THE SEEBECK COEFFICIENT OF TiO 2 THIN FILMS

Transcription

1 Journal of Otoelectronics and Advanced Materials Vol. 7, No. 2, Aril 2005, THE SEEBECK COEICIENT O TiO 2 THIN ILMS D. Mardare * Al. I. Cuza University, aculty of Physics, 11 Carol I Blvd., R , Iasi, Romania Undoed and doed TiO 2 thin films were deosited onto glass substrates, by r.f. suttering technique, at different substrate temeratures. The films were heat treated in order to stabilize the structure of the samles and to decrease the structural defects. The deendence of the Seebeck coefficient, α, versus inverse temerature, has been studied in the range 300 K 425 K. rom the sloe of the linear decrease of these deendences, ermi energies have been obtained. The values of arameter r, which deend on the nature of the scattering mechanism of the current carriers, have been also determined. (Received August 18, 2004; acceted March 23, 2005) Keywords: Titanium dioxide, Semiconducting films, Seebeck coefficient, ermi level 1. Introduction Thin films like titanium dioxide are very attractive materials due to their unique roerties: high thermal and chemical stability in hostile environments, good durability, high transmittance in the visible sectral range, high refractive index and high dielectric constant. Doing is an useful method to induce substantial modifications in electrical and otical roerties of semiconductor materials. The influence of In, Ce, e, or Nb doing on the structure of TiO 2 thin films is already reorted in literature [1-6]. In a series of revious aers [1, 3, 5, 7, 8] we have studied the otical, electrical and dielectric roerties of doed and undoed TiO 2 suttered thin films. The aim of this aer is to obtain information on the change of the conduction tye in TiO 2 thin films by Nb or e doing. 2. Exerimental Undoed and doed TiO 2 thin films were deosited onto glass substrates, by r.f. suttering technique. The substrate temerature varied from room temerature to 250 C. The doants were: 0.35at.% Nb and 1 at. % e. Details on the deosition arameters are given elsewhere [1]. The study includes four different tyes of samles labelled as: A gl, E gl, C gl and D gl. The deosition conditions and the characteristics of the samles are summarized in Table 1. A rofilometer (Alha-Ste 500, Tencor) was used in order to determine the thickness of the thin films. The values are also given in Table 1. Table 1. Doant, weight ercentage of the anatase hase (W A ), substrate temerature (T S ), film thickness (d), surface roughness determined by AM (R AM ) of the studied TiO 2 thin films. Samle Doant (% at.) W A (%) T S ( C) d (nm) R AM (nm) A gl E gl C gl 0.35 at. % Nb D gl 1 at. % e * Corresonding author: dianam@uaic.ro

2 722 D. Mardare X-ray diffraction (XRD) measurements have been erformed in grazing incidence geometry at 5 (Geigerflex, Rigaku). The diffractometer settings were: Cu K α radiation, U = 40 kv and I= 30 ma [1]. The films were heat treated in order to stabilize the structure of the samles and to decrease the structural defects [3]. The heat treatment consisted in two successive heatings and coolings (10 C/min) within a temerature range T = 300 K K. In our exeriments, the aaratus allows for determining the Seebeck voltage, U, between two oints of the film, having the temerature difference T=T 2 T 1 [9]. The temeratures T 1 and T 2 were measured at the surface of the thin film, with the hel of two thermocoules. Because of the high resistance of TiO 2 films [3], a Keithley 6517A electrometer was used, in order to obtain the Seebeck voltage. 3. Results and discussion In another aer [1], we have shown that, being deosited at 250 C, samle A gl (ure TiO 2 thin film) has a mixed anatase/rutile structure with anatase redominating over the rutile hase (Table 1). Adding 0.35 at. % Nb (C gl ), ure anatase TiO 2 films have been obtained. or samle D gl (TiO 2 doed with 1 at. % e) the rutile reflection R(110) is the only one detected. Thin films deosited onto unheated substrates (E gl ) have an amorhous structure and they are smoother than the olycrystalline films (see Table 1) [1,7]. The deendences of the Seebeck coefficient versus inverse temerature, α=f(10 3 /T) were studied in the temerature range 300 K K for all the samles: A gl, E gl, C gl and D gl (ig. 1) α (mv/k) 0.00 A gl E gl C gl D gl /T (K -1 ) ig. 1. The deendences α=f(10 3 /T) in the temerature range 300 K K. It is known that, in n-tye semiconductors [10, 11], the warm end of the samle has a ositive charge and the cold end has a negative charge, because of the diffusion of the electrons from the warm end to the cold one. or a -tye semiconductor, the holes diffuse from the warm end to the cold end, so the Seebeck voltage has an oosite sign. The Seebeck coefficient (thermoelectric coefficient), α, at a temerature T=(T 2 +T 1 )/2, is given by: α= U/ T (1) or undoed semiconductors, or for weakly doed semiconductors, the deendence α=f(t) could be described using the theory of energy bands. When the scattering of the current carriers is roduced by only one scattering mechanism, the following relations corresond to non-degenerate semiconductors of n-tye and of -tye, resectively [10]:

