Effects of physical growth conditions on the structural and optical properties of sputtered grown thin HfO2 films

HfO₂ thin films were prepared by reactive DC magnetron sputtering technique on (100) p-Si substrate. The effects of O₂/Ar ratio, substrate temperature, sputtering power on the structural properties of HfO₂ grown films were studied by Spectroscopic Ellipsometer (SE), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrum, and X-ray photoelectron spectroscopy (XPS) depth profiling techniques. The results show that the formation of a SiO_x suboxide layer at the HfO₂/Si interface is unavoidable. The HfO₂ thickness and suboxide formation are highly affected by the growth parameters such as sputtering power, O₂/Ar gas ratio during sputtering, and substrate temperature. XRD spectra show that the deposited films have (111) monoclinic phase of HfO₂, which is also supported by FTIR spectra. XPS depth profiling spectra shows that highly reactive sputtered Hf atoms consume some of the oxygen atoms from the underlying SiO₂ to form HfO₂, leaving Si-Si bonds behind.     

Photoabsorption properties of β-FeSi2 nanoislands grown on Si(111) and Si(001): Dependence on substrate orientation studied by nano-spectroscopic measurements

Photoabsorption properties of β-FeSi₂ nanoislands epitaxially grown on Si(111) and Si(001) have been discussed using photoabsorption nano-spectroscopy based on scanning tunneling microscope. The obtained spectra exhibit clear features around 0.86-0.91 eV and around 0.71-0.74 eV, which are explained as a direct and an indirect photoabsorption edge of β-FeSi₂, respectively. We also observed a blue shift of spectrum obtained from β-FeSi₂ nanoislands on Si(111) substrates, compared to those on Si(001) substrates. We attributed the dependence on Si-substrate orientation not to a quantum confinement effect but to an effect of elastic strain in the? β-FeSi₂ nanoislands epitaxially grown on the substrate.     

β-FeSi2 growth on Cu-mediated Si substrate and enhancement of photoluminescence

We have investigated the growth of β-FeSi₂ on Cu-mediated (100) Si substrates and photoluminescence (PL) behavior. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that the mediated surface became amorphous-like Si layers due to Cu atomic diffusion from the surface to the inside of Si and recrystallized during β-FeSi₂ deposition, and that the recrystallization may contribute to the improvement of crystallinity of β-FeSi₂ and the hetero-interface. We have observed pronounced enhancement of PL intensity from β-FeSi₂ grown on the Cu-mediated Si substrate. This implies that non-radiative recombination centers may be decreased by the improvement of hetero-interface.     

Epitaxial growth and relaxation of γ-Al2O3 on silicon

0.5-10 nm-thick single crystal γ-Al₂O₃ films was epitaxially grown, at high temperature, on Si(001) and Si(111) substrates using electron-beam evaporation techniques. Reflection High Energy Electron Diffraction studies showed that the Al₂O₃ films grow pseudomorphically on Si (100) up to thickness of 2 nm. For higher thicknesses, a cubic to hexagonal surface phase transition occurs. Epitaxial growth and relaxation were also observed for Si(111). The film surfaces are smooth and the oxide-Si interfaces are atomically abrupt without interfacial layers.     

Growth of Cu2ZnSnS4 thin films on Si (100) substrates by multisource evaporation

Cu₂ZnSnS₄ films were grown on Si (100) by vacuum evaporation using elemental Cu, Sn, S and binary ZnS as sources. X-ray diffraction patterns of films grown at different substrate temperatures indicated that polycrystalline growth was suppressed and the orientational growths were relatively induced in a film grown at higher temperatures. Tetragonal structure of Cu₂ZnSnS₄ films was confirmed by studying RHEED patterns. The existence of c-axis ([001] direction) growth, two kinds of a-axis (〈100〉 direction) growth and four kinds of {112} twins which can be classified as two symmetrical pairs is proposed. Broad emissions at around 1.45 eV and 1.31 eV were observed in the photoluminescence spectrum measured at 13 K.     

Photoluminescence behaviors in ZnGa2O4 thin film phosphors deposited by a pulsed laser ablation

ZnGa₂O₄ thin film phosphors have been deposited using a pulsed laser deposition technique on Si (1 0 0) and Al₂O₃ (0 0 0 1) substrates at a substrate temperature of 550 °C with various oxygen pressures 100, 200 and 300 mTorr, and various substrate temperatures of 450, 550 and 650 °C with a fixed oxygen pressure of 100 mTorr. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. Under the different substrate temperatures, ZnGa₂O₄ thin films show the different crystallinity and luminescent intensity. The crystallinity and photoluminescence (PL) of the ZnGa₂O₄ films are highly dependent on the deposition conditions, in particular, oxygen pressure, substrate temperature, a kind of substrates. The luminescent spectra show a broad band extending from 350 to 600 nm peaking at 460 nm. The PL brightness data obtained from the ZnGa₂O₄ films grown under optimized conditions have indicated that the sapphire is one of the most promised substrates for the growth of high quality ZnGa₂O₄ thin film phosphor.     

