Ultrathin Si Oxide Research Articles

Hydrogenation by catalytically generated atomic hydrogen for passivating contacts in crystalline silicon solar cells

Hydrogenation employing catalytically generated atomic hydrogen (Cat-H) is implemented on stacks of n-type polycrystalline silicon (poly-Si)/Si oxide (SiOx) to enhance passivation quality. The utilization of Cat-H demonstrates a substantial enhancement in surface passivation quality on crystalline Si (c-Si), resulting in an increase of the effective minority carrier lifetime (τeff) from 2.0 to 3.9 ms. The investigation reveals a dependency of τeff on the duration of Cat-H treatment: an initial rise followed by a plateau or slight decline. Secondary ion mass spectrometry (SIMS) measurements revealed the diffusion of H atoms into poly-Si and c-Si, which accumulate at the poly-Si/c-Si interface. This accumulation terminates dangling bonds in the poly-Si layer on the c-Si surface. Furthermore, the presence of an ultra-thin thermal Si oxide layer on the poly-Si layer formed during high-temperature annealing is crucial for protecting the film against etching induced by Cat-H.

Percolation threshold in annealed ultrathin SiOx films by 2D Monte Carlo simulations

In this work, the kinetics of phase separation in ultrathin non-stoichiometric Si oxide (SiOx, x < 2) films during high-temperature annealing is studied by Monte Carlo simulations on the two-dimensional...

Ultrathin Al-doped SiO x passivating hole-selective contacts formed by a simple wet process

Ultrathin Al-doped Si oxide (SiO x ) layers were formed by a simple wet chemical treatment, and their hole-selective passivating contact and electrical properties were investigated. From the evaluated contact resistivity (ρ c) and saturation current density (J 0), carrier selectivity (S 10) was estimated to be 13.3. Moreover, in Si nitride (SiN y )/Al-doped SiO x stacks, negative values of fixed charge density (Q f) were obtained, despite a high positive Q f existing in the single SiN y layer. This result implies that Al-doped SiO x has high negative fixed charges and overcompensates the charge polarity in the stacks, which forms an inversion layer and accumulates holes on the Si surface. Furthermore, the negative fixed charges realize excellent carrier separation by the induced upward band bending. In addition, we proposed a novel device architecture named Al-induced charged oxide inversion layer solar cells and confirmed device operation in a simple device configuration.

Density Functional Theory Study on Stability of Fe, Cu, and Ni Atoms Near (001) Surface of Si Wafer

“Metal gettering” technology is becoming increasingly important to keep the active region of state-of-the-art electronic devices extremely clean. Metal atoms entering the silicon (Si) wafer will be out-diffused at the wafer surface if the internal gettering (IG) does not work well. Therefore, an understanding the stability of metal atoms near the Si (001) wafer surface is important in addition to understanding the IG mechanism. The calculations with density functional theory are preformed to investigate the stability of Fe, Cu, and Ni atoms near the (001) surface with and without ultra-thin oxide film. The following main results are as obtained. (1) Compared to the Si bulk, Fe, Cu, and Ni atoms are more stable up to the fifth atomic layer from the Si (001) surface. (2) Fe0, Fe+, Ni0, and Cu0 atoms at the wafer surface are energetically very stable compared to those internally gettered by B atom, with the order of Fe0 > Fe+ > Ni0 > Cu0, while Cu+ is not so stable at the wafer surface. (3) Just below the ultra-thin Si oxide film, the formation energy of Fe, Cu, and Ni atoms is increased. Furthermore, the segregation coefficient of these metals between the surface region and Si bulk was evaluated.

Nano spin-diodes using FePt-NDs with huge on/off current ratio at room temperature.

Spin transistors have attracted tremendous interest as new functional devices. However, few studies have investigated enhancements of the ON/OFF current ratio as a function of the electron spin behavior. Here, we found a significantly high spin-dependent current ratio—more than 102 at 1.5 V—when changing the relative direction of the magnetizations between FePt nanodots (NDs) and the CoPtCr-coated atomic force microscope (AFM) probe at room temperature. This means that ON and OFF states were achieved by switching the magnetization of the FePt NDs, which can be regarded as spin-diodes. The FePt magnetic NDs were fabricated by exposing a bi-layer metal stack to a remote H2 plasma (H2-RP) on ~1.7 nm SiO2/Si(100) substrates. The ultrathin bi-layers with a uniform surface coverage are changed drastically to NDs with an areal density as high as ~5 × 1011 cm−2. The FePt NDs exhibit a large perpendicular anisotropy with an out-of-plane coercivity of ~4.8 kOe, reflecting the magneto-crystalline anisotropy of (001) oriented L10 phase FePt. We also designed and fabricated double-stacked FePt-NDs with low and high coercivities sandwiched between an ultra-thin Si-oxide interlayer, and confirmed a high ON/OFF current ratio when switching the relative magnetization directions of the low and high coercivity FePt NDs.

