Thin Crystalline Layers Research Articles

Modeling of Electronic Properties of Strained Silicon on a Germanium Substrate

A model for calculation of deformations in a structure consisting of an arbitrary number of thin crystalline layers of Si and Ge on a nonrigid film is constructed. Relative deformations in pseudomorphic layers of Si and Ge are determined as a function of the ratio of thicknesses of contacting semiconductors. The constructed model allows to determine the electron mobility in the strained n-Si on deforming Ge(001) or SiGe(001) Substrate . It is shown that the conduction current is concentrated in the strained channels of silicon transistors made on high-resistance germanium substrates.

Cobalt/copper-decorated carbon nanofibers as novel non-precious electrocatalyst for methanol electrooxidation

In this study, Co/Cu-decorated carbon nanofibers are introduced as novel electrocatalyst for methanol oxidation. The introduced nanofibers have been prepared based on graphitization of poly(vinyl alcohol) which has high carbon content compared to many polymer precursors for carbon nanofiber synthesis. Typically, calcination in argon atmosphere of electrospun nanofibers composed of cobalt acetate tetrahydrate, copper acetate monohydrate, and poly(vinyl alcohol) leads to form carbon nanofibers decorated by CoCu nanoparticles. The graphitization of the poly(vinyl alcohol) has been enhanced due to presence of cobalt which acts as effective catalyst. The physicochemical characterization affirmed that the metallic nanoparticles are sheathed by thin crystalline graphite layer. Investigation of the electrocatalytic activity of the introduced nanofibers toward methanol oxidation indicates good performance, as the corresponding onset potential was small compared to many reported materials; 310 mV (vs. Ag/AgCl electrode) and a current density of 12 mA/cm2 was obtained. Moreover, due to the graphite shield, good stability was observed. Overall, the introduced study opens new avenue for cheap and stable transition metals-based nanostructures as non-precious catalysts for fuel cell applications.

Properties of H+ Implanted 4H-SiC as Related to Exfoliation of Thin Crystalline Films

Ion-beam assisted exfoliation of thin single crystalline layers of 4H-SiC may facilitate heterogeneous integration of SiC devices with silicon CMOS circuits, and lead to lower cost SiC power devices. Practical device structures require H+ ion implantation and wafer bonding in order to achieve transfer of crystalline layers to new handle substrates. However, for initial optimization of the exfoliation step it is sufficient to monitor surface blistering as a function of ion implantation parameters and the subsequent thermal annealing conditions. In this study we show that for 1 μm thick 4H-SiC exfoliated films, there is an optimum implantation dose of ∼ 6 × 1016 cm−2 as well as an optimum implantation temperature of ∼ 300°C. Some exploratory data for 500 keV and 1 MeV implant energies are also shown.

Ultrafast Optical Properties of Dense Electron Gas in Silicon Nanostructures.

We investigate the ultrafast dynamics of carriers in a silicon nanostructure by performing spectrally resolved femtosecond spectroscopy measurements with a supercontinuum probe. The nanostructure consists of a 158-nm-thick crystalline Si layer on top of which a SiO2 passivation layer leads to a very high quality of the Si surface. In addition, a dielectric function approach, including contributions from a Drude part and interband transitions, combined with the Transition Matrix Approximation is used to model the photogenerated carrier dynamics. The spectrotemporal reflectivity reveals two mechanisms. First, an electron–hole plasma is created by the pump pulse and lasts for a few picoseconds. Importantly, its spectral signature is either a positive or a negative change of reflectivity, depending on the probe wavelength. This is complementary to the already reported results obtained with degenerate frequency measurements. The second mechanism is a thermal diffusion of carriers which occurs during several hundreds of picoseconds. The overall dynamics at short and long delays in the whole visible spectrum is well explained with our model which shows that the main contribution to the reflectivity dynamics is due to the Drude dielectric function. The observation of this predominance of free carriers requires both a long lived high density of carriers as well as a little influence of surface scattering as provided by our thin crystalline Si layer with passivated Si/SiO2 interface.

