Silicon Suboxide Research Articles

Coherent states in a coupled quantum dot nanocrystalline silicon transistor

We investigate the coherent coupling of electronic states in a nanocrystalline silicon quantum dot transistor. The device consists of a nanometer-scale point-contact containing only a few silicon grains. The grains form quantum dots, tunnel-coupled across thin silicon sub-oxide grain boundaries. At 4.2K, we observe a pattern of single-electron conductance peaks versus two gate voltages, caused by electrostatic coupling between the quantum dots. Additional peaks are observed when the energy levels from two adjacent quantum dots are resonant, which may be associated with “quasi-molecular” states formed by coherent coupling of the levels. The tunnel splitting obtained from the peak separation is ∼0.4meV, which is from a few times to an order-of-magnitude larger than reported previously in GaAs∕AlGaAs quantum dots.

Experimental characterization of the deposition of silicon suboxide films in a radiofrequency magnetron reactive sputtering system

Sputter deposition of silicon suboxide films in a radiofrequency (rf) magnetron discharge using oxygen gas as a reactive species has been studied by a combination of plasma and film characterization techniques. The deposited films are silicon suboxides (SiO x , 0≤ x≤2). Using an energy-resolved mass spectrometer we found an impingement rate of SiO + ions on the deposition surface, which is comparable in magnitude to that of Si +, so a significant contribution of SiO to the Si and O incorporation in the suboxides cannot be ruled out. Increase of the oxygen flow beyond the point of SiO 2 deposition is characterized by a strong decrease of the Si + ion arrival rate on the growth surface. At the same time, the SiO 2 + arrival rate is increased, indicative for the increased oxidized state of the cathode. Number of atoms of the various kinds in the deposited layers have been determined by using Rutherford backscattering (RBS) and Elastic recoil detection (ERD) and correlated with the results of the plasma characterization.

Inspection of intermediate stress-induced electronic traps in Si/Al2O3 system

The physical characteristics of device-grade thin silicon film at (100) grown on α-Al2O3 substrate using the chemical vapour deposition (CVD) technique has been studied in this paper. Its thickness, crystalline structure, elemental inter-diffusion in the interface region and the quality were characterized by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), core level X-ray photoelectron spectroscopy (XPS) and nuclear resonance reaction 27Al(p,γ)28Si, respectively. The results of stoichiometric defect profile and individual silicon suboxide (such as SiO, and Si2O3 components with respect to the metallic Si element) formation in the intermediate region were observed. The deep traps located around Ec=0.26eV, in ∼500nm thick n-type Si films, were attributed to the defects caused by the strain of the silicon lattice. Raman spectroscopy was used to evaluate the compressive stress in the Si film.

Role of spinodal decomposition in the structure ofSiOx

On the basis of recent calculations we propose that ${\text{SiO}}_{x}$ fulfills the condition for spinodal decomposition. With this in mind we investigated the nanostructure of room temperature, radiofrequent magnetron plasma sputtered ${\text{SiO}}_{x}$ films over the entire composition range $0\ensuremath{\leqslant}x\ensuremath{\leqslant}2$, applying x-ray photoelectron spectroscopy and infrared analysis. We have found that the majority structure of the materials indeed resembles more or less a phase separated mixture, consisting of (small) $a$ -Si regions and (small) ${\text{SiO}}_{2}$ regions with ${\text{Si-SiO}}_{3}$ tetrahedra at the interfaces between the two distinct regions. This structure presumably comes into existence by the operation of the process of spinodal decomposition in the deposition process. This does not seem to hold for sites, where also H (as a minority component in the film) is bonded. We propose that the nanostructure of hydrogenated silicon suboxides is different from that of the nonhydrogenated equivalent material.

