opt is the extrapolated dc conductivity at T =, E c

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1 Jurnal Ovnic Research Vl. 6, N.1, February 21, p EFFECT OF FILM THICKNESS, ANNEALING AND SUBSTRATE TEMPERATURE ON THE OPTICAL AND ELECTRICAL PROPERTIES OF CuGa.25 In.75 Se 2 AMORPHOUS THIN FILMS B. A. MANSOUR a, H. SHABAN a, S. A. GAD a, Y. A. EL-GENDY b, A. M. SALEM c a Slid State Physics Labratry, Natinal Research Centre, Dkki, Cair, Egypt b Physics Departament, Faculty Science, Helwan, Cair, Egypt c Electrn Micrscpe and thin ilm Dept. Natinal Research Center, Dkki, Cair, Egypt Thin ilms CuGa.25 In.75 Se 2 dierent grwth cnditins were depsited n glass substrates by thermal evapratin. X-ray diractin revealed the rmatin amrphus ilms. The eect thickness (d), heat treatment and substrate temperature (T s ) n the ptical energy gap (E pt. ) and n the activatin energy (ΔE) r dc cnductivity and the density lcalized states at the Fermi level N (E ) were carried ut. Fr all ilms depsited at rm temperature, the ptical transitin was und t be indirect. The ptical energy gap increase with increasing ilm thickness and with increasing temperature and time annealing. The band tail (E c ) beys Urabach empirical relatin. Up t Ts > 5 K, it is bvius that the prcess transrmatin t plycrystalline ccurs. As a result, the tw direct energy gaps which attributed t the undamental edge E g1 and band splitting Eg 2 are und. The electrical cnductivity increases with increasing thickness, with increasing the evapratin cnditins (d, Ts, and annealing temperature and time) and exhibits tw types cnductin mechanisms. The crrespnding band is apprximately hal the ptical energy gaps and the eect is interpreted in term density state by Mtt and Davis. (Received January 22, 21; accepted February 17, 21) Keywrds: CuGa.25 In.75 Se 2, Optical, Electrical prperties 1. Intrductin The measurements ptical absrptin and particularly the absrptin edge are imprtant especially in cnnectin with the thery the electrnic structure amrphus materials. In the high absrptin regin, Tauc et al. [1] and Davis and Mtt [2] independently derived an expressin relating the absrptin ceicient α (ν ), t phtn energy, h ν ; m α( ν ) = β ( hν E ) hν (1) pt / Where β is a cnstant ( β = 4 πσ nce c), c is the speed light, σ is the extraplated dc cnductivity at T =, E c is a measure the extent band-tailing, n is the reractive index, E pt. is the ptical energy gap, and m is a number which characterizes the ptical absrptin prcess: m =1/2 r a direct allwed transitin, m=3/2 r a direct rbidden transitin, m=2 r an indirect allwed transitin and m =3 r an indirect rbidden transitin.

