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Thin film pn-junctions based on oxide materials as γ-radiation sensors
Sensors and Actuators A 113 (2004) 307–311
Thin film pn-junctions based on oxide materials as ␥-radiation sensors K. Arshak∗ , O. Korostynska Department of Electronic and Computer Engineering, University of Limerick, Plassey Technological Park, Limerick, Ireland Received 10 June 2003; received in revised form 17 November 2003; accepted 12 January 2004 Available online 25 February 2004
Abstract Throughout this work oxides are considered as appropriate cost-effective materials for radiation sensing. Thin films of TeO2 , In2 O3 and SiO and their mixtures were used in this study for the fabrication of pn-junctions. The detection of radiation is based on the fact that properties of the materials undergo changes by the influence of ␥-rays. A number of various pn-junctions were made that differ in their materials composition. All the devices were exposed to 60 Co or 137 Cs radiation sources and their current–voltage (I–V) characteristics were examined. Samples made with p-type material (TeO2 ) and n-type oxides mixture (50 wt.% of In2 O3 and 50 wt.% of SiO) showed clearly pronounced pn-junction behaviour. However, the response of these devices to radiation was not consistent as samples were too susceptible to environmental conditions. Devices based on TeO2 /In2 O3 oxide materials mixtures (p-type) and sulphur (n-type) were more stable and exhibited Zener diode behaviour. It was found that devices with a mixture of 90 wt.% of TeO2 and 10 wt.% of In2 O3 have sustained a higher breakdown voltage compared to those with pure TeO2 . To improve the stability of the devises response, a number of pn-junctions were manufactured on Si wafers. Samples with TeO2 /Si and (In2 O3 + SiO)/Si configurations were tested. An increase in the values of current with the increase in radiation in a certain dose range makes these devices suitable for dosimetry purposes. © 2004 Elsevier B.V. All rights reserved. Keywords: ␥-Radiation; Dosimetry; pn-Junction; Thin film; Metal oxides
1. Introduction Adequate personal radiation dosimetry is highly essential in a wide range of areas, for example, medicine, industry and science. It is important that people who work with radiation sources wear a dosimeter device to detect and measure any radiation they might be exposed to, and to ensure that they do not receive more than the recommended dose level. Currently, there are three main applications for industrial radiation processing. These are sterilisation of medical devices, food irradiation and modification of polymers. The choice of a particular detector type for an application depends upon the radiation energy range of interest, application’s resolution, efficiency requirements, detector’s performance, timing, environmental suitability and cost. The influence of ␥-radiation onto different types of thin films has been discussed earlier [1,2]. The irradiation of thin films of poly(methyl methacrylate) doped with spirobenzopyran resulted in a permanent change in the materials properties from a non-fluorescent form to a fluorescent,
∗ Corresponding author. Tel.: +353-61-202267; fax: +353-61-338176. E-mail address: [email protected] (K. Arshak).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.01.026
under the excitation of the wavelengths of 488 and 514 nm [1]. Arsenic sulphur thin films deposited by thermo vacuum method were used as radiation-sensitive elements in [2]. They were characterised by irreversible changes in their optical properties. Such dosimeters operate by the combined effect of photo-radiation in arsenic sulphur associated with defect recharging processes [2]. However, these devices are complicated in practice and have a limited working range of doses. Throughout this work, a novel approach is used, where oxides in the form of thermally evaporated thin film pn-junctions are considered as appropriate cost-effective materials for radiation sensing. It is believed that ionising radiation causes structural defects (called colour centres or oxygen vacancies in oxides) leading to change in their density on exposure to ␥-rays [3,4]. Depending on the electronic structure of the material, the nature of oxygen vacancies changes dramatically [5]. Oxygen vacancies (colour or F centres, from Farbe the German word for colour) exist in three states, depending on their electronic charge. Diamagnetic oxygen vacancies can be either neutral, F, or doubly charged, F2+ . In the former case, two electrons are associated to the vacancy. The influence of radiation depends on both the dose and the parameters of the films including their thickness: the degradation is
308
K. Arshak, O. Korostynska / Sensors and Actuators A 113 (2004) 307–311
more severe for the higher dose and the thinner films [6,7]. Deep understanding of the physical properties of the materials under the influence of radiation exposure is vital for the effective design of dosimeter devices.
