- Email: [email protected]
Effects of metallisation on TlBr single crystals for detector applications
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 573 (2007) 212–215 www.elsevier.com/locate/nima
Effects of metallisation on TlBr single crystals for detector applications V. Kozlova,, M. Leskela¨a, M. Vehkama¨kia, H. Sipila¨b a
Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Helsinki, Finland b Oxford Instruments Analytical Oy, P.O. Box 85, FIN-02631, Espoo, Finland Available online 30 November 2006
Abstract The single-crystal TlBr, a promising candidate as g-ray detector, is annoyed by material problems. These problems are manly arisen from purity and quality of the crystal, and the total process of detector manufacturing. In this work, the making of electric contact with different methods was studied. Al, Ti, Cr, Fe, Ni, In, Sn, as well Ag and graphite paste were used for annealed TlBr single crystals. Samples were characterised by I–V measurements and routinely studied by optical, X-ray powder diffraction and rocking curve methods. r 2006 Elsevier B.V. All rights reserved. Keywords: g-Ray detector; TlBr; Metallisation
1. Introduction For hard g-ray radiation, a detector material of the highest possible stopping power is needed. It means that the material components should have high atomic numbers Z and correspondingly high compound density. If the device should operate at room temperature, the band gap of the material has to be of the order 3 eV. Singlecrystalline TlBr is a promising candidate as a room temperature g-ray detector, because of its Z (81+35 ) 58), density (7.56 g/cm3) and band gap (2.68 eV) [1]. A recent success of the authors [2] in achieving an inter-pixel resistance 500 GO (gap 100 mm, 50 V) makes TlBr material attractive for manufacturing of a compact 2Darray detector. However, the carrier transport properties of TlBr are still annoyed by material problems [1]. These problems are manly arisen from purity and quality of the crystal [3], both these properties being affected by the total manufacturing process of the detector [4]. Although, the crystal purity [3], quality and electrical properties can be improved by a hydrothermal annealing [5], the electrical characteristics of the samples vary within time, the reason for this still being unknown. Corresponding author. Tel.: +358 9 191 50 212; fax: +358 9 191 50 198. E-mail address: [email protected].fi (V. Kozlov).
0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.329
In this work, the role of metal electrode made with different methods on a TlBr surface was emphasised. The chemical, physical vapour, metallurgical and glue methods are widely used for the metallisation. The chemical methods, when a metal is formed on the substrate as result of chemical reaction, were out of scope of this work. Among physical vapour deposition methods, electron beam and sputtering deposition are the most common techniques. These methods, however, require preliminary surface treatment and degassing before deposition. The interaction at the interface may be schematically characterised by metal distribution across the interface as presented in Fig. 1. The case marked as ‘‘adhesion’’ corresponds to inert metal. The melting point (m.p.) of metal may roughly be used for evaluations. Thus, metal with m.p. of the order of 1500 1C or more is expected to be inert in respect to TlBr (m.p. ¼ 460 1C), and metals with m.p. below 1000 1C have a higher penetration ability (‘‘diffusion case’’), if there is no chemical reaction (see Table 1). The adhesion for different metals and TlBr–TlI is influenced by chemisorption phenomena [6] and the adhesion stability decreases with the difference between the normal potentials of Tl and condensed metal [6,7]. For the evaluation of the metal–TlHal barrier height, the electro-negativity of the corresponding metal can be used, since direct proportionality between these values was found for ionic compounds [9]. Despite high electro-negativity, Pd, Pt (EN of both is 2.2) and Au, as inert metals, are
ARTICLE IN PRESS V. Kozlov et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 212–215
213
Deposition of metal using glue can be considered as a mechanical contact, where the conductive particles are joined to each other and to the substrate by the use of glue. We have used Ag- and graphite-paste for quick testing of the samples. Samples were characterised by I–V measurements and routinely studied by X-ray powder diffraction and rockingcurve methods. The quality of single crystals and films was controlled by optical reflection spectra and under polarised light. 2. Experimental 2.1. Sample preparation
Fig. 1. A model for metal distribution on a surface.
