Search

International audience; The present work demonstrates the possibility to use liquid phase epitaxy to incorporate nitrogen in epitaxial GaAsN/GaAs and GaInAsN/GaAs heterostructures, including nanoscaled ones. The structures are grown from Ga - and GaIn - melts containing polycrystalline GaN as a nitrogen source. The red shift of the absorption spectra corresponds to nitrogen content in the epitaxial layers near or less than 0.2 at %. Photoluminescence spectra of dilute nitride GaAsN and GaInAsN show emission from localized nitrogen states - N-nanoclusters of more than two N atoms. These studies show that the melt grown dilute GaAsN and GaInAsN nanostructures can be used for solar cells with extended long wavelength edge. (C) 2010 Published by Elsevier Ltd

We report the fabrication of buried contact solar cells using porous silicon as sacrificial layer to create well-defined channels (for buried contacts) in silicon. In this paper, the salient features of the technology have been presented. No detrimental effect was found in the performance of buried contact solar cell with partially filled contact area compared to the solar cells having conventional planar contacts. However, a marked difference in the short circuit current density was seen when channel was fully filled with metal by screen printing, without degradation in the open-circuit voltage. It is expected that improved processing in combination with optimized buried metallic contact parameters may yield higher efficiencies that may result in substantial decrease in solar cell cost.

Samples prepared by ion-beam synthesis are studied. A two-step 56 Fe + ion implantation process was performed on n-type Si wafers with (100) orientation. Subsequently, the samples, implanted with the same dose and energies, were subjected to rapid thermal annealing at two different temperatures – 800 °C and 900 °C for the same time – 90 s. A remarkable difference in the infrared spectra and in the refractive index dispersions near the band edge of samples annealed at different temperatures was found. The behaviour of the optical properties was related to different morphology of the samples.

The current-voltage dependences of heterojunctions between the silicon and the semiconducting metal silicides β-FeSi2 and MgSi2, prepared by ion-beam synthesis, followed by rapid thermal annealing, are reported. It was found that the anisotype junctions showed a rectifying behavior, reflecting the heterojunctions energy band diagrams, constructed on the base of the previous optical and structural investigations.

High-quality tumour DNA is essential for any personalised treatment strategy based on NGS. Here we show that methanol fixation is superior to formalin, greater DNA yield, longer fragment size and more accurate copy-number calling using sWGS. We also show provide a new approach to understand fixation artefacts using non-negative matrix factorization., Background Next-generation sequencing (NGS) of tumour samples is a critical component of personalised cancer treatment, but it requires high-quality DNA samples. Routine neutral-buffered formalin (NBF) fixation has detrimental effects on nucleic acids, causing low yields, as well as fragmentation and DNA base changes, leading to significant artefacts. Patients and methods We have carried out a detailed comparison of DNA quality from matched samples isolated from high-grade serous ovarian cancers from 16 patients fixed in methanol and NBF. These experiments use tumour fragments and mock biopsies to simulate routine practice, ensuring that results are applicable to standard clinical biopsies. Results Using matched snap-frozen tissue as gold standard comparator, we show that methanol-based fixation has significant benefits over NBF, with greater DNA yield, longer fragment size and more accurate copy-number calling using shallow whole-genome sequencing (WGS). These data also provide a new approach to understand and quantify artefactual effects of fixation using non-negative matrix factorisation to analyse mutational spectra from targeted and WGS data. Conclusion We strongly recommend the adoption of methanol fixation for sample collection strategies in new clinical trials. This approach is immediately available, is logistically simple and can offer cheaper and more reliable mutation calling than traditional NBF fixation.

The unpolarized Raman scattering spectra of ion beam synthesized Mg 2 Si phase, embedded in (100) Si matrix are studied. We used two different doses for the Mg ions, 4 × 10 17 cm -2 and 8 × 10 17 cm -2 , each of them with two different energies 40 keV and 60 keV, which were implanted into (100) Si substrates. After implantation, the samples were subjected to rapid thermal annealing for different durations. The samples' surfaces were studied by scanning electron microscopy (SEM). On the basis of the Roman spectra behavior, conclusions were drawn about the influence of the technological parameters on the formation of the Mg 2 Si phase.

