Rahman, A

Wary, G

English

IACS Institutional Repository

Indian J. Phys. 82 (6), 723-728 (2008) wm ^^m^ Comparative studies on optical parameters of CdTe and ZnO thin films G Wary1* and A Rahman2 1 Department of Physics, Cotton College. Guwahati-781 001, Assam, India 2 Department of Physics, Gauhati University, Guwahati-781 014, Assam, India E-mail • ganesh_wary(f/yahoo co in Abstract : The optical parameters of vacuum evaporated (p)CdTe and (n)ZnO thin films were studied and compared using uv-visible spectrophotometer equipped with an integrating sphere The main optical parameters studied are. transmittance, reflectance, absorbance, refractive index, extinction co-efficient etc Comparison at wavelength 500 nm shows that refractive index 'ri as well as imaginary part of dielectric constant, V 'of (n)ZnO films are smaller than that of (p) CdTe films which indicates the more transparency of (n)ZnO films and more efficient semiconductor for using as transparent conducting oxide (TCO) in most of the junction devices Static (dc) electrical conductivity is observed to increase slowly towards the IR spectrum as imaginary part of refractive index 'k' increases towards higher wavelength Keywords : Activation energy, complex dielectnc constant, extinction coefficient. TCO PACS Nos. : 71.55.Gs, 78.20.Ci, 78.40.Fy 1. Introduction ZnO is found as a good candidate of transparent conducting oxide (TCO) and therefore can substitute indium tin oxide (ln203: Sn) and tin oxide (Sn02) in conductive electrodes of other semiconductor solar cells. Due to its high stability [1] in hydrogen plasma, strain free, nontoxicity, controllable resistivity and low cost of the constituent elements make ZnO very attractive for photovoltaic applications [2]. On the other hand CdTe (energy gap [3] =1.5 eV) an ll-VI intermetallic compound is a direct band gap semiconductor. It has a high absorption co-efficient, which is required for good theoretical conversion efficiency. This has led to the investigation of CdTe based heterojunctions and Schottky Barrier devices for using as photovoltaic (PV) devices. The l-V characteristics under illumination of the ZnO/CdTe/Ag junction were reported [4] without PV efficiency calculation. Sputter-Corresponding Author ©2008IACS 724 G Wary and A Rahman deposited (n)ZnO on (p)Zn3P2 devices have been reported [5] with PV efficiencies of only 2.0%. A thin film ZnO/p-CdTe photovoltaic device, which showed an efficiency of 3.7% was reported [6]. Recently PV efficiency of vacuum deposited (p) CdTe/(n)ZnO heteorojunction has been reported [7,8] to be 4.01% using hydrogenation for Na = 1.03 x 1015 cm"3 A/d = 9.95 x 1014 cm'3. 2. Experiment Films of equal area (1.5 x 1.5) cm2 of pure CdTe powder (99.999%, Aldrich Chem. Co., USA) mixed with 2.53% of Sb and ZnO powder (99.995%, Aldrich Chem. Co., USA) mixed with 4.08% of Al were deposited by thermal evaporation from a molybdenum boat on chemically cleaned glass substrates under pressure 10~5 Torr. The substrate temperature was maintained at 423 K at the time of film deposition. The films were then annealed for 5 hours allowing maximum temperature of 528 K and maintaining the pressure at 10~4 Torr. For transmittance, absorbance and reflectance measurement, Cary-300 Scan uv-visible spectrophotometer (Varian, Victoria-3170, Australia) was used. The measurement of the type and carrier concentrations were carried out by the traditional technique involving Hall effect and C-V measurements on the structures of AI/(p)CdTe and Sn/(n)ZnO Schottky junctions were also made. 3. Result and discussion The optical transmittance (T) and absorbance (A) spectra of a typical (p)CdTe and (n)ZnO annealed films have been shown in Figure 1 and