Nanostructural and optical features of amorphous AlxSi0.45−xO0.55 (0 ⩽ x ⩽ 0.05) thin films prepared by RF-magnetron sputter techniques

The nanostructural, chemical, and optical features of Al_xSi_0.45−xO_0.55 (0 ⩽ x ⩽ 0.05) thin films were investigated in terms of Al concentration and post-deposition annealing conditions; the films were prepared by co-sputtering a Si main target and Al-chips, and the annealing was carried out at temperatures of 400–1100 °C. The a-Si_0.45O_0.55 films prepared without Al-chips and annealed at 800 °C contain ∼3.5 nm-sized Si nanocrystallites. The photoluminescence (PL) intensity as well as the volume fraction of Si nanocrystallites increased with increasing the concentration of Al to a certain level. In particular, the intensity of the PL spectra of the Al_0.025Si_0.425O_0.550 films which were annealed at 800 °C increased significantly at wavelengths of ∼580 nm. It is highly likely that the observed increase in the PL intensity is caused by the raise in the total volume of the ∼3.5 nm-sized nanocrystallites in the films. The addition of Al as well as the post-deposition annealing allow adjustment and control of the nanostructural and light-emission features of the a-SiO_x films.     

Enhancement of photoluminescence in SiCxNy nanoparticle films by addition of a Ni buffer layer

This work reports the effect of the presence of a Ni buffer layer on the photoluminescence (PL) of SiC_xN_y nanoparticle films prepared by RF plasma magnetron sputtering process in a reactive N₂ + Ar + H₂ gas mixture. An introduction of a Ni buffer of 80 nm or thicker remarkably improves the PL of the films. Annealing in a temperature range of 400–1100 °C is found to significantly affect the PL intensity. Optimal PL is achievable at 600 °C. X-ray photoelectron and Fourier-transform infrared spectroscopy suggest that the strong PL is directly related to the composition of the SiC_xN_y nanoparticle and the concentration of Si–O, and Si–N bonds. The results are relevant to the development of wide bandgap optoelectronic devices.     

Photoluminescence of Si nanocrystallites in different types of matrices

This paper presents the comparative investigation of photoluminescence (PL) and its temperature dependence for rf-magnetron co-sputtered Si-enriched SiO_x systems and amorphous Si films prepared by hot-wire CVD method with Si nanocrystallites of different sizes. It is shown that PL spectra of Si–SiO_x films consist of the five PL bands peaked at 1.30, 1.50, 1.76, 2.05 and 2.32 eV. Amorphous Si films with Si nanocrystallites are characterized by three PL bands only peaked at 1.35, 1.50 and 1.76 eV. The peak position of the 1.50 eV PL band shifts with the change of Si quantum dot sizes and it is attributed to exciton recombination inside of Si quantum dots. The nature of four other PL bands is discussed as well.     

Photoluminescence of Si or Ge nanocrystallites embedded in silicon oxide

This paper presents the results of photoluminescence, its temperature dependence and Raman scattering investigations on magnetron co-sputtered silicon oxide films with (or without) embedded Si (or Ge) nanocrystallites. It is shown the oxide related defect origin of the visible PL centers peaked at 1.7, 2.06 and 2.30 eV. The infrared PL band centered at 1.44–1.58 eV in Si–SiO_x, system has been analyzed within a quantum confinement PL model. Comparative PL investigation of Ge–SiO_x system has confirmed that high energy visible PL bands (1.60–1.70 and 2.30 eV) are connected with oxide related defects in SiO_x. The PL band in the spectral range of 0.75–0.85 eV in Ge–SiO_x system is attributed to exciton recombination inside of Ge NCs.     

