Effects of electric field and bias voltage on corrosion behavior of tin under a thin electrolyte layer

The effects of electric field and bias voltage on corrosion behavior of tin under a thin electrolyte layer (TEL) were investigated by electrochemical and surface techniques. The results indicated that the order of the corrosion rate of tin under different electric fields with the same electric field gradient was as follows, square wave alternating current electric field (ACEF) > sinusoidal wave ACEF > direct current electric field (DCEF) > blank. Moreover, the corrosion rate of tin increased with increased direct current bias voltage under different electric fields. The contributions of electric field gradient and leakage current to the corrosion rate of tin were quantified under different electric fields with different bias voltages.     

Transparent conductors with an ultrathin nickel layer for high-performance photoelectric device applications

A thin nickel (Ni) layer of thickness 5 nm was inserted in between the indium tin oxide (ITO) layers of thickness 50 nm each so as to increase the conductivity of ITO without affecting much of its transmittance nature. ITO layers with and without Ni film were prepared by reactive DC sputtering on both Si and glass substrates. The influence of Ni layer on the optical and electrical properties of prepared devices was investigated. Due to the insertion of thin Ni layer, the resistivity of ITO/Ni/ITO sample (3.2×10⁻⁴ Ω cm) was reduced 10 times lesser than that of ordinary ITO layer (38.6×10⁻⁴ Ω cm); consequently it increased the mobility of ITO/Ni/ITO device. The external and internal quantum efficiencies (EQE and IQE) of ITO/Ni/ITO device exhibited better performance when compared to ITO layer that has no Ni film. At wavelengths of 350 and 600 nm, the photoresponses of ITO/Ni/ITO device were predominant than that of reference ITO device. This highly conductive and photoresponsive Ni inserting ITO layers would be a promising device for various photoelectric applications.     

Growth of ITO thin films on polyimide substrate by bias sputtering

Transparent conducting indium tin oxide thin films were deposited on polyimide substrates by RF bias sputtering of ITO target. The influences of bias voltage on the structural and electrical properties of the films were investigated. In order to correlate the material characteristics with the plasma parameters during sputtering, we employed Langmuir probe and optical emission spectral studies. The films deposited onto positively biased substrates were poorly crystalline. An improvement in crystallinity was observed with increase in negative bias. The films deposited at a bias voltage of ?20 V showed a preferred orientation in the [1 1 1] direction and has minimum resistivity compared to films grown at other biasing conditions. The measured plasma parameters were correlated to the film properties. The ITO films thus grown have been used as the channel layer for the fabrication of thin film transistor.     

The effect of oxide layer thickness on the quantification of 1.5 MeV γ–radiation induced interface traps in the Ag/SiO2/Si MOS devices

This work presents the effect of varied thickness of oxide layer and radiation dose on electrical characteristics of Ag/SiO₂/Si MOS devices irradiated by 1.5 MeV γ–radiations of varied doses. SiO₂ layers of 50, 100, 150 and 200 nm thickness were grown on Si substrates using dry oxidation and exposed to radiation doses of 1, 10 and 100 kGy. The exposure to radiation resulted in generation of fixed charge centers and interface traps in the SiO₂ and at the Si/SiO₂ interface. Capacitance-conductance-voltage (C-G-V) and capacitance-conductance-frequency (C-G-f) measurements were performed at room temperature for all MOS devices to quantify the active traps and their lifetimes. It is shown that accumulation and minimum capacitances decreased as the thickness of SiO₂ layer increased. For the unexposed MOS devices, the flat band voltage V_FB decreased at a rate of −0.12 V/nm, density of active traps increased by 4.5 times and depletion capacitance C_DP, increased by 2.5 times with the increase of oxide layer thickness from 50 to 200 nm. The density of active traps showed strong dependence on the frequency of the applied signal and the thickness of the oxide layer. The MOS device with 200 nm thick oxide layer irradiated with 100 kGy showed density of active interface traps was high at 50 kHz and was 3.6×10¹⁰ eV⁻¹ cm⁻². The relaxation time of the interface traps also increased with the exposure of γ–radiation and reached to 9.8 µs at 32 kHz in 200 nm thick oxide MOS device exposed with a dose of 100 kGy. It was inferred that this was due to formation of continuum energy states within the band gap and activation of these defects depended on the thickness of oxide layer, applied reverse bias and the working frequency. The present study highlighted the role of thickness of oxide layer in radiation hard environments and that only at high frequency, radiation induced traps remain passivated due to long relaxation times.     