Synthesis of SiGe layered structure in single crystalline Ge substrate by low energy Si ion implantation

Nanometer-thickness SiGe alloy layers were synthesized by direct Si ion implantation in Ge (100) wafers at different fluences followed by high temperature annealing. The cross-sectional transmission electron microscopy and secondary ion mass spectrometry reveal the formation of a thin Si-rich crystalline layer in the near-surface region. The micro-Raman spectroscopy and X-ray reflectivity techniques were used to determine the composition and strain in SiGe alloy layers. The photoluminescence measurements of the annealed samples showed a broad emission, peaking around 500 nm. The peak intensity is, however, dependent on the bombarding fluence.

CdBr2 nanocrystalline layers as nonlinear optical materials

A new type of nanomaterial for optoelectronic is proposed. As such a material cleaved nanolayers of layered CdBr2 single crystals is chosen. A principal possibility to prepare thin hexagonal single crystalline layers of CdBr2 nanocrystals (possessing 6H polytype) with thickness up to several nanometers was shown. The studies of the optical absorption clearly show an occurrence of the blue spectral shift for the absorption edge up to 30 nm. During illumination by the 5 ns nitrogen laser at the wavelength at about 337 nm below the energy band gap it was established a substantial increase of the photoinduced absorption coefficient up to 25 cm−1 at pump power density equal to about 1 GW cm−2. The maximal photoinduced effect was observed for the thinnest film possessing thickness about 4 nm. The linear electrooptical effect was maximal at ambient temperature.

Low-temperature crystallization and hardness enhancement of alumina films using the resputtering technique

In this study, alumina thin films were deposited on Si (100) substrates by reactive magnetron sputtering in an Ar–O2 atmosphere. The resputtering technique without any separated ion source was applied, as well as various substrate biases and deposition temperatures. Suitable oxygen/argon gas flow rate ratio was obtained to deposit stoichiometric Al2O3 films. As investigated by grazing incidence X-ray diffraction (GIXRD) and nanohardness measurements, the amorphous characteristics and hardness values of alumina films aren't altered with the various substrate biases up to −120V at the temperatures up to 600°C. The microstructure of alumina films with thin crystalline Al2O3 layer between the amorphous layers was achieved with the resputtering technique at elevated temperatures (≤300°C). This suggests that the reactive magnetron sputtering with the resputtering technique is a simple and effective method of preparing crystalline alumina films at relatively low temperature. The existence of thin crystalline Al2O3 layers between the amorphous layers makes the film harder than the amorphous films. In general, increasing the deposition temperature with the resputtering technique increased the hardness of the films, thus the crystalline-volume fraction of the films. However, the hardness was not a monotonic function of the deposition temperature.

Atomic layer deposition of high-k dielectrics on carbon nanoparticles

Carbon nanoparticles were synthesized from 5-methylresorcinol and formaldehyde via base catalysed polycondensation reaction and distributed over silicon oxide wafers and aluminium oxide thin films. These particles essentially possessed monocrystalline graphite structure. The particles were covered by hafnium oxide thin films in metal chloride based atomic layer deposition process carried out at 300°C. Upon deposition of HfO2, thin crystalline metal oxide layer containing mostly cubic phase was formed. At the same time, deposition of the metal oxide caused reduction of the sizes of graphite particles and essential increase in the disorder in carbon.

Instability and Pattern Formation Induced in Thin Crystalline Layers of a Conducting Polymer P3HT by Unstable Carrier Films of an Insulating Polymer

We investigate the stability, self-organized morphology, and kinetics of dewetting in a thin polymer bilayer (∼100 nm) of P3HT/polystyrene (PS) supported on Si wafers. The thin film of the conducting polymer, P3HT or regioregular poly(3-hexylthiophene), is driven to microstructure formation by dewetting of the unstable PS underlayer. Eventually, dewetting results in the formation of an undulating P3HT conducting skin covering the isolated islands of PS. The characteristic length scale of the structure is linearly proportional to the thickness of the PS underlayer. The kinetics of hole growth shows that the slippage behavior of the P3HT/PS bilayer which is different from the no-slip seen without the presence of the P3HT over layer. The bending elasticity and slippage induced by confinement of the PS layer by the P3HT layer strongly influence the length scale and kinetics of the dewetting process.

