Chapter 2: Growth Techniques and Characterisation
2.3 Device characterisation
2.3.2 Capacitance Voltage (C-V) measurements
Similarly, the value of φb can be calculated from the intercept Io and is given by:
2.3.2 Capacitance Voltage (C-V) measurements
The technique used to estimate doping concentration (Nd) of the semiconductor and the diffusion voltage of the Schottky diode is the C-V measurement. The depletion width (w) and the barrier height (φb) can also be calculated. A high frequency AC signal of ~1 MHz is used as modulation frequency for the measurement. The junction is probed by the AC signal which contained charges on both side and the depletion region associated with the device. A graph of 1/C2 versus V gives a straight line from which the diffusion voltage (Vd) can be determined from the intercept on the V-axis. A straight line graph is obtained if the material is uniformly doped and its gradient gives the doping
where, Vbi is the built in potential, εs is the permittivity of the semiconductor and other symbols have their usual meanings.
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Equation 2.35 is used to determine the φb which depends on the Vbi [68] and Figure 2.24 shows the Schottky diode for an n-type semiconductor:
b Vbi 2.35
where, ξ is the energy difference between Fermi level and the CB for an n-type semiconductor and VB for a p-type semiconductor. The value of ξ is ~0.1 eV for moderately doped semiconductors.
Figure 2.24: Schottky diode for an n-type semiconductor, reproduced from [55].
Similarly, the depletion width can be calculated from equation 2.36 and is given by [55]:
) 2 (
bi d
s
o V V
W eN
2.36
φb Vbi
ξ
70 References
1. K. L. Chopra and S. R. Das, Thin Film Solar Cells, Plenum Press, NY (1983).
2. V. S. Smehtkowski, Prog. in Surface Science 64, 1 (2000).
3. http://wwwold.ece.utep.edu/research/webedl/cdte/Fabrication/index.htm, Retrieved, December, 2011.
4. S. K. Mandal, S. Chaudhuri and A. K. Pal, Thin Solid Films 357, 102 (1999).
5. P. Taneja, P. Vasa and P. Ayyub, Material Letter 54, 343 (2002).
6. F. El Akkad and M. Thomas, Phys. Stat. Sol. C 2(3), 1177 (2005).
7. H. O. Pierson, Handbook of Chemical Vapour Deposition (CVD): Principles, Technology and Applications, 2nd edition, Noyes Publication, Park Ridge, NJ U.S.A., (1999).
8. A. Ali, N. A. Shah and A. Maqsood, Solid State Electronics 52, 205 (2008).
9. Y. H. Lee, W. J. Lee, Y. S. Kwon, G. Y. Yeon and J. K. Yoon, Thin Solid Films 341, 172 (1999).
10. K. D. Patel, G. K. Solanki, C. J. Panchal, K. S. Hingarajiya and J. R. Ghandi, J.
Nano-Electron.\Phys. 3(1), 41 (2011).
11. K. L. Choy, Progress in Material Science 48, 57 (2003).
12. R. A. Berrigam, N. Maung, S. J. C. Iroine, D. J. Cole-Hamilton and D. Ellis, J.
of Crystal Growth 195, 718 (1998).
13. M. Lindurer, G. F. Schötz, P. Link, H. P. Wagner, W. Kuhn and W. Gebhardt, J.
Phys: Condense Matter 4, 6401 (1992).