Effect of film thickness on the surface,structural and electrical properties of InAlN films prepared by reactive co-sputtering

InAlN films of different thicknesses (150 nm, 250 nm, 380 nm, 750 nm and 1050 nm) were grown on Si (111) by means of reactive co-sputtering at 300 °C. Surface morphology results indicated an increase in the grains size and their spacing with increase of the film thickness. The surface of InAlN remained smooth with a slight variation in its RMS roughness from 1.29 nm to 6.62 nm by varying the film thickness. X-ray diffraction patterns exhibited InAlN diffraction peaks with preferred orientation along (002) plane in the thickness range 250 nm to 750 nm, however, the preferred orientation of the film was changed towards (101) plane at 1050 nm. An improvement in the crystallinity of InAlN was observed with increase of the film thickness. Electrical characterization revealed a decrease in the film's resistivity by increasing its thickness to 750 nm, however, the resistivity was found to increase at 1050 nm. The electron concentration indicated an increasing trend whereas changes in the electron mobility were found to be inconsistent with increase of the film thickness.     

Process optimization of gravure printed light-emitting polymer layers by a neural network approach

We demonstrate that artificial neural network modeling is a viable tool to predict the processing dependence of gravure printed light-emitting polymer layers for flexible OLED lighting applications. The (local) thickness of gravure printed light-emitting polymer (LEP) layers was analyzed using microdensitometry, after which the data was used to train a multi-layer neural network using error back propagation. Cell engraving depth, printing speed, and polymer concentration were used as input parameters of the neural network. Mean printed layer thickness, relative RMS roughness and feature anisotropy were defined as output parameters. The inhomogeneity of the gravure printed LEP layers was defined by two parameters, being the normalized standard deviation from the mean layer thickness, as well as the anisotropy or ‘directionality’ of the roughness features. Despite the limited number of input parameters, a fair prediction accuracy was obtained once new input data was fed into the trained network. The prediction error for the three output parameters was of the order: anisotropy > roughness > mean layer thickness. Calculating the magnitude of the output parameters as a function of the total space determined by the input parameters can be used as a way to find optimal printing conditions. These ‘landscape’ plots also reveal qualitative information on the rheological behavior of the inks during the printing process.     

Electronic and chemical properties of ZnO in inverted organic photovoltaic devices

Photo-conversion efficiency of inverted polymer solar cells incorporating pulsed laser deposited ZnO electron transport layer have been found to significantly increase from 0.8% to up to 3.3% as the film thickness increased from 4 nm to 100 nm. While the ZnO film thickness was found to have little influence on the morphology of the resultant ZnO films, the band structure of ZnO was found to evolve only for films of thickness 25 nm or more and this was accompanied by a significant reduction of 0.4 eV in the workfunction. The films became more oxygen deficient with increased thickness, as found from X-ray photoelectron spectroscopy (XPS) and valence band XPS (VBXPS). We attribute the strong dependence of device performance to the zinc to oxygen stoichiometry within the ZnO layers, leading to improvement in the band structure of ZnO with increased thickness.     

