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Close space vapor transport method for Bi2Te3 thin films deposition: Influence of the type of substrate
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 70 (2009) 365–370
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Close space vapor transport method for Bi2Te3 thin films deposition: Influence of the type of substrate O. Vigil-Gala´n , F. Cruz-Gandarilla, J. Sastre´-Herna´ndez, F. Roy 1, E. Sa´nchez-Meza, G. Contreras-Puente ´ticas—I.P.N., Edificio no. 9, U.P.A.L.M., 07738 Me ´xico D.F., Me´xico Escuela Superior de Fı´sica y Matema
a r t i c l e in fo
abstract
Article history: Received 5 February 2008 Received in revised form 11 July 2008 Accepted 3 November 2008
Bismuth telluride thin films have been grown by close space vapor transport (CSVT) technique as a function of substrate temperature (Tsub). Both N- and P-type samples can be obtained by this method which is a relatively simple procedure, which makes the method interesting for technological applications. The samples were deposited onto amorphous glass and polycrystalline CdTe film substrates in the substrate temperature range 300–425 1C, with a fixed gradient between source and substrate of 300 1C. The influence of the type of substrate and substrate temperature in the CSVT chamber on the physical properties of the films is presented and discussed. & 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films D. Electrical properties
1. Introduction Binary bismuth telluride system and alloys based on Bi2Te3 have been extensively investigated in thin films form [1–6]. The interest in the respective alloys arises due to their potentiality in the microfabrication of integrated thermoelectric devices. The optimal physical properties of these materials like the Seebeck coefficient and the resistivity can be drastically affected when grown by physical evaporation methods, mainly because of the great difference between the vapor pressure of Te with respect to the Bi ones and because the phenomena is related to the reevaporation mechanism of Te in the formation of the Bi2Te3 compound. It is well known that thermoelectric properties of semiconductors such as Bi2Te3 are drastically affected with changes in the stoichiometry. For these reasons, the physical thin films processing methods require a precise control of the composition and some times sophisticate growth conditions must be applied, and these turn them into complicated and expensive growth methods. Furthermore, some phases of Bi–Te system can be obtained from the phase diagram, with P- and N-type conductivities depending on the atomic percentage composition of Te in the sample with respect to the stoichiometric ones. On the other hand, Bi2Te3 thin films have been deposited onto CdTe (111)B substrates using MBE technique, with high crystallinity, high termopower and high electron mobility [1]. Recently we have demonstrated that the CSVT is an efficient technique for the Corresponding author. Tel.: +52 55 5729 6000x55078; fax: +52 55 5586 2957.
E-mail addresses: [email protected], [email protected] (O. Vigil-Gala´n). 1 Permanent address: E´cole Polytechnique de Montre´al, 2500 chemin de Polytechnique, Montre´al, Que´bec, Canada´ H3T 1J4. 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.11.008
deposition of P-type Bi2Te3 and P-type Sb2Te3 films with good thermoelectric properties, and this can be useful in different applications [7]. The CSVT method permits to obtain P- and N-type samples of Bi–Te system. Normally the soda-lime glass is used as substrate for thermoelectric study of Bi2Te3, while in potential solar cells application this material could be used as back contact in CdTe solar cells. The aim of this work is to present the results concerning the Bi–Te thin films system, grown by CSVT in variable range of substrate temperature (Tsub). The influence of the type of substrate and growth temperature, as well as of the post thermal annealing on the properties of layers has been analyzed by using energydispersive X-ray analysis (EDAX), scanning electron microscopy (SEM), Seebeck coefficient and Hall effect measurements.
2. Experimental details Bi–Te layers were deposited on soda-lime glass and polycrystalline CdTe films substrates by CSVT from commercial Bi2Te3 powders. Bi–Te films were deposited in the 300–450 1C range of substrate temperature, using Ar atmosphere, with total final pressure of 1 104 Torr (0.013 Pa). This range of temperature was selected because for the deposition temperature less than 3001, an amorphous peak is observed and for temperature higher than 4251, the re-evaporation process from the substrate limits the film quality. Thermal gradient between the source and the substrate was kept constant in all depositions and the substrate temperature was changed in the range 300–425 1C. Seebeck coefficient characterization was carrier up measuring the thermoelectric voltage variations in temperature intervals of 5 1C. Cu bars were used as ohmic contact for these measurements. Electrical
ARTICLE IN PRESS ´n et al. / Journal of Physics and Chemistry of Solids 70 (2009) 365–370 O. Vigil-Gala
366
conductivity measurements were made by using a dc two-probe configuration. Van der Pauw measurements were used to determine carrier mobility and carrier concentration. All measurements were done at room temperature. The scanning electron microscopy images were obtained with an XL 30 FEG/SIRION form FEI with energy-dispersive X-ray GENESIS 4000 (from EDAX). Post thermal treatments were performed in air at 400 1C for 30 min.
3. Results and discussion 3.1. Growth kinetic The films obtained have a metallic luster and good adherence to the substrate. Fig. 1 displays the growth rate of CSVT–Bi2Te3 layers, deposited onto glass and CdTe substrates. In our experimental conditions, CdTe films grown by CSVT method exhibit zincblende structure with a ¼ 6.48 A˚, with an appreciable {111} preferred orientation, which represents a big lattice mismatch with Bi–Te system. It is seen that the growth rate increases when the samples are grown on polycrystalline CdTe substrates with respect to the amorphous glass ones. Furthermore, in both cases an increase in the growth rate is observed with the substrate temperature. For the samples grown on glass substrate, a near constant activation energy of about (0.1770.2) eV is obtained, whereas for samples grown on polycrystalline CdTe substrates the activation energy is about (0.2270.2) eV. The deposition growth rate varies between (0.1570.01) and (1.2070.01) mm/min for the layers grown on glass and between (0.4370.01) and (3.3070.01) mm/min for the layers grown on CdTe, when the substrate temperature is increased. These results clearly show the growth mechanism dependence of Bi–Te layers on the type of substrate. The influence of the growth parameters and type of substrate on the growth rate of the CSVT–Bi2Te3 is very important for the design of thermoelectric and solar cells devices films. In our case the growth rate depends on the amorphous and polycrystalline nature of glass and CdTe films. Usually the growth rate of the deposited films increases in polycrystalline substrate with respect to the amorphous ones. Other fact to be taken into account is the
3500 (b)
Growth rate (nm/min.)