Self-assembling of supramolecular erbium–oxygen clusters in magnetron plasma,and the optimization of erbium luminescence in a-SiOx:H〈Er, O〉 films

Films of erbium-doped amorphous hydrogenated silicon a-SiO_x:H〈Er, O〉 were fabricated by dc-magnetron sputtering at different concentrations of oxygen in the magnetron plasma and different areas of erbium metallic target. It was demonstrated that the increase of oxygen concentration in the plasma gaseous phase above ∼5 mol% leads to a sharp rise in the amount of oxygen bound to erbium in the a-SiO_x:H〈Er, O〉 films. Simultaneously, a smooth increase in the concentration of oxygen bound to matrix-forming elements (silicon, hydrogen) is observed. The increase of the area of erbium target, corresponding to the rise of concentration of erbium ions in the plasma, also favors the binding of erbium with oxygen. However, the content of erbium in the a-SiO_x:H〈Er, O〉 film (in atomic percents) significantly drops with intense binding of erbium with oxygen. These facts point to the formation of erbium–oxygen clusters, with a large number of oxygen atoms, which are probably formed in the magnetron plasma but are deposited as a separate species on the substrate in the reaction chamber. The intensity of erbium photoluminescence rises significantly in the region of formation of these large erbium–oxygen clusters. A ‘phase-transition’ model is formulated, describing the properties of a-SiO_x:H〈Er, O〉 films, based on the assumption of the formation of large erbium–oxygen clusters in the magnetron plasma. The size and composition of these clusters are determined. The model is semi-quantitatively consistent with all the experimental findings.     

Diffusion and reactions of interstitial oxygen species in amorphous SiO2: A review

This article briefly summarizes the diffusion and reactions of interstitial oxygen species in amorphous SiO₂ (a-SiO₂). The most common form of interstitial oxygen species is oxygen molecule (O₂), which is sensitively detectable via its characteristic infrared photoluminescence (PL) at 1272 nm. The PL observation of interstitial O₂ provides key data to verify various processes related to interstitial oxygen species: the dominant role of interstitial O₂ in long-range oxygen transport in a-SiO₂; formation of the Frenkel defect pair (Si–Si bond and interstitial oxygen atom, O⁰) by dense electronic excitation; efficient photolysis of interstitial O₂ into O⁰ with F₂ laser light (λ = 157 nm, hν = 7.9 eV); and creation of interstitial ozone molecule via reaction of interstitial O₂ with photogenerated O⁰. The efficient formation of interstitial O⁰ by F₂ laser photolysis makes it possible to investigate the mobility, optical absorption, and chemical reactions of interstitial O⁰. The observed properties of O⁰ are consistent with the model that O⁰ takes the configuration of Si–O–O–Si bond. Interstitial O₂ and O⁰ react with dangling bonds, oxygen vacancies, and chloride groups in a-SiO₂. Reactions of interstitial O₂ and O⁰ with mobile interstitial hydrogen species produce interstitial water molecules and hydroperoxy radicals. Interstitial hydroxyl radicals are formed by F₂ laser photolysis of interstitial water molecules.     

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO₂ nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiO_x (a-SiO_x) films with 0 < x < 1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 °C. The series resistance through Ag/[Si-nanodots in SiO₂]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiO_x films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.     

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived xNb₂O₅?(100?x)TiO₂ coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.     

Broad UHF ferromagnetic resonance of iron rich-aluminum pulsed laser deposited thin films

The magnetic susceptibility of Fe–Al off-normal pulsed laser deposited thin films was measured at ultra high frequencies, UHF. Different Fe_1?x–Al_x films from pure Fe to x = 0.2 Al were prepared. The films were ≈40 nm thick and non-crystalline peaks were detected by the X-ray diffractometry studies. The magnetization of the films remained between 2.0 and 1.8 T for composition less than or equal to 20% Al. A magnetic anisotropy, from H_k ≈ 18 Oe for pure Fe to H_k ≈ 130 Oe for 20% Al was measured. These samples exhibited a well-defined ferromagnetic resonance at frequencies between ≈2.0 GHz and 3.8 GHz depending on composition. The broad resonance peaks had a width, at half maximum, w_h, in the interval from 2.5 GHz to 4.0 GHz depending on Al content. After fitting the magnetic hysteresis loops using a simple distribution of anisotropy values, we used the Landau–Lifshitz–Gilbert equation to fit the UHF magnetic susceptibility. From this last fit we obtained a high damping coefficient value (≈4 times higher than that corresponding to Co or CoFe films), explaining this broad ferromagnetic resonance of these Fe_1?x–Al_x films.     