Figure 2 respectively. In both the cases absorbance has its maximum value at the higher energy and decreases with lower energy towards the visible region. Absorption coefficients in the interference-free region were calculated from relation [9], z r = exp[a(f2 - f1 ) ] (1) where, T^ and T2 the transmittances for the samples of thickness ^ and f2 respectively and a, the absorption coefficient. Since CdTe is a direct band gap semiconductor, its band gap was determined from (hva)2 versus hv plots using relation [10], ahv = B(hv-Eg)V2 (2) Figure 1(c) gives a typical plot of (ahvf versus hv for (p)CdTe film from which extrapolation of data to the {ahvf ->0 axis gives the band gap energy Eg = 1.564 eV and for annealed (n)ZnO film, this value is 3.35 eV [Figure 2(c)]. Values of refractive index (n) of (p)CdTe thin film in the interference zone at all transmittance minima [Figurel(a)] were determined from the relation [11], Tmjn = 6n2/(n4 + 3.25n2 + 2.25) (3) Comparative studies on optical parameters of CdTe and ZnO thin films 725 and in the interference-free region by extrapolation of the n values obtained in the interference region using Cauchy's dispersion relation, n=A + B/k2 (4) where, A and 8 are constants and can be obtained from the n versus 1 A2 plots fFioure 1(d) and 2(d)]. From the knowledge of a , the extinction coefficient k(k = oX An) and then real and 400 500 600 700 Wavelength (nm) 800 400 800 500 600 700 Wavelength (nm) Figure 1(a). Transmission spectra of annealed Figure 1(b) Absorption spectra of annealed (p)CdTe film (p)CdTe film 1 4 1 6 1 2 0 2 2 2 4 2 6 2 8 3 0 3 2 X(nm) 4 0 \2(\xm) Figure 1(c). (tva)2 versus tv plots of (p)CdTe Figure 1(d). n versus x* plots of (p)CdTe film film •160 22-T 450 500 550 600 650 700 750 800 X(nm) Figure 1(e). n and k versus A plots of (p)CdTe film 726 G Wary and A Rahman imaginary parts of the dielectric constants e' and e* were calculated using relations [12], E' = rf-k 2 u2 and respectively e' = 2nk Ana ID (5) (6) (n)lnO 001 -J 0 00 (n)ZnO 600 000 Wav«l«ngth (ran) W»veH»noth (nm) Figure 2(a). (n)ZnO film Transmission spectra of annealed Figure 2(b). Absorption spectra of annealed (n)ZnO film 4 6 -40-E 35-> 30-* 25-O O 20-5 -J 0 -e Unannealed film -Annealed film (n)ZnO E -3 32 eV / E8«3 35«V -fd —r—, , , . r-^ . , . . . , , ,—J 1 73 1 72 1 71 1 70 1 69 1 68 1 6* 1 66 -j 1 65 H 1 64 (n)ZnO I 6 6 + ( 0 1 K / X1 ) 2 6 3 0 3 5 hv (cV) 4 0 4 5 6 7 X 2 (^ im) 2 Figure 2(c). film (Aim)2 versus rv plots of (n)ZnO Figure 2(d). n versus K2 plots of (n)ZnO film 1 71 1 70 1 69 H 1 68 A 1 67 0 07 0 06 0 05 0 04 350 400 450 500 550 600 650 700 A.(nm) Figure 2(e). n and k versus X Plots of (n)ZnO film o 03 Comparative studies on optical parameters of CdTe andZnO thin films 727 Where, co is the angular frequency of the incident light and o is the static (dc) electrical conductivity Variations of n and k with wavelength have been shown in Figure 1(e) and Figure 2(e) Normal dispersion is pronounced in both the cases The change of imaginary part of refractive index (K) with wavelength is found to be interesting phenomena in (p)CdTe films Imaginary part of refractive index, k (and so, imaginary part of dielectric constant, t" ) is observed to increase slowly up to the wavelength 550 nm, slows down to 625 nm and then increases rapidly towards the IR region [Figure 1(e)] However, it increases slowly tn case of ZnO films These show that static (dc) electrical conductivity of both (p)CdTe and (n)ZnO films increase towards the IR spectrum[12] Values of some optical parameters of typical (p)CdTe and (n)ZnO films are shown in Table 1 and 2 respectively The comparison chart of some optical parameters of these films for same order of doping