IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 Study o f the Optical Properties for ZnS Thin Film Irradiated by CO2 Laser A. S.Jassim ,N. A.Dahham, M.Sh.Marie Departme nt of Physics, College of Science , Unive rsity of Tik rit Abstract In this study ZnS thin film was p repared by using thermal evap oration vacuum technique under the p ressure (10 -6 ) Torr on glass substrate at room temp erature and annealing at 523 K Samp les were irradiated to CO2 laser of p ower (1 watt) and wave length (10.6) μm at distance 10 cm from the source during (5 sec). The absorbance sp ectra was recorded by using UV-visible sp ectrop hotometer and used to calculated some of op tical p rop erties invest igated includin g their transmittance, reflectance sp ectra, energy gap, and extinction coefficient. From the result of thin films samples at room temp erature and at 523 K, we conclude that t he irradiation by laser causes a decrease in the transmittance and increasing in reflection and extinction coefficient and the irradiation leads to an increase in en ergy gap. Introduction The thin film technique is one of imp ortant methods which is interested in the development and study of semiconductor material. The study of material p rop erties lead to draw att ention p hy sicians toward this technique, from the second half of seventeenths centaury, many of researches were taken out in the field [1, 2].In the nineteenth century there has been an imp rovement in the exp erimental p art of thin film [3, 4].The thin film is one lay er or many lay ers for sp ecific material in the range of tens nanometers thickness in some of micrometers [5]. M etal chalcogenides (sulfides, tellurides and Selenides) are of great imp ortance for research because they are p otential candidates for op toelectronic app lication such as p hoto detectors, solar cells, and thin film transistors etc [6]. Zinc Sulphide is an imp ortant II-VI group semiconductor with a large direct energy gap of (3.50-3.70) eV in the UV range, direct energy gap Eg = 3.68 eV for bulk ZnS [7]. It is used as a key material for light emitt ing diodes and other op toelectronic devices such as electroluminescent disp lay s, Cathodoluminescent disp lay s and multilayer dielectric filters. ZnS is highly suitable as a window lay er in hetrojunction p hotovoltaic solar cells; because the wide band decreases the window absorp tion loses and imp roves the short circuit current of the cell. In the area of op tics, ZnS can be used as a reflector, because of its high refractive index (2.35), and a dielectric filter because of its high transmittance, in the visible range [7, 8, and 9]. ZnS was st udied by M .Y. Nadeem and W. Ahmed. They found that ZnS thin films grown here have energy gap in the range (3.51 -3.84 )eV and found that the refractive index increased for p repared thin film[ 10]. J.P. Borah and K.C. Sarma found that ZnS nanocry st alline films grown on glass substrates using PVA as matrix ,also found that thin films wauld be p hotosensitive. The films kept in air adsorb oxy gen from air and they found that t heir resistance increases as observed from I/V characterist ics. SEM st udy indicated a nanoparticle formation in thin film [9]. Vipin Kuma, , M .K. Sharm, J. Gau and T. P.Sharm show that the energy gap of ZnS sintered film comes out to be 3.50 eV. The films of ZnS are found to be p olycryst alline in nature and have hexagonal wurt zite st ructure. It has been observed that the electrical resist ivity IHJPAS IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 and activation energy of ZnS comes out to be 0.370*10 5 ohm cm and 0.80 eV resp ectively. The conduction in ZnS film is through thermally activated process [7]. Among molecular lasers, the CO2 laser is of greatest p ractical imp ortance. The high level of efficiency with laser in which laser radiation can be generated in continuous wave (CW) and p ulse operation is its most fascinating feature. In atom and ion lasers, laser radiation is the result of the electron transitions close to the limit for single or double ionization. The infrared radiation of the CO2 laser on the other hands is the result of the energy exchange between rotational- vibrational levels within the electron ground level [11]. The objective of this research is to st udy the behavior of the material (ZnS) before