3000
(a) (b)
2500 2000 1500
(a) 1000 500 0 300
320
340 360 Tsub. (°C)
380
400
Fig. 1. Growth rate as a function of the substrate temperature for Bi–Te layers grown on (a) glass substrates and the CSVT-CdTe substrates. Inset; logarithm of variation of growth rate vs. 1/KT (line only as guide to eye).
lattice constant values of the cubic CdTe (6.48 A˚) and hexagonal Bi2Te3 (4.38 A˚), resulting in an in-plane lattice mismatch of 32%. 3.2. Morphological analysis The SEM images in Figs. 2a and b reveal, respectively the morphology of the films deposited on glass and polycrystalline CSVT–CdTe. The morphology of the grains is the plates aligned parallel to the surface of glass substrate, with c-axis perpendicular to the planes, shown in Fig. 3a. Furthermore, there is the growth grain at high substrate temperature. The morphology of the films changes depending on the type of substrate. In the case of CdTe substrates (with morphology of plane faces), the Bi2Te3 is formed with a morphology similar to that of CdTe but different from the ones formed when the layer is deposited onto glass. In Fig. 3 the kinetics of the grain growth (diameter in the films plane) for glass/Bi–Te and CdTe/Bi–Te systems are shown. The grain size dependences on substrate temperature of the samples grown on glass and those grown on CdTe is different. The two grain growths vary exponentially in the substrate temperature T sub y ¼ 1:55 þ 1:3x104 exp 35:8 For the samples grown on glass and T sub y ¼ 0:5 þ 0:16 exp 92:4 For the samples grown on CdTe The influence of the type of substrate on physical properties of the Bi–Te system kinetics is directly related to the grain growth dependence on substrate temperature. 3.3. Electrical properties and composition of the layers Tables 1 and 2 depicts the dependence of values of Seebeck coefficient, resistivity and power factor (measured at room temperature) on the substrate temperature of the Bi–Te layers grown on glass and CdTe substrates, respectively. For the sample grown on glass, it is possible to vary the source temperature to obtain low-resistive films with both types of conductivity deposited by CSVT. Up to Tsub ¼ 350 1C, P-type films are obtained. At Tsub ¼ 375 1C none of the defined types of conductivities was observed to a possible change of phase, while for higher values of Tsub, the N-type conductivity is obtained in the samples. For the P-type samples, the lowest value of resistivity (8 104 O cm) is obtained at Tsub ¼ 325 1C, while for the N-type samples the lowest value of resistivity (5 104 O cm) is reached at Tsub ¼ 425 1C. Samples grown on CdTe show an increase in the resistivity value, with respect to the ones deposited on glass for the same values of the substrate temperature. In this case, P-type samples are obtained up to Tsub ¼ 375 1C, while for substrate temperature higher than 375 1C, N-type conductivity is obtained in the samples. It is found from Table 1 that the highest values of the power factor are obtained, respectively, at Tsub ¼ 350 and 425 1C for the P- and N-type samples grown on glass substrates. From Table 2 the best results for the Bi–Te system are obtained for samples grown at 325 and 400 1C, for the P- and N-type samples; however, the values of the power factor for the samples grown on CdTe are near two orders of magnitude lower than for the ones grown on glass.
ARTICLE IN PRESS ´n et al. / Journal of Physics and Chemistry of Solids 70 (2009) 365–370 O. Vigil-Gala
Tsubs= 325 °C
367
Tsubs= 325 °C
Tsubs= 350 °C
Tsubs= 350 °C
Tsubs= 400 °C
Tsubs= 400 °C
Fig. 2. Scanning electron micrographs of Bi–Te system layers grown on (a) glass substrates and CdTe substrates, at different substrate temperatures.
25
Table 1 Seebeck coefficient, resistivity and power factor dependence on the substrate temperature of the Bi–Te system films grown on glass by CSVT.
glass CdTe
Grain size (µm)
20
15
10
Ts (1C)
a (mV/K)
r (mO cm)
a2/r (104 W/K2 m)
Type of conductivity
300 325 350 375 400 425
180 255 238 33 30 184
8.1 0.8 6.9 8.4 5.5 2.8
4.0 81.2 8.2 0.1 0.2 12.1
P P P – N N
5
0 300
320
340
360
380
400
Tsubs (°C) Fig. 3. Grain size variation as a function of substrate temperature for films grown on glass and CdTe substrates.
Figs. 4a and b displays variation of mobility and carrier concentration of P- and N-type conductivity films with substrate temperature, performed at room temperature. It can be seen that the Hall mobility and the carrier concentration have opposite behaviors for P- and N-type samples, typical of semiconductor materials. Furthermore, Hall mobility and carrier concentration are lower for sample grown on CdTe substrate, compared to those grown on glass substrate.
Table 2 Seebeck coefficient, resistivity and power factor dependence on the substrate temperature of the Bi–Te system films grown on CdTe by CSVT. Ts (1C)
a (mV/K)
r (mO cm)
a2/r (104 W/K2 m)
Type of conductivity
300 325 350 375 400 425
58 76 82 43 11 70
14.7 22.3 2.5 370 136 306
0.23 0.26 2.70 0.005 0.0009 0.02
P P P P N N
In the morphological studies, influence of the type of substrate on growth and distribution of the grains in CSVT-Bi2Te3 layers was analyzed. Of course the morphology of the films has a direct influence on electrical properties and therefore the best values of
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µ carrier concentration
350 300 250 2 µ (cm/V-s)
Carrier concnetration (cm-3)
1020
1019
150 100 50
1018
0
8x1018
200 6x1018 µ (cm2/V-s)
carrier concentration (cm-3)
200
4x1018 2x1018
150
100
50
0 300
320
340
360 380 Tsubstrate (°C)
400
420
Fig. 4. Mobility and carrier concentration of Bi–Te films grown on (a) glass substrates and (b) CdTe substrates, vs. substrate temperature (line only as guide to eye).
conductivity and Seebeck coefficient (which are measured in the parallel direction to the substrate) are obtained for the layer deposited on glass with respect to CdTe substrates. Table 3 lists the films composition of the EDAX analysis as a function of the substrate temperature of the films on glass and CdTe substrates. The atomic percent of the bismuth is nearly a constant in the substrate temperature range 300–320 1C, with values near those of the stoichiometric Bi2Te3 compound. It is well known that one of the tough problems concerning the deposition of Bi2Te3 films by direct evaporation method is the large difference in the vapor pressures of Bi and Te related. This fact limits the preparation of films with the same composition of the source material. To overcome this problem, Zou et al. [2] have reported deposition of Bi2Te3 films with both N- and P-type conductivities by co-evaporator on glass substrates. The type of conductivity in the films was obtained through variation of the substrate temperature and the flux ratio of Bi and Te in the camera. Change from P-type to N-type conductivity was obtained when Te concentration in the samples was lightly increased from 46.62 (Wt%) to 48.05 (Wt%) and the authors assumed that in all cases the Bi2Te3 phase was obtained. In our case concentrations of Bi and Te in Bi2Te3 commercial powders were 40.13 and 59.87 at%, respectively. Therefore, from the results shown in Table 3, the P-type films are obtained with the Bi2Te3 phase. However for N-type samples (grown at 400 and 425 1C), Te concentration is less than that for corresponding Bi2Te3 powders as can be seen in Table 3. From Table 3 Bi–Te layers grown on CdTe substrates show a tendency to a decrease Te concentration for the highest substrate temperatures. Cho et al. [1] have explained the results of Bi2Te3 layers grown onto CdTe in terms of antisite defects. According to them, the excess Te occupies Bi lattice sites and behaves as an N-type dopant. In our case the results from R-x, the samples grown at 400 and 425 1C show a new peak (or peaks) with an
Table 3 Influence of substrate temperature and type of substrate on composition (from EDAX analysis) and thickness of Bi–Te films grown by CSVT method. Tsubstrate (1C)
Bi (At%)
Te (At%)
d (mm)
Possible phases
300 300a 325 325a 350 350a 400 400a 425 425a
39.7 39.1 39.7 39.1 40.3 40.6 42.1 43.1 45.3 46.4
60.3 60.1 60.3 60.1 59.7 59.4 57.9 56.9 54.7 53.6
0.75 2.2 1.40 2.9 2.30 9.5 4.90 13.5 6.20 16.5
Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 ? ? ? ?
a
CdTe substrate.
appreciable shift with respect to the (0 0 6) and (00.15) of the Bi2Te3, instead of a continuous displacement of the peaks (0 0 6) and (00.15) depending on the Bi concentration changes [8], whereas from high-resolution electron microscopy measurements, other phases or phase variants distributed at a microscopic level were found [9]. Taking into account the phase diagram of the Bi–Te system as a function of the Te concentration, we assumed that films at 400 and 425 1C behave as N-type samples with other phases of Bi2Te3. However, a full study is necessary to determine the real phases other than Bi2Te3 present in our samples. The kinetic growth process can be analyzed as follows: Te molecules adsorbed on the surface of the deposited layer and the well-known re-evaporation coefficient (DTe) and density of Te molecules adsorbed are related by [10] DTe ¼ Rt / V 1 re
(1)
ARTICLE IN PRESS ´n et al. / Journal of Physics and Chemistry of Solids 70 (2009) 365–370 O. Vigil-Gala
where R is the flux of Te molecules on the surface, t is the lifetime of an adsorbed Te molecule and Vre is the re-evaporation rate. The lifetime of an adsorbed Te molecule is given by
t ¼ n1 eE=kT S
(2)
where n is a parameter depending on the vibrational frequency of the adsorbed molecule, TS is the substrate temperature and E is the energy related to the adsorption process. From Eq. (1) the reevaporation of tellurium molecules is increased when both R and t decrease. From Eq. (2) the lifetime of the adsorbed Te molecule decreases when temperature of the substrate increases. On the other hand, R depends on the partial pressure in the chamber, which can be controlled by varying the source temperature and/or the final pressure in the chamber. Due to the fact that vapor pressure of Te is higher than that of Bi and taking into account the fact that the pressure in the chamber is nearly 1 104 Torr, the deposited Te atoms tend to reevaporate from the substrate when the temperature is increased. Therefore for high temperature of substrate it is possible to take this behavior as a jump in the structural characteristic. Table 4 presents the properties of the best films grown under optimal conditions of CSVT method. The mobility value of 326 cm2/V s is one of the highest reported for P-type Bi2Te3 films. As the substrate temperature increases, mobility decreases. The
Table 4 Thickness, roughness, mobility, carrier concentration and resistivity of the Bi–Te films deposited on glass and CdTe substrates under the best CSVT growth conditions. Tsubstrate (1C)
d (mm)
Roughness (nm)
m (cm2/V s)
N/P (cm3)
r (O cm)
325 400 325a 400a
1.4 4.9 2.9 13.5
81 120 340 506
326 48 200 20
1.3 1018 2.3 1019 1.4 1018 2.3 1017
8 104 3 104 2 102 4 101
a
CdTe substrate.