Light-induced modification of a-SiOx:H. I: Metastability

Amorphous hydrogenated silicon suboxides (a-SiOx:H) were deposited by plasma enhanced chemical vapor deposition from the source gases SiH4, H2, and CO2. The band gap of the samples can be tuned from 1.9 to 3.0 eV by varying the oxygen content from 0 to 50 at. %. The light-induced degradation of the photoconductivity and the generation of metastable dangling bond defects, known as the Staebler–Wronski effect in hydrogenated amorphous silicon (a-Si:H), were studied in a-SiOx:H for a wide range of oxygen contents up to 44 at. % as a function of illumination time and intensity. Upon intensive illumination above their respective band gaps for several hours at comparable light intensities, all silicon suboxides displayed a similar increase of the subgap defect density and a corresponding decrease of the photocurrent. Also the temporal evolution of the dangling bond density and photoconductivity were found to resemble those of pure amorphous silicon. Apart from an irreversible redshift of the absorption edge for higher oxygen contents (comparable to the effect of photodarkening in chalcogenide glasses), the effects of light degradation could be reversed by an appropriate annealing at temperatures below 200 °C for all oxygen concentrations studied. However, for higher oxygen concentrations ([O]≳30 at. %) partial irreversibilities were observed after repeated degradation cycles.

Light-induced modification of a-SiOx II: Laser crystallization

Amorphous hydrogenated silicon suboxides (a-SiOx:H) were deposited by plasma enhanced chemical vapor deposition from the source gases SiH4, H2, and CO2. The band gap of the samples can be tuned from 1.9 to 3.0 eV by varying the oxygen content from 0 to 50 at. %. H-effused samples were irradiated by ultraviolet laser pulses with intensities up to 480 mJ/cm2. The structural changes and the crystallization behavior were investigated as a function of oxygen content and laser intensity. A decrease of the melting threshold by a factor of two with increasing oxygen content (0–44 at. %) was observed for the SiOx samples. Above the respective melting thresholds, not only a deterioration of the structural properties but also indications of a segregation of Si crystallites were found. Raman spectroscopy and transmission electron microscopy gave evidence for the existence of Si crystallites up to oxygen contents of 40 at. %. The crystal size reached an optimum for oxygen concentrations between 10 and 30 at. %.

Peculiarities of the oxidation of porous silicon during aqueous after-etching

The process of oxidation of the surface of porous silicon in the course of aqueous after-etching has been studied by monitoring the change in the shape of the inverse self-convolution of a SiL23VV peak in the Auger electron spectrum. It was found that the formation of SiOx in the surface layer of samples proceeds in several stages. In the first stage, silicon suboxides composed of SiOnSi4−n tetrahedra (n=1–3) are formed on the surface of silicon quantum filaments. The formation of a SiO2 phase stable with respect to external factors begins only upon 20-h aqueous after-etching and is completed within seven days.

Doping and its efficiency ina−SiOx:H

Amorphous hydrogenated silicon suboxides $(a\ensuremath{-}{\mathrm{SiO}}_{x}:\mathrm{H})$ deposited by plasma enhanced chemical vapor deposition have a band gap which can be tuned from $1.9$ to 3.0 eV by varying the oxygen content [O] from $0$ to 50 at. %. n- and p-type doping is realized by adding ${\mathrm{PH}}_{3}$ and ${\mathrm{B}}_{2}{\mathrm{H}}_{6},$ respectively, to the source gases ${\mathrm{SiH}}_{4},$ ${\mathrm{H}}_{2},$ and ${\mathrm{CO}}_{2}.$ Alloying with increasing amounts of oxygen reduces the average coordination number $〈r〉$ from a value close to $4$ $(a\ensuremath{-}\mathrm{Si}:\mathrm{H})$ to $\ensuremath{\approx}2.7,$ which gradually approaches the ideal value of $〈r〉=2.4$ for network glasses. This goes along with a softening of the amorphous ${\mathrm{SiO}}_{x}$ network, i.e., a reduction of the mechanical hardness of the material, which is also predicted by rigidity percolation theory. Also the incorporation of dopant atoms into electrically active, fourfold coordinated sites becomes more unlikely with increasing [O]. As a consequence, n- and p-type doped ${\mathrm{SiO}}_{x}$ shows increasingly intrinsic character for higher oxygen concentrations. Doping fails for values of $〈r〉<3$ and the doping efficiency tends towards zero. Thus, an overall fourfold coordination was found to be a crucial requirement for efficient doping in amorphous semiconductors.

