IBN AL- HAITHAM J . FO R PURE & APPL. SC I. VO L.22 (4) 2009 The Alternative Electrical Properties of (Al -CdSe0.8Te0.2- Al) Capacitor at Room Temperature G.J. Ibraheem Departme nt of Physics, College of Education, Unive rsity of ALKufa Abstract This research aims to p rep are an (Al-CdSe0.8Te0.2-Al) cap acitor and study the alternating electrical p rop erties of it at room temp erature, and st udy the p ossibility of using these films in electronical app lications. The A.C. conductivity of as-deposited films have been measured in the frequency range (f =100Hz-400KHz), and it has shown that A.C. conductivity ( σa.c ) increases with the frequency increasin g. The study of the variation of each of the cap acitance and real p art of the dielectric constant (Єr ) with frequency has shown t hat their values decrease with frequency increasing. The study of the variation of each of the imaginary p art of dielectric constant (Єi ) and the loss factor with frequency has shown that their values decr ease with frequency increas ing and then they began to increase. Introduction A semiconductor material is the one whose electrical prop erties lie in between those of insulators and good conductors. At 0K, t here are no electrons in the conduction band and the valence band is co mpletely filled[1]. The energy gap for semiconductors is less than 2eV [1, 2] and for Cd Se1-xTex it has been found by many researchers like [3] that Eg=(1.46-1.745)eV for 0≤x≤1, and another researcher [4] found that Eg=(1.47-1.76)eV for 0≤ x≤1. Semiconductors are consider ed as fund amental materials in fabricatin g electronic devices (diodes, transistors, integrated circuits) which are emp loyed in building all electronic circuits[5]. Data st orage with p hase change materials lik e chalcogenide alloys is based up on the reversible switching between the amorphous and the cry stalline phase [6, 7]. The reversible p hase transition between the amorp hous and cryst alline states, which is accomp anied by a consid erable change in electrical r esistivity , is exp loited as a means to st ore bits of information[8, 9]. (II-VI) compounds like (Cd Se1-xTex) arouse great p ractical interest owin g to the unique p rop erties includin g their high p hotosensitivity to electromagnetic and p articular radiation [10], and the main reason of these compounds develop ment is their big d irect en ergy gap, so these comp ounds could be used in lasers and p hoto emitt ing diodes in visible range [11]. Cadmium chalcogn ides have been known to hold great p romise in thin films p hoto electronic devices and photo electrochemical cells [12]. Sulphides, selenides and tellurides of cadmiu m find app lications as p hotoconductors and electro optical devices [13]. A.C Electrical conductivity The electrical p rop erties of semiconductors or insulators in thin layers under metallic electrodes are generally different from the p rop erties of the material itself. The available exp erimental results about the frequency dep endence of A.C. conductivity have revealed a considerable similarity of behavior for a very wide ran ge of materials [14, 15]. The frequency dep endence of a conductivity can be exp ressed by the emp irical relation (16, 17). IBN AL- HAITHAM J . FO R PURE & APPL. SC I. VO L.22 (4) 2009 σa.c (ω) ~ ω s or σa.c (ω)=Ao ω s ………… (1) Where (Ao) is a constant, and (s) is not a constant for all substances, but is a function of temp erature. S= d[Ln σa.c (ω) ] / d [Ln (ω) ] ……..