IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (4) 2009 Effect of Annealing Temperature on the Electrical Conductivity of Amorphous InAs Thin Films B. K.H.Al-Maiyaly Departme nt of physics ,College of Education, Ibn Al-Haitham Unive rsity of Baghdad Abstract In this research the electrical conductivity measurements were made on the amorp hous InAs films p repared by thermal evap oration method in thickness 450 nm and annealed in different t emp eratures in the range (303- 573) K. The electrical conductivity (σ) showed a decreasing trend with the increasing annealing temp erature, while the activation energies (Ea1, Ea2) showed an op p osite trend, where the activation energies are increased with t he annealing temp erature. Introduction In recent y ears, metal or semiconductor thin films were the top ic of an extensive research to reveal the effects of t heir disordered st ructure on their p hy sical prop erties. In p articular, the analysis of the electrical characterist ics has in some cases shown a ty p ical behaviour of an amorp hous structure for both metallic and semiconducting films. [1-3] The III-V semiconducting comp ounds, such as InSb and InAs, app ear to be the most suitable materials for this ty p e of st udy because of the characterist ics of their energy bands and charge carriers. [4-8] It would be of interest to st udy the electrical conductivity of InAs and correlate the data with other p rop erties so that a coherent p icture of the phy sical nature of the material may be obtained. We have already reported [9,10] on the A.C.conductivity and Hall effect measurements on a-InAs films obtained at different deposition p arameters like film thickness and annealing temp erature. Different authors have used different techniques to obtain InAs films, viz.sp uttering [4], electro deposition [11], chemical deposition and heating with a vacuum deposited [12], flash evap oration [13] etc, but the st udies on the p rop erties of vacuum evap orated InAs films are very limited because of the tendency of the material to dissociate at its melting p oint, which makes it difficult t o obtain st oichiometric films. In this p aper, thermal evaporation technique was chosen to obtain a-InAs films as it ensures st oichiometry , and st udied the electrical conductivity behaviour of these films as a function of annealing temp eratures. Experimental Thin InAs films in the thickness 450 nm were obtained on glass substrates from InAs alloy which is p repared by fusing the mixture of the app rop riate quantities of the elements of high p urity (99.999 %) in evacuated fused quartz amp oules at 1273 K. The amp oules were kept at this temp erature for [5-6] hours from the op timum temp erature, then the amp oules quenched rap idly in cold water. These films were p repared by thermal evap oration in a high vacuum sy st em of (3*10 -6 ) torr by using Edward coating unit model E 306 A from molybdenum boat heated to 1273 K. The source to substrate distance was maintained at 15 cm to obt ain uniformly thick films. The rate of dep osition was about 1 nm/sec. IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (4) 2009 Annealing p rocesses were carried out on these films at different temp eratures of (373, 423, 473, 523, and 573) K for one hour. Electrical conductivity measurements for all samples p repared (before and after heat treatment) includes st udy ing the variation of resistivity with temp erature range (303-503) K by using Keithly model 616. The resistivity (ρ) of the films is calculated by using the following equation [14]:- ρ=R.A/L…………………………………………………….. (1) Where, R: is the resist ance. A: is the area of the film (W.t). W: is the width of t he electrode. t: is t he films t hickness. L: is the dist ance between the electrodes. Then the conductivity of the films was determined by using the equation:- σ=1/ρ………………………………………………………… (2) The activation energy (Ea) could be calculated using the formula (14): - σ = σο exp (-Ea / kBT)……………………………. (3) Where σο: is const ant, but change slowly with temp erature. kB: is Boltz man's constant. T: is absolute temperature in kelvin. Results and Discussion The electrical resistance (R) of a- InAs films deposited at and around room temperature (~303 K) were found (67.2*10 6 ) ohm, this result is in agreement with A.M avrokefalos et al. [15] a-Electrical conductivity and annealing behaviour Fig.1 shows the electrical conductivity of a- InAs films with thickness 450 nm, as a function of temp erature for different films with variable annealing temp erature (Ta). This figure app ears to sep arate two temp erature ranges characterized by different conductivity slop es. At low temp erature range (303-383) K, the conduction mechanism due to carriers excited into the localized st ates at the edge of the band by hop p ing and at other range of temperature (383-503) K the conduction mechanism due to carriers excited into extended st ates beyond the mobility edg by thermal excitation. [16] Fig.2 and table (1) shows the electrical conductivity (σ) of a-InAs (450 nm thick) as a function of annealing temp erature (Ta). The noticeable remark is that the electrical conductivity decreases with increasing the Ta.It is decreased from (6.61*10 -4 ) ohm -1 .cm -1 at Ta=303 K to (8.67*10 -5 ) ohm -1 .cm -1 at (Ta=573 K).We can notice that t he decreasing trend in (σ) up on increasing Ta, is in agreement with our result on Hall effect measurements (10). The Hall mobility and carriers concentration showed an decreasing trend with annealing temp erature. This behavior can be att ributed to the imp rovement in the films st ructure with increasing Ta, due to reducing of dangling bonds, and defects like vacancy sites. b-Activation energies Activation energies (Ea1, Ea2) were calculated from lnσ vs 1/T p lots for a-InAs films in the temp erature range (303 – 503) K. IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L.22 (4) 2009 Fig.3 shows the behaviour for a 450 nm thickness film formed at different annealing temp erature (Ta). It is clear from this figure that the activation energies for a film treated at a higher annealing temp erature were higher than those treated at lower annealing temp erature. The increasing trend in Ea up on increasing annealing temp erature is due to the decreasing voides, dangling bonds, and localized st ates through energy gap which seems larger after heat treatment. Conclusions In conclusion, we st udied in detail the influence of annealing temp erature on the electrical conductivity of a-InAs t hin films. The exp erimental results obtained in this work indicate that the electrical conductivity decreasing with increasing annealing temp erature, while activation energies shows as increasing behaviour with increasing annealing temp erature . Finally, we should mention that the behavior of the electrical conductivity as a function of temp erature is a result of the community between two mechanism of transp ort, hop p ing charge transp ort between localized gap st ates near Fermi level and charge transp ort to extended st ate beyond the mobility gap. Re ferences 1- Sharma, A.K. and Reddy, P.J., (1980), “J.Non-Cry st .Solids”, 41:13-19. 2- Burrafato ,G.; M ancini, N.A.; Santagati, S. and Troja, S.O., (1984) "Thin Solid Films" 121(4): 291-301. 3- Chui, T.; Deutscher, G.; Lindenfeld, P. and M clean, W.L. (1981) "Phy s.Rev.B" 23:6172- 6180. 4- Szczy rbowski, J.; Czapla, A. and Jachimowski, M . (1977)"Thin Solid Films", 42:193- 199. 5- Hauser, J.J. (1973), “Phy s.Rev.B”, 8:2678-2685. 6- Chaudhuri ,K.D.; Luthra, A.; Saxena, P.; Agarwal, S.K. and M athur, P.C. (1980), "Phy s.Rev.B", 22: 6319-6325. 7- Clark ,A.H. (1967),"Phy s.Rev." 154:750-755. 8- Chop ra, K.L. and Bahl, S.K., (1970),"Phy s.Rev.B", 1: 2545-2553. 9- Al-M aiy aly , B.K.H., (2000),"J.ofCol.ofEducation", AL-M ust ansiriy ah University , 2: 21-26. 10- Al-M aiy aly , B.K.H., (2000),"J.ofCol.ofEducation", AL-M ust ansiriy ah University , 2: 15-20. 11- Valentin, M .; Benedett o, B. and Luisa, P. (2004),"Journal of alloys and comp ounds", 366(1):152-160. 12- Botha, L.; Shamba, P. and Botha, J.R., (2006), "Semicond.Sci.Technol." 21: 450-461. 13- Sharma A.K., Reddy P.J., (2006), "Phy sica Status Solidi (a)" 1(1):295-299. 14- William, D. Callist er, (2003) "M aterials Science and Engineering, an Introduction”, 6 th edition, John Wiley & Sons, Inc. 15- M avrokefalos, A.; Pettes, M .T.; Zhou, F., and Lishi, (2007) , "Rev.Sci.Inst rum.", 78: 034901. 16- Davis, E.A., M ott, N.F., (1970), "Phil.M ag.", 28:904-911 IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L.22 (4) 2009 Table (1):Liste d the value of σ at a gi ven annealing temperatures Ta for thermal evaporated a-InAs films. Fig.(1):Plot of ln σ versus 1000/T of 450 nm thi ck InAs films treated at different annealing temperature. Fig.(2):Electrical conductivity as a function of anne aling te mperature for a 450 nm thi ck InAs films. Annealing temp erature Ta (K) σ * 10 -4 ohm -1 .cm -1 303 6.61 373 3.86 423 2.22 473 1.58 523 1.25 573 0.867 t=450 nm -10 -8 -6 -4 -2 1.5 2 2.5 3 3.5 1000/T ( K ) - 1 ln  ( o h m .c m ) -1 R. T Ta=373 k Ta=423 K Ta=473 k Ta=523 K Ta=573 K 1.E-05 1.E-04 1.E-03 1.E-02 273 32 3 3 73 4 23 4 73 523 573 T a ( K )  ( o h m .c m ) -1 IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L.22 (4) 2009 Fig.(3): The variati on of the activation energies (Ea1, Ea2) versus the annealing temperature for a 450 nm thi ck InAs films. 0.05 0.15 0.25 0.35 0.45 0.55 273 323 373 423 473 523 573 Ta (K) E a ( e V ) Ea1 Ea2 2009) 4 (22مجلة ابن الهیثم للعلوم الصرفة والتطبیقیة المجلد InAs الكهربائیةألغشیة التوصیلیة فيتأثیر درجة حرارة التلدین العشوائیة بشرى كاظم حسون المیالي جامعة بغداد،كلیة التربیة أبن الهیثم قسم الفیزیاء، الخالصة العـشوائیة والمحـضرة بطریقـة التبخیـر الحــراري InAsأجریـت قیاسـات التوصـیلیة الكهربائیــة الغـشیة فـي هـذا البحـث . K – ( 303 (573 وعند درجات حرارة تلدین مختلفة nm 450بسمك اس معاكـا وقد لوحظ نقصان التوصیلیة الكهربائیة مـع زیـادة درجـات حـرارة التلـدین ، بینمـا أظهـرت طاقـات التنـشیط سـلوك . أزدادت طاقات التنشیط مع زیادة درجة حرارة التلدین،اذ