The crystal structure of beta-FeSi2 phase, prepared by ion beam synthesis (IBS) method, followed by rapid thermal annealing (RTA) is investigated by grazing incident asymmetric X-ray diffraction (GIAXRD). The X-ray spectra, obtained at different grazing angles, indicated that the beta-FeSi2 phase is formed in the whole implantation range. From the comparison of the reflections intensities ratios, it is found that in the metal-deficient regions, where the beta-FeSi2 phase is present in the form of precipitates, the crystallites orientation is influenced by the one of the silicon substrates, while the orientation in the metal-rich region is different and depends on the annealing temperature. (C) 2004 Elsevier Ltd. All rights reserved.

Quasi-continuous layers of β - FeSi 2 phase, formed by ion beam synthesis (IBS), followed by rapid thermal annealing (RTA), are studied. The infrared spectra (IR) of the samples under investigation indicate different prevailing orientation of the β-FeSi 2 crystallites in samples, produced at different fabrication regimes. The optical constants are calculated from the experimental reflectance ( R ) and transmittance ( T ) spectra, using a general matrix method. The value of the optical dielectric function of samples with different orientations of the crystallites is determined from the quantitative interpretation of the refractive index dispersions in the range of transparency.

Monophase β-FeSi2 layers, fabricated using ion beam synthesis with high implantation doses followed by rapid thermal annealing are studied. The structure of the films was analyzed by Rutherford backscattering, X-ray diffraction and atomic force microscopy. The average grain sizes of the layers, formed under different annealing regimes were determined. The room temperature reflectance and transmittance spectra were measured in the range from 0.2 to 1.3 eV . The spectra were analyzed in terms of a multilayered structure. The dispersion of the optical constants was calculated using a generalized matrix method, accounting for the surface and interface roughness. From the optical constants dispersions the effective optical conductivity and that in the grains were estimated. The grain sizes were found to influence the optical conductivity.

The β -FeSi 2 phase was fabricated using ion beam synthesis with high doses of implantation followed by rapid thermal annealing. The as-implanted samples and samples annealed under different conditions are studied by grazing incidence asymmetric X-ray diffraction and atomic force microscopy. The effect of the annealing temperature and time on the β -FeSi 2 crystal structure and surface morphology is investigated.

Microhardness measurements of structures, obtained by a solid-state reaction of iron with silicon are undertaken. The experimental results support the finding that the indentation size effect (ISE) appears in the microhardness-depth profile whenever the indentor crosses an interface. When modelling the experimental microhardness-depth profiles of the multilayered structures, an analytical expression for the coefficient giving the contributions of the film and the substrate hardnesses to the composite one is offered. A value of 9.60 GPa is obtained for the microhardness of β-FeSi2 thin layer. On the base of the microhardness-depth profiles measurement conclusions about the solid-state reaction process are drawn.

The $\ensuremath{\beta}\ensuremath{-}{\mathrm{FeSi}}_{2}$ phase was fabricated using two different techniques: ion-beam synthesis and solid-state reaction of a thin Fe layer with Si substrate. The crystal structure of the films was investigated by grazing incident asymmetric x-ray diffraction. The generalized matrix method was used to obtain the dispersions of the absorption coefficient $\ensuremath{\alpha}(E)$ and of the refractive index $n(E)$ from the experimental transmittance and reflectance spectra, accounting for the surface and interface roughness. From $\ensuremath{\alpha}(E)$ and $n(E)$ dependences a direct band-gap energy, ${E}_{g}=0.80$ eV was determined. When interpreting quantitatively the $\ensuremath{\alpha}(E)$ dependences, the Burstein-Moss effect was considered.

