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Properties of a-SbxSe1−x photoconductors
Journal of Non-Crystalline Solids 299–302 (2002) 998–1001 www.elsevier.com/locate/jnoncrysol
Properties of a-Sbx Se1x photoconductors D. Tonchev, B. Fogal, G. Belev, R.E. Johanson *, S.O. Kasap Department of Electrical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9
Abstract We have examined the thermal stability, and carrier drift mobilities and lifetimes (and hence electron and hole ranges) of a-Sbx Se1x photoconductors. We have found that the most thermally stable a-Sbx Se1x alloys can be obtained for x between 1 and 1.5 at.% which therefore limits the amount of Sb that can be added to improve the X-ray absorption and hence the X-ray photoconductivity of these alloys. On the other hand, we have found that the photoconducting films fabricated from a-Sbx Se1x alloys exhibit better electron and hole ranges when compared with the starting pure a-Se material. The X-ray sensitivity of a-Se:1 at.% Sb is greater than that of the pure a-Se. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 73.61.Jc; 73.50.Dn; 65.60.+a
1. Introduction Amorphous Se alloyed with 0.2–0.5% As and doped with small amount of Cl, called stabilized a-Se, is now a well-established X-ray photoconductor [1,2] used in mammographic flat panel direct-conversion X-ray sensors that have produced excellent images. There is, at present, a close interest in exploring other potential a-Se alloys; antimony is a group III element that has a high Z and acts in principle as a cross-linking agent [3] between Se chains thereby providing thermal stability. We have carried out interrupted field timeof-flight (IFTOF) experiments to measure the electron and hole drift mobilities and lifetimes in aSbx Se1x films to study the variation of the mo-
bility lifetime product (ls) with the alloy composition. We have also examined the relative X-ray sensitivity of a-Se alloyed with 1 at.% Sb. The application of Sb containing glasses has been generally limited because of their strong tendency for crystallization, especially when the Sb content is substantial. A proper characterization of thermal transformations of these glasses under a well-defined thermal history is therefore equally important for a fundamental understanding of their properties and their applications. We have therefore also carried out differential scanning calorimetric measurements on a-Sbx Se1x films to establish the useful range of compositions that can be used.
2. Experimental procedure *
Corresponding author. Tel.: +1-306 966 5456; fax: +1-306 966 5407. E-mail address: [email protected] (R.E. Johanson).
The glasses of the Sbx Se1x system, where x varies from 0 to 0.15 (or 15 at.% Sb), were prepared
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 0 9 2 6 - 2
D. Tonchev et al. / Journal of Non-Crystalline Solids 299–302 (2002) 998–1001
from the pure elements by quenching from the liquid state as described previously [3]. Photoconductive film samples from quenched pure and alloy materials were prepared by conventional vacuum deposition on precleaned, etched and oxidized Al substrates [4]. The substrate temperature (70 °C) was maintained above the glass transition temperature Tg but below the crystallization temperature Tc to obtain good quality photoconductor films [5]. An argon plasma sputtering system was used to place 1 cm2 gold contacts on the top of film samples. The homogeneity and correct composition of Sb–Se alloys and films were determined by means of Kevex superdry X-ray detector connected to a scanning electron microscope, JEOL JSM-T200. The concentration of Sb doping was estimated by means of qualitative and quantitative X-ray microanalysis software Kevex SigmaTM SDP and Quasar. Low concentration Sb samples (less than 1 at.%) have been analyzed by means of a wavelength dispersive X-ray system. The differential scanning calorimetry experiments were performed as described previously [3] on a commercial Temperature Modulated DSC 2910 system from TA Instrument Inc. All samples used in thermal characterizations were given the same thermal history to avoid results and conclusions that are based on thermal history effects. We used conventional and IFTOF transient photoconductivity measurements [6] to evaluate the transport properties of electrons and holes, that is, the drift mobilities le and lh and the lifetimes se and sh . The uncertainty in the measured values were estimated through the standard deviation of repeated measurements. We compared the relative X-ray sensitivity of a-Sbx Se1x films by measuring the X-ray photocurrent generated by exposure to a predefined constant X-ray spectrum. Only two samples, pure a-Se representing the starting material and a-Se:1 at.% Sb were examined since 1 at.% Sb was found to have the longest carrier ranges and also exhibited excellent thermal stability. Both samples had the same thickness (35–36 lm) to enable a meaningful comparison. A dental X-ray source (Siemens Heliodent 57 kVp, 7 mA, 2 mm Al filtration) was adjusted to provide 180 mR exposure over an exposure time of 2.5 s.
