Available nline at www.pelagiaresearchlibrary.cm Advances in Applied Science Research, 2012, 3 (2):743-748 ISSN: 0976-8610 CODEN (USA): AASRFC Study the Optical prperties f Amrphus Structure (Glassy) f B 2 O 3 -CdO Binary System S. A. Babanejad 1, F. Ashrafi 1 and N. Salarzadeh 2, E. Ashrafi 3 1 Department f Physics, Payame Nr University, Tehran, IR f Iran 2 Office f Educatin, Sari, IR f Iran 3 Islamic Azad University, Sciences and Researches, Iran _ ABSTRACT Using ptical absrptin and studying the edge absrptin f either crystalline r amrphus structures, is an efficacius and useful methd fr better realizing the electrnic structure f these materials. In this paper, firstly, we prepared the bulk specimens f different percentages f B 2 O 3 and CdO in frm f binary systems, and then we studied UV- Visible spectrums f specimens in the range f 200 800 nm. After determinatin f absrptin cefficient α (ω,) then we traced ut (αћω) 1/n against (ћω) diagrams fr btaining the value f n. In this research, by cnsidering n=2, we have btained a linear diagram with expnential tail. Therefre, accrding t Mtt and Davis s statement, it was established that, the type f transitin is indirect allwed. By extraplating the linear part f (αћω) 1/2 against (ћω) diagram, we determined the ptical gap energy (E pt ) at (αћω) 1/2 = 0. The btained amunts f E pt (2.505 t 2.965 ev) shw that, E pt decreases with increasing in CdO percentage. By tracing the plt f variatin f ln α (ω) against (ћω), we have determined the slpe f the linear part f diagram, which is equal t width f lcalized states E (abut 0.469 t 0.811 ev). Keywrds: Amrphus structure, edge absrptin, binary system, ptical gap, lcalized states. _ INTRODUCTION Studying ptical absrptin, in particular, shapes and mdificatins f edge absrptin are a significant methd fr understanding essential mechanism and behavir f ptical displacements in crystalline and nne crystalline materials, with useful infrmatin relevant t their related structures [1]. This methd is based n absrptin f phtns by energy and als by relating them t transitin frm the states engaged in bnds capacity t states nnengaged in their cnductin bnd. Tw types f ptical transitins may be ccurring fr basic edges f these materials, a) direct transitin, and b) indirect transitin [2]. In direct transitin, either vectr r electrn s wave directin remains cnstant, but in indirect transitin, phtn applying is necessary fr retaining the amunt f kinetic energy and, electrn s wave directin changes. Optical edge absrptin f the mst amrphus semi cnductrs is defined by an absrptin cefficient α (ω), which varies expnentially with energy, ћω. The variatin f absrptin cefficient with energy, takes an expnential tail at high temperatures. Edge absrptins f nne crystalline an amrphus material has a slpe less than its crystalline frm. Determinatin and interpretatin f edge - adsrptin had significantly helped t understand electrnic structure thery f amrphus materials. 743
In many f amrphus materials the edge absrptin is classified in tw categries: 1) High absrptin zne [α (ω) > 10 4 cm -1 ]; in this zne, absrptin cefficient is btained frm relatin as: α ( ω) = B ( ћω E ) n pt / ћω where B is the prprtin cefficient, E pt is ptical gap energy, ω is angular frequency f transmitted ray, and n may be equal t 1/2, 1, 2 r 3/2, which is depend t the type f electrnic transitin in K space, and t the case whereas the transitin is allwed r nne allwed. Firstly, this equatin was prpsed by Tauk, et al in 1966 as n = 2 [3], then in 1970 by Mtt and Davis, as n = 3/2 [4]. 2) Lw absrptin zne [α (ω) < 10 4 cm -1 ]; in this zne, absrptin cefficient varies expnentially with the energy f descent phtn (ћω). The best relatin, which explain ptical behavir f these materials in this zne have given by Urbach in 1953 [5]: ( ) C exp ( ћ / E) α ω = ω (2) where C is a cnstant, E is the width f lcalized states in the energy f bnds, and (ω) is angular frequency f descent phtn. The general frmula fr the ptical absrptin cefficient α (ω) is given as: ( ) 1/ L ln ( I /I) α ω = (3) Where α (ω) is the absrptin cefficient (cm -1 ), I and I, are the intensity f incident and transmitted light respectively, and L is the thickness f the sample (cm). In this investigatin, we have characterized the type f transmissin vectrs and have calculated the ptical gap energy fr B 2 O 3 CdO binary system as amrphus structure. Amrphus structures have specific characteristics including semi-cnductivity. The electrical prperties f glassy materials have been investigated and reprted in literature [6]. It has been suggested that the presence f interstitials r dangling bnds give rise t a number f lcalized states r traps near the band edges f glassy materials. These lcalized states are respnsible fr the transprt f charge carriers. At lw electric field the small plarn hpping has a dminant rle [4 and7] and at high field this is attributed t either Schttky (electrde-limited) [8] r Ple- Frankle effect (bulk-limited) [9]. The band structure f amrphus slids can be described by the C.F.O mdel [10], which was further imprved by Mtt and Davis [4]. These mdels prvide a clear picture f density distributin f states in the energy gap f amrphus semicnductrs. Brn trixide can frm the vitreus structures with ther xides. Additin f certain xides t brn trixide mdifies the prperties f these glasses. In ur investigatin B 2 O 3 - CdO binary system has been studied. Fr better understanding the electrnic structure and study n the edgeabsrptin, we culd use ptical absrptin. By using a UV-Visible spectrphtmeter the absrptin spectrums were btained. Studying these spectrums by using Urbach [5] and Mtt-Davis [2] appraches, the absrptin cefficient, α (ω), have calculated accrding t Bir-Lambert relatin. By tracing (αћω) 1/n against (ћω) plt, the amunt f n was determined. Fr B 2 O 3 -CdO binary system n = 2 was achieved. By extraplating the plt f (αћω) 1/2 against (ћω), we culd determine ptical gap energy, E pt, at (αћω) 1/2 = 0. Obtained results shw that, E pt decreases with the increase f CdO percentage. By determining the slp f lnα (ω) versus (ћω) diagram, the width f lcalized states in the energy f bnds was cncluded. In this research we were investigated these prperties fr amrphus samples f binary system f brn and cadmium xides, B 2 O 3 CdO, with different amunts f percentages, and we have cncluded that the btained results in this research has the best cnsistency with the theries indicated in literature [11, 12, and 13]. (1) MATERIALS AND METHODS Firstly we have prepared the samples f required binary system B 2 O 3 CdO having the amunts f 75 25, 70 30, 65 35, 60 40, 55 45, 50 50, 45 55, and 40 60 percents f B 2 O 3 and CdO respectively, in bulk frm. Further step was preparatin the samples by fast cling f melted binary systems. Then we have studied ptical prperties f these samples. 2.1. Preparatin the samples An electrical furnace with maximum temperature f 1450 C is used fr preparatin the samples. Firstly, we have prepared the fine pwder f each xide by well milling them individually in an agate mrtar. Afterwards, we have 744
prepared the samples f B2O3 CdO binary system by well mixing B2O3 and CdO xides pwders with percentages as indicated in table 2. Then we have placed all samples, after milling, precise weighing and well hmgenizing, abut 2 hurs in desiccatrs fr eliminating any eventually misture. Afterward, we have put the prepared samples abut 2 hurs in an ven at 250 C, fr eliminating prbable internal stresses and temprary bnds. At that time the samples cmpletely hmgenized fr secnd f time. These accurately prepared hmgenus samples were placed within a furnace fr melting. Dwn t 600 C we increase temperature step by step t 100, 200, 300, 400, 500, and 600 C, and allw them t remain fr 10 minutes at each f these stageing temperatures, befre any further temperature increase. As achieving 600 C we have mixed well, nce mre, every sample fr btaining better hmgeneity. Then, up t 600 C we d the same abve treatment after every 50 C f temperature increasing up t sftening temperature, and allw the samples t remain fr 5 minutes at this temperature. Then we perfrm an increase f 50 C abve sftening temperature, and allw remaining abut 15 minutes at achieved temperature. Then we have unladed the melted material n a ceramic surface at ambient temperature. Fr btaining the best required results, unlading f melted material must perfrm as quickly as pssible, and very rapidly we put anther ceramic plate abve it fr btaining a thin and plate unifrm sample. All samples supprt 2 hurs annealing fr eliminating