Photoelectron spectroscopy as an in situ contact-less method for studies of MOS properties of ultrathin oxides on Si

The electric field across an ultrathin and uniform Si-oxide layer on a Si (111) surface and the positions of the valence band edges at the Si-oxide/Si (111) interface have been probed by high-resolution synchrotron radiation induced photoemission spectroscopy, as an in situ contact-less method. Variation of the “gate bias” is achieved by depositing Sn nanoparticles on the ultrathin oxide surface. These nanoparticles, growing as isolated hemi-spherical islands, attract various quantities of negative charges from the substrate inducing a potential difference between the Sn islands/Si-oxide and Si-oxide/Si (111) interface. This method allows us to study and extract the locally varying electric field and changes in the positions of the edges of the valence bands by measuring the valence band spectra and the Si 2p and Sn 4d core-levels at different Sn coverages. The ultrathin (0.8nm thick) Si-oxide layer is grown in a simple and traceable self-limiting thermal process on a clean Si (111) surface. The oxide grown in this way creates flat bands. The properties of the system of Sn islands grown on this system are also determined. The induced electric field in the oxide varies linearly with the amount of Sn deposited per area.

Photoemission study of the identification of Mn silicate barrier formation on carbon containing low-κ dielectrics

In this study X-ray photoelectron spectroscopy (XPS) has been used to characterize manganese silicate Cu diffusion barrier layer formation on a range of ultralow-κ (ULK) carbon doped oxide (CDO) layers. Ultra-thin Si and Mn oxide films were deposited in order to provide accurate binding energy (BE) references for the Si 2p and O 1s core levels of the dielectric materials. The results indicate that there is a strong correlation between carbon content in the CDO films and the BE position of both the Si 2p and O 1s core level peaks. Furthermore, it has been shown that the full width half maximum (FWHM) of these peaks are significantly larger than those observed in SiO2 leading to complications in the analysis and identification of barrier layer formation on these low-κ substrates. In a separate experiment, the deposition and high temperature annealing of thin fully oxidized Mn layers (MnOy, where y⩾1) on these ULK CDO substrates suggests the formation of an interface layer consistent with MnSiO3 based on analysis of both the O 1s and Mn 2p core level spectra. It is also shown that the use of oxidized Mn films tend to minimize the formation of Mn carbide within the barrier layer region in agreement with previous studies on other CDO substrates.

Evaluation of Chemical Bonding Features and Resistance Switching Behaviors of Ultrathin Si Oxide Dielectric Sandwiched Between Pt Electrodes

Evaluation of Chemical Bonding Features and Resistance Switching Behaviors of Ultrathin Si Oxide Dielectric Sandwiched Between Pt Electrodes

Evaluation of Chemical Bonding Features and Resistance Switching Behaviors of Ultrathin Si Oxide Dielectric Sandwiched Between Pt Electrodes

We investigated the chemical bonding features and resistance switching properties of ultrathin Si-rich oxides sandwiched between Pt electrodes to evaluate the feasibility of SiOx-based resistance random access memories (ReRAMs). In the early stages of SiOx deposition on Pt by radio-frequency (RF) sputtering in Ar + O2 gas mixture at 300 °C, the formation of a PtOx layer at the interface between SiOx and the Pt bottom electrode was observed. This interfacial PtOx layer decreased in thickness with increasing SiOx thickness. With decreasing as-deposited SiOx thickness down to 3.3 nm, the initial electrical state changed from the high resistance state (HRS) to the low resistance state (LRS), and resistance switching behavior was observed without the forming process. To gain a better understanding of the resistance switching mechanism, especially the role of oxygen deficiency in the SiOx network, we investigated the impact of O2 annealing after SiOx deposition on the switching behavior. The resistance switching behaviors were barely detectable for the samples at O2 annealing temperatures of over 500 °C. The results imply that the oxygen vacancies in the SiOx play an important role in resistance switching.

High resolution photoemission study of the formation and thermal stability of Mg silicide on silicon

The focus of this study is to use high resolution synchrotron based photoemission to investigate the initial growth mode of magnesium silicide which has been formed by both stepwise and continuous deposition of metallic Mg onto a thermally grown ultra-thin Si oxide surface. The findings suggest that stepwise deposition of Mg initially results in the growth of Mg silicide islands on the surface. Further magnesium deposition leads to the growth of metallic Mg on the surface of these silicide islands, along with the continued growth of silicide species on the uncovered oxide surface. However, it has been shown that continuous deposition of Mg results in considerably less silicide growth. The thermal stability of Mg silicide and a mechanism for high temperature silicide growth have also been studied using conventional X-ray photoelectron spectroscopy. The results suggest that the presence of oxidised Si acts as a barrier to Si diffusion during vacuum annealing, hence preventing the growth of further Mg silicide. It has also been shown that metallic Mg desorbs from the surface below 300°C, while Mg silicide is not stable at temperatures above 500°C in contrast to other metal silicides.

