The effect of SixNy interlayer on the quality of GaN epitaxial layers grown on Si(1 1 1) substrates by MOCVD

In the present paper, the effects of nitridation on the quality of GaN epitaxial films grown on Si(1 1 1) substrates by metal–organic chemical vapor phase deposition (MOCVD) are discussed. A series of GaN layers were grown on Si(1 1 1) under various conditions and characterized by Nomarski microscopy (NM), atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD), and room temperature (RT) photoluminescence (PL) measurements. Firstly, we optimized LT-AlN/HT-AlN/Si(1 1 1) templates and graded AlGaN intermediate layers thicknesses. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.2 μm. Secondly, the effect of in situ substrate nitridation and the insertion of an Si_xN_y intermediate layer on the GaN crystalline quality was investigated. Our measurements show that the nitridation position greatly influences the surface morphology and PL and XRD spectra of GaN grown atop the Si_xN_y layer. The X-ray diffraction and PL measurements results confirmed that the single-crystalline wurtzite GaN was successfully grown in samples A (without Si_xN_y layer) and B (with Si_xN_y layer on Si(1 1 1)). The resulting GaN film surfaces were flat, mirror-like, and crack-free. The full-width-at-half maximum (FWHM) of the X-ray rocking curve for (0 0 0 2) diffraction from the GaN epilayer of the sample B in ω-scan was 492 arcsec. The PL spectrum at room temperature showed that the GaN epilayer had a light emission at a wavelength of 365 nm with a FWHM of 6.6 nm (33.2 meV). In sample B, the insertion of a Si_xN_y intermediate layer significantly improved the optical and structural properties. In sample C (with Si_xN_y layer on Al_0.11Ga_0.89N interlayer). The in situ depositing of the, however, we did not obtain any improvements in the optical or structural properties.     

Determination of the diffusion coefficient of lithium ions in nano-Si

The diffusion coefficients of lithium ions (D_Li⁺) in nano-Si were determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). D_Li⁺ values are estimated to be ~ 10^? 12 cm² s^? 1 and exhibit a “W” type varying with the lithium concentration in silicon. Two minimum regions of D_Li⁺ (at Li_2.1 ± 0.2Si and Li_3.2 ± 0.2Si) are found, which probably result from two amorphous compositions (a-Li₇Si₃ and a-Li₁₃Si₄). Besides the two minimum regions, one maximum D_Li⁺ is observed at Li₁₅Si₄, corresponding to the crystallization of highly lithiated amorphous Li_xSi.     

AFM measurement of atomic-scale Si surface etching by active oxidation

Atomic-scale etching of a clean Si surface by active oxidation with oxygen molecules was examined using ex-situ atomic force microscopy (AFM). The etch rate was directly determined by measuring the etch depth with AFM. A SiO₂ anti-etching mask was used on a H-terminated Si(001)-2 × 1:H surface prepared by low pH HF treatment followed by annealing in H₂. The etch rate under active oxidation conditions was almost proportional to the O₂ pressure, which was consistent with previous reports. The etch rate exhibited a weak temperature dependence with an apparent activation energy of 0.2 eV. A distinct transition of the reaction mode from etching to oxide formation was observed in detail as an abrupt decrease of the etch rate by lowering the temperature near the boundary condition between the etching and oxide formation conditions.     

Time-evolution of thermal oxidation on high-index silicon surfaces: Real-time photoemission spectroscopic study with synchrotron radiation

The initial oxidation on high-index silicon surfaces with (113) and (120) orientations at 820 K has been investigated by real-time X-ray photoemission spectroscopy (Si 2p and O 1s) using 687 eV photons. The time evolutions of the Siⁿ⁺ (n = 1–4) components in the Si 2p spectrum indicate that the Si^2 + state is suppressed on high-index surfaces compared with Si(001). The O 1s state consists of two components, a low-binding-energy component (LBC) and a high-binding-energy component (HBC). It is suggested that the O atom in strained SiOSi contributes to the LBC component. The reaction rates are slower on high-index surfaces compared with that on Si(001).     

