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Fabrication and characterization of thin films of PbI2 for medical imaging
Journal of Non-Crystalline Solids 338–340 (2004) 81–85 www.elsevier.com/locate/jnoncrysol
Fabrication and characterization of thin films of PbI2 for medical imaging J.F. Condeles a, T.M. Martins a, T.C. dos Santos a, C.A. Brunello a, M. Mulato a, J.M. Rosolen b,* a
Departamento de Fısica e Matematica, Faculdade de Filosofia Ci^encias e Letras de Ribeir~ao Preto, Universidade de S~ao Paulo, Av. Bandeirantes 3900, Ribeir~ao Preto, SP, Brazil b Departamento de Quımica, Faculdade de Filosofia Ci^encias e Letras de Ribeir~ao Preto, Universidade de S~ao Paulo, Av. Bandeirantes 3900, Ribeir~ao Preto, SP 14040-901, Brazil Available online 24 March 2004
Abstract Thin films of lead iodide (PbI2 ) are promising candidates for applications in X-ray digital medical imaging. In this work we present a new methodology used for the deposition of thin films of PbI2 : the spray pyrolysis technique. We investigate the structural properties of the films using X-ray diffraction and Raman scattering experiments. The electronic properties of the obtained films are investigated using dark current versus voltage at room temperature and also using dark current as a function of temperature for a fixed voltage. Post-deposition thermal annealing is also used for induced crystal growth, where the influence of a controlled N2 or ambient atmosphere are compared. 2004 Elsevier B.V. All rights reserved. PACS: 73.61.)r; 81.05.Hd; 81.10.Dm; 87.59.Hp
1. Introduction Researchers seek alternative methods that minimize the time for the deposition of thin films. Among these materials, special attention has been devoted to promising semiconductor candidates for medical applications, such as X-rays detectors for digital radiography [1]. Lead iodide (PbI2 ) has been among those as a good candidate for the fabrication of room temperature detectors. This material has an optical gap of 2.4 eV [2], and an hexagonal structure made of a plane of lead atoms between two planes of iodine atoms [2,3]. The material can be used in the direct detection configuration where the X-ray photons generate electronic charge inside the semiconductor [4,5]. Other authors have fabricated prototype detectors using this material [4]. Their experiments show high resolution and sensitivity for real time imaging, thus showing the material potentiality for medical applications in the future. Nevertheless, one of
the drawbacks of their methods is the long deposition time needed for the fabrication of the thin films. In this work we present a new experimental methodology used for the deposition of thin films of lead iodide (PbI2 ). The alternative growth method is called spray pyrolysis [6]. In addition, we also investigate the structural (X-ray diffraction and Raman scattering) and electronic (dark conductivity as a function of temperature) properties of the obtained films. The effect of postdeposition thermal annealing is also investigated: we compare the influence of controlled N2 and ambient atmospheres on the crystal growth and possible sample contamination. Note that besides the reduced deposition time obtained with this technique, another intrinsic advantage is the fact that it can be easily expanded for large area substrates as desired by the industrial fabrication line.