MOCVD and characterization of Hf-silicate thin films using HTB and TEMAS

Hafnium silicate (HfSi_xO_y) films were deposited by metal-organic chemical vapor deposition (MOCVD) using a combination of precursors: hafnium tetra-tert-butoxide [Hf(OC(CH₃)₃)₄, HTB] and tetrakis-ethylmethylamino silane [Si(N(C₂H₅)(CH₃))₄, TEMAS]. The activation energy was independent on the ratio of precursor amounts in the surface reaction regime. The grown films showed Hf-rich characteristics and the impurity concentrations were less than 1 at.% (below detection limits). Hafnium silicate films were amorphous up to 700 °C annealing. Hf/(Hf + Si) composition ratio and dielectric constant (k) of the Hf-silicate films decreased by increasing the growth temperature above 270 °C.     

Role of ballistic transport in photoluminescence excitation of Si nanocrystals

Photoluminescence of Si NCs with the size (10–300 nm) bigger than the exciton Bohr radius in the bulk Si crystals (4.8 nm) has been considered. Photoluminescence in such NC systems is analyzed from the point of view of new concept based on the effect of hot carrier ballistic transport in excitation of suboxide defect-related photoluminescence at the Si/SiO_x interface. The dependence of the 1.70 eV PL band integrated intensity on Si NC sizes was numerically calculated on the base of the hot carrier ballistic PL model. The well correlation between calculated and experimental results has been obtained for Si NCs with the size from the 30–150 nm range.     

X-ray diffraction measurements for liquid Ge–Si alloys using synchrotron radiation

Energy dispersive X-ray diffraction measurements have been carried out for liquid Ge_1−xSi_x alloys (x = 0.0, 0.3, 0.5, 1.0) using synchrotron radiation at SPring-8. We measured the X-ray diffraction spectra of liquid Ge and Si up to a high temperature range, (liquid Ge from 1270 to1870 K and liquid Si from 1680 to 2020 K), liquid Ge_0.7Si_0.3 at 1620 K, and liquid Ge_0.5Si_0.5 at 1540, 1590, 1670 and 1720 K. The total structure factors of the liquid Ge–Si alloys have a characteristic shoulder on the high-wave-vector side of the first peak. We deduced a pair distribution function from the Fourier transform of the observed structure factor, which was weakly dependent on the temperature. The nearest-neighbor coordination number of liquid Ge–Si alloys is close to that of pure liquid Ge and Si. The first peak of the pair distribution function moved to a shorter distance with increasing Si concentration. These results may indicate that the atomic radii of the Si and Ge atoms in the pure liquid are preserved in the liquid alloys.     

Structural investigations of nitrided Nb2O5 and Nb2O5–SiO2 sol–gel derived films

Sol–gel derived xNb₂O₅–(100 ? x)SiO₂ films (where x = 100, 80, 60, 50, 40, 20, 0 mol%) were nitrided at various temperatures (800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C). The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The XRD results have shown that the temperatures below 1100 °C were too low to obtain a pure NbN phase in the samples. The AFM observations indicate that the formation of the NbN phase and the size of NbN grains are related to the silica content in the layer. NbN grains become more regular and larger as the niobium content increases. The maximum grain size of about 100 nm was observed for x = 100. Preparation of the Nb₂O₅–SiO₂ sol–gel derived layers and the subsequent nitridation is a promising method of inducing crystalline NbN in amorphous matrices. It follows from the XPS results that a small amount of Nb₂O₅ remains in the films after nitridation at 1200 °C and that nitrogen reacted not only with Nb₂O₅ but also with SiO₂.     

