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DEVELOPMENT OF THE INNOVATIVE LEPECVD REACTOR FOR EPITAXIAL GROWTH OF SILICON – GERMANIUM HETEROSTRUCTURES
No full textThe semiconductor materials both in the form of heteropitaxial and homoepitaxial structures, offer huge potentialities due to a large variety of band energy structures which can be exploited in microelectronics (FET, HBTs) and optoeletronic (lasers, LEDs) devices. However, epitaxial integration of different semiconductors for device development present several issues regarding mainly the minimization of the defects within the heterostructures. To achieve this, materials with similar lattice constant should be used, so that the induced elastic strain in the overgrown film is minimized. Other than the physical constraints however, the choice of the high quality substrate must yield to a cost-effective solution to develop the devices. In the field of microelectronics, the silicon has remained the unparallel material of choice for complementary metal-oxide-semiconductor (CMOS) devices due to its large availability and relative low cost of the raw material. Born in the late 1950s and having since grown into an industry with annual revenues currently in excess of $200 bilion, the modern Si-based semiconductor microelectronics industry is an amazing technical and financial accomplishment. The continuous increase in devices performance requirements have highlighted the main limitation of the silicon device employment. The carrier mobility for both electrons and holes is relatively small respect to the III V alloys, which instead combine high electrical performance with equally high radiation interaction efficiency. Furthermore, the maximum velocity that these carriers can attain under high electric fields is also small, and this limits the cut off frequency of the Si-based microelectronic devices. Due to indirect bandgap also, light emission and absorption is fairly inefficient, making impractical its employment in optoelectronics devices. On the other hand, the higher physical and electrical properties of the III V alloys respect to silicon, are also coupled with relatively high costs. Furthermore, these compounds offer an high application flexibility as the relative composition of these alloys can be also tuned in order to obtain a specific optical or electrical properties.
By employment of the Germanium however, the properties of the silicon can be enhanced drastically. SiGe compounds infact exhibit higher electron and hole mobility even if small relative Ge fraction is added to the silicon. Moreover, the application of strain engineering in microelectronic devices using strain-relaxed SiGe buffer layer have brought a drastic enhancement in electrical properties of silicon. These alloys offers in addition the possibility to integrate the high efficiency III V alloys with low cost silicon substrate. The generally large lattice mismatch between these materials and the silicon, give rise to several issue regarding epitaxy integration. High mismatched heterostructure infact, relax plastically the elastic strain by an uncontrollable process which lead to a high density of induced defects in the grown layers. Several methods have been developed to growth high quality Ge layers onto silicon substrate with only a small dislocation content, such as constant composition thin buffer layers, linearly graded buffer layer and terrace graded buffer layers. On the other hand, in case a pure Ge can be directly integrated into the silicon wafer, a subsequent overgrown of III V alloys can be performed generating a low density of induced dislocations. Also the technique employed for deposition play a determinant role on the final quality of the grown layer. Classical CVD techniques, are able to growth high quality epitaxial layers but suffers generally of a low growth rate. Furthermore, the high thermal budgets required for precursor cracking can lead to several defect generation processes which finally degrade the electrical properties of the grown layer.
In this work, high quality pure Ge virtual substrate (VS) have been grown onto silicon substrate using the low energy plasma enhance chemical vapor deposition (LEPECVD) technique. The innovative epitaxial reactor have been developed at the Physics Department of the University of Ferrara in collaboration with Dichroic Cell, and have been installed in the clean room facilities. A very high growth rates, as high as 3 nm/s, can be obtain while maintaining an high crystallographic quality of the epilayers. Furthermore, the substrate temperature have been proved to play a determinant role on the epitaxial processes. Thus a numerical approach have been developed to assess the temperature profile during the epitaxial process within the LEPECVD reactor. The finite element analysis have highlighted several feature useful for the design improvement of the LEPECVD heating stage. Finally, pure Ge VS buffer layers have been obtain with a induced TDD as low as 105 cm-2. The low surface roughness and the high relaxation of the VS buffer layers obtained, put the basis for a cost-effective integration of the high efficient III-V alloys with Silicon substrates
Analysis of Non-Conventional Front Contact Patterns Impact on Concentrator Solar Cells Performances Through a 2.5-D Distributed Electrical Model
No full textA 2.5-D distributed circuit model for concentrator silicon solar cells has been developed in order to assess cell performance in the presence of particular top contact grid pattern geometries. Three kinds of front contact patterns have been considered: a conventional comb-like pattern, a square-like pattern and a fractal (auto-similar) one. Typical electrical parameters have been chosen for bulk resistivity, emitter resistivity, metal-semiconductor contact resistivity and metal resistivity. In this way an accurate assessment of ohmic losses can be carried out. Simulations have been performed by means of ORCAD PSPICE software. A 100 μm-wide, 5 μm-thick contact frame has been applied to the cell perimeter and the current was supposed to be extracted by four points, in correspondence of the mid points of the four sides of the cell. The simulations have been carried out under the assumption of a uniform irradiance distribution incident onto the cell surface (an arbitrary irradiance distribution however can be simulated) with a concentration factor of 50. For each of the three structures geometrical parameters such as finger width, finger spacing, total coverage and number of autosimilarity levels have been varied in order to achieve the optimal performance in terms of power delivered by the cell, for three different values of cell dimensions, 1 cm × 1 cm, 1.5 cm × 1.5 cm and 2 cm × 2 cm. Simulations results show that fractal contact grid pattern provides the best performance in all three cases and the power difference increases with increasing cell dimensions; for a 2 cm × 2 cm cell, difference between fractal and square patterns reaches 13%