(2) The total conductivity at a certain frequency and temperature is: σ (ω) = σa.c (ω) + σd.c ………(3) Where σd.c is a direct or continuous electrical conductivity . The preparation of (CdSe0.8Te0.2) alloy The alloy has been p repared by mixing a certain ratios of Cd, Se and Te (99.999 %) p urity elements with their atomic weight 78.96, 112.40 and 127.60 resp ectively. The weight of each element ratio in the alloy was determined by the equation: The weight of total alloy = (the ratio of the first element in the alloy * its atomic weight) + (the ratio of the second element in the alloy * its atomic weight) +…… …….. [4]. Then the weight of each element ratio was determined by a sensitive balance (M ett ler H 35 AR) and its sensitivity (10 -4 gm), then these weights hav e been p ut in a clean qu artz amp oule and it has been closed, the amp oule has been p ut in a furnace with temp erature T=823 K for four hours, t hen the temp erature has been incr eased to 1023 K for t hree hours, and fin ally the temp erature has been increased to 1223 K for three hours too, after that t he mixed compound has been quench ed in a cold water. (Al-CdSe0.8Te0.2-Al) capacitor preparing A thermal evap oration in high vacuum reaches to (10 -6 mb) has been used in p rep aring the cap acitor by using an Edward 306 sy st em. The aluminu m electrode h as been deposited by using a tungst en boat then CdSe0.8Te0.2 thin films have been deposited on the Al electrode usin g a moly bdenum (M o) boat then the other Al electrode has been d eposited on them. Thin films have been deposited with substrate temp erature of 373 K with (0.3 n m/s) rate of deposition, thin films thickness was (3000±20) nm, the contribute length of electrodes was (0.8cm). A glass substrates have been used with dimensions (2.54*7.62) cm and with (12) cm thickness. A.C Conductivity measurements The (Al -CdSe0.8Te0.2-Al) cap acitor h as been connected to the electrical circuit as shown in figure (2) and the measurements of cap acitance (C) and resistance (R) have been done by a General Radio Capacitance Br idge ty p e (42748 M ultifrequency LCR M eter Hewlet Packard). These instruments p ermitt ed measurements in the frequen cy range (f = 100Hz - 400kHz ). During the measurements the voltage on the sp ecimen was kept at (1 volt), and all measurements were taken at room temp erature. The following equations have been used to comp ute the alternating electrical p rop erties[1,5,16,17] σa.c =t /R.A ……… (5) Where t: film thickness, R: film resistance, A: eff ective area of the capacitor =0.5cm 2 Єr =C.t /Єo .A ………. (6) Where C: the cap acitance of the capacitor, Єo: permitt ivity of the sp ace Єi = t /ω Єo R A ………. (7) Where ω: the an gu lar frequen cy =2 π f, f: the frequency Tan δ = Єi / Єr = 1 / ω R C ………(8) S= d[Ln σa.c (ω) ] / d [Ln (ω) ] this equation has mentioned before equation (2) [16, 17, 18] IBN AL- HAITHAM J . FO R PURE & APPL. SC I. VO L.22 (4) 2009 Results and Discussion Figure (3) shows t he frequency dependence of electrical conductivity (Ln σa.c vs Ln ω) ,there is an increase in σa.c valu es with the increase of frequency values, and it is found from this figure that alternative electrical conductivity nearly or app roximately follows a p ower law of equation [1], with the exp onential S < 1, where(S) has b een computed (S = 0.9257). S is a variable depends on the temp erature and its value arises in the range (1>S >0.5), the alternative electrical conductivity depends on ω at this range. Figure (4) shows