1400
as-dep 12 Gy
1200
24 Gy 36 Gy 66 Gy
1000
2. Experimental procedure Thin films of p-type material (TeO2 ) having thickness of 170 nm were deposited on glass substrates at a rate of 4 nm/s using Edwards E306A vacuum thermal evaporator. This system contains an Edwards FTM5 quartz crystal to monitor the rate of film deposition and to measure the film thickness. The quartz crystal was positioned directly above the evaporation source. The mass deposited on the quartz crystal during the evaporation alters its natural frequency of vibration. This frequency change was recorded on the meter of the film thickness monitor connected to the quartz crystal. Thus, the monitor could record both the thickness and the rate of deposition corresponding to a particular frequency shift. For the deposition of all films, the vacuum chamber was initially evacuated to a partial pressure of 1×10−5 mbar. On top of TeO2 layers, n-type films (homogenous mixture of 50 wt.% of In2 O3 and 50 wt.% of SiO) having a nominal thickness of 160 nm were evaporated at a rate of 0.2 nm/s. Aluminium layers were deposited under the bottom and the top of these layers to serve as the electrical contacts. As a result, pn-junctions with Al–TeO2 –(In2 O3 + SiO)–Al structure were manufactured. Another series of pn-junctions was made on glass substrates, where sulphur was used as an n-type material, whereas TeO2 and mixture of 90 wt.% of TeO2 and 10 wt.% of In2 O3 were used as p-type layers. As well, a number of pn-junctions were manufactured on Si wafers. Thin films of TeO2 having thickness of 245 nm were deposited at a rate of 3 nm/s on polished side of n-type Silicon wafer with orientation N1 1 1 to form pn-junction structures. Thin films with a mixture of 50 wt.% of In2 O3 and 50 wt.% of SiO having thickness of 200 nm were deposited at a rate of 0.2 nm/s on p-type substrate to form pn-junction structures. A one-side polished p-type P1 0 0 Silicon wafer was used with dopant level of 1015 cm−3 on polished side and 1018 cm−3 on unpolished side. 60 Co and 137 Cs radiation sources were used for exposing the samples to ␥-radiation at room temperature. A set of irradiations were performed changing the exposure time and hence the dose. Changes in their current–voltage (I–V) characteristics were measured after each exposure.
3. Results and discussion 3.1. TeO2 /(In2 O3 +SiO) pn-junctions Fig. 1 shows typical I–V plots of as-deposited and ␥-irradiated samples at different doses made with p-type
Current (I), A x 10
-6
96 Gy
800 600 400 200 0
-6
-4
-2
0 -200
2 4 6 Applied voltage (V), V
Fig. 1. I–V plots of as-deposited and ␥-irradiated at different doses TeO2 /(In2 O3 + SiO) pn-junctions.
material (TeO2 ) and n-type oxides mixture (50 wt.% of In2 O3 and 50 wt.% of SiO). These TeO2 /(In2 O3 + SiO) structures showed clearly pronounced pn-junction behaviour. However, the response of these devices to radiation was not consistent. The samples were too sensitive to environmental conditions as slightest variation resulted in permanent alteration of the films structure. When the samples are susceptible to external effects such as temperature, humidity, electromagnetic field, etc., this results in underor overestimation of the absorbed dose. To eliminate such effects, there is a need for corresponding sophisticated signal conditioning. 3.2. TeO2 /S and (TeO2 +In2 O3 )/S pn-junctions To investigate the effect of the material composition on device performance, two types of pn-junctions with different p-type sides were manufactured. The first type was pure TeO2 and the second type was a mixture of 90% of TeO2 and 10% of In2 O3 . The devices showed an enormous difference in their electrical properties. Fig. 2 shows plots of typical I–V characteristics for two types of samples, where type 1 corresponds to TeO2 /S and type 2 to (TeO2 + In2 O3 )/S pn-junctions. The I–V characteristic of a sample with (TeO2 + In2 O3 )/S structure is shifted to the left hand side of the counterpart sample with TeO2 /S pn-junction. This is caused by the difference in the level of doping of p-type material used in the two types of the pn-junctions. The samples with the mixture of In2 O3 and TeO2 sustained higher breakdown voltage. Both types of pn-junctions exhibited Zener diode behaviour. When Zener diode is reverse biased, very little current flows, and the diode is to a first order approximation an open circuit [8]. When reverse bias field is too strong, thermally generated electrons (or holes) acquire enough kinetic
K. Arshak, O. Korostynska / Sensors and Actuators A 113 (2004) 307–311 600
as - dep.
309
600
typ e 1 12 Gy
450
typ e 2
24 Gy
400
36 Gy
300
48 Gy 200 -6
0 -1 5
-1 0
-5
60 Gy
0
5
10
15
-1 5 0
Current (I), A x 10
Current (I), A x 10
-6
150
72 Gy 0 - 10
-5
0
5
10
- 200
-3 0 0 - 400
-4 5 0
-6 0 0
- 600
V olta ge (V ), V
V o lt ag e ( V ) , V
Fig. 2. Plots of typical I–V characteristics for two types of samples, where type 1 corresponds to TeO2 /S and type 2 to (TeO2 + In2 O3 )/S pn-junctions.
