ON THE STRUCTURAL AND OPTICAL PROPERTIES OF ANTIMONY TRISULFIDE THIN FILMS

Transcription

1 Journal of Optoelectroncs and Advanced Materals Vol. 4, No. 4, December, p ON THE STRUCTURAL AND OPTICAL PROPERTIES OF ANTIMONY TRISULFIDE THIN FILMS N. Tgau, G. I. Rusu a, C. Gheorghes Faculty of Scence, Dunarea de Jos Unversty, Galat, R-6, Romana a Faculty of Physcs, Alexandru Ioan Cuza Unversty, Ias, R-66, Romana Thn flms of antmony trsulfde were prepared by thermal vacuum evaporaton technque at substrate temperatures from K. At substrate temperature 473 K, the flms have amorphous structure, whle those prepared at = 498 K has polycrystallne structure as dentfed by X ray dffracton technque (XRD and atomc force mcroscopy (AFM. Absorpton coeffcent, refractve ndex and optcal band gap energy were determned from transmsson spectra. (Receved October 4, ; accepted October 31, Keywords: Antmony trsulfde, Thermal vacuum evaporaton, Optcal propertes 1. Introducton Metal chalcogendes (sulfdes, selendes and tellurdes are mportant materals for applcatons such as photoconductng cells, photovoltac cells and other optoelectrcal devces. Among the group V-IV compounds, antmony trsulfde (Sb S 3 thn flm s used as a target materal n televson cameras, mcrowave devces, swtchng devces and varous optoelectronc devces [1,]. Antmony trsulfde has receved a lttle attenton as a potental canddate n solar energy converson. The band gap ( ev covers the maxmum scan the vsble and near nfrared ranges of the solar spectrum. The optcal and electrcal studes of thn antmony trsulfde flms are mportant both fundamentally and for applcatons [3, 4]. In ths paper the effect of substrate temperature on the fundamental absorpton edge of antmony trsulfde thn flms was nvestgated.. Expermental Antmony trsulfde thn flms were prepared by thermal vacuum evaporaton technque onto glass substrate, at vacuum of torr. Antmony trsulfde powder (Merck of purty 99,99 % was used. The expermental arrangement permtted to prepare thn flms samples under varous condtons: the deposton rate, r d = 1 1 Ås -1, the substrate temperature, = K and vacuum, p = torr. The flm thckness, d, was measured by an nterferometrc method [5] and for nvestgated samples ranged between.35 and 1.5 µm. The structural analyss of flms was performed by X ray dffracton (XRD, usng the CuK α radaton (λ= Å. The surface morphology of the flms was nvestgated by means of atomc force mcroscopy (AFM. The transmsson spectra, n the spectral doman 4 14 nm, were recorded usng a SPECORD UV VIS M-4 double beam spectrophotometer, at room temperature.

2 944 N. Tgau, G. I. Rusu, C. Gheorghes 3. Results and dscusson Dffracton analyss of thn flms deposted at room temperature, or at temperatures up to 473 K, show an amorphous structure, whle those deposted at = 498 K have polycrystallne orthorhombc structure by usng X ray. Intensty (arbtr. unts b a d=.35 µm; < 498K b d=1.17 µm; = 498 K (13 (431 (44 (41 (41 (31 (4 (31 (1 (11 (13 a θ (deg Fg. 1. X ray dffracton pattern of Sb S 3 thn flms. Fg. 1 shows the characterstc peaks of antmony trsulfde. The results are n agreement wth those reported by Droch et al. [6]. Our nvestgatons have shown that the crystallte sze determned by means atomc force mcroscopy (AFM ranged between 1 and 4 nm. The average sze of the crystallte ncreases wth ncreasng substrate temperature durng flm deposton. Ths ncrease s more evdent for thcker flms. The hgher substrate temperatures determne an ncrease of surface moblty of adatoms durng deposton [7]. These condtons favor the growth of flms havng crystalltes wth greater sze. The crystallte sze also ncreases wth ncreasng deposton rate. At very hgh deposton rates, the adatoms nteract strongly wth each other to become chemsorbed wth lttle surface mgraton; consequently the fne graned flms were obtaned. In Fg. are presented AFM mages for two nvestgated samples deposted under dfferent condtons. These mages confrm the mentoned characterstcs of flm structure. a b Fg.. AFM mages (5 µm 5 µm for two Sb S 3 thn flms: a d =.35 µm, = 3 K, r d = 3 Ås -1 ; b d = 1.17 µm, = 498 K, r d = 98 Ås -1.

