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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.

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.

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 $\ensuremath{\beta}\ensuremath{-}{\mathrm{FeSi}}_{2}$ 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.

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.

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 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.

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.

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.