Si–SiO2 interface formation in low-dose low-energy separation by implanted oxygen materials

The evolution of the Si–SiO₂ interface morphology of low-dose low-energy separation by implanted oxygen materials was investigated by transmission electron microscopy and atomic force microscopy. The Si–SiO₂ interface morphology and the RMS roughness are strongly affected by the implantation conditions and the annealing process. Three main types of the domains including round, square, and pyramid shapes with the step-terrace structure were observed on the buried SiO₂ surface. Round domains are observed in the early stage of the annealing process, while the square and pyramid domains are observed after the high temperature annealing. The mean RMS roughness decreases with increasing time and annealing temperature, while in the 1350 °C 4-h annealed samples, the mean RMS roughness decreases with either increasing the implantation dose or decreasing implantation energy. The scaling analysis shows that the Si–SiO₂ interfaces were found to be self-affine on the short length scales with a roughness exponent above 0.50. Qualitative mechanisms of Si–SiO₂ surface flattening are presented in terms of the variations of morphological features with the processing conditions.     

Experimental and theoretical studies of Si-CN bonds to eliminate interface states at Si/SiO2 interface

Cyanide treatment, which includes the immersion of Si in KCN solutions followed by a rinse, effectively passivates interface states at Si/SiO₂ interfaces by the reaction of CN⁻ ions with interface states to form Si-CN bonds. X-ray photoelectron spectroscopy (XPS) measurements show that the concentration of the CN species in the surface region after the cyanide treatment is ∼0.25 at.%. Take-off angle-dependent measurements of the XPS spectra indicate that the concentration of the CN species increases with the depth from the Si/SiO₂ interface at least up to ∼2 nm when ultrathin SiO₂ layers are formed at 450 °C after the cyanide treatment. When the cyanide treatment is applied to metal-oxide-semiconductor (MOS) solar cells with 〈ITO/SiO₂/n-Si〉 structure, the photovoltage greatly increases, leading to a high conversion efficiency of 16.2% in spite of the simple cell structure with no pn-junction. Si-CN bonds are not ruptured by air mass 1.5 100 mW cm⁻² irradiation for 1000 h, and consequently the solar cells show no degradation. Neither are Si-CN bonds broken by heat treatment at 800 °C performed after the cyanide treatment. The thermal and irradiation stability of the cyanide treatment is attributable to strong Si-CN bonds, whose bond energy is calculated to be 1 eV higher than that of the Si-H bond energy using a density functional method.     

Polarized neutron reflectometry from the interface of the heterostructures SiO2(Co)/Si and SiO2(Co)/GaAs

Polarized neutron reflectometry is used to investigate SiO₂(Co) granular films (70 at% of Co nanoparticles in SiO₂ matrix) deposited on Si and GaAs substrates. The aim of the study is to compare magnetization depth profiles in two systems: in SiO₂(Co)/GaAs heterostructure which shows at room temperature giant injection magnetoresistance (IMR) with the system SiO₂(Co)/Si which reveals almost no IMR effect. We found that at room temperature and at the same value of external magnetic field mean magnetization in the SiO₂(Co)/GaAs sample is much higher than in the case of SiO₂(Co)/Si. We also demonstrate that magnetic scattering length density, and hence, magnetization profile strongly depends on the substrate. We show that SiO₂(Co)/Si heterostructure is ferromagnetically ordered within the temperature range between 120 and 460 K what could explain a weak IMR.     

Accurate detection of interface between SiO2 film and Si substrate

Accurate end point detection of interface for multilayers using focused ion beam (FIB) is important in nanofabrication and IC modification. Real-time end point graph shows sample absorbed current as a function of sputtering time during FIB milling process. It is found that sample absorbed current increases linearly with ion beam current for the same material and changes when ion beam is milling through a different material. Investigation by atomic force microscope (AFM) and FIB cross-sectioning shows that accurate SiO₂/Si interface occurs to where the maximum sample absorbed current occurs. Since sample absorbed current can be real-time monitored in focused ion beam machine, the paper provides a viable and simple method for accurately determining the interface during FIB milling process for widely used SiO₂/Si system.     

Annealing effects on structure and laser-induced damage threshold of Ta2O5/SiO2 dielectric mirrors

The effects of annealing on structure and laser-induced damage threshold (LIDT) of Ta₂O₅/SiO₂ dielectric mirrors were investigated. Ta₂O₅/SiO₂ multilayer was prepared by ion beam sputtering (IBS), then annealed in air under the temperature from 100 to 400 °C. Microstructure of the samples was characterized by X-ray diffraction (XRD). Absorption of the multilayer was measured by surface thermal lensing (STL) technique. The laser-induced damage threshold was assessed using 1064 nm free pulsed laser at a pulse length of 220 μs.