Solid-phase crystallization of β-FeSi2 thin film in Fe/Si structure

Dependence of solid-phase growth of β-FeSi₂ thin films on the crystal orientation of Si substrates has been investigated by using a-Fe (thickness: 20 nm)/c-Si(100), (110) and (111) stacked structures. X-ray diffraction (XRD) measurements suggested that the substrate orientation dependence of the formation rate of β-FeSi₂ was as follows: (100)>(111)>(110). This dependence can be explained on the basis of the lattice mismatch between β-FeSi₂ and Si substrates, i.e., the lattice mismatch between β-FeSi₂(100) and Si(100), β-FeSi₂(110) or (101) and Si(111), and β-FeSi₂(010) or (001) and Si(110) of 1.4-2.0%, 5.3-5.5% and 9.2%, respectively. The substrate orientation dependence of solid-phase growth becomes relatively remarkable for very thin films.     

Improved dielectric properties of CaCu3Ti4O12 thin films on oxide bottom electrode of La0.5Sr0.5CoO3

We report on the effect of La_0.5Sr_0.5CoO₃ (LSCO) bottom electrode to the dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) thin films grown on Ir/Ti/SiO₂/Si substrates. Compared with the films grown directly on Ir/Ti/SiO₂/Si substrates, the dielectric constant has been increased greatly about 100%, and the dielectric loss decreased to lower than 0.2 in the frequency range of 1-100 kHz. The origin has been discussed in details based on the analysis of the X-ray diffraction and impedance spectra measurements. Results of the impedance spectra suggest that the absence of undesired interfacial layer between Ir/CCTO thin films might be one of the major reasons of the improvement of the dielectric properties when the LSCO was introduced as the bottom electrode.     

Influence of substrate orientation on low-temperature epitaxial growth of ferromagnetic silicide Fe3Si on Si

Influence of substrate orientation on epitaxial growth of the ferromagnetic silicide Fe₃Si on Si was investigated using low temperature (60-300 °C) molecular beam epitaxy. Transmission electron microscopy (TEM) measurements revealed that Fe₃Si layers were epitaxially grown on Si(110) and Si(111), while random poly-crystal Fe₃Si layers were formed on Si(100). From the Rutherford backscattering spectroscopy measurements, the values of the χ_min of the Fe₃Si layers grown at 60 °C on Si(100), Si(110), and Si(111) were evaluated to be 100%, 97%, and 41%, respectively. This dependence on the substrate orientation was explained on the basis of the atomic alignments at the Fe₃Si/Si interfaces.     

Effect of substrate temperature and deposited thickness on the formation of iron silicide prepared by ion beam sputter deposition

Ion beam sputter deposition (IBSD) method was employed to find optimum conditions for the formation of epitaxial β-FeSi₂ films on Si(100) substrate. It was found that crystal structure of the films as determined by X-ray diffraction (XRD) analysis is dependent on the substrate temperature as well as on the deposited thickness of sputtered Fe. The film with best crystal properties was obtained either at 873 K with the deposited Fe thickness of 15 nm, or at 973 K with the deposited Fe thickness of 30 nm. The obtained results indicate the importance of Fe and/or Si diffusion in determining the crystal properties of β-FeSi₂ film.     

Titanium Interlayer Mediated Epitaxy of CoSi2 on Si1−xGex

Titanium Interlayer Mediated Epitaxy (TIME) has been shown to promote the formation of epitaxial CoSi₂ on Si (100). Similarities between Si and Si_1−xGe_x alloys have motivated a study of whether the TIME process could be successful in forming epitaxial CoSi₂ on Si_1−xGe_x. Titanium layers of varying thickness were deposited as interlayers between a Co layer and c-Si/Si_0.8Ge_0.2 grown epitaxially onto Si (100) to investigate their role in the formation of epitaxial CoSi₂ on Si_1−xGe_x alloys. The effect of Ti interlayer thickness on the orientation of CoSi₂ to the Si_1−xGe_x substrate, and the conditions under which a polycrystalline CoSi₂ film has been formed have been studied. It was found that Ti was beneficial in promoting epitaxy to the substrate in all cases. The experimental results indicate that with a Ti interlayer thickness of ∼ 50 Å, the formation of epitaxial CoSi₂ adjacent to the substrate was achieved, and pinhole formation was minimized. It was also observed that for increased interlayer thickness, Ti reacted with Si to form a titanium silicide.     

