Annealing behavior of TiO<Subscript>2</Subscript>-sheathed Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires

TiO₂-sheathed Ga₂O₃ one-dimensional (1D) nanostructures were synthesized by thermal evaporation of GaN powders and then sputter-deposition of TiO₂. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis results indicate that the Ga₂O₃ cores are of a single crystal nature with a monoclinic structure while the TiO₂ shells are amorphous. Photoluminescence (PL) emission is slightly decreased in intensity by TiO₂ coating, but it is significantly increased by thermal annealing in an oxygen atmosphere. The emission peak is also shifted from ~500 to ~550 nm by oxygen annealing. The increase in the green emission is due to the increase in the concentration of the Ga vacancies in the cores by the inflow of oxygen during oxygen annealing. On the other hand, annealing in a nitrogen atmosphere leads to a red shift of the emission to ~700 nm originating from nitrogen doping.     

RBS,XRR and optical reflectivity measurements of Ti–TiO2 thin films deposited by magnetron sputtering

Single-, bi- and tri-layered films of Ti–TiO₂ system were deposited by d.c. pulsed magnetron sputtering from metallic Ti target in an inert Ar or reactive Ar + O₂ atmosphere. The nominal thickness of each layer was 50 nm. The chemical composition and its depth profile were determined by Rutherford backscattering spectroscopy (RBS). Crystallographic structure was analysed by means of X-ray diffraction (XRD) at glancing incidence. X-ray reflectometry (XRR) was used as a complementary method for the film thickness and density evaluation. Modelling of the optical reflectivity spectra of Ti–TiO₂ thin films deposited onto Si(1 1 1) substrates provided an independent estimate of the layer thickness. The combined analysis of RBS, XRR and reflectivity spectra indicated the real thickness of each layer less than 50 nm with TiO₂ film density slightly lower than the corresponding bulk value. Scanning Electron Microscopy (SEM) cross-sectional images revealed the columnar growth of TiO₂ layers. Thickness estimated directly from SEM studies was found to be in a good agreement with the results of RBS, XRR and reflectivity spectra.     

Epitaxial growth of relaxed germanium layers by reduced pressure chemical vapour deposition on (110) and (111) silicon substrates

The growth of Ge on (110) and (111) oriented Si substrates is of great interest to enhance the mobility of both holes and electrons in complementary metal oxide semiconductor transistors. However, the quality of thick, relaxed Ge layers grown epitaxially on these surfaces is usually much lower than similar layers grown on (100) Si, resulting in both higher defect densities (i.e. threading dislocations and stacking faults) and rougher surfaces. In this work we have investigated the growth of Ge layers on (110) and (111) Si substrates by reduced-pressure chemical vapour deposition using a two temperature process. We have found that the combination of suppressing the Ge seed layer roughness and high temperature post-growth annealing can reduce the rms surface roughness of (110) Ge layers to below 2 nm and the threading dislocation density to below 1 × 10⁷ cm^− 2. Thick (111) Ge layers were found to exhibit a very high density of stacking faults, that could not be reduced by post-growth annealing and a higher rms surface roughness of around 12 nm, which was limited by the Ge seed layer.     

Effect of layer thickness on structural quality of Ge epilayers grown directly on Si(001)

A study of Ge epilayer growth directly on a Si(001) substrate is presented, following the two temperature Ge layer method. In an attempt to minimize the overall thickness while maintaining a good quality Ge epilayer, we have investigated the effect of varying the thickness of both the low and high temperature Ge layers, grown at 400 °C and 670 °C, respectively, by reduced pressure chemical vapor deposition. We find that the surface of the low temperature (LT) seed layer has a threading dislocation density (TDD) to the order of 10¹¹ cm^− 2. On increasing the LT layer thickness from 30 nm to 150 nm this TDD decreases by a factor of 2, while its roughness doubles and degree of relaxation increases from 82% to 96%. Growth of the high temperature (HT) layer reduces the TDD level to around 10⁸ cm^− 2, which is also shown to decrease with increasing layer thickness. Both the surface roughness and degree of relaxation reach stable values for which increasing the thickness beyond about 700 nm has no effect. Finally, annealing the HT layer is shown to reduce the TDD, without affecting the degree of relaxation. However, unless a thick structure is used the surface roughness increases significantly on annealing.     

