Electro-optic metal–insulator–semiconductor–insulator–metal Mach-Zehnder plasmonic modulator

The performance of a CMOS-compatible electro-optic Mach-Zehnder plasmonic modulator is investigated using electromagnetic and carrier transport simulations. Each arm of the Mach-Zehnder device comprises a metal–insulator–semiconductor–insulator–metal (MISIM) structure on a buried oxide substrate. Quantum mechanical effects at the oxide/semiconductor interfaces were considered in the calculation of electron density profiles across the structure, in order to determine the refractive index distribution and its dependence on applied bias. This information was used in finite element simulations of the electromagnetic modes within the MISIM structure in order to determine the Mach-Zehnder arm lengths required to achieve destructive interference and the corresponding propagation loss incurred by the device. Both inversion and accumulation mode devices were investigated, and the layer thicknesses and height were adjusted to optimise the device performance. A device loss of <8 dB is predicted for a MISIM structure with a 25 nm thick silicon layer, for which the device length is <3 μm, and <5 dB loss is predicted for the limiting case of a 5 nm thick silicon layer in a 1.2 μm long device: in both cases, the maximum operating voltage is 7.5 V.     

Optimization of microcrystalline silicon thin film solar cell isolation processing parameters using ultraviolet laser

This study used ultraviolet laser to perform the microcrystalline silicon thin film solar cell isolation scribing process, and applied the Taguchi method and an L₁₈ orthogonal array to plan the experiment. The isolation scribing materials included ZnO:Al, AZO transparent conductive film with a thickness of 200 nm, microcrystalline silicon thin film at 38% crystallinity and of thickness of 500 nm, and the aluminum back contact layer with a thickness of 300 nm. The main objective was to ensure the success of isolation scribing. After laser scribing isolation, using the minimum scribing line width, the flattest trough bottom, and the minimum processing edge surface bumps as the quality characteristics, this study performed main effect analysis and applied the ANOVA (analysis of variance) theory of the Taguchi method to identify the single quality optimal parameter. It then employed the hierarchical structure of the AHP (analytic hierarchy process) theory to establish the positive contrast matrix. After consistency verification, global weight calculation, and priority sequencing, the optimal multi-attribute parameters were obtained. Finally, the experimental results were verified by a Taguchi confirmation experiment and confidence interval calculation. The minimum scribing line width of AZO (200 nm) was 45.6 μm, the minimum scribing line width of the microcrystalline silicon (at 38% crystallinity) was 50.63 μm and the minimum line width of the aluminum thin film (300 nm) was 30.96 μm. The confirmation experiment results were within the 95% confidence interval, verifying that using ultraviolet laser in the isolation scribing process for microcrystalline silicon thin film solar cell has high reproducibility.     

A study on electromagnetic wave absorber for the collision-avoidance radar

In this paper, we designed and fabricated the EM wave absorber for eliminating false image and system-to-system interference in a collision-avoidance radar using Permalloy and CPE, the mixing ratio of which was Permalloy:CPE = 70:30 wt%. The EM wave absorption ability was simulated according to different thicknesses of the EM wave absorber using measured permittivity and permeability, and then the EM wave absorber was fabricated based on the simulated design. The simulated results agree well with the measured ones. As a result, we suggest Permalloy for use as a new EM wave absorber in W-band, and the fabricated EM wave absorber with the thickness of 1.4 mm has absorption ability more than 20 dB in frequency range of 76–77 GHz.     

General regression neural network for prediction of sound absorption coefficients of sandwich structure nonwoven absorbers

In this paper, we propose a more general forecasting method to predict the sound absorption coefficients at six central frequencies and the average sound absorption coefficient of a sandwich structure nonwoven absorber. The kernel assumption of the proposed method is that the acoustics property of sandwich structure nonwoven absorber is determined by some easily measured structural parameters, such as thickness, area density, porosity, and pore size of each layer, if the type of the fiber used in nonwoven is given. By holding this assumption in mind, we will use general regression neural network (GRNN) as a prediction model to bridge the gap between the measured structural parameters of each absorber and its sound absorption coefficient. In experiment section, one hundred sandwich structure nonwoven absorbers are particularly designed with ten different types of meltblown polypropylene nonwoven materials and four types of hydroentangled E-glass fiber nonwoven materials firstly. Secondly, four structural parameters, i.e., thickness, area density, porosity, and pore size of each layer are instrumentally measured, which will be used as the inputs of GRNN. Thirdly, the sound absorption coefficients of each absorber are measured with SW477 impedance tube. The sound absorption coefficient at 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz and their average value are used as the outputs of GRNN. Finally, the prediction framework will be carried out after the desired training set selection and spread parameter optimization of GRNN. The prediction results of 20 test samples show the prediction method proposed in this paper is reliable and efficient.     

