Insertion Of Interlayer Research Articles

Influence of Process Parameters on the Adhesive Strength of CrN Coatings

In order to enhance the corrosion and wear resistance of the STVAX alloy, the CrN film has been deposited by UBM(unbalanced magnetron) sputtering. In this study, the influence of process parameters, such as N2 gas concentration and Cr interlayer insertion, on the adhesive strength was investigated. By increasing the concentration of N2 gas from 30% to 33%, the microstructure of the coating layer was changed from hexagonal Cr2N to cubic CrN. By increasing N2 gas concentration to 38%, a CrN coating layer with a preferred orientation of <111> plane was formed. Nanoindentation testing showed that the mechanical properties of coatings varied according to the coating type and crystallographic orientation. Superior adhesive strength was obtained at the of N2 gas concentration of 38%, and higher hardness at 33% concentration. The adhesive strength of the coating film was higher with a Cr interlayer than that without. These findings showed that the Cr interlayer between the CrN coating and the STAVAX substrate significantly improved the adhesive strength of coating to the substrate.

Examination of Be//Cu joints in hipped Be tiles by using Ion Beam Analysis methods

Be has a tendency to form particularly brittle intermetallics with Cu and a lot of other elements. Insertion of interlayers may be a solution to increase bond quality. Ion Beam Analysis (IBA) methods have been considered together with scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) as complementary techniques for the characterization of the Be//Cu junction. The following work aims at demonstrating how such techniques (used in micro-beam mode) coupled with SEM/EBSD data, can bring valuable information in this area. Interlayer-free mock-ups reveal intermetallics which are mainly BeCu (apparently mixed with lower quantities of BeCu 2 compound). While Cr or Ti interlayers seem to behave as good Be diffusion barriers preventing the formation of BeCu, they strongly interact with Cu to form CuTi 2 or Cr 2Ti intermetallics. In the case of Cr, Be seems to be incorporated into the Cr layer.

Thickness effects on the electrical characteristics of Ba0.7Sr0.3TiO3 capacitors with nano-Cr interlayer

A multilayer Ba0.7Sr0.3TiO3∕Cr∕Ba0.7Sr0.3TiO3 (BST/Cr/BST) structure was sputtered sequentially onto Pt∕Ti∕SiO2∕Si substrate. With the insertion of a 2nm Cr interlayer, the temperature coefficient of capacitance of the BST/Cr/BST dielectric is about 69% lower than that of BST monolayer dielectric. The dielectric constant and dissipation factor as the function of Cr thickness are studied. X-ray diffraction patterns, the analysis results of energy dispersive spectroscopy, and the survey scan profiles of Auger electron spectroscopy reveal the formation of a TiO2 secondary phase after the multilayer is annealed at 800°C in O2 atmosphere. The insertion of nano-Cr interlayer improves the electrical properties of BST capacitors.

The influence of YSZ interlayer on microstructures and dielectric properties of BST thin films prepared by RF magnetron sputtering

(Ba1 − x Sr x )TiO3 (BST) thin films were deposited on Pt/Ti/SiO2/Si and YSZ/Pt/Ti/SiO2/Si substrates by radio frequency (RF) magnetron sputtering. The influence of YSZ interlayer on microstructures and dielectric properties of BST thin films were investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy and dielectric frequency spectra. It was found that the preferred orientation of BST thin films could be tailored by insertion of YSZ interlayer and adjusting the thickness of YSZ interlayer. The BST thin films deposited on YSZ interlayer exhibited a more compact and uniform grain structure than that deposited directly on Pt electrode. Dielectric measurement revealed that the BST thin films deposited on 10 nm YSZ interlayer have the largest dielectric constant and a low dielectric loss tangent. The enhanced dielectric behavior is mainly attributed to the YSZ interlayer which serves as an excellent seeding layer to enhance the crystallization of subsequent BST films layer, and a smaller thermal stress field built up at the interface between YSZ interlayer and BST film layer.