369
lattice scattering mechanism could be causing this behavior. For the N-type samples low values or low mobility are obtained. For samples grown on CdTe substrates, degradation in the electrical parameters is observed probably as a consequence of stress due to the difference between lattice constants of CdTe and Bi–Te systems. Measuring the Hall effect for the sample grown at 3751 was not possible and in Tables 1 and 2 the lowest value of the Seebeck coefficient is observed . For the sample grown on CdTe at this substrate, a light tendency to P-type was observed. These results could be related to possible phase change at this temperature.
3.4. Influence of the post thermal treatment Post thermal treatments were performed in air in samples grown on glass at 400 1C for 30 min and on CdTe at 350 1C. The influence of thermal treatment in morphology of samples is shown in Fig. 5. From this figure, we can observe that the thermal treatment has different influence in samples grown on glass with respect to those grown on CdTe. The thermal treatment leads to an increase in the grain size average over the film surface in the first case, whereas for the samples grown on CdTe a tendency to form a more compact film is observed. The number of grains per unit area becomes smaller but the occupied surface increases. The post thermal treatment has a different influence on the electrical properties of the samples (see Table 5). For the samples grown at low substrate temperatures (300, 325 and 350 1C) the electrical conductivity decreases with the thermal treatment, while for the sample grown at higher substrate temperatures a light increment in the conductivity was observed, whether the samples are grown on glass or on CdTe substrates. Also, the value of the Seebeck coefficient for the samples deposited at low substrate temperature (up to 375 1C), showed a tendency to decrease while for high substrate temperature it was near constant, before and after the thermal treatment. Grain boundaries determine changes in carrier’s conduction mechanism. Usually the main effect of thermal annealing is to decrease the
Tsubs = 350 °C
Tsubs = 350 °C
Tsubs = 350 °C; TT 400
Tsubs = 350 °C; TT 400
Fig. 5. Grain size variation as a function of substrate temperature for films grown on (a) glass substrates and (b) CdTe substrates.
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Table 5 Values of the Seebeck coefficient and resistivity of Bi–Te layers deposited on glass substrates, before and after the thermal treatment. Tsubstrate (1C)
300 325 350 375 400 425
a (mV/K)
r (mO cm)
Before TT
After TT
Before TT
After TT
180 255 238 33 30 184
126 230 190 30 25 180
8.1 0.8 6.9 8.4 5.5 2.8
11.6 1.7 9.3 10.5 6.6 3.1
grain boundary barrier. In this case diminution of the grain boundary barrier increases the minority electron carrier injection through the conduction band, with a partial compensation of the majority carriers (holes). The net result is the decrease in the Seebeck voltage and therefore in the Seebeck coefficient. In N-type samples where phase transition occurs, the electron carrier injection has a negligible effect on compensation of majority carrier concentration, and the Seebeck coefficient does not change significantly, in agreement with our experimental results.
4. Conclusions In summary, we demonstrated the possibility of preparing Bi–Te films, depositing on glass and CdTe substrates, by using close space vapor transport and Bi2Te3 powders. The physical properties of the layers are dependent of the type of substrate. In general, layers with better electrical properties are obtained when the films are deposited on glass substrates. Both types of conductivity can be obtained choosing growth parameters. N-type samples can be obtained in samples deposited on glass as well as in those on polycrystalline CdTe. For substrate
temperature higher than 375 1C phases other than Bi2Te3 were assumed. Growth processes of CSVT–Bi2Te3 films on glass and polycrystalline CSVT-CdTe films have been established and then permit control of growth parameters in their future applications.
Acknowledgments This work was partially supported by CONACyT-Me´xico and SIP-IPN (Project 2006216) and Conacyt. O. Vigil-Gala´n and F. CruzGandarilla acknowledge support from COFAA-IPN. F. Roy acknowledges support from the government of Mexico through the Ministry of Foreign Affairs. References [1] S. Cho, Y. Kirn, A. DiVenere, G.K.L. Wong, J.B. Ketterson, J.R. Meyer, Appl. Phys. Lett. 75 (1999) 1401. [2] Helin Zou, D.M. Rowe, Gao Min, J. Cryst. Growth 222 (2001) 82. [3] H. Zou, D.M. Rowe, S.G.K. Williams, Thin Solid Films 408 (2002) 270. [4] H. Noro, K. Sato, H. Kagechika, J. Appl. Phys. 73 (1993) 1252. [5] J. Walachova´, R. Zeipl, J. Zelinka, V. Malina, M. Pavelka, M. Jelı´nek, V. Studnika, P. Losˇt’a´k, Appl. Phys. Lett. 87 (2005) 1902. [6] L. Scidone, S. Diliberto, N. Stein, C. Boulanger, J.M. Lecuire, Mater. Lett. 59 (2005) 746. [7] O. Vigil-Gala´n, F. Cruz-Gandarilla, J. Sastre´-Herna´ndez, F. Roy, E. Sa´nchezMeza, Close space vapor transport method for Bi2Te3 and Sb2Te3 films deposition: application to thermoelectric and solar cells devices, J. Cryst. Growth (2008). ˜ as-Moreno, J. Sastre´-Herna´ndez, F. [8] F. Cruz-Gandarilla, O. Vigil-Gala´n, G. Caban Roy, Structural and microstructural characterization of Bi2Te3 films deposited by close space vapor transport method using SEM and DRX techniques, Thin Solid Films (2008). [9] F. Cruz-Gandarilla, O.Vigil-Gala´n, J.E. Stabubbs, S. Peripolli, C.A. Achete, H.A. Caldero´n, Phase formation as a function of substrate temperature in thin films of Bi–Te grown by CSVT, accepted by Presentation at the 2008 Microscopy and Microanalysis Meeting, Symposium Understanding the Synthesis and properties of Nanostructures and Nanomaterials, Alburquerque, USA. [10] Y.A. Boikov, O.S. Gribanov, V.A. Danilov, A. Kutasov, Sov. Phys. Solid State 33 (1991) 1926–1929.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Close space vapor transport method for Bi2Te3 thin films deposition: Influence of the type of substrate O. Vigil-Gala´n , F. Cruz-Gandarilla, J. Sastre´-Herna´ndez, F. Roy 1, E. Sa´nchez-Meza, G. Contreras-Puente ´ticas—I.P.N., Edificio no. 9, U.P.A.L.M., 07738 Me ´xico D.F., Me´xico Escuela Superior de Fı´sica y Matema
a r t i c l e in fo
abstract
Article history: Received 5 February 2008 Received in revised form 11 July 2008 Accepted 3 November 2008
Bismuth telluride thin films have been grown by close space vapor transport (CSVT) technique as a function of substrate temperature (Tsub). Both N- and P-type samples can be obtained by this method which is a relatively simple procedure, which makes the method interesting for technological applications. The samples were deposited onto amorphous glass and polycrystalline CdTe film substrates in the substrate temperature range 300–425 1C, with a fixed gradient between source and substrate of 300 1C. The influence of the type of substrate and substrate temperature in the CSVT chamber on the physical properties of the films is presented and discussed. & 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films D. Electrical properties
1. Introduction Binary bismuth telluride system and alloys based on Bi2Te3 have been extensively investigated in thin films form [1–6]. The interest in the respective alloys arises due to their potentiality in the microfabrication of integrated thermoelectric devices. The optimal physical properties of these materials like the Seebeck coefficient and the resistivity can be drastically affected when grown by physical evaporation methods, mainly because of the great difference between the vapor pressure of Te with respect to the Bi ones and because the phenomena is related to the reevaporation mechanism of Te in the formation of the Bi2Te3 compound. It is well known that thermoelectric properties of semiconductors such as Bi2Te3 are drastically affected with changes in the stoichiometry. For these reasons, the physical thin films processing methods require a precise control of the composition and some times sophisticate growth conditions must be applied, and these turn them into complicated and expensive growth methods. Furthermore, some phases of Bi–Te system can be obtained from the phase diagram, with P- and N-type conductivities depending on the atomic percentage composition of Te in the sample with respect to the stoichiometric ones. On the other hand, Bi2Te3 thin films have been deposited onto CdTe (111)B substrates using MBE technique, with high crystallinity, high termopower and high electron mobility [1]. Recently we have demonstrated that the CSVT is an efficient technique for the Corresponding author. Tel.: +52 55 5729 6000x55078; fax: +52 55 5586 2957.
E-mail addresses: [email protected], [email protected] (O. Vigil-Gala´n). 1 Permanent address: E´cole Polytechnique de Montre´al, 2500 chemin de Polytechnique, Montre´al, Que´bec, Canada´ H3T 1J4. 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.11.008
deposition of P-type Bi2Te3 and P-type Sb2Te3 films with good thermoelectric properties, and this can be useful in different applications [7]. The CSVT method permits to obtain P- and N-type samples of Bi–Te system. Normally the soda-lime glass is used as substrate for thermoelectric study of Bi2Te3, while in potential solar cells application this material could be used as back contact in CdTe solar cells. The aim of this work is to present the results concerning the Bi–Te thin films system, grown by CSVT in variable range of substrate temperature (Tsub). The influence of the type of substrate and growth temperature, as well as of the post thermal annealing on the properties of layers has been analyzed by using energydispersive X-ray analysis (EDAX), scanning electron microscopy (SEM), Seebeck coefficient and Hall effect measurements.
2. Experimental details Bi–Te layers were deposited on soda-lime glass and polycrystalline CdTe films substrates by CSVT from commercial Bi2Te3 powders. Bi–Te films were deposited in the 300–450 1C range of substrate temperature, using Ar atmosphere, with total final pressure of 1 104 Torr (0.013 Pa). This range of temperature was selected because for the deposition temperature less than 3001, an amorphous peak is observed and for temperature higher than 4251, the re-evaporation process from the substrate limits the film quality. Thermal gradient between the source and the substrate was kept constant in all depositions and the substrate temperature was changed in the range 300–425 1C. Seebeck coefficient characterization was carrier up measuring the thermoelectric voltage variations in temperature intervals of 5 1C. Cu bars were used as ohmic contact for these measurements. Electrical
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366
conductivity measurements were made by using a dc two-probe configuration. Van der Pauw measurements were used to determine carrier mobility and carrier concentration. All measurements were done at room temperature. The scanning electron microscopy images were obtained with an XL 30 FEG/SIRION form FEI with energy-dispersive X-ray GENESIS 4000 (from EDAX). Post thermal treatments were performed in air at 400 1C for 30 min.
3. Results and discussion 3.1. Growth kinetic The films obtained have a metallic luster and good adherence to the substrate. Fig. 1 displays the growth rate of CSVT–Bi2Te3 layers, deposited onto glass and CdTe substrates. In our experimental conditions, CdTe films grown by CSVT method exhibit zincblende structure with a ¼ 6.48 A˚, with an appreciable {111} preferred orientation, which represents a big lattice mismatch with Bi–Te system. It is seen that the growth rate increases when the samples are grown on polycrystalline CdTe substrates with respect to the amorphous glass ones. Furthermore, in both cases an increase in the growth rate is observed with the substrate temperature. For the samples grown on glass substrate, a near constant activation energy of about (0.1770.2) eV is obtained, whereas for samples grown on polycrystalline CdTe substrates the activation energy is about (0.2270.2) eV. The deposition growth rate varies between (0.1570.01) and (1.2070.01) mm/min for the layers grown on glass and between (0.4370.01) and (3.3070.01) mm/min for the layers grown on CdTe, when the substrate temperature is increased. These results clearly show the growth mechanism dependence of Bi–Te layers on the type of substrate. The influence of the growth parameters and type of substrate on the growth rate of the CSVT–Bi2Te3 is very important for the design of thermoelectric and solar cells devices films. In our case the growth rate depends on the amorphous and polycrystalline nature of glass and CdTe films. Usually the growth rate of the deposited films increases in polycrystalline substrate with respect to the amorphous ones. Other fact to be taken into account is the
3500 (b)
Growth rate (nm/min.)
