# Probing the transient response to improve the stability of diamond devices under pulsed periodic excitation

Philippe Bergonzo, Hassen Hamrita, Dominique Tromson, Caroline Descamps, Christine Mer, Milos Nesladek

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Philippe Bergonzo, Hassen Hamrita, Dominique Tromson, Caroline Descamps, Christine Mer, et al. Probing the transient response to improve the stability of diamond devices under pulsed periodic excitation. 2007 MRS Fall Meeting - Symposium P - Diamond Electronics - Fundamentals to Applications II, Nov 2007, Boston, United States. pp.1039-P06-01. cea-05462632

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# Probing the transient response to improve the stability of diamond devices under pulsed periodic excitation.

P. Bergonzo, H. Hamrita, D. Tromson, C. Descamps, C. Mer, M. Nesladek;

CEA, LIST, Centre d'Etudes de Saclay, Gif-sur-Yvette, France

## ABSTRACT

CVD diamond combines attractive properties for the fabrication of detection devices operating in specific environments. One problem that remains critical for device stability is the presence of defect levels that alter the detection performances, and the detection characteristics often appear as they are very depending on time, temperature, and history of the preceding irradiations.

One issue we have proposed is to adapt one technique that is commonly used for time of flight spectroscopy in order to maintain a uniform electric field in the probed device, and based on the synchronisation of the device bias with the period of a periodical excitation. This can be applied to several types of detection applications, as long as we can rely on periodical triggering in order to synchronise the device polarisation. We apply it here to a LINAC electron accelerator used for photon pulse generation at the frequency of 25Hz. The result is a remarkable improvement the performance of a polycristalline diamond detector that exhibits a particularly defective response when used in the steady state excitation, to that of a perfectly stable and reproducible device response in the pulsed mode. We claim this method to be applicable to several types of excitations and particularly it presents a high interest for monitoring accelerator sources, e.g. for medical dosimetry applications.

## INTRODUCTION

The attractive properties of diamond for detection device fabrication have been well documented [bouquin]. Diamond combines specific advantages that are of high benefit for its operation as a detector in specific environments. One problem however and also well documented is the presence of defect levels that are altering the detection characteristics [domi, bruzzi, souw, ]. These are present in most synthetic materials just like they were observed in natural diamonds. They result in unstable responses and carrier losses. The result is that of course it motivated the challenge to grow materials with the least defect concentrations: synthetic single crystals (SC) are now forseen as good candidates for most detection applications (Domi, J. Morse, Kagan ? ). However, SC diamond are remaining difficult to grow, especially on large areas, and only small size substrates can be used, typically in the 5x5 mm range at reasonable costs, that imposes to selectively grow each SC diamond detector. Further, it is now evidenced that the detection performances of the SC diamond strongly depend on the surface treatment made on the crystal used for growth, since dislocations and defects propagates during synthesis from the bulk of the substrate to the grown layer [nico]. For such aspects the possibility to grow polycristalline diamonds over large areas remains a choice since it enables not only large detection areas but also high uniformity between synthetic devices when grown in similar conditions. However, due to the much higher
defect concentrations in polycrystalline diamonds (typ. by a factor of $10^3$), unstable performances are common, and namely depend on the progressive filling of trapping levels (including e.g. pumping and polarisation effects) [1,2], with a detrimental effect on stability and response time. Several studies have aimed at identifying the origin of defect levels but the question remains open: impurities, dislocations, and grain boundaries are often associated with the presence of those electrical defects highly detrimental to the detection performances.

Here after a clear explanation on how the defects contribute to alter the probed signal in diamond detectors, we propose to look at the temporal photoresponses to pulsed excitation, and to show the effect of their temporal progressive evolution. The case of high energy X-rays (14 MV electrons on W target) is discussed, at a repetition frequency of 25Hz.

One application indirectly addressed here is the possibility to apply this technique to medical dosimetry where the devices require to be particularly stable and reliable. Common medical accelerators are always pulsed, at repetition frequencies of typically a few 100Hz, a range at which this technique does not find any difficulty to be used.

## THE EFFECT OF DEFECT POPULATIONS ON THE DETECTION RESPONSE

In Polycrystalline diamond, severe detrimental effects of defects do occur, which alter the detection properties of devices by reducing the number of electron-holes pair available for the formation of the electrical signal. If the device state is modified either because of the ionisation mechanism, or because of a significant elevation in the temperature, the internal electric field as well as the carrier lifetime of the device can be modified thereby altering the detector response [2]. This capture mechanism of trapping levels induces either a release current that is probed for example during thermally stimulated measurements (TSC) [1] or a glow signal that consists of the signal of interest for thermoluminescent dosimeters (TL) [3, 4]. According to the technique used to calculate the energy levels that are released at approximately 550K, values from 1.3 to 1.8 eV were probed [5, 6] although 1.1eV is now more commonly agreed [7]. Similar levels are commonly observed in natural diamonds that can be attributed to defects induced by nitrogen. In the present case on CVD diamond, no clear origin has been attributed to the levels observed, and it is not the object of this communication. Figure 1 displays the typical TSC and TL spectra from 3 different diamonds. Clearly they exhibit defect levels which when emptied give rise to corresponding detrapping currents and the glow signal lobes observed. Two populations can be identified which can arbitrarily be classified as *deep* and *shallow* from the temperatures at which they are emptied. Deep levels are released at temperatures close to 550K : when filled during pre-irradiation, they are stable when the device is used at room temperature. In fact, the trapped population in this *deep* level constitutes the measured signal of thermoluminescent dosimeters [4, 8]. Exposure to light may also affect the stability of those deep traps [9] but this is extremely low under the opaque electrical contacts. Also, if the impinging radiation tends to alter the trapping level status, and particularly non-homogeneously through the device volume, the modification of the electric field distribution may result, therefore leading to shifts in the detection signal. These effects relative to the radiation-induced *polarisation* are predominant in thick layers when used
for short-range particle detection, therefore another way to enhance the stability is to use a thickness that fits the penetration depth of the particles to be detected [10]. On the other hand, shallow levels are emptied at temperatures close to 350 K. The problem here is that being close to ambient, this population will continuously evolve : if the device is left unirradiated at room temperature, this population will progressively be emptied till it vanishes. On the opposite, between two prompt successive irradiations, this trapped population has no time to be detrapped and therefore remains filled. Thus the device response is clearly not the same whether it was left for a long or short time unirradiated. This is the basis of non reproducible detector responses that often render devices improper for accurate measurements.

<figure><img src="image_2.png" /><figcaption>Figure 1: Thermally stimulated current (TSC) and thermoluminescence (TL) signatures for 3 typical detectors: two are from IIa type natural diamond and one is a polycrystalline CVD. Deep traps are released at high temperatures and are therefore considered stable at room temperatures. Shallow traps however may continuously evolve and are very detrimental since they affect the device stability and reproducibility.</figcaption></figure>

From this distinction made in the previous paragraph between deep and shallow levels, and that is only based on the temperature at which defects are emptied, there is one further consideration that needs to be taken into account and that concerns the trapped population on shallow levels: if this population is progressively emptied in the material, due to permanent thermal detrapping, it comes that it tends to be more filled in a region where a lot of carriers circulate. To explain this we can consider for example the case of an electron trap: electron-hole pairs are generated through the volume of the diamond layer (figure 2). If we suppose a uniform interaction in the diamond volume (the typical case for photons above 100keV or high energy electrons), the generated electrons all drift towards the anode. Therefore, when emptied an electron trap located close to the anode may statistically be more often filled than one located close to the
cathode. In other words, even assuming a uniform thermal detrapping, more electrons will appear trapped close to the anode than to the cathode. Therefore, the volume distribution of electrons trapped on shallow levels through the diamond volume is non homogeneous: the shallow electron traps remaining more frequently filled in the region close to the anode since more electrons are circulating there. On the other side, close to the cathode, electron-hole pairs are also generated but fewer electrons are transiting there and therefore only few electron traps will be filled on these shallow levels progressively emptied. We deduce from this discussion that the filled electron trap distribution profile is inhomogeneous in a device exhibiting shallow traps, and is greater in the vicinity of the anode with respect to that at the cathode. During irradiation and use of the device, there is therefore a permanent built-in charge that is non-homogeneously distributed in the diamond, and considering the case of shallow electron traps this results in a negative charge located close to the anode.

<figure><img src="image_3.png" /></figure>

**Figure 2:** Here illustrated for the case of electron traps: deep traps get progressively trapped and end-up filled throughout the entire device. After being filled, they are supposed stable and not perturbing the photoresponse. However, shallow traps are progressively emptied due to thermal agitation. Therefore, the probability to be refilled is greater in the region where more electron circulate, thus e.g. for electrons closer to the anode side. This implies the existence of a charge of negative sign on the anode side, that builds-up during use of the device, and that vanishes progressively when irradiation is stopped. This negative charge lowers the overall electric field, thus is responsible for a decrease of the sensitivity. Similarly, the same illustration can be made for holes, on
other deep and shallow traps, and resulting in the build-up during use of a charge of positive sign on the anode sign, also resulting in lowering the sensitivity.

Exactly the same discussion can be made on hole shallow traps : if uniformly generated through the volume of the layer, they are all travelling towards the cathode, and since shallow hole traps only capture them for a certain time, there will be more holes captured in shallow hole traps in the vicinity of the cathode of the device. Thus a positive charge accumulates where more holes circulate and therefore in the material depth with higher concentration at the cathode electrode side. This therefore implies that during the use of a diamond detector, there exists a built-in charge in the material, that is caused by the trapped population in shallow levels, and this charge is of positive sign close to the cathode side, and of negative sign close to the anode side. It therefore appears on figure 2 that during use of the device, this built-in charge reduces the electrical field externally applied to the diamond layer. The device response is therefore lowered by the built-in charge. From the qualitative approach used here, it is to be said that the same discussion can also be extended to several distributions of traps. The resulting localised created charge modifying the electric field in the same way, and therefore always leading to a similar effect on the detector behaviour.

We then have probed the effect of the shallow levels on the progressive evolution of the diamond device. Here we used a 210 µm thick detector grade polycrystalline device grown in an ASTeX AX5400 reactor at 130torr and 850°C, and using H₂/CH₄/O₂ mixtures of 478/20/2 sccm respectively. The device is mounted using an ionisation chamber configuration with one metallic contact on each side of the device. It is assembled on a SMA connector enabling biasing and signal measurement, and it is encapsulated in a wax in order to avoid any possible air ionisation around the device that would affect the signal response. More detail on the device preparation and assembly are also given in [¹¹]. Here we aim at explaining the effect of deep and shallow traps in diamond and their effect on the device temporal response and including persistent photocurrent transients. Then we will be able to concentrate to the mechanisms involved when a pulsed excitation is used, and namely using a LINAC accelerator driving 14MV electrons onto a W target for X-ray pulses generation at a 25 Hz frequency.

From the observations of figure 3 we see that the response is from the first aspects highly not compatible with what could be expected from a perfect device : several minutes are required to attain an almost stable response, and strong transients are observed when the device is left unirradiated. During the first irradiation, we can comment on the progressive rise of the net photoresponse : it is due to the progressive trapping of deep traps, that is here dominant. At the same time, shallow levels are also likely to be trapped, non-uniformly through the material depth, but this is not particularly visible here. However, when the device is left unirradiated for a little time, after re-irradiation, we observe a transient, that is negative : the signal being stronger at the beginning of the irradiation, to decay progressively before it catches up with the signal rise from where it had been left during the preceding irradiation. This transient has also been called overshoot in some communications [8]. From the preceding explanations,
indeed the net current measured at equilibrium after the transient corresponds to a built-
in charge within the material, thus of negative side on the anode side and of positive
sign on the cathode side, and it lowers the applied electric field on the detector, thus the
photoresponse. If the device is left unirradiated, this charge vanishes progressively, due
to thermal detrapping and therefore the applied electric field increases in the material,
and therefore higher net currents are probed. The longer the device is left unirradiated,
the more the trapped charge vanishes, and therefore the higher the overshoot. Our
explanation clearly confirms the behaviour observed on this poor quality device as
displayed in figure 3.

<figure><img src="image_4.png" /></figure>

Figure 3 : Photo-response of a polycrystalline diamond detector to X-rays, 50kV, 1mA. This plot illustrates the progressive instabilities due to deep trap priming, as well as the overshoot effect due to the space charge build-up, and also the non reproducibility of the device response as a function of the time during which the device is left unirradiated.

In order to highlight the shift with respect to time as observed in **figure 3**; we propose here to
plot them using a multichannel analyser (MCA) that probes the pulse height spectra (PHS) of the
pulses. The full range of the MCA (2048 channels) corresponds to 8V. Figure 1 display the sets
of PHS recorded every 5 seconds (125 counts each). For clarity each PHS is shifted vertically.
The first measurement (fig. 1-a-) is probed at 0.5V/µm bias field. It displays that the PHS is
progressively shifting towards higher energies, thus demonstrating that the photoconductivity
rises. This can be related to the progressive filling of deep traps (*priming*), and we reach after
approx 20 s (500 pulses) an equilibrium that supposes complete deep level trapping, and non-
homogeneous shallow level trapping with built-in charge reducing the internal field. We then
stop the irradiation for several seconds, change the bias to 1V/µm, and reset the MCA and re-
start X-ray excitation, we probe the set of PHS probed in fig. 1-b-. Here, as opposed to fig. 1-a-
the deep levels were filled during the first irradiation, and no *priming* is visible. On the opposite,
a weak shift towards the lower energies in the PHS can be associated with *polarisation* effects due to shallow traps. After typically 30s equilibrium has been reached, with values of the photoconductive signals that are below that observed during the first 5s, since the built-in charge in shallow levels is reducing the internal field.