2 14 * Crrespnding authr: samiagad2@yah.cm Absrptin at lwer phtn energy usually llws the Urbach rule [3], i.e. α( ν ) = α exp hν (2) E c E c where is the Urbach energy which is interperated as the width the tails lcalized states in the band gap. In the present wrk was cncerned with sme experimental bservatin n eect ilm thickness, thermal annealing and substrate temperature n the (i) structure, (ii) ptical and electrical cnductin the CuGa.25 In.75 Se 2 amrphus thin ilms. 2. Experimental CuGa.25 In.75 Se 2 thin ilms were prepared by thermal evapratin in vacuum 4-6 ( 1 Pa, i.e.abut 1 trr) synthetic chalcpyrite grwn in labratry. A plycrystalline ingt was prepared rm stichimetric mixture pure elements (5 N Mathey Chem. Ltd.). The cmpsitin and the hmgeneity the ally were determined by micrprbe analysis which was perrmed at the physical chemistry Institute Munster University Germany. Excess Se (apprximately 3 ml %) was included in the starting cmpsitin t cmpensate r any ptential lss Se vapur during evapratin and t avur p-type electrical cnductin in these ilms. The details the preparatin and evapratin ingt and thin ilms are given in Re. [4] and [5], respectively. The structure the as-depsited and the eect annealed, substrate temperature and ilm thickness were analyzed using an X-ray diractmeter (XRD) (Philips PW 1373). The transmissin spectra were measured in the wavelength range 3-25 nm using a duble beam spectrphtmeter, mdel (Shimaza mdel 311). The electrical cnductivity, σ was measured by cnventinal ur prbe methd using speciically cnstructed sample hlder [6], keithly electrmeter type 614 was used r vltage and current measurements. Ohmic cntacts were abricated by use liquid silver. Each experimental pint pltted in the curves electrical cnductivity is the mean value at least tw ilms having the same thickness and tw dierent successive runs r each thickness. 3. Results and discussin 3.1 X-ray diractin Fig.1 shw the XRD diagrams crrespnding t a pwder sample and thin ilms dierent thickness heat treatment r 1 and 2 hur at 373 K and evaprated at dierent substrate temperature, T s. It is revealed that the bulk sample has a crystalline structure, while thin ilms, bth as-depsited and dierent thickness and annealed r 1h and 2h at 373 K, shw n evidence mlecular units crrespnding t the crystallizatin, indicating the amrphus nature. The grain size the ilm was s small and the disrder within these grains s high that n speciic diractin peaks culd be detected. But, the ilms btained by evapratin n substrate with temperature ranging rm rm temperate t T s < 473 K cnsist mainly the amrphus phase and the amrphus diminishes with the raise T s. Up t T s > 523 K; it is bvius that the prcess transrmatin t plycrystalline ccurs.

3 15 Fig.1 XRD patterns pwder and CuGa.25 In.75 Se 2 thin ilms at dierent evapratin cnditins. 3.2 Optical prperties The ptical prperties were studied by measuring the transmissin spectra in the wavelength range 3-25 nm. The reractive index was determined using a prcedure prpsed by Swanepel [7] which is based n drawing envelpe curves thrugh the minima, (T m ) and maxima, (T M ) in the transmissin spectrum, the variatin in the transmissin spectrum is principally due t the intererence phenmena. This apprach yields [ N + ( N n ) ] ni = s TM Tm ns + 1 (3) N = 2ns + TMTm 2 where n i is the reractive index the ilm, and n s is the reractive index the glass substrate (n s =1.5 in ur case). Using Eq.3, n (λ) can be calculated. Accrding t Tauc [8] it is pssible t separate three distinct regins in the absrptin edge spectrum r amrphus semicnductrs; (i) the weak absrptin tail, which riginate rm deects and impurities, (ii) the expnential l randmness the system, and (iii) the high absrptin regin which determines the ptical energy gap. 1 2

4 16 On the ther hand, thickness the prepared ilms has been determined using tw dierent techniques and the results were cmpared. First it has been calculated rm the transmissin curves Fig. 2 using the relatin λ1λ2 d = 2( n2λ1 n1λ2 ) (4) where n 1 and n 2 are the reractive indices at tw adjacent maxima (r minima) at λ 1 and λ 2. Secnd, the multiple beams Fizeau ringes methd [9] has been used t determine the ilm thickness. The values thickness, d r CuGaInSe thin ilms determined by dierent methds were und cincide t within 5 %. m The methd used t determine the value E pt invlves pltting a graph ( αh ν ) versus h ν. Frm apprpriate value E pt will be given the interceptin n the hν axis. Dierent authrs have suggested dierent values m r dierent amrphus materials. Davis and Mtt [2] btained m = 2 r mst amrphus semicnductrs. Hwever, r mre cmplicated materials Fagen and Fritzsche [1] btained m = 3 and m =1 was btained by thers [11, 12]. Fig. 2 (a, b, c and d) illustrates the transmissin spectra (T-λ), the absrptin ceicient, 2 α, ( αh ν ) 1 and ln α versus h ν, r all investigated samples depsited at rm temperature. The 2 graph ( αh ν ) 1 versus hν in Fig. 2c has a well deined linear regin thus cnirming that Eq.1 hlds r m=2 indicating that indirect phtn transitin are invlved and the ptical energy gap, E pt was calculated rm the curves. Fig.4 (c, d) shws the direct energy gaps r thin ilms depsited at 533 K and at 533 K and annealed at 373 K r 2h respectively a: Eect thickness As shwn in Fig.2c and Table 1 the ptical energy gap increases with increasing thickness, this can be explained as due t the presence deects in amrphus materials [13] in terms eliminatin deects in the amrphus structure. The insuicient number atms depsited in the amrphus ilm results in the existence unsaturated bnds [14]. The unsaturated bnds are respnsible r the rmatin sme deects in the ilms which prduced lcalized states in amrphus slids [15]. Thicker ilms are characterized by mre hmgeneus netwrk, which minimizes the number deects and lcalized states, and thus the ptical band gap increases. The calculated values E pt. are listed in Table 1. The values E c are very much larges than.5 ev and decreases with increasing thickness. The Tauc mdel [16] based electrnic transitins between lcalized states in the band tails may be valid in ur materials, and the extending deeper band tail in the gap is decrease and led t decrease in the value E c and n increase in the value E pt. Table I Optical parameters. Thickness (nm) Subs. Temp (K) An. Temp. (K) Time (h) E gind. (ev) E c (ev) E gdir. (ev) E g1 E g h 2h 2h