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3. Results In comparing the crystallization behavior of glasses with limited stability it is useful to determine the crystallization and glass transition temperature difference rather than the absolute values. A wider difference is generally associated with higher kinetic resistance to crystallization [7]. The plot of Tc Tg vs. Sb content in Fig. 1 shows that the thermal stability is maximum when the Sb content reaches 1 at.%. Although Tc Tg decreases after 1 at.% Sb, it is still higher than that for pure a-Se. Error estimates can be found in Ref. [3]. The results of the conventional and IFTOF measurements are shown in Figs. 2–4. Sb addition to a-Se affects the hole mobility slightly whereas there is a substantial decrease in the electron mobility with the Sb content. There is an initial increase in both the electron and hole lifetimes with small amounts of Sb addition as shown in Fig. 3. Both lifetimes reach a maximum at about 1–1.5 at.% Sb beyond which they both decrease with Sb addition. The initial enhancement in the carrier lifetimes is more than the drop in the drift mobilities with the Sb content, which leads to enhanced electron and hole ranges (mobility lifetime products) as shown in Fig. 4. We performed X-ray photoconductivity experiments to examine the relative X-ray sensitivity of a-Se:0.1 at.% Sb films with respect to the starting pure a-Se film (the control material). The results
Fig. 1. Dependence of Tc Tg on the composition of Se–Sb glass alloys from the temperature modulated DSC total heat flow heating scans at an underlying heating rate of 1 °C/min. The error in the temperatures is smaller than the symbol size, about 1°.
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D. Tonchev et al. / Journal of Non-Crystalline Solids 299–302 (2002) 998–1001
Fig. 2. Dependence of the mobility of electrons (le ) and holes (lh ) on the composition of Sb–Se films from TOF experiments (lines are spline fits). The error in the hole mobility is estimated to be 0:005 cm2 /V s and in the electron mobility 0:2 103 cm2 /V s.
Fig. 4. Dependence of the range ls of electrons and holes on the composition of Sb–Se films from TOF/IFTOF experiments (lines are spline fits). The error in both hole and electron ls is estimated to be 0.05 cm2 /V, about the same as the symbol size.
are summarized in Table 1 in terms of the X-ray photocurrent as described in Section 2. There is a distinct improvement in the X-ray sensitivity with Sb alloying.
4. Discussion
Fig. 3. Dependence of the lifetime of electrons (se ) and holes (sh ) on the composition of Sb–Se films from IFTOF experiments (lines are spline fits). The error in hole lifetime is estimated to be 0.5 ls and in electron lifetime 10 ls.
What is remarkable in the results shown in Figs. 1 and 4 is the fact that both curves peak at about the same composition, 1 at.% Sb, at the composition around the maximum thermal stability. Thus, Se:1 at.% Sb alloy is a potential candidate as a suitable X-ray photoconductor in which carriers of both polarity can be transported without any deep trapping; lsE L, where L is the photoconductor thickness. Other recent measurements of the charge transport properties of a-Sbx Se1x [8–11] indicate that both electron and hole trapping increases with the Sb content which contrasts with
Table 1 1 cm2 electrode, E is the field and the polarity refers to the radiation receiving electrode (Au top electrode with respect to Al substrate) Photoconductor
Photocurrent
E ¼ þ8 V=lm
E ¼ 8 V=lm
E ¼ 20 V=lm
a-Se a-Se:1 at.% Sb
Iph ðnA=cm2 Þ Iph ðnA=cm2 Þ
145 190
150 170
350 410
Sample thickness L is the same, 35 lm. 57 kVp exposure at 180 mR.