2. Experiments *
Corresponding author. Fax: +55-16 633 8151. E-mail address: rosolen@ffclrp.usp.br (J.M. Rosolen).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.026
Spray pyrolysis is the name of a technique used for the production of thin films from the vaporization of a
82
J.F. Condeles et al. / Journal of Non-Crystalline Solids 338–340 (2004) 81–85
solution [6,7]. In our case the films were obtained from the vaporization of a spray of water-dissolved PbI2 , which was directed toward a glass substrate sitting at varying temperatures (from 150 up to 270 C). In order to obtain the original PbI2 powder we used the chemical reaction described by Eq. (1): PbðNO3 Þ2 þ 2KI ! PbI2 þ 2KNO3 :
ð1Þ
3. Results and discussion 3.1. Structural properties X-ray experiments were performed using Cu Ka from a Siemens D5005 (40 kV, 40 radiation (1.5405 A) mA) diffraction system. The angular 2h scanning was performed from 5 up to 65 C using a step of 0.02 degrees. The results corresponding to a sample deposited at 225 C and without any thermal treatment are shown in Fig. 2. The main identified peaks were (0 0 1) at 12.8, (1 0 1) at 25.6, (0 0 3) at 38.7 and (2 0 2) at 52.5. These were identified using the Joint Committee on Powder Diffraction Standards, reference number 07-0235. Note that the samples were always microcrystalline: no complete amorphous sample was obtained in this work. Following the analysis of the highest intensity peak of Fig. 2 and using Eq. (2) below [9], an average grain size of about 26 nm was obtained for a sample deposited at 225 C. 0:9k : ð2Þ Dh cos h In Eq. (2), d is the grain size, k is the wavelength of the scattered radiation, h is the scattering angle and Dh is the full-width at half-maximum of the scattering peak. It should be noted that the diffraction results shown in Fig. 2 are somehow comparable to the ones obtained by other researches using other deposition techniques: the peak positions are the same with the peaks showing different relative intensities [4].
d¼
Intensity (arb. units)
After the creation of PbI2 , the powder was washed using room temperature DI water and ethylic alcohol. For the production of the film, the PbI2 powder was again dissolved in DI water at 100 C. At this temperature, a solubility of about 4.2 g/l is obtained, which is about 10 times higher than at 0 C [8]. After complete dissolution, the mixture was cooled down to room temperature and the excess material was filtered. As it can be seen below, the low solubility of lead iodide in water was not a limiting factor for the production of thin films. The schematic experimental set-up for the spray pyrolysis technique is presented in Fig. 1. The deposition process is as follows: (i) vacuum is made inside a glass chamber, (ii) the desired deposition temperature is controlled in a heated substrate holder (from 150 up to 270 C in the case of the samples of the present work), (iii) a nitrogen flow is established inside the chamber through one entrance of the spray-system. The chamber then sits at positive pressure, i.e. above atmospheric pressure, (iv) the water-dissolved PbI2 is also added to the spray-system through the second entrance. The resulting spray is then directed to the substrate (corning glass) that sits 16.5 cm below. The spray-system to substrate distance was kept constant for all the samples of the present work. The exit-gases of the deposition process are properly eliminated through an exhaustion hood. A total deposition time of about 2.5 h was adopted.
After the deposition of the films, we also thermally treated some samples for the study of post-deposition crystal growth through a solid fi solid phase process. The thermal treatment was performed for 3 h at 350 C under two different conditions: either atmospheric or a controlled N2 atmosphere was used during the process leading to extremely different results as discussed below.
(001)
3
X-ray diffraction
2
1 (101)
(003)
(202)
0 10 Fig. 1. Schematic of the spray pyrolysis deposition system.
20 30 40 50 60 Scattering angle 2θ (degrees)
Fig. 2. X-ray diffraction data for a PbI2 sample deposited at 225 C.
J.F. Condeles et al. / Journal of Non-Crystalline Solids 338–340 (2004) 81–85
Original Films
28 24 20 16 150 175 200 225 250 275 Deposition Temperature (˚C)
Fig. 3. Crystal size as a function of deposition temperature.
