- Email: [email protected]
A new material for imaging in the UV: CVD Diamond
Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
A new material for imaging in the UV: CVD Diamondq L. Barberini, S. Cadeddu, M. Caria* Physics Department, University of Cagliari, 09042 Monserrato (Cagliari), Italy
Abstract We critically discuss the possibility of using diamond films, grown with the Chemical Vapour Deposition (CVD) technique, as imaging detector in the extreme UV energy range. We present results on electrical tests and on irradiation studies, under UV source, of CVD films, bought from market. We show that the behaviour of the film under irradiation in the energy interval of 190–350 nm would prevent its use as imaging detector, if special measures on the deposition qualities are not taken. We discuss the mechanisms of the behaviour under irradiation, in terms of the crystal defects. We have extensively studied the charge-up effect of the film and the influence on the detection efficiency. We find a dependence on the irradiation time and methods. We can address the explanation of the behaviour in terms of the decay time of the traps. This leads to an important conclusion on the homogeneity and on the defect sites. We claim that this effect depends on the defect types and it is rarely reproducible. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamonds; CVD diamond; UV; Photodetectors; Electrical properties
1. Introduction Imaging in UV is of great importance for a very wide range of applications. As an example, researchers in the fields of biology investigate the bio-molecular compounds in the region of the energy spectrum they absorb the most. Their basic constituents always contains ATP which absorbs most in the interval 190–260 nm [1]. The material which best suits imaging in UV is diamond. Its gap is 5.47 eV corresponding to l=227 nm. The advantage of using diamond in this interval is that, for a perfect material, there should be a sharp cut off, at wavelengths below and above this value. This would allow us to
q
Presented by Mario Caria. *Corresponding author. E-mail address: [email protected] (M. Caria).
operate a UV imaging detector under room-light illumination or close to heating source, without deteriorated performance from infrared irradiation. The non-availability of pure diamond material in nature limits its use as a commercial imaging detector. We have investigated the possibility of using what is available on the shelf as synthetic diamond films grown with CVD techniques. Until now, these are the most studied [2] as radiation detectors and they are available in the market. In this paper, we report an investigation on the feasibility of using imaging detector based on CVD films. We have studied, in detail this behaviour under realistic conditions of irradiation. We show that the composition of the material questions, the use of CVD diamond as UV imaging detector. We discuss the causes and special measures needed to suppress the effects,
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 1 0 7 - 4
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L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
Fig. 1. Sketch of the sample and reference mapping of the positions of the metal contacts for the measurements.
which in any case still prevent us from using it as UV detector. In the first part, we report results on basic electrical characterisation tests on metal homogeneity, resistance, IV and CV measurements and their reproducibility from one sample to another. In the second part, we show results on irradiation with a deuterium lamp and mono-chromator wavelength selector, as a function of irradiation time and history. The so-called priming1 effect is observed and for the first time its dependence on the energy is reported. We conclude reviewing the steps needed to have a commercial CVD film for UV imaging.
2. CVD diamond samples
1 Also called with some confusion pumping or hysteresis or memory or charging-up effect.
2 By Norton, Goddard Road, Northboro, MA 01532–1545, Tel.: +1-508-351-7733.
Although there are many methods for producing synthetic diamonds, the choice for Chemical Vapour Deposition (CVD) [3,4] films was unavoidable as these are the ones available in the market as radiation sensors. We have purchased2 three samples of CVD film detectors with the characteristics indicated in Fig. 1. They are 1cm2 films with a thickness of 150–250 mm. On the growth face, a metallization, with the geometry indicated in Fig. 1, formed by three metals (Ti (0.1 mm)/Pt (0.2 mm)/Au (1 mm)), was done. The principle of detection for diamond films sensors is much simpler than for other semiconductor detectors. There is no need for
L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
implantations, like the more common silicon photodiodes, which could make the sensor easy to build and potentially cheaper. Due to the low energy, the UV photons are absorbed in the diamond on the metallization side. They create an electron–hole pair, which by applying an electric field between the electrodes is collected from the two different electrodes on the same surface. It is, therefore, mandatory to study the quality of the electrodes and the crystal surface. This is described in the next sections.
3. Experimental set-up The measurements were performed with a probe station inside a metallic box in order to isolate the detector from external electromagnetic fields and from the possible influence of environmental light. The measurements on the resistance and the current were performed with a picoamperometer, with few tenths of pA accuracy3. The measurements of capacitance were performed at 100 kHz, with an accuracy of 10 fF4. The optical system consists of an ultraviolet source5, a monochromator (model H10 Jobin Yvon) and a bundle of optical fibres. The set-up was used for an illumination in the interval 190–310 nm with 2 nm accuracy while the UV source provides a continuous spectrum from 185 nm to about 400 nm. The exit slit of the monochromator is replaced by the entrance of the bundle of optical fibres. The bundle is 1 m long, it has a rectangular entry terminal of 4.3 mm0.2 mm and a circular exit terminal of diameter 1.1 mm.
