Chattopadhyay, K K

Kundoo, S

Maity, R

IACS Institutional Repository

Indian J. Phys. 78(8). 837-840 (2004)U P  ?Electro-optical properties of ZnO : A1 thin films producedby sol-gel routeR Maity, S Kundoo and K Kfchallopadhyay*Department of Physics, Jadavpur University. Kolkata-700 032, India.E-mail: kkc®>jupliys.dmct.m (Abstract t Both undoped and aluminium doped ZnO thin films of high t|ual|ty were prepared by sol-gel route on glass substrates. Zinc acetate and isopropyl alcohol were taken as precursor materials. The conditions for preparations were optimized. Aluminium nitrate (AlfNOjXr^lljO ) was diiping material for aluminium doping and Zn/Al ratio was varied in the solution to vary doping concentration in the film. Structural characterizations were performed by XRD and TEM studies. The optical and electrical properties of the films were studied in detail and they were correlated with the doping concentrations and preparations conditions. The films deposited at optimized conditions on glass substrates showed electrical conductivity -1.39 (SI cm) * and a transmission of about 90% in the visible region. Electrical properties were studied in detailed which indicated that the films showed activated electronic conduction and the activation encigies were found to depend on the doping concentrations in the films. Room temperature Seebcck co-cfficicnt was obtained -91.65 /rV/K in optimized condition.Keywards : Zinc oxide, sol-gel route, aluminium doping.PACS Nos. ; 73.61.Gb, 78.30.Fs1. IntroductionTransparent conducting oxides (TCO) are known during the last four decades and extensive researches have been (lone on these materials. Among these materials indium tin oxides (ITO) and zinc oxide (ZnO) are two most important semiconductors because of their excellent combinations of electrical and optical properties. ZnO films have several promising applications in solar cells [1], gas sensors [2], transducers [3], luminescent materials[4] etc. only to mention a few. Doped ZnO, especially ZnO ; Al, have high temperature stability because the doping effect is not caused by oxygen deficit. They do not degrade active solar cell m aterials owing to interdiffusion of constituents as it occurs with ITO.Zinc oxide films have been prepared by a wide variety of methods such as spray pyrolysis [5J, metal organic chemical vapor deposition [6], reactive magnetron sputtering [7,8], ion assisted evaporation [9] and sol gel "methods [10-12]. All these methods have their advantages ®ud disadvantages and cho ice o f any particu larCorresponding Authortcchnological route is dictated by many conditions such as cost effectiveness, capability of the technique to coat a large area, quality and reproducibility of the methods. While the last two factors are generally better in sophisticated deposition techniques, such as magnetron sputtering or MOCVD than the sol-gel method, but the first two factors dictate that sol-gel process will be the only industrially viable process in future. There are not so many reports on the synthesis of ZnO thin films by sol-gel methods in the literature, and more research is necessary to improve the properties and reproducibility of the ZnO films by sol-gel process. In this paper we report synthesis and characterization of ZnO and ZnO : Al thin films by sol-gel method on glass and Si substrates.2. Experimental procedureAluminium doped zinc-oxide (AZO) films were prepared on glass substra tes from  (A 1(N 0 3 )3-9H 2 0 ), (Zn(CH3COO)2*2H20) and isopropyl alcohol (Pr*-OH). Diethanolamine (DEA)' was added to get transparent© 2004 lACS838 R Maity, S  Kundoo and K  K  Chattopadhyaysolutions due to low solubility of zinc acetate into the isopropyl alcohol. Initially (Zn(CH3C00)2-2H20) and (A1(N03)3'9H20) were dissolved in isopropyl alcohol mixed with alkanolamine (DEA) and the resultant solution was stirred keeping the temperature fixed ~70°C for one hr. Distilled water (~5 ml) and small amount of acetic acid were added for better stable of the solution and avoid from geletion or precipitation. After stirring the solution, it was aged also for one hr. The percentage of atomic ratio of Al/Zn were varied from 0.32 x 10'  ^ to 1.62 x 10“- in solution and the concentration of zinc acetate was fixed at 0.85 mol 1 '. Now the clean and ultrasonicated glass substrate was dipped into solution and withdrawn at a slow rate to coat it. The coated substrate was dried at room temperature for 10 minutes and heated at 250°C for 10 minutes in open atmosphere. This process was continued 3 to 5 times getting to desire thickness. Finally the film was heated at the same temperature for one hr.X-ray diffractometer (Philips PW 