D. Tonchev et al. / Journal of Non-Crystalline Solids 299–302 (2002) 998–1001
the results in Figs. 3 and 4. Previous lower ls may be in part due to the room temperature substrate temperature that was used for the deposition of the films or differences in the starting materials. The observed effect of Sb on the electron transport is similar to the effect of As addition [12]; not unexpected as both are group III elements. Mikla et al. [11] observe that the electron mobility activation energy for pure a-Se and a-Se:1 at.% Sb are the same, which means that Sb addition must increase the total concentration of the shallow electron traps that control the drift mobility. Within the shallow controlled transport model, the measured carrier range ls is given by l ls ¼ l0 s0 ¼ 0 ; Ct Nt where l0 is the microscopic mobility (in the transport band), s0 is the intrinsic deep trapping time in the absence of shallow traps, Ct is the capture coefficient, and Nt is the concentration of deep traps. Thus, the improvement in the ls product with small additions of Sb as in Fig. 4 implies that the deep trap population, Nt , is reduced for both holes and electrons upto 1 at.% Sb; we make the reasonable assumption that there is no substantial change in the microscopic mobility and Ct . The exact explanation for the improvement in the X-ray sensitivity in Table 1 must include not only the charge transport parameters (ls) but also the ionization energy and the absorption coefficient [13]; the latter measurements are currently in progress.
5. Conclusions We have prepared a range of a-Sbx Se1x alloys and photoconductor type films from these alloys by using conventional vacuum deposition techniques, with x up to 0.15. Based on differential scanning calorimetry experiments, the most stable films have been obtained for compositions containing about 1–1.5 at.% Sb.
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The electron drift mobility decreases sharply with Sb addition whereas the hole drift mobility is only slightly affected. There is a distinct enhancement in the electron and hole lifetimes, and hence carrier ranges, which attain their maximum values when the film composition has about 1 at.% Sb. The X-ray sensitivity of Se:1 at.% Sb film is greater than that of pure a-Se film made from the starting pure a-Se material. a-Se:1 at.% Sb alloy is a potential candidate as a suitable X-ray photoconductor in which carriers of both polarity need to be transported without any deep trapping. Acknowledgements This project was made possible by financial support from NSERC (Strategic Grant) and Anrad (Montreal, Quebec, Canada). We thank Stephen O’Leary, University of Regina, for his interest in the project. References [1] J.A. Rowlands, S.O. Kasap, Phys. Today 50 (11) (1997) 24. [2] J.A. Rowlands, J. Yorkston, in: J. Beutel, H.L. Kundel, R.L. Van Metter (Eds.), Handbook of Medical Imaging, vol. 1, SPIE, Washington, 2000 (Chapter 4 and references therein). [3] D. Tonchev, S.O. Kasap, J. Non-Cryst. Solids 248 (1999) 28. [4] S.O. Kasap, in: A. Diamond (Ed.), Handbook of Imaging Materials, 2nd Ed., Marcel Dekker, New York, 2002 (Chapter 9). [5] S.O. Kasap, J.A. Rowlands, J. Mater. Sci. Mater. Electron. 11 (2000) 179, and references therein. [6] S.O. Kasap, B. Polischuk, D. Dodds, Rev. Sci. Instrum. 61 (1990) 2081. [7] S. Mahadevan, A. Giridhar, A.K. Singh, J. Non-Cryst. Solids 88 (1986) 11. [8] V.I. Mikla, I.P. Mikhalko, Y.Y. Nagy, J. Phys.: Condens. Matter 6 (1994) 8269. [9] V.I. Mikla, V.M. Rubish, Phys. Stat. Sol. B 182 (1994) 325. [10] V.I. Mikla, Y.Y. Nagy, V.V. Mikla, A.V. Mateleshko, Mater. Sci. Eng. B 64 (1999) 1. [11] V.I. Mikla, A.V. Mateleshko, V.V. Mikla, Y.Y. Nagy, J. Non-Cryst. Solids 246 (1999) 46. [12] S.O. Kasap, C. Juhasz, J. Phys. D 18 (1985) 703. [13] S.O. Kasap, J. Phys. D 33 (2000) 2853.