The effects of low temperature buffer layer on the growth of pure Ge on Si(001)

We investigated the effects of low temperature (LT) Ge buffer layers on the two-step Ge growth by varying the thickness of buffer layers. Whereas the two-step Ge layers using thin (< 40 nm) Ge buffer layers were roughened due to the formation of SiGe alloy, pure and flat Ge layers were grown by using thick (> 50 nm) LT Ge buffer layers. The lowest threading dislocation density of 1.2 × 10⁶ cm⁻² was obtained when 80-nm-thick LT Ge buffer layer was used. We concluded that the minimum thickness of buffer layer was required to grow uniform two-step Ge layers on Si and its quality was subject to the thickness of buffer layer.     

Fabrication and characteristics of porous germanium films

Abstract

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm^?3) and a maximum of Hall mobility at ～225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Fabrication and characteristics of porous germanium films

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm⁻³) and a maximum of Hall mobility at ∼225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Poly(methyl methacrylate)-TiO<Subscript>2</Subscript> nanocomposite obtained by non-hydrolytic sol–gel synthesis

Poly(methyl methacrylate) (PMMA)/titanium dioxide (TiO₂) nanocomposites were prepared by means of in situ generation of TiO₂ through a non-hydrolytic sol–gel process (NHSG), starting from titanium chloride, as titania precursor, benzyl alcohol, as oxygen donor, and commercial PMMA. TiO₂ nanoparticles (average size of 30 nm) were obtained in the anatase and amorphous forms. The in situ generation led to a very homogeneous distribution of particulate fillers within the polymeric matrix avoiding the problems related to distributive and dispersive mixing of conventional compounding methods (top down approach). A slight increase of glass transition temperature was observed for all prepared composites with respect to the pristine PMMA. The NHSG process did not affect the molecular weight of the polymers indicating the absence of any degradation reaction for PMMA. The presence of titania in the anatase phase increases the photodegradation of the PMMA matrix due to UV irradiation.     

Effect of high-temperature annealing on solid-state reactions in hydroxyapatite/TiO<Subscript>2</Subscript> films on titanium substrates

Hydroxyapatite (HA) films 0.5 and 1 μm thick with a 0.2-μm TiO₂ underlayer have been grown on titanium by rf magnetron sputtering. After annealing in an argon atmosphere at 900, 950, and 1000°C for 30 and 60 min, the phase composition, elemental composition, and surface morphology of the films have been determined by scanning electron microscopy, X-ray microanalysis, and X-ray diffraction. The surface microstructure of the films is shown to depend on annealing temperature. X-ray microanalysis results indicate that, at HA layer thicknesses of both 1 and 0.5 μm, the Ca/P ratio increases with annealing temperature. In addition, the Ca/P ratio in the films produced under identical conditions depends on the film thickness. X-ray diffraction data indicate the presence of reaction intermediates. A TiO₂ interlayer in the HA/Ti system inhibits titanium oxidation, and the reaction intermediates forming during heat treatment improve the adhesion between the titanium and HA.     

Formation of buried insulating layers by high dose oxygen implantation under controlled temperature conditions

A specially designed sample holder was used to form SOI structures by high dose oxygen implantation under controlled temperature conditions. This system uses a layer of molten tin to provide a good thermal contact between the silicon wafer and a resistively heated support. The layers formed under these conditions were characterized by RBS and XTEM. After annealing at 1150°C for 2 h the only remaining defects in the top silicon layers are dislocations with a density of less than 10⁷ cm^?2 and SiO₂ precipitates. Annealing at 1185°C for 6 h does not change the density of dislocations but leads to the formation of a 200 nm thick silicon layer free of SiO₂ precipitates, suitable for VLSI processing without growing an epitaxial layer.     