Intensity (arb. units)
The crystallite size varies as a function of deposition temperature as can be observed in Fig. 3. As expected, the higher the temperature the higher the crystal size. Crystal size values from 16 up to 32 nm can be obtained for deposition temperatures from 150 up to 270 C. As presented before, in addition to the effect of substrate temperature, we also investigated the structural changes due to thermal annealing after sample growth. We used original samples deposited at 225 C for this investigation. They were thermally treated at 350 C for 3 h under different atmospheres, and the resulting X-ray diffraction data after the treatment is presented in Fig. 4. The upper curve of Fig. 4 corresponds to a treatment under ambient atmosphere while the bottom curve of Fig. 4 corresponds to a treatment under controlled N2 atmosphere. We expected an induced crystal growth under thermal treatment, with an eventual variation of the intensities of the XRD peaks. Nevertheless, the resulting XRD data are totally different from each other. Only a main peak can be observed for the upper curve at 30.7. This peak is identified as (1 0 3) and the material is identified as Pb5 O4 I2 using the reference
(103)
600 450
Thermal Treatment at 350˚C, for 3h Ambient atmosphere x2
300 (001)
N2 atmosphere
150 (101)
0
15
30
(003)
(202)
45
60
Scattering angle 2θ (degrees) Fig. 4. X-ray diffraction data for samples deposited at 225 C, and thermally treated at 350 C for 3 h under N2 atmosphere (bottom) and under ambient atmosphere (top). Note that the upper curve was multiplied by 2 for better visualization.
number 31-0690 from the same Joint Committee on Powder Diffraction Standards. This fact shows the strong incorporation of oxygen inside the material leading to the formation of an alloy. Another important effect of the incorporation of oxygen inside the sample is not shown here: the color change of the thin film. The original films are orange, while the films treated under ambient atmosphere look yellowish. No change of color is observed for the films treated under controlled N2 atmosphere. Raman scattering experiments were performed using a HeNe laser (k ¼ 632 nm) at low power (below 0.3 mW). The top curve of Fig. 5 corresponds to the data of an original sample deposited at 225 C. No scattering peak was observed. The broad line is probably due to a strong luminescence effect that covers the Raman signal. On the other hand, the bottom curve of Fig. 5 corresponds to the data of an original sample deposited at 225 C, that was thermally treated at 350 C for 3 h under ambient atmosphere. Once again a strong influence of oxygen seems to be present, and some peaks can be observed. Note that this curve was multiplied by 5 for better visualization, and that the main peak is observed at about 140 cm1 . Previous studies [10] of PbI2 inserted in a silica matrix with varying porous sizes have identified three Raman peaks between 60 and 150 cm1 : (i) 75 cm1 for the TO2 mode; (ii) 96 cm1 for the LO2 mode and (iii) 116 cm1 for the LO1 mode. Besides the main peak at about 140 cm1 , our thermally treat sample also shows other satellite peaks around 240 and 300 cm1 . We believe that the variations come from two effects: (a) different network environment and (b) strong influence of the incorporated oxygen. Finally, our results suggest that oxygen incorporation might contribute either to an increase of the optical gap (change of color) or to the reduction of the radiative recombination processes that masks the Raman signal. Nevertheless, as
50 Intensity (arb. units)
Crystal Size (nm)
32
83
Original Sample
40 30
Thermaly treated under ambient atmosphere
20
at 350˚C for 3h
x5
10 0 100
200
300 400 500 600 Raman Shift (cm-1)
700
Fig. 5. Raman scattering data for an original sample deposited at 225 C (top), and another sample deposited at the same temperature but thermally treated at 350 C for 3 h under ambient atmosphere (bottom). The bottom curve was multiplied by 5 for better visualization.