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A joint thermal-electrical analysis of void formation effects on concentrator silicon solar cells solder layer
No full textIt is known that the formation of voids in solar cell solder joints leads to a worsening of their heat sinking capabilities, causing an increase in the average device temperature and thermal resistance. This phenomenon can be detrimental for the solar cell's performance, since the open-circuit voltage linearly decreases with temperature. The performances of silicon solar cells, in the presence of voids in the cell solder joint, are studied by means of numerical simulations and of an analytical thermal model which can assess the local temperature increase at cell surface due to a single isolated void in the solder joint. The results show that for small isolated voids the analytical model gives temperature peaks above the voids which match very well with the simulations result, within 5% of relative error. The analytical model also gives an estimation of the whole device thermal resistance, in the presence of a regular pattern of non-interacting voids all with the same surface area. For a 10×7 pattern of small area voids the analytical value for the device thermal resistance matches well with the results of numerical simulations, with a maximum error of 17.3% at 70% void coverage. To determine the temperature profile of the device surface we have implemented a thermal finite element analysis (FEA) which employs a detailed 3D model of the real morphology of voids in the solar cell, obtained by X-ray inspection. The resulting temperature map has been used as an input parameter for the subsequent electrical simulations performed by means of PSPICE software, which is based on a distributed 2.5 D electrical model of the solar cell. Results show that, for a concentrating factor of 100×, a real void pattern with 36.6% void coverage does not noticeably affect the performances of a concentrator silicon solar cell
Numerical simulation of the temperature distortions in InGaP/GaAs/Ge solar cells working under high concentrating conditions due to voids presence in the solder joint
No full textThe presence of voids in solar cell solder joints causes a modification of the heat fluxes inside the device during its operation, which in turn leads to local increases in cell temperature and thermal resistance. These temperature increases at device surface lead to a modification of the photovoltaic cell voltage map which translates in a drop in cell output power. Moreover, for pv cells working under high concentration conditions such as III-V multi-junction solar cells for terrestrial application, very high temperature increases can arise as a consequence of void presence, both above the void volume and around it, leading in extreme cases to irreversible damages of the device. In this paper a matlab script is implemented to assess the temperature increase at the top surface of a InGaP/GaAs/Ge solar cell, in the device regions lying outside the void coverage area, for different void sizes found in concentrator solar cells. The obtained results are compared to those of finite element method (FEM) simulations, which are based on an equivalent 2.5 D thermal representation of the cell. A good agreement between FEM simulations and the developed thermal model is observed for small and medium size voids, while for larger voids the error between FEM simulations and the developed model becomes not-negligible. An analytical expression is obtained to assess the device thermal resistance in presence of a random distribution on not-interacting voids. The developed model can be used as starting point to assess the influence of void presence on the power performances of InGaP/GaAs/Ge multi-junction solar cells working under high concentrating conditions
Effects of Irradiance Distribution Unevenness on the Ohmic Losses of CPV Receivers
No full textConcentrating photovoltaic (CPV) systems use solar collectors to focus the incoming solar radiation
onto PV receivers equipped with small area solar cells. The irradiance profile at the PV receiver plane is strongly
dependent on the concentrator architecture and has deep influence on the performances of the solar cells. In this
work we will describe an approach matching ray-tracing simulations and circuit analysis to assess the ohmic losses of
the PV receiver operating under uneven illumination.
In this investigation we considered three types of solar concentrator optics (Fresnel lens, parabolic mirror and
freeform mirror) having different flux distribution in the focal region. The optical simulations have been carried out
taking into account the real radial distribution (sunshape) of the incident solar energy. The irradiance profile at the
PV cells plane has then been used as a map for the photo-generated current to be collected and drawn from the
devices. The cells have been binned into a grid of generators connected between them by a resistive network
simulating the metal contacts and the semiconductor layers. The current-voltage characteristics of real solar cells has
been measured and modeled by an array of diodes connected in parallel to each generator. The nonlinear circuit has
been solved by a simulation program with integrated circuit emphasis (SPICE
Finite Element Model for Thermal Analysis of a High-Concentration Modular CPV System
No full textAiming at assess and optimize the thermal budget during operative condition of an innovative modular point-focus concentrating photovoltaic (CPV) system, a finite element thermal analysis have been carried out in this work. The CPV system has been designed and developed at University of Ferrara, as a part of Europen APOLLON project. Each module is comprised of a free-form 20 x 20 cm2 polycarbonate reflector and a secondary reflector in Cassegrain configuration. An aluminum high reflection homogenizer is placed in front of the solar cell and provides a further concentration level. The particular concentrating optic design offers unique possibility to tune the concentrating factor, ranging from 100x up to 400x, while maintaining an outstanding evenness of the irradiance on the cells, only by selecting the aperture angle of the aluminum homogeneizer. 3D simulations have been performed by means of a detailed finite element (FE) model in which both structure and fluid element are included. Solar cell temperature has been simulated and optimized as a function of thermal coupling technical solution with the heat sink. Heat transfer due to natural air convection induced by the thermal buoyancy force inside the CPV enclosure has been also taken into account. Results predicts a quasi equilibrium temperature of solar cell in operative conditions always under 80°C, even without any finned heat sink on the back of the module
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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