Thermal stability and electric properties of Ba 0.7Sr 0.3TiO 3 parallel plate capacitor with nano-Cr interlayer

A novel sandwich structure of Ba 0.7Sr 0.3TiO 3/Cr/Ba 0.7Sr 0.3TiO 3 (BST/Cr/BST) was sputtered onto Pt/Ti/SiO 2/Si substrate. With the insertion of a Cr layer, the leakage currents are decreased and the thermal stability of the specimens is enhanced. Temperature coefficient of capacitance (TCC) of specimens with BST(200 nm)/Cr(2 nm)/BST(200 nm) multifilms can achieve about 83% lower than those with BST (400 nm) monolayer. However, the dielectric constant of the BST(200 nm)/Cr(2 nm)/BST(200 nm) multifilms decreases to about 37% of that BST monolayer. The leakage current densities under an electric field of 125 kV/cm at 90 °C are 4 × 10 − 4 A/cm 2 and 9 × 10 − 1 A/cm 2 for BST (200 nm)/Cr (2 nm)/BST (200 nm) and monolayer BST (400 nm), respectively. X-ray diffraction results indicate the formation of a CrO 3 secondary phase after annealing at 700 °C or above in O 2 atmosphere. The root causes for the improvement of leakage currents and thermal stability with the insertion of nano-Cr interlayer are explored. The results show the insertion of Cr-nanolayer improves the electric properties for application in capacitors.

Efficacy of single and double SiNx interlayers on defect reduction in GaN overlayers grown by organometallic vapor-phase epitaxy

We report on the growth of and evolution of defects in GaN epilayers having single- and double-layer SiNx nanoporous insertion layers. The SiNx was formed in situ in the growth chamber of an organometallic vapor-phase epitaxy system by simultaneous flow of diluted silane and ammonia. The GaN epilayers and SiNx interlayers were grown on 6H-SiC substrates using three different nucleation layers, namely, low-temperature GaN, high-temperature GaN, and high-temperature AlN nucleation layers. X-ray-diffraction rocking curves and cross-sectional and plan-view transmission electron microscope analyses indicated that a nanoporous SiNx layer can reduce the dislocations density in the GaN overgrown layer to ∼3×108cm−2 range; below this level the defect blocking effect of SiNx would saturate. Therefore the insertion of a second SiNx layer becomes much less effective in reducing dislocations, although it continues to reduce the point defects, as suggested by time-resolved photoluminescence measurements. The insertion of SiNx interlayers was found to improve significantly the mechanical strength of the GaN epilayers resulting in a much lower crack line density.

Wavelength-tunable (1.55-μm region) InAs quantum dots in InGaAsP∕InP (100) grown by metal-organic vapor-phase epitaxy

Growth of wavelength-tunable InAs quantum dots (QDs) embedded in a lattice-matched InGaAsP matrix on InP (100) substrates by metal-organic vapor-phase epitaxy is demonstrated. As∕P exchange plays an important role in determining QD size and emission wavelength. The As∕P exchange reaction is suppressed by decreasing the QD growth temperature and the V∕III flow ratio, reducing the QD size and emission wavelength. The As∕P exchange reaction and QD emission wavelength are then reproducibly controlled by the thickness of an ultrathin [zero to two monolayers (MLs)] GaAs interlayer underneath the QDs. An extended interruption after GaAs interlayer growth is essential to obtain well-defined InAs QDs. Submonolayer GaAs coverages result in a shape transition from QD to quantum dash at low V∕III flow ratio with a slightly shorter emission wavelength. Only the combination of reduced growth temperature and V∕III flow ratio with the insertion of GaAs interlayers above ML thicknesses allows wavelength tuning of QDs at room temperature in the technologically important 1.55-μm wavelength region for fiber-optical telecommunication systems. A GaAs interlayer thickness just above one ML produces the highest photoluminescence (PL) efficiency. Temperature-dependent PL measurements reveal zero-dimensional carrier confinement and defect-free InAs QDs.