Si/SiO_2系のための電荷移動原子間ポテンシャルの開発(J08-1 解析手法・モデリング,J08 原子系の計算力学シミュレーション)

It is important to simulate Si/SiO_2 systems which are essential for LSI technology. The molecular dynamics simulation that can deal with thousands of atoms is able to evaluate macroscopic interface physical properties, and also microscopic interface natures derived from atomic stuructures. Becase I think charge transfer effects mainly govern Si/SiO_2 interface natures, I developed an accurate Si/SiO_2 potential based on the band theory that can apply to the charge equilibration method. I fitted this potential with genetic algorithm to density functional theory calculations of silicon crystal, silica crystal, oxygen interstitials in silicon, oxygen vacancies in silica, suboxides of silicon, and c-Si/c-SiO_2.

Magnetic resonances of Fe and Ni nanoparticles in films of silicon suboxide produced by ion irradiation of triethoxysilane gels containing Fe or Ni solute atoms

The structure and magnetic properties of composite films obtained by ion irradiation or thermal treatment of triethoxysilane gels containing iron or nickel in solution are investigated by means of X-ray diffraction, electron microscopy and electron spin resonance. Ion irradiated gels are converted into silica glasses containing metallic nanoparticles with a narrow range of sizes. These particles exhibit a magnetic resonance with a preferential alignment of magnetic moments perpendicular to the surface. This unusual out of plane anisotropy seem to be ascribed to an interaction of the moments with radiation defects. The magnetization in irradiated film is comparable to that in films of same materials submitted to a thermal conversion at 1000 °C in vacuum. But the latter films are porous and contain particles of metal and silicide or silicate with a wider range of sizes.

On the deposition process of silicon suboxides by a RF magnetron reactive sputtering in Ar–O 2 mixtures: theoretical and experimental approach

Magnetron reactive sputtering of silicon oxides produced by Ar–O 2 RF capacitive plasmas has been studied from a theoretical and experimental point of view. In this way a plasma model has been developed, that allows to know the densities of all the species in the discharge, as well as the flow of ions. This model is coupled with a sputtering model, which relates the plasma properties to the deposition process. Therefore, the substrate stoichiometry, as well as the deposition rate, is obtained as a function of the model input parameters, i.e. the argon and oxygen flows, RF power and pumping speed. These results are compared with experimental data on the deposited layers. The stoichiometry of the SiO x layers was determined by Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy measurements. A good agreement between theoretical predictions and experimental data is found in several deposition conditions. A discussion about these results is carried out.

Photoluminescence ofEr3+-implanted amorphous hydrogenated silicon suboxides

Erbium ions incorporated into amorphous hydrogenated silicon suboxides $(a\ensuremath{-}{\mathrm{SiO}}_{x}:\mathrm{H})$ allow to overcome the disadvantages of ${\mathrm{Er}}^{3+}$ in $c\ensuremath{-}\mathrm{Si}$ such as the limited solubility, the strong quenching of the luminescence at room temperature, and the need for co-doping with electronegative atoms. $a\ensuremath{-}{\mathrm{SiO}}_{x}:\mathrm{H}$ alloys have an enhanced Er solubility and easily variable oxygen content, thereby providing favorable atomic environments for an efficient Er luminescence and reduced excitation backtransfer due to deeper localized band-tail states. In the present study, ${\mathrm{Er}}^{3+}$ doses up to $7\ifmmode\times\else\texttimes\fi{}{10}^{14}{\mathrm{cm}}^{\ensuremath{-}2}$ were implanted into a-${\mathrm{SiO}}_{x}:\mathrm{H}$ with oxygen content between 0 and 44 at. %. Optical properties such as the absorption coefficients and the photoluminescence (PL) spectra of the ${\mathrm{Er}}^{3+}$ ions and of the ${\mathrm{SiO}}_{x}$ host were investigated as a function of erbium implantation dose, oxygen content, defect density, temperature, and annealing treatment. It was found that annealing is a requirement for activating the characteristic Er PL at 1.54 \ensuremath{\mu}m mainly due to a reduction of implantation induced defects. The intensity of both the intrinsic ${\mathrm{SiO}}_{x}$ and the Er PL was found to be inversely proportional to the defect density as measured by electron spin resonance or subgap absorption. The Er PL is additionally enhanced upon annealing, probably as a result of better structural arrangements of the Er ions. The Er PL intensity increases approximately linearly with the implantation dose. An increase of the oxygen content (and correspondingly of the optical band gap) of $a\ensuremath{-}{\mathrm{SiO}}_{x}:\mathrm{H}$ causes no drastic changes in the erbium luminescence energy and intensity, whereas the intrinsic PL shifts to higher photon energies according to the larger band gap. Already low O concentrations of a few percent provide favorable Er environments. The main advantage of $a\ensuremath{-}{\mathrm{SiO}}_{x}:\mathrm{H}$ as a host matrix is revealed by temperature-dependent PL measurements. For high oxygen contents, the thermal quenching of both the ${\mathrm{Er}}^{3+}$ and the intrinsic PL is strongly reduced. In an $a\ensuremath{-}{\mathrm{SiO}}_{x}$ sample with 44 at. % oxygen, the Er PL is only quenched by 20% between 77 and 300 K. In contrast, the quenching of the intrinsic PL for all [O] is roughly one order of magnitude stronger than that of the Er PL. These PL measurements were complemented by PL excitation experiments over a wide spectral range. We have observed that the ${\mathrm{Er}}^{3+}$ PL is excited about one order of magnitude more efficiently when pumped with sub-band-gap light compared to band-to-band excitation. The experimental results are discussed with regard to the two currently proposed ${\mathrm{Er}}^{3+}$ excitation models, the defect-related Auger effect and the F\orster transfer mechanism.