84
J.F. Condeles et al. / Journal of Non-Crystalline Solids 338–340 (2004) 81–85
will be shown below, this might also lead to a worsening of the transport properties of the material. 3.2. Electrical transport properties The coplanar configuration was used for the electrical transport characterization of the samples. For this aim, the PbI2 samples were deposited over corning glass substrates previously coated with evaporated Pd lines. Palladium was chosen as metallic contact because it makes ohmic contact with PbI2 [11]. The Pd lines were 1 mm thick and were spaced by 2 mm. The PbI2 film covered about 5 mm of the lines. The dark current of the samples were studied as a function of temperature, for an external bias voltage of 50 V. Fig. 6(a) shows the results corresponding to an original sample deposited at 225 C. Note that we could not measure precisely the thickness of the sample yet (neither using a Step-scan profilometer, AFM, nor optical transmission) what prevents us from estimating the current density. An activated behavior is observed, with corresponding activation energy of 0.43 eV. Other authors have reported an activation energy of about 0.30 eV for the same temperature range [2]. We do not have a definite explanation for the difference between these values, but we believe that the electronic quality of the samples deposited by spray pyrolysis can be further improved with some basic changes: (i) a better initial vacuum can be obtained before the beginning of the deposition, (ii) nitrogen carrying gas with higher purity could be used; (iii) starting PbI2 powder with higher purity could also be used; etc. Fig. 6(b) shows the data for the dark current as a function of bias voltage for samples sitting at room temperature. The bottom curve (open squares) corresponds to the same as in Fig. 6(a), while the top curve
Thermally treated under ambient atmosphere 100 at 350˚C for 3h
10 10 Original
1 1 3.0 3.2 3.4 3.6 3.8 1000/T ( K -1 )
In this work we have studied the deposition of PbI2 thin film using the spray pyrolysis technique. We show that this technique might be promising for future industrial applications for large area fabrication of digital medical imaging systems. We investigated the structural and electronic properties of the obtained films as a function of deposition temperature from 150 up to 275 C. In this temperature range a corresponding crystallite size from 16 up to 32 nm was obtained. The original sample deposited at 225 C shows an activated transport property with activation energy of about 0.43 eV. Post-deposition thermal annealing under ambient atmosphere at 350 C for 3 h leads to a strong oxygen incorporation, with the material changing to Pb5 O4 I2 . The strong oxygen influence is not only observed in the X-ray diffraction data, but also in the Raman scattering and electronic transport experiments. In the first case, oxygen seems to eliminate a possible luminescence signal, while in the second an increase of at least one order of magnitude in the dark current is observed. The same effects are not observed for samples thermally treated under controlled N2 atmosphere. Finally, we believe that the electronic quality of the samples deposited by spray pyrolysis can be further improved with some basic changes: (i) a better initial vacuum can be obtained before the beginning of the deposition, (ii) nitrogen carrying gas with higher purity could be used; (iii) starting PbI2 powder with higher purity could also be used; etc.
(A)
(b)
4. Conclusion
-12
(a)
Dark Current x 10
Dark Current
x 10 -11 (A)
1000 100
(dark squares) corresponds to the same sample after thermal treatment under ambient atmosphere at 350 C for 3 h. The current increases by at least one order of magnitude after thermal treatment, an effect once again related to the strong oxygen incorporation inside the material.
Acknowledgements We thank A.R. Zanatta for the Raman Scattering experiments and F.C. Marques for the substrates with metallic Pd contacts. This work has been supported by FAPESP (proc. 01/08221-9) and CNPq Brazilian agencies.
20 40 60 80 100 Bias (V)
Fig. 6. (a) Dark current as a function of temperature for an original sample deposited at 225 C, with an external bias of 50 V. (b) Dark current as a function of bias voltage for samples at room temperature: bottom curve – same as in (a); top curve – same sample after thermal treatment under ambient atmosphere at 350 C for 3 h.
References [1] L. Fornaro, E. Saucedo, L. Mussio, L. Yerman, X. Ma, A. Burger, Nucl. Instrum. Methods Phys. Res. A 458 (2001) 406. [2] T. Unagami, J. Electrochem. Soc. 146 (1999) 3110.
J.F. Condeles et al. / Journal of Non-Crystalline Solids 338–340 (2004) 81–85 [3] V. Deich, M. Roth, Nucl. Instrum. Methods Phys. Res. A 380 (1996) 169. [4] K.S. Shah, R.A. Street, Y. Dmitriyev, P. Bennett, L. Cirignano, M. Klugerman, M.R. Squillante, G. Entine, Nucl. Instrum. Methods Phys. Res. A 458 (2001) 140. [5] R.A. Street, S.E. Ready, K. Van Schuylenbergh, J. Ho, J.B. Boyce, P. Nylen, K. Shah, L. Melekhov, H. Hermon, J. Appl. Phys. 91 (2002) 3345. [6] J. Hao, S.A. Studenikin, M. Cocivera, J. Appl. Phys. 90 (2001) 2287.
