Thin Silicon Oxide Layer Research Articles

Synthesis and Properties of Single-Crystal FeSi Nanowires

We report for the first time the chemical synthesis of free-standing single-crystal nanowires (NWs) of FeSi, the only transition-metal Kondo insulator and the host structure for ferromagnetic semiconductor Fe(x)Co(1-x)Si. Straight and smooth FeSi nanowires are produced on silicon substrates covered with a thin layer of silicon oxide through the decomposition of the single-source organometallic precursor trans-Fe(SiCl3)2(CO)4 in a simple chemical vapor deposition process. Unlike typical vapor-liquid-solid (VLS) NW growth, FeSi NWs form without the addition of metal catalysts, have no catalyst tips, and depend strongly on the surface employed. X-ray spectroscopy verifies the identity and the room-temperature metallic nature of FeSi NWs. Room-temperature electrical transport measurements using NW devices show an average resistivity of 210 micro Omega cm, similar to the value for bulk FeSi. Investigations into the low-temperature physical properties of the first one-dimensional Kondo insulator and the possible new NW growth mechanism are underway. This unique synthetic approach to FeSi NWs will be generally applicable to many other transition-metal silicides.

Reduction of current instabilities in silicon nanogaps

Silicon nanogaps are electrodes for connecting nanoscale elements to silicon technology. Two silicon electrodes are separated by a thin silicon dioxide layer, which is removed by selective etching to create a nanometer sized accessible gap. Parasitic gap currents with large instabilities occur after the etching. These are compared to the instabilities in ordinary MOS (metal oxide silicon) devices after soft breakdown. Clear similarities are seen when subjecting the devices to constant voltage stress, storing them and annealing them. We discuss a percolation-based model for the instable current, in which the percolation clusters are defects in the oxide surface created during the etching, or possibly contaminating residuals from the same process.

Subattoampere current induced by single ions in silicon oxide layers of nonvolatile memory cells

A single ion impinging on a thin silicon dioxide layer generates a number of electron/hole pairs proportional to its linear energy transfer coefficient. Defects generated by recombination can act as a conductive path for electrons that cross the oxide barrier, thanks to a multitrap-assisted mechanism. We present data on the dependence of this phenomenon on the oxide thickness by using floating gate memory arrays. The tiny number of excess electrons stored in these devices allows for extremely high sensitivity, impossible with any direct measurement of oxide leakage current. Results are of particular interest for next generation devices.

RuO 2/SiO 2/Si and SiO 2/porous Si/Si interfaces analysed by SIMS

Compared are interfaces between thin silicon oxide layers and two substrates bulk silicon and porous silicon. About 2 μm thick porous silicon was prepared by an electrochemical etching of Si in HF solution. Oxide layers were formed by thermal oxidation. Part of SiO 2/Si samples were covered with 50 nm RuO 2 oxide. SIMS depth profiles of these interfaces were performed using ultra-low energy (880 eV) and low energy (5 keV) argon ion beams at several incidence angles. Positive and negative secondary ions were analysed by quadrupole mass spectrometer QMA 410 Balzers during Ar + bombardment with the use of 06-350E Physical Electronics ion gun. Charge build-up effects during positive secondary ion detection were observed dependent of the thickness of silicon oxide layer. The SiO 2/Si interface regions characterised by SIMS show differences related to the kind of a substrate used. In case of 50 nm SiO 2/porous Si interface, characteristic oxygen depletion region (20 nm thick) is present on the porous silicon side. The observed depletion was confirmed also by Auger depth profiling. The depletion formation mechanism is explained on the basis of ion beam induced redeposition of oxygen inside the pores.

Subpicosecond conduction through thin SiO2 layers triggered by heavy ions

Heavy ions impinging on thin silicon dioxide layers generate a dense plasma of electrons and holes. Under particular conditions, such as in thin oxide layer surrounding isolated conductive lines, this plasma can act as a conductive medium, able to carry current for very short times (10−14s). We studied this phenomenon by using large data set obtained on state-of-the art floating gate memory arrays. Floating gates hit by ions experience a charge loss linearly dependent on ion linear energy transfer and on the electric field across the tunnel oxide. Despite its absolute low value, reaching at most thousands of electrons, charge lost from floating gate exceeds by orders of magnitude that expected from relatively simple models, such as generation and recombination. The model we are proposing is fully consistent with a broad range of theoretical and experimental results, and has excellent fitting capabilities.