Structural and electrical characterization of platinum (~15 at%) doped nickel silicides on Si(100) and SiGe/Si(100) substrates

Ni(Pt~15 at%)Si/Si(100) and Ni(Pt~15 at%)SiGe/SiGe/Si(100) films corresponding to rapid thermal annealing (RTA1) temperatures of 220, 230 and 240 °C with constant RTA2 (at 420 °C) have been investigated for sub 20 nm devices. X-ray reflectometry (XRR), X-ray diffraction (XRD), four point probe, and atomic force microscopy (AFM) techniques were employed for the characterization of NiSi and NiSiGe films. XRR results indicated that NiSi and NiSiGe film thicknesses increased with RTA1 temperatures. NiSi films densities increased with layer thickness but NiSiGe films displayed an opposite trend. The diffractograms revealed that NiSi and NiSiGe layers contain identical phases and possessed fiber texture at 220 °C. Whereas, the peaks shift were observed for NiSi (211) and NiSi (021) at higher RTA1 temperatures which appear due to Pt diffusion (hexagonal structures of larger grain size were noted). NiSiGe crystallites self-alignment was observed because of strained SiGe/Si(100) substrate. At 240 °C, NiSiGe layer showed the smallest crystallites. This is believed to be due to Pt distributed along the silicide grain boundaries which obstructs silicide grain growth. NiSi and NiSiGe sheet resistance decreased significantly with increase in RTA1 temperatures and found to correlate with multiple grain orientation. AFM revealed a smooth-stable surface morphology for all films.     

The influence of thickness and ammonia flow rate on the properties of AlN layers

Undoped AlN layers have been grown on c-plane sapphire substrates by metal-organic chemical vapor deposition in order to study the effects of ammonia (NH₃) flow rate and layer thickness on the structural quality and surface morphology of AlN layers by high-resolution X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Lower NH₃ flow rate improves crystallinity of the symmetric (0 0 0 2) plane in AlN layers. Ammonia flow rate is also correlated with surface quality; pit-free and smooth AlN surfaces have been obtained at a flow rate of 70 standard cm³ per minute. Thicker AlN films improve the crystallinity of the asymmetric (1 0 1̄ 2) plane.     

Synthesis and some surface studies of laminated ZnO/TiO2 transparent bilayer by two-step growth

A laminated bilayer was prepared by first depositing titanium dioxide (TiO₂) nanocrystals on indium tin oxide (ITO) coated glass by a two-electrode cell. Zinc oxide (ZnO) thin film was thereafter deposited on the TiO₂ by two different techniques: electrochemical deposition and vacuum evaporation. The films were characterized by some surface probing techniques. Morphological study revealed that particle size of the TiO₂ underlayer increases between 110 and 138 nm with increase in deposition voltage. It also showed that ZnO thin film (overlayer) completely covered the underlying TiO₂ without chemical interaction between constituents of both layers. Cross-sectional FESEM study gave values of layered film thickness below 55 µm. Exhibition of strong diffraction peak at plane (121) indicated preference of TiO₂ film's growth orientation. It also suggested a feature of phase-pure brookite. Optical studies showed that each film exhibited strong absorption edge at λ=~330 nm and transmitted fairly across visible light region. Energy band gap lied between 3.24 and 3.43 eV. This study demonstrated successive layer deposition of transparent metal oxide structures from inorganic reagents. It also reaffirmed TiO₂ as a recipe for barrier layer that can hinder transition of holes from absorber to transparent front contact of nanostructured photonic devices.     

Active layer thickness effect on the recombination process of PCDTBT:PC71BM organic solar cells