85
[7] A. Bouzidi, N. Benramdane, A. Nakrela, C. Mathieu, B. Khelifa, R. Desfeux, A. Da Costa, Mater. Sci. Eng. B 95 (2002) 141. [8] J.P. Ponpon, M. Amann, Thin Solid Films 394 (2001) 277. [9] B.D. Cullity, Elements of X-ray Diffraction, 2nd Ed., AddisonWesley, USA, 1978. [10] R. Mu, Y.S. Tung, A. Ueda, D.O. Henderson, J. Phys. Chem. 100 (1996) 19927. [11] K.S. Shah, P. Bennett, M. Klugerman, L. Moy, L. Cirignano, Y. Dmitriyev, M.R. Squillante, F. Olschner, W.W. Moses, IEEE Trans. Nucl. Sci. 44 (1997) 448.
Fabrication and characterization of thin films of PbI2 for medical imaging J.F. Condeles a, T.M. Martins a, T.C. dos Santos a, C.A. Brunello a, M. Mulato a, J.M. Rosolen b,* a
Departamento de Fısica e Matematica, Faculdade de Filosofia Ci^encias e Letras de Ribeir~ao Preto, Universidade de S~ao Paulo, Av. Bandeirantes 3900, Ribeir~ao Preto, SP, Brazil b Departamento de Quımica, Faculdade de Filosofia Ci^encias e Letras de Ribeir~ao Preto, Universidade de S~ao Paulo, Av. Bandeirantes 3900, Ribeir~ao Preto, SP 14040-901, Brazil Available online 24 March 2004
Abstract Thin films of lead iodide (PbI2 ) are promising candidates for applications in X-ray digital medical imaging. In this work we present a new methodology used for the deposition of thin films of PbI2 : the spray pyrolysis technique. We investigate the structural properties of the films using X-ray diffraction and Raman scattering experiments. The electronic properties of the obtained films are investigated using dark current versus voltage at room temperature and also using dark current as a function of temperature for a fixed voltage. Post-deposition thermal annealing is also used for induced crystal growth, where the influence of a controlled N2 or ambient atmosphere are compared. 2004 Elsevier B.V. All rights reserved. PACS: 73.61.)r; 81.05.Hd; 81.10.Dm; 87.59.Hp
1. Introduction Researchers seek alternative methods that minimize the time for the deposition of thin films. Among these materials, special attention has been devoted to promising semiconductor candidates for medical applications, such as X-rays detectors for digital radiography [1]. Lead iodide (PbI2 ) has been among those as a good candidate for the fabrication of room temperature detectors. This material has an optical gap of 2.4 eV [2], and an hexagonal structure made of a plane of lead atoms between two planes of iodine atoms [2,3]. The material can be used in the direct detection configuration where the X-ray photons generate electronic charge inside the semiconductor [4,5]. Other authors have fabricated prototype detectors using this material [4]. Their experiments show high resolution and sensitivity for real time imaging, thus showing the material potentiality for medical applications in the future. Nevertheless, one of
the drawbacks of their methods is the long deposition time needed for the fabrication of the thin films. In this work we present a new experimental methodology used for the deposition of thin films of lead iodide (PbI2 ). The alternative growth method is called spray pyrolysis [6]. In addition, we also investigate the structural (X-ray diffraction and Raman scattering) and electronic (dark conductivity as a function of temperature) properties of the obtained films. The effect of postdeposition thermal annealing is also investigated: we compare the influence of controlled N2 and ambient atmospheres on the crystal growth and possible sample contamination. Note that besides the reduced deposition time obtained with this technique, another intrinsic advantage is the fact that it can be easily expanded for large area substrates as desired by the industrial fabrication line.