Metalorganic chemical vapor deposition of β-FeSi2 on β-FeSi2 seed crystals formed on Si substrates

We have fabricated a β-FeSi₂ film by metalorganic chemical vapor deposition on a Si(001) substrate with β-FeSi₂ seed crystals grown by molecular beam epitaxy, and investigated the crystallinity, surface morphology and temperature dependence of photoresponse properties of the β-FeSi₂ film. The surface of the grown β-FeSi₂ film was atomically flat, and step-and-terrace structure was clearly observed. Multi-domain structure of β-FeSi₂ whose average size was approximately 200 nm however was revealed. The photoresponse was obtained in an infrared light region (~ 0.95 eV) at temperatures below 200 K. The external quantum efficiency reached a maximum, being as large as 25% at 100 K when a bias voltage was 2.0 V.     

Low-temperature vapor synthesis of 1D β-Ga2O3 nanostructures on Si substrate by inert salt-assisted route

The attractive feature of one-dimensional inorganic nanomaterials grown on substrate at low temperature is the good adhesion between the deposited material and the substrate. In this work inert salt-assisted route has been extended to prepare Ga₂O₃ product on bare Si substrate. By using CaF₂ powder as a dispersant, the vapor pressure of metallic Ga is increased greatly as compared to its non-dispersed state. This allows for the β-Ga₂O₃ nanostructures to be formed at a relatively low temperature of 650 °C as a result of direct oxidation of the well-mixed metallic Ga and CaF₂ powder. This temperature is much lower than the synthetic temperature required in the cases of direct oxidation of metallic Ga as reported by others. The as-prepared Ga₂O₃ nanowires and nanobelts are characterized by XRD, SEM, EDX, TEM and PL. The vapor–solid growth process was also discussed for the as-prepared Ga₂O₃ product on Si substrate. The interesting results indicate the wide applications of inert salt-assisted route to vapor growth of other nanomaterials at relative low temperature.     

Growth, structure and luminescence properties of multilayer Si/β-FeSi2NCs/Si/…/Si nanoheterostructures

Multilayer structures (up to 15 layers) with β-FeSi₂ nanocrystallites (NCs) buried in silicon crystalline lattice were grown by successive repetition of reactive deposition epitaxy (RDE) or solid phase epitaxy (SPE) of thin iron film on Si(100) or Si(111) substrates and silicon molecular beam epitaxy (MBE) (100-200 nm). Cross-section high resolution transmission electron microscopy (HR TEM) images and ex situ optical and Raman spectroscopy data prove that NCs formed in silicon matrix have the structure and optical properties of β-FeSi₂. The growth conditions provide no dislocations in silicon lattice were found in the course of TEM analysis. Two types of NCs depth distribution were observed: (i) layered that corresponds to iron RDE and (ii) uniform that occurs in the case of iron SPE. The uniform NCs distribution points out the fact that during a growth process NCs moves up to the surface. In spite of small nanocrystallites size (5-50 nm) and their distribution in silicon cap layers the significant photoluminescence (PL) signal at 0.8 eV was observed for all grown samples.     

Effect of target compositions on the crystallinity of β-FeSi2 prepared by ion beam sputter deposition method

Targets with the elemental composition of Fe, Fe₂Si and FeSi₂ were employed in the present study to grow β-FeSi₂ film on Si (100) substrate by means of ion beam sputter deposition (IBSD) method. The results revealed that when FeSi₂ target was employed, a Si-rich phase, α-FeSi₂ (Fe₂Si₅), was predominant at temperatures above 973 K, while β-FeSi₂ phase was observed only in the limited temperature range at around 873 K. In this case, Si was originated both from the sputtered target and the substrate, thus, the supply of Si was considered to be excessive to sustain β structure. On the other hand, the films prepared with Fe target became polycrystalline as they grow thicker than 100 nm. In order to optimize the supply of Fe and Si for epitaxial growth, Fe₂Si target was employed, where highly (100)-oriented β-FeSi₂ layer of 120 nm in thickness was obtained at 973 K.     