3 On the structural and optcal propertes of antmony trsulfde thn flms 945 The optcal transmsson spectra as a functon of wavelength n the range of 4 to 14 nm were measured for dfferent samples deposted at substrate temperatures from 3 to 498 K. Fg. 3 show typcal transmsson spectra for two nvestgated flms. 1 8 Transmsson (% 6 4 d =.35 µm, = 3 K d = 1.17 µm, = 498 K Wavelength (nm Fg. 3. Transmsson spectra of Sb S 3 thn flms. The number of the nterference fngers n the transmsson curves s determned by the thckness of the flms. If the flms thckness, d, s unform, nterference effects gve a bg number of nterference frnges n the regon of weak and medum absorpton. The optcal constants lke refractve ndex, n and the absorpton coeffcent, α of the flms have been determned from the transmttance spectrum followng the method of Swanepoel [8,9]. If two envelopes are drawn through the maxma and mnma of the oscllatng transmttance the refractve ndex s gven as: where: s n 1 / 1 / n = [ N( λ + ( N ( λ n ] (1 ns + n TM ( λ Tm ( λ N( λ = + nsn ( T ( λ T ( λ In relatons (1 and ( n s = 1.5 s the refractve ndex of the glass substrate, n = 1 s the refractve ndex of ar and T M and T m are the maxmum and mnmum transmsson n the transmsson spectra. The absorpton coeffcent, α, of the thn flms can be calculated by the expresson: M m 1 1 α = ln (3 d x In the regon of weak and medum absorpton parameter x were calculaton from the relaton: 3 EM [ EM ( n 1 ( n ns ] x = (4 3 ( n 1 ( n n s 4 1/

4 946 N. Tgau, G. I. Rusu, C. Gheorghes where: E M 8n ns = + ( n 1( n ns (5 T M In the regon of strong absorpton the values of x can be calculated usng the relaton: 3 ( n + 1 ( n + ns x T (6 16 n n The varaton of the absorpton coeffcent wth the ncdent photon energy at and near the fundamental absorpton edge for Sb S 3 thn flms of dfferent substrate temperature s shown n Fg 4. The flm prepared at = 3 K posses the characterstc features of the optcal absorpton edge of amorphous semconductors. The flm prepared at = 498 K has a structure polycrystallne. Ths flm shown dfferent behavor of absorpton spectra than that for amorphous flms, due to the change n the structure. Ths behavor s probably due to the ncrease n the crystallte sze. It s known that ntercrystallte boundares contan structural defects, mpurtes etc. These factors have a strong nfluence on the absorpton processes. In flms wth greater crystalltes the ntergran domans play a less mportant role n optcal absorpton. The absorpton coeffcent s slghtly affected by the change of substrate temperature at lower energy values. Ths reflects a dfference n the degree of dsorder of amorphous flms [1]. s 5 d =,35 µm, = 3 K d = 1,17 µm, = 498 K α (1 3 cm ,8 1, 1, 1,4 1,6 1,8,, hν (ev Fg. 4. Varaton of the absorpton coeffcent wth the ncdent photon energy for Sb S 3 thn flms. where Optcal band gap of the amorphous flms were calculated from the relaton: 1 / g ( α hν = A( hν E (7 E g s the optcal band gap whch corresponds to ndrect transton and A s a constant. g Consequently, the optcal band gap, E, cold be found by takng the ntercept of a plot of ( α hν versus photon energy h ν (Fg /

