Browsing by Author "Siripala, W.P."

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Electronic Energy States in Nano Particles
(University of Kelaniya, 2012) Jayalalani, J.A.D.; Hewageegana, P.S.; Siripala, W.P.
An electron in the conduction band of a particle is nearly free to move inside the particle and this situation can be pictured as an “Electron inside a finite depth potential well”. The energy equations for this system can be derived by applying the “Time Independent Schrödinger equation” and corresponding boundary conditions in terms of one and three dimensions. In this study we have employed computer software and numerical root finding methods to obtain the numerical values of the legitimate energy states as it is more reliable than the conventional graphical methods. According to these numerical solutions, we could demonstrate that the number of allowed energy states and the spacing between adjacent levels inside a nano particle depend on both particle size and the magnitude of the attractive potential. Further, “Quantum tunneling effect” is significant when the particle size is below 20 nm and lowering the magnitude of the attractive potential, would extend the wave function far beyond its boundary. The energy levels obtained by employing the computer software and numerical root finding methods to the energy equation were plotted and compared with reported experimental observations and they are in good agreement. The most interesting size dependent property related to the semiconducting nano particles is that, we can obtain every colour of the visible spectrum by changing the size within the nano range, while the composition is unchanged.
Experiments on interference
(University of Kelaniya, 2008) Chandana, A.W.S.; Siripala, W.P.; de Silva, N.
We report a few experiments carried out to demonstrate the formation of interference patterns with thin Aluminium sheets placed along zero probability positions (positions where the probability of finding a particle is zero). The presence of the Aluminium sheets did not destroy the interference patterns though one would have expected the particles to interact with the sheets and wash out the patterns. In the experiment arrangement, Laser beam (He/Ne gas laser, wave length 633nm, maximum power <1mW ), a double-Slit (a= O.lmm, b = 1mm), digital camera, Aluminium sheets (0.056cm x 3cm x 40cm), two lenses and traveling microscope were needed. Eight thin Aluminium sheets of dimensions 0.56cm x 3cm x 30cm were placed along zero probability positions within the central maxima of interference pattern, and 20 more thin small Aluminium sheets of dimension 0.056cm x 3cm x 6cm were paced along zero probability positions of the diffraction pattern. The interference patterns were not changed as can be seen in the figure 1 Now the front edge of the first Aluminium sheet from the left of the eight sheets that were placed within the central maxima was moved through 4mm without moving the rear end of the sheet. The diffraction and interference patterns were washed off as shown in the figure 2.
Study of double junction effect on the quantum efficiency of optical devices
(University of Kelaniya, 2008) Hansameenu, W.P.T.; Siripala, W.P.
In practical optical devices one of the most important parameter which determines the performance of the device is the quantum efficiency. Generally, an optical device is fabricated using a junction between two semiconductors or a junction between a metal and a semiconductor. However, in practice formation of unwanted junctions during the fabrication stage of the optical devices is a major problem and it will result in the reduction of the overall performance of the device. In this investigation, a theoretical study was carried out on an optical device made by using a semiconductor/ metal junction to study this eff�ct. The formation of an additional junction at the back contact was considered as the reason for the formation of a double junction. In the study, photoinduced charge separation at the two space charge layers was considered as the main contribution to the photocurrents. Both drift current and diffusion current of the majority and minority carriers are considered in the continuity equation for calculating the total current produced by the device. Absorption of photons in the semiconductor was calculated using the variation of the absorption coefficient with the wavelength of the direct band gap semiconductor Cu20. In solving the continuity equation, the absorption of light in both space charge regions and creation of electron- hole pairs and the motion of the carriers in opposite directions were considered. The computed variation of the quantum efficiency with the wavelength of the incident light agreed well with the reported experimental results. We could observe that the efficiency of the device is drastically reduced as a result of the double junction effect. Further, with our model it was possible to explain the observation of the variation of direction of photocurrent with the wavelength, as a result of the existence of a double junction. In conclusion, using the computed quantum efficiency variation with the wavelength it was possible to obtain the junction parameters of an optical device using experimental data.
Study of double junction effect on the quantum efficiency of optical devices
(Sri Lanka Association for the Advancement of Science, 2009) Hansameenu, W.P.T.; Siripala, W.P.