I–V characteristics of ZnO/Cu2O thin film n–i–p heterojunction

ZnO/Cu₂O thin film n–i–p heterojunctions were fabricated by magnetron sputtering. The microstructure, optical, and electrical properties of n-type (n‐) ZnO, insulating (i‐) ZnO, and p-type (p‐) Cu₂O films deposited on glass substrates were characterized by X-Ray diffraction (XRD), spectrophotometer, and the van der Pauw method, respectively. XRD results show that the mean grain size of i-ZnO film is much larger than that of n-ZnO film. The optical band gap energies of n-ZnO, i-ZnO, and p-Cu₂O film are 3.27, 3.47, and 2.00 eV, respectively. The carrier concentration of n-ZnO film is two orders of magnitude larger than that of p-Cu₂O film. The current–voltage (I–V) characteristics of ZnO/Cu₂O thin film n–i–p heterojunctions with different i-ZnO film thicknesses were investigated. Results show that ZnO/Cu₂O n–i–p heterojunctions have well-defined rectifying behavior. All ideality factors of these n–i–p heterojunctions are larger than 2.0. The forward bias threshold voltage and ideality factor increase when i-ZnO layer thickness increases from 100 to 200 nm. An energy band diagram was proposed to analyze the I–V characteristics of these n–i–p heterojunctions.     

Device-level vacuum packaged uncooled microbolometer on a polyimide substrate

Uncooled infrared detectors (IR) on a polyimide substrate have been demonstrated where amorphous silicon (a-Si) was used as the thermometer material. New concepts in uncooled microbolometers were implemented during the design and fabrication, such as the integration of a germanium long-pass optical filter with the device-level vacuum package and a double layer absorber structure. Polyimide was used for this preliminary work towards vacuum-packaged flexible microbolometers. The detectors were fabricated utilizing a carrier wafer and low adhesion strength release layer to hold the flexible polyimide substrate during fabrication in order to increase the release yield. The IR detectors showed a maximum detectivity of 4.54 × 10⁶ cm Hz^1/2/W at a 4 Hz chopper frequency and a minimum noise equivalent power (NEP) of 7.72 × 10⁻¹⁰ W/Hz^1/2 at a biasing power of 5.71 pW measured over the infrared wavelength range of 8–14 μm for a 35 μm × 35 μm detector. These values are comparable to other flexible microbolometers with device-level vacuum packaging which are found in literature.     

Photoemission study of the reactivity of barium towards SiOx thermal films

Barium was deposited at room temperature on a thermal silicon oxide layer and the interfacial reaction was monitored by synchrotron induced photoemission (both core level and valence band). The first step of the growth consists of an interfacial reaction which leads to the formation of an interfacial silicate layer. The next step consists in formation of barium oxide while metallic barium occurs subsequently. The deposit can be also homogenized by annealing above 575 K. This results in the formation of several layers of silicate by consumption of silicon oxide. In the case of fractional coverage, subsequent annealing at 975 K induces the decomposition of barium silicate. However, such a decomposition process is strongly dependent on the initial film thickness. It can be avoided for deposits thicker than 3 eqML.     

Surface and interface effects on structural transformation of vapor-deposited ethylbenzene films

We have investigated how the structures of vapor-deposited glassy films change with increasing temperature by using time-of-flight secondary ion mass spectrometry and ion scattering spectroscopy. It is found that intermixing of the topmost layer of an ethylbenzene film occur at temperature (~ 80 K) considerably lower than the glass transition temperature (T_g = 118 K) when the film is deposited at 20 K. This phenomenon can be interpreted as the occurrence of a two-dimensional liquid that diffuses into pores of the film, which is evidenced from comparison with surface diffusivity measurements using a porous silicon layer. For nonporous films deposited at higher temperatures, the molecules intermix gradually prior to the abrupt film morphology change at T_g. This phenomenon can be interpreted as decoupling between translational diffusivity and viscosity in the bulk. The film thickness has no significant effects on the evolution of supercooled liquid at T_g except for the monolayer film, whereas crystallization is quenched for the films thinner than 8 monolayers. The roles of the 2D liquid on the surface and an immobilized layer formed at the interface are discussed in finite-size effects on the glass-liquid transition and crystallization.     