5 On the structural and optcal propertes of antmony trsulfde thn flms (αhν 1/ (cm -1/ ev 1/ =3 K ; E g = 1,79 ev =333 K ; E g = 1,77 ev =373 K ; E g = 1,7 ev =43 K ; E g = 1,69 ev =473 K ; E g = 1,6 ev 5,8 1, 1, 1,4 1,6 1,8,, hν (ev 1 / Fg. 5. Varaton of ( hν α versus photon energy ν h for amorphous thn flms. A slght decrease n the ndrect optcal band gap, from 1.79 to 1.61 ev as the substrate temperature of the deposted flm ncreases from 3 to 473 K was observed. The values of optcal band gap are n agreement wth that reported prevously [1-13]. Ths feature can be related to the growth of ordered domans n the flms. In general, the propertes of the glasses wth Sb S 3 component or Sb O 3 depend on the structure of the antmony sulfde or oxde clusters as shown by Sava [14] and Naln et al. [15]. The optcal band gap of polycrystallne flm prepared at 498 K s due to the drect transton accordng to the relaton: d g ( α hν = A( hν E (8 d where E g s the drect optcal band gap and B s a constant. The dependence of ( α hν versus photon energy h ν s represented n Fg. 6, from whch the drect optcal band gap s equal to 1.8 ev. 35 (αhν (1 8 cm - ev T s =498 K ; E g d =1,8 ev 5,8 1, 1, 1,4 1,6 1,8,, hν (ev Fg. 6. Varaton of ( hν α versus photon energy, h ν, for polycrystallne thn flm.

6 948 N. Tgau, G. I. Rusu, C. Gheorghes 5. Conclusons From the present study, t can be concluded that the flms deposted at 473 K have amorphous structure, whle those deposted at = 498 K have polycrystallne structure as dentfed by X ray dffracton analyss. As the substrate temperature ncreases, the optcal band gap of the amorphous flms lnearly decreases from 1.79 to 1.6 ev. The degree of dsorder decreases wth ncreasng the substrate temperature up to 473 K. At = 498 K, the optcal band gap for drect transton s equal to 1.8 ev due to the structural transformaton of polycrystallne. References [1] M. S. Ablova, A. A. Andreev, T. T. Degegkaev, B. T. Melekh, A. B. Pevtsov, N. S. Shendel, L. N. Shurnlova Sov. Phys. Semcond. 1, 6 (1976. [] G. Ghosh, B. P. Varma, Thn Sold Flms 6, (1979. [3] L. P. Deshmukh, S. G. Holkatt, B. P. Rane, B. M. More, P. P. Hankare, J. Electrochem. Soc., 41, 1779 (1994. [4] M. T. S. Nar, Y. Pena, J. Campos, V. M. Garca, P. K. Nar, J. Electrochem. Soc. 141, 113 (1998. [5] K. L. Chopra, Thn Sold Fenomena, McGraw-Hll, New York, [6] M. S. Droch, F. Vallant, E. Bustarret, D. Jousse, J. Non-Cryst. Solds 11, 151 (1988. [7] L. I. Massel, R. Glang, Handboock of Thn Flm Technology, McGraw-Hll Press, New York, 198. [8] R. J. Swanepoel, J. Phys. E. Sc. Instrum. 16, 114 (1983. [9] E. Marquez, J. B. Ramrez-Malo, P. Vllares, R. Jmenez-Garay, R. J. Swanepoel, Thn Sold Flms, 54, 83 (1993. [1] I. K. El Zawaw, A. Abdel-Moez, F. S. Terra, M. Mounr, Thn Sold Flms, 34, 3 (1998. [11] E. I. Zorna, N. I. Gndash, Sov. Physcs-Semconductors 4, 1 (1971. [1] A. G. Vedeshwar, J. Phys. II France, 5, 1161 (1995. [13] A. M. Salem, M. Solman Selm, J. Phys.D: Appl. Phys. 34, 1 (1. [14] F. Sava, J. Optoelectron. Adv. Mater. 3(, 45 (1. [15] M. Naln, Y. Messadeq, S. J. L. Rbero, M. Poulan, V. Bros, J. Optoelectron. Adv. Mater. 3(, 553 (1.