Electrically active defects at AlN/Si interface studied by DLTS and ESR

AbstractA combined deep‐level transient spectroscopy (DLTS) and electron spin resonance (ESR) study is performed to identify the electrically active defects at the AlN/Si (111) interface. It is shown that the density of deep‐level states not only depends on the thermal budget of the epitaxial deposition but also on the strain built up during growth and upon cooling to room temperature (RT). At the same time, diffusion of Si into the 200 nm thick AlN layer produces a thin crystalline Si3N4 interfacial layer, identified by transmission electron microscopy (TEM). This gives rise to so‐called dangling bond Pb centres at the Si3N4/Si (111) interface. In addition, a strong evolution of the electrically active defect clusters in the silicon substrate close to the interface has been observed both in DLTS and ESR.

Sn whisker growth during thermal cycling

Pure Sn plating on ceramic chip capacitors was tested by thermal cycling both in air and in vacuum for up to 3000 cycles and the whisker growth mechanism was clarified. A thin crystalline SnO layer is formed on the surface of Sn plating and whiskers, which exhibits a high level of cracking. Thermal cycling whiskers exhibit two distinct features: fine striation rings on the whisker side face vertical to the whisker growth axis; and deep grooves at the root of the whiskers. One ring of the fine striations corresponds to each thermal cycle. The formation of the grooves can be attributed to thermal cycle cracking along grain boundaries of Sn followed by oxidation and growth of whiskers from the root grains. The characteristic winding feature observed for thermal cycling whiskers can be attributed to the formation of root grooves with severe oxidation. Whisker growth in vacuum is faster than in air. Whiskers grown in vacuum are thinner and longer than whiskers grown in air, while the whisker density is not influenced by atmosphere. The interval of striation rings is wider for vacuum-grown whiskers as compared with air-grown whiskers.

Synthesis and characterization of ZrO2 capsules and crystalline ZrO2 thin layers on Fe2O3 powders

A novel solution deposition process has been developed to prepare thin crystalline ZrO2 layers on particle surfaces. The thickness and morphology of the ZrO2 layers can be controlled by using surfactants under different reaction conditions. A needle-shaped monoclinic ZrO2 array was formed by a multi-step deposition process with surfactants. Hollow ZrO2 capsules that were obtained by dissolving the nanoparticle cores exhibited obvious photocatalytic properties for the degradation of Rhodamine B. This is a general process that can form crystalline layers coated on matrix core particles in solution.

ChemInform Abstract: Reversed Crystal Growth: Implications for Crystal Engineering

AbstractReview: 20 refs.

Formation of dual-layer melt-spun ribbon through liquid phase separation

A dual-layer ribbon of Co–Si–B–Cu alloy was prepared by the single-roller melt-spinning method. The substrate surface comprised Co–Si–B-based amorphous phase with Cu crystalline nanoscale globules, while the free surface showed a thin crystalline Cu layer. A macroscopically separated unique structure was obtained by the simultaneous occurrence of liquid phase separation of the Co–Cu-based alloy system and amorphous phase formation in the Co–Si–B alloy layer.

Effects of the front surface field in n-type interdigitated back contact silicon heterojunctions solar cells

Abstract Previous simulations of interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cells have indicated that front surface passivation is a critical factor in the performance of such cells. This is why we here focus on the effect of a front surface field (FSF) layer by 2D numerical modelling. A FSF layer made of a highly doped thin crystalline silicon top layer makes the cell performance insensitive to the surface recombination velocity up to quite high values (5000 cm/s). It also reduces the lateral resistance losses due to the increased lateral current through the doped layer particularly in IBC-SiHJ solar cells with large pitches. A FSF layer can also be produced by doped hydrogenated amorphous silicon due to the induced accumulation layer at the crystalline silicon surface. The positive effect of such layer strongly depends on the a-Si:H/c-Si interface quality.

