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20320140502001
20320140502001
20320140502001
20320140502001
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20320140502001

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  • 1. International Journal of Advanced Research in Engineering RESEARCH IN ENGINEERING INTERNATIONAL JOURNAL OF ADVANCED and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online)TECHNOLOGY (IJARET) pp. 01-05, © IAEME AND Volume 5, Issue 2, February (2014), ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 5, Issue 2, February (2014), pp. 01-05 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2014): 4.1710 (Calculated by GISI) www.jifactor.com IJARET ©IAEME EFFECT OF DOPING ON D.C. CONDUCTIVITY FOR ZnS:Pb THIN FILMS Farah T. M. Noori1, Salma M. Shaban1, Zaidoon T. M. Noori2 and Raad M. S. Al-Haddad1 1 Department of physics, College of science, University of Baghdad 2 Dijla University College, Baghdad, Iraq ABSTRACT Pure ZnS and ZnS:Pb thin films were prepared by vacuum thermal evaporation onto glass substrates at room temperature and 200 nm film thickness to study effect of doping on electrical measurements. Hall measurements indicated that the type of conductivity for pure films is n-type but transforms to p-type after doping. The behavior of I-V characteristics can be considered as ideal diode. The ideality factor for the I-V characteristics at forward bias voltage was calculated. The concentration of carriers increases with doping of Pb atoms. Keywords: ZnS, Junction, Thin Films. 1. INTRODUCTION Thin films of lead and Zinc sulfate show promise in many physical applications, such as optoelectronics, infrared photo detectors, optical switches, and solar cells J. Twidell et al. (2006)[1], A. Shah et al. (1999)[2] studied the doping effect of Cu & Al on thin films structure of PbS and ZnS which have generated significant interest because they offer the advantage of tunable optical and opto-electronic properties of PbS, ZnS. The band gap, electrical conductivity, and structural properties doping ZnS were studied by A. Goetzberger et al. (2002)[3] and R. Strandberg et al. (2010)[4] . From such results, doping of PbS by Cd has been found to influence film growth and result in reductions in crystallite size. In the present work, the effect of doping on Hall measurements and D.C. conductivity were studied for ZnS:Pb thin films which prepared by thermal evaporation. The Hall Effect is important in the analysis of semiconductor material (in which the charge carriers can be either positively or negatively charged). It also has many important practical applications in detecting and measuring magnetic fields. 1
  • 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 2, February (2014), pp. 01-05, © IAEME 2. EXPERIMENTAL PART Pure and ZnS:Pb thin films were prepared by thermal evaporation using an Edward E306 A coating unit onto glass substrates which were cleaned by distilled water , pure alcohol , and then by ultrasonic vessel in order to deposit thin films at room temperature of 200 nm film thickness for electrical measurements. A molybdenum boat was used as the source holder and the pressure inside the chamber was better than 2*10-5 mbar. Pure ZnS and ZnS:Pb thin films were evaporated thermally with a deposit rate of 0.8 nm/sec to get 200 nm thickness. Aluminum electrodes were evaporated from a molybdenum boat by the previous coating unit for electrical measurements. The electrical resistance and then D.C. conductivity were calculated from I-V measurements. Also Hall measurements were obtained using two sensitive digital electrometer type Keithley 616 to measure the voltage and current as shown in Fig. 1. Fig. 1 the circuit diagram for Hall measurement 3. RESULTS AND DISCUSSION In general for D.C. measurements the electrical conductivity σ was calculated from equation [5]: σ = (1/ρ) = (R*A)/L ………………..