Optical properties of (1- x-y)b203-xli20-ymci2 (M = Cd, Zn) glasses

Transcription

1 Bull. Mater. Sci., Vl. 22, N. 7, December 1999, pp Indian Academy f Sciences. Optical prperties f (1- x-y)b203-xli20-ymci2 (M = Cd, Zn) glasses P S VIJOY and M ABDUL KHADAR* Schl f Pure and Applied Physics, Mahatma Gandhi University, Kttayam , India *Department f Physics, University f Kerala, Kariavattm, Thiruvananthapuram 69 8t, India MS received 2 February 1999; revised 7 May 1999 Abstract. Optical absrptin and transmissin spectra f (1-x-y)B203-xLi20-yMCl2 (M = Cd, Zn) glasses f varying cmpsitins were recrded in the UV-visiblf regin. Frm the spectra, varius ptical parameters such as ptical energy gap (Ept), refractive index (n), ptical dielectric cnstant (e'), width f the tail f lcalized states in the frbidden gap (AE), rati f carrier cncentratin t the effective mass (N/m*) and the cnstant B were evaluated. The effects f cmpsitin f glasses n these parameters are discussed. Keywrds. Brate glasses; ptical energy gap; refractive index. 1. Intrductin There has been interest in recent years n halide glasses because f the pssibility f develping lw lss IR waveguide (Jiang et al 1990). Szukwei et al (1982) reprted a systematic investigatin f the ptical and electrical prperties f Ge-Se-Te glass system in rder t find ut the relatinship between the glass cmpsitin and band gaps. Mrsi et al (1986) investigated the additin f Ti 4 and C z~ ins t brate glasses which intrduced an intense charge transfer band in the UV regin that shifts twards lnger wavelength. Kh Shkr (1992) explained the increase f ptical gap with increase f Se cntent and thickness f film in terms f decrease in the percentage f tellurium and eliminatin f defects in the amrphus film. Refractive index measurements (Ma et al 1993) had been perfrmed n different tellurium halide based glasses. Banff et al (199) investigated the nn linear respnse f semicnductr dped glasses at a phtn energy f 1-17 ev. They fund that the third-rder nnlinearity is vershadwed by a fifth-rder ne when the band gap f the semicnductr crystallites becme smaller than twice the phtn energy. The fifth-rder nnlinearity is due t the refractin f the free carriers generated by tw-phtn absrptin (TPA). The effect f cmpsitin f leadbismuth-titanate glasses n varius ptical parameters had been reprted by Burghate et al (199). The influence f cmpsitin and film thickness n the ptical band gap and width f the band tail f thin slid films f chalcgenide glass system As-Ge-Se were investigated by Abd *Authr fr crrespndence EI-Rahman et al (1998). Sedeek (1998) investigated the ptical and electrical prperties f amrphus GeSe and Ge0.97Se2Cd0.03 films. Cd 2 incrpratin in these systems affects greatly the thermal activatin energy but has little effect n the ptical gap. The ptical energy gap f a number f amrphus xide glasses (Rashed and Abd E1- Ghani Salem 1984; A1-Ani and Higazy 1991; Sabry and El-samanudy 199; Sharma et al 199) have been analysed n the basis f the thery suggested by Davis and Mtt (1970) and Tauc et al (1966). Values f the ptical energy gap f ZnO-P20~ glasses were reprted and were fund t decrease linearly with the ZnO cncentratin (Rashed and Abd EI-Ghani Salem 1984). A series