4. Electrical measurements The imaging detector has to be reliable, homogeneous, with low electric noise, highly efficient and, possibly, economic and easy to build. 3
Keithley 2400 and Keithley 237 for IV curves at high field. Keithley CV590. 5 30W D2 model L591 Hammamatsu Corporation. 4
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We, therefore, have evaluated the homogeneity of the electrodes and of the crystal by measuring the resistance along several positions of the electrodes and for several electrodes. The film is structurally highly non-homogenous due to its polycrystalline nature. The dimensions of the grains and their shapes may vary and influence the response, influencing the electrical characteristics in the three spatial directions. To estimate the expected noise in input to the associated readout electronics, we have investigated how the values of the capacitance depend on the crystal homogeneity. We have measured the leakage current, which was found to be dependent on the sample as expected. To have an indication of a non-reproducible behaviour, we have measured the IV curves, even at very high field. All these tests were performed at room temperature, without irradiation. 4.1. Results on electrical measurements The measurements on resistance have already been discussed elsewhere [5] as well as the ones on the capacitance, which amounted on average to about 1 pF/cm, as in other semiconductor photon detectors. In Fig. 2, we report a summary of the measurements for three samples on the resistance. It is clearly seen that the samples vary among each other and that the quality of the crystal in terms of its homogeneity also reflects the metallization quality. Resistance values of few ohms were measured in the external part of both electrodes, indicating a good quality of metallization, while at the fingers of the electrodes (also for points far apart not more than few tens of micrometers) the metallization was interrupted in several points. Fig. 3 illustrates the IV curves. The voltage applied between two adjacent electrodes, is run in a very broad interval, to observe the reliability of the sample, at its highest efficiency. Up to a bias voltage of few tenths of volts the current stays in the order of 0.5 mA (3 mA/cm2 for 2.5 kV/cm), which is quite a high leakage current. Furthermore, increasing the voltage up to 900 V, we observe that it saturates only at very high field (45 kV/cm). This indicates that the device may not be fully efficient
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L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
Fig. 2. Graph of the resistance measurements. Sample A, top; sample B, middle; sample C, bottom.
if very high fields are not applied. This is the only way to evaluate the efficiency, in this detector. Alternative measurements on the depletion voltage cannot be performed here as there are no
junctions, so it is not reverse polarized and therefore the depletion bias value cannot be extracted from the CV curve. Quantitative measurements on the efficiency should be done by
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Fig. 3. Curves of the current without irradiation as a function of voltage. The insets show, respectively, the hysteresis effect (top left) and the low voltage current (bottom right)
measuring quantum efficiency with calibrated diodes and sources. We have judged that this is not necessary as the films showed an irreproducible behaviour of the current measurements. The values of the currents were found to depend not only on the applied voltage values but also on the order, the values were set. The curve shows a sort of hysteresis. At values between 550 V and 750 V, the current measured when increasing the voltage, is about a factor of 5 less when decreasing it, for the same values of the bias. This is related to the charge up or memory or priming effect discussed in the next section.
5. UV irradiation measurements The UV irradiation tests were performed on the three samples. The applied bias voltage was set to 180 V between the main transversal electrodes. Three measurements were performed for each sample, with the light spot incident at the centre
of each sample and the probe tips put at three different positions of the external part of the electrodes to verify if there was any dependence on the homogeneity and we found that there was none. The tests to be done had to verify that the sample was sensitive to UV irradiation, by measuring a photo-current above the dark current; how different was the photo-current with respect to the dark current (i.e. the gain factor); how sharp was the raise in the photo-current curve with respect to the leakage current and at what wavelength, under irradiation, the current equaled the dark current. This allowed to evaluate how blind the detector is to visible light or higher wavelength. Fig. 4 illustrates the response curves for the three samples in the interval 190–310 nm. For each sample, the current decreases rapidly above 230 nm. The non-null response above the 227 nm band-gap limit is caused by internal trapping centres and also surface defects. This
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Fig. 4. Curves of the current under irradiation for samples A–C, as a function of the wavelength.
