علوم الصرفة والتطبيقية المجلد 2009) 3( 22مجلة ابن الهيثم لل الرقیقة CdTe:In صائص التركیبة والكهربائیة الغشیةدراسة الخ أقبال سهام ناجي ، جامعة بغدادقسم الفیزیاء، كلیة العلوم الخالصة علـى أرضـیات مـن الزجـاج 300nmبسـمك المطعمـة باالنـدیوم لقد تم إنمـاء أغشـیة تلوریـد الكـادمیوم المتعـددة البلـورات درســت الخصــائص التركیبیــة والكهربائیــة لهــذه . باســتخدام تقنیــة التبخیــر الحـراري المــزدوج 423K عنـد درجــة حــرارة تســاوي ددة تحلیــل األشــعة الســینیة أوضــح ان جمیــع النمــاذج هــي متعــ .K(323,373,423)األغشــیة دالــة للمعاملــة الحراریــة عنــد او أي Cd, Te ود تقابـل العناصـرحیـلـم تالحـظ قمـم ، و [111] ب المكعبي مع توجه مفضـل باالتجـاهالبلورات وتمتلك التركی مـن %1.5 بنسـبة CdTeوتركیـز الحـامالت یـزداد عنـد تطعـیم غشـاء ،وجـد ان المقاومیـة الكهربائیـة تهـبط .مركبات أخـرى .لفةاالندیوم وعند معاملتها عند درجات حرارة مخت IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 Study The Structural And Electrical Properties Of CdTe:In Thin Films I. S. Naji Departme nt of Physics, College of Science, Unive rsity of Baghdad Abstract Indium dop ed CdTe p oly crystalline films of thickness equals to 300nm were grown on cornin g glass substrates at temp erature equals to 423K by thermal co-evaporation technique. The st ructural and electrical p rop erties for these films were st udied as a function of heat treatment (323,373,423)K. T he x-ray analysis showed that all sa mples are polycrystalline and have the cubic zincb lende st ructure with p referential orientation in the [111] direction, no diffraction p eaks corresp onding to metallic Cd, Te or other compounds were observed. It was found that the electrical resistivity drops and the carrier concentration incr eases when the CdTe film dop ed with 1.5% indium and treated at different annealin g temp eratures. Introduction There is a good nu mber of comp ound semiconductors from both of the III-V and II- VI group s widely emp loyed in technological devices. M ost of the optoelectronic industry is based on III-V compounds while II-VI comp ounds can be found in diverse app lication such as IR detection, X-ray detection, p hotovoltaic devices, etc. However, the interest to address unsolved technology needs and imp roves current device p erformance, drives the research to obtain new semiconductor compounds (1). The unique p rop erties of CdTe make it an ideal material for several app lications: p hotovoltaic cells, nuclear detectors, high p erformance electro-op tic modulators and p hotorefractive devices. It can exhibit both n and p types of conductivity , which makes diode technology and field effect transistors p ossible, and it can exh ibit a semi- insulating st ate as well. Device grade CdTe thin films can be formed in both homojunction and heterojunction configuration (2,3). The app licability of CdTe to be substrates for HgCdTe epitaxial lay ers was st udied for a long time (4). CdTe-based semimagn etics, like CdM nTe, disp lay extremely excitin g p rop erties which have not so far been comp letely exp loited. CdTe is a comp onent of the ternary alloy CdHgTe, one of the major indust rial materials for infrared detection (2). M ost afa Abd El-Raheem (5) in 1996 st udied the conductivity of CdTe sample p repared by travelling heater method technique, undop ed, In, Ga, and p b-dop ed. In-dop ed sample showed a gapless semiconductor behaviour. In 1997 Becerril et al (6) st udied the electrical, st ructural and op tical p rop erties as a function of the concentration of elements for indium dop ing of CdTe films p repared by co-sp uttering of CdTe-In-Cd targets. They found the electrical resistivity drop s and carrier concentration in creases when Cd and In are simultaneously incorp orated. The efficiency of these devices is strongly determined by the electrical and op tical p rop erties of the comp onent films. A study of these prop erties and their dep endence on the p rep aration conditions for obtaining films which to assure a highly and st able efficiency of resp ective devices (7). Gri ll R. et al (8) showed theoretically the p ossibility to p repare semi-insulating CdTe with a deep -level dop ing below the limit 10 13 cm -3 demanded in detector industry . The heavy metals exhibit a low diffusivity in the host CdTe lattice and, due to their similar electronegativity and ionic rad ius related to Cd ions, t hey comp ensate the native p oint defects (mostly Cd vacancies) and occup y the Cd sites (9). In this p aper, combined st udies of the st ructural and electrical p rop erties of CdTe thin films dop ed