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n + p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n + emitter. The electrical properties of the samples with PS were improved with decrease of the n + p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm 2 using n + region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.

The performance improvement of solar cells due to the formation of a porous silicon layer on deep-junction n+/p silicon structures has been investigated. The photovoltaic properties of two identical sample groups with and without porous silicon layer are compared. More than a 50% improvement in Isc and about a 5% increase in Voc was demonstrated in structures with PS layer. No change in the fill factor was observed. The performance of the porous silicon solar cell has been found to be sensitive to the PS layer thickness. Optimal thickness was determined using the electrical measurement. The main result is that the photovoltaic energy conversion efficiency of solar cells can be increased by the PS layer. This low-cost PS formation process can be applied in the photovoltaic cell technology as a standard procedure. © 1997 Elsevier Science S.A.

The semiconducting metal silicides and Mg2Si in particular have been extensively studied in the last few years because of the following reasons: the materials have the potential to be successfully integrated in the optoelectronic silicon technology, the materials are nontoxic and the constituting elements are found in a large amount in nature, and lastly the formation of nanocrystals in a perticular matrix offers the possibility of semiconducting structures with new properties. The Mg-Si phase diagram [1] shows that Mg2Si is the only silicide in this system and it melts congruently at 1085 ◦C. Therefore, crystal growth from the melt should be possible, but one has to be careful about the considerable evaporation of the Mg component as the boiling point of pure Mg is very close to the compound melting temperature. Growth by chemical vapor transport is hardly possible because of the lack of stable magnesium halides and the growth of Mg2Si bulk material has been accomplished exclusively from a melt [2–5]. The compound Mg2Si has, similar to Si, a cubic structure with a larger lattice parameter. The lattice mismatch for the Mg2Si (111) face on the Si (111) face is 1.9%. Due to the comparatively small lattice mismatch high quality epitaxial growth of Mg2Si onto monocrystalline silicon appears to be possible [6]. However thin film formation is difficult because of the low condensation and high vapor pressure of Mg and that is why thin film preparation and studies are scarce. The first reports on Mg2Si thin film formation [7, 8] note that the silicidation proceeds rather rapidly in Mg/Si thin film substrates at relatively low temperatures and silicide columns matching the monocrystalline silicon substrate are formed. There has been an attempt to fabricate Mg2Si by laser melting of magnesium film deposited on a Si substrate [9]. The synthesized layers contain both silicides and Si crystallites. Mahan et al. [10] have prepared Mg2Si films, textured in the (111) direction by molecular-beam epitaxy. They find that intended reactive deposition of magnesium onto silicon substrate at temperatures from 200 to 500 ◦C results in no accumulation of magnesium. However, codeposition of magnesium and silicon at 200 ◦C, using a magnesium-rich flux ratio, gives stoichiometric Mg2Si films. A solidphase growth technique with preliminary formation of templates was used by Galkin et al. [11] to prepare continuous Mg2Si films with a thickness of the order of 20 nm on Si (111) substrates. The very low condensation coefficient of magnesium as well as the limited Mg2Si films thickness due to the barrier behavior of Mg2Si can be overcome using the ion-beam synthesis (IBS) method, reported in this paper. The samples were prepared by IBS, followed by rapid thermal annealing (RTA). The implantation of 24Mg+ ions was performed using an ion accelerator type ILU-4, allowing a high current density. As an ion plasma source 4N pure Mg, heated at 500 ◦C, was used. The vapor pressure of Mg at this temperature is about 10−2–10−1 Torr. During the implantation, in order to avoid amorphization [12], the substrates were heated to a temperature of about 230 ◦C by means of the incident ion beam (current density 10–12 μA · cm−2). The mass separated 24Mg+ ions were implantated into n-type Si wafers with (100) orientation and resistivity 4.3–4.5 ·cm. Four types of samples were prepared. The first two types were fabricated by two-step ion implantation with two different doses—5 × 1016 and 1 × 1017 cm−2, with energies 15 and 40 keV respectively. The other two types of samples were prepared by one-step implantation of two different higher doses—2 × 1017 and 4 × 017 cm−2 with energy 40 keV. The Mg implantation profiles were simulated by SRIM (Stopping and Range of Ions in Matter) and the initial profiles of the implanted Mg ions were estimated as