concentration and at same wavelength (500 nm) has been shown in Table 3 Companson at wavelength Table 1 Values of some optical parameters of a typical (p)CdTe film (doping concentration Nt - 2 87 x 1016 cm-3) for film thickness 5304 A k (nm) 400 450 500 550 600 650 700 800 T% 27 324 54 336 75 436 78 134 70 232 80 054 75 587 84 044 n 2 838 2 504 2 320 2 120 1 997 1 948 1 725 1 725 (/ (104 cm 1) 3 10 3 06 2 83 2 60 2 33 2 18 2 24 2 44 Table 2. Values of some optical parameters of a 1017 cm3) for film thickness 3050A X (nm) 350 400 450 500 550 600 650 T% 89 286 94 805 95 129 95 779 96 428 96 429 97 362 n 1 710 1 698 1 690 1 684 1 680 1 677 1 674 a (104 cm"1) 1 364 1 319 1 285 1 257 1 211 1 152 1 121 k (10"* ) 98 676 109 578 112 602 113 796 111 249 112 761 124 777 155 335 typical (n)ZnO k 0 038 0 042 0 046 0 050 0 053 0 055 0 058 / 8 045 6 744 5 369 4 481 3 976 5 095 3 972 2 952 film (doping i 2 922 2 881 2 853 2 833 2 819 2 809 2 799 i" 0 560 0 544 0 522 0 482 0 444 0 509 0 498 0 536 E<,l*V) 1 56 concentration Nd = 2 28 x t" 0 129 0 143 0 155 0 169 0 178 0 184 0 194 £ff(eV) 3 35 728 G Wary and A Rahman 500 nm shows that refractive index n as well as imaginary part of dielectric constant, t* of (n)ZnO films is smaller than that of (p)CdTe film which indicates more transparency of (n)ZnO film. This indicates that (n)ZnO is more efficient semiconductor for using as transparent conducting oxide (TCO) as well as window layers in most of the junction devices. Table 3. Comparison of some optical parameters of typical (p)CdTe, and (n)ZnO films (for same order of doping concentration and in same wavelength =500 nm). Sample (p)CdTe (n)ZnO N (1016cnr3) 2.87 2.88 a ( *T 1 cm'1) 8.47 x 10-8 3.37 x 1(T7 a (cm"1) 2.83 x 104 1.26 x 104 n 2.32 1.68 e' 5.37 2.83 f ' 0.52 0.17 4. Conclusion Optical parameters of vacuum deposited CdTe and ZnO thin films have been determined and compared. Higher absorption coefficient has been observed in (p)CdTe film, which is required for a good conversion efficiency and has led to the investigation of CdTe-base heterojunctions and Schottky barrier devices for its use as a photovoltaic (PV) device. On the other hand lower refractive index of (n)ZnO film in comparison to (p)CdTe indicates that (n)ZnO is more efficient semiconductor for using as transparent conducting oxide (TCO) in most of the junction devices. References [1] B Joseph, K G Gopchandran, P K Manoj, P Koshy and V K Vaidyan Bull. Mater. Sci. 22 921 (1999) [2] K L Chopra, S Manoj and D K Pandya Thin Solid Film 102 1 (1983) [3) J J Lofesrki J. Appl. Phys. 27 777 (1956) [4] B R Mehta, S Kumar, K Singh and K L Chopra Thin Solid Films 164 265 (1988) [5) K W Mitchell Physics of Solar Cells, (eds.) S C Join, S Radhakrishna and T R S Reddy (1st edition) (International Council of Scientific Union, Madras, India) 159 (1984) [6) M S Tomar Thin Solid Films 164 295 (1988) [7] G Wary, T Kachary and A Rahman Intl. J. Thermophysics 27 332 (1) (2006) [8) G Wary, T Kachary and A Rahman Indian J. Pure & Appl. Phys. Vol. 44 754 (2006) [9] F Abeles Optical Properties of Solids 23 (Amsterdam : North Holand Publishing Co.) (1972) [10] J I Pankove Optical Processes In Semi-conductors pgs. 36, 94 (New York : Dover Publications, Inc) (1971) [11] I Martil and G Gonzalez Diaz Am. J. Phys. 60(1) 83 (1992) [12] K L Chopra Thin Film Phenomena pgs. 729, 734 (New Work : MacGraw Hill Book Co.) (1969) 

Comparative Studies on Optical Parameters of CdTe and ZnO Thin Films

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/7879/1/Comparative%20studies%20on%20optical%20parameters_By%20G%20Wary.pdf

Comparative Studies on Optical Parameters of CdTe and ZnO Thin Films

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository