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 Departme nt of Physics ,College of Education , Unive rsity of Baghdad Abstract The behaviour of the electrical conductivity (σ) and the activation energies (Ea1, Ea2) have been invest igated on a-InAs t hin 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 ty p es of transp ort mechanisms, and the electrical conductivity (σ) increases whereas the activation energy (Ea) would decrease as the films t hickness increases. Introduction The method of p reparation p lay s an important role in preparing InAs films [1] because of the tendency of the material to dissociate at its melting p oint, which makes it difficult to obtain st oichiometric films [2-7]. Several authors have used co-evaporation [3], sp uttering [4, 5], electro deposition [6], flash evap oration [2, 8], chemical deposition and heating with a vacuum deposited [9] etc. In the p resent p aper, thermal evap oration technique for a-InAs films formation as it ensures st oichiometric 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 p repared at different thickness. Experiment Amorp hous InAs films were p repared 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 obt ained by fusing the mixture of the app rop riate quantities of the elements in evacuated fused quartz amp oules at (1273) K. The amp oules quenched rapidly in cold water. Film thickness varied from 250 nm to 650 nm. The distance from moly bdenum 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 temp erature 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 t he resist ivity by equation [11]:- IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010   1 c.d ………………………………………… (2) Re sults and Discussion Fig.1 shows t he lnσ of a- InAs film versus 10 3 /T results obt ained for different thickness on a various samples for as-deposited films. This figure also shows two mechanisms for electrical conductivity at lower and higher temp eratures with two values of activation energy (Ea1, Ea2) for all films, which means that there is two mechanism of transp ort, at a higher temp erature range (383-503) K the conduction mechanism of this st age is due to carriers excited into extended st ates beyond the mobility edge .At other range of temp erature (298- 383) K the conduction mechanism due to carriers excited into t he localized st ates at the edge of the bands and hop p ing. Fig.2 (a, b) shows the variation of the electrical conductivity (σ) of an InAs film, as a function of thickness and annealing temp erature. In conclusion, it is seen that the films with lower thickness had lower conductivities than those with higher thickness. We should p oint out that t he 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 up on increasing thickness is due to the decreasing scatt ering at grain boundaries in thicker films [12]. We believe that the increase in film thickness (t) decreases t he trapp ing centers of charge carriers, this is, p erhap s, duo of the decreases of the grain boundary scatt ering, moreover it y ields more p acking density , this result is in an agreement with Sharma and Reddy [2], who p repared InAs films by the different method and different dep osition 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 op p osite trend for all films p repared because of the decreased number of carriers available for transp ort, this behaviour can be att ributed to the decrease of the density of st ates in the gap, the reduce of the dangling bonds, and defects like vacancy sites in the films st ructure with the increasing Ta. Conclusions The electrical conductivity and activation energies of thin a-InAs films, p repared by thermal evap oration, 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 ty p es of transp ort mechanisms, and the electrical conductivity decreases with t he increasing Ta while the activation energies decrease. References 1- Sharma, A.K. and Reddy , P.J., (1984), Ap p l.phy s.A, 34: 69-71. 2- Sharma, A.K., (1984), Czech.J.Phy s.B, 34(7): 705-711. 3- Howson, R.P.and M alina, V., (1970), J.Phy s.D. 3: 854-861 . 4- Jha, K.N.and Korgaonkar, A.V., (1972), Thin Solid films, 9:133-139. 5- Szczy rbowski, J.; Czap la, A.and Jachimowski ,M ., (1977) Thin Solid films, 42: 193-198. 6- Valentin, M .; Benedett o, B. and Luisa, P. (2004), Journal of alloys and comp ounds, 366 (1):152-160. IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 7- M y ers, H. P. (1997) Introductory Solid State Phy sics, Second edition, Chalmers University of Technology , Sweden. 8- Sharma, A.K.and Reddy , P.J. (2006), Phy sica 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-M aiy aly , B.K.H.,(2000), J.ofCol.ofEducation",AL-M ust ansiriy ah University , 2:15-20. 11- William, D. Callist er, (2003), M aterials Science and Engineering, an Introduction, 6 th edition, John Wiley & Sons, Inc. 12- Sharma, A.K. and Reddy , P.J., (1983), “Phy s.Stat.Sol.(a)”, 78(1):K27. Fig.(1): Electrical conductivity behaviour as a function of temperature for a-InAs thi n films as-deposited at different thicknesses. (a) (b) (a) (b) Fig.(2): Electrical conductivity of a-InAs thi n films as a function of:- (a) thi ckne ss at different annealing te mperature. (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 Thickne ss (t) nm  ( o h m .c m ) -1 R.T Ta=373 K Ta=423 K Ta=473 K 1.E-0 5 1.E-0 4 1.E-0 3 1.E-0 2 273 323 373 423 473 523 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 e nergies versus thi ckness 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 Thickne ss (t) nm E a 1 ( e V ) R.T T a=373 K T a=423 K T a=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 T a=373 K T a=423 K T a=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 ولـدنت بـدرجات حـرارة تلــدین ، nm (250,350,450,550,650)حضـرت االغشـیة بأسـماك مختلفـة وقـد، لتغیـر السـمك (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