Strain and composition of ultrasmall Ge quantum dots studied by x-ray scattering and in situ surface x-ray absorption spectroscopy

Investigations of the structural properties of ultrasmall Ge dots, grown through a thin silicon oxide layer on Si(001), have been performed by combining (i) grazing incidence x-ray diffraction, (ii) grazing incidence small angle x-ray scattering (GISAXS), and (iii) surface extended x-ray absorption fine structure (SEXAFS). Grazing incidence x-ray diffraction observations show that the dots are highly strained to the Si(001) substrate. The dot shape and composition, as determined by GISAXS experiments and in situ SEXAFS, strongly depend on the thickness and the composition of the silicon oxide layer covering the surface of the silicon substrate. Ge dots grown through 1.2nm thick SiO2 are round shaped with a Ge content of about 60% whereas dots grown through 0.3nm thick SiOx are facetted and composed of almost pure Ge.

Oxidation of Si nanocrystals fabricated by ultralow-energy ion implantation in thin SiO2 layers

The effect of thermal treatments in nitrogen-diluted oxygen on the structural characteristics of two-dimensional arrays of Si nanocrystals (NCs) fabricated by ultralow-energy ion implantation (1 keV) in thin silicon dioxide layers is reported. The NC characteristics (size, density, and coverage) have been measured by spatially resolved electron-energy-loss spectroscopy by using the spectrum-imaging mode of a scanning transmission electron microscope. Their evolution has been studied as a function of thermal treatment duration at a temperature (900 °C) below the SiO2 viscoelastic point. An extended spherical Deal-Grove [J. Appl. Phys. 36, 3770 (1965)] model for self-limiting oxidation of embedded silicon NCs has been carried out. It proposes that the stress effects, due to oxide deformation, slow down the NC oxidation rate and lead to a self-limiting oxide growth. The model predictions show a good agreement with the experimental results. Soft oxidation appears to be a powerful way for manipulating the NC size distribution and surface density.

Growth of silicon bump induced by swift heavy ion at the silicon oxide-silicon interface

Thin silicon oxide layers on silicon substrates are investigated by scanning probe microscopy before and after irradiation with 210 MeV Au+ ions. After irradiation and complete chemical etching of the silicon oxide layer, silicon bumps grown on the silicon surface are observed. It is shown that each impinging ion induces one silicon bump at the interface. This observation is consistent with the thermal spike theory. Ion energy loss is transferred to the oxide and induces local melting. Silicon-bump formation is favored when the oxide and oxide-silicon interface are silicon rich.

Investigation of Ta 2O 5/SiO 2/4H-SiC MIS capacitors

Tantalum pentoxide (Ta 2O 5) deposited by pulsed DC magnetron sputtering technique as the gate dielectric for 4H-SiC based metal-insulator-semiconductor (MIS) structure has been investigated. A rectifying current–voltage characteristic was observed, with the injection of current occurred when a positive DC bias was applied to the gate electrode with respect to the n type 4H-SiC substrate. This undesirable behavior is attributed to the relatively small band gap of Ta 2O 5 of around 4.3 eV, resulting in a small band offset between the 4H-SiC and Ta 2O 5. To overcome this problem, a thin thermal silicon oxide layer was introduced between Ta 2O 5 and 4H-SiC. This has substantially reduced the leakage current through the MIS structure. Further improvement was obtained by annealing the Ta 2O 5 at 900 °C in oxygen. The annealing has also reduced the effective charge in the dielectric film, as deduced from high frequency C– V measurements of the Ta 2O 5/SiO 2/4H-SiC capacitors.

InP dies transferred onto silicon substrate for optical interconnects application

We bonded quantum well InP dies on a photonic layer transferred on silicon CMOS processed wafer using direct molecular bonding. This approach is suitable for new applications, viz., photonics on silicon, 3D packaging and integrated sensors. The chips are diced from a bulk substrate and bonded directly onto a silicon substrate without any organic nor metallic adhesive layer. A thin silicon dioxide layer can be added on both assembled surfaces to enhance bonding quality. After bonding, the dies can mechanically be thinned down to 20 μm and chemically etched. The InAsP quantum well stack of the InP dies keeps its optoelectronics features and performances after being transferred onto a silicon substrate.