3000
(a) (b)
2500 2000 1500
(a) 1000 500 0 300
320
340 360 Tsub. (°C)
380
400
Fig. 1. Growth rate as a function of the substrate temperature for Bi–Te layers grown on (a) glass substrates and the CSVT-CdTe substrates. Inset; logarithm of variation of growth rate vs. 1/KT (line only as guide to eye).
lattice constant values of the cubic CdTe (6.48 A˚) and hexagonal Bi2Te3 (4.38 A˚), resulting in an in-plane lattice mismatch of 32%. 3.2. Morphological analysis The SEM images in Figs. 2a and b reveal, respectively the morphology of the films deposited on glass and polycrystalline CSVT–CdTe. The morphology of the grains is the plates aligned parallel to the surface of glass substrate, with c-axis perpendicular to the planes, shown in Fig. 3a. Furthermore, there is the growth grain at high substrate temperature. The morphology of the films changes depending on the type of substrate. In the case of CdTe substrates (with morphology of plane faces), the Bi2Te3 is formed with a morphology similar to that of CdTe but different from the ones formed when the layer is deposited onto glass. In Fig. 3 the kinetics of the grain growth (diameter in the films plane) for glass/Bi–Te and CdTe/Bi–Te systems are shown. The grain size dependences on substrate temperature of the samples grown on glass and those grown on CdTe is different. The two grain growths vary exponentially in the substrate temperature T sub y ¼ 1:55 þ 1:3x104 exp 35:8 For the samples grown on glass and T sub y ¼ 0:5 þ 0:16 exp 92:4 For the samples grown on CdTe The influence of the type of substrate on physical properties of the Bi–Te system kinetics is directly related to the grain growth dependence on substrate temperature. 3.3. Electrical properties and composition of the layers Tables 1 and 2 depicts the dependence of values of Seebeck coefficient, resistivity and power factor (measured at room temperature) on the substrate temperature of the Bi–Te layers grown on glass and CdTe substrates, respectively. For the sample grown on glass, it is possible to vary the source temperature to obtain low-resistive films with both types of conductivity deposited by CSVT. Up to Tsub ¼ 350 1C, P-type films are obtained. At Tsub ¼ 375 1C none of the defined types of conductivities was observed to a possible change of phase, while for higher values of Tsub, the N-type conductivity is obtained in the samples. For the P-type samples, the lowest value of resistivity (8 104 O cm) is obtained at Tsub ¼ 325 1C, while for the N-type samples the lowest value of resistivity (5 104 O cm) is reached at Tsub ¼ 425 1C. Samples grown on CdTe show an increase in the resistivity value, with respect to the ones deposited on glass for the same values of the substrate temperature. In this case, P-type samples are obtained up to Tsub ¼ 375 1C, while for substrate temperature higher than 375 1C, N-type conductivity is obtained in the samples. It is found from Table 1 that the highest values of the power factor are obtained, respectively, at Tsub ¼ 350 and 425 1C for the P- and N-type samples grown on glass substrates. From Table 2 the best results for the Bi–Te system are obtained for samples grown at 325 and 400 1C, for the P- and N-type samples; however, the values of the power factor for the samples grown on CdTe are near two orders of magnitude lower than for the ones grown on glass.
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Tsubs= 325 °C
367
Tsubs= 325 °C
Tsubs= 350 °C
Tsubs= 350 °C
Tsubs= 400 °C
Tsubs= 400 °C
Fig. 2. Scanning electron micrographs of Bi–Te system layers grown on (a) glass substrates and CdTe substrates, at different substrate temperatures.
25
Table 1 Seebeck coefficient, resistivity and power factor dependence on the substrate temperature of the Bi–Te system films grown on glass by CSVT.
glass CdTe
Grain size (µm)
20
15
10
Ts (1C)
a (mV/K)
r (mO cm)
a2/r (104 W/K2 m)
Type of conductivity
300 325 350 375 400 425
180 255 238 33 30 184
8.1 0.8 6.9 8.4 5.5 2.8
4.0 81.2 8.2 0.1 0.2 12.1
P P P – N N
5
0 300
320
340
360
380
400
Tsubs (°C) Fig. 3. Grain size variation as a function of substrate temperature for films grown on glass and CdTe substrates.
Figs. 4a and b displays variation of mobility and carrier concentration of P- and N-type conductivity films with substrate temperature, performed at room temperature. It can be seen that the Hall mobility and the carrier concentration have opposite behaviors for P- and N-type samples, typical of semiconductor materials. Furthermore, Hall mobility and carrier concentration are lower for sample grown on CdTe substrate, compared to those grown on glass substrate.