the variation of cap acitance with the frequency, it has shown that cap acitance values decrease with the frequency increase and at the last p oints in the figure, the chan ge is very small. Figure (5) shows the variation of the real p art of dielectric constant (Єr) with frequency, the values of Єr decr ease with frequency incr ease till f = 100 kHz , where at the last p oints the change is very small and the points try to be nearly constant. Figures (6 and 7) show the variation of imaginary p art of dielectric constant (Єi) and the loss factor (Tan δ ) with frequency, the values of (Єi) and (Tan δ) decrease with frequen cy increase till f = 20kHz where the last p oints begun to increase, the beh avior of (Єi and Tan δ) with frequency are similar to each other and that’s clear if we see the equation (8) where the p rop ortion between them is direct, and the behavior of (Єr and C) with frequen cy are si milar to each other and that’s clear if we see the equation (6) where the prop ortion between them is direct t oo. Generally , crystallization occurs by a mechanism of nucleation and growt h. In such a mechan ism, small cryst alline nuclei for m initially, which subsequently grow. The formation of nuclei can proceed at t he surface / interface and / or t he bulk of the material. To understand the kinetics of crystallization, the activation ener gy for nucleation needs t o be considered f irst. The net change in free ener gy is the sum of the decrease in volume free ener gy due to crystallization and the increase in free ener gy due to the surface energy [19]. This volume chan ge upon crystallization causes changes in the film capacitance. Dep ending on the equation (3) σa.c will ascendant at high fr equencies and with low temp eratures, whereas σd.c will ascendant at low frequencies and with hi gh temp eratures. Conclusions The (ω s) dependence of conductivity can be considered as an indication of the exist ence of a wide d istribution of transition p robabilities for charge carr iers. Carr iers t ransfer through interfaces either between the sp ecimen and electrodes or b etween grain boundaries also contribute to (ω s) dep endence [16]. - Successive lay ers of (Al -CdSe0.8Te0.2-Al) structure can be obtained by vacuum evap oration in the range of frequency (100Hz -400kHz). - At the range f = (100Hz -20kHz ) where the values of loss factor decrease with frequency the material shows a good insulating p rop erties because when Tan δ decrease the cap acitor would be near to the ideal state(5 ) - This cap acitor can be used in engineering app lications as insulators of field effect transistor, JFET amp lifier, in applying alternative current (AC) t o direct current (DC) in biased transist or circuit[1]. Re ferences 1- Theraja, B.L. and Theraja, A.K., (1988), “ATEXT-Book of Electrical Technology ”, Nirja Const ruction and Development Co. (P) Ltd., Ram Nagar, New Delhi. IBN AL- HAITHAM J . FO R PURE & APPL. SC I. VO L.22 (4) 2009 ت ترجمة رفع الطبعة األولى، ،“المواد والنبائط للمهندسین الكهربائیین والفیزیائیین ” ، )1992( ،.اي .ار ،كولك الیسر -2 .جامعة البصرة ،مشاعل متي وریتا یعقوب ،رزق 3- Rassam, N.T ., (2000), “Study ing the Structure and Transp ort Electrical and Opt ical Prop erties of Evaporated CdSe1-xTex Thin Films”, Ph.D. T hesis, Baghdad University . 4- M angalhara, J.P.; Thangaraj, R. and Angihotri, O.P., (1989), Solar Energy M aterials, 00SEM 00611. جامعة دار الكتب للطباعة والنشر، ،“ فیزیاء األلكترونیات ” ،)2001(، ظفر أنورالنعمة، م ;كاع فرمانمحمد، و -5 .