5 17 a α (cm-1) Ga.25 dierent thickness, d d= 29 nm d=776 nm d= 126 nm Phtn energy, hν (ev) b (αhν) 1/2 (.cm -1.eV) 1/2 8 4 ln (α) Phtn energy, hν (ev) hν (ev) c d Fig.2 Eect thickness n (a) Transmissin spectra (b) Absrptin ceicient (c) (αhν) 1/2 vs. (hν) (d) Ln (α) vs. (hν) r CuGa.25 In.75 Se 2 thin ilms.

6 b: Eect annealing and substrate temperature n the ptical band gap The degree disrder and deects present in the amrphus structure change due t heat treatment [17]. The decrease in the disrder and deects in the structural bnding is knwn t 2 increase the ptical band gap E pt. Figs. 3(c, d) shw ( αh ν ) 1 and ln α versus h ν, r the investigated ilms annealed at 373 K r 1 and 2 h, respectively. The values E pt. increase as the temperature and time annealing increase, but the values E c decrease. This can be explained by assuming that during the annealing prcess the ilms will have time enugh r sme atmic rearrangement t take place. Sme deects will be remved which, reducing the density dangling bnds, redistribute atmic distances and bnd angles and E pt. will then increase. Fr substrate temperature Ts = 533 K, and annealed at 373 K r 2h, there are tw direct energy gaps Eg 1 = (1.11,1.14), Eg 2 = (1.25,1.28) shwn in Fig. 4 (c, d) due t the existence plycrystalline phase as a result amrphus phase transrmed t plycrystalline phase Fig.(1). These energies are attributed t the undamental edge and band splitting. This result is in gd agreement with ur previus wrk [5] r plycrystalline CuGa x In 1-x Se 2 depsited at 63 K. 1 2.E E+4 Transmissin, T 6 4 α (cm) -1 1.E+4 2 Ga.25 Ga.25 & ann. at 1 C r 1 h Ga.25 & ann. at 1 C r 2 h 5.E Wavelenght (λ).e hν (ev) a b (αhν) 1/2 1 ln (α) hν (ev) hν (ev) c d Fig. 3 Eect annealing n (a) Transmissin spectra (b) Absrptin ceicient (c) (αhν) 1/2 vs. (hν) (d) Ln (α) vs.(hν) r CuGa.25 In.75 Se 2.