the effect of laser radiation and what hap p en after irradiation by CO2 laser Experime ntal and theoretical part ZnS thin film was p repared by using thermal evap oration vacuum technique under the p ressure (10 -6 ) Torr on glass substrate at room temp erature. Thickness of the films has been carried out by weighting method and the measured thicknesses were about (300 nm) then the films were annealed at 523K by using electric oven. Samp les were irradiated to CO2 laser of p ower (1 watt) and wave length (10.6 μm) at distance (10 cm) from the source during (5 sec). The absorbance ad transmission sp ectra were recorded using uv-visible sp ectrop hotometer ty p e centra-5 in the range of wave length (180-1100) nm at room temp erature .Some of op tical p rop erties was calculated from absorbance and transmittance sp ectra. The transmitt ance (T) was calculated from the relation[12]: T=Log 1/A …………………………………. (1) Where A is the absorbance. The reflection from the surface of the prepared thin films was calculated from the relation [12]: R=1-A-T…………………………………….. (2) The extinction coefficient K° was calculated from the relation [13]: K°=αλ/4π…………………………………….. (3) Where λ is the wave length and α is absorp tion coefficient. The relation between the absorp tion coefficient and p hoton energy hυ is given by [14]: α hυ= A (hυ-Eg±Ep) n …………………………(4) Where Ep is p hoton energy and Eg is the en ergy gap in a d irect t ransition and (n) is equal to 1/2. Results and Discussion 1. Transmittance The plot of t ransmission data versus wave length (μm) is shown in figure (1). We can see the maximu m value for transmission at room temp erature before irradiation with CO2 laser is 0.999% at wave len gth 0.184 μm that means at (UV) and the maximum value for transmission after irradiation with laser is 0.998% at t he wave len gth 0.4 μm (visible region) as shown in figure (2), so we can note the transmission is decreasing after irradiation with laser because laser causes to rearrange the atoms. Figure (3) represents the transmittance as a function to wave length (μm) with annealing at 523K before irradiation with laser, so we note the maximum value of transmission is 0.9997% at the wave len gth 0.4 μm. We note a sm all incr ease in value of transmission before and after annealin g, the maximum v alue in transmittance happ ened in visible region, while f igure (4) represents the transmission as a function to wave length (μm) with annealing at 523 K after irradiation with laser and the maximum value for transmittance is 0.9999 % at the wave len gth 0.488 (μm). So we can note an increase in the transmittance value and shiftin g in wave length IHJPAS IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 because laser causes cryst al defect in the sa mple after irradiation ,this defect causes to increase the localized electronic st ates which increasin g the absorp tion and decreases the transmission . (2) Re flectance Figure (5) shows a plot of reflectance against wave length (μm) it can be noticed that t he maximu m value of reflectance before irradiation with laser at room temp erature is 0.033% at t he wave len gth 0.304 μm and after irradiation the maximum valu e is 0.058 % at the wave len gth 0.304 (μm) as shown in figure (6). And we note an in crease at reflection after irrad iation .We can exp lain this increase in the reflectance value because some of sub levels app eared in the crystal lattice for ZnS lattice. Figure (7) shows t he reflectance as a function to wave length (μm) with annealing in 250 ° C before irradiation with CO2 laser. In this p lot we note the maximum v alue for r eflectance is 0.0168 % at t he wave length 0.296 (μm) and figure (8) represents the reflectance as a function to wave len gth (μm) and the maximum value is 0.0137 % at the wave length 0.296 (μm), that is means the reflectance is decreasing after irradiation with laser .This decr ease may be happ ened because the transmittance was increased. 