Laryngeal squamous cell carcinoma (laryngeal SCC) is a frequently occurring cancer of the head and neck area. Epigenetic changes of tumor-related genes contribute to its genesis and progression.We assessed promoter methylation status of the selected genes (CDKN2A, MGMT, MLH1, and DAPK) using methylation-sensitive high resolution melting (MS-HRM) in 100 patients with laryngeal SCC and studied the correlations with clinical characteristics.The prevalence of promoter methylation in MGMT, CDKN2A, MLH1, and DAPK was 59 of 97 (60.8%), 46 of 97 (47.4%), 45 of 97 (46.4%), and 41 of 97 patients (42.3%), respectively. Significantly increased methylation of CDKN2A was observed in heavy smokers. Epigenetic inactivation of CDKN2A and MLH1 were found to be associated with lymph node involvement. An inverse correlation was present between MLH1 methylation and alcohol consumption.Our results strongly suggest that deregulation of p16-associated, and MLH1-associated pathways, because of promoter hypermethylation, is associated with increased cancer cell migration, tumor invasiveness, and, thus, aggressive phenotype.

The aim of this study was to evaluate the prevalence of and risk factors for chronic venous disease (CVD) in the Bulgarian adult population seeking medical help from general practitioners.The design was a cross-sectional descriptive survey. Each general practitioner (GP) enrolled 50 consecutive patients aged 18 and over attending for a routine consultation. A subsample of these patients was reappraised by a group of specialists to confirm the GPs diagnosis.A total of 576 GPs selected 26 785 subjects to participate in the survey. In the GP survey, 11 724 subjects (44%) were found to be suffering from CVD. Specialist reappraisal of a subset of 373 subjects confirmed the initial diagnosis in 91.2% of cases. CVD was more prevalent in females (51%) than in males (32%) (P0.0001) and prevalence increased with age and body mass index.Heavy legs, pain in the legs, and a sensation of swelling were the most frequently reported symptoms. Varicose veins and telangiectasias were the most common signs in both males and females, but with a higher frequency of varicose veins in males and telangiectasias in females. Increasing age, pregnancy, and a positive family history were found to be risk factors for CVD.

Lightly-doped-drain MOS transistors with an inverse-T-gate and an active polysilicon spacer are investigated. The application of these devices in VLSI integrated circuit fabrication is examined from a practical point of view. It is shown that in comparison to the minimum gate length of a conventional transistor the designed gate length can be decreased, with the double spacer width. The results of the ring-oscillator measurements show that the increase in overlap capacitance is not a restriction for circuit application of these devices.

An analytical model for the estimation of the minimum channel length which ensures a long channel behaviour in the MOS transistor is presented. The two-dimensional doping of the region under the gate is taken into account in the calculations. The model can be used in the case of graded-drain devices. Some of the results obtained are compared with those published in the literature.

The interface, oxide and transport characteristics of the silicon/thin thermal SiO 2 (20–35 nm) system after nitridation in a low temperature, “soft” ammonia plasma are investigated. Plasma conditions are found which lead to full transformation of the initial thermal oxide (thickness about 20 nm) into stoichiometric oxynitride film. The nitridation process consists of two stages: (i) formation of an oxynitride layer with worse dielectric and interface characteristics than those of the initial thermal oxide; and (ii) a further nitridation process leading to improvement of the properties of the formed oxynitride layer and its interface with Si without any stoichiometric change. It is shown that the “soft” r.f. ammonia plasma, reacting with the native oxide on the silicon, induces growing of ultrathin oxynitride film. The properties of this layer in terms of its dependence on the substrate temperature (293–573 K) and plasma nitridation time (2–180 min) are investigated.