manifests as another aspect of the charge up or memory or priming effect which we have studied extensively. We have irradiated the detector at different wavelengths, at intervals of 10 nm. The detector was kept at dark and the light was open and closed regularly at each wavelength and at fixed time intervals, making use of the shutter of the lamp, without opening the insulation box. In this way, we could prevent the contamination of the environmental light on the memory effect. For each measurement the irradiation time was the same, equal to 2000 s. The dark time was the same as well, equal to 500 s. The curves obtained, shown in Fig. 5, indicate that there is a clear dependence on the time and on the history of the irradiation, and that these effects vary from one sample to another and as a function of wavelength. Details of one of these curves are shown in Fig. 6. A relatively sudden rise of the current is observed as soon as the photons hit the diamond sample. The current continues to rise for several tenths of seconds and it then reaches saturation, or a level nearly constant. This again depends on the wavelength and on the sample and on its history. At the end of the exposure, the dark current
decreases slowly, reaching a value higher than before exposure. It reaches a level comparable to the dark current prior to irradiation only after hundreds of seconds. The tail is not a simple exponential, probably due to contributions of different types of defects acting as trapping sites. The long tail after exposure is caused by traps with long lifetime. Only when these traps have released the charge, the dark current acquires its initial value. It is therefore necessary to wait for a relatively longtime in order to make sure that the detector is operating again in the same manner as before illumination and that another irradiation can take place, without affecting its response. To quantify how the current is influenced by the time of operation, we have calculated the rise time and fall time for each value of wavelength. They are defined as (respectively) the time needed for reaching 90% (called rise time) of the constant value (or in several case, the highest value, not exactly constant) under irradiation and the time needed to reach the same value of the current as before irradiation with a difference of 10% (called fall time). We find a clear dependence of these time intervals on the wavelength, which so far has not been reported. Figs. 7 and 8 illustrate these
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Fig. 5. Curves of the current under irradiation, at different wavelengths. The values in the vertical axis are scaled for the sake of clarity: the highest values curve is for sample A; second highest, sample B; sample C is the lowest values curve.
Fig. 6. Curve of the current under irradiation versus time. The curve qualitatively illustrates what is shown on previous picture, on the time needed for the detector to reach a steady current value, both before and after irradiation.
phenomena. The rise time decreases as the wavelength increases, while it is the opposite for the fall time. The plots of Fig. 5 are taken following a time order. The curves at higher wavelengths were
taken after those at lower wavelengths. In principle, this should not influence the response, if the fall time was long enough to let the sensor reach a steady value of the current. However, there is a clear dependence on the fact that the film has been
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Fig. 7. The rise time of the current under irradiation at different wavelengths. It is defined as the time needed by the sensor to reach 90% of the maximum value of the current.
Fig. 8. The fall time of the current under irradiation at different wavelengths. It is defined as the time needed for the sensor to reach 10% more of the minimum value of the current.
irradiated. This is manifested by the fact that at longer wavelengths, so at later times, the current constant value appears faster and easier to reach; and Vice versa if the fall time is longer, indicating that the current measured is only indirectly related
to the irradiation. It is as if more and more centres are occupied, as the time goes, so the free charges liberated by the photons are no longer trapped in the defect sites. Since more and more sites are filled up, the longer will be the decay time, as these
L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
charges are left free in the film. The time needed for trapping depends on the history, namely, how much was the photo-current before or after irradiation. When the photo-current was higher, the fall time is lower and vice versa. There is a turning point at the value around the gap (slightly lower than the theoretical value, as it is not pure), in which the opposite occurs. The mechanism underlying the behaviour is the same. Due to the traps left free, the time for trapping and relaxation is long; vice versa, when all the gap is available, the charges left free are less, so the decay time is less.
6. Discussion The possibility of using commercially available synthetic diamond, CVD grown, as imaging detector in the UV, is questioned by the behaviour observed. Several results of the measurements indicate this fact. Generally, we can say that the behaviour depends on the sample. This leads to conclusion that there is a limited reliability on the market production. More specifically, there are quantities affecting the behaviour as imaging sensors. They are re-discussed here. The leakage current is high, up to about 1 mA, even at very low fields. This may introduce high noise in a readout amplifier. Furthermore, the leakage current depends on the history on how the film has been biased, by increasing or decreasing the voltage. This is related to the priming effect discussed below. The resistance on the contact indicates that there are zones, not easily optically identifiable, where the metallization is poor or definitely interrupted. This is probably due to the nonhomogeneous crystal, which does not allow the regular deposit of the metals alloy. A sensitive single strip detector cannot yet be made reliably. The behaviour observed on the bias history is similar to the one under irradiation. We have observed it more dramatically on the tests with UV source. The simple biasing of the crystal makes up free charge [6]. When the bias voltage increases then the amount of these charges increases. When