We investigated the effect of active layer thickness on recombination kinetics of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) based solar cells. Analysis of the fitted Lambert W-function of illuminated current density–voltage (J–V) characteristics revealed increased recombination processes with increased active layer thicknesses. The ideality factor extracted from PCDTBT:PCBM solar cells continuously increased from 1.89 to 3.88 when photoactive layer thickness was increased from 70 to 150 nm. We found that such increase in ideality factor is closely related to the defect density which is increased with increased photoactive layer thickness beyond 110 nm. Therefore, the different density of defect states in PCDTBT:PCBM solar cells causes the different recombination paths where solar cells with a thicker active layer (?110 nm) are considered to undergo coupled trap-assisted recombination processes while single-defect trap-assisted recombination is dominant for thinner (70–90 nm) PCDTBT:PCBM solar cells. As a result, we found that the optimal efficiencies of PCDTBT:PC₇₁BM solar cells were limited to the active layers between 70 and 90 nm. Particularly, when PCDTBT:PC₇₁BM solar cells were optimized with an active layer thickness of 70 nm, energy conversion efficiency reached 6.5% while an increase in thickness led to the reduction of efficiency to 4.7% at 133 nm but then an increase to 5.02% at 150 nm.     

Thermal oxidation of tin layers and study of the effect of their annealings on their structural and electrical properties

The main objective of this article is the control of tin dioxide preparation process on glass substrate. Layers of pure tin with thicknesses of 500 and 1000 Å are first deposited. Their enrichment with oxygen is ensured by thermal annealing for 1 and 2 h in a continuous tube furnace with temperatures varying between 300 and 500 °C.The tin film formed by vacuum evaporation has tetragonal crystalline structure, and is composed of grains of various sizes separated by grain boundaries. After annealing in oxygen, the formed phases consist of a mixture of SnO and SnO₂ crystalline mixtures and sometimes amorphous tin oxide. The more the time or the temperature of annealing, the more the quantity of SnO₂ and SnO. For an annealing at 500 °C for 10 h the size of grains increases more than annealing for 2 h. This is confirmed by the study of their micrographs.The electrical resistivity of these layers, measured by the 4 point method, is correlated to the size of the oxide particles: the smaller the particle size, the lower the electrical resistivity.     

Wetting layer formation in superlattices with Ge quantum dots on Si(1 0 0)

The influence of parameters of germanium deposition on wetting layer thickness was studied during the growth on the Si(1 0 0) surface. A non-monotone dependence of the thickness on growth temperature was discovered and accounted for by changing the mechanism of the layer-by-layer growth. The conclusion was supported by changing the mode of oscillations of the reflection high-energy electron diffraction (RHEED) specular beam. In addition, wetting layer thickness is strongly affected by the replication number and thickness of the silicon spacer due to accumulation of elastic strains throughout the structure.     

Effect of AlN buffer layer thickness on the properties of GaN films grown by pulsed laser deposition

The GaN films are grown by pulsed laser deposition (PLD) on sapphire, AlN(30 nm)/Al₂O₃ and AlN(150 nm)/Al₂O₃, respectively. The effect of AlN buffer layer thickness on the properties of GaN films grown by PLD is investigated systematically. The characterizations reveal that as AlN buffer layer thickness increases, the surface root-mean-square (RMS) roughness of GaN film decreases from 11.5 nm to 2.3 nm, while the FWHM value of GaN film rises up from 20.28 arcmin to 84.6 arcmin and then drops to 31.8 arcmin. These results are different from the GaN films deposited by metal organic chemical vapor deposition (MOCVD) with AlN buffer layers, which shows the improvement of crystalline qualities and surface morphologies with the thickening of AlN buffer layer. The mechanism of the effect of AlN buffer layer on the growth of GaN films by PLD is hence proposed.     

Optical and electronic interaction at metal–organic and organic–organic interfaces of ultra-thin layers of PTCDA and SnPc on noble metal surfaces

