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Particle and Radiation Detectors Based on Diamond
P. Bergonzo, D. Tromson, C. Mer, B. Guizard, F. Foulon, and A. Brambilla
LIST(CEA-Recherche Technologique)/DIMRI/SIAR, CEA/Saclay,F-91191 Gif-sur-Yvette, France
(Received January 16, 2001; accepted March 12, 2001)
Subject classification: 61.80.–x; 68.55.Ac; 73.50.Pz; S5
CVD diamond is a remarkable material for the fabrication of particle and photon radiation detec-tors. The improvement of the electronic properties of the material has been under intensive inves-tigations and led to the development of a few applications that are addressing specific industrialneeds. In particular, we have used diamond layers for industrial applications where it exhibits at-tractive characteristics as compared with other materials: e.g., radiation and corrosion hardness fora-counters or high gamma-meters at high fluxes; high transparency to low energy X-rays for syn-chrotron beam line monitoring devices, etc. These specific properties can motivate the use of dia-mond even though the detection properties remain relatively poor. Indeed, one inherent problemwith diamond is the presence of defect levels that are altering the detection characteristics. Theseare observed in all CVD materials but also in very high quality natural diamonds. They result inunstable responses and carrier losses. Also, it has been observed that high sensitivities may resultfrom the progressive filling of deep levels, e.g. pumping effects, with a detrimental effect on thestability and the response time. Also, the polycrystalline nature is somewhat detrimental as it in-duces significant non-uniformities of the device response with respect to the position of interaction.We have investigated these features by imaging the response of CVD diamond using a micrometersize focused X-ray beam. The comparison with the grain structure showed that it has a stronginfluence on the field distribution. We present here recent developments studied at CEA in Saclayfor the optimisation of the material with respect to the specific requirements of several applica-tions. They include radiation hard counters; X-ray intensity, shape and beam position monitors,solar blind photodetectors, and high dose rate gamma-meters.
1. Introduction
Diamond exhibits several superior properties in comparison to other semiconductingmaterials, such as a high band gap, high electron–hole pair mobility, short carrierlifetime, and extreme resilience to harsh environments. Early in the seventies, theattractive properties of diamond for radiation detection were demonstrated by Ko-zlov et al. [1] using mono-crystalline diamond stones of extremely high electronicquality. The principle of radiation detection in diamond relies on the creation ofelectron–hole pairs within the diamond from the interaction of the incident particlesor photons to be detected. The local displacement of these carriers driven by anelectric field in the material will induce a transient signal on the device electrodes.A typical configuration consists of applying an electric field through the volume ofthe diamond layer in a sandwich configuration where electrical contacts are depos-ited on both sides. The electronic properties of diamond can be assessed from themeasurement of the detection response characteristics under charged particles. Afigure of merit deduced from this measurement is the charge collection efficiency hdefined as the ratio of the electrical charge Qind induced in the external circuit tothe charge Q0 induced by a particle in the detector. This ratio is assumed to beequal to that of the mean charge collection distance of free carriers, i.e. the product
phys. stat. sol. (a) 185, No. 1, 167–181 (2001)
mtE, by the detector thickness L,
h = Qind/Q0 = mtE/L (1)
under the approximation of a uniform field profile distribution through the materialand if L is large with respect to mtE. Few natural IIa type diamond stones have beenreported as exhibiting h values reaching 100% (mtE products of several millimetres[2, 3]). However, it appears in fact that these properties are only obtained on very fewgems, and that the average quality monocrystal, even though from the IIa type, wouldnot present these attractive features. Among the materials available, strong differencesexist in their detection properties according to their nature and characteristics. Figure 1shows spectra measured under a 241Am a-source (5.5 MeV) in four qualities of dia-monds. The top spectrum is obtained from a high quality detector fabricated from ahighly selected IIa type monocrystalline natural diamond. This detector was charac-terised in the frame of a collaboration with the Triniti Institute in Troitsk (Russia). Itexhibits a response that is close to perfection, i.e. the ha value is 100%: the electricalcharge probed is very close to that created by the impinging a-particles, assuming13.1 eV for the electron–hole pair creation energy [4, 5]. The spectrum below obtainedfrom a selected but average quality IIa type natural diamond reveals a lower collectionefficiency (ha � 15%). This relative poor characteristic results from the high concentra-tion of defects and impurities generally found in natural diamond gems. This motivatedstudies on material synthesis for electronic applications. Of these, one technique,namely the microwave plasma enhanced chemical vapour deposition, particularly whenthe gaseous precursors are excited in a microwave plasma (PECVD), ensures the fabri-cation of large area polycrystalline diamond substrates with reproducible impurity con-tent and properties. In Fig. 1 the a-peaks recorded from two synthetic diamonds grownusing this technique are shown, one grown at CEA/Saclay and one commercially avail-able (polished detector grade). The former exhibits a most probable collection effi-ciency value of 40% whereas values reaching 80% are observed, corresponding to crys-
168 P. Bergonzo et al.: Particle and Radiation Detectors Based on Diamond
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Collection efficiency (%)
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.)
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CVD (commercial-detector grade)(300 µm - polished)
CVD (CEA), (25 µm)
Commercially available IIa type (200 µm)
Highly selected IIa type, Triniti (Russia) (200 µm)
Fig. 1. 241Am detection spectra re-corded on four grades of diamonds.From top to bottom, on a highly se-lected IIa type diamond, on a com-mercially available random IIa typenatural diamond, on a 25 mm thickCVD diamond grown at CEA/Saclay,and on a 300 mm thick commerciallyavailable detector grade CVD dia-mond. Measurements are made in va-cuum and using a constant 10 kV/cmbias field
tallites with higher electronic quality than the average. This indicates promising electro-nic perspectives for CVD even though its polycrystalline nature results in a poor en-ergy resolution (typically 50%) rendering the detector unsuitable for spectrometry. Thiscould partly be caused by the spread of the electric field alongside the material becauseof the non-uniform thickness as well as from geometrical non-uniformities caused bythe polycrystalline nature of the material ([6], and see below). Anyway, since the peakminimum lies well above the electronic noise threshold, these detectors can be used asa-particle counters with high detection efficiency. CVD diamond can therefore benefitto detection applications for activity measurements in the counting mode: as comparedwith natural diamonds, the material can be fabricated on large areas and in a reprodu-cible manner. For comparison, the bottom spectrum reveals a relative poor characteris-tic for a-particle counting of the commercially available detector grade material thatcould be caused by an altered surface layer resulting from the polishing process (therange of 5.5 MeV a-particle is of 15 mm in diamond).
2. CVD Diamond Growth
We have used the PECVD technique to grow diamond, and have concentrated on theoptimisation of the process conditions for best electronic properties [7]. The electroniccharacteristics were enhanced from the tuning of the growth parameters, namely, thesubstrate temperature, the microwave power density, and the gaseous mixture (CH4
highly diluted in H2). Care is required in order to reduce the background pressure asnitrogen has proven to be of extremely bad effect on the detection properties. From10 ppm N2 in the precursor mixture, the electronic properties start to be altered. Also,it was found that the best properties were obtained at low methane concentrations,typically of the order of 1% CH4 in H2, with the effect of reducing the growth ratedown to typically a few tens of micrometer per hour. For radiation detection purposes,the aim is to measure the perturbation induced by the incoming charged particles orphotons within the device, and therefore this imposes the device thickness with respectto the stopping range, typically from 10 mm for heavy charged particles up to severalmillimetres for low ionising radiations. Therefore, the limitation in thickness reflectinglow growth rates will limit the areas of application of diamond radiation detectors.Also, the relative poor quality of very thin diamond coatings generally prevents fromthe fabrication of devices being a few micrometer thick. In our case, we essentiallyfocus on devices that rely on thicknesses between 10 and 500 mm.
