I. Backscattering and Channeling Effect Studies on Semiconductor-Metal Systems. II. Low Temperature Migration of Silicon through Metal Films

PART I Channeling measurements by backscattering of He and H ions have been made on &lt;111&gt;- and &lt;110&gt;-oriented Si covered with evaporated layers of Al and Au. The energy range was 0.4 - 1.8 MeV and the film thicknesses ranged between 100 and 1100Å for Au, and between 900 and 3000Å for Al. As a first approach to analysis of disorder in crystals, we have investigated the effects of simulated disorder in form of metal layers on the surface of Si and Ge. This has an advantage in that particle scattering in the metal films can be controlled independently of scattering in the underlying substrate. The minimum yield, half-width of the angular-yield profile and the depth dependence of aligned yield have been studied as a function of metal-film thickness and beam energy. Comparisons between experimental and calculated values have been made on the basis of two different treatments of plural scattering. The minimum yield values obtained by applying only a step-function approximation to the angular yield profile were first evaluated as a function of film thickness. The minimum yield, angular-yield profiles and dechanneling dependence on depth obtained witl1 Al films follow the predictions of Meyer's treatment of plural scattering. A detailed study of minimum yield values on covered Si was then made. In this case the minimum yield was calculated from the Meyer treatment and probability curves determined from (i) a step-function approximation to the angular-yield profile, and (ii) two different axial scans on uncovered Si, one of which is azimuthally averaged. The minimum yields calculated using the step-function approximation and average probability curves are in good agreement with experimental results. This suggests that the step-function approximation, although less accurate than the azimuthally averaged procedure, is adequate for use with investigations of disorder in crystals by channeling-effect measurements. On the basis of the step function approximation, we have established universal curves from which minimum yield values as a function of disorder may be obtained. PART II The backscattering spectrometry using 2 MeV He⁺ ions have been employed to study the phenomenon of low temperature migration of Si through thin films of Au and Ag evaporated on &lt;110&gt; and &lt;111&gt; Si single crystal substrates. The thicknesses of Au films ranged from 200 to 4000Å, and those of Ag from 200 to 800Å. Migration of Si into these metal films is observed when the systems are heat treated in an oxidizing ambient at low temperatures (150°C for Au, 400°C for Ag), well below their eutectic points (375°C for Au and 830°C for Ag). The migration of Si is followed by formation of a silicon-oxide layer on top of the metal film. The initial growth of this oxide layer is proportional to (time)½. The factors controlling this low-temperature oxide formation have been investigated. Both oxidizing ambient and orientation of the substrate influence the oxide growth rate, and the thickness of evaporated film determines the final thickness of the oxide. A model to explain the oxide-growth mechanism is presented. The migration of Si also has been studied through layers of Au with superimposed layers of Ag, and vice versa. It is found that the interface between Si and the metal film plays a leading role in these low-temperature migration studies.</p

Technical, Economic and Sustainability Considerations of a Solar PV Mini Grid as a Tool for Rural Electrification in Uganda

The challenges facing rural electrification in Uganda are diverse with less than 3% of the rural population having access to electricity. The establishment of mini-grids powered by renewable energy sources makes it possible to electrify remote areas at affordable rates. In this study, an assessment of a solar PV mini-grid system to provide electricity to forty households in rural Uganda was carried out. The considered system comprised six solar modules each rated 175 Wp, a controller, off-grid inverter and batteries with a capacity of 600 Ah. Manufactured by SMA, the Sunny Island inverter proposed for the mini-grid would ensure provision of grid-quality electricity. The study aimed to investigate the mini-grid’s technical design with focus on optimal distribution against constraints of voltage drops, electrical losses and increasing load. Customised load limiters shared between households using thermistors were included to reduce costs and limit consumption. The incomes of rural households are often seasonal and thus issues pertaining to affordability and sustainability were also considered. Results of the economic analysis showed a payback period of less than 5 years given an affordable fixed monthly tariff for the case study area

A heuristic model for planning of single wire earth return power distribution systems

G. Bakkabulindi, M. R. Hesamzadeh, M. Amelin, I. P. Da Silva, and E. Lugujjo - A Heuristic Model for Planning of Single Wire Earth Return Power Distribution Systems - The IASTED International Conference on Power and Energy Systems and Applications, PESA, Pittsburgh, USA 7-9 November 2011The planning of distribution networks with earth return is highly dependent on the ground's electrical properties. This study incorporates a load flow algorithm for Single Wire Earth Return (SWER) networks into the planning of such systems. The earth's variable conductive properties are modelled into the load flow algorithm and the model considers load growth over different time periods. It includes optimal conductor selection for the SWER system and can also be used to forecast when an initially selected conductor will need to be upgraded. The planning procedure is based on indices derived through an iterative heuristic process that aims to minimise losses and investment costs subject to load flow constraints. A case study in Uganda is used to test the model's practical application