7 Transmissin, T % 5 25 R.T Absrptin ceicient, α [cm -1 ] R.T Ts = 533 K Ts = 533 and ann. At 373 K r 2 h Wavelength λ [nm] Phtn energy [hν] a b 3.E+8 1.6E+9 1.E K & annealed at 373 r 2h 3.E K (αh ν) 2 (cm -1.eV -1 ) 2 2.E+8 1.E+8 1.2E+9 8.E+8 4.E+8 (αh ν) 2 (cm -1.eV -1 )2 (αh ν) 2 (cm -1. ev -1 ) 2 8.E+8 4.E+8 2.E+8 1.E+8 (αh ν) 2 (cm -1.eV -1 ) 2.E Phtn energy, h ν c.e+.e+.e h ν (ev) d Fig.4 Eect substrate temperature n (a) Transmissin spectra. (b) Absrptin ceicient. (c) (αhν) 2 vs. (hν) (d) Eect substrate and annealing n (αhν) 2 vs. (hν) r CuGa.25 In.75 Se Electrical cnductins The dc electrical cnductivity, σ as shwn in Fig. 5 was measured as a unctin temperature in the range 3-45 K. The general beahviur σ (T ) in temperature range 37 K < T < 476 K r all samples was increased expnentially with T and exhibit tw types cnductin mechanisms. The activatin energy, Δ E can be estimated accrding Eq.5 σ = σ exp( ΔE kt) (5) where σ is the pre-expnential actr and k is the Bltzmann cnstant and Δ E is the crrespnding activatin energy, which is a unctin the electrnic energy levels the chemically interacting atms in the glass and hence the energy gap. The estimated values Δ E and σ R. T (electrical cnductivity at rm temperature) r all samples are given in Table II. Frm Table II and Fig.5 it is bserved that σ R. T and ΔE increase with increasing thickness, annealing and substrate temperature and the value ΔE is apprximately hal the ptical energy gap shw Table I. These results may be due t the enhancement hmgeneity the ilms, which cntain lwer deects [18].

8 2 Fig. 5. Temperature dependence d.c. cnductivity r CuGa.25 In.75 Se 2 thin ilms with annealing and substrate temperature. The inset shws the plt Lg (σt 1/2 ) versus T -1/4 thin ilms. Fig. 6. Temperature dependence d.c. cnductivity r CuGa.25 In.75 Se 2 thin ilms with annealing and substrate temperature. The inset shws the plt Lg (σt 1/2 ) versus T -1/4 thin ilms. Sample Thickn ess (nm) As-dep As-dep Ann T s Table II Electrical parameters σ R.T (Ω -1.cm -1 ) A (K -1 ) T (K) ( 1 7 ) N( E ) ( 1 18 ev cm -1 ) γ ( 1-7 cm 1 ) R (cm) ( 1-7 ) W ΔE (ev) In rder t interpret the electrical data we shall assume that the mdel r the electrnic density states in amrphus semicnductrs as prpsed by Mtt et al. [15] is applicable. The essential eature this mdel is the existence narrw tails lcalized states at the extremities the valence and cnductin band [15]. The slight increase in σ (T ) culd be explained as llws: the amrphus semicnductr, due t lcal statisactin all valence requirements,