Table 2 Seebeck coefficient, resistivity and power factor dependence on the substrate temperature of the Bi–Te system films grown on CdTe by CSVT. Ts (1C)
a (mV/K)
r (mO cm)
a2/r (104 W/K2 m)
Type of conductivity
300 325 350 375 400 425
58 76 82 43 11 70
14.7 22.3 2.5 370 136 306
0.23 0.26 2.70 0.005 0.0009 0.02
P P P P N N
In the morphological studies, influence of the type of substrate on growth and distribution of the grains in CSVT-Bi2Te3 layers was analyzed. Of course the morphology of the films has a direct influence on electrical properties and therefore the best values of
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µ carrier concentration
350 300 250 2 µ (cm/V-s)
Carrier concnetration (cm-3)
1020
1019
150 100 50
1018
0
8x1018
200 6x1018 µ (cm2/V-s)
carrier concentration (cm-3)
200
4x1018 2x1018
150
100
50
0 300
320
340
360 380 Tsubstrate (°C)
400
420
Fig. 4. Mobility and carrier concentration of Bi–Te films grown on (a) glass substrates and (b) CdTe substrates, vs. substrate temperature (line only as guide to eye).
conductivity and Seebeck coefficient (which are measured in the parallel direction to the substrate) are obtained for the layer deposited on glass with respect to CdTe substrates. Table 3 lists the films composition of the EDAX analysis as a function of the substrate temperature of the films on glass and CdTe substrates. The atomic percent of the bismuth is nearly a constant in the substrate temperature range 300–320 1C, with values near those of the stoichiometric Bi2Te3 compound. It is well known that one of the tough problems concerning the deposition of Bi2Te3 films by direct evaporation method is the large difference in the vapor pressures of Bi and Te related. This fact limits the preparation of films with the same composition of the source material. To overcome this problem, Zou et al. [2] have reported deposition of Bi2Te3 films with both N- and P-type conductivities by co-evaporator on glass substrates. The type of conductivity in the films was obtained through variation of the substrate temperature and the flux ratio of Bi and Te in the camera. Change from P-type to N-type conductivity was obtained when Te concentration in the samples was lightly increased from 46.62 (Wt%) to 48.05 (Wt%) and the authors assumed that in all cases the Bi2Te3 phase was obtained. In our case concentrations of Bi and Te in Bi2Te3 commercial powders were 40.13 and 59.87 at%, respectively. Therefore, from the results shown in Table 3, the P-type films are obtained with the Bi2Te3 phase. However for N-type samples (grown at 400 and 425 1C), Te concentration is less than that for corresponding Bi2Te3 powders as can be seen in Table 3. From Table 3 Bi–Te layers grown on CdTe substrates show a tendency to a decrease Te concentration for the highest substrate temperatures. Cho et al. [1] have explained the results of Bi2Te3 layers grown onto CdTe in terms of antisite defects. According to them, the excess Te occupies Bi lattice sites and behaves as an N-type dopant. In our case the results from R-x, the samples grown at 400 and 425 1C show a new peak (or peaks) with an
Table 3 Influence of substrate temperature and type of substrate on composition (from EDAX analysis) and thickness of Bi–Te films grown by CSVT method. Tsubstrate (1C)
Bi (At%)
Te (At%)
d (mm)
Possible phases
300 300a 325 325a 350 350a 400 400a 425 425a
39.7 39.1 39.7 39.1 40.3 40.6 42.1 43.1 45.3 46.4
60.3 60.1 60.3 60.1 59.7 59.4 57.9 56.9 54.7 53.6
0.75 2.2 1.40 2.9 2.30 9.5 4.90 13.5 6.20 16.5
Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 ? ? ? ?
a
CdTe substrate.
appreciable shift with respect to the (0 0 6) and (00.15) of the Bi2Te3, instead of a continuous displacement of the peaks (0 0 6) and (00.15) depending on the Bi concentration changes [8], whereas from high-resolution electron microscopy measurements, other phases or phase variants distributed at a microscopic level were found [9]. Taking into account the phase diagram of the Bi–Te system as a function of the Te concentration, we assumed that films at 400 and 425 1C behave as N-type samples with other phases of Bi2Te3. However, a full study is necessary to determine the real phases other than Bi2Te3 present in our samples. The kinetic growth process can be analyzed as follows: Te molecules adsorbed on the surface of the deposited layer and the well-known re-evaporation coefficient (DTe) and density of Te molecules adsorbed are related by [10] DTe ¼ Rt / V 1 re
(1)
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where R is the flux of Te molecules on the surface, t is the lifetime of an adsorbed Te molecule and Vre is the re-evaporation rate. The lifetime of an adsorbed Te molecule is given by
t ¼ n1 eE=kT S
(2)
where n is a parameter depending on the vibrational frequency of the adsorbed molecule, TS is the substrate temperature and E is the energy related to the adsorption process. From Eq. (1) the reevaporation of tellurium molecules is increased when both R and t decrease. From Eq. (2) the lifetime of the adsorbed Te molecule decreases when temperature of the substrate increases. On the other hand, R depends on the partial pressure in the chamber, which can be controlled by varying the source temperature and/or the final pressure in the chamber. Due to the fact that vapor pressure of Te is higher than that of Bi and taking into account the fact that the pressure in the chamber is nearly 1 104 Torr, the deposited Te atoms tend to reevaporate from the substrate when the temperature is increased. Therefore for high temperature of substrate it is possible to take this behavior as a jump in the structural characteristic. Table 4 presents the properties of the best films grown under optimal conditions of CSVT method. The mobility value of 326 cm2/V s is one of the highest reported for P-type Bi2Te3 films. As the substrate temperature increases, mobility decreases. The
Table 4 Thickness, roughness, mobility, carrier concentration and resistivity of the Bi–Te films deposited on glass and CdTe substrates under the best CSVT growth conditions. Tsubstrate (1C)
d (mm)
Roughness (nm)
m (cm2/V s)
N/P (cm3)
r (O cm)
325 400 325a 400a
1.4 4.9 2.9 13.5
81 120 340 506
326 48 200 20
1.3 1018 2.3 1019 1.4 1018 2.3 1017
8 104 3 104 2 102 4 101
a
CdTe substrate.