Optical and structural study of Ge/Si quantum dots on Si(1 0 0) surface covered with a thin silicon oxide layer

The formation of Ge quantum dots (QDs) grown on an ultrathin interlayer of SiO 2 on top of a Si(1 0 0) substrate was investigated, as a function of the thicknesses of the SiO 2 interlayer (0.5, 0.75 or 1 monolayer (ML)) and the Ge layer (0.3, 0.6 or 0.9 nm). The structural characterization was performed by Rutherford backscattering spectroscopy (RBS). Photoluminescence (PL) studies were done to characterize the optical behavior of all samples. Hydrogen treatment was performed in order to passivate non-radiative recombination channels, thus enhancing the PL intensity. The results suggest the formation of Ge nanoislands (quantum dots, QDs), for the sample with 1 ML of SiO 2 and 0.9 nm of Ge, and exclude their formation for samples with lower SiO 2 and Ge layer thicknesses. We also observe an influence of the SiO 2 interlayer thickness in the QDs formation.

Praseodymium silicate layers with atomically abrupt interface on Si(100)

Synchrotron radiation x-ray photoelectron spectroscopy was applied to study the solid state reaction between praseodymium and thin silicon dioxide layers on Si(100). Nondestructive depth profiling studies by variation of the incident photon energy indicate after praseodymium deposition at room temperature the reaction of the upper silicon dioxide to praseodymium oxide and silicide. High-temperature annealing of films with an appropriate praseodymium / silicon dioxide ratio results in homogeneous praseodymium silicate films with an atomically abrupt interface. Ab initio calculations corroborate the results of the photoemission study.

Fabrication of deep single trenches from N-type macroporous silicon

Since its discovery in 1956, porous silicon has found various applications according to its wide range of morphologies (chemical sensing structures, Micro-Electro-Mechanical Systems—MEMS, silicon based optoelectronics, etc.). In this paper, we present a macroporous material, with pore diameters larger than 1 μm, produced in a polarized HF/H 2O mixture ([HF]=4.5 wt.%). Some square apertures were realized in a highly doped poly-silicon deposited on a thin (300 nm) silicon oxide layer. Then, electrochemical etching of the N-type 〈100〉 oriented silicon with total current densities of 5.65 and 7 mA/cm 2 during 2 h were performed with a backside illumination. We observed deep trenches near the mask border, perpendicular to the surface, over 120 μm from the surface of the poly-silicon. Current density distributions have been simulated in two-dimensional structures in order to explain these phenomena.

Local order around Er3+ ions in thin silicon oxide layers grown in Si by MBE

Er doped Si represent an interesting class of materials for the realization of light emitting devices based on Si technology. The local structure around Er ions is known to strongly affect the luminescence properties of the Rare Earth. In this work the Er site in Er + O doped silicon samples prepared by Molecular Beam Epitaxy has been investigated by x-ray Absorption Spectroscopy at the Er LIII edge. Samples with different preparation parameters such as the O/Er ratio and thermal treatments were investigated. Data were fitted using a model consisting in a first shell of O atoms and a second shell of Si atoms and accounting for multiple scattering effects. The rare earth is found to be linked to 5–6 O atoms at around 2.23 Å whereas a well defined Er-O-Si bond angle of ≈136 deg is evidenced.

Simplified tunnelling current calculation for MOS structures with ultra-thin oxides for conductive atomic force microscopy investigations

As charge tunnelling through thin and ultra-thin silicon dioxide layers is regarded as the driving force for MOS device degradation the determination and characterisation of electrically week spots is of paramount importance for device reliability and failure analysis. Conductive atomic force microscopy (C-AFM) is able to address this issue with a spatial resolution smaller than the expected breakdown spot. For the determination of the electrically active oxide thickness in practice an easy to use model with sufficient accuracy and which is largely independent of the oxide thickness is required. In this work a simplified method is presented that meets these demands. The electrically active oxide thickness is determined by matching of C-AFM voltage–current curves and a tunnelling current model, which is based on an analytical tunnelling current approximation. The model holds for both the Fowler–Nordheim tunnelling and the direct tunnelling regime with one single tunnelling parameter set. The results show good agreement with macroscopic measurements for gate voltages larger than approximately 0.5–1 V, and with microscopic C-AFM measurements. For this reason arbitrary oxides in the DT and the FNT regime may be analysed with high lateral resolution by C-AFM, without the need of a preselection of the tunnelling regime to be addressed.