الموصل 6- Ovshinsky , S.R., (1968), Phys. Rev. Lett . 21, 20. 7- Libera, M . and Chen, M ., (1990), M ater. Res. Soc. Bull. 15, 40. 8- Koo, Y.W.; Kim, J.H.; Cho, W.J. and Chun g, H.B., (2007), M icroelectron. Eng. Online: doi: 10.1016 / j. mee., (2007), 01,.245. 9- Ghanlouche, H.; M ahmoud, S.T.; Qamhieh, N.; Ahm ed, S. and AL- Shamisi, H., (2008), Phys. Scr. 77 015701. 10- Ermolovich, I.B.; Parelets, A.M . and Khanat, L.N., (1986), Thin Solid Films, 143 225. 11- Solymar S. L. and Walsh D., (1998), “Electrical p rop erties of materials”, O xford, New York. 12- M angalhara, J.P., (1989), “Ph. D.Thesis”, Indian Institute of Technology . 13- Padmanabha Sarma, H.; Subramanian, V. ; Rangarajan, N. and M urali, K.R., (1995), “Bull. M ater. Sci.”,. 7, Nove mber, PP. 875-881 Printed in India 14- Jonscher, A.K., (1972), “J. Non-Cry st. Solids”, 8-10, 293. 15- Jonscher, A.K., (1973), “J. Phy s.”, C, 6 L 235 16- Kao, K.C. and Hwang, W., (1981), “Electron Transp ort in Solids”,. 14, Pergamon Press. 17- Tauc,J., “Amorp hous and liquid Semiconductors”, Plenum Publishing Co mpany Ltd., London, (1974). ان هادي عبیدالحساني، -18 M(دراسة الخواص البصریة والتوصیلیة المتناوبة لزجاج ” ،)1992 (،إحس gO-p 2O5( “، .رسالة ماجستیر مقدمة إلى جامعة بغداد 19- M einders, E.R.; M ijiritskii, A.V.; Pi eterson, L. and Wutt ig, M ., (2006), “Optical Data Storage Phase Change M edia and R ecording", (B erline: Sp ringer). a-Th e mask u sed for film d eposition. b-Th e mask used for Al -electrodes deposition Fig. (1) The masks used in preparing the capacitor IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.22 (4) 2009 Fig. (2) Shows the circuit diagram of alternative condu ctivity measurements -22 -20 -18 -16 -14 -12 6 8 10 12 14 L n a .c C o n d u c ti v it y (o h m . c m )- 1 Fig. (3) Shows the frequ ency depend ence of alternative electrical conductivity 30 40 50 60 70 100 1 000 10000 100000 1000000 Frequency (HZ) C a p a c it a n ce *1 0 ^ -9 (f a ra d ) Fig. (4) Shows the variation of capacitance with frequ ency Wires for cond ction LCR The lo wer Al el ectrode Film 2 CdS e0.8T e0. fu rnace The up per Al electrod e T he substrate T he substrate IBN AL- HAITHAM J . FO R PURE & APPL. SC I. VO L.22 (4) 2009 25 30 35 40 45 50 55 60 6 8 10 12 14 Ln(W) W in(ra d/s ) R e a l p a rt o f d ie le c tr ic c o n s ta n t( E r) Fig. (5) Sh ows the variation of real part of dielectri c constant Єr with frequ ency 3 5 7 9 11 6 9 12 15 Ln(W) W in(rad/s) Im a g in a ry p a rt o f d ie le c tr ic c o n s ta n t (E i) Fig. (6) Shows the variation of imaginary p art of dielectric constant Єi with frequ ency 0.14 0.16 0.18 0.2 0.22 6 9 12 15 Ln(W) W in HZ L o s s f a c to r Fig. (7) Shows the variation of loss factor tan δ with frequ ency 2009) 4 (22 المجلد مجلة ابن الھیثم للعلوم الصرفة والتطبیقیة الخواص الكهربائیة المتناوبة للمتسعة (Al-CdSe0.8Te0.2-Al) درجة حرارة الغرفةعند غصون جلیل إبراهیم ، جامعة الكوفةكلیة التربیة للبناتقسم الفیزیاء ، الخالصة عند ودراسة الخواص الكهربائیة المتناوبة لها ،Al-CdSe0.8Te0.2-Al)( یهدف هذا البحث إلى تحضیر المتسعة . لتطبیقات األلكترونیةألغشیة في اودراسة إمكانیة استخدام هذه ادرجة حرارة الغرفة، .ولوحظ أنها تزداد بزیادة التردد) f=100Hz-400KHz(التوصیلیة الكهربائیة المتناوبة في مدى الترددات قیست ض بزیادة یقي من ثابت العزل الكهربائي وأظهرت دراسة تغیر كل من السعة والجزء الحق مع التردد أن قیمها تنخف مع التردد أن قیمها تنخفض من ثابت العزل الكهربائي تغیر كل من عامل الفقد والجزء الخیالي دراسة أظهرت و .التردد .بزیادة التردد ثم تبدأ بالزیادة