f glass specimens were prepared frm the binary glasses (MgO-P2Os), and their densities and ptical absrptin edges in the UV-visible range were measured by AI-Ani and Higazy (1991). Sabry and El-samanudy (199) reprted that the ptical band gap increases as the B203 cntent in the TeO2-B203 system increases frm -30 mle percentage. Sharma et al (199.) reprted the ptical absrptin spectra and energy band gap in prasedymium brphsphate glasses. Studies n the ptical prperties f brate glasses had als been reprted by Terashima et al (199). But infrmatin available n the ptical prperties f glasses cntaining a mixture f an xide and a halide is very limited (Dalba et al 1983). It has been reprted that the ptical absrptin f the (AgI)x(Ag20-B203)l_x glasses shifts linearly n increasing the AgI cntent, x. We reprt here the results f a systematic study f the ptical prperties f (1 -x -y)b203-xlizo-ymci2 (M = Cd, Zn) glasses and the variatin f these prperties with cmpsitin. 10

2 106 P S Vijy and M Abdul Khadar 2. Experimental (1 - x - y)b203-xli20-ymci2 (M = Cd, Zn) glasses f different cmpsitins were prepared frm apprpriate amunts f analar grade H3BO3, Li2CO3, and CdCI2 r ZnC12 (table 1). Calculated quantities f the chemicals were mixed thrughly in an agate mrtar and heated t 90 C in a crucible s that a hmgeneusly mixed melt was btained. Glass discs f ram thickness, and diameter ~ 2 cm were made using melt quenching technique. The glass samples were annealed at a temperature f abut 40 C fr 4 h. The samples were cled t rm temperature and were then plished. The grinding and plishing f the glass samples are nly pssible with thse glasses annealed at 40 C. If the annealing temperature is belw 40 C it will break during plishing. The transmissin and absrptin spectra f glasses were recrded using Shimadzu 160A spectrpbtmeter in the wave- length range nm (figures 1 and 2). All the spectra were recrded at rm temperature. The glassy nature f the samples was cnfirmed by recrding the X-ray diffractin pattern. 3. Thery Fr absrptin cefficient ~> 104cm -t, the fllwing relatin is beyed (Tauc et al 1966; Davis and Mtt 1970; Tauc and Menth 1972) a(co)hoj = B(h) - Ep t )r, (1) where hf is the phtn energy, B a cnstant, Ept the ptical gap and r a number which characterizes the transitin prcess. The expnent r can take the values 2, 3, 1/2, and 3/2 fr indirect allwed, indirect frbidden, direct allwed, and direct frbidden transitins, respec- Table 1. Variatin f ptical energy gap (Ept), infinitely high frequency dielectric cnstant (t'.), refractive index (n), cnstant (B), rati f carrier cncentratin t the effective mass (NIm*), and the extent f band tailing (AE) with different glass cmpsitins. Sample Ep t B (N/m*) 1022 AE Glass cmpsitin cde (ev) E'. n (cm-t ev -I/2) (cm-3) (cm-t ev -1/2) x "7 B Li ZnCI2 BZLI B Li ZnCI2 BZL B Li ZnCIz BZL B Li20--O.20ZnC12 BZLA B Li ZnC12 BZL B Li ZnCI2 BZL B Li ZnCI2 BZL B Li CdC12 BCL B Li CdCI2 BCL B LizO-0-20CdCIz BCL B Li20--O.20CdCIz BCL B Li CdCI2 BCL B Li CdCI2 BCL BzO3-0.20Li CDC12 BCL W < BQ.4 IK:CS ICLI -f 1.] ma~ 1!1-0. ~. ~ ~ w.v~w~ (m.) ~T El(X) Wavelength (nm) 20 Figure 1. Transmissin and absrptin spectra f BCL glasses. Figure 2. Transmissin and absrptin spectra f BZL glasses.