Fig. 3. Plots of I–V characteristics that were recorded for as-deposited and ␥-irradiated Al-TeO2 /S-Al thin film pn-junctions.
energy to ionise the atoms within the crystal structure. These in turn ionise other atoms leading to a very swift multiplication effect and a large current. It is the high accelerating field and narrow depletion region that allow the electrons to tunnel through. The reverse bias voltage leading to Zener breakdown is adjustable during manufacture of the device [8]. The two types samples were exposed to 60 Co radiation source with a dose rate of 6 Gy/min. The I–V characteristics of as-deposited and ␥-irradiated samples were recorded. Samples with a mixture of 90 wt.% of TeO2 and 10 wt.% of In2 O3 as p-type material showed unstable behaviour, probably due to the compensation process took place when mixing p- and n-type materials (TeO2 and In2 O3 , respectively). Fig. 3 shows the plots of I–V characteristics that were recorded for as-deposited and ␥-irradiated samples having Al/TeO2 /S/Al structure. The dose was increased in steps of 12–72 Gy. To trace the changes in I–V characteristics behaviour for these samples with the increase in radiation dose see Fig. 4. This figure shows the dependence of the normalised current (I − I0 )/I0 with radiation dose under an applied voltage of −6 V for Al–TeO2 /S–Al thin film structures. In reverse biased region it was difficult to determine precisely the breakdown voltage. The applied voltage of −6 V was chosen to monitor the changes in values of current caused by ␥-radiation as it gives almost linear current–dose dependence. As one may consider the fabricated pn-junction
as a simple model of radiation sensor, this dependence could serve as a reference to receive information about the radiation dose absorbed (see Fig. 4). These samples were damaged by the increase in the radiation dose, exceeding a dose level of 72 Gy. In general, these samples were more stable to environmental conditions than the counterpart TeO2 /(In2 O3 + SiO) pn-junctions. However, elevated temperatures could cause irreversible damage to sulphur layers and to maintain such devices would be cost-consuming. 1.8 y = 0.0235x - 0.0948 R2 = 0.9816
1.5
(I-I0)/I0
1.2 0.9 0.6 0.3 0 0
20
40
60
80
Radiation dose (D), Gy
Fig. 4. Dependence of normalised current (I − I0 )/I0 with radiation dose under an applied voltage of −6 V for Al–TeO2 /S–Al thin film pn-junctions.
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342 µGy
33
Current (I), A x10-6
0 Gy
1000
28
500 Current (I), A x10 -6
23 18 13 8 3
-5
-3
-1
-2
0
-5
1
-7
3
-3
-1
1
3
5
Voltage (V), V
-500
-1000
5
Voltage (V), V
-12
-1500
Fig. 5. I–V plots of as-deposited and ␥-irradiated with a dose of 342 Gy TeO2 /Si samples.
-2000 Fig. 7. I–V plots of backward diode, where In2 O3 + SiO thin films were deposited on p-type Si wafer as a substrate.
3.3. TeO2 /Si and (In2 O3 + SiO)/Si pn-junctions Fig. 5 shows typical I–V plots of TeO2 /Si samples as-deposited and ␥-irradiated with a dose of 342 Gy. As-deposited TeO2 /Si structures exhibited a strong pn-junction behaviour. These samples were gradually exposed to a 137 Cs ␥-radiation source. To trace the radiationinduced changes in the values of current under the reverse biased voltage of −1.2 V, see Fig. 6. It depicts linear changes in the value of current with the increase in radiation dose to a level of 171 Gy. Further increase in radiation dose resulted a change in their behaviour towards a non-linear resistor-type after a dose of 342 Gy. Fig. 7 shows typical I–V characteristics of as-deposited (In2 O3 + SiO)/Si junctions that behaved like backward diodes. This type of diode behaviour was first reported by Esaki for pn-junctions formed between very heavily doped P and N regions due to the tunnelling phenomenon [9]. The differences among the different semiconductor materials
are primarily traceable to the differences in their energy band gaps and direct versus indirect tunnelling effects [10]. The wider the energy gap, the larger the voltage scale of the forward characteristic [10]. Backward diodes are widely used for very fast switching in high frequency applications as they show no appreciable charge-storage effects. Fig. 8 shows current-voltage characteristics of (In2 O3 + SiO)/Si backward diodes for as-deposited and ␥-irradiated samples with a dose of 2736 Gy. The value of current under the reverse-biased conditions increased considerably with
0 -3
-2
-1
0
1
-500
0.18
0 Gy
0.16
2736 µGy
0.14 0.12
-1000
0.10 0.08 0.06
-1500
0.04 0.02 0.00 0
50 100 Radiation dose (D), Gy
150
200
Fig. 6. Dependence of normalised current (I − I0 )/I0 with dose under an applied voltage of −1.2 V for TeO2 /Si junctions.
-2000
Current (I), A x10 -6
(I-I0)/I0
2 3 Voltage (V), V
Fig. 8. I–V plots of as-deposited and ␥-irradiated with a dose of 2736 Gy (In2 O3 + SiO)/Si backward diodes.