Structural and optical properties of chromotrope 2R thin films of different thicknesses

Chromotrope 2R (CHR) films of different thicknesses have been prepared using spin coater. The material has been characterized using FT-IR, DTA and X-ray diffraction. The XRD of the material in powder and thin film forms showed polycrystalline structure with triclinic phase. Preferred orientation at the (1 1 4) plane is observed for the deposited films. Initial indexing of the XRD pattern was performed using “Crystalfire” computer program. Miller indices, h k l, values for each diffraction line in X-ray diffraction (XRD) spectrum were calculated and indexed for the first time. The DTA thermograms of CHR powder have been recorded in the temperature range 25–350 °C with different heating rates. The spectra of the infra-red absorption allow characterization of vibration modes for the powder and thin film. The effect of film thickness on the optical properties has been studied in the UV-visible-NIR regions. The films show high transmittance exceeding 0.90 in the NIR region λ > 800 nm. The intensity of the absorption peaks for λ < 800 nm are enhanced as the film thickness increase. The absorption bands are attributed to the (π–π*) and (n–π*) molecular transitions. The optical properties have been analyzed according to the single-oscillator model and the dispersion energy parameters as well as the free charge carrier concentration have been determined. The optical energy gap as well as the oscillator strength and electric dipole strength have been calculated.     

Highly flexible,transparent, conductive and antibacterial films made of spin-coated silver nanowires and a protective ZnO layer

We prepared highly flexible, transparent, conductive and antibacterial film by spin coating a silver nanowire suspension on a poly (ethylene terephthalate) (PET) substrate. The ZnO layer covered the conductive silver nanowire (AgNW) network to protect the metal nanowires from oxidization and enhance both wire-to-wire adhesion and wire-to-substrate adhesion. It is found that the number of AgNW coatings correlates with both the sheet resistance (R_s) and the transmittance of the AgNW/ZnO composite films. An excellent 92% optical transmittance in the visible range and a surface sheet resistance of only 9 Ω sq⁻¹ has been achieved, respectively. Even after bending 1000 times (5 mm bending radius), we found no significant change in the sheet resistance or optical transmittance. The real-time sheet resistance measured as a function of bending radius also remains stable even at the smallest measured bending radius (1 mm). The AgNW/ZnO composite films also show antibacterial effects which could be useful for the fabrication of wearable electronic devices.     

Characteristics of ZnO MSM UV photodetector with Ni contact electrodes on poly propylene carbonate (PPC) plastic substrate

In this study, we report the fabrication of ZnO metal-semiconductor-metal UV photodetector (MSM UV PD) by deposition ZnO thin film on poly propylene carbonate (PPC) plastic substrate using direct current (DC) sputtering technique, and Nickel (Ni) contact as electrodes. The structural, optical and electrical properties of the ZnO thin film were investigated by using atomic force microscopy (AFM), X-Ray diffraction (XRD) measurement, and photoluminescence (PL). The electrical characteristics of the detector were investigated using the current–voltage (I–V) measurements, the dark- and photo-currents were found to be 1.04 and 93.80 μA, respectively. Using forward dark conditions at 5 volt; the barrier height Φ_B was calculated to be 0.675 eV. Under incident wavelength of 385 nm, it was found that the maximum responsivity (R) of the Ni/ZnO/Ni MSM PD was found to be 1.59 A/W.     

Effect of GZO thickness and annealing temperature on the structural,electrical and optical properties of GZO/Ag/GZO sandwich films

The GZO/Ag/GZO sandwich films were deposited on glass substrates by RF magnetron sputtering of Ga-doped ZnO (GZO) and ion-beam sputtering of Ag at room temperature. The effect of GZO thickness and annealing temperature on the structural, electrical and optical properties of these sandwich films was investigated. The microstructures of the films were studied by X-ray diffraction (XRD). X-ray diffraction measurements indicate that the GZO layers in the sandwich films are polycrystalline with the ZnO hexagonal structure and have a preferred orientation with the c-axis perpendicular to the substrates. For the sandwich film with upper and under GZO thickness of 40 and 30 nm, respectively, it owns the maximum figure of merit of 5.3 × 10⁻² Ω⁻¹ with a resistivity of 5.6 × 10⁻⁵ Ω cm and an average transmittance of 90.7%. The electrical property of the sandwich films is improved by post annealing in vacuum. Comparing with the as-deposited sandwich film, the film annealed in vacuum has a remarkable 42.8% decrease in resistivity. The sandwich film annealed at the temperature of 350 °C in vacuum shows a sheet resistance of 5 Ω/sq and a transmittance of 92.7%, and the figure of merit achieved is 9.3 × 10⁻² Ω⁻¹.     