Classic and novel methods of dislocation reduction in heteroepitaxial nitride layers

Due to the lack of commercially available large size native substrates, reduction of threading dislocations remains one of the main challenges of group III-nitride based technology. This paper reviews major methods aimed at producing high-quality heteroepitaxial nitride films with reduced densities of dislocations. Methods such as, incorporation of buffer layers, substrate nitridation, interlayer insertion, silane-ammonia treatment, lateral overgrowth and growth of thick nitride layers are reviewed. Advantages and drawbacks of these methods are discussed and compared. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrochemical Performance of (La, Sr)(Co, Fe)O3-(Ce, Sm)O3 Composite Cathodes for Low Temperature Solid Oxide Fuel Cells

La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) - Ce 0.7 Sm 0.3 O 3 (SDC) composite cathodes were investigated for demonstration of SOFCs working at reduced-temperatures of ca. 800°C. Interlayer of SDC or Ce 0.7 Gd 0.3 O 3 (GDC) was inserted between the composite cathode and Sc 0.10 Ce 0.01 Zr 0.89 O 2-α (ScSZ) electrolyte to achieve stable operation. Ni-ScSZ was used as anode. The performance of the composite cathode and the effect of the interlayer were evaluated based on current-voltage and impedance measurements. It was found that insertion of SDC and GDC interlayers was effective in improving power generation characteristics, and especially the SDC interlayer resulted in better performance. In the case of the GDC interlayer, the GDC/LSCF-SDC interface could be responsible for inferior performance to the cell with the SDC interlayer, possibly due to poor adhesion in the GDC/LSCF-SDC interface.

Effect of Various Interlayers on Epiwafer Bowing in AlGaN/GaN High-Electron-Mobility Transistor Structures

We have systematically studied the effect of various interlayers on epiwafer bowing in AlGaN/GaN high-electron-mobility transistor (HEMT) structures on sapphire substrates by using an interlayer insertion method. The interlayers including aluminum increase the epiwafer bowing independently of growth temperature. However, both InGaN-based interlayers and low-temperature-deposited GaN interlayers suppress the epiwafer bowing. When multilayers are used as interlayers, the effect on epiwafer bowing is enhanced compared with that in the case of using single interlayers. The minimum bowing values were lower than 10 µm, which were accomplished by using multilayers. The crystalline qualities of AlGaN/GaN HEMTs with interlayers were satisfactory and the performance of these HEMTs was better than that of conventional AlGaN/GaN HEMTs without interlayers. Epiwafers with reduced bowing exhibited a reduced strain in GaN layers compared with that of GaN layers with larger bowing, which indicates that strain control in the epitaxial layers is important for reducing the epiwafer bowing.

Suppression of cracks and V-shaped defects, and improvement of reflectivity of GaN/AlGaN distributed Bragg reflectors by insertion of multiple interlayers

GaN/AlxGa1–xN distributed Bragg reflectors (DBRs) for the near-ultraviolet region have been grown on sapphire substrates by metal-organic chemical vapor deposition. In order to suppress the generation of cracks and V-shaped defects on the surface of the DBRs, AlGaN/AlN superlattices and step-graded multiple AlGaN layers were inserted prior to or during the growth of the DBRs. By optimizing those layers, the reflectivity of a 40-pair GaN/Al0.35Ga0.65N DBR was increased up to 96% without any cracks and V-shaped defects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement in optical properties and surface morphologies of GaN films using low-temperature GaN interlayers

High temperature (HT)-GaN films inserted with low temperature (LT)-GaN interlayers were grown on (0001) sapphire substrates by alternate supply of metal alkyls (trimethylgallium (TMG) and trimethylaluminum (TMA)) and ammonia (NH3) in an inductively heated quartz reactor operated at atmospheric pressure. Based upon the results of scanning electron microscopic (SEM) observations, it was found that the mosaic structure in a thin HT-GaN film could be eliminated by using a LT-GaN interlayer. The HT-GaN film inserted with a LT-GaN interlayer having optimized thickness shows an enhanced room temperature (RT) photoluminescence (PL) intensity of the near band-edge emission by more than one order of magnitude compared with that of a HT-GaN film without any LT-GaN interlayer. The results of transmission electron microscopy (TEM) and double-crystal X-ray diffractometry (DCXRD) indicate that improvement in PL results is primarily due to the elimination of mosaic structure in the GaN film. Moreover, the insertion of LT-GaN interlayers in HT-GaN films was found to reduce the compressive strain in the HT-GaN films.