K. Arshak, O. Korostynska / Sensors and Actuators A 113 (2004) 307–311
position. Samples made with p-type material (TeO2 ) and n-type oxides mixture (50 wt.% of In2 O3 and 50 wt.% of SiO) showed clearly pronounced pn-junction behaviour, but the response of these devices to radiation was not consistent as samples were too susceptible to environmental conditions. TeO2 /S structures showed an increase in the values of leakage current up to the dose level of 72 Gy. The devices with (In2 O3 + SiO)/Si structures were considered as the most reliable and suitable for dosimetry applications.
1 0.9 0.8 0.7 (I-I0)/I0
311
0.6 0.5 0.4 0.3 0.2 0.1 0 0
500
1000
1500
2000
2500
3000
Radiation dose (D), Gy
Fig. 9. Dependence of normalised current (I − I0 )/I0 with dose at −1.8 V for (In2 O3 + SiO)/Si backward diodes.
the increase in radiation dose to a level of 1026 Gy, while no significant changes were observed in the values of current when the diode was forward-biased. Fig. 9 illustrates the dependence of normalised current (I −I0 )/I0 with dose under the applied voltage of −1.8 V for (In2 O3 + SiO)/Si backward diodes. This curve may roughly be considered to consist of three regions. Linear increase in the values of reverse current was monitored with the increase in radiation to a dose of around 400 Gy. In the second region from 400 to around 1200 Gy, a very fast increase in the values of current was detected. Further increase in radiation dose led to some kind of saturation in I–V plots, which is clearly seen in Fig. 9. Similar feature of dose response was found in TL materials and lies in the dependence of the supralinearity on ionisation density, i.e., on ␥-ray energy and on the type of radiation. The value of maximum supralinearity rapidly decreased with decreasing ␥-ray energy: for 1.25 MeV ␥-rays in LiF:Mg,Cu,P showed no supralinearity [11]. The difference in the properties of the materials used emerges as reasonable explanation to the changes of supralinear region in dependence of electrical parameters on radiation dose. However, the region of linear response is preferable in radiation dosimetry. The devices with (In2 O3 + SiO)/Si structures were considered as the most reliable and suitable for dosimetry applications.
4. Conclusions Throughout this work, metal oxides such as TeO2 , In2 O3 and SiO and their mixtures were used for the manufacture of radiation sensing layers in the form of thermally evaporated thin film pn-junctions. The detection of radiation was based on the fact that properties of the materials undergo changes by the influence of ␥-rays. A number of various pn-junctions were made that differ in their materials com-
Acknowledgements This work was performed with the assistance of AMT Ireland at University of Limerick, as part of the RADSENAD project (ref. PRP00/AMT/06), sponsored by Enterprise Ireland Development Funds. References [1] G.W. Phillips, A.K. Readshaw, G.O. Brown, R.G. Weiss, N.A. Guardala, J.L. Price, S.C. Mueller, M. Moscovitch, Appl. Radiat. Isotopes 50 (1999) 875–881. [2] O.I. Shpotyuk, Radiat. Phys. Chem. 46 (1995) 1279–1282. [3] L.N. Trefilova, A.M. Kudin, L.V. Kovaleva, T.A. Charkina, A.I. Mitichkin, L.E. Belenko, Radiat. Meas. 33 (2001) 687–692. [4] R.y. Zhu, Nucl. Instrum. Methods A 413 (1998) 297–311. [5] G. Pacchioni, Solid State Sci. 2 (2000) 161–179. [6] E. Atanassova, A. Paskaleva, R. Konakova, D. Spassov, V.F. Mitin, Microelectron. J. 32 (2001) 553–562. [7] K. Arshak, O. Korostynska, J. Harris, Proc. MIEL 1 (2002) 357–360. [8] B.V. Zeghbroeck, Principles of Semiconductor Devices, Prentice-Hall, University of Colorado at Boulder, 2001. [9] L. Esaki, Y. Miyahara, Solid State Electron. 1 (1960) 13–14. [10] Hunter, L.P. in: L.P. Hunter (Ed.), Handbook of Semiconductor Electronics, McGraw-Hill, New York, 1970, pp. 3-1–3-66. [11] Y.S. Horowitz, Nucl. Instrum. Methods B 184 (2001) 68–84.
Biographies Prof. Khalil I. Arshak received BSc from Basrah University, Iraq in 1969, MSc from Salford University, UK in 1979, and the PhD and DSc from Brunel University, UK in 1986 and 1998, respectively. He joined the University of Limerick in 1986 where he leads the Microelectronic and Semiconductor Research Group. Prof. Arshak has authored more than 125 research papers in the area of microelectronics and thin- and thick-film technology. His current research interests include lithography process modelling, TSI processes characterisation, mixed oxide thin- and thick-film sensor development, and application specific integrated circuit design. Dr. Olga Korostynska received her BSc and MSc from National Technical University of Ukraine (KPI) in 1998 and 2000, respectively in biomedical electronics, and the PhD from University of Limerick, Ireland in 2003. Her research interests are in thin and thick film technologies, material properties characterisation and namely radiation sensors.
