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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.

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.

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.

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 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 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

International audience; Dilute III-nitrides, such as GaAsN and GaInAsN, are of considerable current interest both from a fundamental point of view and for applications in solar cells, GaAs-based long-wavelength photodetectors and diode lasers. The addition of nitrogen leads to material properties that deviate strongly from those expected for conventional III-V solid solutions. The possibility was investigated to use liquid phase epitaxy to incorporate nitrogen in epitaxial GaAsN/GaAs and GaInAsN/GaAs heterostructures. The structures were grown from Ga- and Ga-In- melts containing powder GaN as a nitrogen source. The initial growth temperature was varied in the range 560°C − 660°C. The low temperature growth favors nitrogen incorporation in the epilayers. The optical transmission and photoluminescence spectra of a set of structures grown at different temperatures were studied showing ternary and quaternary dilute nitride solid solutions with nitrogen content about 0.2 at.%. The photoluminescence spectra show emission from localized nitrogen states as well.

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.

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

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.

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.