1730/10) with Cu- Ka radiation was used for crystal structure determination. UV-visible spectrophotometer (Hitachi, U-3410) was used for the optical characterization. Scanning electron microscopy and transmission electron microscopy were carried out by (JEOL, JSM-5200) and (Hitachi, H-600) respectively. For conductivity measurements, standard four- probe method has been used with silver paste as Ohmic contact.2. Results and discussions3.1. X-ray diffraction analysis :X-ray diffraction (XRD) pattern of an A1 doped ZnO thin film deposited on glass substrates is shown in Figure 1. Most of the peaks are from (100), (002), (101), (102), (110), (103) and (112) reflections of hexagonal ZnO having a = h = 3.249 A  and c = 5.205 A .  Information on crystallite size (L) and strain (e) obtainedfrom XRD data are related by the following equation 113]Pcos9 1 esinO—  = 1 ^  — ’ ('Jwhere f i  is the fullwidth at half maxima (FWHM).Figure 2 shows the plot of f i  cosOIA vs sin<9/y^ . From slope of the line and the intercept at x = 0 we calculated the strain (e )  and crystallite size (L) respectively. The0.03 1- -----------------------------------------------0.02 ]II  o.ot -I0.00'0.1.5 0.20 0.25 0.30sinOA (A-')F ig u re  2. Graph for size-strain analysis.0.35 0.40crystallite size was found to be ~88.5 nm and the strain -5.82 X 10" .^ We have also confirmed from X-ray diffraction that there is no peak of A1 even at higher percentage (1.96%) of Al.3.2. SEM and TEM :Surface morphology was studied by scanning electron microscope. Figure 3 shows the SEM micrograph of an Al doped ZnO thin film deposited by sol-gel process. Average grain size found from the SEM micrograph was -0.12 fjm. Cross sectional SEM measured the thickness of the film -0.6 pm.Also structural characterization was carried out by TEM studies. For TEM measurement, samples were prepared by dispersing the materials into methanol and copper grid was immersed into the ultrasonicated methanolFigure 1. XRD pattern of ZnO : Al thin film for %A1 s  0.65. Figure 3. SEM micrograph of ZnO film.Electro-optical properties o f ZnO : Al thin films produced by sol-gel route 839solution. The diffraction pattern (not shown here) shows the characteristic spots with the lattice spacing (d) values 0(2 .81,2 .25,1 .91, 1.62 and 1.463 A which correspond to ihc (100), (101), (102), (110) and (103) reflections respectively of hexagonal ZnO crystal.f,3. Optical properties :Al doped ZnO thin films have been u-sed for their high conductivity and extensive transmission of visible light. The transmittance spectra of undoped and Al doped ZnO deposited on glass substrates has shown in Figure 4. From the transmittance data, using Manifacier model |I4]Wavelength (nm)Figure 4. UV vis transmittance spectra of ZnO : Al film for different A) (.occnlralions.we have calculate absorplion coefficients (r3r).at the region of strong absorption. Absorplion coefficient {a) and photon energy {hv) are related by the formula [15]( a h v y  = A(/iV -  E  ), (2)where A is a constant and slope of this curve represents band gap energy (£^) of this material.Wc have observed that band gap energies has increased Irom 3.22 to 3.31 eV as the percentage of Al has increased from 0.32 to L62 respectively. These values are smaller than of 3.35 eV found for bulk crystalline ZnO (I6j. Inspite o f small increase in band gap in Al doped ZnO films, its conductivity increases because the conduction electron obtained by ionization of the introduced donor levels dominate the electrical transport.^^ 4. Electrical properties :The temperature dependence of dc conductivity (d) of Al doped ZnO films were measured by Keithley electrometer (Model 6514) in the temperature range 300-500 K. Figure 5 shows the In a  V5 1000/7 plots for different Al percentage. The conductivity can be expressed as in the simplest from,^  cJb exp(-Eyjt7), (3)65 - 4 - 3 - 2 - ? 1 0 ■ -1 ■ -2 - -3 ' 1.8V :Q V* a ■ • % ■“u4■ AUO.32% • Al=065% o Al  ^0 97% D Al s 1 62%;a■°D2.0 2.2 2.4 2.6 2.8 I0Yr(K *)o o o o o o  O c • • • • • • • •DOOOoJi^3.0 3.2 3.4FiBufe 5. TemjKraturc dependence of conductivity for different Al perc<jniagc.whetc Ea is the activation energy, the minimum energy reqt^ i^red to transfer electrons from donor level to conduction band, obtained from slope of the plots of Incr vs. 10V7. Comparative study of activation energy (£ J  and room temperature conductivity (rr^r) arc shown in Table 1 for different Al percentage. The activation (E„) energies arc different for different ranges of 1000/7 but we have shown the values of activation energy in the 1'able 1, calculated from slope at middle portion of the plot.Table 1. Comparauve study of activation energy (£«) and room temperature conductivity {(Tfn).Sample no. % Al Ort i£■„ (meV)(42 em) 'ZO-42 0 32 0.12 1192ZO-40 0.65 0.68 779ZO-35 0.97 1.392 638ZO-41 1.62 0.08 886Undoped ZnO has smaller conductivity at roomtemperature and even at higher temperature (-600°K)117] al.so. But Al doped ZnO has large conductivity at room and high (~500°K) temperature. Room temperature conductivity (£ ^ )  increases from 0.12 to 1.39 (Q cm)"' whenever percentage of Al increases from 0.32 to 0.97 and further increase of Al (1.62%) percentage, decreases the conductivity to 0.08 (£2 cm.)"'