The material obtained has a polycrystalline structure with a grain size of the order of15% of the layer thickness. Raman analysis showed one intense peak at 1332 cm––1 andno other non-diamond carbon species. By probing the 1332 cm––1 peak width across thesurface of a detector grade CVD diamond, we have observed that the peak width liesbetween 2.2 and 2.5 cm––1, demonstrating the quality of the grown material. In fact, ithas been observed that h values and Raman line widths are directly anti-correlated,with a sharp increase of h below 2.5 cm––1 [8].
One important step in diamond synthesis is the initial growth on non-diamond sub-strates, relying on the presence of sites on the substrate surface where nucleation oc-curs. The sites can be obtained from mechanical abrasion, typically using diamond pow-der in an ultrasonic bath, or from the ionic bombardment of the silicon surface duringthe first initial steps of growth (Bias Enhanced Nucleation (BEN) technique). This gen-
phys. stat. sol. (a) 185, No. 1 (2001) 169
erally leads to greater nucleation densities and results in the local orientation of thenucleation sites with respect to the silicon lattice. As a result, the polycrystalline dia-mond film grown on top of such a prepared substrate reveals a preferred orientation ofthe grains. These films are supposedly exhibiting better electronic properties from anenhanced carrier mobility [9]. We have grown such films where grains are aligned with-in a few degrees from the h100i direction and over a 2 inch diameter substrate (Fig. 2).In order to quantify the uniformity of the film orientation over 2 inch, we have probedthe local variation of the intensity of the reflected light at grazing incidence when thesample is rotated around its symmetry axis. Since the grains exhibit a pyramidal shape,we can quantify the reflected light from the sides of the pyramids. As the 2 inch sampleis rotated around its axis, the reflected light passes through a maximum every p/2.From the angle of maximum reflection, and limiting the region of interest to a5 � 5 mm2 region using a perforated foil, we measure the ratio by which the reflected
170 P. Bergonzo et al.: Particle and Radiation Detectors Based on Diamond
Fig. 2. CVD diamond polycrystallinestructure observed on a highlyoriented 50 mm thick layer grownafter bias enhanced nucleation
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Fig. 3. Probing the uniformity of the film orientation over a substrate with 5 cm diameter (seetext)
light intensity decays from its maximum as the sample is rotated by p/4. The map ofthis ratio is plotted in Fig. 3 and reveals the high uniformity of the film orientation overthe entire 2 inch substrate.
However, for radiation detection, no significant improvement of the detection proper-ties has to date been observed in such a material, further studies being currently con-ducted on these matters. In fact, it is difficult to assess the real improvement in detec-tion characteristics since the growth process can be influenced by the initial nucleationstep. Indeed, nitrogen is often introduced in the growth gases to encourage <100>growth, and this strongly alters the detection properties. As a result, studies either tendto conclude that highly oriented materials do not perform as well as non-oriented poly-crystalline diamonds [10, 11], or compare materials not grown exactly under the sameconditions [12]. In the following, devices and results address non-oriented materialsgrown using the PECVD technique on ultrasonically prepared materials.
3. Radiation Hardness
The high bonding energy of the sp3 C–C bond gives diamond a natural resilience tohigh levels of ionising radiations, as well as to corrosive aggressions. In order to evalu-ate the hardness of diamond to very high g-dose rates, a series of devices were fabri-cated and their properties evaluated under 60Co irradiation with a source activity of3.5 � 1014 Bq (9.5 � 103 Ci). The dose rate was of 5 kGy/h. The measurement wascarried out during 500 hours, corresponding to an integrated dose level of 2.5 MGy(250 MRad). The devices were biased and the photo-induced current under irradiationwas recorded. Figure 4 shows the evolution of the signal measured on three deviceswith time over several days. The measured current appears to rise progressively from adose of 700 kGy. However, the observation of the leakage current in the PVC cableused for the experiment revealed its degradation with a progressive increase from a fewnA to 70 nA. This reveals that the observed deviation of the device signal was causedby the degradation of the cable and not by that of the diamond detector itself. Post-irradiation measurements have shown that the performances of the detectors in termsof sensitivity and dark current remained unchanged after such a high integrated g-raydose. This experiment also demonstrates that CVD diamond devices can be used to
phys. stat. sol. (a) 185, No. 1 (2001) 171
0 500 1000 1500 2000 2500
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10
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Sig
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Dose (kGy)
Fig. 4. Evolution of the signalsmeasured on three CVD diamonddetectors (solid circles) (variousgrades and thicknesses) when ex-posed to a 5 kGy/h g-ray fluence(60Co, 10 kCi) as a function of theintegrated dose. The bottom curve(open circles) shows the progres-sive rise of the cable signal result-ing from its degradation
monitor the g-ray dose level at very high fluxes. The perfect linearity of the deviceresponse with respect to the dose level was assessed from other measurements from10 mGy/h to 5 kGy/h [13, 14]. There exist several industrial applications for processcontrol out core in nuclear reactors or in nuclear fuel recycling processes that are re-quiring such performances.