2. Experiments *
Corresponding author. Fax: +55-16 633 8151. E-mail address: rosolen@ffclrp.usp.br (J.M. Rosolen).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.026
Spray pyrolysis is the name of a technique used for the production of thin films from the vaporization of a
82
J.F. Condeles et al. / Journal of Non-Crystalline Solids 338–340 (2004) 81–85
solution [6,7]. In our case the films were obtained from the vaporization of a spray of water-dissolved PbI2 , which was directed toward a glass substrate sitting at varying temperatures (from 150 up to 270 C). In order to obtain the original PbI2 powder we used the chemical reaction described by Eq. (1): PbðNO3 Þ2 þ 2KI ! PbI2 þ 2KNO3 :
ð1Þ
3. Results and discussion 3.1. Structural properties X-ray experiments were performed using Cu Ka from a Siemens D5005 (40 kV, 40 radiation (1.5405 A) mA) diffraction system. The angular 2h scanning was performed from 5 up to 65 C using a step of 0.02 degrees. The results corresponding to a sample deposited at 225 C and without any thermal treatment are shown in Fig. 2. The main identified peaks were (0 0 1) at 12.8, (1 0 1) at 25.6, (0 0 3) at 38.7 and (2 0 2) at 52.5. These were identified using the Joint Committee on Powder Diffraction Standards, reference number 07-0235. Note that the samples were always microcrystalline: no complete amorphous sample was obtained in this work. Following the analysis of the highest intensity peak of Fig. 2 and using Eq. (2) below [9], an average grain size of about 26 nm was obtained for a sample deposited at 225 C. 0:9k : ð2Þ Dh cos h In Eq. (2), d is the grain size, k is the wavelength of the scattered radiation, h is the scattering angle and Dh is the full-width at half-maximum of the scattering peak. It should be noted that the diffraction results shown in Fig. 2 are somehow comparable to the ones obtained by other researches using other deposition techniques: the peak positions are the same with the peaks showing different relative intensities [4].
d¼
Intensity (arb. units)
After the creation of PbI2 , the powder was washed using room temperature DI water and ethylic alcohol. For the production of the film, the PbI2 powder was again dissolved in DI water at 100 C. At this temperature, a solubility of about 4.2 g/l is obtained, which is about 10 times higher than at 0 C [8]. After complete dissolution, the mixture was cooled down to room temperature and the excess material was filtered. As it can be seen below, the low solubility of lead iodide in water was not a limiting factor for the production of thin films. The schematic experimental set-up for the spray pyrolysis technique is presented in Fig. 1. The deposition process is as follows: (i) vacuum is made inside a glass chamber, (ii) the desired deposition temperature is controlled in a heated substrate holder (from 150 up to 270 C in the case of the samples of the present work), (iii) a nitrogen flow is established inside the chamber through one entrance of the spray-system. The chamber then sits at positive pressure, i.e. above atmospheric pressure, (iv) the water-dissolved PbI2 is also added to the spray-system through the second entrance. The resulting spray is then directed to the substrate (corning glass) that sits 16.5 cm below. The spray-system to substrate distance was kept constant for all the samples of the present work. The exit-gases of the deposition process are properly eliminated through an exhaustion hood. A total deposition time of about 2.5 h was adopted.
After the deposition of the films, we also thermally treated some samples for the study of post-deposition crystal growth through a solid fi solid phase process. The thermal treatment was performed for 3 h at 350 C under two different conditions: either atmospheric or a controlled N2 atmosphere was used during the process leading to extremely different results as discussed below.
(001)
3
X-ray diffraction
2
1 (101)
(003)
(202)
0 10 Fig. 1. Schematic of the spray pyrolysis deposition system.
20 30 40 50 60 Scattering angle 2θ (degrees)
Fig. 2. X-ray diffraction data for a PbI2 sample deposited at 225 C.