3 The Seebeck coefficient of TiO 2 thin films 723 k 5 E k 5 N n = r + = r + + ln (2) e 2 kt e 2 n k 5 E + E g = r + + (3) e 2 kt c Here, k is the Boltzmann constant, e is the absolute value of the electron charge, E is the energy of the ermi level (the energy is measured from the inferior limit of the conduction band, E c ), E g is the energy bandga, n is the concentration of the electrons, N c is the density of the states in the conduction band, r is a arameter which deend on the nature of the scattering mechanism of the current carriers. One can see the deendence of Seebeck coefficient on the temerature and on the semiconductor nature. Because n<n c, the sign of α is given by the sign of the current carriers. The relations (2) and (3) have been obtained in the hyothesis that E slowly deends on the temerature. Also, the lower limit of the conduction band, E c, is assumed to be E c = 0, and the uer limit of the valence band E v is E v = E g. rom ig. 1, one can observe the negative values of the Seebeck coefficient for A gl, C gl and E gl samles, which indicate the n-tye conduction in these semiconductors. The ositive values of the Seebeck coefficient for D gl samle indicate -tye conduction. The values of the Seebeck coefficient at 400 K, are given in Table 2. The highest value for α, belongs to the samle E gl. Oxygen vacancies and imurities influence electrical roerties of TiO 2 thin films. While e imurities act as accetors [12], Nb imurities, as well as oxygen vacancies, act as donors [13]. In e doed films, e imurities determine a decrease of the electron concentration, while the oxygen vacancies increase it. Bally et al. [6] established that for an iron concentration higher than 0.13 at. % the ratio between vacancies and e imurities concentration is lower than 0.5 and the conductivity switches from n to -tye. Comaring our data with those obtained for TiO 2 single crystals at 310 K, we have found for E gl the value α 310K = 0.2 mv/k, very closed to α 310K = 0.28 mv/k, obtained by orro et al. [14] for anatase single crystals. or Nb doed samle and for e doed samle we have found 0.07 mv/k and 0.03 mv/k resectively. Thurber et al. [15] found for Nb doed rutile single crystals, α 310K = 0.7 mv/k, ten times higher than ours. Cristea [16] obtained, at the same temerature, α 310K = 0.4 mv/k 0.6 mv/k, for Nb doed rutile single crystals, deending on Nb concentration. Table 2. Seebeck coefficients at 400 K (α 400K ), ermi energies (E ), arameter r and the correlation coefficients (R) of the linear deendences. Samle α 400K E (ev) r R (µv/k) A gl -3,50-0,033-1,54 0,97 E gl -7,50-0,018-2,01 0,94 C gl -1,20-0,0054-2,35 0,99 D gl 1,07-2,970-1,64 0,97 or all the studied samles, the absolute value of the Seebeck coefficient resents a shar decrease with the increasing temerature, near room temerature. Then, it follows a slow linear decrease, in higher temerature range (ig. 2). On this range, from the sloe of the linear decrease, ermi energies have been obtained. The correlation coefficients, R, and the values of ermi energies, E, are given in Table 2. So, for an n-tye semiconductor, the energy of the ermi level, E, can be obtained from the sloe of the linear deendence, α n (1/T) which is observed in the higher temerature range. In that range, we can choose a small domain, T"-T', on which r and E do not deend on the temerature: k 5 E ' n = r +, (4) e 2 kt'