any internal stresses and break dwn eventual prvisinal bundaries frmed during melting prcess. Finally, the thicknesses f samples were measured preciusly by a micrmeter. RESULTS AND ISCUSSION 3.1 UV-Visible and XRD Spectra and Calculatin methd Absrptin spectra f samples were btained by an UV-Visible spectrphtmeter (G.B.C. Cintra20 mdel) within a range f 200 t 800 nm. The shape f spectrums shw that the edge absrptin f them is nt sharp, which is the characteristic f vitreus structures (Fig. 1). As the spectrum (shws, there is nt any sharp edges f absrptin. This characteristic is specific fr vitreus structures, and nt fr crystalline structures. When an electrmagnetic wave having intensity I radiate the surface f a slid with the thickness L, the intensity f incident phtn decreases expnentially as passing thrugh it. Intensity f transmit radiatin may btain by fllwing equatins (A. A. Hsseini, and C. A. Hgarth, 1983), ( )L I / I = e α ω (4) Ln I / I = α ( ω) L (5) α ( ω ) = 1/ L Ln I / I (6) Finally, rearrangement f equatin (6) gives the fllwing equatin as: α ( ω ) = 1/ L 2.303 Lg I / I = 1/ L 2 / 303 A (7) Where A is Lg I / I. Therefre, fr calculating α (ω), we need t measure the thickness f sample, L, and calculate Lg I /I. We have measured L by means f a micrmeter, in cm, and determined Lg I /I frm absrptin plt f crrespnding spectrum f each sample. Cnsequently, we culd btain the value f α (ω) (cm -1 ). Thus, we can study the variatin f (αћω) 1/n versus (ћω). Thereafter, angular frequencies f incident phtn have been determined fr any pint selected n absrptin spectrum, and ω may be btained frm fllwing relatin as: ω = 2 πν = 2 π c / λ (8) Where c is the velcity f light in vacuum, and λ is the wave length f incident phtn which may be btain frm selected pints n absrptin spectrum. Determining n fr binary system B 2 O 3 CdO, we traced the plt f variatin f (αћω) 1/n with different n amunts (i.e., 1, 2, 1/2 and 3/2), in relatin with emitted phtn energy (ћω), accrding t the btained data that was shwn fr a typically sample n table 1 (fr example, N. 1). S, we plt the diagrams f (αћω), (αћω) 1/2, (αћω) 2 and (αћω) 2/3 versus (ћω). The btained diagrams shw that the plt f (αћω) 1/2 versus (ћω) has mst linear tail in cmparisn with the ther nes. Thus, fr this binary system, n is cnsidered as 2, and this shws that, absrptin prcess is indirect and allwed. Then we have btained E pt fr the samples by extraplating the linear part f plt f (αћω) 1/2 versus ћω. Thus, we have calculated (ћω), α (ω) and (αћω) ½ fr all 745
pints n the spectrum f figure 1 [14]. The results f such calculatins were carrying ut fr a typical sample n table 1 (fr example, N. 1). Calculating the width f lcalized states ( E), we have pltted the variatin f ln α (ω) against (ћω) and determined the slpe f the linear part f diagram. The XRD spectrum f this typical sample (fr example, N. 1), als, cnfirms the vitreus structure f prepared samples (Fig. 2). Figure 3 and 4 shws the diagrams f variatin f (αћω) 1/2 and ln α (ω) versus (ћω) fr the typically sample (fr example, N. 1). Then, by the same methd described abve, we have pltted (αћω) 1/2 and ln α (ω) versus (ћω) fr the rest prepared samples. The results btained frm this prceeding fr the rest prepared samples have reprted in table 2. Just, the XRD spectra f these samples, als cnfirms the vitreus structure f them. In additin, fr shwing dependence f E pt n thickness f samples we have prepared tw samples with the same percentages f cmpnents, but different in thickness. The results f this experiment have shwn n table 2. Nte that the melting pint (sftness) f samples increases with increasing f CdO percentage, and vitreus characteristics decrease inversely. Fig.1. Absrptin spectrum f B 2 O 3 CdO binary system, fr a typical sample (N. 1) Fig. 2. XRD spectrum btained fr a typical sample (N.1) 746