369
lattice scattering mechanism could be causing this behavior. For the N-type samples low values or low mobility are obtained. For samples grown on CdTe substrates, degradation in the electrical parameters is observed probably as a consequence of stress due to the difference between lattice constants of CdTe and Bi–Te systems. Measuring the Hall effect for the sample grown at 3751 was not possible and in Tables 1 and 2 the lowest value of the Seebeck coefficient is observed . For the sample grown on CdTe at this substrate, a light tendency to P-type was observed. These results could be related to possible phase change at this temperature.
3.4. Influence of the post thermal treatment Post thermal treatments were performed in air in samples grown on glass at 400 1C for 30 min and on CdTe at 350 1C. The influence of thermal treatment in morphology of samples is shown in Fig. 5. From this figure, we can observe that the thermal treatment has different influence in samples grown on glass with respect to those grown on CdTe. The thermal treatment leads to an increase in the grain size average over the film surface in the first case, whereas for the samples grown on CdTe a tendency to form a more compact film is observed. The number of grains per unit area becomes smaller but the occupied surface increases. The post thermal treatment has a different influence on the electrical properties of the samples (see Table 5). For the samples grown at low substrate temperatures (300, 325 and 350 1C) the electrical conductivity decreases with the thermal treatment, while for the sample grown at higher substrate temperatures a light increment in the conductivity was observed, whether the samples are grown on glass or on CdTe substrates. Also, the value of the Seebeck coefficient for the samples deposited at low substrate temperature (up to 375 1C), showed a tendency to decrease while for high substrate temperature it was near constant, before and after the thermal treatment. Grain boundaries determine changes in carrier’s conduction mechanism. Usually the main effect of thermal annealing is to decrease the
Tsubs = 350 °C
Tsubs = 350 °C
Tsubs = 350 °C; TT 400
Tsubs = 350 °C; TT 400
Fig. 5. Grain size variation as a function of substrate temperature for films grown on (a) glass substrates and (b) CdTe substrates.
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Table 5 Values of the Seebeck coefficient and resistivity of Bi–Te layers deposited on glass substrates, before and after the thermal treatment. Tsubstrate (1C)
300 325 350 375 400 425
a (mV/K)
r (mO cm)
Before TT
After TT
Before TT
After TT
180 255 238 33 30 184
126 230 190 30 25 180
8.1 0.8 6.9 8.4 5.5 2.8
11.6 1.7 9.3 10.5 6.6 3.1
grain boundary barrier. In this case diminution of the grain boundary barrier increases the minority electron carrier injection through the conduction band, with a partial compensation of the majority carriers (holes). The net result is the decrease in the Seebeck voltage and therefore in the Seebeck coefficient. In N-type samples where phase transition occurs, the electron carrier injection has a negligible effect on compensation of majority carrier concentration, and the Seebeck coefficient does not change significantly, in agreement with our experimental results.
4. Conclusions In summary, we demonstrated the possibility of preparing Bi–Te films, depositing on glass and CdTe substrates, by using close space vapor transport and Bi2Te3 powders. The physical properties of the layers are dependent of the type of substrate. In general, layers with better electrical properties are obtained when the films are deposited on glass substrates. Both types of conductivity can be obtained choosing growth parameters. N-type samples can be obtained in samples deposited on glass as well as in those on polycrystalline CdTe. For substrate
temperature higher than 375 1C phases other than Bi2Te3 were assumed. Growth processes of CSVT–Bi2Te3 films on glass and polycrystalline CSVT-CdTe films have been established and then permit control of growth parameters in their future applications.
Acknowledgments This work was partially supported by CONACyT-Me´xico and SIP-IPN (Project 2006216) and Conacyt. O. Vigil-Gala´n and F. CruzGandarilla acknowledge support from COFAA-IPN. F. Roy acknowledges support from the government of Mexico through the Ministry of Foreign Affairs. References [1] S. Cho, Y. Kirn, A. DiVenere, G.K.L. Wong, J.B. Ketterson, J.R. Meyer, Appl. Phys. Lett. 75 (1999) 1401. [2] Helin Zou, D.M. Rowe, Gao Min, J. Cryst. Growth 222 (2001) 82. [3] H. Zou, D.M. Rowe, S.G.K. Williams, Thin Solid Films 408 (2002) 270. [4] H. Noro, K. Sato, H. Kagechika, J. Appl. Phys. 73 (1993) 1252. [5] J. Walachova´, R. Zeipl, J. Zelinka, V. Malina, M. Pavelka, M. Jelı´nek, V. Studnika, P. Losˇt’a´k, Appl. Phys. Lett. 87 (2005) 1902. [6] L. Scidone, S. Diliberto, N. Stein, C. Boulanger, J.M. Lecuire, Mater. Lett. 59 (2005) 746. [7] O. Vigil-Gala´n, F. Cruz-Gandarilla, J. Sastre´-Herna´ndez, F. Roy, E. Sa´nchezMeza, Close space vapor transport method for Bi2Te3 and Sb2Te3 films deposition: application to thermoelectric and solar cells devices, J. Cryst. Growth (2008). ˜ as-Moreno, J. Sastre´-Herna´ndez, F. [8] F. Cruz-Gandarilla, O. Vigil-Gala´n, G. Caban Roy, Structural and microstructural characterization of Bi2Te3 films deposited by close space vapor transport method using SEM and DRX techniques, Thin Solid Films (2008). [9] F. Cruz-Gandarilla, O.Vigil-Gala´n, J.E. Stabubbs, S. Peripolli, C.A. Achete, H.A. Caldero´n, Phase formation as a function of substrate temperature in thin films of Bi–Te grown by CSVT, accepted by Presentation at the 2008 Microscopy and Microanalysis Meeting, Symposium Understanding the Synthesis and properties of Nanostructures and Nanomaterials, Alburquerque, USA. [10] Y.A. Boikov, O.S. Gribanov, V.A. Danilov, A. Kutasov, Sov. Phys. Solid State 33 (1991) 1926–1929.