Table 1 Metal–Tl halide interface parameters Metal
m.p. (1C)
Tensile strength
Stability
EN
Cu Ag Au Cd Al In Tl Sn Pb Ti Cr Fe Co Ni Mn Mg
1083 962 1064 321 660 157 304 232 328 1660 1857 1535 1495 1453 1244 649
24 27 27 36 Peel off 18
Atm Atm Atm
1.9 1.9 2.4 1.7 1.5 1.7 1.8 1.8 1.8 1.5 1.6 1.8 1.8 1.8 1.5 1.2
6 29 18 14 23 22 22 X X
The transparent TlBr slices 5 5 2 mm3 in size and of 99.99% purity were supplied by GIREDMET (Moscow, Russia) [5]. The samples were chemically washed or etched by a mixture of HBr and H2O2 (5:1). In case of an annealing, vacuum (2 103 mbar), nitrogen (99,999%) or argon (99,9999%) atmospheres were used. A hydrothermal annealing in pure water (2–3 MO) was used as well. 2.2. Characterisation
Bad Good
Good Good Good Atm Good Atm Bad Bad
The tensile strength [6] measured on films of 0.5 mm was in units MN/m2 without explanation. Stability by Ref. [8]; EN—electronegativities of Pauling, Atm.—unstable in atmosphere, X—chemical reaction.
widely used for ohmic contacts, but some problems with Au were observed [4]. For the current study, Ti, Cr, Fe and Ni were chosen because of their rather high m.p. and lower electro-negativity. The metallurgical method is used for metallisation of materials by the melt metal, for example zinc. The interface quality strictly depends on the pre-treatment of material surface, since good contact is formed due to diffusion of metal into substrate or formation of an intermediate compound (Fig. 1). The formation of the compound between metal and the surface may provide a reliable and reproducible Schottky barrier [9]. Al, In, Sn and Tl were chosen for this study, because of their low m.p. and since they are capable to form compounds with TlBr.
A D8 advanced X-ray diffractometer (Bruker AXS, Analytical X-ray Systems, GMBH) was used to control the crystal quality by the rocking curve method. An express evaluation of the crystal quality was carried out using a polarisation microscope. By-products of the experiments were analysed by Philips powder diffractometer PW1710/00. The device is used, as well, for the measuring of a possible X-ray response of the TlBr detectors. I–V measurements were made by a Keithley 2400 Source Meter at room temperature in dark. The bias voltage was swept between 7200 V with a step of 0.5 V. Al, Ti and Cr electrodes were made by the means of electron beam evaporation with a thickness 25 or 40 nm. Fe and Ni were tested in direct contact with the TlBr melt. In, Sn and Tl were molten on the TlBr surface. In case of annealing, nitrogen atmosphere at 300 1C was used. A commercial Agglue Biston or graphite paste were used to make electrodes or attach them to metallic contacts. 3. Results and discussion Ti and Cr deposited by electron beam evaporation gave good contacts that were stable for year in a laboratory environment. Both metals give good gettering that improve the vacuum conditions in situ during deposition. Pure Fe and Ni were inert in contact with molten TlBr under inert atmosphere or at high vacuum. However, the surface of both metals became black, in case of an incomplete degassing of system or/and if metal of lower purity was used. Thus, these four elements can be used for a reliable contact with TlBr.