Resonant Raman scattering by ion beam synthesized in silicon matrix ${\mathrm{Mg}}_{2}\mathrm{Si}$ phase is studied. The samples are prepared with the implantation of $^{24}\mathrm{Mg}^{+}$ ions with dose $4\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ and with two different energies 40 and $60\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ into (100)Si substrates. The far infrared spectra are used as criteria for the formation of the ${\mathrm{Mg}}_{2}\mathrm{Si}$ phase. The Raman spectra are excited with different lines of ${\mathrm{Ar}}^{+}$ laser, with energies of the lines lying in the interval from $2.40\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The resonant scattering can be investigated using these laser lines, as far as according to the ${\mathrm{Mg}}_{2}\mathrm{Si}$ band structure, there are direct gaps with energies in the same region. The energy dependences of the scattered intensities in the case of the scattering by the allowed ${F}_{2g}$ and the forbidden LO-type modes are experimentally obtained and theoretically interpreted. On the base of the investigation energies of the interband transitions in the ${\mathrm{Mg}}_{2}\mathrm{Si}$ are determined. It is found also that the resonant Raman scattering appears to be a powerful tool for characterization of a material with inclusions in it. In the particular case it is concluded that the ${\mathrm{Mg}}_{2}\mathrm{Si}$ phase is present in the form of a surface layer in the sample, prepared with implantation energy $40\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and as low-dimensional precipitates, embedded in the silicon matrix, in the sample, prepared with the higher implantation energy.

The apparent expression of MOST threshold voltage distribution with arbitrary mask-level gate length is obtained. It is shown that calculated and experimental distributions are in good agreement in the case of a short channel device.

Continuous layers and precipitates of the β-FeSi2 phase were formed by ion-beam synthesis followed by rapid thermal annealing. Two steps of Fe+ ion implantation in Si, at energies of 20 and 60 keV, were performed, using three different doses – 5×1015, 5×1016, and 1017 cm−2. The infrared spectra were used as a criterion for the formation of the β-FeSi2 phase. The refractive-index-energy dependences were calculated from the transmittance and reflectance spectra, using the general matrix method. From these dependences, the energy band gap and the energies of the higher-energy transitions in the vicinity of the gap were determined.

This paper presents investigation of electronic transport properties of GaAsN and InGaAsN epitaxial layers with low nitrogen content, the so called dilute nitrides, grown by liquid-phase epitaxy (LPE). The layers up to 2 microns thick have been grown from Ga- and In-rich melt at different initial epitaxy temperatures in the range 660?620 ?C. Polycrystalline GaN has been used as a source for nitrogen. As grown, unintentionally doped GaAsN and InGaNAs are n-type with free carrier concentrations one order of magnitude higher than those for the reference nitrogen free undoped GaAs and InGaAs layers. Lattice matched to GaAs substrate InGaAsN layers exhibit Hall mobility values higher than 2000 V/cm2.s

Low-cost single-junction GaAs solar cells on the base of p-AlGaAs/p-GaAs/ n-GaAs heterostructures grown by low-temperature liquid-phase epitaxy have been fabricated. For analyzing the factors contributing to solar cell efficiency the program products PC1D have been used. The optimal doping concentrations and thicknesses of the layers which correspond to the maximum values of the solar cell output parameters have been determined. The multilayer solar cell structure has been optimized on the base of this simulation. Low cost technological processes and masks, necessary for fabrication of the cells on the base of the grown heterostructures, have been developed, too. The front and back metallization of the cell is Ni/Al system deposited by magnetron sputtering. A double layer Al2O3 /ZrO2 antireflection coating is deposited on the front side of a solar cell structure by the spin coating technique. The conversion efficiencies of 19 % at one sun AM1.5 conditions have been measured.