Novel clays: Solid-state synthesis,characterization and cation exchange selectivity

Several novel swelling mica-type clays have been synthesized by solid-state processes. Synthetic clays of ideal compositions such as Na₂Si₆Al₂Mg₆O₂₀F₄ · xH₂O (Na-2-mica), Na₃Si₅Al₃Mg₆O₂₀F₄ · xH₂O (Na-3-mica) and Na₄Si₄Al₄Mg₆O₂₀F₄ · xH₂O (Na-4-mica) have been prepared and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and ²⁷Al and ²⁹Si solid-state magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Powder XRD showed that all syntheses yielded water swollen micas with c-axis spacing of ∼1.2 nm except in the case of Na-2-mica which also showed a small peak of anhydrous mica phase with a c-axis spacing of 0.96 nm. Solid-state ²⁷Al MASNMR spectroscopy revealed that almost all the Al is present in the tetrahedral environment of the different micas. Solid-state ²⁹Si MASNMR spectroscopy revealed different Si (Al) nearest neighbor environments depending upon the composition of the various micas. Selective cation exchange studies were performed on the various micas using 0.5 N NaCl solution containing 12.9 ppm Sr²⁺ or 8.12 ppm of La³⁺. The results showed, for the first time, that Na-3-mica has a high selectivity for the trivalent cation tested. The previously reported high selectivity of these synthetic micas for the divalent cations has been confirmed. These selective cation exchange studies are of relevance in cation separations from drinking and waste waters.     

Laser ablation time-of-flight mass spectrometry (LA-TOF-MS) of “nitrogen doped diamond-like carbon (DLN) nano-layers”

Nitrogen-doped diamond-like carbon (DLC) layers (a-C:H:N, N-DLC or DLN) were prepared by the plasma-enhanced chemical vapor deposition (PECVD) technique, using a RF capacitive discharge (13.56 MHz), at low pressures (20 Pa), produced from a mixture of methane, nitrogen and hexamethyldisiloxane (HMDSO), deposited on single-crystalline silicon wafers placed on steel samples. The films, of differing deposition times, were subjected to laser ablation time-of-flight (LA-TOF) mass spectrometric measurements, using different commercial instrumentation to characterize their structures. The analysis of mass spectra was made and the following positively singly charged species were detected and identified: C_n⁺ (n=4–30), Si_n⁺ (n=2, 3), Si_nH⁺ (n=2, 3), SiOK⁺, Si₃H₄⁺, Si₂N⁺, Si₂NH₂⁺, and Si₃C⁺. The later three species could reflect the presence of nitrogen–silica and carbon–silica chemical bonds in the structure of the DLN layer. The stoichiometry of all species was confirmed by isotopic pattern simulation. In the negative detection mode, the C_n⁻ (n=2–12) clusters were observed. The findings are discussed in the light of the current research concerning analysis of the DLN thin layers and it is concluded that namely Si₂N⁺, Si₂NH₂⁺ and Si₃C⁺ species are reflecting the chemical structure of the DLN layer. LA-TOF-MS was found useful supplementary method for the characterization of DLN nano-layers.     

In-situ formation of SiC nanocrystals by high temperature annealing of SiO2/Si under CO: A photoemission study

We have studied CO interaction with SiO₂/Si system at high temperature (~ 1100 °C) and 350 mbar by core-level photoemission. Even for short annealing time (5 min) the signal from Si2p and C1s core levels shows a clear change upon CO treatment. Shifted components are attributed to formation of SiC. This is confirmed by TEM imaging which further shows that the silicon carbide is in the form of nano-crystals of the 3C polytype. Photoemission spectroscopy moreover reveals the formation of silicon oxicarbide which could not be evidenced by other methods. Combining these results with previous Nuclear Resonance Profiling study gives a deeper insight into the mechanisms involved in the nanocrystals growth and especially for the reaction equation leading to SiC formation. We show that CO diffuses as a molecule through the silica layer and reacts with the silicon substrate according the following reaction: 4 CO + 4 Si → SiO₂ + 2SiC + SiO₂C₂.     