3 Optical prperties f (I - x - y)b2oz-xli20-ymc12 glasses 107 tively. The cnstant B is related t the parameter AE, which is a measure f extent f band tailing, by the fllwing equatin (Mtt and Davis 1979), B = 4ffO'mi n/ncae, (2) where O'mi n is the extraplated dc-cnductivity at T =, n the refractive index and C the velcity f light. Ignring the variatin f n with hc and taking n as the average value f n, it fllws that (nb)-1 ~ AE is a measure f extent f band tailing. The reflectance f glass samples was calculated using the equatin t = (l-r) 2 exp(-a), (3) where R is the reflectance, t the transmittance and A the absrbance. Absrptin cefficient tz is btained frm the absrbance (A) with an accuracy _ 0.02 thrugh the relatin A = trd, where d is the thickness f the sample (measured with an accuracy f _ mm). The ptical dielectric cnstant (e') and the square f wavelength (X 2) are related (Spitzer and Fan 197) thrugh the fllwing equatin e' = n 2 = [(1 + x/-r) / (1 - x~")] 2 = e',.- e2n,~2/;'rc2rn *, (4) 40 where e'is the infinitely high frequency dielectric cnstant, e the electrnic charge, and (N/m*) the rati f carrier cncentratin t the effective mass. A 3-2- l --I-- BC L1 ~~b(b BCL2 -..a,-- BC L3 BCL (.) ~ 1. & (a) (11 "7 E I 2 /x. > E v I If/ --=--~CL 1! > Ib) ~,,,..m.-" (.) I, I, I, I, I i hv(ev) Figure 3. Plt f (tzhv) tt2 vs (hv) fr different mle% f ZnC12 and Li20 in BZL glasses. I0, I, I, I, I t 2, hv(ev) Figure 4. Plt f (O~hw) 1/2 VS (hv) fr different mle% f CdC12 and Li20 in BCL glasses. I

4 108 P S Vijy and M Abdul Khadar 4. Results and discussin The varius ptical parameters f the glass samples were calculated and the results are given in table 1. The nature f plts (figures 3 and 4) is fund t cnsist f tw linear regins (a) and (b); in the regin (b), r = 2, suggesting the absrptin prcess t be due t an indirect allwed transitin. The ptical energy gap (Ept) f the glass samples and the cnstant B were btained frm the extraplatin f the linear regin (b) and frm the slpes f the abve-derived curves, respectively. The values f Ept (determined with an accuracy f ev), as a func e- v~t~ e ~-,-- (~),,~ t~ b ICL ~ J ^ -'*- ~'~. *""*" "" 0"* ~'~ ~,",~.. I /" tin f the glass cmpsitin are shwn in figure 6. There is a tendency fr the ptical band gap t becme larger with increasing Li20 r ZnCI2 cntent fr BZL glasses. But the values btained fr Ept are fund t exhibit a maximum at 10 mle per cent f Li20 (BCL2) and minimum with 1 mle per cent f Li20 (BCL3). Similarly fr CdCI2 variatin, Ep t are fund t exhibit a maximum at mle per cent f CdC12 (BCL) and minimum with 1 mle per cent f CdCI2 (BCL7). The ptical band gap f the glass system M20-B203 (M = Li, Na, K, Rb, Cs, Ag) was studied by Terashima et al (199). There is a tendency fr the ptical band gap t becme smaller with increasing M20 cntent fr each glass system. The cmpsitinal dependence f refractive index n as shwn in figure reveals that n is increasing with increasing cncentratin f Li20 r ZnC12 in BZL glasses. Fr BCL glasses, n shws a maximum at 1 mle per cent f Li20 (BCL3) and a minimum value with 10 mle per cent f Li20 (BCL2). While fr CdCI2 variatin, n shws a maximum at mle per cent f CdCI2 (BCL) and a minimum value with 10 mle per cent f CdCI2 (BCL6). 2.O" 1.0" " Mle percen~ fx r y 3AI 3A 3.2 3~0 l ICl.0 l A llol7 Figure. Variatin f refractive index (n) with mle per- centage f Li20 (x) and CdCI 2 r ZnC12 (y) i J I II II I I III II d a. / --m-- L~O ~mlilnn hl 1 = t~c~_, va~aecn ~ ect emmm U~ ~mlm In OZI. ilmms ---- ZnC~ ~'~n ~ nzl im~e 2.4 t~ m.!. I m - I, l. l Mle ~ f x r y Figure 6. Variatin f ptical bandgap (Ept) with mle percentage f Li20 (x) and CdCl 2 r ZnC12 0'). Figure 7.?2 (nm 2) (X10-6) Variatin f dielectric cnstant (g) with ~2.