We investigate the interaction mechanisms at metal–organic and organic–organic interfaces in highly-ordered ultra-thin layers of the dye molecules 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and tin(II)-phthalocyanine (SnPc) on single crystalline noble metals. The ultra-thin films are characterized by means of in situ differential reflectance spectroscopy (DRS), followed by an extraction of the optical functions by application of a numerical algorithm. For the first time, DRS data of PTCDA and SnPc films on Ag(1 1 1) are presented. We found that for the contact layers of PTCDA and SnPc the well-known covalent interaction between adsorbate and substrate is manifested in broad and structureless absorption spectra. Surprisingly, the optical spectra of the respective first monolayers on Ag(1 1 1) are almost identical despite of the rather different electronic structure of the free molecules. The special character of the optical spectra is emphasized by a comparison with PTCDA and SnPc monolayers on Au(1 1 1) where the electronic interaction at the metal–organic interface is much weaker. Quite differently from the contact layer, the second layer of the same molecule on Ag(1 1 1) clearly shows monomeric behavior which can only be observed if the electronic and optical coupling with the surrounding molecules and the substrate is faint. However, a very weak out-of-plane electronic interaction remains as concluded from the comparison with the spectra obtained on inert mica substrates. We also present structural data acquired with low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) of SnPc on Au(1 1 1).     

Enhanced performance of PbPc photosensitive organic field effect transistors by inserting different-thickness pentacene inducing layers

Lead phthalocyanine (PbPc) based photosensitive organic field effect transistors (PhOFETs) with different-thickness pentacene inducing layers (INLs) inserted between SiO₂ and PbPc layer were fabricated and characterized. The photoelectric measurements demonstrate that the device with 2-nm-thick pentacene INL exhibits the largest photoresponsivity of 505.75 mA/W and maximum photo/dark current ratio of 405.35 in all devices. For this, we give an overall explanation that different-thickness INLs display different continuity and crystallinity and thus produce strong or weak template inducing. Especially, when the INL thickness (δ) is 2 nm a quasi-continuous and highly crystalline approximate-monolayer INL forms on SiO₂ surface, which may play a strong role of template inducing, thus causing its upper PbPc film to demonstrate the strongest triclinic (3 0 0) line and the strongest NIR absorption in series PbPc films. When δ = 1 nm, pentacene does not form a continuous film. And when δ = 5 or 10 nm, a continuous multilayer INL with a declined crystallinity due to possible lattice mismatch forms on SiO₂ surface and gives a weakened template inducing. Thereby, it can be recognized that inserting a pentacene INL can markedly enhance the performance of single layer PbPc PhOFET and the optimum INL thickness is proved ∼2 nm in present conditions.     

Depression of the superconducting critical temperature and finite-size scaling relation in YBa2Cu3O7−δ/La2/3Ca1/3MnO3 bilayers

High-quality YBa₂Cu₃O_7?δ/La_2/3Ca_1/3MnO₃ (YBCO/LCMO) bilayers were fabricated on (0 0 1)-oriented SrTiO₃ (STO) substrates by dc-sputtering technique. Bottom layer was always LCMO since it grows better on STO than on YBCO. The thickness of the ferromagnetic layer varied between 5 and 35 monolayers (～2–13 nm) and that of the top YBCO was fixed at 10 monolayers (～12 nm). The transport properties of the YBCO layers as well as the magnetic properties of the LCMO counterparts were studied as a function of the LCMO layer thickness. A sizeable depression of the Curie temperature (T_C) of the LCMO layers from the bulk to lower temperatures is observed when decreasing their layer thickness d_LCMO, which might be ascribed to intrinsic dimensionality effects or strain-induced phenomena. On the contrary, the superconducting critical temperature of the YBCO layer T_S displays a sudden strong decrease at a critical LCMO thickness of ～12 nm. Since no dramatic change of the structural and morphological quality of the YBCO top layers with increasing d_LCMO is observed, the suppression of T_S of the YBCO layers should take place via proximity effect due to the increasing magnetization strength in the LCMO layer. However, extrinsic factors like interface strain, interdiffusion of cations between YBCO and LCMO or injection of spin-polarized carriers from the magnetic into the superconducting layer could also play an important role or even to be directly responsible for the observed depressed T_S.     