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the voltage is decreased again to certain values, then these charges are found free and they contribute to the leakage current. They may be trapped again but this may take long time, as discussed below. A possible approach would be to switch off the bias to recover, then switch it on again during illumination. However, this would give a variation in the response. The number and the appearance at certain voltages of the free charges, depend on the type of defects and their amount. So far, there are hundreds of trapping contaminant centres which by analysis have been found in CVD diamond films [7,8]. They vary in numbers and type, so the behaviour is hardly predictable since their appearance depends on the energy they acquire and therefore on the field. There are no specs about the preferred biasing for commercial films. In order to disentangle the effect of the field from the one due to irradiation, we have biased the film at relatively low voltage which for our samples guaranteed a non-hysteresis behaviour of the current. All this goes generally also under the name of steady current, memory or charge up. Strictly speaking, this leads to the priming behaviour. Even at low field the effect appears again. The energy jump to conduction is now given by the photons creating free charges. In comparison with silicon irradiated in UV, assuming that the behaviour may be similar, the amount of free charge should be exactly in a ratio about one (without taking into account other impact effects) to the number of photons. In the case of CVD diamond films, the amount of charge liberated is of course proportional to the energy and number of photons but not all the trapping centres are liberated in short time following the photon impact. There are long travelling and relaxing times of the order of tenths of seconds to liberate and again trap the free charges. Their nature being different according to the contaminant materials in the bulk, this effect depends on the energy. This is demonstrated by our measurements on the photo-current as a function of wavelength and time (Figs. 7 and 8). Therefore, the proportionality, between number of photons and the current is smeared by the current left free after irradiation. As a consequence, the quantum
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efficiency is a quantity difficult to define, even more to measure. It would be possible to use the film as imaging detector, only if the time needed to reach a steady state or the time needed to recover after irradiation is not too long (of the order of seconds at most). Actually, this should match the readout time. For application in slow devices for imaging, it indicates a potential working frequency (1 Hz or less) lower than that of conventional imaging systems based on CRT technologies. Furthermore, the response may be fast at certain energies and slow at others. All this doesn’t make an imaging device based on CVD synthetic diamond films, appealing. Recent studies [9–11] show that special treatment of the surface with a methane-based mixture, allows to lower the fall time by several orders of magnitude. The process corresponds to a kind of passivation of the surface, filling up the trapping centres, prior to irradiation. How this can control the rise time and, most important, the fall time, by tuning the mixture composition, is to be verified on films available in the market. Furthermore, the priming effect is not avoided in this case but may be suppressed for detectors for which most of the radiation detection is at the surface. This is particularly appealing for UV detectors. It should be stressed that even recent studies on CVD films for higher energy radiation studies [12] like minimum ionising particles or irradiation tests at the fluxes expected in high-energy physics experiments, manifest the priming effect. A solution for that is presented as prior to charging up of the diamond, by making all the traps occupied, then using it as such. This is very much similar to the passivation, instead of being performed at the surface alone, it is done on the bulk and on the surface. We have investigated this possibility also for our film but instead of irradiating it, as this is not practical for an imaging device, we have tried this with room light. We have found that the room light either did not influence the film at all (for good films), or made it saturated (for bad films) so that it became blind to any other radiation. On the other hand this is one of the reasons for using diamond in the UV energy interval. All these studies indicate that the priming effect is still
present even with high-energy particles. We have demonstrated that in order to have the film as an efficient detector, one has to reach a high field in which the current shows the hysteresis effect. But the hysteresis prevents any use of the film in any normal set up in which the bias voltage has to be changed. Furthermore, at present, studies are lacking on the time needed to recover after the film has been pumped or charged up. Studies similar to the ones presented here are advisable also for High-Energy Physics devices.
7. Conclusions We claim that the CVD synthetic diamond films, currently available in the market do not satisfy the requirements for an UV imaging detector. Moreover, we believe that, irrespective of older findings, the CVD films cannot be used as radiation sensors in normal experimental conditions in which there cannot be a suitable control on the history of irradiation and bias. The future appears bright as we believe that recent developments on surface treatment will shorten the duration of the priming effect even though it cannot be completely eliminated.
Acknowledgements Gratitude is expressed to all the people who have contributed during several years to our knowledge in the field. Among them, in random order we recall, Manfred Krammer, Alex Howard, Kevin Gray, Ricardo Sussmann, Rache Chechik, Brian Stoner, Werner Haenni.
References [1] A. Ito et al., Photochem. Photobiol. 44 (3) (1986) 355. [2] D.R. Kania, Proceedings of the International School E.Fermi Course CXXXV, The Physics of Diamonds, IOS Press, Amsterdam, 1997, pp. 555–573. [3] P. Gonon et al., Diamond Related Mater. 6 (1997) 860. [4] A. Mainwood et al., Diamond Related Mater. 7 (1998) 504.
L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137 [5] L. Barberini et al., Nucl. Instr. and Meth. A 442 (2000) 400. [6] C.E. Nebel et al., Diamond Related Mater. 7 (1998) 556. [7] P. Brovetto, University of Cagliari, private communications.