High-yield synthesis of semiconductive type-II Si clathrates with low Na content

To develop an efficient synthesis of type-II Si clathrates with low Na content (Na_xSi₁₃₆: x = 0–24), various conditions for annealing the Zintl phase NaSi were examined. The addition of a pre-annealing process under vacuum at 250 °C following the preparation of NaSi resulted in a decrease in the Na content of type-II Si clathrates from 4 to 2 when the pre-annealing duration ranged from 0 to 60 h, while the volume fraction of type-II Si clathrate crystals in the synthesized specimens (type-II/(type-I + type-II + d-Si)) deduced by powder X-ray diffraction and Rietveld analysis was maintained at approximately 85%. These preparation techniques that enable the high-yield synthesis of semiconductive type-II Si clathrates open opportunities for the application of these substances to semiconductor devices.     

Absorption and luminescence in amorphous SixGe1-xO2 films fabricated by SPCVD

Optical absorption and photoluminescence of Ge-doped silica films fabricated by the surface-plasma chemical vapor deposition (SPCVD) are studied in the 2–8 eV spectral band. The deposited on silica substrate films of about 10 μm in thickness are composed as x·GeO₂-(1-x)·SiO₂ with x ranging from 0.02 to 1. It is found that all as‐deposited films do not luminesce under the excitation by a KrF (5 eV) excimer laser, thus indicating lack of oxygen deficient centers (ODCs) in them. After subsequent fusion of silicon containing (x < 1) films by a scanning focused CO₂ laser beam absorption band centered at 5 eV as well as two luminescence bands centered at blue (3.1 eV) and UV (4.3 eV) wavelengths arise, highlighting the formation of the ODCs. The excitation of unfused SPCVD films by an ArF (6.4 eV) excimer laser yields a luminescence spectrum with two bands typical for the ODCs, but with a faster decay kinetics. Intensities of these bands grow up with samples cooling down to a temperature of 80–60 K. Unfused films excited by the ArF laser also demonstrate luminescence due to recombination of a trapped charge resulted from the excitation of localized electron states of the glass network. In the unfused GeO₂ film luminescence related to a self-trapped exciton (STE) typical for GeO₂ crystals with α-quartz structure is observed. The observed STE luminescence can be indicative of the crystalline fraction availability in the film. Whereas GeO₂ crystals are known as not containing twofold coordinated germanium, a polycrystalline inclusion in the SPCVD GeO₂ film serves as a factor explaining the absence of any spectroscopic manifestation of this type of defects in it even after fusion. On the other hand, lack of STE luminescence in other unfused films with x < 1 testifies truly amorphous state of the matter in them.     

Influence of nickel content on the electrochemical behavior of Finemet type amorphous and nanocrystalline alloys

The aim of this work has been the systematic study of the influence of partial substitution of Fe by Ni in the Ni_xFe_73.5−xSi_13.5B₉Nb₃Cu₁ alloy within the range 0 ⩽ x ⩽ 10% atom (x = 0, 2, 4, 6, 8, 10) on electrochemical behavior, corrosion rate and the structural changes in amorphous and nanocrystalline alloys. The amorphous nature of the alloys was confirmed by X-ray diffraction and the chemical compositions were determined by ICP. The glass transition and kinetic crystallization of amorphous alloys were studied by DSC. The technique of XPS was used for evaluating the chemical states of elements present in native oxide films. The electrochemical behavior of amorphous and nanocrystalline alloys have been investigated in 0.5 M KOH using cyclic voltammetry. The experimental results show that the formation of different nanocrystalline phases is not excessively transformed by the addition of small amount of nickel and the electrochemical behavior is improved as nickel content increased.     

Glassy state and structure of Sn–Sb–Se chalcogenide alloy

The effect of Sn addition on the glass transition and structure of c-Sb₂₀Se₈₀ chalcogenide alloy have been studied by X-ray diffraction and differential scanning calorimetric studies. The increase in the glass forming region and the glass transition temperature with the addition of Sn is discussed by considering the formation of [SnSe₄] tetrahedra, another type of network former, which inhibits the crystallization. The differential scanning calorimetric studies on Sn_xSb₂₀Se_80−x (8 ⩽ x ⩽ 18) glassy samples reveal a single glass transition temperature for all values of x while a single crystallization peak was obtained only for 10 ⩽ x < 12. The X-ray diffraction studies reveal that the glass crystallizes to Sb₂Se₃ and SnSe₂ phases upon annealing. The glass formation and composition dependence of glass transition temperature in the Sn–Sb–Se chalcogenide alloy could be understood by considering the topological phase transitions and a chemically ordered network model.     

