Physics
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Item A Study of CuInS2 Thin Films for Photovoltaic Applications(Proceedings of the Technical Session of Institute of Physics, 1999) Wijesundera, R.P.; Nadesalingam, M.P.; Siripala, W.; Jayasuriya, K.D.; Kalingamudali, S.R.D.; Jayanetti, J.K.D.S.; de Silva, K.T.L.Thin films of copper indium disulphide (CuInS2) on Ti Substrate were prepared by annealing potentiastatically electrodeposited Cu-In alloy in H2S gas at 5500 C. Films were characteristised by X-ray diffraction (XRD), scanning electron microscopy (SEM), and spectral response in a polysulphide electrolyte. XRD measurements revealed the formation of the polysrystaline CuInS2 thin films and the abscence of any other phases. SEM showed the formation of crystallites having the size about 0.2 ?m. Variations of spectral response, open-circuit voltage (Voc) and short circuit current (Isc) with annealing in air have been studied. As deposited CuInS2 films exhibit a direct band gap of 1.5 eV, and shows n-type behaviour when used in a Photoelectrochemical (PEC) cell. Heat treatment shows a considerable enhancement of the photoresponse.Item Computational Study of I-V characteristics of ITO/Cu2 O/Metal junctions, Technical Session of Institute of Physics(Processing of the Technical Session of Institute of Physics, 1999) Wijesinghe, W.M.P.L.; Siripala, W.; Jayasuriya, K.D.; Kalingamudali, S.R.D.A theoretical model for current-voltage (I-V) characteristics of back-to-back diode systems was developed using the ideal diode equation. A computer model was developed using the language C++ to fit the experimental data to the theoretical equation and to determine the ideality factors and reverse saturation currents of each diode. This model was tested with commercial back-to-back diode systems. The values obtained for the above parameters from the theoretical fits were in very good agreement with the standard values. The experimental I-V characteristics data obtained for fabricated ITO/Cu2O/Metal (Au, Ag and Hg) structures were fitted to the model and values for the relevent parameters were obtained. These values indicate that the fabricated systems are back to back diodes except the ITO//Cu2O/Hg structure. Using this model, a good understanding of I-V characteristics of metal-semiconductor-metal diodes can be gained and thereby the quality of junction devices can be tested.Item Construction of a Near Ideal Nanoammeter(1998) Wijesundera, R.P.; Kalingamudali, S.R.D.; Jayasuriya, K.D.A near ideal ac/dc nanoammeter was constructed using two operational amplifiers (op-amp) and a voltmeter. Small currents were converted into a voltage by the first op-amp and this was amplified into a larger voltage by the second op-amp. The amplified voltage was measuredby the voltmeter. The constructed meter has 10 and 100 nA ranges for the dc current measurements and 100 nA range for the ac current measurements.All the ranges were calibrated using a digital picoammeter. As compared to commercially available nanoammeters with similar features this meter has higher advantages such as almost zero internal resistance and negligible operating bias current. The construction cost of the meter is very low compared to a commercial ac/dc nanoammeter.Item Thermally Stimulated Current-Voltage Characteristics of Cu2O/CuxS Diode(1988) Kalingamudali, S.R.D.; Siripala, W.A simple method was developed to fabricate a p-n junction diode of CU20/CuxS and it Was observed that the stability as well as the current-voltage characteristics of the diode could be improved significantly by controlling the system parameters at the fabricating stage. The diode characteristics of the fabricated diode was compared with a commercial germanium diode. High temperature measurements under the reverse bias conditions revealed that there are two distinct current transport mechanisms that would lead to high leakage current across the junction. Namely, two distinct current peaks were observed at the reverse bias. This was interpreted as the thermally enhanced tunneling at the junction due to the existance of defect interface states. We beleive that proper surface treatment might reduce the density of interface state, and then lead to better diode characteristics. Comparing with the ideal diode equation, it was observed�that the current transport mechanisms are complicated in the CU20/CuxS diode. �Item New Experimental Interpretation for the Emitter-size Effect of AIGaAs/GaAs heterojunction bipolar transistors.(1995) Kalingamudali, S.R.D.; Woods, R.C.; Wismayer, A.C.AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have an excellent frequency performance. One of the major limitations of these devices is current gain degradation with decreased emitter-base junction size, which is known as the emitter-size effect. Several experimental and theoretical studies have been reported on the emitter-size effect in AIGaAs/GaAs HBTs. These papers suggest that it is due to the extrinsic base surface recombination current. However, in thts paper. experimental evidence is reported which suggests that it is the perimeter recombination current which causes the emitter-size effect. Further analysis or previously published results, from rectangular geometry AIGaAs/GaAs HBTs fabricated with and without an overgrown A1GaAs layer approximately 0.5 um thick around the emitter-mesas, has been carried out. This analysis suggests that the perimeter recombination current rather than the extrinsic base surface recombination current causes the emitter-size effect. Devices with the overgrown layer and with the same emitter-base area. but significantly different perimeters, had similar current gains and n=2 recombination current values. In addition, for similar geometry devices without the overgrowth layer, the n=2 recombination current was roughly proportional to the device perimeter and due to the emitter-size effect current gains were decreased when the perimeter was increased. Since the reduction in the perimeter recombination current leads to a reduction of the emitter-size effect, it is suggested