Study on Er3+ emission from the erbium-doped hydrogenated amorphous silicon suboxide film

The erbium-doped hydrogenated amorphous silicon suboxide films containing amorphous silicon clusters were prepared. The samples exhibited photoluminescence peaks at around 750 nm and 1.54 μm, which could be assigned to the electron-hole recombination in amorphous silicon clusters and the intra-4f transition in Er3+, respectively. Correlations between the intensities of these two photoluminescence peaks and oxidation and dehydrogenation of the films during annealing were studied. It was found that the oxidation is triggered by dehydrogenation of the films even at low annealing temperatures, which decisively changes the intensities of the two photoluminescence peaks. On the other hand, the increase of Er content in the erbium-doped hydrogenated amorphous silicon suboxide film will enhance Er3+ emission at 1.54 μm, while quench amorphous silicon cluster emission at 750 nm, such a competitive relationship, was also observed in the erbium-doped silicon nanocrystals embedded in SiO2 matrix. Moreover, we found that Er3+ emission is not sensitive to whether silicon clusters are crystalline or amorphous. The amorphous silicon clusters can be as sensitizer on Er3+ emission as that of silicon nanocrystals.

Infrared spectra of photochemically grown suboxides at the Si/SiO2 interface

Infrared (IR) spectra of ultrathin silicon-oxide layers, grown by pulsed F2-laser (157 nm) photoinduced oxidation of H-terminated Si(111) and Si(110) in an oxygen atmosphere, were monitored in situ with monolayer sensitivity. Oxygen atoms play an essential role in this laser-induced oxidation process occurring at room temperature. The IR spectra of the various oxidation states of silicon (suboxides) in the interfacial region recorded for oxides grown by the self-limiting photochemical oxidation process are compared with those of thermal oxide layers of 22 nm thickness obtained by a top-down wet-chemical thinning procedure. The characteristic features of the suboxide spectra recorded for normal and oblique incidence of the analyzing IR beam are discussed.

Controlled photoluminescence from silicon nanocrystals in a vertical optical cavity with distributed Bragg reflectors

Optical transmission and photoluminescence (PL) spectra of silicon nanocrystals (nc-Si) dispersed in a vertical optical cavity are measured at room temperature. The optical microcavity is constructed between a distributed Bragg reflector (DBR) and a gold film. The samples including the DBR structure are based on silicon suboxide (SiOx with x < 2.0) deposited by a plasma-enhanced chemical vapour deposition method. Subsequent thermal annealing of the deposited film leads to the formation of nc-Si of 2–3 nm in size. The bandwidth of the PL from nc-Si in the optical cavity is narrowed to ∼15 nm, whereas it is originally ∼180 nm. The results are interpreted as effects of controlled spontaneous emission due to the optical cavity.