Properties of a-Sbx Se1x photoconductors D. Tonchev, B. Fogal, G. Belev, R.E. Johanson *, S.O. Kasap Department of Electrical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9
Abstract We have examined the thermal stability, and carrier drift mobilities and lifetimes (and hence electron and hole ranges) of a-Sbx Se1x photoconductors. We have found that the most thermally stable a-Sbx Se1x alloys can be obtained for x between 1 and 1.5 at.% which therefore limits the amount of Sb that can be added to improve the X-ray absorption and hence the X-ray photoconductivity of these alloys. On the other hand, we have found that the photoconducting films fabricated from a-Sbx Se1x alloys exhibit better electron and hole ranges when compared with the starting pure a-Se material. The X-ray sensitivity of a-Se:1 at.% Sb is greater than that of the pure a-Se. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 73.61.Jc; 73.50.Dn; 65.60.+a
1. Introduction Amorphous Se alloyed with 0.2–0.5% As and doped with small amount of Cl, called stabilized a-Se, is now a well-established X-ray photoconductor [1,2] used in mammographic flat panel direct-conversion X-ray sensors that have produced excellent images. There is, at present, a close interest in exploring other potential a-Se alloys; antimony is a group III element that has a high Z and acts in principle as a cross-linking agent [3] between Se chains thereby providing thermal stability. We have carried out interrupted field timeof-flight (IFTOF) experiments to measure the electron and hole drift mobilities and lifetimes in aSbx Se1x films to study the variation of the mo-
bility lifetime product (ls) with the alloy composition. We have also examined the relative X-ray sensitivity of a-Se alloyed with 1 at.% Sb. The application of Sb containing glasses has been generally limited because of their strong tendency for crystallization, especially when the Sb content is substantial. A proper characterization of thermal transformations of these glasses under a well-defined thermal history is therefore equally important for a fundamental understanding of their properties and their applications. We have therefore also carried out differential scanning calorimetric measurements on a-Sbx Se1x films to establish the useful range of compositions that can be used.
2. Experimental procedure *
Corresponding author. Tel.: +1-306 966 5456; fax: +1-306 966 5407. E-mail address: [email protected] (R.E. Johanson).
The glasses of the Sbx Se1x system, where x varies from 0 to 0.15 (or 15 at.% Sb), were prepared
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 0 9 2 6 - 2
D. Tonchev et al. / Journal of Non-Crystalline Solids 299–302 (2002) 998–1001
from the pure elements by quenching from the liquid state as described previously [3]. Photoconductive film samples from quenched pure and alloy materials were prepared by conventional vacuum deposition on precleaned, etched and oxidized Al substrates [4]. The substrate temperature (70 °C) was maintained above the glass transition temperature Tg but below the crystallization temperature Tc to obtain good quality photoconductor films [5]. An argon plasma sputtering system was used to place 1 cm2 gold contacts on the top of film samples. The homogeneity and correct composition of Sb–Se alloys and films were determined by means of Kevex superdry X-ray detector connected to a scanning electron microscope, JEOL JSM-T200. The concentration of Sb doping was estimated by means of qualitative and quantitative X-ray microanalysis software Kevex SigmaTM SDP and Quasar. Low concentration Sb samples (less than 1 at.%) have been analyzed by means of a wavelength dispersive X-ray system. The differential scanning calorimetry experiments were performed as described previously [3] on a commercial Temperature Modulated DSC 2910 system from TA Instrument Inc. All samples used in thermal characterizations were given the same thermal history to avoid results and conclusions that are based on thermal history effects. We used conventional and IFTOF transient photoconductivity measurements [6] to evaluate the transport properties of electrons and holes, that is, the drift mobilities le and lh and the lifetimes se and sh . The uncertainty in the measured values were estimated through the standard deviation of repeated measurements. We compared the relative X-ray sensitivity of a-Sbx Se1x films by measuring the X-ray photocurrent generated by exposure to a predefined constant X-ray spectrum. Only two samples, pure a-Se representing the starting material and a-Se:1 at.% Sb were examined since 1 at.% Sb was found to have the longest carrier ranges and also exhibited excellent thermal stability. Both samples had the same thickness (35–36 lm) to enable a meaningful comparison. A dental X-ray source (Siemens Heliodent 57 kVp, 7 mA, 2 mm Al filtration) was adjusted to provide 180 mR exposure over an exposure time of 2.5 s.