with 1.5% Indium under varying annealin g temp erature were st udied. IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 Experimental The CdTe alloy was p repared from high p urity (99.999% p ure obtained from Balzers, Switzerland) of cadmium and tellurium. The p olycryst alline films of thickness 300nm dop ed with 1.5% indium grown on corning glass slides at t emp erature equals t o 423K by thermal co- evap oration by using Edward coating unit model 306A. The sour to substrate distance was 15cm and the deposition rate was fixed at 0.5nm/sec. The cry st alline structure of the film was determined by x-ray diffraction p att ern, measured with an x-ray diffractometer (Phillips PW 139) around 60 o in the 2θ scale. The samples annealed at different annealing temp erature (323,373,423)K in air for one hour. Then the films were p rovided with suitable masks to deposit the aluminum and indium electrodes. The electrical contacts were made by fine copp er wires soldered to the electrodes by indium using low p ower soldering iron (30 watt). The electrical conductivity of CdTe films as function of temp erature is st udied in the range of temp erature (300- 473K). D.C p ower sup p ly ty p e (PE-1540) and digital electrometer Keithley (616) were used. The D.C. conductivity was determined according to the relation  1.................................. .. twR L  Where L is the dist ance between the electrodes, w is the width of t he film, t is the thickness of the film and R represents the measured electrical resistance of the film. Hall effect was carried out by using D.C. p ower sup p ly (0-40 volt), and two digital electrometer (Keithley) to measure the p assing current (I) and Hall voltage (VH) that emerge after app lied constant transverse magnetic field (B=0.25 Tesla). Hall coefficient (RH), concentration (n) and mobility (μ) of the carrier for all samples were obtained by using the following equations resp ectively.  2............. . . BI tV R HH   3............... . 1 HRe n   4................ .ne H    Where t thickness of the sample which is determined by using op tical interference fringes (fizeau fringes). Re sult and Discussion These results include the X-ray diffraction examination of CdTe alloy, p ure CdTe film, and films dop ed with 1.5% indium which treated at different annealing temp erature (323,373,423)K, also the result of electrical measurements ( D.C. conductivity , Hall effect) of films emp loyed on which Al, and In electrodes were analyzed. S tructural propertie s X-ray diffraction sp ectra of CdTe p owder and CdTe films have p olycryst alline st ructure as shown in figure (1). The two sp ectra disp lay the characteristic diffraction p eaks of cubic p hase of CdTe. The diffraction p eaks of p owder at 23.90 o , 39.50 o , 46.60 o , and 56.80 o corresp ond to the (111), (220), (311), and (400) cryst alline planes of this p hase resp ectively. While p ure CdTe film deposited at substrate temp erature equals to 423K revealed the cubic p hase with sharp p eaks at 2θ equal to 23.71 o , 39.50 o , and 46.50 o corresp ond to the (111), (220), and (311) cry st alline planes of this p hase resp ectively. Both the p eak height and peak p ositions of the sp ectra are in a good agreement with ASTM x-ray p owder file data for cubic CdTe. The fourth p eak, which represents the reflection from (400) p lane was vanished and disapp eared in the diffraction p att erns of the films and the residue p eaks became lower intensity while there is an increase of the peak height in the [111] direction, this mains there is a good orientation in the [111] direction. IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L.22 (3) 2009 In figure (2) ty p ical x-ray diffraction sp ectra are p resented showing the influence of dop ing with 1.5% In as deposited film and the films which treated at different heat treatment( 323,373,423)K in air for one hour. It can be noticed that all the sp ectra disp lay the characterist ic diffraction p eaks of the cubic p hase of CdTe. The diffraction p eaks were at 23.74 o , 39.23 o , and 46.45 o , and no CdTe hexagonal p hase was observed in the films. Similar results were reported by Becerril et al. (6) also Bon et al. (10) found that the CdTe films dop ed with low indium concentration ratio (do not exceeds 8%) p roduced a low level of In dop ing in the CdTe cubic lattice. All films deposited show the highest p eak near 2θ equals to 24 o , suggesting that the cryst al st ructure of CdTe films in zincblende with a p referential orientation