Other measurements have addressed the radiation hardness of diamond to neutronirradiation, and particularly on the resulting degradation of the a-detection properties.Irradiations were carried out in the experimental nuclear reactor facility ISIS at CEA.The detectors were placed in a cadmium box in order to absorb the thermal neutroncomponent up to 0.6 eV. The total neutron integrated dose was above 3 � 1015 neutronscm––2 (1 MeV equivalent in Si) with a broad energy distribution centred approximatelyat 2 MeV. Nevertheless, the results of the irradiation on the device response to a-parti-cles at 5.5 MeV is shown in Fig. 5. A slight deterioration of the detection spectrumafter irradiation can be observed, which corresponds to a decrease of the mean ampli-tude of the recorded pulses by approximately 25%, while keeping a good discrimina-tion between the lowest energy pulses and the electronic noise, thus demonstrating thatdiamond based a-counters would well tolerate such a high neutron dose.
4. Defects
Severe detrimental effects of defect levels may occur in diamond and significantly alterthe detection properties of devices. In fact, it is clear that their response is reducedbecause of defects and impurities. If their state is modified either because of the ionisa-tion mechanism (trapping) or because of a significant elevation in temperature (traprelease), the internal electric field in the device as well as the carrier lifetime can bemodified thus altering the response of the detector [15]. This capture/release mechan-ism of trapping levels induces a space charge build up that has been clearly evidencedby thermally stimulated measurements [16]. According to the technique used to calcu-late the energy levels that are released around 300 �C, values close to 1.2 eV areprobed. Similar levels are commonly observed in natural diamonds and have been at-tributed to nitrogen levels. The filling of those deep traps prior to measurements leadsto improved transport properties. This can be achieved by irradiating the material un-der a 50 kV X-ray tube at a typical dose of 10 Gy. This procedure called the ‘‘priming”
172 P. Bergonzo et al.: Particle and Radiation Detectors Based on Diamond
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Channel number (a.u.)
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Fig. 5. Shift of the pulse heightspectrum recorded under 241Ama-particles after thermal neutronirradiation with a fluence greaterthan 3 � 1015 neutrons cm––2
(1 MeV equivalent in Si)
or ‘‘pumping” effect is known to improve dramatically the collection efficiency [17].The progressive increase of the detector sensitivity can be probed during X-ray irradia-tion and is shown in the insert of Fig. 6. One simple other way to pump a thin diamondlayer is to use an e-beam evaporator for contact fabrication : the intense electron fluxhitting the metal source results in secondary electrons and X-ray emission that irradiatethe sample [18] and give rise to carrier trapping.
Also, the presence of charge trapping may induce a modification of the space chargein the material within the duration of the experiment, which could in turn result in theevolution of the effective applied field on the material and therefore modify the detec-tor response. When the traps are filled, the sensitivity remains stable providing thesample is kept at the same temperature. Exposure to light may also affect the stabilityof the filled traps but is extremely low under the opaque electrical contacts.
Also, if the impinging radiation tends to alter the status of the trapping level, particu-larly non-homogeneously throughout the device volume, a modification of the electricfield distribution may result, therefore leading to shifts in the detection spectrum. Theseeffects relative to the radiation-induced polarisation are predominant in thick layerswhen used for short-range particle detection. Therefore, one other way to enhance thestability is to use a thickness that fits the penetration depth of the particles to be de-tected. In the case of 5.5 MeV a-particles, exhibiting a 14 mm range in diamond, detec-tors with thicknesses below 30 mm have been shown to exhibit best performances. Dif-ferences between materials, however, may still occur, and it is observed that mostmaterials tend to exhibit responses that vary with time [15]. We have observed, how-ever, that it was possible from the growth conditions to synthesise CVD diamond mate-rials that exhibit a stable response. This is illustrated in Figs. 7a and c, where the re-sponses of two diamonds to bremsstrahlung X-ray excitation at 50 keV emitted througha lead chopper rotating at a typical frequency of 15 Hz are compared. The correspond-ing TSC spectra of both materials are given in Figs. 7b and d, respectively. It is shownthat the slow device (Fig. 7a) also exhibits the highest sensitivity, whether the fast de-vice (Fig. 7c) has a very particular TSC signature with extremely low detrapping cur-rents. It appears here feasible to grow such material where deep trap levels are less
phys. stat. sol. (a) 185, No. 1 (2001) 173
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rent
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Fig. 6. TSC spectrum mea-sured on a typical CVD dia-mond revealing the deep traplevels located around 1.2 eV(after subtraction of the darkcurrent). The insert shows theprogressive increase of sensi-tivity during the primary step
predominant (Fig. 7d) and exhibit a faster response (Fig. 7c) compatible with the fre-quency used here.
The presence of traps may also affect the decay time of the detector after a pulsedexcitation. In Fig. 8 the responses of two devices under X-ray pulsed excitation gener-ated by a synchrotron light source (at LURE, Laboratoire pour l’Utilisation du Rayon-nement Electromagnetique, Orsay, France) are shown. The pulse duration is estimatedat 600 ps at FWHM with a period of 120 ns expected to be long with respect to thesignal decay time in diamond. The curve reveals extreme differing qualities of the CVD
174 P. Bergonzo et al.: Particle and Radiation Detectors Based on Diamond
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Fig. 7. Comparison of the responses of two CVD diamond detectors to chopped X-ray (typically50 keV) excitation (15 Hz). One exhibits a slow response (part a) whether the other (part c) has aresponse time that fits with the speed requirement of the experiment. The corresponding TSCspectra are given in b) and d), respectively (electric field is 10 kV/cm for all curves)
-10 0 10 20 30 400.001
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τ=340 ps
Time (ns)
Sign
al(V
)
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10-2
Fig. 8. Response obtained on two CVD diamond devices (same contact geometry and field) to1 keV X-ray pulses (FWHM �800 ps). The top device exhibits a slow decay component that is notvisible on the bottom one (the small oscillations just after the pulses are caused by signal bouncesin non-perfectly adapted cables). The inserts show the shapes of the detection signals observedunder pulsed X-ray excitations at 20 keV and 0.3 GHz (see [21, 24])
diamonds probed, as the signal on the top seems to exhibit a slow decay characteristics,whereas the response on the bottom is much faster. In fact, this fast characteristic isobtained on the same material as that of Figs. 7c and d. The slow decay characteristiccan be closely fitted by a double exponential with a slow decay characteristic at 17 nsthat has a much greater contribution to the overall signal. The fast device has beensuccessfully used for the characterisation of ultra-fast X-ray pulses at extreme repetitionrates of 0.3 GHz at the ESRF (European Synchrotron Radiation Facility) (see [19]).However, the same measurement performed on the slow device would have been se-verely affected by a rising baseline level, resulting from the slow decay time, and super-imposed to the signal. This is illustrated in the inserts of Fig. 8. It further demonstratesthat small changes in the growth conditions can result in films with extremely differingcharacteristics for their uses as radiation detectors.
5. Influence of the Polycrystalline Structure on the Non-Uniformityof the Sensitivity
Since the material is polycrystalline, with grain sizes typically of the order of 10% to20% of the thickness, one inherent drawback to CVD diamond is that regions ofdegraded properties are likely to be observed within the grain boundaries, which mayresult in the non-uniformity of the detector response. Using a microfocused X-ray beam,we have imaged sensitivity maps of detector grade polycrystalline diamonds using micro-focused X-ray beams. The experiments were performed at the European SynchrotronRadiation Facility (ESRF) using the scanning X-ray microscope (SXM) of the ID21X-ray microscopy beamline [20] which provided a focused X-ray microprobe (Fig. 9).The beam was monochromatised to provide an incident photon energy of 6 keV using adouble-crystal fixed-exit monochromator (Si 111). Harmonic rejection was performedusing two Ni coated mirrors at grazing incidence angles of 8 mrad. The beam wasfocused using a 1 mm diameter Ta zone plate (outer zone width 150 nm) giving an X-ray
phys. stat. sol. (a) 185, No. 1 (2001) 175
Fig. 9. Experimental set-up used for imaging the uniformity of the CVD diamond sensitivity fromthe signals generated under a micrometer size X-ray beam (at the European Synchrotron Radia-tion Facility, ESRF)
beam flux of approximately 1010 photons/sto a spot size of 1mm (vertical) � 3 mm(horizontal). The diamond sample wasplaced perpendicular to the beam axis inthe focus plane of the zone plate lens, theX-rays impinging the diamond through a100 �A gold layer. At 6 keV, the attenua-tion of the incident X rays induced by this100 �A gold layer is below 1%. The detec-tor was biased and x–y scanned in theplane perpendicular to the X-ray beam di-rection using a combination of piezoelec-tric actuated flexure and mechanicalstages giving a total accessible scan regionof 10 � 10 mm2. Hence, the response of
the detector was measured as a function of the interaction position of the focused X-raybeam on the diamond surface. Using the piezoelectric translation stage, the sample wasmoved in front of the X-ray beam with a 1 mm step and pixel dwell times of 10 ms whilethe photo-induced current was measured using a Keithley 487 electrometer. The result-ing mesh gives an image of the detector response. Measurements were made on theregion shown in the scanning electron microscopy (SEM) image of Fig. 10a. The pictureis taken through a 100 �A gold layer, and reveals that the Au coating is uniformly cover-ing the diamond rough surface (no local polarisation). The device is 150 mm thick anddoes not display any preferred orientation of the diamond crystallite grain morphology.Figure 10b displays the map of photosensitivity recorded as the focused X-ray beam isscanned across the region shown in Fig. 10a. Strong variations in the sample sensitivityare observed along its surface. Comparison of the SEM image and the map of X-rayphoto-induced currents reveals a strong correlation between the polycrystalline structureand the response of the detector.
Figure 11 presents a plot of the variations of the photo-induced currents with themeasured thickness profile between two points marked as A and B in Fig. 10. The plotreveals a significant correlation between the grain structure and the response of thedetector. The sensitivity is increased as the probing beam is moved away from grain
176 P. Bergonzo et al.: Particle and Radiation Detectors Based on Diamond
A
B
50 µm
A
B
50 µm
50 µm
A
B
(a)
(b)
--4000nA--2522--1591--1003--632--398--251100--158
Fig. 10. a) SEM (secondary electron mode)image of the region of interest. The sample is150 mm thick, grown from microwave plasma-enhanced CVD in a pure CH4/H2 precursormixture. b) Map of the X-ray response ob-served on the region displayed in Fig. 10ausing a micrometer size focused X-ray beam at6 keV. The sensitivity of the detector appearsto be closely dependent on the material as thegrain structure can be clearly recognized. Theblack/white gradient spreads on a log scalefrom 100 to 4000 nA
boundaries and edges. Indeed, the response measured when the beam spot hits therecess region between two grains is small with respect to that measured when the beamhits the middle of a grain. The sensitivity variation lies in the �30% range, a value farin excess of what could be expected from the observed thickness variations alone. Also,similar observations can be made regarding the top edge of a grain, as the sensitivity islower in this region. These trends can be observed in the entire probed surface by closecomparison or superposition of Figs. 10a and b. This agrees with similar observationsmade on coplanar surface devices and under UV illumination, using a varying distancebetween the electrodes. Other works [21–23] have shown that there was a strong effectof the grain boundaries as they seemed to alter significantly the detector sensitivity.
6. Applications
One of the first difficulty in using diamond for radiation detection in the frame ofindustrial applications is of course that the direct competitor to diamond, namelysilicon, is generally more efficient, more stable, with perfect homogeneity, thus greaterresolution, cheaper and readily available. Also, we showed earlier the extremely poorresolution that a typical CVD diamond detector exhibits to monochromatic a-particles(typically 30%), therefore, forbidding its use for all spectrometry measurements wherethe incoming energy needs to be probed. This consequently reduces the diamond inter-ests to areas where there exist several constraints that could only be overcome becauseof the extreme superior properties of diamond.
One of the first applications that motivated CVD diamond studies for radiation de-tection was for the characterisation of high-energy particle interactions for physical ex-periments [24]. This is by far the most demanding application. At high energy, particlesare very penetrating, therefore, the level of interaction remains low. Diamond has a lowcross section and high electron–hole pair creation energy and thus very few carriers aregenerated per interaction. The deposited energy here varies with the thickness L, andEq. (1) shows that the detector efficiency becomes directly equal to the product mtE,being here given as figure of merit. Also, the aim to measure the position of inter-action, requires devices with very high uniformity. This application thus combines re-quirements for high sensitivity, thickness, stability with respect to time, and uniformity
phys. stat. sol. (a) 185, No. 1 (2001) 177
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Fig. 11. Evolution of the mea-sured thickness and of thedetector sensitivity along theline A–B in Fig. 10
178 P. Bergonzo et al.: Particle and Radiation Detectors Based on Diamond
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neu
tro
nco
un
ters
sou
rces
or
nu
clea
rre
acto
rs
–st
abili
tyw
ith
tim
e–
low
gse
nsi
tivi
ty–
rad
iati
on
har
dn
ess
(n+
g)
con
vert
ing
med
ia(e
.g.,
10B
)
–th
in(f
or
hig
hh
and
low
gse
n-
siti
vity
)–
pri
med
dia
mo
nd
:ty
p.40
%,
dev
ice:
typ.
1%[3
2]
hig
h-e
ner
gyn
eutr
on
sfu
sio
nin
stal
lati
on
s–
sen
siti
vity
–st
abili
ty
12C
(n,
a)9 B
ere
acti
on
–th
in(f
or
hig
hh
)–
pri
med
typ.
10––
4co
un
tscm
2 s/n
[33]
fast
X-r
ay-p
uls
ed
etec
tors
fast
cam
eras
,la
sers
and
syn
chro
tro
ns
–fa
stsi
gnal
turn
-off
–ra
dia
tio
nh
ard
–h
eat
resi
lien
t
curr
ent
mo
de
–lo
wca
rrie
rap
par
ent
lifet
ime
–lif
etim
e<
70p
s–
rep
etit
ion
rate
ste
sted
up
to0.
3G
Hz
[19,
26]
syn
chro
tro
nX
-ray
bea
mm
on
ito
rin
g
syn
chro
tro
nb
eam
lines
–th
inn
ess
–st
able
wit
hti
me
and
tem
per
atu
re
curr
ent
mo
de
on
san
dw
ich
stru
ctu
res
–th
inm
emb
ran
es–
dev
ice
geo
met
ryo
pti
mis
edfo
rp
osi
tio
no
rp
rofi
lem
easu
rem
ents
–b
eam
po
siti
on
mo
nit
ori
ng
(res
.�
1m
m)
–b
eam
pro
file
mo
nit
ors
(typ
.re
s.�
10m
m)
[26,
34,
35,
36]
low
-en
ergy
X-r
ayse
nso
rsas
tro
ph
ysic
s–
sola
rb
lind
nes
s–
sen
siti
vity
–st
abili
ty
curr
ent
mo
de
or
ph
oto
-cat
ho
de
–th
in–
pre
par
edsu
rfac
e–
rati
oV
UV
/UV
sen
siti
vity
:ty
p.40
0–
low
ener
gyX
-ray
sen
siti
vity
typ.