[8] [9] [10] [11] [12]
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De Beers, UK, private communications. M. Whitfield et al., Diamond Related Mater. 5 (1996) 829. R. McKeag et al., Diamond Related Mater. 7 (1998) 513. H.J. Looi et al., Diamond Related Mater. 7 (1998) 550. T. Behnke et al., Nucl. Instr. and Meth. A 414 (1998) 340.
A new material for imaging in the UV: CVD Diamondq L. Barberini, S. Cadeddu, M. Caria* Physics Department, University of Cagliari, 09042 Monserrato (Cagliari), Italy
Abstract We critically discuss the possibility of using diamond films, grown with the Chemical Vapour Deposition (CVD) technique, as imaging detector in the extreme UV energy range. We present results on electrical tests and on irradiation studies, under UV source, of CVD films, bought from market. We show that the behaviour of the film under irradiation in the energy interval of 190–350 nm would prevent its use as imaging detector, if special measures on the deposition qualities are not taken. We discuss the mechanisms of the behaviour under irradiation, in terms of the crystal defects. We have extensively studied the charge-up effect of the film and the influence on the detection efficiency. We find a dependence on the irradiation time and methods. We can address the explanation of the behaviour in terms of the decay time of the traps. This leads to an important conclusion on the homogeneity and on the defect sites. We claim that this effect depends on the defect types and it is rarely reproducible. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamonds; CVD diamond; UV; Photodetectors; Electrical properties
1. Introduction Imaging in UV is of great importance for a very wide range of applications. As an example, researchers in the fields of biology investigate the bio-molecular compounds in the region of the energy spectrum they absorb the most. Their basic constituents always contains ATP which absorbs most in the interval 190–260 nm [1]. The material which best suits imaging in UV is diamond. Its gap is 5.47 eV corresponding to l=227 nm. The advantage of using diamond in this interval is that, for a perfect material, there should be a sharp cut off, at wavelengths below and above this value. This would allow us to
q
Presented by Mario Caria. *Corresponding author. E-mail address: [email protected] (M. Caria).
operate a UV imaging detector under room-light illumination or close to heating source, without deteriorated performance from infrared irradiation. The non-availability of pure diamond material in nature limits its use as a commercial imaging detector. We have investigated the possibility of using what is available on the shelf as synthetic diamond films grown with CVD techniques. Until now, these are the most studied [2] as radiation detectors and they are available in the market. In this paper, we report an investigation on the feasibility of using imaging detector based on CVD films. We have studied, in detail this behaviour under realistic conditions of irradiation. We show that the composition of the material questions, the use of CVD diamond as UV imaging detector. We discuss the causes and special measures needed to suppress the effects,
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 1 0 7 - 4
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L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
Fig. 1. Sketch of the sample and reference mapping of the positions of the metal contacts for the measurements.
which in any case still prevent us from using it as UV detector. In the first part, we report results on basic electrical characterisation tests on metal homogeneity, resistance, IV and CV measurements and their reproducibility from one sample to another. In the second part, we show results on irradiation with a deuterium lamp and mono-chromator wavelength selector, as a function of irradiation time and history. The so-called priming1 effect is observed and for the first time its dependence on the energy is reported. We conclude reviewing the steps needed to have a commercial CVD film for UV imaging.
2. CVD diamond samples
1 Also called with some confusion pumping or hysteresis or memory or charging-up effect.
2 By Norton, Goddard Road, Northboro, MA 01532–1545, Tel.: +1-508-351-7733.
Although there are many methods for producing synthetic diamonds, the choice for Chemical Vapour Deposition (CVD) [3,4] films was unavoidable as these are the ones available in the market as radiation sensors. We have purchased2 three samples of CVD film detectors with the characteristics indicated in Fig. 1. They are 1cm2 films with a thickness of 150–250 mm. On the growth face, a metallization, with the geometry indicated in Fig. 1, formed by three metals (Ti (0.1 mm)/Pt (0.2 mm)/Au (1 mm)), was done. The principle of detection for diamond films sensors is much simpler than for other semiconductor detectors. There is no need for
L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
implantations, like the more common silicon photodiodes, which could make the sensor easy to build and potentially cheaper. Due to the low energy, the UV photons are absorbed in the diamond on the metallization side. They create an electron–hole pair, which by applying an electric field between the electrodes is collected from the two different electrodes on the same surface. It is, therefore, mandatory to study the quality of the electrodes and the crystal surface. This is described in the next sections.