9 exhibits the ability r sel-cmpensatin. This sel-cmpensatin prperty amrphus slid results in an intrinsic σ and well-deined activatin energy. Arund rm temperature up t 33 K regin, the electrical cnductivity σ (T ) is insensitive t temperature and crrespnding t transprt via variable-range hpping charge carriers in the lcalized states near the Fermi level, and is characterized by Mtt' s variable range hpping relatin [ 19] 4 σ ( T ) = σ T )[exp( T T ) 1 ] (6) ( T represents the degree disrder in the material and can be given apprximately by: 21 T = 18α 3 kn( E ) (7) where N E ) the density lcalized states at the Fermi level, k is the Bltzman cnstant and ( α is the ceicient the expnential decay the lcalized state wave unctin. A plt 1 4 ln( σ T ) versus T r as-prepar ed ilms with dierent thickness and heat treatment is shwn in Fig.5. The calculated values T and N( E ) are given in Table II. It is bserved that T decreased while N E ) increased with increasing thickness and annealing and substrate ( temperatures. This decrease T results in a decrease in disrder [2]. Tw ther hpping parameters R(cm) the hpping distance and W (ev ) the average hpping energy accrding t Mtt and Davis [21] and Hill [22] are given as 1 4 R = [9 8παkTN( E )] and (8) 3 W = 4πR N( E ) 3 The calculated values R and W r thin ilms are als listed in Table Cnclusins The ptical and electrical prperties CuGa.25 In.75 Se 2 amrphus ilms dierent grwth cnditins (thickness, d, substrate temperature T s and heat treatment) were investigated. The ptical transitin r all ilms depsited at rm and annealed temperature was und indirect and the band tails E c beys Urabach empirical relatin. The ptical energy gaps E pt increases and E c decreases with increasing d and with annealing temperature and time, this can attributed t decrease in the degree disrder. Tw direct energy gaps which attributed t the undamental edge E g1 and band splitting E g2 are und r ilms depsited at T s = 533K as a result phase transrmatin rm amrphus phase t plycrystalline phase. The electrical cnductivity depends n the thickness and annealing temperature and exhibits tw types cnductin mechanisms. The density lcalized states at the Fermi level N (E ) and the activatin energy ΔE are increased with evapratin grwth and the crrespnding band is apprximately hal the cnductin ptical energy. It is reasnable t assume that the bserved changes in the ptical and electrical prperties n annealing and high temperature depsitin are a result micrstructural re-arrangement initiated either during depsitin r during annealing and the eects are interpreted in terms density states mdel prpsed by Mtt and Davis.

10 22 Reerences [1] T. Tauc, R.Grigkvici, A.Vancu, Phys.Status Slidi 15, 627 (1966). [2] E. A.Davis, N. F. Mtt, Phils. Mag.22 ( 197). [3] F. Urbach, Phys. Rev. 92, 1324 (1953). [4] B. A. Mansur, I. K. El Zawawi, H. Shaban, J. Mat.Sci. Elect. 14, 63 (23). [5] B.A. Mansur, S.A. Abd El- Hady, A. Abdel-All,, I. K.El Zawawi, H.Shaban, Fizika A, 12(2), 75 (23). [6] B.A. Mansur, B.S. Farag and S.A. Khdier, Thin Slid Films, 247, 112 (1994). [7] R. Swanepel, J. Phys. E. Sci. Instrum., 16, 1214 (1983). [8] J. Tauc, in Amrphus and Liquid Semicnductrs, edit by J. Tauc, Plenum, New Yrk, (1974) Ch.4. [9] H. E. Bennet, J. M. Bennet, in physics Thin Films, Ed. G.Haas and R.E. Thun, Academic Press, New Yrk, 4 (1967), pp [1] E. A.Fagen, Fritzsche, J. Nn- Cryst. Slids, 2 (197) 18. [11] N.A. Hegab, M. Fadel, M. M. El Samanudy, J. Mater. Sci. 3, 5461 (1995). [12] K. Sedeek, Fadel, Thin Slid Films, 229, 223 (1993). [13] S. Chaudhuri, S. K.Biswas, A. Chudhury, J. Mater. Sci. 23, 447 (1988). [14] S. K.Biswas, S. Chaudhuri, A. Chudhury, Phys. Status. Slidi (a) 15 (1988) 467. [15] K. I. Arshak, C. Ahgarth, Thin Slid Films 137, 281 (1986). [16] J. Tauc, " The Optical Prperties Slids' edited by F. Abeles ( Nrth Hlland, Amterdam, 197 ) P.227. [17] M. Suzuki, H. Ohdaira, T.Matsumi, T. Matsumi, M. Kumeda, T. Shimizu, J. Appl. Phys. 16, 221 (1977). [18] B. A. Mansur, H. Shaban, S. H. Mustaa, Phys. Lw.Dim. Struct. 314, 127 (24). [19] Z. S. El Manduh, J. Appl. Phys. 78(12), 7158 (1995). [2] N. F. Mtt, E. A. Davis, R. A. Street, Philns. Mag. 32, 961 (1975). [21] N. F. Mtt, E. A. Davis, Electrnic Prcesses in Nn- Crystalline Materials (Clarrendn, Oxrd. 1979) [22] R. M. Hill. Philns. Mag. 24, 137 (1971).