Grazing incidence x-ray diffraction and atomic force microscopy investigations of germanium dots grown on silicon (001) by successive depositions of germanium through a thin silicon oxide layer

Structural properties of high density, nanometric size germanium dots grown on a silicon (001) surface covered by a very thin (1.2 nm thick) silicon oxide layer have been investigated by in-situ grazing incidence x-ray diffraction (GIXD) and ex-situ atomic force microscopy. Growth under molecular nitrogen partial pressure of 10−5 Torr yielded a high density (∼4×1010/cm2) of dots. The dot size can be progressively increased by successive depositions of germanium. GIXD investigations show that dots grow in epitaxial relationship to the silicon (001) substrate and that after few successive depositions, the dots are composed of pure and fully relaxed germanium.

Large Scale Engineered Nanostructured Surfaces by Reactive Ion Etching with Kinetically Self-Assembled Non-continuous Metal Film as Etching Mask

ABSTRACTIn this study, we explored the possibility of using annealed non-continuous metal film enabled etching technique to produce large nano-structured surfaces. Non-continuous Ag film is deposited on silicon wafer with a thin layer of silicon dioxide using E-beam deposition, and then vacuum thermal annealing was applied on the deposited films, causing nano-scaled Ag particles to migrate and agglomerate into self-assembled islands of larger nanometer dimensions. We controlled the density and average size of the metal islands through thickness of the initial film and subsequent annealing rates. Reactive ion etching through the metal islands mask into the underneath silicon dioxide layer was performed following the annealing process. Preliminary hydrophobicity experiments were carried out using the engineered nano-structured surfaces.

Leakage Currents through Thin Silicon Oxide Grown on Atomically Flat Silicon Surfaces

ABSTRACTAtomically flat surfaces can be obtained by high-temperature annealing in UHV of specially patterned silicon samples. Thin silicon oxide layers were grown by dry oxidation on three types of surfaces: (a) atomically flat surfaces, (b) normal (stepped) surfaces cleaned in UHV by the same high-temperature annealing and (c) normal wafer surfaces, which underwent just an RCA chemical cleaning before oxidation. Atomic force microscopy (AFM) was performed to reveal the topography of the surfaces. Aluminum pads were deposited on these oxidized surfaces using photolithography techniques. The leakage current through the oxide was measured for all three cases. Our results show that the smoother the surface before oxidation, the smaller the leakage current.

Oxidation of Si nanocrystals fabricated by ultra-low energy ion implantation in thin SiO2 layers

ABSTRACTThe effect of annealing in diluted oxygen on the structural characteristics of thin silicon dioxide layers with embedded Si nanocrystals fabricated by ultra-low energy ion implantation (1 keV) is reported. The nanocrystal characteristics (size, density, coverage) have been measured by spatially resolved Electron Energy Loss Spectroscopy using the spectrum-imaging mode of a Scanning Transmission Electron Microscope. Their evolution has been studied as a function of the annealing duration under N2+O2 at 900°C. An extended spherical Deal-Grove model for the self-limiting oxidation of embedded silicon nanocrystals has been carried out. It shows that stress effects, due to the deformation of the oxide, slows down the chemical oxidation rate and leads to a self-limiting oxide growth. The model predictions show a good agreement with the experimental results.

Growth of ZnO:Ga thin films at room temperature on polymeric substrates: thickness dependence

In this paper, we present results concerning the thickness dependence (from 70 to 890 nm) of electrical, structural, morphological and optical properties presented by gallium-doped zinc oxide (GZO) deposited on polyethylene naphthalate (PEN) substrates by r.f. magnetron sputtering at room temperature. For thicknesses higher than 300 nm an independent correlation between the electrical, morphological, structural and optical properties are observed. The lowest resistivity obtained was 5×10 −4 Ω cm with a sheet resistance of 15 Ω/□ and an average optical transmittance in the visible part of the spectra of 80%. It is also shown that by passivating the surface of the polymer by depositing a thin silicon dioxide layer the electrical and structural properties of the films are improved nearly by a factor of two.