Thin film pn-junctions based on oxide materials as ␥-radiation sensors K. Arshak∗ , O. Korostynska Department of Electronic and Computer Engineering, University of Limerick, Plassey Technological Park, Limerick, Ireland Received 10 June 2003; received in revised form 17 November 2003; accepted 12 January 2004 Available online 25 February 2004
Abstract Throughout this work oxides are considered as appropriate cost-effective materials for radiation sensing. Thin films of TeO2 , In2 O3 and SiO and their mixtures were used in this study for the fabrication of pn-junctions. The detection of radiation is based on the fact that properties of the materials undergo changes by the influence of ␥-rays. A number of various pn-junctions were made that differ in their materials composition. All the devices were exposed to 60 Co or 137 Cs radiation sources and their current–voltage (I–V) characteristics were examined. Samples made with p-type material (TeO2 ) and n-type oxides mixture (50 wt.% of In2 O3 and 50 wt.% of SiO) showed clearly pronounced pn-junction behaviour. However, the response of these devices to radiation was not consistent as samples were too susceptible to environmental conditions. Devices based on TeO2 /In2 O3 oxide materials mixtures (p-type) and sulphur (n-type) were more stable and exhibited Zener diode behaviour. It was found that devices with a mixture of 90 wt.% of TeO2 and 10 wt.% of In2 O3 have sustained a higher breakdown voltage compared to those with pure TeO2 . To improve the stability of the devises response, a number of pn-junctions were manufactured on Si wafers. Samples with TeO2 /Si and (In2 O3 + SiO)/Si configurations were tested. An increase in the values of current with the increase in radiation in a certain dose range makes these devices suitable for dosimetry purposes. © 2004 Elsevier B.V. All rights reserved. Keywords: ␥-Radiation; Dosimetry; pn-Junction; Thin film; Metal oxides
1. Introduction Adequate personal radiation dosimetry is highly essential in a wide range of areas, for example, medicine, industry and science. It is important that people who work with radiation sources wear a dosimeter device to detect and measure any radiation they might be exposed to, and to ensure that they do not receive more than the recommended dose level. Currently, there are three main applications for industrial radiation processing. These are sterilisation of medical devices, food irradiation and modification of polymers. The choice of a particular detector type for an application depends upon the radiation energy range of interest, application’s resolution, efficiency requirements, detector’s performance, timing, environmental suitability and cost. The influence of ␥-radiation onto different types of thin films has been discussed earlier [1,2]. The irradiation of thin films of poly(methyl methacrylate) doped with spirobenzopyran resulted in a permanent change in the materials properties from a non-fluorescent form to a fluorescent,
∗ Corresponding author. Tel.: +353-61-202267; fax: +353-61-338176. E-mail address: [email protected] (K. Arshak).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.01.026
under the excitation of the wavelengths of 488 and 514 nm [1]. Arsenic sulphur thin films deposited by thermo vacuum method were used as radiation-sensitive elements in [2]. They were characterised by irreversible changes in their optical properties. Such dosimeters operate by the combined effect of photo-radiation in arsenic sulphur associated with defect recharging processes [2]. However, these devices are complicated in practice and have a limited working range of doses. Throughout this work, a novel approach is used, where oxides in the form of thermally evaporated thin film pn-junctions are considered as appropriate cost-effective materials for radiation sensing. It is believed that ionising radiation causes structural defects (called colour centres or oxygen vacancies in oxides) leading to change in their density on exposure to ␥-rays [3,4]. Depending on the electronic structure of the material, the nature of oxygen vacancies changes dramatically [5]. Oxygen vacancies (colour or F centres, from Farbe the German word for colour) exist in three states, depending on their electronic charge. Diamagnetic oxygen vacancies can be either neutral, F, or doubly charged, F2+ . In the former case, two electrons are associated to the vacancy. The influence of radiation depends on both the dose and the parameters of the films including their thickness: the degradation is
308
K. Arshak, O. Korostynska / Sensors and Actuators A 113 (2004) 307–311
more severe for the higher dose and the thinner films [6,7]. Deep understanding of the physical properties of the materials under the influence of radiation exposure is vital for the effective design of dosimeter devices.
