Low loss optical channel waveguides for the infrared range using niobium based hybrid sol-gel material

In this work, we report the fabrication of single-mode Nb₂O₅ based hybrid sol-gel channel waveguides. Nb₂O₅ based hybrid sol-gel material has been deposited by spin-coating on silicon substrate and channel waveguides have been fabricated by a UV direct laser writing process. Optical guided modes have been observed to confirm single-mode conditions and optical propagation loss measurements have been performed using the cut-back technique. Optical propagation losses were measured to be 0.8 dB/cm and 2.4 dB/cm at 1.31 μm and 1.55 μm respectively. These experimental results demonstrate low loss optical waveguiding within the infrared range and are very promising in view of material choice for the development of integrated optical devices for telecommunication.     

Preparation of five-layered Si/SixGe1−x nano-films by RF helicon magnetron sputtering

Five-layered Si/Si_xGe_1−x films on Si(1 0 0) substrate with single-layer thickness of 30 nm, 10 nm and 5 nm, respectively were prepared by RF helicon magnetron sputtering with dual targets of Si and Ge to investigate the feasibility of an industrial fabrication method on multi-stacked superlattice structure for thin-film thermoelectric applications. The fine periodic structure is confirmed in the samples except for the case of 5 nm in single-layer thickness. Fine crystalline Si_xGe_1−x layer is obtained from 700 °C in substrate temperature, while higher than 700 °C is required for Si good layer. The composition ratio (x) in Si_xGe_1−x is varied depending on the applied power to Si and Ge targets. Typical power ratio to obtain x = 0.83 was 7:3, Hall coefficient, p-type carrier concentration, sheet carrier concentration and mobility measured for the sample composed of five layers of Si (10 nm)/Si_0.82Ge_0.18 (10 nm) are 2.55 × 10⁶ /°C, 2.56 × 10¹² cm⁻³, 1.28 × 10⁷ cm⁻², and 15.8 cm⁻²/(V s), respectively.     

Photoluminescence of Eu-doped CaMgSi2xO6+2x (1.00?x?1.20) phosphors in UV-VUV region

Alkaline-earth silicate phosphors CaMgSi_2xO_6+2x:Eu²⁺ (1.00?x?1.20) were prepared by traditional solid-state reaction. The phosphors showed an intense blue emission centered around 453 nm, with both 254 and 147 nm excitations. The host absorption below 200 nm in the excitation spectra consisted of two bands around 160 and 190 nm. The band around 160 nm was ascertained to be associated with the SiO₄-tetrahedra and MgO₆-polyhedra, and that around 190 nm was due to the CaO₈-polyhedra or some impurities. The incorporation of excess Si of less than 15% would not lead to formation of impurities and the results indicated that an appropriate Si excess could improve the Photoluminescence (PL) intensity in both ultraviolet (UV) and vacuum ultraviolet (VUV) regions     

Luminescence studies of Nd- and Yb-doped α-Y(IO3)3 and β-Y(IO3)3, transparent host matrix in the mid- and beginning of the far-infrared

α-Y(IO₃)₃ and β-Y(IO₃)₃ are transparent until 12.8 and 13.4 μm, respectively; thus they are interesting as a potential laser matrix in the mid- and beginning of the far-infrared. So, in order to investigate the properties of lanthanides- doped anhydrous yttrium iodate, polycrystalline samples of α-Y_1−xNd_x(IO₃)₃ (0.01?x?0.05), β-Y_1−xNd_x(IO₃)₃ (0.001?x?0.1), α-Y_1−xYb_x(IO₃)₃ (0.01?x?0.33) and β-Y_1−xYb_x(IO₃)₃ (0.01?x?0.25) were synthesized. For Nd³⁺ ions, fluorescent emissions from the ⁴F_3/2 multiplet were observed at 300 K under pulsed laser excitations at 750 nm and for Yb³⁺, fluorescent emissions from the ²F_5/2 multiplet were observed at 300 K under pulsed laser excitations at 980 nm. The decays of all these emissions were measured. They are exponential and the fluorescence lifetimes are in the range 0.093-0.193 ms for Nd³⁺ and 0.370-0.541 ms for Yb³⁺, depending on the nature of the host and the concentration of doping.     

BiFeO3/Zn1−xMnxO bilayered thin films

BiFeO₃/Zn_1−xMn_xO (x = 0-0.08) bilayered thin films were deposited on the SrRuO₃/Pt/TiO₂/SiO₂/Si(1 0 0) substrates by radio frequency sputtering. A highly (1 1 0) orientation was induced for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO thin films demonstrate diode-like and resistive hysteresis behavior. A remanent polarization in the range of 2P_r ∼ 121.0-130.6 μC/cm² was measured for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO (x = 0.04) bilayer exhibits a highest M_s value of ∼15.2 emu/cm³, owing to the presence of the magnetic Zn_0.96Mn_0.04O layer with an enhanced M_s value.     

Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ thin films were deposited on LaNiO₃(LNO)/SiO₂/Si(1 0 0) and Pt/Ti/SiO₂/Si(1 0 0) substrates by metal-organic decomposition, and the effects of bottom electrodes LNO and Pt on the ferroelectric, dielectric and piezoelectric properties were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy, respectively. For the thin films deposited on LNO and Pt electrodes, the remnant polarization 2P_r are about 22.6 and 8.8 μC/cm² under 375 kV/cm, the dielectric constants 238 and 579 at 10 kHz, the dielectric losses 0.06 and 0.30 at 10 kHz, the statistic d_33eff values 95 and 81 pm/V. The improved piezoelectric properties could make (Na_1−xK_x)_0.5Bi_0.5TiO₃ thin film as a promising candidate for piezoelectric thin film devices.     

Magneto-transport properties of [Co/Bi]n wire structures

The present study examines the artificial control of grain-boundary resistance and its contribution to the magneto-transport properties of [Co(1 nm)/Bi(2.5 nm)]_n (n=10 or 20) line structures on the Si(0 0 1)/SiN_x substrate. Conventional patterning and deposition processes are applied for the fabrication of a device that consists of five-line structures with a line width of 2 μm. A ΔR/R=80% ratio was observed in the five-line structure of [Co(1 nm)/Bi(2.5 nm)]₁₀ multilayers at 10 K. Our measurements indicate that grain-boundary effects can be associated with the large ΔR/R ratio of transverse magnetoresistance.     

Spectroscopic properties of the rubidium and cesium aluminum double molybdate crystals

Infrared, Raman, electron absorption, excitation and emission spectra were measured for RbCr_xAl_1−x(MoO₄)₂ and CsCr_xAl_1−x(MoO₄)₂ crystals (x=0-2%) at the temperatures ranging from 7 to 300 K. A very rich vibronic structure of the emission band was explained and assigned to the respective vibrational modes. One Cr³⁺ center characterized by 2.35 ms lifetimes for rubidium derivative and 1.3 ms for cesium one at 7 K for the ²E→⁴A₂ transition was identified for both crystals. The local structure of the Cr³⁺ surrounding is discussed in terms of the spectroscopic results and the crystal field parameters are derived for both materials.     

Hf1−xSixOy dielectric films deposited by UV-photo-induced chemical vapour deposition (UV-CVD)

Hf_1−xSi_xO_y is an attractive candidate material for high-k dielectrics. We report in this work the deposition of ultra-thin Hf_1−xSi_xO_y films (0.1 ≤ x ≥ 0.6) on silicon substrate at 450 °C by UV-photo-induced chemical vapour deposition (UV-CVD) using 222 nm excimer lamps. Silicon(IV) and hafnium(IV) organic compounds were used as the precursors. Films from around 5 to 40 nm in thickness with refractive indices from 1.782 to 1.870 were grown. The deposition rate was found to be of 6 nm/min at a temperature of 450 °C. The physical, interfacial and electrical properties of hafnium silicate (Hf_1−xSi_xO_y) thin films were investigated by using X-ray photoelectron spectroscopy, ellipsometry, FT-IR, C-V and I-V measurements. XRD showed that they were basically amorphous, while Fourier transform infrared spectroscopy (FT-IR), clearly revealed Hf-O-Si absorption in the photo-CVD deposited Hf_1−xSi_xO_y films. Surface and interfacial properties were analysed by TEM and XPS. It is found that carbon content in the films deposited by UV-CVD is very low and it also decreases with increasing Si/(Si + Hf) ratio, as low as about 1 at.% at the Si/(Si + Hf) ratio of 60 at.%.     

Determination of the molecular dipole moment of bromofluoromethane: Microwave Stark spectra and ab initio calculations

The electric dipole moment of bromofluoromethane, CH₂⁷⁹BrF, has been determined with a good accuracy by observing the second order ΔM_J = 0 Stark spectrum of the J = 3_2,1 ← 3_1,2, J = 5_2,3 ← 5_1,4 and J = 5_2,4 ← 5_1,5 rotational transitions. In addition, the equilibrium geometry and dipole moment have been evaluated using highly accurate ab initio calculations. By comparing the experimental [μ_a = 0.3466(11) D and μ_b = 1.704(26) D] and theoretical [μ_a = −0.339 D and μ_b = −1.701 D] dipole moment components, a very good agreement has been found.     

X-ray multiple diffraction in the characterization of TiNO and TiO2 thin films grown on Si(0 0 1)

TiO₂ and TiN_xO_y thin films grown by low pressure metal-organic chemical vapor deposition (LP-MOCVD) on top of Si(0 0 1) substrate were characterized by X-ray multiple diffraction. X-ray reflectivity analysis of TiO₂[1 1 0] and TiNO[1 0 0] polycrystalline layers allowed to determine the growth rate (−80 Å/min) of TiO₂ and (−40 Å/min) of TiNO films. X-ray multiple diffraction through the Renninger scans, i.e., ?-scans for (0 0 2)Si substrate primary reflection is used as a non-conventional method to obtain the substrate lattice parameter distortion due to the thin film conventional deposition, from where the information on film strain type is obtained.     

Influence of temperature on structure as well as adhesion and friction and wear behavior of hydrogenated carbon nitride films prepared on silicon substrate

Hydrogenated-carbon nitride (CN_x:H) films were synthesized on silicon substrate in a large quantity by the pyrolysis of ethylenediamine in a temperature range of 700-950 °C. The influence of temperature on the morphology, structure, adhesion to substrate, and friction and wear behavior of CN_x:H films was investigated. It has been found that CN_x:H films obtained at 700 °C and 800 °C are amorphous, and those prepared at 900 °C and 950 °C consist of carbon nitride nanocrystal. Besides, CN_x:H film sample obtained at 700 °C has the maximum N content of 9.1 at.% but the poorest adhesion to Si substrate, while the one prepared at 900 °C has the lower N content and the highest adhesion to substrate. As a result, nanocrystalline CN_x:H (nc-CN_x:H) film synthesized at 900 °C possesses the best wear resistance when slides against stainless steel counterpart. N atom is incorporated into the graphitic network in three different bonding forms, and their relative content is closely related to temperature, corresponding to different adhesion as well as friction and wear behavior of the films obtained at different temperatures. Furthermore, the friction coefficient and antiwear life of as-deposited CN_x:H films vary with varying deposition temperature and thickness, and the film with thickness of 1.3 μm, obtained at 900 °C, has the longest antiwear life of over 180,000 s.     

Enhanced formation of periodic arrays of low-resistivity NiSi nanocontacts on (0 0 1)Si0.7Ge0.3 by nanosphere lithography with a thin interposing Si layer

In this study, we demonstrated significant enhancement of the formation of low-resistivity NiSi nanocontacts with controlled size on (0 0 1)Si_0.7Ge_0.3 substrates by combining the nanosphere lithography with the use of a new Ni/a-Si bilayer nanodot structure. Low-resistivity NiSi with an average size of 78 nm was observed to be the only silicide phase formed in samples annealed at 350-800 °C. The presence of the interposing Si layer with appropriate thickness was found to effectively prevent Ge segregation and maintain the interface stability in forming NiSi nanocontacts on (0 0 1)Si_0.7Ge_0.3. As the annealing temperature was increased to 900 °C, amorphous SiO_x nanowires were observed to grow from silicide nanocontact regions. The NSL technique in conjunction with a sacrificial Si interlayer process promises to be applicable in fabricating periodic arrays of other low-resistivity silicide nanocontacts on Si_1−xGe_x substrates without complex lithography.     

Infrared optical properties of Ba(Zr0.20Ti0.80)O3 and Ba(Zr0.30Ti0.70)O3 thin films prepared by sol-gel method

Ba(Zr_xTi_1−x)O₃ (BZT) (x = 0.20 and 0.30) thin films are deposited on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrate by sol-gel method. X-ray diffraction patterns show that the thin films have a good crystallinity. Optical properties of the films in the wavelength range of 2.5-12 μm are studied by infrared spectroscopic ellipsometry (IRSE). The optical constants of the BZT thin films are determined by fitting the IRSE data using a classical dispersion formula. As the wavelength increases, the refractive index decreases, while the extinction coefficients increase. The effective static ionic charges are derived, which are smaller than that in a purely ionic material for the BZT thin films.     

Omnidirectional antireflective properties of porous tungsten oxide films with in-depth variation of void fraction and stoichiometry

We report on the fabrication of porous hot-wire deposited WO_x (hwWO_x) films with omnidirectional antireflective properties coming from in-depth variation of both (i) void fraction from 0% at the Si substrate/hwWO_x interface to 30% within less than 7 nm and to higher than 50% at the hwWO_x/air interface, and (ii) x, namely hwWO_x stoichiometry, from 2.5 at the Si/hwWO_x to 3 within less than 7 nm. hwWO_x films were deposited by means of hw deposition at rough vacuum and controlled chamber environment. The films were analyzed by Spectroscopic Ellipsometry to extract the graded refractive index profile, which was then used in a rigorous coupled wave analysis (RCWA) model to simulate the antireflective properties. RCWA followed reasonably the experimental reflection measurements. Void fraction and x in-depth variation, controlled by the hw process, greatly affect the antireflective properties, and improve the omnidirectional and broadband characteristics. The reflection suppression below 10% within the range of 500-1000 nm for angles of incidence up to more than 60° is demonstrated.     