Magnetic properties and structures of Fe/MgF2 multilayered films

Multilayers composed of Fe and MgF₂ with layer thicknesses between 9 Å and 100 Å and of 30 Å, respectively, were prepared with an ultrahigh-vacuum deposition technique. Medium-angle X-ray data show that the Fe layers in the BCC phase have considerable (1 1 0) texture. Periodicity due to multilayered structures was confirmed by a small-angle X-ray diffraction study and cross-section transmission electron microscope for films with Fe layer thicknesses >45 Å. In an Fe/MgF₂(9 Å/30 Å) sample, an island structure for the Fe layers was suggested by the existence of superparamagnetism in a film. At 4.2 K, enhancements of both magnetization and hyperfine field were observed in films having Fe layers thinner than 40 Å. The maxima in the magnetization (233 emu/g of Fe) and in the average hyperfine field (390 kOe) at 4.2 K were found in an Fe/MgF₂(9 Å/30 Å) film and were approximately 105% and 115% that of the bulk α-Fe, respectively. The thickness dependence suggests a 12% enhancement in the magnetic moment of interface Fe atoms. No exchange bias was found in the films, implying that antiferromagnetic fluorides are not formed at the interface, which is different from the case of Fe/LiF and Fe/CaF₂ multilayers.     

Planar sulfur-doped silicon detectors for high-speed infrared thermography

Planar extrinsic sulfur-doped silicon detectors for infrared (IR) semiconductor-discharge gap image converters intended for use in high-speed thermography of remote objects have been developed. The detectors were fabricated by high-temperature diffusion of sulfur into silicon wafers from the vapor phase. The dependence of doping efficiency on the sulfur vapor pressure in the course of diffusion was analyzed. The detector fabrication technology was optimized to meet the specific requirements for their operation in the microdischarge devices considered. The detectors were tested in a laboratory setup comprising a blackbody source of IR light, an image converter, and a pulsed CCD camera for recording the converted images. The converter equipped with the detector can provide imaging of objects heated to a temperature, T_min ≈ 200 °C, with a temporal resolution on the order of 10^?6 s and spatial resolution of about 5 lines/mm.     

Epitaxial ZnO thin films grown by pulsed electron beam deposition

In this work, the pulsed electron beam deposition method (PED) is evaluated by studying the properties of ZnO thin films grown on c-cut sapphire substrates. The film composition, structure and surface morphology were investigated by means of Rutherford backscattering spectrometry, X-ray diffraction and atomic force microscopy. Optical absorption, resistivity and Hall effect measurements were performed in order to obtain the optical and electronic properties of the ZnO films. By a fine tuning of the deposition conditions, smooth, dense, stoichiometric and textured hexagonal ZnO films were epitaxially grown on (0001) sapphire at 700 °C with a 30° rotation of the ZnO basal plane with respect to the sapphire substrate. The average transmittance of the films reaches 90% in the visible range with an optical band gap of 3.28 eV. Electrical characterization reveals a high density of charge carrier of 3.4 × 10¹⁹ cm^?3 along with a mobility of 11.53 cm²/Vs. The electrical and optical properties are discussed and compared to ZnO thin films prepared by the similar and most well-known pulsed laser deposition method.     

1.55 μm silicon-based reflection-type waveguide-integrated thermo-optic 2 × 2 switch

In this work a waveguide-integrated 2 × 2 switch operating at the infrared communication wavelength of 1550 nm is proposed and theoretically discussed. The device is based on the total internal reflection (TIR) phenomenon and the thermo-optic effect (TOE) in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). It takes advantage of a bandgap-engineered a-Si:H layer to explore the properties of an optical interface between materials showing similar refractive indexes but different thermo-optic coefficients. In particular, thanks to modern plasma-enhanced chemical vapour deposition (PECVD) techniques, the refractive index of the amorphous film can be properly tailored to match that of c-Si at a given temperature. TIR may be therefore achieved at the interface by acting on the temperature. The device is integrated in a 4 μm-wide and 3 μm-thick single-mode rib waveguide. The substrate is a silicon-on-insulator (SOI) wafer with an oxide thickness of 500 nm. We calculated an output crosstalk always better than 24 dB and insertion losses as low as 3.5 dB.     