It was found that the center wavelength shifted to long wavelength gradually as the annealing temperature increased, and kept its non-crystalline structure even after annealing. The absorbance of the reflectors decreased after annealing. A remarkable increase of the laser-induced damage threshold was found when the annealing temperature was above 250 °C.     

The role of multiple damaged layers at the Si/SiO2 interface on the dielectric breakdown of MOS capacitors

Oxide breakdown in metal-oxide-semiconductor (MOS) devices in the nanometer scale is simulated as a cluster growth depending process in which the local electric field is a function of a randomly varying local dielectric permissivity. Effects of MOS device bias polarity, film thickness and non-uniform defect distributions through the entire oxide film on the breakdown are studied. The slope of the Weibull distribution increases with the oxide thickness in agreement with experimental results.     

Study of the interface Si-nc/SiO2 by infrared spectroscopic ellipsometry and X-ray photoelectron spectroscopy

SiO_x films (1<x<2), 0.5 μm thick, have been elaborated by electron-gun evaporation. A thermal annealing of these films induced a phase separation leading to the formation of Si nanocrystals embedded in a SiO₂ matrix. These films have been studied by infrared spectroscopic ellipsometry and by X-ray photoelectron spectroscopy (XPS). The effective dielectric function of the thin films has been extracted in the 600–5000 cm⁻¹ range which allowed us to deduce the dielectric function of the matrix surrounding the Si-nc. A study of the Transverse Optical (TO) vibration mode has revealed the presence of SiO_x into the matrix. Before XPS measurements, the films have been etched in fluorhydric acid to remove the superficial SiO₂ layer formed during air exposure. The Si 2p core-level emission has been recorded. The decomposition of the Si 2p peak into contributions of the usual five tetrahedrons Si-(Si_4−nO_n) (n=0–4) has also revealed the presence of a SiO_x phase. Consistency between infra-red and XPS results is discussed.     

Suppression of boron segregation by interface Ge atoms at SiGe/SiO2 interface

We investigate the migration pathway and barrier for B diffusion at SiGe/SiO₂ interface through first-principles density functional calculations. Similar to the diffusion mechanism reported for Si/SiO₂ interface, a substitutional B, which initially forms a B-self-interstitial complex in SiGe, diffuses to the interface and then to the oxide in form of an interstitial B. At the defect-free interface, where bridging O atoms are inserted to remove interface dangling bonds, it is energetically more favorable for the interstitial B to intervene in the Ge–O bridge bond rather than the Si–O bridge bond at the interface. As a result of the B intervention, interface Ge atoms significantly enhance the stability of B-related defects in the interface region and thereby act as traps for B dopants. At the interface with the Ge–O bridge bond, the overall migration barrier for B diffusion from SiGe to SiO₂ is estimated to be about 3.7 eV, much higher than the reported value of about 2.1 eV at Si/SiO₂ interface. Our results provide a clue to understanding the experimental observation that B segregation toward the oxide is suppressed in SiGe/SiO₂ interface.     

Luminescence of rare earth-doped Si–ZrO2 co-sputtered films

Er-doped Si-yttria-stabilized zirconia (YSZ) thin film samples were prepared by rf co-sputtering. Chemical composition of the samples was determined using energy-dispersive spectroscopy (EDS) and the structure of the films by X-ray diffraction (XRD). The samples were annealed to 700 °C. Photoluminescence (PL) measurements were performed for the visible and infrared. By exciting with the 488-nm-laser line the Er³⁺ emissions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and a narrow ⁴I_13/2→⁴I_15/2 emission were observed. The ⁴I_11/2→⁴I_15/2 emissions for the same excitation wavelength were weak. Excitation wavelength dependence of the ⁴I_13/2→⁴I_15/2 emissions indicated that the emissions were due to a combination of energy transfer from Si nanoparticles (np) to Er ions and energy transfer from defects in the matrix to the Er ions for excitations resonant with the energy levels of such defects. ⁴I_13/2→⁴I_15/2 emission decay measurements show two decaying populations of Er ions according to their locations with respect to other ions or any non-radiative defects. ⁴I_11/2→⁴I_15/2 emission dependence on ⁴I_13/2→⁴I_15/2 emission showed that the former was possibly due to a combination of downconversion from higher levels of the Er ions, energy transfer from Si nanoparticles and upconversion transfer processes. We concluded that Er-doped Si-YSZ is a promising material for photonic applications being easily broadband excited using low-pumping powers.     