Influence of europium ions on structure and crystallization properties of bismuth borate glasses and glass ceramics

Glasses of the xEu₂O₃ · (100?x)[2Bi₂O₃ · B₂O₃] system with 0 ? x ? 25 mol% have been characterized by X-ray diffraction and FTIR spectroscopy measurements. Melting at 1100 °C and the rapid cooling at room temperature permitted us to obtain glass samples. In order to improve the local order and to develop crystalline phases, the glass samples were kept at 625 °C for 24 h. After heat treatment two crystalline phases were put into evidence. One of the crystalline phases was observed for the host glass matrix, the x = 0 mol% sample, and belongs to the cubic system. The second one was observed for the x = 25 mol% sample and was find to be orthorhombic with two unit cell parameters very close to each other. For the samples with 0 < x < 25 mol% there is a mixture of the two mentioned phases. FTIR spectroscopy data suggest that both Bi₂O₃ and B₂O₃ play the glass network former role while the europium ions play the network modifier role in the studied glasses.     

Crystallization of bismuth titanate and bismuth silicate grown as thin films by atomic layer deposition

Bismuth silicate and bismuth titanate thin films were deposited by atomic layer deposition (ALD). A novel approach with pulsing of two Bi-precursors was studied to control the Si/Bi atomic ratio in bismuth silicate thin films. The crystallization of compounds formed in the Bi₂O₃–SiO₂ and Bi₂O₃–TiO₂ systems was investigated. Control of the stoichiometry of Bi–Si–O thin films was studied when deposited on Si(1 0 0) and crystallization was studied for films on sapphire and MgO-, ZrO₂- and YSZ-buffered Si(1 0 0). The Bi–Ti–O thin films were deposited on Si(1 0 0) substrate. Both Bi–Si–O and Bi–Ti–O thin films were amorphous after deposition. Highly a-axis oriented Bi₂SiO₅ thin films were obtained when the Bi–Si–O thin films deposited on MgO-buffered Si(1 0 0) were annealed at 800 °C in nitrogen. The full-width half-maximum values for 200 peak were also studied. An excess of bismuth was found to improve the crystallization of Bi–Ti–O thin films and the best crystallinity was observed with Ti/Bi atomic ratio of 0.28 for films annealed at nitrogen at 1000 °C. Roughness of the thin films as well as the concentration depth distribution were also examined.     

Effects of tantalum doping on microstructure and ferroelectric properties of Bi4Ti3O12 thin films prepared by a sol–gel method

Tantalum-substituted Bi₄Ti₃O₁₂ (Bi₄Ti_3-x/5Ta_x/5O₁₂, BTTO) thin films were fabricated on Pt(111)/Ti/SiO₂/Si(100) substrates by sol–gel technology. The effects of various processing parameters, including Ta content (x=0～0.08) and annealing temperature (500～800 °C), on the growth and properties of thin films were investigated. X-ray diffraction analysis shows that the BTTO thin films have a bismuth-layered perovskite structure with preferred (117) orientation. With the increase of Ta content, the grain size of film decreased slightly, and highly (117)-oriented BTTO films were obtained in the composition of x=0.06. Ta doping on the B-site of Bi₄Ti₃O₁₂ could induce the distortion of oxygen octahedral and decrease the oxygen vacancy concentration by a compensating effect. The highly (117)-oriented BTTO thin films with x=0.06 exhibits the maximum remanent polarization (2P_r) of 50 μC/cm² and a low coercive field (2E_c) of 104 kV/cm, fatigue free characteristics up to ≧ 10⁸ switching cycles.