. The increase in electrical conductivity brought in by the aluminium doping can be explained as follows : the concentration of free charge carriers in ZnO increases by the aluminium doping because aluminium has one valance electron more than zinc. Wc may consider that aluminium substitutes the zinc atom or it occupies interstitial sites. In both cases, aluminium acts as a donor. For higher percentage of Al doping, though the carrier concentration increases, due to much scattering of carriers, conductivity decreases. The840 R Maity, S Kundoo and K K Chattopadhyaymain scattering mechanisms for chaiged carriers in thin films arc lattice scattering, ionized impurities scattering and grain boundary scattering [18]. So optimum percentage of A1 is 0.97 for getting highest conductivity at room temperature. It may be mentioned here, that in the optical transmission spectra, at the wavelength corresponding to the activation energy, some signature of absorption should also come. The maximum activation energy obtained by us was ~1.2 eV which corresponds to wavelength 1035 nm. This is beyond our measured spectral range as is evident from Figure 4.Thermoelectric power measurements indicated negative value of Seebcck coefficient (5), thus confirmed n-type nature of tlie material. .Seebeck coefficients were calculated from the derivatives of the thermoemf vs temperature difference plot (not shown here). Figure 6 shows the temperature variation of Seebeck coefficient for 0.97%AI-0.302.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8looorr (K-‘)F ig u re  6. Seebcck coefficient vs tem perature plot.doped ZnO film. Value of room temperature Seebeck coefficient was found — 91.65 //V/K for 0.97% of Al.4. ConclusionsUndoped and Al doped ZnO thin films were prepared by sol-gel processes. X-ray diffraction (XRD) pattern and selected area diffraction pattern confirm the hexagonal crystal structure of ZnO. Dopant Al slightly increases the bandgap from 3.20 to 3.31 eV. Maximum roomtemperature conductivity obtained was 1.39 {Q cm) ’ Thermoelectric power measurements confirm the material is n-type and Seebeck coefficient was found out -91.65 //V/K.AcknowledgmentsTwo of us (RM) and (SK) wish to thank the Council of Scientific and Industrial Research, Govt, of India, for awarding them junior research fellowship (JRF) during this work.References111 K Ellmer J. Phys. D33 R17 (2000)(2) S P S Arya and 0  N Srivaslava Cryst. Res, TechnoL 23 669 (J9K8)f3] J K Srivaslava. L Agarwal and B Bhattacharyya J. Electrocliem Soc. 11 3414(1989)14] H C Pan and B W Wesscls Mater. Res. Soc. Symp. Proc. 152 3414 (1989)[5] C Ebcrspacher, A L Fahrenbruch and R H Bubc Thin Solid Films 136 1 (1986)[6] J Hu and R G Gordon Sol. Cells 30 437 (1991)[7J T Minami, H Sato, H Imamolo and S Takata Jpn. J. A p p l Phys 31 L257(1992)(8) G L Harding, B Window and E C Horrigan Sol. Energy Mater 22 69(1991)[9] A Kuroyanagi J. Appl. Phys. 66 5492 (1989){101 A E Jimenez-Gonzalez, J A Soto Urueta and R Suarez-Parra J Crvst Growth 192 430 ( i m )[111 Y Ohya. H Saiki and Y Takahashi J. Mater. Sci. 29 4099 (1994)[12] M Ohyama, H Kozuka, T Yoko and S Sakka J. Ceram Soc. Jpn, 104 296 (1996)[13] SB Quadri, E F Skelton, D Hsu. D Dinsmorc, J Yang, H F Gray and B R Ratna Physical Review B (Third Series) 60 9191 (1999)[14] J C Manifacier, M De Murcia, J P Fillard and E Vicario Thin Solids Films 41 127(1977)[15] J I Pankove O ptical Proceses in Semiconductors (New Jersey Prentice-Hall) p34 (1971)[16] S M Szc Physics o f  Semiconductor D evices (New York : Wiley) p849(1981)[17] MS Tokumoto, A Smith, C V Santiili, S H Pulcinelli, A F Craicvich. E Elkaim, A Traverse and V Briois Thin Solid Films 416 284 (2002)[18] D H Zhang and H L Ma Appl. Phys. A : Matter. ScL Process. 62 487 (1996)

Electro-Optical Properties of ZnO : A1 Thin Films Produced by Sol-Gel Route

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/7113/1/Electro-optical%20Properties%20of%20ZnO_By%20R%20Maity.pdf

Electro-Optical Properties of ZnO : A1 Thin Films Produced by Sol-Gel Route

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository