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 irradiationstudies, under UV source, of CVD films, bought from market. We show that the behaviour of the film under irradiationin the energy interval of 190–350 nm would prevent its use as imaging detector, if special measures on the depositionqualities 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 adependence on the irradiation time and methods. We can address the explanation of the behaviour in terms of the decaytime 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 avery wide range of applications. As an example,researchers in the fields of biology investigate thebio-molecular compounds in the region of theenergy spectrum they absorb the most. Their basicconstituents always contains ATP which absorbsmost in the interval 190–260 nm [1].The material which best suits imaging in UV is

diamond. Its gap is 5.47 eV corresponding tol=227 nm. The advantage of using diamond inthis interval is that, for a perfect material, thereshould be a sharp cut off, at wavelengths belowand above this value. This would allow us to

operate a UV imaging detector under room-lightillumination or close to heating source, withoutdeteriorated performance from infrared irradia-tion.The non-availability of pure diamond material

in nature limits its use as a commercial imagingdetector. We have investigated the possibilityof using what is available on the shelf as syntheticdiamond films grown with CVD techniques.Until now, these are the most studied [2] asradiation detectors and they are available in themarket.In this paper, we report an investigation on the

feasibility of using imaging detector based onCVD films. We have studied, in detail thisbehaviour under realistic conditions of irradiation.We show that the composition of the materialquestions, the use of CVD diamond as UVimaging detector. We discuss the causes andspecial measures needed to suppress the effects,

qPresented by Mario Caria.

*Corresponding author.

E-mail address: caria@ca.infn.it (M. Caria).

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

which in any case still prevent us from using it asUV detector.In the first part, we report results on basic

electrical characterisation tests on metal homo-geneity, resistance, IV and CV measurements andtheir reproducibility from one sample to another.In the second part, we show results on irradia-

tion with a deuterium lamp and mono-chromatorwavelength selector, as a function of irradiationtime and history. The so-called priming1 effect isobserved and for the first time its dependence onthe energy is reported.We conclude reviewing the steps needed to have

a commercial CVD film for UV imaging.

2. CVD diamond samples

Although there are many methods for produ-cing synthetic diamonds, the choice for ChemicalVapour Deposition (CVD) [3,4] films was un-avoidable as these are the ones available in themarket as radiation sensors. We have purchased2

three samples of CVD film detectors with thecharacteristics indicated in Fig. 1. They are 1cm2

films with a thickness of 150–250 mm. On thegrowth face, a metallization, with the geometryindicated 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 othersemiconductor detectors. There is no need for

Fig. 1. Sketch of the sample and reference mapping of the positions of the metal contacts for the measurements.

1Also called with some confusion pumping or hysteresis or

memory or charging-up effect.

2By Norton, Goddard Road, Northboro, MA 01532–1545,

Tel.: +1-508-351-7733.

L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137128

implantations, like the more common siliconphotodiodes, which could make the sensor easyto build and potentially cheaper. Due to the lowenergy, the UV photons are absorbed in thediamond on the metallization side. They createan electron–hole pair, which by applying anelectric field between the electrodes is collectedfrom the two different electrodes on the samesurface.It is, therefore, mandatory to study the quality

of the electrodes and the crystal surface. This isdescribed in the next sections.

3. Experimental set-up

The measurements were performed with a probestation inside a metallic box in order to isolate thedetector from external electromagnetic fields andfrom 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 measure-ments 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 JobinYvon) and a bundle of optical fibres. The set-upwas used for an illumination in the interval190–310 nm with 2 nm accuracy while the UVsource provides a continuous spectrum from185 nm to about 400 nm. The exit slit of themonochromator is replaced by the entrance ofthe bundle of optical fibres. The bundle is 1mlong, it has a rectangular entry terminal of4.3mm�0.2mm and a circular exit terminal ofdiameter 1.1mm.

4. Electrical measurements

The imaging detector has to be reliable, homo-geneous, with low electric noise, highly efficientand, possibly, economic and easy to build.

We, therefore, have evaluated the homogeneityof the electrodes and of the crystal by measuringthe resistance along several positions of theelectrodes and for several electrodes. The film isstructurally highly non-homogenous due to itspolycrystalline nature. The dimensions of thegrains and their shapes may vary and influencethe response, influencing the electrical character-istics in the three spatial directions.To estimate the expected noise in input to the

associated readout electronics, we have investi-gated how the values of the capacitance depend onthe crystal homogeneity. We have measured theleakage current, which was found to be dependenton the sample as expected. To have an indicationof a non-reproducible behaviour, we have mea-sured the IV curves, even at very high field.All these tests were performed at room tem-

perature, without irradiation.

4.1. Results on electrical measurements

The measurements on resistance have alreadybeen discussed elsewhere [5] as well as the ones onthe capacitance, which amounted on average toabout 1 pF/cm, as in other semiconductor photondetectors.In Fig. 2, we report a summary of the measure-

ments for three samples on the resistance. It isclearly seen that the samples vary among eachother and that the quality of the crystal in terms ofits homogeneity also reflects the metallizationquality. Resistance values of few ohms weremeasured in the external part of both electrodes,indicating a good quality of metallization, while atthe fingers of the electrodes (also for points farapart not more than few tens of micrometers) themetallization was interrupted in several points.Fig. 3 illustrates the IV curves. The voltage

applied between two adjacent electrodes, is run ina very broad interval, to observe the reliability ofthe sample, at its highest efficiency. Up to a biasvoltage of few tenths of volts the current stays inthe 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 900V, we observe thatit saturates only at very high field (45 kV/cm). Thisindicates that the device may not be fully efficient

3Keithley 2400 and Keithley 237 for IV curves at high field.4Keithley CV590.530W D2 model L591 Hammamatsu Corporation.

L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137 129

if very high fields are not applied. This is the onlyway to evaluate the efficiency, in this detector.Alternative measurements on the depletion voltagecannot be performed here as there are no

junctions, so it is not reverse polarized andtherefore the depletion bias value cannot beextracted from the CV curve. Quantitative mea-surements on the efficiency should be done by

Fig. 2. Graph of the resistance measurements. Sample A, top; sample B, middle; sample C, bottom.

L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137130

measuring quantum efficiency with calibrateddiodes and sources. We have judged that this isnot necessary as the films showed an irreproduci-ble behaviour of the current measurements. Thevalues of the currents were found to depend notonly on the applied voltage values but also on theorder, the values were set. The curve shows a sortof hysteresis. At values between 550V and 750V,the current measured when increasing the voltage,is about a factor of 5 less when decreasing it, forthe same values of the bias. This is related to thecharge up or memory or priming effect discussed inthe next section.

5. UV irradiation measurements

The UV irradiation tests were performed on thethree samples. The applied bias voltage was set to180V between the main transversal electrodes.Three measurements were performed for eachsample, with the light spot incident at the centre

of each sample and the probe tips put at threedifferent positions of the external part of theelectrodes to verify if there was any dependence onthe homogeneity and we found that there wasnone.The tests to be done had to verify that the

sample was sensitive to UV irradiation, bymeasuring a photo-current above the dark current;how different was the photo-current with respectto the dark current (i.e. the gain factor); how sharpwas the raise in the photo-current curve withrespect to the leakage current and at whatwavelength, under irradiation, the current equaledthe dark current. This allowed to evaluate howblind the detector is to visible light or higherwavelength.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 abovethe 227 nm band-gap limit is caused by internaltrapping centres and also surface defects. This

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)

L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137 131

manifests as another aspect of the charge up ormemory or priming effect which we have studiedextensively.We have irradiated the detector at different

wavelengths, at intervals of 10 nm. The detectorwas kept at dark and the light was open and closedregularly at each wavelength and at fixed timeintervals, making use of the shutter of the lamp,without opening the insulation box. In this way,we could prevent the contamination of theenvironmental light on the memory effect. Foreach measurement the irradiation time was thesame, equal to 2000 s. The dark time was the sameas well, equal to 500 s. The curves obtained, shownin Fig. 5, indicate that there is a clear dependenceon the time and on the history of the irradiation,and that these effects vary from one sample toanother and as a function of wavelength. Detailsof 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 diamondsample. The current continues to rise for severaltenths of seconds and it then reaches saturation, ora level nearly constant. This again depends on thewavelength and on the sample and on its history.At the end of the exposure, the dark current

