IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L. 23 (1) 2010 D.C. Conductivity of a-InAs Films Prepared at Different Thickness B. K.H.al-Maiyaly Department of Physics ,College of Education , University of Baghdad Abstract The behaviour of the electrical conductivity (σ) and the activation energies (Ea1, Ea2) have been investigated on a-InAs thin films as a function of thickness (250,350,450,550,650) nm, before and after heat treatment. The films were annealed at (373, 423, 473) K for one hour. The films contain two types of transport mechanisms, and the electrical conductivity (σ) increases whereas the activation energy (Ea) would decrease as the films thickness increases. Introduction The method of preparation plays an important role in preparing InAs films [1] because of the tendency of the material to dissociate at its melting point, which makes it difficult to obtain stoichiometric films [2-7]. Several authors have used co-evaporation [3], sputtering [4, 5], electro deposition [6], flash evaporation [2, 8], chemical deposition and heating with a vacuum deposited [9] etc. In the present paper, thermal evaporation technique for a-InAs films formation as it ensures stoichiometric was used. Detailed measurements are made on electrical conductivity and activation energies of this material , by varying thickness for both as deposited and annealed films at (373, 423, 473) K for one hour. We have already reported [10] on the Hall effect measurements on a- InAs films prepared at different thickness. Experiment Amorphous InAs films were prepared onto a glass slide substrate (using a suitable mask) in a vacuum of (3*10-6) torr by using Edward coating unit model E 306 A. The alloy was obtained by fusing the mixture of the appropriate quantities of the elements in evacuated fused quartz ampoules at (1273) K. The ampoules quenched rapidly in cold water. Film thickness varied from 250 nm to 650 nm. The distance from molybdenum boat to substrate was about 15 cm. The deposition rate was about 1nm/s for all the films. Al electrodes were used as contact material for making the electrical connections. For D.C. measurement Keithly model 616 was used to measure the variation of electric resistance (R) with temperature range (298-503) K, then calculated the resistivity (ρ) by the formula [11]:- L tbR   ……………………………………… (1) Where t is film thickness, b is electrodes width; L is the distance between two Al electrodes. The conductivity (σ) is related to the resistivity by equation [11]:- IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L. 23 (1) 2010   1 c.d ………………………………………… (2) Results and Discussion Fig.1 shows the lnσ of a- InAs film versus 103/T results obtained for different thickness on a various samples for as-deposited films. This figure also shows two mechanisms for electrical conductivity at lower and higher temperatures with two values of activation energy (Ea1, Ea2) for all films, which means that there is two mechanism of transport, at a higher temperature range (383-503) K the conduction mechanism of this stage is due to carriers excited into extended states beyond the mobility edge .Atother range of temperature (298- 383) K the conduction mechanism due to carriers excited into the localized states at the edge of the bands and hopping. Fig.2 (a, b) shows the variation of the electrical conductivity (σ) of an InAs film, as a function of thickness and annealing temperature. In conclusion, it is seen that the films with lower thickness had lower conductivities than those with higher thickness. We should point out that the electrical conductivity (σ) increased dramatically with increasing (t) and reached the maximum value (1.6 *10-3) ohm-1.cm-1 at t=650 nm for film Ta=298 K. The increasing trend in σ upon increasing thickness is due to the decreasing scattering at grain boundaries in thicker films [12]. We believe that the increase in film thickness (t) decreases the trapping centers of charge carriers, this is, perhaps, duo of the decreases of the grain boundary scattering, moreover it yields more packing density , this result is in an