3. Experimental set-up The measurements were performed with a probe station inside a metallic box in order to isolate the detector from external electromagnetic fields and from the possible influence of environmental light. The measurements on the resistance and the current were performed with a picoamperometer, with few tenths of pA accuracy3. The measurements of capacitance were performed at 100 kHz, with an accuracy of 10 fF4. The optical system consists of an ultraviolet source5, a monochromator (model H10 Jobin Yvon) and a bundle of optical fibres. The set-up was used for an illumination in the interval 190–310 nm with 2 nm accuracy while the UV source provides a continuous spectrum from 185 nm to about 400 nm. The exit slit of the monochromator is replaced by the entrance of the bundle of optical fibres. The bundle is 1 m long, it has a rectangular entry terminal of 4.3 mm0.2 mm and a circular exit terminal of diameter 1.1 mm.
4. Electrical measurements The imaging detector has to be reliable, homogeneous, with low electric noise, highly efficient and, possibly, economic and easy to build. 3
Keithley 2400 and Keithley 237 for IV curves at high field. Keithley CV590. 5 30W D2 model L591 Hammamatsu Corporation. 4
129
We, therefore, have evaluated the homogeneity of the electrodes and of the crystal by measuring the resistance along several positions of the electrodes and for several electrodes. The film is structurally highly non-homogenous due to its polycrystalline nature. The dimensions of the grains and their shapes may vary and influence the response, influencing the electrical characteristics in the three spatial directions. To estimate the expected noise in input to the associated readout electronics, we have investigated how the values of the capacitance depend on the crystal homogeneity. We have measured the leakage current, which was found to be dependent on the sample as expected. To have an indication of a non-reproducible behaviour, we have measured the IV curves, even at very high field. All these tests were performed at room temperature, without irradiation. 4.1. Results on electrical measurements The measurements on resistance have already been discussed elsewhere [5] as well as the ones on the capacitance, which amounted on average to about 1 pF/cm, as in other semiconductor photon detectors. In Fig. 2, we report a summary of the measurements for three samples on the resistance. It is clearly seen that the samples vary among each other and that the quality of the crystal in terms of its homogeneity also reflects the metallization quality. Resistance values of few ohms were measured in the external part of both electrodes, indicating a good quality of metallization, while at the fingers of the electrodes (also for points far apart not more than few tens of micrometers) the metallization was interrupted in several points. Fig. 3 illustrates the IV curves. The voltage applied between two adjacent electrodes, is run in a very broad interval, to observe the reliability of the sample, at its highest efficiency. Up to a bias voltage of few tenths of volts the current stays in the order of 0.5 mA (3 mA/cm2 for 2.5 kV/cm), which is quite a high leakage current. Furthermore, increasing the voltage up to 900 V, we observe that it saturates only at very high field (45 kV/cm). This indicates that the device may not be fully efficient
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L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
Fig. 2. Graph of the resistance measurements. Sample A, top; sample B, middle; sample C, bottom.
if very high fields are not applied. This is the only way to evaluate the efficiency, in this detector. Alternative measurements on the depletion voltage cannot be performed here as there are no
junctions, so it is not reverse polarized and therefore the depletion bias value cannot be extracted from the CV curve. Quantitative measurements on the efficiency should be done by
L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
131
Fig. 3. Curves of the current without irradiation as a function of voltage. The insets show, respectively, the hysteresis effect (top left) and the low voltage current (bottom right)
measuring quantum efficiency with calibrated diodes and sources. We have judged that this is not necessary as the films showed an irreproducible behaviour of the current measurements. The values of the currents were found to depend not only on the applied voltage values but also on the order, the values were set. The curve shows a sort of hysteresis. At values between 550 V and 750 V, the current measured when increasing the voltage, is about a factor of 5 less when decreasing it, for the same values of the bias. This is related to the charge up or memory or priming effect discussed in the next section.
5. UV irradiation measurements The UV irradiation tests were performed on the three samples. The applied bias voltage was set to 180 V between the main transversal electrodes. Three measurements were performed for each sample, with the light spot incident at the centre
of each sample and the probe tips put at three different positions of the external part of the electrodes to verify if there was any dependence on the homogeneity and we found that there was none. The tests to be done had to verify that the sample was sensitive to UV irradiation, by measuring a photo-current above the dark current; how different was the photo-current with respect to the dark current (i.e. the gain factor); how sharp was the raise in the photo-current curve with respect to the leakage current and at what wavelength, under irradiation, the current equaled the dark current. This allowed to evaluate how blind the detector is to visible light or higher wavelength. Fig. 4 illustrates the response curves for the three samples in the interval 190–310 nm. For each sample, the current decreases rapidly above 230 nm. The non-null response above the 227 nm band-gap limit is caused by internal trapping centres and also surface defects. This
132
L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137
Fig. 4. Curves of the current under irradiation for samples A–C, as a function of the wavelength.