Table1. Typical results btained frm the plt shwn in figure1, fr a typical sample (N. 1) λ (nm) 475.47 450.50 441.03 434.15 422.96 412.63 400.57 391.10 383.36 369.58 355.81 350.65 342.90 333.43 328.26 319.66 310.44 A 0.59493 0.60266 0.60668 0.60821 0.61499 0.62482 0.64617 0.67395 0.71272 0.82135 1.0477 1.1727 1.4162 1.8119 2.0883 2.5470 3.1847 ω 10-15 (s -1 ) 3.962 4.182 4.272 4.340 4.454 4.566 4.703 4.817 4.914 5.098 5.295 5.373 5.494 5.650 5.739 5.894 6.069 ħω (ev) 2.608 2.753 2.812 2.857 2.932 3.005 3.095 3.170 3.234 3.355 3.485 3.536 3.616 3.719 3.777 3.879 3.995 L 10-1 (cm) α(ω) (cm -1 ) 9.928 10.057 10.125 10.150 10.263 10.427 10.783 11.247 11.894 13.707 17.484 19.570 23.634 30.238 34.850 42.505 53.147 (αħω) 1/2 (cm -1/2 ev 1/2 ) 5.088 5.262 5.336 5.385 5.485 5.597 5.777 5.971 6.202 6.781 7.806 8.318 9.244 10.604 11.473 12.840 14.571 lnα(ω) 2.295 2.308 2.315 2.317 2.328 2.344 2.378 2.420 2.476 2.618 2.861 2.974 3.163 3.409 3.351 3.750 3.973 Figure 3 Variatin f (αћω) 1/2 in relatin with ћω fr sample N. 101 with 25 % CdO and mm thickness and E pt = 2.965 ev. Figure 4 Variatin f ln α (ω) in relatin with ћω fr sample N. 101 with 25 % CdO and mm thickness and E = 0.469 ev. Table 2 Experimental and calculated results fr all samples N. f Sample B 2O 3 wt% CdO wt% Thickness (mm) E pt (ev) E (ev) 1 75 25 2.965 0.469 2 70 30 1.12 2.505 0.584 3-A 65 35 1.11 2.660 0.734 3-B 65 35 1.16 2.650 0.745 4-A 60 40 1.12 2.670 0.784 4-B 60 40 1.42 2.644 0.811 5-A 55 45 1.02 2.810 0.619 5-B 55 45 1.22 2.610 0.716 6-A 50 50 1.07 2.930 0.564 6-B 50 50 1.09 2.920 0.596 7 45 55 0.91 2.755 0.652 8 40 60 1.09 2.780 0.563 747
CONCLUSION Studying the spectra, and accmplished data lead us t cnclude the fllwing results: The edge absrptin f these spectra is nt sharp, in cmparisn with the spectra f crystalline, and this is specifying characteristic f amrphus (vitreus) structures. The btained experimental data shw that, the edge absrptin tends twards lng waves (lw energies), when CdO percentage increases and B 2 O 3 percentage decreases. This effect may be interpreted by ptical gap energy (E pt ) decreasing. In fact, this may be due t certain structural changes f materials with changing in cmpsitin. Increasing f cntents f CdO may mre lcalize sme f the lcalized states present in deep energy levels. Of curse, an ESR spectrum will shw mre acceptable interpretatins. Studying spectra we bserve that, the edge absrptin, when the thickness f a determined specimen with the same percentages increases, tends twards lng waves; which may be explicate by ptical gap energy decreasing. The same reasning which has described abve, als, explicates increasing in the width f lcalized states ( E). Interpretatin is the same. As a matter f fact, we may interpret these latter phenmena, mre prbably, by increasing f electrn wave functins verlap, due t increasing f specimen thickness, the width f lcalized states increases and the ptical gap energy decreases. The same explicatin has reprted fr MO 3 In 2 O 3 binary system [15]. The variatin in ptical gap energy with variatin in the width f lcalized states tail, reinfrce Mtt and Davis s theries fr lcalized states within energy gap f amrphus semicnductrs [4, 14]. The XRD spectrum des nt shw any peak s that it is a prf fr (amrphus) vitreus structure f these samples. REFERENCES [1] A. A. Hsseini, C. A. Hgarth and J. Beynn, J. Mat. Sci., Vl.18, (1994), pp. 4444 4445. [2] N. F. Mtt and E. A. Davis, Phil. Mag., (1970), Vl.22, pp. 903 922. [3] J. Tauk, R. Grigrivici and A. Bunc, Amrphus and Liquid Semicnductr, Physica Status Slidi, Vl.15, (1966), pp. 627. [4] N. F. Mtt and E. A. Davis, Electrnic Prcesses in Nn Crystalline Materials, Clarendn Press, Oxfrd Univ., (1971), pp. 273 300. [5] F. Urbach, The Lng-Wavelength Edge f Phtgraphic Sensitivity and f the Electrnic Absrptin f Slids, Phys. Rev. Vl. 92, (1953) pp. 1324. [6] M.A. Ghauri, A.R. Mir, C.A. Hgarth, and M.M. Ahmed, Internat. J.Electrnics, 53(1982), pp. 311. [7] M. Y. Nadeem, T. B. Sadhana, M. Altaf and M. A. Chaudhry, Jurnal f Research (Science), Vl.15, N.3, (2004), pp. 245-251. [8] M.Y. Nadeem and C.A. Hgarth, J. Mat. Sci., 21(1986), pp. 3829. [9] C.A. Hgarth, and M.A. Ghauri, J. Mat. Sci., 15, (1980), pp. 409. [10] M.H. Chen, H. Fritzsche, and R. Ovshinsky, Phys. Rev. Lett., 22, (1969), pp. 1065. [11] C. A. Hgarth, A. A. Hsseini, J. Mat. Sci.,Vl. 18, (1983), pp. 2697-2705. [12] A. P. Thrne, Spectrphysics, (1988), (Halsted Press). [13] A. A. Hsseini, C. A. Hgart and J. Beynn, J. Mat. Sci. Lett., Vl. 15, (1994), pp.1144 1145. [14] S. A. Babanejad, F. Ashrafi, Internatinal Jurnal f Chem Tech Research, Vl.2, N.3, (2010), pp. 1831-1837. [15] M. Anwar and A. A. Hsseini, J. Mater. Sci. Lett., Vl. 3, (1984), pp. 1035. 748