of extracts is a viable approach to producing thin zirconia coatings on coreless silicon carbide fibers and that further research is needed to optimize the microstructure of zirconia coatings.     

X-ray diffraction study of compounds in the Ag<Subscript>2</Subscript>S-Cu<Subscript>2</Subscript>S system

Ag_1.55Cu_0.45S and Ag_0.93Cu_1.07S single crystals have been grown by the Bridgman method, and their polymorphic transformations have been studied by high-temperature x-ray diffraction.