Study on TiO2 thin films grown by advanced pulsed laser deposition on ITO

TiO₂ films were grown by an advanced pulsed laser deposition method (PLD) on ITO substrates to be used as functional electrodes in the manufacturing of solar cells. A pure titanium target (99.99%) was irradiated by a Nd:YAG laser (355 and 532 nm, 5 ns, 35 mJ, 3 J/cm²) in an oxygen atmosphere at different pressures (20-160 mTorr) and at room temperature. After deposition, the films were subjected to an annealing process at 350 °C. The film structure, surface morphology, thickness, roughness, and optical transmission were investigated. Regardless of the wavelength used, the films deposited at room temperature presented only Ti₂O and TiO peaks. After thermal treatment, the TiO₂ films became strongly crystalline, with a tetragonal structure and in the anatase phase; the threshold temperature value was 250 °C. The deposition rate was in the range of 0.035-0.250 nm/pulse, and the roughness was 135-305 nm. Optical transmission of the films in the visible range was between 40% and 60%.     

Epitaxy of In0.01Ga0.99As on Ge/offcut Si (001) virtual substrate

In_0.01Ga_0.99As thin films free of anti-phase domains were grown on 7° offcut Si (001) substrates using Ge as buffer layers. The Ge layers were grown by ultrahigh vacuum chemical vapor deposition using ‘low/high temperature’ two-step strategy, while the In_0.01Ga_0.99As layers were grown by metal-organic chemical vapor deposition. The etch-pit counting, cross-section and plane-view transmission electron microscopy, room temperature photoluminescence measurements are performed to study the dependence of In_0.01Ga_0.99As quality on the thickness of Ge buffer. The threading dislocation density of Ge layer was found to be inversely proportional to the square root of its thickness. The threading dislocation density of In_0.01Ga_0.99As on 300 nm thick Ge/offcut Si was about 4 × 10⁸ cm^− 2. Higher quality In_0.01Ga_0.99As can be obtained on thicker Ge/offcut Si virtual substrate. We found that the threading dislocations acted as non-radiative recombination centers and deteriorated the luminescence of In_0.01Ga_0.99As remarkably. Secondary ion mass spectrometry measurement indicated as low as 10¹⁶ cm^− 3 Ge unintended doping in In_0.01Ga_0.99As.     

Dye-sensitized solar cells using TiO<Subscript>2</Subscript> nanoparticles transformed from nanotube arrays

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ mesoporous layers obtained by very simple method—transformation of TiO₂ nanotube (NT) films grown by electrochemical oxidation to nanoparticle (NP) films. This transformation is based on thermal annealing of TiO₂ NT arrays formed by anodization of titanium foil in fluorine ambient. Performance of DSSCs fabricated with different size NPs was studied in the range from 35 to 350 nm. Highest nominal efficiency (9.05%) was achieved for DSSC with NP size 65 nm while the lowest nominal efficiency (1.48%) was observed for DSSCs with NP size 350 nm. The dependence of the solar cell parameters with NP size is discussed.     

Surface characterization of titanium hydride powder

Titanium hydride is well known for its stability in air because of formation of thin TiO₂ oxide layer on top of TiH₂ powder particles. Formation of the oxide layer is directly related to high affinity of Ti to oxygen and it can be formed in few seconds also on top of TiH₂ particles in low vacuum. On the other hand, thickness of TiO₂ layer on the formed TiH₂ powder is in the range of 130 nm. Existence of oxide layer on top of powder particles was confirmed with AES, XPS and EDXS cross section line scans. Formation of thin oxide was confirmed also with calculations based on chemical and TG analyses.     

Microstructure and magnetic properties of (0 0 1)-oriented L10 FePt films: Role of Ag underlayer and Fe/Pt ratio

FePt multilayer films were deposited on Si(1 0 0) substrate with thermally grown SiO₂ film and sputtered Ag underlayer at room temperature by dc magnetron sputtering and subsequently annealing in vacuum. Experimental results suggest that proper thickness of Ag underlayer and slightly rich of Fe content can effectively induce the (0 0 1) texture of FePt films. A Fe_57.4Pt_42.6 thin film on the 8 nm Ag underlayer exhibits a large perpendicular coercivity of 7.6 kOe with magnetic remanence close to 1.     