Using high haze (> 90%) light-trapping film to enhance the efficiency of a-Si:H solar cells

The high haze light-trapping (LT) film offers enhanced scattering of light and is applied to a-Si:H solar cells. UV glue was spin coated on glass, and then the LT pattern was imprinted. Finally, a UV lamp was used to cure the UV glue on the glass. The LT film effectively increased the Haze ratio of glass and decreased the reflectance of a-Si:H solar cells. Therefore, the photon path length was increased to obtain maximum absorption by the absorber layer. High Haze LT film is able to enhance short circuit current density and efficiency of the device, as partial composite film generates broader scattering light, thereby causing shorter wave length light to be absorbed by the P layer so that the short circuit current density decreases. In case of lab-made a-Si:H thin film solar cells with v-shaped LT films, superior optoelectronic performances have been found (V_oc = 0.74 V, J_sc = 15.62 mA/cm², F.F. = 70%, and η = 8.09%). We observed ~ 35% enhancement of the short-circuit current density and ~ 31% enhancement of the conversion efficiency.     

Effect of thickness on the structural,optical and electrical properties of thermally evaporated PbI2 thin films

This work describes the physical properties of lead iodide (PbI₂) thin films with different thicknesses that were deposited on ultrasonically cleaned glass substrates using a thermal evaporation technique at 5×10^-6 torr. The initial material was purified by the zone refining technique under an atmosphere of argon gas. XRD analysis of the material demonstrates that the thin films were preferably oriented along the (001) direction. The size of the crystallites was calculated from the Scherer relation and found to be in the range of ～5–10 nm, with higher values being observed for increasing film thicknesses. The optical energy band gaps were evaluated and determined to belong to direct transitions. Because the band gap increased with decreasing film thickness, a systematic blue shift was observed. The surface morphologies of PbI₂ films exhibited a clear increase in grain size with increasing film thickness. The photoluminescence and dc conductivity of the thin films are also discussed.     

Performance of mid-wave T2SL detectors with heterojunction barriers

A heterojunction T2SL barrier detector which effectively blocks majority carrier leakage over the pn-junction was designed and fabricated for the mid-wave infrared (MWIR) atmospheric transmission window. The layers in the barrier region comprised AlSb, GaSb and InAs, and the thicknesses were selected by using k · P-based energy band modeling to achieve maximum valence band offset, while maintaining close to zero conduction band discontinuity in a way similar to the work of Abdollahi Pour et al. [1] The barrier-structure has a 50% cutoff at 4.75 μm and 40% quantum efficiency and shows a dark current density of 6 × 10⁻⁶ A/cm² at −0.05 V bias and 120 K. This is one order of magnitude lower than for comparable T2SL-structures without the barrier. Further improvement of the (non-surface related) bulk dark current can be expected with optimized doping of the absorber and barrier, and by fine tuning of the barrier layer design. We discuss the effect of barrier doping on dark current based on simulations. A T2SL focal plane array with 320 × 256 pixels, 30 μm pitch and 90% fill factor was processed in house using a conventional homojunction p–i–n photodiode architecture and the ISC9705 readout circuit. High-quality imaging up to 110 K was demonstrated with the substrate fully removed.     

Measurement of optical and surface properties of PLZT thin films

Lanthanum-modified lead zirconate titanate (Pb_0.93La_0.07(Zr_0.3Ti_0.7)_0.93O₃, PLZT7/30/70) thin films with and without a seeding layer of PbTiO₃ (PT) were successfully deposited on indium-doped tin oxide (ITO) coated glass substrate via spin coating in conjunction with a sol–gel process, and a top transparent conducting thin film of SnO₂ was also prepared in the same way. The thicknesses of PLZT and PT layers are 0.5 μm and 24 nm, respectively. The retardance of PLZT film was measured by a new heterodyne interferometer and enhanced by application of a seeding layer of PT. The Pockels linear electro-optical coefficient of PLZT film with a PT layer was determined to be 3.17 × 10^?9 m/V when the refractive index is considered as 2.505, which is one order larger than 1.4 × 10^?10 m/V for PLZT12/40/60 doped with Dy reported in the literature. The root-mean-square (rms) roughness of PLZT thin film with a PT layer (R_rms = 6.867 nm) was larger than that of PLZT film (R_rms = 0.799 nm). From the comparisons, the average transmittance of PLZT film with a PT seeding layer was 77.01%, which was a little smaller than that of PLZT film (around 80.75%). Experimental results imply that the PT seeding layer plays a key role in the increase of retardance value, leading to a higher Pockels coefficient.