(1) where ρ , R , A , and L are the resistively , the resistance , cross section area , and the distance between electrodes respectively . The resistivity can found from the relation between current and the applied voltage. Figure 2 shows the current voltage characteristics of the electrical conductivity for ZnS and ZnS:Pb films . As seen from fig ure 2, the relation is exponential with the increase of positive bias voltage and it shows saturated current with the increase of reverse bias voltage. So the I-V characteristics for ZnS:Pb films can be considered as ideal diode. Also with doping of Pb atoms in pure ZnS, the increase of saturated current with the increase of reverse bias voltage and this can be attributed to increase the electrical conductivity with doping then these films can be used as intermediate layer in solar cell devices[6]. From figure 2, the initial part of the forward branch of the plot (V<0.3) represents the recombination current while tunneling current dominates in the rising part of the forward branch of the plot (V>0.3 ) volt. Figure 3 shows the logarithm of the initial part of the forward branch, the ideality factor for the diode can be calculated as listed in Table 1 .The values of the ideality factor are in agreement with M. Green whose thin films were prepared by (Chemical Bath Deposition) method. 2
  • 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 2, February (2014), pp. 01-05, © IAEME Fig. 2 I- V characteristics of pure ZnS and ZnS:Pb Films Fig. 3 the logarithm of the initial part of the forward branch of pure ZnS and ZnS:Pb Films Table 1 Values of ideality factor for ZnS :Pb Films ZnS Films β Pure 9.1 Doped 9.3 Figs. 4 and 5 represent the relation between Hall voltage VH and the current I for pure ZnS and ZnS:Pb films. Hall coefficient RH, carrier concentration n, and mobility µ can be calculated from the slope of the figure and by using equations: RH = (VH/I)*(t / B) (3) n = (-1 / RH*q) (4) µ = (σ RH) (5) where t , B represent the thickness and magnetic field respectively . 3
  • 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 2, February (2014), pp. 01-05, © IAEME Fig.5 Hall voltage as a function of current for pure ZnS Fig.4 Hall voltage as a function of current for ZnS:Pb films Values of Hall parameters were indicated in Table 2. From the Table we noticed that the type of conductivity transforms from n-type to p-type and the carrier concentration increases after doping[7,8,9]. Table 2 Values of Hall measurements Type of Type of Hall Carrier Conductivity Thin Conductivity effect Concentration (1/ῼ Cm) Films RH ( cm-3 ) Carrier Mobility cm2/v. s. Pure ZnS n - type 1.69E+7 3.68 *1011 1.38E-6 24.8 ZnS:Pb p- type 1.01E+7 6.12 *1011 1.46E-5 142 4
  • 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 2, February (2014), pp. 01-05, © IAEME 4. CONCLUSION The behavior of I-V characteristics for Pure ZnS and ZnS:Pb can be considered as good ideal diode. The doped films ZnS:Pb have higher concentration and p-type conductivity. From these results, ZnS:Pb films can be used as intermediate layer in solar cell devices. 5. REFERENCES [1] J. Twidell and T. Weir, Renewable Energy Resources. Taylor & Francis Ltd, second ed., 2006. [2] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, and H. Keppner, “Photo-voltaic technology: the case for thin-film solar cells,” Science, vol. 285,no. 5428, p. 692, 1999. [3] A. Goetzberger, J. Luther, and G. Willeke, “Solar cells: past, present, future,” Solar energy materials and solar cells, vol. 74, no. 1-4, pp. 1–11, 2002. [4] R. Strandberg, Theoretical studies of the intermediate band solar cell. PhD thesis, 2010. [5] Y. Subbaiah, P. Prathap, and K. Reddy, “Structural, electrical and optical properties of ZnS films deposited by close-spaced evaporation,” Applied Surface Science, vol. 253, no. 5, pp. 2409–2415, 2006. [6] M. Green, Third Generation Photovoltaics: Advanced Solar Energy Conversion. SpringerVerlag Berlin Heidelberg, 2006. [7] A. Luque and A. Martí, “Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels,” Physical Review Letters, vol. 78, no. 26, pp. 5014–5017, 1997. [8] A. Luque, A. Martí, A. Bett, V. Andreev, C. Jaussaud, J. Van Roos- malen, J. Alonso, A. Rauber, G. Strobl, and W. Stolz, “FULLSPEC-TRUM: a new PV wave making more efficient use of the solar spectrum,” Solar energy materials and solar cells, vol. 87, no. 1-4, pp. 467–479, 2005. [9] K. Seeger, Semiconductor Physics. Springer-Verlag Berlin Heidelberg New York, 9 ed., 2004. [10] Eman M. Nasir, “Characterization of Zns and Zns: Al Thin Films”, International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 4, Issue 7, 2013, pp. 266 - 275, ISSN Print: 0976-6480, ISSN Online: 0976-6499. 5

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