Reduction of the bowing in MOVPE AlGaN/GaN HEMT structures by using an interlayer insertion method

AlGaN/GaN high electron mobility transistor (HEMT) structures with reduced bowing were demonstrated using an interlayer insertion method. The InGaN interlayers had an effect of suppressing the epiwafer bowing up to 20 μm. By using InGaN/GaN multilayers as an interlayer, the epiwafer bowing was drastically decreased by up to 9.4 μm. The mobility of AlGaN/GaN HEMT structures with 10-period InGaN/GaN interlayers was 1083 cm2/Vs with the sheet carrier concentration, Ns, of 1.1 × 1013 at room temperature and 4997 cm2/Vs with Ns of 1.1 × 1013 cm2/Vs at 77 K. The FWHM of the X-ray rocking curve for (0004) reflection was 259 arcsec. The fabricated 2-μm-length and 15-μm-width high electron mobility transistor showed a high drain current (IDSmax = 617 mA/mm) and a high transconductance (gmmax = 174 mS/mm). The interlayer insertion method is effective for suppressing the epiwafer bowing. It may be a good solution for reducing the bowing of large-diameter epiwafers. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical and electrical characterization of n-GaAs surfaces passivated by N2–H2 plasma

The passivation of GaAs (100) surface has been performed by using remote N2–H2 (3% in H2) RF plasma nitridation. The samples, consisting of n-doped GaAs wafers, show photoluminescence enhancement when the nitridation time and exposure to the plasma are in a narrow temporal window, so that a very thin (about 10Å) GaN layer is deposited on the GaAs surface. Pure N2 nitridation does not provide an efficient passivation, because it results in GaN layers with As and AsNx segregation at the GaN/GaAs interface. Increase of Au–GaAs Schottky barrier with the insertion of GaN interlayer and improvement of current–voltage characteristic have been observed.

Effect of LaNiO3 interlayer on dielectric properties of (Ba0.5Sr0.5)TiO3 thin films deposited on differently oriented Pt electrodes

The (Ba0.5Sr0.5)TiO3 (BST) thin films were deposited on the LaNiO3/Pt(111)/Ti/SiO2/Si [LNO/Pt(111)] and LaNiO3/Pt(200)/SiO2/Si [LNO/Pt(200)] substrates by metalorganic deposition process. The BST films directly grown on the Pt(200) and Pt(111) substrates exhibited random orientation. In case of the LNO/Pt(111) substrates, the BST film was also randomly oriented, but the insertion of LNO interlayer on the Pt(200) substrates caused the growth of (100) textured BST films. The dielectric constant of the 350-nm-thick BST films on LNO/Pt(200) was around 400, which was higher than that of the films on LNO/Pt(111) (around 340). This result was attributed to the fact that the polar axis of the (100) oriented film was more tilted away from the normal to the film surface than that of randomly oriented film. Also, the tunabilities of BST films deposited on the LNO/Pt(200) and LNO/Pt(111) substrates were nearly 60% and 51% at the applied electric field of 400 kV/cm, respectively. Improved tunability has been attributed to the (100) texture of the film leading to an enhancement of the in-plane oriented polar axis.

Effect of LiF/metal electrodes on the performance of plastic solar cells

The insertion of thin interlayers of LiF under the negative metal electrode (Al and Au) of bulk heterojunction solar cells significantly enhances the fill factor and stabilizes high open circuit voltages. Compared to devices without the LiF interfacial layer, the white light efficiencies increase by over 20% up to ηeff∼3.3%. Substitution of the LiF by another insulating interlayer SiOx results in lower overall efficiencies. In the case of a LiF/Au electrode, substantial efficiency enhancement is observed compared to a pristine Au electrode and white light efficiencies up to ηeff∼2.3% are reported.