1400
as-dep 12 Gy
1200
24 Gy 36 Gy 66 Gy
1000
2. Experimental procedure Thin films of p-type material (TeO2 ) having thickness of 170 nm were deposited on glass substrates at a rate of 4 nm/s using Edwards E306A vacuum thermal evaporator. This system contains an Edwards FTM5 quartz crystal to monitor the rate of film deposition and to measure the film thickness. The quartz crystal was positioned directly above the evaporation source. The mass deposited on the quartz crystal during the evaporation alters its natural frequency of vibration. This frequency change was recorded on the meter of the film thickness monitor connected to the quartz crystal. Thus, the monitor could record both the thickness and the rate of deposition corresponding to a particular frequency shift. For the deposition of all films, the vacuum chamber was initially evacuated to a partial pressure of 1×10−5 mbar. On top of TeO2 layers, n-type films (homogenous mixture of 50 wt.% of In2 O3 and 50 wt.% of SiO) having a nominal thickness of 160 nm were evaporated at a rate of 0.2 nm/s. Aluminium layers were deposited under the bottom and the top of these layers to serve as the electrical contacts. As a result, pn-junctions with Al–TeO2 –(In2 O3 + SiO)–Al structure were manufactured. Another series of pn-junctions was made on glass substrates, where sulphur was used as an n-type material, whereas TeO2 and mixture of 90 wt.% of TeO2 and 10 wt.% of In2 O3 were used as p-type layers. As well, a number of pn-junctions were manufactured on Si wafers. Thin films of TeO2 having thickness of 245 nm were deposited at a rate of 3 nm/s on polished side of n-type Silicon wafer with orientation N1 1 1 to form pn-junction structures. Thin films with a mixture of 50 wt.% of In2 O3 and 50 wt.% of SiO having thickness of 200 nm were deposited at a rate of 0.2 nm/s on p-type substrate to form pn-junction structures. A one-side polished p-type P1 0 0 Silicon wafer was used with dopant level of 1015 cm−3 on polished side and 1018 cm−3 on unpolished side. 60 Co and 137 Cs radiation sources were used for exposing the samples to ␥-radiation at room temperature. A set of irradiations were performed changing the exposure time and hence the dose. Changes in their current–voltage (I–V) characteristics were measured after each exposure.
3. Results and discussion 3.1. TeO2 /(In2 O3 +SiO) pn-junctions Fig. 1 shows typical I–V plots of as-deposited and ␥-irradiated samples at different doses made with p-type
Current (I), A x 10
-6
96 Gy
800 600 400 200 0
-6
-4
-2
0 -200
2 4 6 Applied voltage (V), V
Fig. 1. I–V plots of as-deposited and ␥-irradiated at different doses TeO2 /(In2 O3 + SiO) pn-junctions.
material (TeO2 ) and n-type oxides mixture (50 wt.% of In2 O3 and 50 wt.% of SiO). These TeO2 /(In2 O3 + SiO) structures showed clearly pronounced pn-junction behaviour. However, the response of these devices to radiation was not consistent. The samples were too sensitive to environmental conditions as slightest variation resulted in permanent alteration of the films structure. When the samples are susceptible to external effects such as temperature, humidity, electromagnetic field, etc., this results in underor overestimation of the absorbed dose. To eliminate such effects, there is a need for corresponding sophisticated signal conditioning. 3.2. TeO2 /S and (TeO2 +In2 O3 )/S pn-junctions To investigate the effect of the material composition on device performance, two types of pn-junctions with different p-type sides were manufactured. The first type was pure TeO2 and the second type was a mixture of 90% of TeO2 and 10% of In2 O3 . The devices showed an enormous difference in their electrical properties. Fig. 2 shows plots of typical I–V characteristics for two types of samples, where type 1 corresponds to TeO2 /S and type 2 to (TeO2 + In2 O3 )/S pn-junctions. The I–V characteristic of a sample with (TeO2 + In2 O3 )/S structure is shifted to the left hand side of the counterpart sample with TeO2 /S pn-junction. This is caused by the difference in the level of doping of p-type material used in the two types of the pn-junctions. The samples with the mixture of In2 O3 and TeO2 sustained higher breakdown voltage. Both types of pn-junctions exhibited Zener diode behaviour. When Zener diode is reverse biased, very little current flows, and the diode is to a first order approximation an open circuit [8]. When reverse bias field is too strong, thermally generated electrons (or holes) acquire enough kinetic
K. Arshak, O. Korostynska / Sensors and Actuators A 113 (2004) 307–311 600
as - dep.
309
600
typ e 1 12 Gy
450
typ e 2
24 Gy
400
36 Gy
300
48 Gy 200 -6
0 -1 5
-1 0
-5
60 Gy
0
5
10
15
-1 5 0
Current (I), A x 10
Current (I), A x 10
-6
150
72 Gy 0 - 10
-5
0
5
10
- 200
-3 0 0 - 400
-4 5 0
-6 0 0
- 600
V olta ge (V ), V
V o lt ag e ( V ) , V
Fig. 2. Plots of typical I–V characteristics for two types of samples, where type 1 corresponds to TeO2 /S and type 2 to (TeO2 + In2 O3 )/S pn-junctions.