that the emitter-size effect is not due to the extrinsic base surface recombination current. Consequently, these results suggest that the emitter-size effect is due to the perimeter recornbination current.Item IV characteristics of ITO/Cu2O/ metal junctions(1996) Perera, L.D.R.D.; Siripala, W.; Kalingamudali, S.R.D.; Jayanetti, J.K.D.S.; de Silva, K.T.L.Cuprous oxide is an inexpensive and non-toxic semiconductor material having the potential for use in low-cost photovoltaic devices. Electrodeposition is a low-cost method t0 produce thin Cu2O films and electrodeposited CU2O has been reported to be n-type, In order to investigate the possibility of using electrodeposited Cu2O in solar cells, Cu2O/metal junctions were studied using their I-V characteristics. Thin films of Cu20 were electrodeposited on Indium tin oxide (ITO) coated glass substrate using an aqueous solution of cupric acetate. Electrodeposition was carried out under potentiostatic condition of -250 mV vs SCE at 55"C for a period of 1 h. These Cu20 samples were used to make junctions with Al, Hg,Cu, Ag and Au. The I-V plots of these junctions were obtained in dark and when illuminated. The I-V characteristics of the Cu2O/Hg junction show good rectifying properties.The plots for the junction in dark and under illumination indicate that the electrodeposited Cu2O behaves as an n-type semiconductor. The I-V profile fits into the standard diode equation with I0 = 2.8 x 10 -10 A, ideality factor n=0.48 and series resistance Rs = 40U. Also, the Junction exhibits a contact potential of about 150 mV.Item Photoelectrochemical Characterisation of ZnSe coated Copper Indium Sulphide Thin Film Electrodes(1999) Wijesundera, R.P.; Siripala, W.; Jayasuriya, K.D.; Kalingamudali, S.R.D.; de Silva, K.T.L.; Jayanetti, J.K.D.S.Capper Indium Sulphide thin films were prepared by sulphidation of Cu-In alloy on Ti substrate. Cu-In alloy was potentiostatically electrodeposited at ~1.4 V Vs SCE in an aqueous bath containing 5 mM CuCl2) 37.5m1 InCl3,1% (V/V) TEM and .75% (V/V) ammonia. Sulphidation was carried out in saturated H2S gas at 550�C for 30 min. XRD measurement reveals that the crystal structure of the films is CuInl1S17. ZnSe was deposited on CuInS electrode by electrodeposition in an aqueous bath of 0.1 M ZnS04 and 10 -5 M Se02 at -0.5 V Vs SCE for 90 min. XRD measurement reveals that the ZnSe films are amorphous. Ti/CulnS/ ZnSe thin film system in a PEC cell containing KI produces n-type photocondutivity. Dark and illuminated I-V measurement shows the existence of a main junction. However, some departure is also evident suggesting the possibility of existence of another junction. Spectral response of the Ti/CuInS/ZnSe system in a PEC cell shows the photoresponse of born n-CuInS/p-ZnSe and p-ZnSe/electrolyte interfaces. Normally, for shorter wavelength the spectral response is p-type while for the long wavelength it is n-type. The preliminary results of the study suggest the possibility of utilising electrodeposited n-CuInS in combination with electrodeposited p-ZnSe in developing a low-cost thin film solar cell.Item Computational Study of I-V characteristics of ITO/Cu2/Metal junctions(1999) Wijesinghe, W.M.P.L.; Siripala, W.; Jayasuriya, K.D.; Kalingamudali, S.R.D.A theoretical model for current-voltage (I-V) characteristics of back-to-back diode systems was developed using the ideal diode equation. A computer model was developed using the language C++ to fit the experimental data to the theoretical equation and to determine the ideality factors and reverse saturation currents of each diode. This model was tested with commercial back-to-back diode systems. The values obtained for the above parameters from the theoretical fits were in very good agreement with the standard values. The experimental I-V characteristics data obtained for fabricated ITO/Cu2O/Metal (Au, Ag and Hg) structures were fitted to the model and values for the relevent parameters were obtained. These values indicate that the fabricated systems are back to back diodes except the ITO//Cu2O/Hg structure. Using this model, a good understanding of I-V characteristics of metal-semiconductor-metal diodes can be gained and thereby the quality of junction devices can be tested.Item Growth and Characterisation of CuInS2 Thin Films(1999) Wijesundera, R.P.; Siripala, W.; Jayasuriya, K.D.; Kalingamudali, S.R.D.; de Silva, K.T.L.; Jayanetti, J.K.D.S.; Samantilleke, A.P.; Dharmadasa, I.M.Copper Indium Disulphide thin films were grown by electrodeposition of Cu-ln alloy followed by sulphurisation in H2S gas. It was observed that the ionic concentration of Cu2+/ In3+ in the electrodepositing bath determines the composition of the materials formed after the sulphurisation. CuInS2 thin films having the chalcopyrite crystal structure can be produced using this technique and the films are n-type semiconductors.Item Experimental evaluation of separate contributions to ideality factor for the base surface recombination current in heterojunction bipolar transistors(Materials Science and Engineering: B, 1994) Kalingamudali, S.R.D.; Wismayer, A.C.; Woods, R.C.In this paper it is shown experimentally, for the first time, that the ideality factor n for the extrinsic base surface recombination current in AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is unity and is bias independent. The devices with various emitter radii have been fabricated and the n = 1 and n = 2 current components of the emitter-base junction current were calculated. It was observed experimentally that the n = 1 current component was proportional to the total area of the emitter-base junction plus the exposed extrinsic base surface, both at lower biases such as emitter-base voltage VBE=0.60V (when the n = 2 current component dominates), and at higher biases such as VBE=1.0V (when the n = 1 current component dominates). The ideality factor value for the diffusion current is 1. Therefore, these results suggest that the ideality factor of the extrinsic base surface recombination current in HBTs is unity and is independent of the emitter-base bias.