Growth of single-layer and multilayer Co/TiO<Subscript>2</Subscript> nanostructures with bulklike properties

We have studied the surface morphology, interfaces, and structure of individual Co and TiO₂ films 2 to 10 nm in thickness and those of Co/TiO₂ multilayers up to 100 nm in thickness. Auger depth profiling and transmission electron microscopy results show that [Co(2 nm)/TiO₂(2 nm)]₁₅, [Co(2 nm)/TiO₂(4 nm)]₁₅, and [Co(4 nm)/TiO₂(4 nm)]₁₂ structures are composed of continuous layers well adherent to one another, with sharp interfaces. The conclusion is made that such multilayer structures can be used as model systems in designing magneto-optical and spintronic devices.     

The effect of sputtering gas pressure on structure and photocatalytic properties of nanostructured titanium oxide self-cleaning thin film

Titanium oxide thin films were deposited by DC reactive magnetron sputtering on ZnO (80 nm thickness)/soda-lime glass and SiO₂ substrates at different gas pressures. The post annealing on the deposited films was performed at 400 °C in air atmosphere. The results of X-ray diffraction (XRD) showed that the films had anatase phase after annealing at 400 °C. The structure and morphology of deposited layers were evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface grain size and roughness of TiO₂ thin films after annealing were around 10-15 nm and 2-8 nm, respectively. The optical transmittance of the films was measured using ultraviolet-visible light (UV-vis) spectrophotometer and photocatalytic activities of the samples were evaluated by the degradation of Methylene Blue (MB) dye. Using ZnO thin film as buffer layer, the photocatalytic properties of TiO₂ films were improved.     

Growth of stoichiometric TiO2 thin films on Au(100) substrates by molecular beam epitaxy

The present study reports on the growth of thin TiO₂ films onto Au(100) single crystals by Ti evaporation in a reactive O₂ atmosphere at two different substrate temperatures: room temperature (RT) and 300 °C. The growth of the oxide films was monitored by means of X-ray photoemission spectroscopy, while the valence and conduction band electronic structure was investigated by UV and inverse photoemission spectroscopy, respectively.The TiO₂ film grows epitaxially on the Au(100) substrate at 300 °C exhibiting the rutile (100) surface. The evolution of the Ti 2p lineshape with the oxide coverage shows the presence of reduced oxide species (characterized by Ti^3 + ions) at the Au(100) interface. A crystalline and stoichiometric TiO₂ oxide is produced at high substrate temperature, while growth at RT gives a measurable concentration of defects. Post growth annealing in ultra-high vacuum of the RT grown film increases this concentration, while subsequent annealing in O₂ atmosphere restores the sample to the as-grown conditions.     

Preparation and characterization of ultra-thin cobalt silicide for VLSI applications

Ultra-thin cobalt silicide (CoSi₂) was formed from 10 nm cobalt film by solid phase reaction of Co and Si by use of rapid thermal annealing (RTA). The Ge⁺ ion implantation through Co film caused the interface mixing of the cobalt film with the silicon substrate and resulted in a homogeneous silicide layer. XRD was used to identify the silicide phases that were present in the film. The metallurgical analysis was performed by RBS. XRD and RBS investigations showed that final RTA temperature should not exceed 800°C for thin (< 50 nm) CoSi₂ formation.     

Growth and characterization of nc-Ge prepared by microwave annealing

Ge nanocrystals embedded in Silicon oxide matrix have been synthesized on Si substrate by co-sputtering of SiO₂ and Ge using RF magnetron sputtering technique. The as deposited films were subjected to microwave annealing at 800 and 900 °C. The structural characterization was performed by using X-ray diffraction (XRD) and Raman spectrometry. XRD measurements confirmed the formation of Ge nanocrystals. Raman scattering spectra showed a peak of Ge-Ge vibrational mode around 299 cm⁻¹, which was caused by quantum confinement of phonons in the Ge nanocrystals. Variation of the nanocrystal size with annealing temperature has been discussed. Advantages of microwave annealing are explained in detail.