Fig. 3. Plots of I–V characteristics that were recorded for as-deposited and ␥-irradiated Al-TeO2 /S-Al thin film pn-junctions.
energy to ionise the atoms within the crystal structure. These in turn ionise other atoms leading to a very swift multiplication effect and a large current. It is the high accelerating field and narrow depletion region that allow the electrons to tunnel through. The reverse bias voltage leading to Zener breakdown is adjustable during manufacture of the device [8]. The two types samples were exposed to 60 Co radiation source with a dose rate of 6 Gy/min. The I–V characteristics of as-deposited and ␥-irradiated samples were recorded. Samples with a mixture of 90 wt.% of TeO2 and 10 wt.% of In2 O3 as p-type material showed unstable behaviour, probably due to the compensation process took place when mixing p- and n-type materials (TeO2 and In2 O3 , respectively). Fig. 3 shows the plots of I–V characteristics that were recorded for as-deposited and ␥-irradiated samples having Al/TeO2 /S/Al structure. The dose was increased in steps of 12–72 Gy. To trace the changes in I–V characteristics behaviour for these samples with the increase in radiation dose see Fig. 4. This figure shows the dependence of the normalised current (I − I0 )/I0 with radiation dose under an applied voltage of −6 V for Al–TeO2 /S–Al thin film structures. In reverse biased region it was difficult to determine precisely the breakdown voltage. The applied voltage of −6 V was chosen to monitor the changes in values of current caused by ␥-radiation as it gives almost linear current–dose dependence. As one may consider the fabricated pn-junction
as a simple model of radiation sensor, this dependence could serve as a reference to receive information about the radiation dose absorbed (see Fig. 4). These samples were damaged by the increase in the radiation dose, exceeding a dose level of 72 Gy. In general, these samples were more stable to environmental conditions than the counterpart TeO2 /(In2 O3 + SiO) pn-junctions. However, elevated temperatures could cause irreversible damage to sulphur layers and to maintain such devices would be cost-consuming. 1.8 y = 0.0235x - 0.0948 R2 = 0.9816
1.5
(I-I0)/I0
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20
40
60
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Radiation dose (D), Gy
Fig. 4. Dependence of normalised current (I − I0 )/I0 with radiation dose under an applied voltage of −6 V for Al–TeO2 /S–Al thin film pn-junctions.
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342 µGy
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Fig. 5. I–V plots of as-deposited and ␥-irradiated with a dose of 342 Gy TeO2 /Si samples.
-2000 Fig. 7. I–V plots of backward diode, where In2 O3 + SiO thin films were deposited on p-type Si wafer as a substrate.
3.3. TeO2 /Si and (In2 O3 + SiO)/Si pn-junctions Fig. 5 shows typical I–V plots of TeO2 /Si samples as-deposited and ␥-irradiated with a dose of 342 Gy. As-deposited TeO2 /Si structures exhibited a strong pn-junction behaviour. These samples were gradually exposed to a 137 Cs ␥-radiation source. To trace the radiationinduced changes in the values of current under the reverse biased voltage of −1.2 V, see Fig. 6. It depicts linear changes in the value of current with the increase in radiation dose to a level of 171 Gy. Further increase in radiation dose resulted a change in their behaviour towards a non-linear resistor-type after a dose of 342 Gy. Fig. 7 shows typical I–V characteristics of as-deposited (In2 O3 + SiO)/Si junctions that behaved like backward diodes. This type of diode behaviour was first reported by Esaki for pn-junctions formed between very heavily doped P and N regions due to the tunnelling phenomenon [9]. The differences among the different semiconductor materials
are primarily traceable to the differences in their energy band gaps and direct versus indirect tunnelling effects [10]. The wider the energy gap, the larger the voltage scale of the forward characteristic [10]. Backward diodes are widely used for very fast switching in high frequency applications as they show no appreciable charge-storage effects. Fig. 8 shows current-voltage characteristics of (In2 O3 + SiO)/Si backward diodes for as-deposited and ␥-irradiated samples with a dose of 2736 Gy. The value of current under the reverse-biased conditions increased considerably with
0 -3
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Fig. 6. Dependence of normalised current (I − I0 )/I0 with dose under an applied voltage of −1.2 V for TeO2 /Si junctions.
-2000
Current (I), A x10 -6
(I-I0)/I0
2 3 Voltage (V), V
Fig. 8. I–V plots of as-deposited and ␥-irradiated with a dose of 2736 Gy (In2 O3 + SiO)/Si backward diodes.