Irradiation induced modifications in morphology and magnetic property of Mn/Si structure

Mn films of ∼50 nm has been deposited by electron beam evaporation technique on cleaned and etched Si [(1 0 0), 8–10 Ω cm] substrates to realize a Mn/Si interfacial structures. The structures have been irradiated from energetic (∼100 MeV) ion beam from Mn side. The irradiated and unirradiated structures have been characterized from atomic force microscopy, X-ray diffractometry, magnetic force microscopy, and vibrating sample magnetometer facilities. It has been found that surface/interfacial granular silicide phases (of Mn_xSi_y) are formed before and after the irradiation with a irradiation induced modifications of surface morphology and magnetic property. The surface/interface roughness has been found to increase on the irradiation from the atomic force microscopy data. The magnetic property on the irradiation shows an interesting and significant feature of an increased coercivity and a ferromagnetic like behavior in the Mn–Si structure. The observed increased coercivity has been related to the increased roughness on the irradiation. The ferromagnetism after the irradiation is a curious phenomenon which seems due to the formation of Mn–C–Si compound from the carbon dissolved in silicon.     

A simple method for well-defined and clean all-SiC nano-ripples in ambient air

Well-defined and clean all-SiC nano-ripples with a period of about 150 nm are produced via the combination of 800-nm femtosecond laser irradiation and chemical selective etching with mixture solution of 65 wt% HNO₃ acid (20 mL) and 40 wt% HF acid (20 mL). The incorporation mechanism of oxygen (O) species into the laser induced obscured nano-ripples is attributed to femtosecond laser induced trapping effect of dangling bonds, while that of chemical etching induced well-defined and clean nano-ripples is assigned to chemical reactions between mixture acid solution and amorphous silicon carbide (SiC) or silicon oxide (SiO₂). Results from EDX analysis show that the incorporated foreign O species (atomic percentages of 9.39%) was eliminated effectively via chemical etching, while the atomic percentages of silicon (Si) and carbon (C) were about 47.82% and 52.18% respectively, which were similar to those of original SiC material. And the influences of laser irradiation parameters on the nano-ripples are also discussed.     

X-ray photoelectron spectroscopic studies of black silicon for solar cell

The black silicon has been produced by plasma immersion ion implantation (PIII) process. The microstructure and optical reflectance are characterized by field emission scanning electron microscope and spectrophotometer. Results show that the black silicon appears porous or needle-like microstructure with the average reflectance of 4.87% and 2.12%, respectively. The surface state is investigated by X-ray photoelectron spectroscopy (XPS) technique. The surface of the black silicon is composed of silicon, carbon, oxygen and fluorine element. The formation of Si_xO_yF_z in the surface of black silicon can be proved clearly by the O 1s, F 1s and Si 2p XPS spectra. The formation mechanism of the black silicon produced by PIII process can be obtained from XPS results. The porous or needle-like structure of the black silicon will be formed under the competition of SF_x⁺ (x ≤ 5) and F⁺ ions etching effect, Si_xO_yF_z passivation and ion bombardment.     

N2-plasma nitridation on Si(111): Its effect on crystalline silicon nitride growth

This study uses a combination of atom-resolved scanning tunneling microscopic and spectroscopic (STM/STS) techniques to investigate the effects of N₂-plasma nitridation on a crystalline β-Si₃N₄ ultrathin film grown on the Si(111) substrate. The proposed two-step growth process, including N₂-plasma nitridation followed by vacuum annealing both at high temperature (~ 900 °C), can substantially improve the atomic and electronic structures of the β-Si₃N₄ having an atomically uniform morphology and stoichiometry. The effects of nitridation and post-annealing temperatures were examined by monitoring the morphological and electronic–structural evolutions of a non-stoichiometric surface. Moreover, the two-step N₂-plasma nitridation process has extremely low activation energy and thus minimizes the thermal energy from the substrate for practical growth of β-Si₃N₄ ultrathin film.     