ARTICLE IN PRESS 214
V. Kozlov et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 212–215
Although Al–TlHal interface was considered in Ref. [6] as unstable even in vacuum (Table 1), in our case 40 nm layer (against cited 500 nm) did not peel off in a laboratory environment for several days. This increased stability may be associated with a finer layer of metal or electron beam deposition method itself, since it is performed at higher vacuum and at lower temperature than vacuum deposition method used in Ref. [6]. However, the m.p. of Al is rather low, moreover, AlBr3 has a high heat of formation (126.6 kcal) and vapour pressure of 1 bar at 256 1C. Therefore, Al–TlBr interface was studied additionally by the deposition of TlBr on a clean Al surface (Fig. 2). The films received below 100 1C were stable for year. Corresponding optical reflection spectra showed that the TlBr films were uniform and fine. When the deposition temperature was raised to 150 1C, the destruction of Al surface and formation of amorphous phase with the Tl-drops was observed. The corresponding reaction can be written as follows: Al(s)+3TlBr(g) ) 3Tl (drop)+AlBr3(g). Therefore, although Al is an attractive electrode material for many applications, it can be used with TlBr only at moderated temperatures and low current flow. In, Sn and Tl were deposited on TlBr crystal by the metallurgical method. The metals were briefly melted at 310 1C under nitrogen atmosphere. Sn formed drops on the TlBr surface without traces of a reaction. Indium became formless and a yellow-layer compound was formed between In and underlaying TlBr. In case of Tl–TlBr interface, the intermediate browndark layer formed is easily peeled off from substrate. The prolonged annealing at 300 1C instigated the active reaction followed with TlBr evaporation. Opposite sides of samples treated with In and Sn for a short time at 250 1C gave well-formed drops on the surfaces without visual formation of any underlayer. I–V curves of Sn–TlBr–Sn were asymmetrical without any rectifier effect. Probably higher temperature is needed for the intermediate compound formation. In case of In–TlBr–layer–In, the rectifier effect was observed (Fig. 3).
Fig. 3. I–V curve of In–TlBr–layer–In interface.
However, a breakdown took place at field of 200 V/cm that was considerably lower than the working range (500 V/cm) in Ref. [2]. The breakdown seems to be connected to low crystal quality, inner block boundaries. Al and graphite particles of paste-glue used as electrodes were burned into a body of TlBr with the formation of craters on its surface at fields of order 2 kV/cm. However, when samples were properly polished and annealed, not any burning effects were noted at 2.5 kV/cm. 4. Conclusions Ti, Cr, Fe and Ni elements were shown to be inert to TlBr and Ti and Cr could be used as reliable electrodes. Al contact can be used at low current without heating. The same can be said about Ag- and graphite-paste. Tl is reacting aggressively with TlBr. Sn electrode was reliable at our conditions. In was shown to form intermediate layer compound. In/TlBr interface possesses rectifier properties; however, the crystal quality currently limits the use of this interface. Acknowledgement The work was supported by the Finnish Technology Agency TEKES. References
Fig. 2. XRD of TlBr film on the Al substrate.
[1] A. Owens, A. Peacock, Nucl. Instr. and Meth. A 531 (2004) 18. [2] A. Owens, M. Bavdaz, G. Brammertz, V. Gostilo, N. Haack, A. Kozorezov, I. Lisjutin, A. Peacock, S. Zatoloka, Nucl. Instr. and Meth. A 497 (2003) 359. [3] V. Kozlov, M. Leskela¨, T. Prohaska, G. Schultheis, G. Stingeder, H. Sipila¨, Nucl. Instr. and Meth. A 531 (2004) 165. [4] V. Gostilo, D. Gryaznov, I. Lisjutin, Nucl. Instr. and Meth. A 487 (1–2) (2002) 13. [5] V. Kozlov, M. Leskela¨, H. Sipila¨, Nucl. Instr. and Meth. A 546 (2005) 200.
ARTICLE IN PRESS V. Kozlov et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 212–215 [6] S.A. Varchena, A.A. Simanovskii, S.V. Stolyarova, G. Upite, Latv. PSR Zinat. Akad., Fiz. Inst., Preprint LAFI-067, Salaspils, 1984, p. 11. [7] A.A. Simanovskii, S.V. Stolyarova, G. Upite, Inst. Fiz., Riga, USSR. Poverkhnost 11 (1985) 58.
215
[8] S.A. Varchena, A.A. Simanovskii, S.V. Stolyarova, G. Upite, Latv. PSR Zinat. Akad., Fiz. Inst., Preprint LAFI-068, Salaspils, 1984, p. 39. [9] S.M. Sze, Physics of Semiconductor Devices, second ed, John Wiley and Sons, Inc., 1981, p. 276.
Nuclear Instruments and Methods in Physics Research A 573 (2007) 212–215 www.elsevier.com/locate/nima
Effects of metallisation on TlBr single crystals for detector applications V. Kozlova,, M. Leskela¨a, M. Vehkama¨kia, H. Sipila¨b a
Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Helsinki, Finland b Oxford Instruments Analytical Oy, P.O. Box 85, FIN-02631, Espoo, Finland Available online 30 November 2006
Abstract The single-crystal TlBr, a promising candidate as g-ray detector, is annoyed by material problems. These problems are manly arisen from purity and quality of the crystal, and the total process of detector manufacturing. In this work, the making of electric contact with different methods was studied. Al, Ti, Cr, Fe, Ni, In, Sn, as well Ag and graphite paste were used for annealed TlBr single crystals. Samples were characterised by I–V measurements and routinely studied by optical, X-ray powder diffraction and rocking curve methods. r 2006 Elsevier B.V. All rights reserved. Keywords: g-Ray detector; TlBr; Metallisation
1. Introduction For hard g-ray radiation, a detector material of the highest possible stopping power is needed. It means that the material components should have high atomic numbers Z and correspondingly high compound density. If the device should operate at room temperature, the band gap of the material has to be of the order 3 eV. Singlecrystalline TlBr is a promising candidate as a room temperature g-ray detector, because of its Z (81+35 ) 58), density (7.56 g/cm3) and band gap (2.68 eV) [1]. A recent success of the authors [2] in achieving an inter-pixel resistance 500 GO (gap 100 mm, 50 V) makes TlBr material attractive for manufacturing of a compact 2Darray detector. However, the carrier transport properties of TlBr are still annoyed by material problems [1]. These problems are manly arisen from purity and quality of the crystal [3], both these properties being affected by the total manufacturing process of the detector [4]. Although, the crystal purity [3], quality and electrical properties can be improved by a hydrothermal annealing [5], the electrical characteristics of the samples vary within time, the reason for this still being unknown. Corresponding author. Tel.: +358 9 191 50 212; fax: +358 9 191 50 198. E-mail address: [email protected].fi (V. Kozlov).
0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.329
In this work, the role of metal electrode made with different methods on a TlBr surface was emphasised. The chemical, physical vapour, metallurgical and glue methods are widely used for the metallisation. The chemical methods, when a metal is formed on the substrate as result of chemical reaction, were out of scope of this work. Among physical vapour deposition methods, electron beam and sputtering deposition are the most common techniques. These methods, however, require preliminary surface treatment and degassing before deposition. The interaction at the interface may be schematically characterised by metal distribution across the interface as presented in Fig. 1. The case marked as ‘‘adhesion’’ corresponds to inert metal. The melting point (m.p.) of metal may roughly be used for evaluations. Thus, metal with m.p. of the order of 1500 1C or more is expected to be inert in respect to TlBr (m.p. ¼ 460 1C), and metals with m.p. below 1000 1C have a higher penetration ability (‘‘diffusion case’’), if there is no chemical reaction (see Table 1). The adhesion for different metals and TlBr–TlI is influenced by chemisorption phenomena [6] and the adhesion stability decreases with the difference between the normal potentials of Tl and condensed metal [6,7]. For the evaluation of the metal–TlHal barrier height, the electro-negativity of the corresponding metal can be used, since direct proportionality between these values was found for ionic compounds [9]. Despite high electro-negativity, Pd, Pt (EN of both is 2.2) and Au, as inert metals, are
ARTICLE IN PRESS V. Kozlov et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 212–215
213
Deposition of metal using glue can be considered as a mechanical contact, where the conductive particles are joined to each other and to the substrate by the use of glue. We have used Ag- and graphite-paste for quick testing of the samples. Samples were characterised by I–V measurements and routinely studied by X-ray powder diffraction and rockingcurve methods. The quality of single crystals and films was controlled by optical reflection spectra and under polarised light. 2. Experimental 2.1. Sample preparation
Fig. 1. A model for metal distribution on a surface.