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position. Samples made with p-type material (TeO2 ) and n-type oxides mixture (50 wt.% of In2 O3 and 50 wt.% of SiO) showed clearly pronounced pn-junction behaviour, but the response of these devices to radiation was not consistent as samples were too susceptible to environmental conditions. TeO2 /S structures showed an increase in the values of leakage current up to the dose level of 72 Gy. The devices with (In2 O3 + SiO)/Si structures were considered as the most reliable and suitable for dosimetry applications.
1 0.9 0.8 0.7 (I-I0)/I0
311
0.6 0.5 0.4 0.3 0.2 0.1 0 0
500
1000
1500
2000
2500
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Radiation dose (D), Gy
Fig. 9. Dependence of normalised current (I − I0 )/I0 with dose at −1.8 V for (In2 O3 + SiO)/Si backward diodes.
the increase in radiation dose to a level of 1026 Gy, while no significant changes were observed in the values of current when the diode was forward-biased. Fig. 9 illustrates the dependence of normalised current (I −I0 )/I0 with dose under the applied voltage of −1.8 V for (In2 O3 + SiO)/Si backward diodes. This curve may roughly be considered to consist of three regions. Linear increase in the values of reverse current was monitored with the increase in radiation to a dose of around 400 Gy. In the second region from 400 to around 1200 Gy, a very fast increase in the values of current was detected. Further increase in radiation dose led to some kind of saturation in I–V plots, which is clearly seen in Fig. 9. Similar feature of dose response was found in TL materials and lies in the dependence of the supralinearity on ionisation density, i.e., on ␥-ray energy and on the type of radiation. The value of maximum supralinearity rapidly decreased with decreasing ␥-ray energy: for 1.25 MeV ␥-rays in LiF:Mg,Cu,P showed no supralinearity [11]. The difference in the properties of the materials used emerges as reasonable explanation to the changes of supralinear region in dependence of electrical parameters on radiation dose. However, the region of linear response is preferable in radiation dosimetry. The devices with (In2 O3 + SiO)/Si structures were considered as the most reliable and suitable for dosimetry applications.
4. Conclusions Throughout this work, metal oxides such as TeO2 , In2 O3 and SiO and their mixtures were used for the manufacture of radiation sensing layers in the form of thermally evaporated thin film pn-junctions. The detection of radiation was based on the fact that properties of the materials undergo changes by the influence of ␥-rays. A number of various pn-junctions were made that differ in their materials com-
Acknowledgements This work was performed with the assistance of AMT Ireland at University of Limerick, as part of the RADSENAD project (ref. PRP00/AMT/06), sponsored by Enterprise Ireland Development Funds. References [1] G.W. Phillips, A.K. Readshaw, G.O. Brown, R.G. Weiss, N.A. Guardala, J.L. Price, S.C. Mueller, M. Moscovitch, Appl. Radiat. Isotopes 50 (1999) 875–881. [2] O.I. Shpotyuk, Radiat. Phys. Chem. 46 (1995) 1279–1282. [3] L.N. Trefilova, A.M. Kudin, L.V. Kovaleva, T.A. Charkina, A.I. Mitichkin, L.E. Belenko, Radiat. Meas. 33 (2001) 687–692. [4] R.y. Zhu, Nucl. Instrum. Methods A 413 (1998) 297–311. [5] G. Pacchioni, Solid State Sci. 2 (2000) 161–179. [6] E. Atanassova, A. Paskaleva, R. Konakova, D. Spassov, V.F. Mitin, Microelectron. J. 32 (2001) 553–562. [7] K. Arshak, O. Korostynska, J. Harris, Proc. MIEL 1 (2002) 357–360. [8] B.V. Zeghbroeck, Principles of Semiconductor Devices, Prentice-Hall, University of Colorado at Boulder, 2001. [9] L. Esaki, Y. Miyahara, Solid State Electron. 1 (1960) 13–14. [10] Hunter, L.P. in: L.P. Hunter (Ed.), Handbook of Semiconductor Electronics, McGraw-Hill, New York, 1970, pp. 3-1–3-66. [11] Y.S. Horowitz, Nucl. Instrum. Methods B 184 (2001) 68–84.
Biographies Prof. Khalil I. Arshak received BSc from Basrah University, Iraq in 1969, MSc from Salford University, UK in 1979, and the PhD and DSc from Brunel University, UK in 1986 and 1998, respectively. He joined the University of Limerick in 1986 where he leads the Microelectronic and Semiconductor Research Group. Prof. Arshak has authored more than 125 research papers in the area of microelectronics and thin- and thick-film technology. His current research interests include lithography process modelling, TSI processes characterisation, mixed oxide thin- and thick-film sensor development, and application specific integrated circuit design. Dr. Olga Korostynska received her BSc and MSc from National Technical University of Ukraine (KPI) in 1998 and 2000, respectively in biomedical electronics, and the PhD from University of Limerick, Ireland in 2003. Her research interests are in thin and thick film technologies, material properties characterisation and namely radiation sensors.