B-MQMAS and Si–{B} Double-Resonance NMR Studies on the Structure of Binary B2O3–SiO2 Glasses

Glasses in the system x B₂O₃·(1−x)SiO₂ (0.2≤x≤1.0) were studied using ¹¹B multiple quantum magic angle spinning NMR spectroscopy (MQMAS), ²⁹Si–\{¹¹B\} rotational echo adiabatic passage double-resonance and ²⁹Si–\}¹¹B\{ CP heteronuclear correlation spectroscopy. The results can be quantitatively interpreted in terms of a phase separation of the borosilicate glasses into a virtually SiO₂-free B₂O₃ phase and a mixed borosilicate phase. While the MQMAS spectra allowed the site speciation and resolution of at least two different ¹¹B resonances, attributable to BO_3/2 units consumed in boroxol rings, BO_3/2 units connecting the boroxol rings and BO_3/2 units involved in B–O–Si linkages, the analysis of the double-resonance data further elucidated the structure of the mixed borosilicate phase. The results indicate that only a fraction of 0.48 mol B₂O₃ can be accommodated per mole SiO₂, building a mixed borosilicate network.     

Nanostructuring Si surface and Si/SiO2 interface using porous-alumina-on-Si template technology. Electrical characterization of Si/SiO2 interface

In this work, anodic porous alumina thin films with pores in the nanometer range are grown on silicon by electrochemistry and are used as masking material for the nanopatterning of the silicon substrate. The pore diameter and density are controlled by the electrochemical process. Through the pores of the alumina film chemical oxidation of the silicon substrate is performed, leading to the formation of regular arrays of well-separated stoichiometric silicon dioxide nanodots on silicon, with a density following the alumina pores density and a diameter adjustable by adjusting the chemical oxidation time. The alumina film is dissolved chemically after the SiO₂ nanodots growth, revealing the arrays of silicon dioxide dots on silicon. In a next step, the nanodots are also removed, leaving a nanopatterned bare silicon surface with regular arrays of nanopits at the footprint of each nanodot. This silicon surface structuring finds interesting applications in nanoelectronics. One such application is in silicon nanocrystals memories, where the structuring of the oxidized silicon surface leads to the growth of discrete silicon nanocrystals of uniform size. In this work, we examine the electrical quality of the Si/SiO₂ interface of a nanostructured oxidized silicon surface fabricated as above and we find that it is appropriate for electronic applications (an interface trap density below 1–3×10¹⁰ eV⁻¹ cm⁻² is obtained, indicative of the high quality of the thermal silicon oxide).     

XPS study of annealing induced effects on surface and interface electronic properties of Si/Ge nanostructures

The present study is focused on the influence of vacuum thermal treatment on surface/interface electronic properties of Si/Ge multilayer structures (MLS) characterized using X-ray photoelectron spectroscopy (XPS) technique. Desired [Si(5 nm)/Ge(5 nm)]_×10 MLS were prepared using electron beam evaporation technique under ultra high vacuum (UHV) conditions. The core-level XPS spectra of as-deposited as well as multilayer samples annealed at different temperatures such as 100 °C, 150 °C and 200 °C for 1 h show substantial reduction in Ge 2p peak integrated intensity, whereas peak intensity of Si 2p remains almost constant. The complete interdiffusion took place after annealing the sample at 200 °C for 5 h as confirmed from depth profiling of annealed MLS. The asymmetric behaviour in intensity patterns of Si and Ge with annealing was attributed to faster interdiffusion of Si into Ge layer. However, another set of experiments on these MLS annealed at 500 °C suggests that interdiffusion can also be studied by annealing the system at higher temperature for relatively shorter time duration.     

Optical properties of Ag–Al2O3 nano-array composite structure

Ag/Al₂O₃ nano-array composite structures were obtained by alternating current (AC) electrodeposition of Ag into the pores of anodic alumina membrane (AAM). The ordered wire-grid structure of Ag nanowires formed along a preferential direction, typically 20 nm in diameter, was fabricated. XRD revealed that Ag₂O is also preserved in the pores of AAM. Optical properties of Ag/Al₂O₃ prepared at different annealing temperatures were measured by using spectrometer. Transmission spectra indicate that the transmittance of Ag/Al₂O₃ increases obviously with the increasing of annealing temperature. Polarization spectra indicate that Ag/Al₂O₃ has good polarization in the range of 900–2000 nm, and the extinction ratio increases with the increasing of the annealing temperature or incident angle.     