Table 1 Metal–Tl halide interface parameters Metal
m.p. (1C)
Tensile strength
Stability
EN
Cu Ag Au Cd Al In Tl Sn Pb Ti Cr Fe Co Ni Mn Mg
1083 962 1064 321 660 157 304 232 328 1660 1857 1535 1495 1453 1244 649
24 27 27 36 Peel off 18
Atm Atm Atm
1.9 1.9 2.4 1.7 1.5 1.7 1.8 1.8 1.8 1.5 1.6 1.8 1.8 1.8 1.5 1.2
6 29 18 14 23 22 22 X X
The transparent TlBr slices 5 5 2 mm3 in size and of 99.99% purity were supplied by GIREDMET (Moscow, Russia) [5]. The samples were chemically washed or etched by a mixture of HBr and H2O2 (5:1). In case of an annealing, vacuum (2 103 mbar), nitrogen (99,999%) or argon (99,9999%) atmospheres were used. A hydrothermal annealing in pure water (2–3 MO) was used as well. 2.2. Characterisation
Bad Good
Good Good Good Atm Good Atm Bad Bad
The tensile strength [6] measured on films of 0.5 mm was in units MN/m2 without explanation. Stability by Ref. [8]; EN—electronegativities of Pauling, Atm.—unstable in atmosphere, X—chemical reaction.
widely used for ohmic contacts, but some problems with Au were observed [4]. For the current study, Ti, Cr, Fe and Ni were chosen because of their rather high m.p. and lower electro-negativity. The metallurgical method is used for metallisation of materials by the melt metal, for example zinc. The interface quality strictly depends on the pre-treatment of material surface, since good contact is formed due to diffusion of metal into substrate or formation of an intermediate compound (Fig. 1). The formation of the compound between metal and the surface may provide a reliable and reproducible Schottky barrier [9]. Al, In, Sn and Tl were chosen for this study, because of their low m.p. and since they are capable to form compounds with TlBr.
A D8 advanced X-ray diffractometer (Bruker AXS, Analytical X-ray Systems, GMBH) was used to control the crystal quality by the rocking curve method. An express evaluation of the crystal quality was carried out using a polarisation microscope. By-products of the experiments were analysed by Philips powder diffractometer PW1710/00. The device is used, as well, for the measuring of a possible X-ray response of the TlBr detectors. I–V measurements were made by a Keithley 2400 Source Meter at room temperature in dark. The bias voltage was swept between 7200 V with a step of 0.5 V. Al, Ti and Cr electrodes were made by the means of electron beam evaporation with a thickness 25 or 40 nm. Fe and Ni were tested in direct contact with the TlBr melt. In, Sn and Tl were molten on the TlBr surface. In case of annealing, nitrogen atmosphere at 300 1C was used. A commercial Agglue Biston or graphite paste were used to make electrodes or attach them to metallic contacts. 3. Results and discussion Ti and Cr deposited by electron beam evaporation gave good contacts that were stable for year in a laboratory environment. Both metals give good gettering that improve the vacuum conditions in situ during deposition. Pure Fe and Ni were inert in contact with molten TlBr under inert atmosphere or at high vacuum. However, the surface of both metals became black, in case of an incomplete degassing of system or/and if metal of lower purity was used. Thus, these four elements can be used for a reliable contact with TlBr.