Thin-film formation of Si clathrates on Si wafers

In this study, we prepared Si clathrate films (Na₈Si₄₆ and Na_xSi₁₃₆) using a single-crystalline Si substrate. Highly oriented film growth of Zintl-phase sodium silicide, which is a precursor of Si clathrate, was achieved by exposing Na vapour to Si substrates under an Ar atmosphere. Subsequent heat treatment of the NaSi film at 400 °C (3 h) under vacuum (<10⁻² Pa) resulted in a film of Si clathrates having a thickness of several micrometres. Furthermore, this technique enabled the selective growth of Na₈Si₄₆ and Na_xSi₁₃₆ using the appropriate crystalline orientation of Si substrates.     

Giant magnetoresistance in the Ge-rich magnetocaloric compound,Gd5(Si0.1Ge0.9)4

We have measured the zero-field electrical resistivity in the temperature range 5–295 K and magnetoresistance in magnetic fields of up to 12 T of Gd₅(Si_0.1Ge_0.9)₄. The resistivity changes drastically at the magnetostructural first-order transition (T_C≅80 K on heating). This transition can be induced reversibly by the application of an external magnetic field above T_C, producing a concomitant giant magnetoresistance (GMR) effect, Δρ/ρ≅−50%. This study demonstrates that (in addition to giant magnetocaloric and magnetoelastic effects) GMR can be tuned between ∼20 and ∼290 K in Gd₅(Si_xGe_1−x)₄ with x⩽0.5 by simply adjusting the Si : Ge ratio.     

Magnetic anisotropy of epitaxial Fe layers grown on Si(0 0 1)

The magnetic properties of epitaxial iron films up to 80 monolayers (ML) thickness grown on Si(0 0 1) by using a template technique were investigated by means of superconducting quantum interference device and magneto-optic Kerr effect techniques. The thinnest films investigated (∼3 ML) exhibit a composition close to Fe₃Si with a Curie temperature below room temperature (RT) and strong out-of-plane remanent magnetization that reflects the presence of a dominant second order surface anisotropy term. Thicker films (⩾4 ML) are ferromagnetic at RT with remanent magnetization in film-plane and a composition closer to pure Fe with typically 8–10% silicon content. When deposited at normal incidence such films show simple in-plane fourfold anisotropy without uniaxial contribution. The relevant fourth-order effective anisotropy constant K₄^eff was measured versus film thickness and found to change its sign near 18 ML. The origin of this remarkable behavior is investigated by means of a Néel model and mainly traced back to fourth-order surface anisotropy and magneto-elastic effects related to the large biaxial in-plane compressive strain up to 3.5% in the thinnest (⩽25 ML) films.     

Formation of binary clusters by molecular ion irradiation

In the present study, we have observed silicon–carbon cluster ions (Si_nC_m⁺) emitted from a Si(1 0 0) surface under irradiation of reactive molecular ions, such as C₆F₅⁺, at 4 keV, 1 μA/cm². The cluster Si_n up to n=8 and “binary” cluster Si_nC up to n=6 are clearly detected for the C₆F₅⁺ irradiation. Stoichiometric clusters (Si_nC_m n=m) except SiC⁺ and other binary clusters which contain more than two carbon atoms (m≥2) were scarcely observed. The observed clusters show a yield alternation between odd and even n. The intensities of Si₄, Si₆ and Si₅C clusters are relatively higher than those of the neighboring clusters. In the case of Si₅C, it is considered that doped carbon atom acts as silicon atom. These results imply that the recombination through the nascent cluster emission and subsequent decomposition takes place during the cluster formation.     