J.F. Condeles et al. / Journal of Non-Crystalline Solids 338–340 (2004) 81–85
Original Films
28 24 20 16 150 175 200 225 250 275 Deposition Temperature (˚C)
Fig. 3. Crystal size as a function of deposition temperature.
Intensity (arb. units)
The crystallite size varies as a function of deposition temperature as can be observed in Fig. 3. As expected, the higher the temperature the higher the crystal size. Crystal size values from 16 up to 32 nm can be obtained for deposition temperatures from 150 up to 270 C. As presented before, in addition to the effect of substrate temperature, we also investigated the structural changes due to thermal annealing after sample growth. We used original samples deposited at 225 C for this investigation. They were thermally treated at 350 C for 3 h under different atmospheres, and the resulting X-ray diffraction data after the treatment is presented in Fig. 4. The upper curve of Fig. 4 corresponds to a treatment under ambient atmosphere while the bottom curve of Fig. 4 corresponds to a treatment under controlled N2 atmosphere. We expected an induced crystal growth under thermal treatment, with an eventual variation of the intensities of the XRD peaks. Nevertheless, the resulting XRD data are totally different from each other. Only a main peak can be observed for the upper curve at 30.7. This peak is identified as (1 0 3) and the material is identified as Pb5 O4 I2 using the reference
(103)
600 450
Thermal Treatment at 350˚C, for 3h Ambient atmosphere x2
300 (001)
N2 atmosphere
150 (101)
0
15
30
(003)
(202)
45
60
Scattering angle 2θ (degrees) Fig. 4. X-ray diffraction data for samples deposited at 225 C, and thermally treated at 350 C for 3 h under N2 atmosphere (bottom) and under ambient atmosphere (top). Note that the upper curve was multiplied by 2 for better visualization.
number 31-0690 from the same Joint Committee on Powder Diffraction Standards. This fact shows the strong incorporation of oxygen inside the material leading to the formation of an alloy. Another important effect of the incorporation of oxygen inside the sample is not shown here: the color change of the thin film. The original films are orange, while the films treated under ambient atmosphere look yellowish. No change of color is observed for the films treated under controlled N2 atmosphere. Raman scattering experiments were performed using a HeNe laser (k ¼ 632 nm) at low power (below 0.3 mW). The top curve of Fig. 5 corresponds to the data of an original sample deposited at 225 C. No scattering peak was observed. The broad line is probably due to a strong luminescence effect that covers the Raman signal. On the other hand, the bottom curve of Fig. 5 corresponds to the data of an original sample deposited at 225 C, that was thermally treated at 350 C for 3 h under ambient atmosphere. Once again a strong influence of oxygen seems to be present, and some peaks can be observed. Note that this curve was multiplied by 5 for better visualization, and that the main peak is observed at about 140 cm1 . Previous studies [10] of PbI2 inserted in a silica matrix with varying porous sizes have identified three Raman peaks between 60 and 150 cm1 : (i) 75 cm1 for the TO2 mode; (ii) 96 cm1 for the LO2 mode and (iii) 116 cm1 for the LO1 mode. Besides the main peak at about 140 cm1 , our thermally treat sample also shows other satellite peaks around 240 and 300 cm1 . We believe that the variations come from two effects: (a) different network environment and (b) strong influence of the incorporated oxygen. Finally, our results suggest that oxygen incorporation might contribute either to an increase of the optical gap (change of color) or to the reduction of the radiative recombination processes that masks the Raman signal. Nevertheless, as
50 Intensity (arb. units)
Crystal Size (nm)
32
83
Original Sample
40 30
Thermaly treated under ambient atmosphere
20
at 350˚C for 3h
x5
10 0 100
200
300 400 500 600 Raman Shift (cm-1)
700
Fig. 5. Raman scattering data for an original sample deposited at 225 C (top), and another sample deposited at the same temperature but thermally treated at 350 C for 3 h under ambient atmosphere (bottom). The bottom curve was multiplied by 5 for better visualization.