Integrity of functional self-assembled monolayers on hydrogen-terminated silicon-on-insulator wafers

Silicon-on-insulator (SOI) wafers are commonly used to design microelectronics, energy conversion, and sensing devices. Thin solid films on the surfaces of SOI wafers have been a subject of numerous studies. However, SOI wafers modified by self-assembled monolayers (SAMs) that can also be used as functional device platforms have been investigated to a much lesser extent. In the present work, tert-butoxycarbonyl (t-boc, (CH₃)₃-C-O-C(O)-)-protected 1-amino-10-undecene monolayers were covalently attached to a H-terminated SOI (1 0 0) surface. The modified wafers were characterized by X-ray photoelectron spectroscopy to confirm the stability of the SAM/Si interface and the integrity of the secondary amine in the SAM. The transmission electron microscopy investigation suggested that this t-boc-protected 1-amino-10-undecene SAM produces atomically flat interface with the 2 μm single crystalline silicon of the SOI wafer, that the SiO_x and both available Si/SiO_x interfaces are preserved, and that the organic monolayers are stable, with apparent thickness of 1.7 nm, which is consistent with the result of the density functional theory modeling of the molecular features within a SAM.     

Luminescent properties of Tb activated GdTaO4 with M and M′ type structure under UV-VUV excitation

The photoluminescence of Tb³⁺ doped M and M′ type gadolinium orthotantalate Gd_1−xTb_xTaO₄ (0.01≤x≤0.20) was investigated under ultraviolet and vacuum ultraviolet excitation. For the samples of Gd_1−xTb_xTaO₄ with different crystallographic structures, emission spectra were the same in addition to intensity; the optimal concentration for Tb³⁺ was about 10 mol % in M type Gd_1−xTb_xTaO₄ but 5 mol % in M′ type Gd_1−xTb_xTaO₄. These differences could be corresponding with the difference in structures. In addition, compared to commercial Zn₂SiO₄: Mn²⁺, the integrated intensity of M and M′ type GdTaO₄: Tb³⁺ could reach 67% and 85%, respectively, of that at 147 nm excitation, which indicates that GdTaO₄: Tb³⁺ would be a promising vacuum ultraviolet phosphor for application in PDP and Hg-free lamp.     

Effect of boric acid on structure,morphology and luminescent properties of divalent europium doped calcium aluminate phosphors

Blue phosphors Ca_1 − xAl₂O₄: xEu²⁺ were prepared by high temperature solid-state method. Their structure, morphology and luminescent properties were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and fluorescence spectroscopy. The effect of different amounts of fluxing agent H₃BO₃ on structure, morphology and luminescent properties of blue phosphors Ca_1 − xAl₂O₄: xEu²⁺ luminous intensity caused by different amount of H₃BO₃ was also investigated. The amount of H₃BO₃ doped Ca_1 − xAl₂O₄: xEu²⁺ in optimal luminous intensity had been determined. The results showed that both the excitation and emission spectra of samples were all broad bands, and that the peak of emission spectra was near 442 nm, which was corresponding to the 4f⁶5d → 4f⁷ transition of Eu²⁺ illuminating blue light. Ca_1 − xAl₂O₄: xEu²⁺ (x = 3.5 mol%) could be gained with good morphology and the best luminous intensity when H₃BO₃ mass ratio was 0.5 wt%.     

Hybrid-DFT study for the initial oxidation steps on silicon cluster surface

We performed a hybrid density functional theory calculation for the successive adsorption of nitrous oxide (N₂O) on Si(1 0 0)-Si₉H₁₂O_x (x = 0 and 1) cluster surfaces to elucidate N₂O decomposition and the subsequent surface oxidation processes. N₂O decomposed into N₂ and O fragments, and the latter fragment inserted into either surface-dimer bonds or back-bonds with similar activation barriers on both the clean and partially oxidized Si surfaces. The Si₉H₁₂ cluster surface was eventually oxidized to five distinct structures of Si₉H₁₂O₂.     

Tunable light emission and decaying process of photoluminescence from a nanostructured Si-in-SiNx film

Strong photoluminescence (PL) covering the green-violet band was measured at room temperature in an as-deposited amorphous Si-in-SiN_x film, which was prepared by plasma-enhanced chemical vapor deposition on cold (below 60 °C) Si(1 0 0) wafer. With an increase in photon energy of excitation, the PL shifts its peak position from 510 to 416 nm at yet-comparable intensities, thus allowing an energy-selected excitation in practical application. Also, a time-resolved analysis was performed for the emissions at various wavelengths, which showed a decay time shorter than 1.0 ns. These results indicate that the nanostructured Si-in-SiN_x can be a promising candidate material for the fabrication of silicon-based optical interconnections and switches.