manifests as another aspect of the charge up or memory or priming effect which we have studied extensively. We have irradiated the detector at different wavelengths, at intervals of 10 nm. The detector was kept at dark and the light was open and closed regularly at each wavelength and at fixed time intervals, making use of the shutter of the lamp, without opening the insulation box. In this way, we could prevent the contamination of the environmental light on the memory effect. For each measurement the irradiation time was the same, equal to 2000 s. The dark time was the same as well, equal to 500 s. The curves obtained, shown in Fig. 5, indicate that there is a clear dependence on the time and on the history of the irradiation, and that these effects vary from one sample to another and as a function of wavelength. Details of one of these curves are shown in Fig. 6. A relatively sudden rise of the current is observed as soon as the photons hit the diamond sample. The current continues to rise for several tenths of seconds and it then reaches saturation, or a level nearly constant. This again depends on the wavelength and on the sample and on its history. At the end of the exposure, the dark current
decreases slowly, reaching a value higher than before exposure. It reaches a level comparable to the dark current prior to irradiation only after hundreds of seconds. The tail is not a simple exponential, probably due to contributions of different types of defects acting as trapping sites. The long tail after exposure is caused by traps with long lifetime. Only when these traps have released the charge, the dark current acquires its initial value. It is therefore necessary to wait for a relatively longtime in order to make sure that the detector is operating again in the same manner as before illumination and that another irradiation can take place, without affecting its response. To quantify how the current is influenced by the time of operation, we have calculated the rise time and fall time for each value of wavelength. They are defined as (respectively) the time needed for reaching 90% (called rise time) of the constant value (or in several case, the highest value, not exactly constant) under irradiation and the time needed to reach the same value of the current as before irradiation with a difference of 10% (called fall time). We find a clear dependence of these time intervals on the wavelength, which so far has not been reported. Figs. 7 and 8 illustrate these
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Fig. 5. Curves of the current under irradiation, at different wavelengths. The values in the vertical axis are scaled for the sake of clarity: the highest values curve is for sample A; second highest, sample B; sample C is the lowest values curve.
Fig. 6. Curve of the current under irradiation versus time. The curve qualitatively illustrates what is shown on previous picture, on the time needed for the detector to reach a steady current value, both before and after irradiation.
phenomena. The rise time decreases as the wavelength increases, while it is the opposite for the fall time. The plots of Fig. 5 are taken following a time order. The curves at higher wavelengths were
taken after those at lower wavelengths. In principle, this should not influence the response, if the fall time was long enough to let the sensor reach a steady value of the current. However, there is a clear dependence on the fact that the film has been
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Fig. 7. The rise time of the current under irradiation at different wavelengths. It is defined as the time needed by the sensor to reach 90% of the maximum value of the current.
Fig. 8. The fall time of the current under irradiation at different wavelengths. It is defined as the time needed for the sensor to reach 10% more of the minimum value of the current.
irradiated. This is manifested by the fact that at longer wavelengths, so at later times, the current constant value appears faster and easier to reach; and Vice versa if the fall time is longer, indicating that the current measured is only indirectly related
to the irradiation. It is as if more and more centres are occupied, as the time goes, so the free charges liberated by the photons are no longer trapped in the defect sites. Since more and more sites are filled up, the longer will be the decay time, as these
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charges are left free in the film. The time needed for trapping depends on the history, namely, how much was the photo-current before or after irradiation. When the photo-current was higher, the fall time is lower and vice versa. There is a turning point at the value around the gap (slightly lower than the theoretical value, as it is not pure), in which the opposite occurs. The mechanism underlying the behaviour is the same. Due to the traps left free, the time for trapping and relaxation is long; vice versa, when all the gap is available, the charges left free are less, so the decay time is less.