ARTICLE IN PRESS 214
V. Kozlov et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 212–215
Although Al–TlHal interface was considered in Ref. [6] as unstable even in vacuum (Table 1), in our case 40 nm layer (against cited 500 nm) did not peel off in a laboratory environment for several days. This increased stability may be associated with a finer layer of metal or electron beam deposition method itself, since it is performed at higher vacuum and at lower temperature than vacuum deposition method used in Ref. [6]. However, the m.p. of Al is rather low, moreover, AlBr3 has a high heat of formation (126.6 kcal) and vapour pressure of 1 bar at 256 1C. Therefore, Al–TlBr interface was studied additionally by the deposition of TlBr on a clean Al surface (Fig. 2). The films received below 100 1C were stable for year. Corresponding optical reflection spectra showed that the TlBr films were uniform and fine. When the deposition temperature was raised to 150 1C, the destruction of Al surface and formation of amorphous phase with the Tl-drops was observed. The corresponding reaction can be written as follows: Al(s)+3TlBr(g) ) 3Tl (drop)+AlBr3(g). Therefore, although Al is an attractive electrode material for many applications, it can be used with TlBr only at moderated temperatures and low current flow. In, Sn and Tl were deposited on TlBr crystal by the metallurgical method. The metals were briefly melted at 310 1C under nitrogen atmosphere. Sn formed drops on the TlBr surface without traces of a reaction. Indium became formless and a yellow-layer compound was formed between In and underlaying TlBr. In case of Tl–TlBr interface, the intermediate browndark layer formed is easily peeled off from substrate. The prolonged annealing at 300 1C instigated the active reaction followed with TlBr evaporation. Opposite sides of samples treated with In and Sn for a short time at 250 1C gave well-formed drops on the surfaces without visual formation of any underlayer. I–V curves of Sn–TlBr–Sn were asymmetrical without any rectifier effect. Probably higher temperature is needed for the intermediate compound formation. In case of In–TlBr–layer–In, the rectifier effect was observed (Fig. 3).
Fig. 3. I–V curve of In–TlBr–layer–In interface.
However, a breakdown took place at field of 200 V/cm that was considerably lower than the working range (500 V/cm) in Ref. [2]. The breakdown seems to be connected to low crystal quality, inner block boundaries. Al and graphite particles of paste-glue used as electrodes were burned into a body of TlBr with the formation of craters on its surface at fields of order 2 kV/cm. However, when samples were properly polished and annealed, not any burning effects were noted at 2.5 kV/cm. 4. Conclusions Ti, Cr, Fe and Ni elements were shown to be inert to TlBr and Ti and Cr could be used as reliable electrodes. Al contact can be used at low current without heating. The same can be said about Ag- and graphite-paste. Tl is reacting aggressively with TlBr. Sn electrode was reliable at our conditions. In was shown to form intermediate layer compound. In/TlBr interface possesses rectifier properties; however, the crystal quality currently limits the use of this interface. Acknowledgement The work was supported by the Finnish Technology Agency TEKES. References
Fig. 2. XRD of TlBr film on the Al substrate.
[1] A. Owens, A. Peacock, Nucl. Instr. and Meth. A 531 (2004) 18. [2] A. Owens, M. Bavdaz, G. Brammertz, V. Gostilo, N. Haack, A. Kozorezov, I. Lisjutin, A. Peacock, S. Zatoloka, Nucl. Instr. and Meth. A 497 (2003) 359. [3] V. Kozlov, M. Leskela¨, T. Prohaska, G. Schultheis, G. Stingeder, H. Sipila¨, Nucl. Instr. and Meth. A 531 (2004) 165. [4] V. Gostilo, D. Gryaznov, I. Lisjutin, Nucl. Instr. and Meth. A 487 (1–2) (2002) 13. [5] V. Kozlov, M. Leskela¨, H. Sipila¨, Nucl. Instr. and Meth. A 546 (2005) 200.
ARTICLE IN PRESS V. Kozlov et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 212–215 [6] S.A. Varchena, A.A. Simanovskii, S.V. Stolyarova, G. Upite, Latv. PSR Zinat. Akad., Fiz. Inst., Preprint LAFI-067, Salaspils, 1984, p. 11. [7] A.A. Simanovskii, S.V. Stolyarova, G. Upite, Inst. Fiz., Riga, USSR. Poverkhnost 11 (1985) 58.
215
[8] S.A. Varchena, A.A. Simanovskii, S.V. Stolyarova, G. Upite, Latv. PSR Zinat. Akad., Fiz. Inst., Preprint LAFI-068, Salaspils, 1984, p. 39. [9] S.M. Sze, Physics of Semiconductor Devices, second ed, John Wiley and Sons, Inc., 1981, p. 276.