of the (111) p lane, no diffraction p eak corresp onding to metallic Cd, Te or other comp ounds was observed. The [111] direction is the close- p acking direction of the zincblende st ructure and this t y p e of textured growt h has often been observed in polycryst alline CdTe films grown on amorp hous substrates (11). Dawar et al. (12), Zelay a et al. (13), M . Bayhan (14), and Rusu et al. (15) reported that CdTe films grown by different deposition methods had essentially a cubic st ructure with a [111] p referred orientation, while Winn et al. (16) Found the x-ray diffraction p att erns of films deposited by thermal evap oration are att ributed to cubic CdTe, hexagonal CdTe, and hexagonal Te. Annealing the CdTe:In thin films at (323,373,423)K about one hour showed a dramatic change, where the diffraction lines in the sp ectra of samples which treated at different annealing temp erature exhibit a gradual broadening and a decrease in their intensity , the latter indicates a decrease in the cryst alline quality of these films, including a reduction in p article size. This happ ens due to the increase of the Cd vacancies with the increase of the annealing temp erature, then In atoms substitut e Cd atoms in these films, since In has an ionic and covalent radii smaller than Cd, it would be exp ected a latt ice deformation when In substitut es Cd. Electrical Propertie s For t he electrical measurements, the samp les with p lanar geometry were used. It is known that t he study of the temp erature dep endence of the electrical conductivity of semiconducting thin films offers a lot of information about the correlation between the st ructure and the electrical p rop erties of the films. In order to st udy conductivity mechanisms, it is convenient to p lot logarithm of the conductivity (lnσ), as a function of recip rocal of temperature. Fig.(3 a, b, and c) show the variation of lnσ with 1000/ T for CdTe film, and CdTe:In films with Al and In electrodes resp ectivily. These figurs reveal that there is dependence of conductivity on temp erature and there are two t ransp ort mechanisms giving rise to two activation energies Ea1,and Ea2. This result is in contrast to the observation of Abd El-Raheem (5), who found that the conductivity of In dop ed samp le is insensitive to t emp erature within the whole considered range of Tam b. The conductivity of CdTe film was 0.99x 10 -5 (Ω.cm) -1 and increased to 1.1x10 -5 and 5.4x 10 -5 (Ω.cm) -1 when the film dop ed with indium for Al and In electrodes resp ectivily, as shown in table (1).We believe that the increment in conductivity when the films dop ed with In att ributed to this donor imp urities. (In) p revents the formation of Cd vacancies which act as electron traps. Similar results were obtained by Becerril et al. (6), they have observed that the resistivity which reduced two orders of magnitude for films dop ed with 2.5 % indium, and to achieve that change in the resist ivity , a much larger amount of In in the film was needed. They believe the lower resistivity is due to alloy ing effects in the CdTe-In sy st em. M ohammed et al. (17) rep orted that the resist ivity of CdTe films is lowered by more than one order of magnitude due to indium dop ing. While Bon et al. (10) found that t he electrical IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 resistivity was decreased by almost three orders of magnitude when the films were dop ed with 10% indium. There is also an increment in conductivity for CdTe:In films treated at different annealing temp erature, where they were increased from 1.1x10 -5 ,and 5.9x10 -5 (Ω.cm) -1 to 6x10 -4 and 3.1x10 -3 (Ω.cm) -1 for films annealed at 423K which p rovided with Al, and In electrodes resp ectively. It is wort h to mention that the effect of the annealing, lead to a great reduction of the resistivity , which is caused by some intermediate st ages of st ructure created during annealing. Hall effect measurements give a comp lementary information about the electrical p rop erties such as ty p e of charge carriers of the semiconductor material, concentration (n) and their mobilities. Fig.