Pulsed laser ultrahigh vacuum deposited silicon in the presence of excess cesium and oxygen studied with x-ray photoelectron spectroscopy and atomic force microscopy

Si/Cs/O clusters were prepared in an ultrahigh vacuum system with pulsed laser ablation of Si in the presence of Cs vapor and molecular oxygen. The effects of an O2 ambient, the pressure, and the deposition sequence of Cs and O2 on the oxidation state of cesium oxides and silicon oxides, as well as on the formation of Si/Cs/O clusters, were studied with x-ray photoelectron spectroscopy and atomic force microscopy (AFM). Oxygen 1s spectra were deconvoluted to cesium oxide(s) at around 530.5 eV and silicon oxide(s) at 532.3 eV. From the low binding energy side to the high binding energy side, Si 2p spectra were assigned to negatively charged Si clusters (94 and 96 eV), silicon suboxide (102 eV), SiO2 (104 eV), and Si/Cs/O clusters (106 eV), respectively. The high binding energy species was explained by the formation of Si(CsO1+x)n clusters. Surface morphologies were influenced by the oxidation state of cesium oxides and their amount. The work function decrease as much as 0.5±0.1 eV was measured with ex situ AFM.

Role of amorphous silicon domains on Er 3+ emission in the Er-doped hydrogenated amorphous silicon suboxide film

An investigation on the correlation between amorphous Si (a-Si) domains and Er3+ emission in the Er-doped hydrogenated amorphous silicon suboxide (a-Si:O:H ) film is presented. On one hand, a-Si domains provide sufficient carriers for Er3+ carrier-mediated excitation which has been proved to be the highest excitation path for Er3+ ion; on the other hand, hydrogen diffusion from a-Si domains to amorphous silicon oxide (a-SiOx) matrix during annealing has been found and this possibly decreases the number of nonradiative centres around Er3+ ions. This study provides a better understanding of the role of a-Si domains on Er3+ emission in a-Si:O:H films.

Investigation of Si nanocluster formation in sputter-deposited silicon sub-oxides for nanocluster memory structures

In silicon nanocluster (Si-NC) memories, Si-NC embedded in the gate oxide of an MOSFET are used to store and release electrons thereby modifying the threshold of the transistor. This paper describes the formation of its core functional structure, the Si-NC MOS memory capacitor, by annealing a SiO 2/SiO x ( x<2) stack, deposited onto a thin direct tunneling oxide on silicon by a sputtering method. To achieve a high density of isolated Si-NC, both, the initial silicon excess in the SiO x layer and the thermal annealing treatment must be optimized. Optimum conditions are expected just at the end of the nucleation stage of the NC ensemble. Therefore, the effect of various rapid thermal anneals (RTA) on SiO x /Si structures with different silicon excess x was investigated focusing on two optical methods: infrared absorption and photoluminescence (PL). The rate of nucleation and phase separation were found to depend strongly on both, silicon excess and annealing temperature. The characteristic size dependence of Si-NC PL was used to differentiate annealing regimes yielding NC ensembles being ‘frozen’ in the process of nucleation or in their growth/ripening phase, respectively. Additionally, Si-NC MOS memory capacitors were prepared and characterized using both, optical and electrical tests. The devices prepared under optimized conditions yield a high flat-band shift and good charge retention characteristics.

Characterization and modeling of the nitrogen passivation of interface traps in SiO2/4H–SiC

The relationship between nitrogen content and interface trap density (Dit) in SiO2/4H–SiC near the conduction band has been quantitatively determined. Nitridation using NO significantly reduces Dit near the conduction band, but the effect saturates after ≈2.5×1014 cm−2 of nitrogen. These results are consistent with a model of the interface in which defects such as carbon clusters or silicon suboxide states produce traps with energies corresponding to the sizes of the defects. Nitrogen passivation results in the dissolution of the defects, which then lowers the energies of the traps in the band gap.

Surface functionalization of amorphous silicon and silicon suboxides for biological applications

We have studied the chemical modification and functionalization of the surface of hydrogenated amorphous silicon (a-Si:H) and silicon suboxides (a-SiO x :H) for the purpose of selective cell adhesion. The preparation of hydrophobic/hydrophilic contrast on the amorphous surfaces has been investigated systematically as a function of the oxygen content by contact angle measurements. Successful functionalization of a-Si:H by hydrosilylation with methyl acrylate is reported and analyzed by thermal desorption studies as well as enzyme linked immunosorbent assays using RGD-peptide derivatives.