agreement with Sharma and Reddy [2], who prepared InAs films by the different method and different deposition parameters. Fig.3 (a, b) shows the activation energies (Ea1, Ea2) of an InAs film, as a function of thickness for different films, before and after heat treatment. It is clear from these figures that both Ea1 and Ea2 decrease with increasing thickness (t). It is clear from figures (2, 3) that σ decreases after heat treatment, while the activation energies showed opposite trend for all films prepared because of the decreased number of carriers available for transport, this behaviour can be attributed to the decrease of the density of states in the gap, the reduce of the dangling bonds, and defects like vacancy sites in the films structure with the increasing Ta. Conclusions The electrical conductivity and activation energies of thin a-InAs films, prepared by thermal evaporation, are seen to be dependent on the film thickness. The electrical conductivity shows an increasing behaviour with an increasing thickness before and after heat treatment, whereas the activation energies decrease as the thickness increases. The films contain two types of transport mechanisms, and the electrical conductivity decreases with the increasing Ta while the activation energies decrease. References 1- Sharma, A.K. and Reddy, P.J., (1984), Appl.phys.A, 34: 69-71. 2- Sharma, A.K., (1984), Czech.J.Phys.B, 34(7): 705-711. 3- Howson, R.P.and Malina, V., (1970), J.Phys.D. 3: 854-861 . 4- Jha, K.N.and Korgaonkar, A.V., (1972), Thin Solid films, 9:133-139. 5- Szczyrbowski, J.; Czapla, A.and Jachimowski ,M., (1977) Thin Solid films, 42: 193-198. 6- Valentin, M.; Benedetto, B. and Luisa, P. (2004), Journal of alloys and compounds, 366 (1):152-160. IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L. 23 (1) 2010 7- Myers, H. P. (1997) Introductory Solid State Physics, Second edition, Chalmers University of Technology, Sweden. 8- Sharma, A.K.and Reddy, P.J. (2006), Physica Status Solidi (a), 61(1):295-299. 9- Botha, L.; Shamba, P.and Botha, J.R., (2006), Semicond.Sci.Technol., 21: 450-461. 10- Al-Maiyaly, B.K.H.,(2000), J.ofCol.ofEducation",AL-Mustansiriyah University, 2:15-20. 11- William, D. Callister, (2003), Materials Science and Engineering, an Introduction, 6 th edition, John Wiley & Sons, Inc. 12- Sharma, A.K. and Reddy, P.J., (1983), “Phys.Stat.Sol.(a)”, 78(1):K27. Fig.(1): Electrical conductivity behaviour as a function of temperature for a-InAs thin films as-deposited at different thicknesses. (a) (b) (a) (b) Fig.(2): Electrical conductivity of a-InAs thin films as a function of:- (a) thickness at different annealing temperature. (b) annealing temperature formed at different thicknesses. -9 -7 -5 -3 -1 1.5 2 2.5 3 3.5 4 1000/T ( K ) -1 ln  ( o h m .c m ) -1 t=250 nm t=350 nm t=450 nm t=550 nm t=650 nm 1.E-05 1.E-04 1.E-03 1.E-02 150 250 350 450 550 650 750 Thickness (t) nm  ( o h m .c m ) -1 R.T Ta=373 K Ta=423 K Ta=473 K 1.E-05 1.E-04 1.E-03 1.E-02 273 323 373 423 473 523 Ta ( K )  ( o h m .c m ) -1 t=250 nm t=350 nm t=450 nm t=550 nm t=650 nm IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L. 23 (1) 2010 (a) (b) Fig.3 Fig.(3):Activation energies versus thickness of a-InAs thin films at different annealing temperatures 0 0.05 0.1 0.15 0.2 0.25 150 250 350 450 550 650 750 Thickness (t) nm E a 1 ( e V ) R.T Ta=373 K Ta=423 K Ta=473 K 0.3 0.35 0.4 0.45 0.5 0.55 150 250 350 450 550 650 750 Thickness (t) nm E a 2 ( e V ) R.T Ta=373 K Ta=423 K Ta=473 K 2010) 1( 23مجلة ابن الھیثم للعلوم الصرفة والتطبیقیة المجلد العشوائیة InAsالتوصیلیة المستمرة ألغشیة المحضرة عند أسماك مختلفة كاظم حسون المیالي بشرى جامعة بغداد ،أبن الهیثم -كلیة التربیة قسم الفیزیاء، الخالصة العشـوائیة دالــة InAsیهـدف هـذا البحـث الـى دراسـة سـلوك التوصـیلیة الكهربائیـة وحسـاب طاقـات التنشـیط الغشـیة ة وقـد، لتغیـر السـمك ولـدنت بـدرجات حـرارة تلــدین ، nm (250,350,450,550,650)حضـرت االغشـیة بأسـماك مختلفـ (373, 423, 473) K ومدة ساعة واحدة. ولـوحظ زیـادة التوصـیلیة الكهربائیـة مـع نقصـان طاقـات التنشـیط بزیـادة ،وقـد أظهـرت االغشـیة آلیتـین لالنتقـال االلكترونـي .سمك االغشیة المحضرة