84
J.F. Condeles et al. / Journal of Non-Crystalline Solids 338–340 (2004) 81–85
will be shown below, this might also lead to a worsening of the transport properties of the material. 3.2. Electrical transport properties The coplanar configuration was used for the electrical transport characterization of the samples. For this aim, the PbI2 samples were deposited over corning glass substrates previously coated with evaporated Pd lines. Palladium was chosen as metallic contact because it makes ohmic contact with PbI2 [11]. The Pd lines were 1 mm thick and were spaced by 2 mm. The PbI2 film covered about 5 mm of the lines. The dark current of the samples were studied as a function of temperature, for an external bias voltage of 50 V. Fig. 6(a) shows the results corresponding to an original sample deposited at 225 C. Note that we could not measure precisely the thickness of the sample yet (neither using a Step-scan profilometer, AFM, nor optical transmission) what prevents us from estimating the current density. An activated behavior is observed, with corresponding activation energy of 0.43 eV. Other authors have reported an activation energy of about 0.30 eV for the same temperature range [2]. We do not have a definite explanation for the difference between these values, but we believe that the electronic quality of the samples deposited by spray pyrolysis can be further improved with some basic changes: (i) a better initial vacuum can be obtained before the beginning of the deposition, (ii) nitrogen carrying gas with higher purity could be used; (iii) starting PbI2 powder with higher purity could also be used; etc. Fig. 6(b) shows the data for the dark current as a function of bias voltage for samples sitting at room temperature. The bottom curve (open squares) corresponds to the same as in Fig. 6(a), while the top curve
Thermally treated under ambient atmosphere 100 at 350˚C for 3h
10 10 Original
1 1 3.0 3.2 3.4 3.6 3.8 1000/T ( K -1 )
In this work we have studied the deposition of PbI2 thin film using the spray pyrolysis technique. We show that this technique might be promising for future industrial applications for large area fabrication of digital medical imaging systems. We investigated the structural and electronic properties of the obtained films as a function of deposition temperature from 150 up to 275 C. In this temperature range a corresponding crystallite size from 16 up to 32 nm was obtained. The original sample deposited at 225 C shows an activated transport property with activation energy of about 0.43 eV. Post-deposition thermal annealing under ambient atmosphere at 350 C for 3 h leads to a strong oxygen incorporation, with the material changing to Pb5 O4 I2 . The strong oxygen influence is not only observed in the X-ray diffraction data, but also in the Raman scattering and electronic transport experiments. In the first case, oxygen seems to eliminate a possible luminescence signal, while in the second an increase of at least one order of magnitude in the dark current is observed. The same effects are not observed for samples thermally treated under controlled N2 atmosphere. Finally, we believe that the electronic quality of the samples deposited by spray pyrolysis can be further improved with some basic changes: (i) a better initial vacuum can be obtained before the beginning of the deposition, (ii) nitrogen carrying gas with higher purity could be used; (iii) starting PbI2 powder with higher purity could also be used; etc.
(A)
(b)
4. Conclusion
-12
(a)
Dark Current x 10
Dark Current
x 10 -11 (A)
1000 100
(dark squares) corresponds to the same sample after thermal treatment under ambient atmosphere at 350 C for 3 h. The current increases by at least one order of magnitude after thermal treatment, an effect once again related to the strong oxygen incorporation inside the material.
Acknowledgements We thank A.R. Zanatta for the Raman Scattering experiments and F.C. Marques for the substrates with metallic Pd contacts. This work has been supported by FAPESP (proc. 01/08221-9) and CNPq Brazilian agencies.
20 40 60 80 100 Bias (V)
Fig. 6. (a) Dark current as a function of temperature for an original sample deposited at 225 C, with an external bias of 50 V. (b) Dark current as a function of bias voltage for samples at room temperature: bottom curve – same as in (a); top curve – same sample after thermal treatment under ambient atmosphere at 350 C for 3 h.
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