999
3. Results In comparing the crystallization behavior of glasses with limited stability it is useful to determine the crystallization and glass transition temperature difference rather than the absolute values. A wider difference is generally associated with higher kinetic resistance to crystallization [7]. The plot of Tc Tg vs. Sb content in Fig. 1 shows that the thermal stability is maximum when the Sb content reaches 1 at.%. Although Tc Tg decreases after 1 at.% Sb, it is still higher than that for pure a-Se. Error estimates can be found in Ref. [3]. The results of the conventional and IFTOF measurements are shown in Figs. 2–4. Sb addition to a-Se affects the hole mobility slightly whereas there is a substantial decrease in the electron mobility with the Sb content. There is an initial increase in both the electron and hole lifetimes with small amounts of Sb addition as shown in Fig. 3. Both lifetimes reach a maximum at about 1–1.5 at.% Sb beyond which they both decrease with Sb addition. The initial enhancement in the carrier lifetimes is more than the drop in the drift mobilities with the Sb content, which leads to enhanced electron and hole ranges (mobility lifetime products) as shown in Fig. 4. We performed X-ray photoconductivity experiments to examine the relative X-ray sensitivity of a-Se:0.1 at.% Sb films with respect to the starting pure a-Se film (the control material). The results
Fig. 1. Dependence of Tc Tg on the composition of Se–Sb glass alloys from the temperature modulated DSC total heat flow heating scans at an underlying heating rate of 1 °C/min. The error in the temperatures is smaller than the symbol size, about 1°.
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D. Tonchev et al. / Journal of Non-Crystalline Solids 299–302 (2002) 998–1001
Fig. 2. Dependence of the mobility of electrons (le ) and holes (lh ) on the composition of Sb–Se films from TOF experiments (lines are spline fits). The error in the hole mobility is estimated to be 0:005 cm2 /V s and in the electron mobility 0:2 103 cm2 /V s.
Fig. 4. Dependence of the range ls of electrons and holes on the composition of Sb–Se films from TOF/IFTOF experiments (lines are spline fits). The error in both hole and electron ls is estimated to be 0.05 cm2 /V, about the same as the symbol size.
are summarized in Table 1 in terms of the X-ray photocurrent as described in Section 2. There is a distinct improvement in the X-ray sensitivity with Sb alloying.
4. Discussion
Fig. 3. Dependence of the lifetime of electrons (se ) and holes (sh ) on the composition of Sb–Se films from IFTOF experiments (lines are spline fits). The error in hole lifetime is estimated to be 0.5 ls and in electron lifetime 10 ls.
What is remarkable in the results shown in Figs. 1 and 4 is the fact that both curves peak at about the same composition, 1 at.% Sb, at the composition around the maximum thermal stability. Thus, Se:1 at.% Sb alloy is a potential candidate as a suitable X-ray photoconductor in which carriers of both polarity can be transported without any deep trapping; lsE L, where L is the photoconductor thickness. Other recent measurements of the charge transport properties of a-Sbx Se1x [8–11] indicate that both electron and hole trapping increases with the Sb content which contrasts with
Table 1 1 cm2 electrode, E is the field and the polarity refers to the radiation receiving electrode (Au top electrode with respect to Al substrate) Photoconductor
Photocurrent
E ¼ þ8 V=lm
E ¼ 8 V=lm
E ¼ 20 V=lm
a-Se a-Se:1 at.% Sb
Iph ðnA=cm2 Þ Iph ðnA=cm2 Þ
145 190
150 170
350 410
Sample thickness L is the same, 35 lm. 57 kVp exposure at 180 mR.
D. Tonchev et al. / Journal of Non-Crystalline Solids 299–302 (2002) 998–1001
the results in Figs. 3 and 4. Previous lower ls may be in part due to the room temperature substrate temperature that was used for the deposition of the films or differences in the starting materials. The observed effect of Sb on the electron transport is similar to the effect of As addition [12]; not unexpected as both are group III elements. Mikla et al. [11] observe that the electron mobility activation energy for pure a-Se and a-Se:1 at.% Sb are the same, which means that Sb addition must increase the total concentration of the shallow electron traps that control the drift mobility. Within the shallow controlled transport model, the measured carrier range ls is given by l ls ¼ l0 s0 ¼ 0 ; Ct Nt where l0 is the microscopic mobility (in the transport band), s0 is the intrinsic deep trapping time in the absence of shallow traps, Ct is the capture coefficient, and Nt is the concentration of deep traps. Thus, the improvement in the ls product with small additions of Sb as in Fig. 4 implies that the deep trap population, Nt , is reduced for both holes and electrons upto 1 at.% Sb; we make the reasonable assumption that there is no substantial change in the microscopic mobility and Ct . The exact explanation for the improvement in the X-ray sensitivity in Table 1 must include not only the charge transport parameters (ls) but also the ionization energy and the absorption coefficient [13]; the latter measurements are currently in progress.
5. Conclusions We have prepared a range of a-Sbx Se1x alloys and photoconductor type films from these alloys by using conventional vacuum deposition techniques, with x up to 0.15. Based on differential scanning calorimetry experiments, the most stable films have been obtained for compositions containing about 1–1.5 at.% Sb.
1001
The electron drift mobility decreases sharply with Sb addition whereas the hole drift mobility is only slightly affected. There is a distinct enhancement in the electron and hole lifetimes, and hence carrier ranges, which attain their maximum values when the film composition has about 1 at.% Sb. The X-ray sensitivity of Se:1 at.% Sb film is greater than that of pure a-Se film made from the starting pure a-Se material. a-Se:1 at.% Sb alloy is a potential candidate as a suitable X-ray photoconductor in which carriers of both polarity need to be transported without any deep trapping. Acknowledgements This project was made possible by financial support from NSERC (Strategic Grant) and Anrad (Montreal, Quebec, Canada). We thank Stephen O’Leary, University of Regina, for his interest in the project. References [1] J.A. Rowlands, S.O. Kasap, Phys. Today 50 (11) (1997) 24. [2] J.A. Rowlands, J. Yorkston, in: J. Beutel, H.L. Kundel, R.L. Van Metter (Eds.), Handbook of Medical Imaging, vol. 1, SPIE, Washington, 2000 (Chapter 4 and references therein). [3] D. Tonchev, S.O. Kasap, J. Non-Cryst. Solids 248 (1999) 28. [4] S.O. Kasap, in: A. Diamond (Ed.), Handbook of Imaging Materials, 2nd Ed., Marcel Dekker, New York, 2002 (Chapter 9). [5] S.O. Kasap, J.A. Rowlands, J. Mater. Sci. Mater. Electron. 11 (2000) 179, and references therein. [6] S.O. Kasap, B. Polischuk, D. Dodds, Rev. Sci. Instrum. 61 (1990) 2081. [7] S. Mahadevan, A. Giridhar, A.K. Singh, J. Non-Cryst. Solids 88 (1986) 11. [8] V.I. Mikla, I.P. Mikhalko, Y.Y. Nagy, J. Phys.: Condens. Matter 6 (1994) 8269. [9] V.I. Mikla, V.M. Rubish, Phys. Stat. Sol. B 182 (1994) 325. [10] V.I. Mikla, Y.Y. Nagy, V.V. Mikla, A.V. Mateleshko, Mater. Sci. Eng. B 64 (1999) 1. [11] V.I. Mikla, A.V. Mateleshko, V.V. Mikla, Y.Y. Nagy, J. Non-Cryst. Solids 246 (1999) 46. [12] S.O. Kasap, C. Juhasz, J. Phys. D 18 (1985) 703. [13] S.O. Kasap, J. Phys. D 33 (2000) 2853.