4 724 D. Mardare k 5 E " n = r +. (5) e 2 kt" or an n-tye semiconductor, the energy of the ermi level E is given by: E " n ' n = e (6) 1 1 T" T' Analogously, for a -tye semiconductor, using the relation (3), one gets: E " ' = e E g (7) 1 1 T" T' The ermi energy decreases from ev for the undoed samle, to ev by doing with Nb imurities, and the ermi level aroaches to the bottom limit of the conduction band, E c. or samle D gl, E = 2.97 ev, so, e doing determines an aroach of the ermi level to the the uer limit of the valence band, at a distance of ev, if we consider E g 3 ev, a characteristic value for rutil hase which is resent in the e doed samle [1] α (mv/k) A gl E gl C gl D gl /T (K -1 ) ig. 2. The deendences α=f(10 3 /T) in the range 355 K K. The free term in the relations (2) and (3) offers the value of arameter r, which deend on the nature of the scattering mechanism of the current carriers (see Table 2). We found out that, in the undoed samle, on the temerature range 375 K K and in e doed samle on the range 355 K K, revails the carrier scattering on acoustic longitudinal honons (r 3/2). By Nb doing, r becomes 2.35, in the range 360 K-415 K. This value may indicate a simultaneous action of scattering mechanisms. 4. Conclusions In this aer we have shown that the undoed samles and Nb (0.35 at. %) doed samle, have a n-tye conduction, indicated by the negative values for the Seebeck coefficient, while ositive values for α, obtained for e (1 at. %) doed samle, indicate a -tye conduction. While Nb doing determines an aroach of the ermi level to the bottom limit of the conduction band, e

5 The Seebeck coefficient of TiO 2 thin films 725 doing determines the shift of the ermi level towards the uer limit of the valence band. The carrier scattering on the acoustic longitudinal honons revails in the undoed samle, in the temerature range 375 K-425 K, and in e doed samle in the range 355 K-400 K. Acknowledgements The author is indebted to rofessor. Lévy and A. R. Bally from IPA, EPL Lausanne, Switzerland, for the rearation of the samles. References [1] D. Mardare, M. Tasca, M. Delibas, G. I. Rusu, Alied Surface Science 156, 200 (2000). [2] W. A. Badawy, R. S. Momtaz, E. M. Elgiar, Phys. Stat. Sol. (a) 118, 197 (1990). [3] D. Mardare, G. I. Rusu, Materials Science and Engineering, B 75(1), 68 (2000). [5] D. Mardare, Peter Hones, Materials Science and Engineering, B68, 42 (1999). [6] A. R. Bally, E. N. Korobeinikova, P. E. Schmid,. Lévy,. Bussy, J. Physics D: Al. Phys. 31(10) 1149 (1998). [7] D. Mardare, G. I. Rusu, J. Otoelectron Adv. Mater. 3(1), 95 (2001). [8] D. Mardare, G. I. Rusu, J. Otoelectron. Adv. Mater. 6(1), 333 (2004). [9] G. G. Rusu, J. Otoelectron. Adv. Mater. 3(4), 861 (2001). [10] R. Smith, Semiconductors, (Cambridge University Press, London, 1980). [11] I. V. Savelyev, Physics, A General Course, vol. III, (Mir Publishers, Moscow, 1979). [12] M. Radecka, M. Rekas, K. Zakrzewska, Solid State Phenomena, 39-40, 113 (1994). [13] A. Bernasik, M. Radecka, M. Rekas, M. Sloma, Alied Surface Science 65/66, 240 (1993). [14] L. orro, O. Chauvet, D. Emin, L. Zuiroli, H. Berger,. Lévi, J. Al. Phys. 75(1), 633 (1994). [15] W. R. Thurber, J. H. Mante, Phys. Rev. 5A, 139, 1655 (1965). [16] V. Cristea, Rev. Roum. Phys. 23(6), 537 (1978).