Reversed Crystal Growth: Implications for Crystal Engineering

The discovery of reversed crystal growth routes in zeolite analcime and zeolite A implies that crystal growth does not always follow the classic theory established 100 years ago. Aggregation of nanoparticles may dominate in the early stages of crystal growth, followed by surface crystallization, and then extension from surface to core of the disordered aggregates. A perfect polyhedral morphology can be developed in a thin surface crystalline layer of a particle with a disordered core. Evidence of such a novel crystal growth phenomenon can be also found in many other materials. This article highlights the recent achievements in this topic, which might have a significant impact on crystal engineering, materials science, and mineralogy.

Possibilities for measurement of nano-crystallites size with use of parametric X-ray radiation

Spectral and angular properties of the parametric X-ray radiation (PXR) of relativistic particles emitted in the backward direction from a thin crystalline layer and polycrystal are considered. The shapes of the natural spectral peak of the PXR from a crystalline ball and spectral distribution of PXR from a polycrystal are estimated. It is shown that the natural spectral peak width determines the PXR spectral peak width emitted in backward direction from crystalline layer and grains with size of nanometer range. The method for measurements of crystalline layer thickness and crystalline grains size of nanometer range with use of spectral peak width of the PXR emitted in the backward direction is discussed.

Arithmetic Device Based on Multiple Schottky-like Junctions

Photoelectrodes containing cadmium sulfide deposited as powders and crystalline thin layers combined with semi-solid ionic liquid electrolyte behave like Schottky photodiodes. Appropriate connection of these devices results in simple optoelectronic logic gates (AND, XOR) and enables construction of an optoelectronic binary half-adder.

Pt−K/Al2O3 NSR Catalysts: Characterization of Morphological, Structural and Surface Properties

Morphological, textural, and surface properties of a NSR (NOx storage reduction) Pt−K/Al2O3 model catalyst (Pt 1 wt %; K 5.4 wt %) were characterized by means of XRD, HRTEM, and FT-IR spectroscopy. Thin crystalline K-containing layers, in the form of cubic K2O and monoclinic K2CO3 and very small roundish Pt particles with a mean diameter of 1.5 nm, have been observed. Monoclinic K2CO3 disappears, and a certain degree of Pt sintering occurs (dPt ≈ 3.4 nm) after use. However, the presence of potassium limits the Pt sintering which occurs on the Pt/Al2O3 reference sample (Pt 1 wt %). FT-IR spectra of CO adsorbed at RT, compared with those recorded for Pt/Al2O3, revealed a marked interaction between the Pt and K phases that is much higher than the interaction between the Pt and Ba phases observed for the classic Pt−Ba/Al2O3 catalyst. CO2 adsorption at RT indicated a high heterogeneity of the K phase, evidenced by the formation of a variety of surface-carbonate-like species (mainly bridging carbonates on K sites). Minor amounts of nitrites and nitrates were formed at RT under NO admission, while the uptake was sensibly higher under NO/O2 or NO2 admission; nitrites (mono- and bidentate) and nitrates (ionic and bidentates) were formed in different amounts, both relative and absolute, and the nitrate to nitrite ratio increased in parallel with the NO/O2 ratio. Also, at each contact time, the amount of the stored NOx species increased upon increasing the NO/O2 ratio.

Scale-dependent subsurface deformation of metallic materials in sliding

Using laser speckle decorrelation, TEM, optical microscopy and AFM we study deformation structures generated in subsurface layers of metals and alloys by sliding wear. Strain localization process in sliding wear as well as its effect on sliding-induced structures is considered. As shown, strain localization leads to intense fragmentation in subsurface layers and generation of shear bands in previously fragmented materials. These shear bands first exist at the microscale level under mild wear but may reveal at the mesoscale deformation level when there occurs mild-to-catastrophic wear mechanism transition. The result of such a transition is a thick (tens and hundreds of micrometers) nanocrystalline layer at the surface of metals. Hadfield steel shows another type of tribological behavior when only thin nano crystalline layer is formed. We relate such a behavior to the specificity of fragmentation in this steel. High wear resistance of high manganese steel is analyzed too.