(4 a, b, and c) illustrate the relation between Hall voltage (VH) and the current of CdTe film, and CdTe:In films with Al & In electrodes resp ectively. From this figure it is clear that all samples have a negative Hall coefficient, i.e. the ty p e of carrier is n-ty p e, the electrons are predominant in the conduction p rocess, this happ ens because the introduction of III group donors into CdTe latt ice allows to get n-t y p e low-resist ive material. This result is in an agreement with the result of Becerril et al. (6). They reported that the films become n-ty p e after the indium incorporation. Table (1) summaries the values of Hall coefficient, carrier concentration, and mobility . It can be noticed that the carrier concentration was 1.36x10 14 cm -3 for CdTe film and was increased to 4.0x10 15 and 6.5x10 14 cm -3 for CdTe:In films with Al & In electrodes resp ectively. Such result was found by Becerril et al. (6), where the carrier concentration is increased from 10 13 cm -3 to almost 10 16 cm -3 when the film dop ed with 2.5% indium. Also it was increased when the films annealed at different annealing temp eratures. The carriers mobility were increased when the films annealed, sp ecially for CdTe:In films with In electrodes which means In is a good material for ohmic contact with CdT e films. Conclusion In this work we have shown that all CdTe and CdTe:In films have cubic zincblende st ructure, and no app earance of other cryst alline p hases are observed, this means that the adding In does not change the phase of CdTe film, while it makes lattice deformation when In substitut es Cd because the small radii of In atoms. The electrical resistivity of n-ty p e CdTe films which are p repared by thermal co-evaporation technique drop s one order of magnitude lower and carrier concentrations increase one order larger than intrinsic films. Refe rences 1- M elendez-Lira, M .; Becerril- Silv a, M .; Zap ata-Torres, M.; M endoza-Galvan, A. and Jimenez-Sandov al S., (2005), “Sup erficies Vacio”, 18 ( 3): 22-26. 2- Triboulet, R., (2003), “Cry st. Res. Technol”, 38,(3-5): 215-224. 3- Zengir, B.; B ayhan, M . and Kavasoglu, S., (2006), “Journal of Arts and Sciences Sayt”, 5, (103-116). 4- Jeong, T.S. and Yu, P.Y., (2003), “Journal of the Korean Phy sical So ciety ”, 43, (6), (1101-1104). 5- Abd El-Raheem, M .M ., (1996), “Indian Journal of Pure & App lied Phy sics”, 34, (398- 404). 6- Becerril, M .; Zelay a-Angel, O.; Ramirez-Bon, R.; Esp inoza-Beltran, F.J. and Gonz alez-Hernandez, J., (1997), “App l. Phy s. Lett.”, 70(4): 452-454. 7- Rusu, G.G., (2001), “Journal of Op toelectronics and Advanced M aterials”, 3, (4), (861-866). 8- Grill, R.; Franc, J.; Turkevych, I.; Hoschl, P.; Belas, E. and M oravec, P., (2005), “Journal of Electronic M aterials”, 34, (6), (939-942). 9- Suh, J.H.; Cho, S.H.; Won, J.H.; Hon g, J.K.; Kim, S.U.; Ki m, K.H.; Kim, H. and Kim, S.S., (2006), “Journal of the Korean phy sical society ”, 49, (S750-S754). IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 10- Ramirez-Bon, R.; Lop ez, R.N.; Beltran, F.J.; Angel, O.Z . and Hernandez, J.G., (1997), “J. Phys. Solids”, 58, (5): (807-811). 11- Lee,J.H.; Lim, D.G. and Yi, J. S., (2003), “Solar Ener gy M aterials & solar cells”, 75, 235-242. 12- AL.Dawar; Jagadish, C.; Ferdin and, K.V.; Kumar, A. and Mathur, P.C, (1985), “App l. Surf. Sci”, 22-23, 846-858. 13- Zelaya, O.; Sinencio, F.S.; Alvarez, J.G.; Farias, M .H.; Araiza, L. and Flores, G.H., (1988), “J. Appl. Phys.”, 63, (2): 410-413. 14- Bayhan, M ., (1998), “Tr. J. of p hysics”, 22, 929-937 15- Rusu, G.G. and Caraman, I., (2000), “AL. I. CUZA”, 158-165. 16- Winn, M .B. and Ly ons, L.E., (1985), “App lications of surface scien ce”, 22-23, 724- 730. 17- M ohammed, W.F. and Yousif M .A., (2002), “Renewable Ener gy ”, 26, 285-294. Table (1) The electrical paramete rs of CdTe and CdTe:In films obtain from D.C and Hall effect measurements. Films Ta(K) σX10 -5 (Ω.cm) -1 Ea1 (eV) Ea2 (eV) RH (cm 3 /C) n H X10 15 (cm -3 ) μH (cm 2 /V.s) CdTe 300 0.99 0.765 0.274 45840 0.136 0.4550 CdTe:In With Al electrodes 300 1.10 0.643 0.179 1560 4.006 0.0166 323 4.70 0.637 0.173 1080 5.787 0.0505 373 14.0 0.587 0.171 720 8.680 0.1020 423 60.0 0.523 0.166 480 13.02 0.2880 CdTe:In With In electrodes 300 5.9 0.538 0.162 9600 0.651 0.571 323 24 0.509 0.158 8880 0.703 2.140 373 121 0.436 0.148 5040 1.240 6.080 423 318 0.406 0.144 3960 1.578 12.60 Fi g. (1) X-ray diffracti on patte rn of C dTe . a- al loy b- film de posite d at 423 K. Fi g.(2) X-ray di ffra cti on pattern of C dTe :In films. a- as de posite d b- anneale d at 323K c- annealed at 373K d- anneale d at 423 K. Fig. (4) The rel ationshi p between Hall voltage and Passing current for a- CdTe film b- CdTe:In films with Al electrodes c- CdTe:In films with In e lectrodes. Fig. (3) The plots of l n σ vs. 1000/T fora- CdTe fi lm b- CdTe:In films with Alelectrodes c- CdTe:In fi lms with In e lectrodes.