14. J. R. Arthur, Surface Science 500, 189 (2002).
15. P. Boieriu, R. Sporken, Y. Xin, N.D. Browaning and S. Sivawanthan, J. of Electronic Mater. 29(6), 718 (2000).
16. J. Zhao, Y. Zeng, C. Liu and Y. Li, Appl. Surface Science 256, 6881 (2010).
17. S. N. Alamri, Phys. Stat. Sol. a 200(2), 352, (2003).
71
22. P. S. Patil, Materials Chemistry and Physics 59, 185 (1999).
23. J. Toušková, D. Kindl and J. Konanda, Thin Solid Films 214, 92 (1992).
24. J. De Merchant and M. Coceiver, J. Electrochem. Soc., 143(12), 4054 (1996).
25. A. K. Berry, Mater. & Engineering B 8, 57 (1996).
26. A. Luque and S. Hegedus, Handbook of Photovoltaic Science and Engineering, John Wiley & Sons, Ltd., U.S.A., (2003).
27. T. Arita, A. Hanafusa, N. Ueno, Y. Nishiyama, S. Kitamura and M. Murozona, Solar Energy Material 23, 371 (1991).
28. V. Kumar, J. K. Gaur, M. K. Sharma and T. P. Sharma, Chalcogenide Letters 5(8), 171 (2006).
29. E. Tekin, J. Smith, S. Hoeppener, A. M. J. Vanden Berg, A. S. Susha, A. L.
Rogach, J. Feldmann and U. S. Schubert, Advance Functional Materials 17, 23 (2003).
30. R. Parashkov, E. Becker, T. Riedl, H-H Johnnes and W. Kowalsky, In Proc:
IEEE 93(7), 1321 (2008).
31. F. Ullik, Ber. Akad. Wien 52, 115 (1865).
32. M. Ddero, Compt. Rend. Acad. Sci. Paris 109, 556 (1934).
33. J. J. Combo and R. J. Gambino, J. Electrochem. Soc. 115(7), 755 (1968).
34. R. C. DeMattei, D. Elwell and R. S. Feigelson, J. Crystal Growth 43, 643 (1978).
72
35. G. Hodes, J. Manassen and C. Cahen, Nature 261, 403 (1976).
36. B. Miller and A. heller, Nature 262, 680 (1976).
37. M. P. R. Panicker, M. Knaster and F. A. Kröger, J. Electrochem. Soc. 125, 566 (1978).
38. W. J. Danaher and L. E. Lyon, Nature 271, 139 (1978).
39. J. F. MaCann and K. M. Skyllas, J. Electroanal. Chem. 119, 409 (1981).
40. I. M. Dharmadasa and J. Haigh, J. Electrochem. Soc. 153(1), G47 (2006).
41. S. Dennison, J. Mater. Chem. 4(1), 41 (1994).
42. R. K. Pandey, S. N. Sahu and S. Chandra, Handbook of Semiconductor Electrodeposition, Marcel Dekker, Inc. New York, (1996).
43. D. Cunnigham et al. Conf. rec. 16th EPSSEC, 281 (2000).
44. M. A. Green, K. Emery, Y. Hisikawa and W. Warta, Prog. Photovolt: Res. Appl.
15, 425 (2007).
45. K. Premaratne, S. N. Akuranthilaka, I. M. Dharmadasa and A. P. Samantilleka, Renewable Energy 29, 549 (2003).
46. R. B. Gore, R. K. Pandey and S. K. Kulkarni, Solar Energy Material 18, 159 (1989).
47. A. G. Voloshchuk, N. I. Tsipishchuk, Inorganic Materials 38(11), 1114 (2002).
48. J. F. McCann and M. Sk. Kazacos, J. Electroanal. Chem. 119, 409 (1981).
49. T. Mahalingam, V. S. John, S. Rajindran, G. Ravi and P.J. Sebastian, Surface and Coating Technol. 155, 245 (2002).
50. W. J. Danaher and L. E. Lyons, Aust. J. Chem. 37, 689 (1984).
51. B. D. Cullity and S. R. Stock, Elements of X-ray Diffraction 3rd edition, Prentice Hall, Inc. Upper Saddle River, New Jersey, U. S. A., (2001).
73
52. http://Serc.carleton.edu/research./geochemsheets/technique/XRD.html.
Retrieved, October 2011.
53. C. R. Brundle, C. A. Evans Jr, and S. Wilson, Encyclopaedia of Mater. Charact.:
Surfaces, Interfaces and Thin Films, Butterworth-Heinemann Inc. USA, (1992).
54. G. J. Tolan, PhD. Thesis, Sheffield Hallam University, UK (2008).
55. S. M. Sze and Ng. K. Kwok, Physics of Semiconductor Devices 3rd Edition, Wiley-Interscience, (2007).
56. http://www.wellesley.spectrophotometer. Retrieved, October 2011.