Optical and microstructural properties of p-type SrCu2O2: First principles modeling and experimental studies

In this paper, we report first principles calculations and experimental studies of the optical and microstructural properties of both bulk and thin films of SrCu₂O₂. Polycrystalline SrCu₂O₂ films were grown by a conventional Pulsed Laser Deposition method in a flowing oxygen environment on corning glass 7059 and silicon substrates. Several characterization techniques, including X-ray diffraction (XRD), Fourier Transform IR (FTIR), Raman, spectroscopic ellipsometry, reflectance/transmission spectrophotometry and Atomic Force Microscopy have been used for the investigation of the microstructural and vibrational properties of both bulk and thin films of SrCu₂O₂.XRD shows that bulk SrCu₂O₂ is polycrystalline and assumes the pure tetragonal phase of SrCu₂O₂. The vibrational properties of the tetragonal phase of SrCu₂O₂ have been inferred from Raman and FTIR spectroscopies and for the first time both Raman and IR active modes have been assigned. The bulk polycrystalline SrCu₂O₂ optical band gap determined from spectroscopic ellipsometry was 3.34 ± 0.01 eV. XRD results confirmed that pure non-textured polycrystalline phase SrCu₂O₂ thin films with a smooth surface can be grown by PLD at low temperature (300 °C).     

Fabrication of SmBa2Cu3O7 Films on CeO2 Buffered Sapphire

SmBa₂Cu₃O_7−δ (SmBCO) thin films and CeO₂ buffer layers were deposited on γ-cut sapphire by pulsed laser deposition (PLD) and characterized with X-ray diffraction (XRD) and atomic force microscope (AFM). The θ–2θ XRD scans of the SmBCO/CeO₂/sapphire structures revealed that the CeO₂ and SmBCO films were grown with c-axis perpendicular to the substrate. In Φ-scan XRD patterns, four (103) peaks of the SmBCO film were observed and the peak positions were shifted by 45° from (202) peaks of the CeO₂ films. From the peak shifts we could conclude that the [110]_SmBCO crystal axis is parallel to the [100]_CeO2 crystal axis. Moreover, pole figure also confirmed that SmBCO films were grown on the substrates epitaxially along in-plane direction. The SmBCO films show very flat surfaces with root mean square (RMS) about 5 nm. In agreement with this crystalline perfection, SmBCO thin films present excellent superconducting properties: T _c0 > 90 K, transition width 0.4 K, and J _c(77 K) > 2 MA/cm².     

Gold catalysts for the abatement of environmentally harmful materials: Modeling the structure dependency

FeO_x, TiO₂ and CeO_x layers were deposited by pulsed laser deposition (PLD) technique onto Au films or Au nanoparticles supported on SiO₂/Si(100). The samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and their reactivity was studied in catalytic CO oxidation. Comparison was made with reference samples of FeO_x/SiO₂/Si(100), TiO₂/SiO₂/Si(100), CeO_x/SiO₂/Si(100) and Au/SiO₂/Si(100) layers. The catalytic activity of the metal-oxide/Au/SiO₂/Si(100) samples must be attributed to active sites located on the metal-oxides overlayer modified by gold underneath, since no Au was exposed to the surface according to the XPS and SIMS. We found a promoting effect of gold on the catalytic activity of the FeO_x overlayer and an inhibiting effect of gold on the TiO₂ and CeO_x overlayers. These findings are discussed in terms of electronic interactions at the Au/metal oxide interface.     

SiOx-doped DLC films: Charge transport, dielectric properties and structure

In the present study, SiO_x-doped diamond-like carbon (DLC) films were synthesized by ion beam deposition on different substrates. Electrical properties, morphology and structure of the DLC films were investigated. Poole-Frenkel emission was the main carrier transport mechanism in all investigated metal-SiO_x-doped DLC-metal samples. Dielectric properties of the samples were dependent on both the bottom and top electrode metal. The trans-polyacetylene chain vibrations detected from the Raman spectra have been observed for all the SiO_x-doped DLC films. Different dielectric properties of the film deposited onto the different metal interlayers were explained both by different roughness of the metal films and by different structure of the ion beam-synthesized SiO_x-doped DLC films.     

Combined spectroscopic ellipsometry and attenuated total reflection analyses of Al2O3/HfO2 nanolaminates

The microstructure of thin HfO₂-Al₂O₃ nanolaminate high κ dielectric stacks grown by atomic vapor deposition has been studied by attenuated total reflection spectroscopy (ATR) and 8 eV spectroscopic ellipsometry (SE). The presence of Al₂O₃ below HfO₂ prevents the crystallisation of HfO₂ if an appropriate thickness is used, which depends on the HfO₂ thickness. A thicker Al₂O₃ is required for thicker HfO₂ layers. If crystallisation does occur, we show that the HfO₂ signature in both ATR and 8 eV SE spectra allows the detection of monoclinic crystallites embedded in an amorphous phase.