Coverage-dependent chemisorption of Cl on Si(1 1 4)

The dissociative chemisorption of Cl₂ on Si(1 1 4) was studied at room temperature using scanning tunneling microscopy. Filled and empty state imaging revealed a multitude of possible adsorption configurations with adsorption preferring rebonded atom and dimer sites over tetramer sites. The dissociation and subsequent adsorption processes were sufficiently exothermic that Cl could interrogate the potential energy landscape and find local energy minima. Annealing revealed configurations that preserved the strongest π-bonds of the (1 1 4) reconstruction.     

Thermal stability of the Co/β-Si3N4/Si(111) interface: A photoemission study

The thermal stability of the Co/β-Si₃N₄/Si(111) interface has been studied by high-resolution photoemission spectroscopy in a temperature range extending from room temperature to 650 °C. It is demonstrated the ability of a very thin crystalline buffer layer of silicon nitride to prevent the interfacial reaction between cobalt and silicon at room temperature. The behaviour of the interface at higher temperature shows the formation of cobalt silicides already at 300 °C. Moreover, the presence of new components in the decomposition of the photoemission spectra is discussed in the light of the existing literature.     

Structural and photoluminescence investigation on the hot-wire assisted plasma enhanced chemical vapor deposition growth silicon nanowires

High density of silicon nanowires (SiNWs) were synthesized by a hot-wire assisted plasma enhanced chemical vapor deposition technique. The structural and optical properties of the as-grown SiNWs prepared at different rf power of 40 and 80 W were analyzed in this study. The SiNWs prepared at rf power of 40 W exhibited highly crystalline structure with a high crystal volume fraction, X_C of ～82% and are surrounded by a thin layer of SiO_x. The NWs show high absorption in the high energy region (E>1.8 eV) and strong photoluminescence at 1.73 to 2.05 eV (red–orange region) with a weak shoulder at 1.65 to 1.73 eV (near IR region). An increase in rf power to 80 W reduced the X_C to ～65% and led to the formation of nanocrystalline Si structures with a crystallite size of <4 nm within the SiNWs. These NWs are covered by a mixture of uncatalyzed amorphous Si layer. The SiNWs prepared at 80 W exhibited a high optical absorption ability above 99% in the broadband range between 220 and ～1500 nm and red emission between 1.65 and 1.95 eV. The interesting light absorption and photoluminescence properties from both SiNWs are discussed in the text.     

Influence of the antireflection,window, and active layers on optical properties of exponential-doping transmission-mode GaAs photocathode modules

In order to well study the influence of the thickness and doping concentration on optical properties of transmission-mode GaAs photocathode, three exponential-doping and one uniform-doping photocathode modules were prepared by molecular beam epitaxy with a structure of glass/Si₃N₄/Ga_1 ? xAl_xAs/GaAs. By use of the spectrophotometer, the optical properties were separately measured including the reflectivity and transmissivity curves. Based on thin film optical principles, the optical properties and their integral values are calculated by matrix formula for the four-layer photocathode module. The result shows that the antireflection and window layers affect the peak and valley of the optical property curves and the active layer influences the absorptivity values of the transmission-mode cathode modules. The photocathode module has high absorptivity within the response waveband when the optimal module has the Si₃N₄ antireflection layer of 0.1 μm, the Ga_1 ? xAl_xAs window layer of more than 0.4 μm, and the GaAs active layer of 1.5 μm–2 μm and low average doping concentration.     

Theoretical analysis of microdisk resonators based on Si/Si1−xGex:Er light-emitting structures

Parameters of dielectric microdisk resonators based on Si/Si_1?xGe_x:Er light-emitting structures were investigated using the theory of whispering-gallery modes. Theoretical analysis was carried out for a cylinder made of Si_0.7Ge_0.3 with a radius a and a height h varying in a wide range. The data obtained are of interest of a laser realization on Si basis operating at 1.54 μm.