Characterization of semiconductor nanostructures formed by using ultrathin Si oxide technology

We present a method to form semiconductor nanodots on Si substrates by using ultrathin Si oxide technology and the results on their optical properties. We can form ultra-small semiconductor nanodots with the size of ∼5 nm and ultra-high density of ∼10 12 cm −2 on Si surfaces covered with ultrathin SiO 2 films of ∼0.3 nm thickness. We focus on photoluminescence and electroluminescence properties of Ge nanodots embedded in Si films. These structures exhibit intense luminescence in the energy region of about 0.8 eV.

In situ monitoring of nucleation and evolution of Ge nanodots on faintly oxidized Si(1 1 1) surfaces

We have investigated the nucleation and evolution of germanium (Ge) nanodot (ND)s taking place while depositing Ge onto the silicon (Si) (1 1 1) surfaces with ultra-thin Si oxide films by using ultra-high vacuum in situ high-resolution transmission electron microscopy in the profile-imaging geometry. Various types of growth phenomena such as nucleation, growth and coalescence of Ge NDs have successfully been observed. The results show that the growth phenomena of the Ge NDs are dramatically rapid after their size reaches the size of the critical nucleus. The critical nucleus size estimated from a model using the cohesive energy of the Ge NDs has been consistent with observed one.

Opto-Electronic Properties of Ge and Si Related Nanostructures on Ultrathin Si Oxide Covered Si Surfaces

AbstractWe present a method to form semiconductor nanodots on Si substrates by using ultrathin Si oxide technology and the results on their opto-electronic properties. We can form ultra-small semiconductor nanodots with the size of ˜5nm and ultra-high density of ˜1012 cm−2 on Si surfaces covered with ultrathin SiO2 films of ˜0.3nm thickness. We focus on the Ge and GeSn nanodots on Si substrates and those embedded in Si films. These structures exhibit quantum confinement effects and intense luminescence in the energy region of about 0.8 eV.

Chemical phase transitions of a Si oxide film on SiC by MeV electron beam irradiation

An ultrathin Si oxide film grown on a 6H-SiC(0001) wafer was irradiated with 1MeV electron beam to examine its effect on the chemical species of a Si oxide/SiC wafer, where the Si oxide film was composed of SiO2, Si suboxides (Si3+, Si2+, and Si1+), and Si oxycarbides (Si–C–O). Scanning photoelectron microscopy and Si 2p core-level spectroscopy show that e-beam irradiation induces chemical phase transitions from the Si suboxides and Si oxycarbides to SiO2. This suggests that e-beam irradiation is an efficient and simple method of producing a chemically uniform SiO2 film on SiC without thermal and chemical treatments.

Observation of the quantum-confinement effect in individual Ge nanocrystals on oxidized Si substrates using scanning tunneling spectroscopy

Scanning tunneling spectroscopic studies revealed the quantum-confinement effects in Ge nanocrystals formed with ultrahigh density (>1012cm−2) by Ge deposition on ultrathin Si oxide films. With decreasing crystal size, the conduction band maximum upshifted and the valence band minimum downshifted. The energy shift in both cases was about 0.7 eV with the size change from 7 to 2 nm. This shows that the energy band gaps of Ge nanocrystals increased to ∼1.4eV with decreasing size. This size dependence can be explained by the quantum-confinement effect in Ge nanocrystals.

Formation of strained iron silicide nanodots by Fe deposition on Si nanodots on oxidized Si (111) surfaces

We studied the epitaxial growth of iron silicide ({epsilon}-FeSi,{beta}-FeSi{sub 2}, and {alpha}-FeSi{sub 2}) nanodots on Si (111) substrates by Fe deposition on Si nanodots on Si (111) substrates with ultrathin Si oxide films using reflection high-energy electron diffraction, scanning tunneling microscopy, and transmission electron microscope (TEM). We formed almost single phase iron silicide nanodots by controlling the Fe deposition conditions; growth temperature, deposition rate, and amount. The {epsilon}-FeSi or {alpha}-FeSi{sub 2} nanodots were epitaxially grown in a dome shape with an average size of {approx}5 nm and an ultrahigh density (>10{sup 12} cm{sup -2}) on the surface. We formed {approx}2-nm high and {approx}8-nm wide {beta}-FeSi{sub 2} nanodots in a dome shape with a density of {approx}5x10{sup 11} cm{sup -2} on the surface. Cross-sectional TEM images revealed that the {beta}-FeSi{sub 2} growth continued beneath the Si surface. The part of the {beta}-FeSi{sub 2} nanodot beneath the surface was a disk shape, which was {approx}5 nm thick and {approx}20 nm wide, with an abrupt interface parallel to the Si (111) plane. The nanodots were epitaxially grown and had almost no misfit dislocations near the {beta}-FeSi{sub 2}/Si interface, which were strained by the lattice mismatch. The size distribution of the iron silicide dotsmore » was determined by Ostwald ripening with the activation energy of dot edge mobility of {approx}1.3 eV.« less