Stacking effects on dielectric properties of sol-gel derived Pb(Zr0.53Ti0.47)O3/PbTiO3 thin films

A thin film multilayer structure consisting of Pb(Zr0.53Ti0.47)O3(PZT) and PbTiO3(PT) were fabricated by a sol-gel process. The effects of the number of PZT/PT layers upon microstructure and dielectric characteristics were investigated. For a pure PZT thin layer annealed at 600 °C, the microstructure observed was a rosette type, whereas the insertion of PT interlayers yielded thin films with homogeneous grain distribution regardless of the number of PZT/PT layers. With increasing number of PZT/PT layers, the leakage current density and coercive field effectively decreased, while the dielectric constant increased. Loss tangent and fixed charge were found to be independent of the number of PZT/PT layers. These results are possibly explained by the enhanced crystallization resulting from the introduction of large number of nucleation sites in the multilayer film, and by the stacking of stable and dense PZT/PT layers. The thin film composed of three PZT/PT layers with a thickness of ∼450 nm exhibited dielectric constant of 1000, remnant polarization of 20 μC/cm2, coercive field of 40 kV/cm, and tan δ of 0.03. The relaxorlike ferroelectric behavior was observed with an increasing number of PZT/PT layers.

The effects of metal interlayer insertion on the friction, wear and adhesion of TiC coatings

TiC coatings on 440C stainless steel (SS) substrates that contain different kinds of interlayers (Ti, Cr and Mo) of different thickness were grown by magnetron sputtering. The friction, wear and adhesion behaviors were investigated for the TiC coatings using pin-on-disk configuration. The wear tracks of the TiC coatings and alumina pin were characterized by optical microscope or scanning electron microscope (SEM). Inserting Cr or Ti interlayer between TiC coating and substrate could improve greatly the wear, hardness and adhesion. In wear tests the TiC coating with Cr interlayer usually showed better characteristics than that with Ti interlayer. In addition, the TiC coating with 50 nm Ti interlayer performed better than that with 500 nm Ti interlayer in the adhesion and wear tests. It is noticed that the TiC coating could not be improved on the friction and wear behavior when metallic Mo was inserted between the TiC and substrate resulting from the poor adhesion of the coating.

TiC coatings prepared by pulsed laser deposition and magnetron sputtering

Tribological properties of TiC coatings grown by pulsed laser deposition (PLD) and magnetron sputtering were investigated. The PLD TiC coatings grown at room temperature were found to be much harder than the TiC coatings grown by magnetron sputtering under the given experimental conditions. The hardness of PLD coatings deposited at room temperature was as high as 44 GPa, in contrast to ~20 GPa of the magnetron sputtered ones. The coefficient of friction of the PLD films measured with a pin-on-disk type tribometer had a typical value of about 0.2 when using a 440C stainless steel pin. Scratch tests indicated that magnetron-sputtered TiC coatings adhered well to the stainless steel substrates. The relatively poor adhesion of the PLD coatings obtained from this scratch test was probably partly due to their high brittleness and the relatively weak adhesive bond. However, the adhesion of PLD coatings could be improved by raising the substrate temperature slightly to 300 °C or by in situ laser annealing. The former was also able to maintain their hardness at a relatively high level. The adhesion of magnetron-sputtered TiC coatings could be modified by inserting a metallic interlayer between the coating and stainless steel substrate. Mo interlayer had a detrimental effect on the adhesion caused probably by the poor stress bearing capability of the porous Mo film deposited at low temperature. However, the insertion of both Ti and Cr interlayers enhanced the adhesion of TiC by as much as 25%.

Sandwich magnetoresistive films based on 3d-transition metal alloys

The effects of nonmagnetic Cu, Cr and TiN interlayers on the hysteresis and magnetoresistive properties of Fe 19Ni 81 and Fe 15Co 20Ni 65 films annealed at 200–600°C have been investigated. Interlayer insertion into the film leads to a considerable decrease in the coercive force with practically no loss of magnetoresistance.