5 x y)b2oy-xlizo-ymcl2 Optical prperties f (1 - - glasses zt"[ 0 IZLtl ezl7 indicating that the degree f disrder is markedly influenced by the change in cncentratin f Li20, ZnC12 r CdCI2. The bservatin that the magnitude f the band tailing is small pints t the generatin f states in the band gap which are lcalized near the mbility edge. Variatin f AE with cmpsitin f the glass als shws that the extent f the lcalized states is disrdered dependent, which is in agreement with the thery f Mtt and Davis (1979).. Cnclusins :l.li 12.r lo.o 8. "w 6.0 v O~q.3 6ZL4 The ptical gap (Ept) and band tailing (AE) f the glass systems (1 -x- y)b203-xli20-ymc12 (M = Cd, Zn) were fund t vary with the variatin f Li20, CdC12, and ZnC12 cncentratin in the glass cmpsitin. The refractive index (n) is fund t increase with increasing cncentratin f Li20 r ZnCI2 cntent in the BZL glasses, while fr BCL glasses, it shws a maximum and a minimum value fr increasing cncentratin f Li20 r CdC12. It is bserved that the transitin in (1 - x - y)b203-xli20- ymci2 (M = Cd, Zn) glass system is indirect allwed in nature. The values f band tailing (AE) are fund t be very small leading t large values f ptical band gap. References Figur~ 8. i ?.0 " 9. 2 (nm 2) 0(10 "s) Variatin f dielectric cnstant (e ) with ~2. Equatin (4) is verified and the variatin f e' with A, 2 is shwn in figures 7 and 8. The values f e~. and e2n/rtczm * were determined frm the extraplatin f the plt t ;I, 2 = 0 and their slpes, respectively. Then the quantity f (N/m*) is cmputed and its values listed in table 1 as functins f glass cmpsitin. The derived values f (nb) -l, which is a measure f the extent f band tailing (AE), are fund t decrease with increasing cncentratin f Li20 r ZnC12 in BZL glass. But the values f AE shw a maximum and minimum with increasing cncentratin f CdCI2 r Li20 cntent in BCL glasses (table 1). The small values f band tailing energy indicate the presence f sharp lcalized states in the band gap. The changes in the brate netwrk prduced by the additin f Li20 may be the reasn fr the generatin f lcalized states in the band gap. The intrductin f xygen frm a mdifier xide t bric xide glasses cnverts brn frm a 3-crdinated state t a 4-crdinated state. In the BOa and BO3 grups, the xygens are fully bridging fr small cncentratin f mdifying xygen and it will give mre rigidity t the structure. Further additin f mdifier xide causes creatin f nn bridging xygens in the structure and induces mre disrder in the glass structure. Frm table 1 it is seen that there is almst a systematic variatin in Ept and AE 8.0 Abd E1-Rahman A A, Eid A M, Sanad M and El-cker R M 1998 J. Phys. Chem. Slids 9 82 A1-Ani S K J and Higazy A A 1991 J. Mater. Sci Banff G P, Degirgi V and Tan H M 199 J. Opt. Sc. Am. B Burghate D K, Degankar V S, Pakade S V and Yawale S P 199 Indian J. Pure & Appl. Phys Dalba G, Fntana A, Frnasini P, Maritt G, Masull M R and Rcca F 1983 Slid State lnics 9&10 97 Davis E A and Mtt N F 1970 Phils. Mag Jiang H, Sun H and Gan F 1990 J. Mater. Sci. Lett Kh Shkr E 1992 Indian J. Pure & Appl. Phys Ma H L, Zhang X H, Lucas J, Iizuka R, Yamagishi T and Yamashita T 1993 J. Nn-Cryst. Slids Mrsi M Mrsi, EI-Knsl S and El-shahawy M I 1986 J. Nn- Cryst. Slids Mtt N F and Davis E A 1979 Electrnic prcesses in nn-crystalline materials (Oxfrd: Clarendn Press) Secnd ed., p. 289 Rashed I H and Abd E1-Ghani Salem M 1984 Indian J. Pure & Appl. Phys Sabry A I and El-samanudy M M 199 J. Mater. Sci Sedeek K 1998 Indian J. Pure & Appl. Phys Sharma Y K, Mathur S C, Dube D C and Tandn S P 199 J. Mater. Sci. Lett Spitzer W G and Fan H Y 197 Phys. Rev Szukwei M, Hanmei Y and Zngcai C 1982 J. Nn-Cryst. Slids Tauc J and Menth A 1972 J. Nn-Cryst. Slids Tauc J, Grigrvici R and Vancu A 1966 Phys. Status Slidi (a) Terashima K, Kim S H and Tshinbu Yk 199 J. Am. Ceram. Sc