57. R. F. Egerton, Physical Principles of Electron Microscopy: An Introduction to TEM, SEM and AFM, Springer Science+Business Media, Inc. (2005).
58. A. T. Hubbard, The Handbook of Surface Imaging and Visualization, CRC Press Ltd., (1995).
59. P. M. Martin, Handbook of Deposition Techniques 3rd edition, Elsevier Sci. &
Technol., USA, (2010).
60. I. Salaoru, P. A. Buffat, D. Laub, A. Amariel, N. Apetroaei and M. Rusu, J. of Optoelectronics & Adv. Mater. 8(3), 939 (2006).
61. C. V. Raman and K. S. Krishnan, Nature 21, 501 (1928).
62. A. Smekal, Naturwissenchaften 43, 873 (1923).
63. E. Smith and G. Dent, Modern Raman Spectroscopy: A Practical Approach, John Wiley & Sons, Ltd., England, (2005).
64. D. Briggs and M. P. Seah, Practical Surface Analysis, John Wiley, Chichester (1990).
65. K. Siegbahn, Alpha- Beta- and Gamma-Ray Spectroscopy, North-Holland, Amsterdam, (1965).
74
66. J. F. Moulder, J. Chastain, R. C. King, in: Handbook of X-Ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for identification and Interpretation of XPS Data, Physical Electronics, Eden prairie, MN, (1995).
67. M. Pagliaro, G. Palmisano and R. Ciriminna, Flexible Solar Cells, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, (2008).
68. E. H. Rhoderick and R. H. Williams, Metal-Semiconductor Contacts 2nd edition, Oxford University Press, NY, (1988).
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Chapter 3: Experimental
3.1 Introduction
This chapter describes the experimental techniques used to electrodeposit two window materials (CdS and ZnTe) onto glass/FTO substrate and an absorber material CdTe onto glass/FTO/CdS substrate. Due to the peeling off of ZnTe in acidic CdS and CdTe baths, the complete structure of the devices is based only on glass/FTO/CdS/CdTe/Au contact.
Prior to deposition of CdS and ZnTe, the substrates were washed thoroughly with soapy water, methanol, acetone, dilute HNO3 and finally rinsed in glacial acetic acid. The glass/FTO substrates were rinsed in de-ionized water between the different solvents, while only glacial acetic acid is used to clean the glass/FTO/CdS layer and rinsed in de-ionized water prior to CdTe deposition. All chemicals used for electrodeposition were analytical reagent grade of purity 5N (99.999%) from Fisher Scientific Ltd., and Sigma-Alderich, UK. Details of substrate cleaning process are discussed in the next section.
The working and counter electrodes were held vertically using Teflon tape.
Potentiostatic deposition in a conventional single compartment cell was carried out using a two-electrode set-up. The glass/FTO or glass/FTO/CdS conductive substrate formed the working electrode and a graphite carbon electrode as a counter electrode. A typical experimental set-up is shown in Figure 2.10 (b) in section 2.1.6.2 of chapter 2.
The electrolyte was stirred constantly using a magnetic stirrer throughout deposition at moderate rate. The electrodeposition mechanism was studied using a cyclic voltammogram. Cyclic voltammetry was performed using a Gill AC computerised potentiostat system (S/N 1313 ACM Instruments), the details of which are discussed in chapters 4, 5 and 6.
The deposited layers were rinsed with de-ionized water, dried under nitrogen air at room temperature and subjected for further analysis. Conductivity type, optical, structural, surface morphology, molecular structure, chemical composition and atomic percentage of the deposited films were studied using Photoelectrochemical (PEC) studies, UV-Vis spectrophotometer, X-ray diffraction (XRD), scanning electron microscopy (SEM)/
atomic force microscopy (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) measurements respectively.
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Furthermore, the electrical characterisation of fully fabricated devices was performed using current-voltage (I-V) and capacitance-voltage (C-V) measurements.