In this paper, we present the preparation of multilayer AlGaAs/GaAs heterostructures for photovoltaic application. The structures developed consist of several layers grown on a highly conducting GaAs substrate: n-GaAs base, p-GaAs emitter embedded between two AlGaAs layers, and heavily doped p+GaAs capping contact layer. The second 0.03 μm thick 'window' AlGaAs layer has band gap energy of 2.1 eV and ensures penetration of the higher-energy photons in the active region of the p-n junction. The electrical and optical parameters of the multilayer heterostructure as well as the layers thickness are designed by numerical simulation using computer modeling. The optimized heterostructure has a back-surface-field AlGaAs layer of several microns and an ultra thin (20-40 nm) window layer providing the best conversion efficiency and a maximum spectral response in the range 300-900 nm.

Interface and oxide characteristics of a O 2 +4% HCl grown SiO 2 after different type low temperature plasma treatment (He, H 2 , Ar) have been investigated. It is shown that all the plasmas used introduce radiation defects in the Si-SiO 2 structures: oxide fixed charge (in order of 10 11 cm −2 ) and interface acceptor states (shallow and deep with a density depending on the plasma regime). These defects strongly influence the scattering of electrons in the inversion channel. It is shown also that a partial annealing of the radiation defects in Si-SiO 2 structures caused by heavy-particle plasmas (e.g. Ar plasma) is typical of He- and H 2 -plasmas; H 2 plasma effect is stronger.

Combining measurements of inversion channel I-V and high frequency C - V characteristics it is shown that argon plasma treatment leads to an increase of positive fixed oxide charge and generation of deep acceptor-type interface states (with a charge density of about 10 11 cm −2 ) accompanied by a reduction of the density of shallow acceptor-type interface states and a decrease of the inversion layer mobility. The radiation induced oxide charge is in the order of (1–3.5) × 10 11 cm −2 depending on the oxide thickness. The mobility decrease and the plasma-introduced charges in the Si-SiO 2 structures are assumed mainly to be trivalent Si and non-bridging oxygen due to the radiation-induced fracture of weak and strained bonds in the oxide and at the interface. The temperature behaviour of the mobility indicates a dominance of Coulomb and surface potential fluctuation scattering. Hence a severe degradation of Si-SiO 2 structures after low-temperature argon plasma treatment is found.

Experimental data are presented for the influence of helium plasma on the electrical properties of Si−SiO 2 structures with dry thermal oxide (d=16−72 nm). It is found that for strongly damaged structures low temperature helium plasma introduces acceptor-type interface states near the conduction band edge and has an annealing effect on the deep acceptor-type states. It has also shown that the initial characteristics of the Si−SiO 2 structures and the gas used are critical for the plasma-forming of both the interface and inversion channel properties.

A polysilicon sealed pad oxide local oxidation process has been investigated. The oxidation mask in this method has a polysilicon frame formed along the perimeter of a conventional mask pattern by a self-aligned technique. This technology is completely free of “bird's beak” formation which is particularly important for increasing the packing density.

Radiation damage is investigated in Si−SiO 2 structures with thin (34 nm) SiO 2 film exposed to the action of the ultraviolet component of a “soft” argon plasma. To obtain information about the electrical properties of the structures, inversion channel current-voltage characteristics are measured. It is found that low energy ultraviolet readiation (up to 8.7 eV) from the low-temperature argon plasma creates shallow acceptor-like interface states. As a result of the screening effect of the trapped electrons in these states a significant increase of the channel mobility is observed. It is assumed that the plasma-induced defects are connected with breaking of weak and strained bonds in the oxide and at the Si−SiO 2 interface. The temperature dependence of the channel mobility is interpreted in terms of several scattering mechanisms.

A new transistor structure with inverse-T gate and active polysilicon spacer (APOS) is presented. The spacer is formed from doped polysilicon and is in electrical contact with both the transistor gates and the polysilicon interconnections. The effective extension of the polysilicon lines is calculated and good agreement with the experimental results is achieved.