Growth Kinetics and Electrical Properties of Ultrathin Si Oxide Film Fabricated Using Krypton-Diluted Oxygen Plasma Excited by Electron Cyclotron Resonance

The growth characteristics and electrical properties of a thin Si oxide film formed by Kr and O2 mixed ECR plasma irradiation at a low temperature of 130°C have been investigated. The Kr gas is better than Ar for diluting the O2 plasma for Si oxidation according to the current–voltage characteristics of the Si oxide film. It is found that the surface-reaction-limited growth dominates the initial stage of oxidation while the diffusion-limited growth dominates the thicker region. A large positive bias and a small O2 gas flow rate lead to the decrease in the leakage current and the increase in the breakdown electric field of the Si oxide film, implying that the oxidation in the initial stage is essential for obtaining good electrical properties. Under the optimum condition, the as-grown Si oxide film shows a breakdown electric field of 10–12 MV/cm, and for more than 80% of the samples a leakage current density at an electric field of 5 MV/cm distributes in the range of 10-9 or 10-8 A/cm2.

Surface morphology of three-dimensional Si islands on Si(0 0 1) surfaces

Silicon deposition on ultrathin Si oxide films was used for growing three-dimensional (3D) Si islands on Si(0 0 1) substrates. While it was previously demonstrated that at growth temperatures from 450 to 570 °C hemispherical Si islands form which grow epitaxially with respect to the Si(0 0 1) substrate, we show that these islands exhibit an irregular surface structure. Our LEED data also reveal that pyramidal Si islands, appearing at growth temperatures between 570 and 680 °C, have {1 1 3} facets with a 3 × 1 reconstruction on the sidewalls. With increasing growth temperature, the decomposition rate of the Si oxide film gradually increases and the film completely disappears at T∼640 °C, whereas the pyramidal islands are observed at the growth temperatures up to 680 °C. This result indicates that the local minimum of the Si surface energy at {1 1 3} orientation supports the growth of pyramids on Si(0 0 1) surfaces under the flux of Si atoms even after complete decomposition of the Si oxide film.

Defects at the interface of ultra-thin VUV-grown oxide on Si studied by electron spin resonance

Ultra-thin (∼2nm) Si-oxide films, grown by vacuum ultraviolet (VUV) enhanced oxidation of Si at 300K, were studied using electron spin resonance monitoring of Si dangling bond-type interface defects. As a major impact of VUV photons, large densities (up to ∼9×1012cm−2) of Pb and Pb0 centers (interfacial (Si3Si)) are observed in VUV-grown (111) and (100) Si/SiO2, respectively. Their features indicate that, as compared to standard thermal Si/SiO2, the VUV Si/SiO2 interface is under much enhanced stress. No Pb1 defects are observed in VUV (100) Si/SiO2, ascribed to lack of high temperature oxide relaxation. This may appear pertinent as to the understanding of the defect’s specific role in the interface structure. Microscopic understanding is provided for the known inferior electrical interface quality threatening low thermal budget oxide fabrication.

Paramagnetic defects at the interface of ultrathin oxides grown under vacuum ultraviolet photon excitation on (111) and (100) Si

Electron-spin-resonance monitoring of Si dangling-bond-type interface defects is used to study ultrathin (∼20 Å) Si-oxide films grown by vacuum ultraviolet (VUV)-enhanced oxidation of Si at 300 K. Large densities (up to ∼9×1012 cm−2) of Pb and Pb0 centers (interfacial Si3≡Si⋅) are observed in VUV-grown (111) and (100) Si/SiO2, respectively. As compared to standard thermal Si/SiO2, two major differences emerge: the VUV Si/SiO2 interface is under substantially enhanced stress, while no Pb1 defects are discerned in VUV (100) Si/SiO2. It is inferred that Pb1 generation requires a minimum amount of oxide relaxation. Microscopic understanding is provided for the known inferior electrical interface quality threatening low-thermal-budget oxide fabrication.