decreases slowly, reaching a value higher thanbefore exposure. It reaches a level comparable tothe dark current prior to irradiation only afterhundreds of seconds. The tail is not a simpleexponential, probably due to contributions ofdifferent types of defects acting as trapping sites.The long tail after exposure is caused by traps withlong lifetime. Only when these traps have releasedthe charge, the dark current acquires its initialvalue. It is therefore necessary to wait for arelatively longtime in order to make sure that thedetector is operating again in the same manner asbefore illumination and that another irradiationcan take place, without affecting its response.To quantify how the current is influenced by the

time of operation, we have calculated the rise timeand fall time for each value of wavelength. Theyare defined as (respectively) the time needed forreaching 90% (called rise time) of the constantvalue (or in several case, the highest value, notexactly constant) under irradiation and the timeneeded to reach the same value of the current asbefore irradiation with a difference of 10% (calledfall time). We find a clear dependence of thesetime intervals on the wavelength, which so far hasnot been reported. Figs. 7 and 8 illustrate these

Fig. 4. Curves of the current under irradiation for samples A–C, as a function of the wavelength.

L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137132

phenomena. The rise time decreases as thewavelength increases, while it is the opposite forthe 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 princi-ple, this should not influence the response, if thefall time was long enough to let the sensor reach asteady value of the current. However, there is aclear dependence on the fact that the film has been

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.

L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137 133

irradiated. This is manifested by the fact that atlonger wavelengths, so at later times, the currentconstant value appears faster and easier to reach;and Vice versa if the fall time is longer, indicatingthat the current measured is only indirectly related

to the irradiation. It is as if more and more centresare occupied, as the time goes, so the free chargesliberated by the photons are no longer trapped inthe defect sites. Since more and more sites are filledup, the longer will be the decay time, as these

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.

L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137134

charges are left free in the film. The time neededfor trapping depends on the history, namely, howmuch was the photo-current before or afterirradiation. When the photo-current was higher,the fall time is lower and vice versa. There is aturning point at the value around the gap (slightlylower than the theoretical value, as it is not pure),in which the opposite occurs. The mechanismunderlying the behaviour is the same. Due to thetraps left free, the time for trapping and relaxationis long; vice versa, when all the gap is available, thecharges left free are less, so the decay time is less.

6. Discussion

The possibility of using commercially availablesynthetic diamond, CVD grown, as imagingdetector in the UV, is questioned by the behaviourobserved. Several results of the measurementsindicate this fact.Generally, we can say that the behaviour

depends on the sample. This leads to conclusionthat there is a limited reliability on the marketproduction. More specifically, there are quantitiesaffecting the behaviour as imaging sensors. Theyare re-discussed here.The leakage current is high, up to about 1 mA,

even at very low fields. This may introduce highnoise in a readout amplifier. Furthermore, theleakage current depends on the history on how thefilm has been biased, by increasing or decreasingthe voltage. This is related to the priming effectdiscussed below.The resistance on the contact indicates that

there are zones, not easily optically identifiable,where the metallization is poor or definitelyinterrupted. This is probably due to the non-homogeneous crystal, which does not allowthe regular deposit of the metals alloy. A sensitivesingle strip detector cannot yet be madereliably.The behaviour observed on the bias history is

similar to the one under irradiation. We haveobserved it more dramatically on the tests with UVsource. The simple biasing of the crystal makes upfree charge [6]. When the bias voltage increasesthen the amount of these charges increases. When

the voltage is decreased again to certain values,then these charges are found free and theycontribute to the leakage current. They may betrapped again but this may take long time, asdiscussed below. A possible approach would be toswitch off the bias to recover, then switch it onagain during illumination. However, this wouldgive a variation in the response.The number and the appearance at certain

voltages of the free charges, depend on the typeof defects and their amount. So far, there arehundreds of trapping contaminant centres whichby analysis have been found in CVD diamondfilms [7,8]. They vary in numbers and type, so thebehaviour is hardly predictable since their appear-ance depends on the energy they acquire andtherefore on the field. There are no specs about thepreferred biasing for commercial films.In order to disentangle the effect of the field