3 .Extinction Coefficient Figure (9) shows the behavior of the extinction coefficient versus wave len gth (μm). It can be noticed that the maximu m valu e for extinction coefficient before irrad iation with laser is 0.000166 at the wave length 0.304 (μm) and after irrad iation the maximum v alue for extinction coefficient is 0.00031 at the same wav e length 0.304 (μm) as shown in figure (10) .This increasin g was h app ened because the absorp tion was increasing and the relation between the absorption coefficient and extinction coefficient is lin ear. Figure (11) shows the extinction coefficient as a function of wave length (μm) with annealin g 250 ° C before irr adiation with laser the maximu m value is 7.97*10 -5 at t he wave len gth 0.296 (μm), and figure (12) represents to the extinction coefficient as a function of wave len gth (μm) with annealing in 250 ° C after irradiation with laser and the maximu m value is 6.47*10 -5 at the wave length 0.296 (μm). And we note the value for extinction coefficient is decreasin g after irradiation with laser because the relation b etween the absorp tion coefficient and extinction coefficient is linear. 4. Energy gap a p lot of (αhυ)2 as a function of p hoto energy is show in figure (13) which represents t he direct op tical forbidden ener gy gap to allow direct electronic transmission at room temp erature before irradiation with laser and the value of this gap is 3.45 eV, and figur e (14) represents the direct op tical forbidden ener gy gap to allow direct electronic transmission at room temp erature after irradiation with laser and the valu e of this gap is 3.62 eV. This result coherent with theoretical direct ener gy gap[7] . We note the incr ease in ener gy gap valu e this increase in gap may be hap p ened because some of cryst al defects were ap p eared in cryst al lattice after irradiation with laser. These defect caused the increase of the density of localize st ates in the Eg . Figure (15) shaws the relation between (αhυ) 2 as a function of p hoton energy from this relation we calculated the value of ener gy gap in (eV) with annealin g in 250 ° C b efore irradiation with laser which equals to 4 eV. And figure (16) represents the relation b etween (αhυ) 2 as a function of p hoton energy from this relation we calculated the value of ener gy gap in (eV) with annealin g in 250 ° C after irrad iation with laser which equals t o 4.15 (eV), this incr ease in ener gy gap may happ en in result because some of cryst al defects appeared in cryst al lattice after irradiation with laser for the same r easons we mentioned before. IHJPAS IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 Conclusion 1. Irradiation with CO2 laser leads to a decrease in the transmittance 2. Reflectance and extinction coefficient increase after irrad iation with laser, which means the absorption is not attributed to the free carriers only, but to defect or localized electronic st ates 3. The laser radiation causes t he increase in energy gap. Re ference 1.Al-Ani, S.K.J.; M akadsi,M .N. and Al Shakarchi,I.K. (1993). Journal of materials science, 28, 2.Smith,R.A. (1987). Semicondu ctors" Cambridge University , Press 2 nd ed. 3.Al-Ameen,A.F. (1992). optical p rop erties of CdS and PbS thin films and their mixture"M .Sc.Thesis, University of Baghdad 4.Segger, K. (1980)." Semiconductor p hysics", 2 nd ed.New y ork 5.Ckertor,L.E. (1977). Phy sics oh thin films "p lenar press New y ork, 6.Sahay ,P. P. ; Nath, R. K. and Tewari, S. (2007).Opt ical p rop erties of thermally evap orated CdS thin films, Cry st. Res. Technol. 42( 3): 275 – 280 7.Kuma,V.; Sharm,M .K. ; Gau,J. and Sharm,T. P. (2008),.Poly cryst alline ZnS thin films by screen p rinting method and its characterization ",Chalcogenide Lett ers ,5(11): November p . 289 – 295. 8.Ant ony ,A. ; M irali,K.V.; M anoj,R.and M ing.K. (2005).Jayaraj, M ater.Chem.Phy s.90, 106 9. J.P. Borah and K.C. Sarma, "Op tical and Op toelectronic Prop erties of ZnS Nanost ructured Thin Film", ACTA physica polonica Vol. 114, No. 4. (2008) 10. Nadeem,M .Y. and Ahmed ,W. (2000). Op tical Prop erties of ZnS Thin Films, Turk J Phy , 24: 651 – 659 11. Wood ,J.D.L.H. and Pearson, P.R. ( 1990) A Rep etitively p ulsed, Journel De Phy sique, , p .351. 12. Obaid,Y.N. ;A gool ,and Alwan, I.R. (2002). S.Th. SC. J.Iraqi AtomicEner gy Commission, 2, 13. Habubi, N.F.; Abdula, H.I. and M ansour,H.L. (1999). Al-Fateh journal, No.5, 14. Pathinettam,D. ;Padiyon; M arikani,A.and M urali,K.R. (2000). "Cry st. Res. Technol.", 35, 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 1.01 0 0.2 0.4 0.6 0.8 1 1.2 T ra n sm it ta n ce T ra n sm it ta n c e (% ) Wave length (μm) Fig.