6. Discussion The possibility of using commercially available synthetic diamond, CVD grown, as imaging detector in the UV, is questioned by the behaviour observed. Several results of the measurements indicate this fact. Generally, we can say that the behaviour depends on the sample. This leads to conclusion that there is a limited reliability on the market production. More specifically, there are quantities affecting the behaviour as imaging sensors. They are re-discussed here. The leakage current is high, up to about 1 mA, even at very low fields. This may introduce high noise in a readout amplifier. Furthermore, the leakage current depends on the history on how the film has been biased, by increasing or decreasing the voltage. This is related to the priming effect discussed below. The resistance on the contact indicates that there are zones, not easily optically identifiable, where the metallization is poor or definitely interrupted. This is probably due to the nonhomogeneous crystal, which does not allow the regular deposit of the metals alloy. A sensitive single strip detector cannot yet be made reliably. The behaviour observed on the bias history is similar to the one under irradiation. We have observed it more dramatically on the tests with UV source. The simple biasing of the crystal makes up free charge [6]. When the bias voltage increases then the amount of these charges increases. When
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the voltage is decreased again to certain values, then these charges are found free and they contribute to the leakage current. They may be trapped again but this may take long time, as discussed below. A possible approach would be to switch off the bias to recover, then switch it on again during illumination. However, this would give a variation in the response. The number and the appearance at certain voltages of the free charges, depend on the type of defects and their amount. So far, there are hundreds of trapping contaminant centres which by analysis have been found in CVD diamond films [7,8]. They vary in numbers and type, so the behaviour is hardly predictable since their appearance depends on the energy they acquire and therefore on the field. There are no specs about the preferred biasing for commercial films. In order to disentangle the effect of the field from the one due to irradiation, we have biased the film at relatively low voltage which for our samples guaranteed a non-hysteresis behaviour of the current. All this goes generally also under the name of steady current, memory or charge up. Strictly speaking, this leads to the priming behaviour. Even at low field the effect appears again. The energy jump to conduction is now given by the photons creating free charges. In comparison with silicon irradiated in UV, assuming that the behaviour may be similar, the amount of free charge should be exactly in a ratio about one (without taking into account other impact effects) to the number of photons. In the case of CVD diamond films, the amount of charge liberated is of course proportional to the energy and number of photons but not all the trapping centres are liberated in short time following the photon impact. There are long travelling and relaxing times of the order of tenths of seconds to liberate and again trap the free charges. Their nature being different according to the contaminant materials in the bulk, this effect depends on the energy. This is demonstrated by our measurements on the photo-current as a function of wavelength and time (Figs. 7 and 8). Therefore, the proportionality, between number of photons and the current is smeared by the current left free after irradiation. As a consequence, the quantum
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efficiency is a quantity difficult to define, even more to measure. It would be possible to use the film as imaging detector, only if the time needed to reach a steady state or the time needed to recover after irradiation is not too long (of the order of seconds at most). Actually, this should match the readout time. For application in slow devices for imaging, it indicates a potential working frequency (1 Hz or less) lower than that of conventional imaging systems based on CRT technologies. Furthermore, the response may be fast at certain energies and slow at others. All this doesn’t make an imaging device based on CVD synthetic diamond films, appealing. Recent studies [9–11] show that special treatment of the surface with a methane-based mixture, allows to lower the fall time by several orders of magnitude. The process corresponds to a kind of passivation of the surface, filling up the trapping centres, prior to irradiation. How this can control the rise time and, most important, the fall time, by tuning the mixture composition, is to be verified on films available in the market. Furthermore, the priming effect is not avoided in this case but may be suppressed for detectors for which most of the radiation detection is at the surface. This is particularly appealing for UV detectors. It should be stressed that even recent studies on CVD films for higher energy radiation studies [12] like minimum ionising particles or irradiation tests at the fluxes expected in high-energy physics experiments, manifest the priming effect. A solution for that is presented as prior to charging up of the diamond, by making all the traps occupied, then using it as such. This is very much similar to the passivation, instead of being performed at the surface alone, it is done on the bulk and on the surface. We have investigated this possibility also for our film but instead of irradiating it, as this is not practical for an imaging device, we have tried this with room light. We have found that the room light either did not influence the film at all (for good films), or made it saturated (for bad films) so that it became blind to any other radiation. On the other hand this is one of the reasons for using diamond in the UV energy interval. All these studies indicate that the priming effect is still
present even with high-energy particles. We have demonstrated that in order to have the film as an efficient detector, one has to reach a high field in which the current shows the hysteresis effect. But the hysteresis prevents any use of the film in any normal set up in which the bias voltage has to be changed. Furthermore, at present, studies are lacking on the time needed to recover after the film has been pumped or charged up. Studies similar to the ones presented here are advisable also for High-Energy Physics devices.
7. Conclusions We claim that the CVD synthetic diamond films, currently available in the market do not satisfy the requirements for an UV imaging detector. Moreover, we believe that, irrespective of older findings, the CVD films cannot be used as radiation sensors in normal experimental conditions in which there cannot be a suitable control on the history of irradiation and bias. The future appears bright as we believe that recent developments on surface treatment will shorten the duration of the priming effect even though it cannot be completely eliminated.
Acknowledgements Gratitude is expressed to all the people who have contributed during several years to our knowledge in the field. Among them, in random order we recall, Manfred Krammer, Alex Howard, Kevin Gray, Ricardo Sussmann, Rache Chechik, Brian Stoner, Werner Haenni.
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De Beers, UK, private communications. M. Whitfield et al., Diamond Related Mater. 5 (1996) 829. R. McKeag et al., Diamond Related Mater. 7 (1998) 513. H.J. Looi et al., Diamond Related Mater. 7 (1998) 550. T. Behnke et al., Nucl. Instr. and Meth. A 414 (1998) 340.