from the one due to irradiation, we have biased thefilm at relatively low voltage which for our samplesguaranteed a non-hysteresis behaviour of thecurrent. All this goes generally also under thename of steady current, memory or charge up.Strictly speaking, this leads to the priming beha-viour. Even at low field the effect appears again.The energy jump to conduction is now given by thephotons creating free charges.In comparison with silicon irradiated in UV,

assuming that the behaviour may be similar, theamount of free charge should be exactly in a ratioabout one (without taking into account otherimpact effects) to the number of photons. In thecase of CVD diamond films, the amount of chargeliberated is of course proportional to the energyand number of photons but not all the trappingcentres are liberated in short time following thephoton impact. There are long travelling andrelaxing times of the order of tenths of seconds toliberate and again trap the free charges. Theirnature being different according to the contami-nant materials in the bulk, this effect depends onthe energy. This is demonstrated by our measure-ments on the photo-current as a function ofwavelength and time (Figs. 7 and 8). Therefore,the proportionality, between number of photonsand the current is smeared by the current left freeafter irradiation. As a consequence, the quantum

L. Barberini et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 127–137 135

efficiency is a quantity difficult to define, evenmore to measure.It would be possible to use the film as imaging

detector, only if the time needed to reach asteady state or the time needed to recover afterirradiation is not too long (of the order of secondsat most). Actually, this should match the readouttime. For application in slow devices for imaging,it indicates a potential working frequency (1Hz orless) lower than that of conventional imagingsystems based on CRT technologies. Furthermore,the response may be fast at certain energies andslow at others. All this doesn’t make an imagingdevice based on CVD synthetic diamond films,appealing.Recent studies [9–11] show that special treat-

ment of the surface with a methane-based mixture,allows to lower the fall time by several orders ofmagnitude. The process corresponds to a kind ofpassivation of the surface, filling up the trappingcentres, prior to irradiation. How this can controlthe rise time and, most important, the fall time, bytuning the mixture composition, is to be verifiedon films available in the market. Furthermore, thepriming effect is not avoided in this case but maybe suppressed for detectors for which most of theradiation detection is at the surface. This isparticularly 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 testsat the fluxes expected in high-energy physicsexperiments, manifest the priming effect. A solu-tion for that is presented as prior to charging up ofthe diamond, by making all the traps occupied,then using it as such. This is very much similar tothe passivation, instead of being performed at thesurface alone, it is done on the bulk and on thesurface. We have investigated this possibility alsofor our film but instead of irradiating it, as this isnot practical for an imaging device, we have triedthis with room light. We have found that the roomlight either did not influence the film at all (forgood films), or made it saturated (for bad films) sothat it became blind to any other radiation. On theother hand this is one of the reasons for usingdiamond in the UV energy interval. All thesestudies indicate that the priming effect is still

present even with high-energy particles. We havedemonstrated that in order to have the film as anefficient detector, one has to reach a high field inwhich the current shows the hysteresis effect. Butthe hysteresis prevents any use of the film in anynormal set up in which the bias voltage has to bechanged. Furthermore, at present, studies arelacking on the time needed to recover after thefilm has been pumped or charged up. Studiessimilar to the ones presented here are advisablealso for High-Energy Physics devices.

7. Conclusions

We claim that the CVD synthetic diamondfilms, currently available in the market do notsatisfy the requirements for an UV imagingdetector. Moreover, we believe that, irrespectiveof older findings, the CVD films cannot be used asradiation sensors in normal experimental condi-tions in which there cannot be a suitable controlon the history of irradiation and bias.The future appears bright as we believe that

recent developments on surface treatment willshorten the duration of the priming effect eventhough it cannot be completely eliminated.

Acknowledgements

Gratitude is expressed to all the people whohave contributed during several years to ourknowledge in the field. Among them, in randomorder we recall, Manfred Krammer, Alex Howard,Kevin Gray, Ricardo Sussmann, Rache Chechik,Brian Stoner, Werner Haenni.

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