20%
[28,
37]
of the sensitivity along the surface in thickness for electric field uniformity; the lattercondition being reached from the mechanical polishing of the diamond layers. All theseaspects are the frames of the RD42 collaboration at CERN [25] in close collaborationwith industrial CVD diamond growers. It led to materials the qualities of which nowreach on an industrial basis mtE values close to 200 mm. However, we have observedfrom Fig. 1 that such a high quality material for low ionisation particles is not so re-markable for the characterisation of a-particles, probably because of the degradation ofthe surface layers after mechanical lapping steps. Other applications that are not asconcerned with the challenge to the highest mtE values as the detection of high-energyparticles and where the specifications impose other characteristics (e.g., small thickness,fast response) have been pointed out.
One example can be for the detection of X-ray beam light generated from a synchro-tron light source. Here the interest of diamond stands in its low atomic number so thatone thin layer remains highly transparent to the incoming beam. That way the diamondcan be used as a semi-transparent monitor for probing the beam intensity. Also, thequantity of photons being high (typically 1010––1013 photons cm2s––1), there is no needfor highly sensitive materials. We have fabricated such devices the geometry of whichenables the measurement of intensity, profile, and position of synchrotron beams with-out significant attenuation [26].
Using similar approaches, it has been made feasible to develop at CEA/Saclay otherdevices that are currently available on an almost commercial basis for several applica-tions as summarised in Table 1. The necessity is to adjust the material properties to thecharacteristics required for the device use. Further, for geometrical concerns (thickness,sensor area, electrode configuration), essentially four criteria are involved: sensitivity,stability, uniformity, and temporal response, that need to be optimised as required.Some compromises may have to be taken into consideration, for example an ultra-fastdevice relying on the presence of recombination centres or defects generally exhibits alow sensitivity. Further, the state of the device has to be stable, i.e. no variation of thesensitivity nor ageing can be tolerated for a long term use. This way, since diamond isknown to exhibit deep levels that are likely to get charged under the interaction withionising radiation, it is necessary to charge these defects and to keep the device in astate where no release can occur. Fortunately, thermal detrapping occurs at relativelyhigh temperatures (> 200 �C). Above these values, the sensitivity is known to collapse[27]. To date, the deep levels (�1.2 eV) in CVD diamond have not been attributed toany impurity or crystal structure. We have, however, observed that few materials grownunder particular conditions are less subject to these levels as was shown in Figs. 7c andd. It is hoped that future works will give better insights in order to ensure a stableresponse of diamond detectors at high temperatures.
7. Conclusion
Recent progresses in the synthesis of CVD diamond led to significant improvements ofits electronic properties that especially concern the synthesis of films that do not exhibitspace charge build up effects which are often encountered in diamond materials andthat are highly detrimental for detection devices. Further, special care is required forthe preparation of devices for increased stability and sensitivity. On a pre-industrialbasis, CVD diamond detectors have been fabricated for several applications in hostile
phys. stat. sol. (a) 185, No. 1 (2001) 179
environments as encountered in the nuclear industry. Such devices can operate in harshenvironments and overcome the limitations encountered with standard semiconductormaterials.
Acknowledgements The measurements under focused X-ray beams were performed atthe European Synchrotron Radiation Facility (ESRF) in Grenoble (France) by R. Bar-rett. The authors also acknowledge V. Amosov (Triniti Institute, Moscow) for his helpand fruitful discussions on his almost perfect gems.
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