Enhanced cyclability of CdS/TiO2 photocatalyst by stable interface structure

CdS nanoparticles were in situ deposited on TiO₂ nanosheets and nanorods under hydrothermal conditions, respectively. The effect of CdS–TiO₂ interface structure on hydrogen production activity was mainly investigated under visible light irradiation. The results showed that the TiO₂ nanosheet-based CdS/TiO₂ showed a higher activity and a higher cyclability than the nanorod-based sample due to the stronger interaction of CdS with the (0 0 1) facets of TiO₂ than with the (1 0 1) facets. It was proposed that the strong interaction between CdS nanoparticles and TiO₂ nanosheets effectively refrains the recombination of electrons and holes.     

Measurement of nanosize etched pits in SiO2 optical fiber conduit using AFM

Fission fragment tracks from ²⁵²Cf have been observed in SiO₂ optical fiber, using an atomic force microscope (AFM), after a very short chemical etching in hydrofluoric acid solution at normal temperature. The nuclear track starting and evolution process is followed by the AFM direct measurements on the material surface and beyond a fine layer of the surface material. The images of the scanned cones were determined observing the two predominant energies from ²⁵²Cf fission fragments and the development of the tracks in the 150 μm diameter optical fiber conduit.     

Regular step arrays at the SiO2/Si(111) interface

A set of slightly misoriented Si(11) wafers have been oxidized and annealed in different ways. After removal of the oxide the structure of the Si/SiO₂ interface has been studied by a high-resolution LEED system. The resulting LEED spots showed relatively sharp splitting for out-of-phase condition, indicating a nearly regular step array. The spot profiles are described precisely by a closed form terrace width distribution. The energy dependence of the spot profiles yields the vertical roughness of the samples. It can be shown that the selection of appropriate oxidation parameters decreases the deviations from the periodic structure.     

Effects of annealing on laser-induced damage threshold of TiO2/SiO2 high reflectors

The mechanism of improving 1064 nm, 12 ns laser-induced damage threshold (LIDT) of TiO₂/SiO₂ high reflectors (HR) prepared by electronic beam evaporation from 5.1 to 13.1 J/cm² by thermal annealing is discussed. Through optical properties, structure and chemical composition analysis, it is found that the reduced atomic non-stoichiometric defects are the main reason of absorption decrease and LIDT rise after annealing. A remarkable increase of LIDT is found at 300 °C annealing. The refractive index and film inhomogeneity rise, physical thickness decrease, and film stress changes from compress stress to tensile stress due to the structure change during annealing.     

Interfaces analysis of the HfO2/SiO2/Si structure

The physical and chemical properties of the HfO₂/SiO₂/Si stack have been analyzed using cross-section HR TEM, XPS, IR-spectroscopy and ellipsometry. HfO₂ films were deposited by the MO CVD method using as precursors the tetrakis 2,2,6,6 tetramethyl-3,5 heptanedionate hafnium—Hf(dpm)₄ and dicyclopentadienil-hafnium-bis-diethylamide—Сp₂Hf(N(C₂H₅)₂)₂.The amorphous interface layer (IL) between HfO₂ and silicon native oxide has been observed by the HRTEM method. The interface layer comprises hafnium silicate with a smooth varying of chemical composition through the IL thickness. The interface layer formation occurs both during HfO₂ synthesis, and at the annealing of the HfO₂/SiO₂/Si stack. It was concluded from the XPS, and the IR-spectroscopy that the hafnium silicate formation occurs via a solid-state reaction at the HfO₂/SiO₂ interface, and its chemical structure depends on the thickness of the SiO₂ underlayer.     

Giant magnetoimpedance in FM/SiO2/Cu/SiO2/FM films at GHz frequencies

We study the magnetic properties at high frequency of new structures of the tri-layer samples. The magnetoimpedance effect was analyzed in FM/i/Cu/i/FM sandwiched layers, where the ferromagnetic layer (FM) is, in fact, a multilayered film [F (10 nm)+Cu (1 nm)]×50 and F is the amorphous ferromagnetic alloy Fe_73.5Cu₁Nb₃Si_13.5B₉ and i is an isolating layer produced by magnetron sputtering. The effect of, both, the probe current frequency (in the range 10 MHz–1.8 GHz) and the dimensions of the magnetic and non-magnetic layers of the MI response were investigated. A comparison between samples with and without the isolating layer is discussed. MI ratios of 220% were obtained for samples at 180 MHz with a ferromagnetic and Cu width layers of 2 and 1 mm, respectively.