(1) transmittance as a function to wave length at room te mperature before irradiation IHJPAS IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 T ra n sm it ta n ce ( % ) Wave length (μm) 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 0 0.2 0.4 0.6 0.8 1 1.2 T ra n sm it ta n ce T ra n sm it ta n c e (% ) Wave length (μm) Fig.(2) transmittance as a function to wave length at room te mperature after irradiation 0.965 0.97 0.975 0.98 0.985 0.99 0.995 1 1.005 0 0.2 0.4 0.6 0.8T ra n sm it ta n ce ( % ) Wave length (μm) Fig.(3) transmittance as a function to wave length at 0.97 0.975 0.98 0.985 0.99 0.995 1 1.005 0 0.2 0.4 0.6 0.8 1 T ra n sm it ta n ce ( % ) Wave length (μm) Fig.(4) trans mittance as a function to wave length at 523 K after irradiation IHJPAS IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0 0.2 0.4 0.6 0.8 1 1.2 R ef le ct a n ce ( % ) Wave length (μm) Fig.(5) Reflectance as a function to wave length at room temperature before irradiation 0 0.005 0.01 0.015 0.02 0 0.2 0.4 0.6 0.8 R ef le ct a n ce ( % ) Wave length (μm) Fig.(7) Reflectonce as a function to wave length at 523 K before i rradiation 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 0.2 0.4 0.6 0.8 1 1.2 R ef le ct a n ce ( % ) Wave length (μm) Fig.(6) Reflectonce as a function to wave length at room temperature after irradiation IHJPAS IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0 0.2 0.4 0.6 0.8 1 R ef le ct a n ce ( % ) Wave length (μm) Fig.(8) Reflectonce as a function to wave length at 523 K after irradiation 0 0.00002 0.00004 0.00006 0.00008 0.0001 0.00012 0.00014 0.00016 0.00018 0 0.2 0.4 0.6 0.8 1 1.2E x ti n ct io n c o ef fi ci en t Wave length (μm) Fig(9) Extinction coefficient as a function to wave length at room temperature before irradiation 0 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003 0.00035 0 0.2 0.4 0.6 0.8 1 1.2 E x ti n ct io n c o ef fi ci en t Wave length (μm) Fig.(10) Extinction coefficient as a function to wave length at room temperature after irradiation IHJPAS IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 0 0.00002 0.00004 0.00006 0.00008 0.0001 0 0.2 0.4 0.6 0.8 E x ti n ct io n c o ef fi ci en t Wave length (μm) Fig.(11) Extinction coefficient as a function to wave length at 523 K before irradiation 0 0.00001 0.00002 0.00003 0.00004 0.00005 0.00006 0.00007 0 0.2 0.4 0.6 0.8 1E x ti n ct io n c o ef fi ci en t Wave length (μm) Fig.(12) Extinction coefficient as a function to wave length at 523 K after irradiation Fig.(13) Optical forbidden ene rgy gap for direct electronic transmi ssi on at room temperature before irradiation with laser 0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 (α h υ )2 ( cm -1 * eV )2 Photon energy (eV) IHJPAS IBN AL- HAITHAM J. FO R PURE & APPL. SC I. VO L.23 (3 ) 2010 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 (α h υ )2 ( cm -1 * eV )2 Photon energy (eV) Fig.(14) Optical forbidden ene rgy gap for direct electronic transmi ssi on at room temperature after irradiation with laser 0 0.00005 0.0001 0.00015 0.0002 0.00025 0 1 2 3 4 5 (α h υ )2 ( cm -1 * eV )2 Photon energy (eV) Fig.(15) Optical forbidden ene rgy gap for direct electronic transmi ssi on at 523 K before i rradiation with laser 0 0.00002 0.00004 0.00006 0.00008 0.0001 0.00012 0.00014 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 (α h υ )2 ( cm -1 * eV )2 Photon energy (eV) Fig. (16) Optical forbidden ene rgy gap for direct electronic transmi ssi on at 523 K after irradiation with laser IHJPAS 2010) 3( 23مجلة ابن الھیثم للعلوم الصرفة والتطبیقیة المجلد CO2 شعع بلیزرلما ZnS دراسة الخواص البصریه لغشاء محمد شیاع مرعي، نجاة احمد دحام ، عواطف صابر جاسم جامعة تكریت م الفیزیاء، كلیة العلوم، قس الخالصة )تحت ضغط والمحضرة بطریقة التبخیر الحراري الفراغي ZnSتم في هذه الدراسة تحضیر اغشیة رقیقة من مادة 10 -6 یزرتشعیع العینات بلو وتم ) K 523(وتم تلدین العینات بدرجة حرارة ،من الزجاج في درجة حرارة الغرفة على قواعد ( CO2 طاقة يذ(1watt) وطول موجي(10.6μm) وعلى بعد) (10Cmومدة , من المصدر)(5 sec ةبعض الخواص البصریه التي تتضمن النفاذی ومنه حسبت UV-visible وسجل طیف االمتصاصیه باستخدام مطیاف بدرجة ةوالملدن ةحرارة الغرف ةبدرج ةلمحضر ا ةالرقیق ةومن نتائج نماذج االغشی. ةومعامل الخمود وفجوة الطاق ةواالنعكاسی (523 K)بلیزر ةوالمشععCO2 لیزر ادى الى نقصان في النفاذیه وزیاده في االمتصاصیه ومعامل لاستنتج ان التشعیع با .ةفي فجوة الطاق ةزیاد التشعیع ادى الى الخمود وكذلك IHJPAS