Investigation of optical and electronic properties in Al–Sn co-doped ZnO thin films

Al-Sn co-doped ZnO thin films were deposited onto quartz substrates by sol-gel processing. The surface morphology and electrical and optical properties were investigated at different annealing temperatures. The surface morphology showed a closely packed arrangement of crystallites in all the doped films. As prepared co-doped films show a preferred orientation along an (0 0 2) plane. This preferred orientation was enhanced by increasing the annealing temperature to between 400 °C and 500 °C, but there was a shift to the (1 0 1) plane when the annealing temperature rose above 500 °C. These samples show, on average, 91.2% optical transmittance in the visible range. In this study, the optical band gap of all the doped films was broadened compared with pure ZnO, regardless of the different annealing temperature. The carrier concentration and carrier mobility of the thin films were also investigated.     

Thickness dependent barrier performance of permeation barriers made from atomic layer deposited alumina for organic devices

Organic devices like organic light emitting diodes (OLEDs) or organic solar cells degrade fast when exposed to ambient air. Hence, thin-films acting as permeation barriers are needed for their protection. Atomic layer deposition (ALD) is known to be one of the best technologies to reach barriers with a low defect density at gentle process conditions. As well, ALD is reported to be one of the thinnest barrier layers, with a critical thickness – defining a continuous barrier film – as low as 5–10 nm for ALD processed Al₂O₃. In this work, we investigate the barrier performance of Al₂O₃ films processed by ALD at 80 °C with trimethylaluminum and ozone as precursors. The coverage of defects in such films is investigated on a 5 nm thick Al₂O₃ film, i.e. below the critical thickness, on calcium using atomic force microscopy (AFM). We find for this sub-critical thickness regime that all spots giving raise to water ingress on the 20 × 20 μm² scan range are positioned on nearly flat surface sites without the presence of particles or large substrate features. Hence below the critical thickness, ALD leaves open or at least weakly covered spots even on feature-free surface sites. The thickness dependent performance of these barrier films is investigated for thicknesses ranging from 15 to 100 nm, i.e. above the assumed critical film thickness of this system. To measure the barrier performance, electrical calcium corrosion tests are used in order to measure the water vapor transmission rate (WVTR), electrodeposition is used in order to decorate and count defects, and dark spot growth on OLEDs is used in order to confirm the results for real devices. For 15–25 nm barrier thickness, we observe an exponential decrease in defect density with barrier thickness which explains the likewise observed exponential decrease in WVTR and OLED degradation rate. Above 25 nm, a further increase in barrier thickness leads to a further exponential decrease in defect density, but an only sub-exponential decrease in WVTR and OLED degradation rate. In conclusion, the performance of the thin Al₂O₃ permeation barrier is dominated by its defect density. This defect density is reduced exponentially with increasing barrier thickness for alumina thicknesses of up to at least 25 nm.     

Electrodeposition of CuIn1−xGaxSe2 solar cells with a periodically-textured surface for efficient light collection

The photoresponse of CuIn_1?xGa_xSe₂ (CIGS) solar cells is improved using a periodically-textured structure as an antireflection layer. The CIGS absorber layers were prepared by one-step electrodeposition from an aqueous solution containing 12 mM CuSO₄, 25 mM In₂(SO₄)₃, 28 mM Ga₂(SO₄)₃, and 25 mM SeO₂. The electrodeposited CIGS films exhibit the (112)-preferred orientation of the chalcopyrite structures and feature improved film stoichiometry after the selenization process. In addition, the lower bandgap value of 0.97 eV is caused by the discrepancy of the reduction potentials for each constituent, resulting in insufficient Ga content in the deposited films. Using self-assembled silica nanoparticles as the etching mask, periodically-textured structures can be easily formed on an indium tin oxide (ITO)-coated soda lime glass to achieve a low average reflection (<10.5%) in a wide wavelength and incident angle range. With the periodic textured structures suppressing light reflections from the front surface, the photogenerated current in the semi-transparent CIGS solar cells made with transparent conducting electrodes is 1.82 times higher than they otherwise would be.