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Pulsed height spectra (PHS) of the photoconductive responses under high energy X-rays and recorded every 5 seconds (the spectra are shifted vertically upwards every 5 s for clarity). -a- at 100V, initial rise of the photoconductivity probed at 100V, and -b- after stabilisation and weak observation of the polarisation effect : the spectra are here shifted to lower energies.</figcaption></figure>

If we heat up the device to temperatures close to 70°C to 100°C, as reported by Guerrero et al, the trapped population in shallow levels is instantly emptied, thus the trapped charge is instantly released, and those transient are not observed [8, 12]. Indeed the overshoot corresponds to the time it takes to reach the equilibrium in the diamond trapped population in shallow levels, and if we consider these shallow levels fully emptied, this effect vanishes. We had therefore proposed the opportunity to use a device that is maintained at elevated temperatures, of the order of 70 to 100°C, a temperature to be optimised depending on the material characteristics and deduced after one simple TSC signature measurement. This solution could be used although we admit it may not be highly practical in the case of a radiotherapy dosimeter device, especially for measurements where the detector is in contact with the patient.

As demonstrated previously, it comes that the measured sensitivity of the device is in fact only an equilibrium, that is affected by the presence of the built-in charge in the detector. Therefore, the measured signal is the result of the equilibrium between the trapped population in the shallow levels, and their progressive thermal release at the temperature of the device. The net current signal is always minored by the built up charge within the material, and this charge varies with the incident fluency. The measured normalised signal is therefore reduced at higher fluencies. This is the basis for a non linear response : since the sensitivity varies with the fluency.

SYNCHRONISING THE BIAS TO THE PULSED EXCITATION
The issue proposed here is therefore to apply one trick that has been commonly used for TOF measurement spectroscopy on materials with high defect concentrations such as a-Si:H, and namely to synchronise the applied bias on the device with the incident pulsed excitation. We developed an electronic apparatus that allowed us to switch off the bias applied on the sample between pulses: since the excitation lasts 2µs every 40 ms, we have plenty time between two pulses so that the bias can be zeroed on the sample during 20ms between two pulses (bias is thus applied 10ms before and 10ms after each pulse only). For this setup we used a HP 81110A pulser that provided us with repeating pulses that enabled the synchronisation and the triggering of a DC bias source supply with the incident pulses. For this a high voltage switch box (GRX-1.5K-E) was used and enable the control of the bias (up to several 100V) changes at high rate (<few µs). In this way we were able to switch from DC bias supply (the bias is continuously applied to the sample) to pulsed bias supply (the bias is applied only when the X-ray pulse is about to fire). We expect here that during the time the bias is kept at zero volts, part of the built-in charge in shallow levels has time to be emptied.

We define the arrangement set-up using the description DC mode when the bias is NOT pulsed and left as commonly used on the diamond detection device. As opposed to this DC mode, the pulsed mode 1/1 implies that we are synchronising the bias pulsed at the same frequency of the excitation frequency, as displayed in fig XX-b.

We compare the pulses obtained in DC mode with respect to that in the pulse mode on figure 3 It comes that we observe a very strongly gain in photoresponse, the signal shifting from approx 3.8V (ch 730) to more than 6V (ch 1030). This improvement of the photosensitivity is also visible on the PHS spectra of figure 3. This clearly efficiently demonstrates how pulsing the bias field improves the photoresponse. Also, the PHS is much narrower, thus demonstrating the improved stability of the photocurrents probed for each pulses, and this because the field is more uniform according to the position of interaction. The use of alternative electric biasing on the device thus appears to be of high benefit to the device response.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Improvement of the photosensitivity: DC mode corresponds to a constant bias voltage of 1V/µm on the device, whether in the pulse mode, the bias voltage (also at 1V/µm) is zeroed between 2 pulses.</figcaption></figure>
To explain the observed improvement we have to go back to considering the state of the traps. As discussed earlier when a uniform excitation is creating carriers through the diamond layer, polarisation is liquely to occur, i.e. an internal electric field build ups, opposite to the applied electric field, and that lowers the photocurrent. This internal build-up field is associated with the non homogeneous carrier trapping in shallow levels. During the half period when the applied bias is kept at zero volts, thermal excitation provides sufficient energy for shallow levels to be detrapped, and the internal charge build-up vanishes. This way the shallow level trapping situation is considered as emptied, and for each following pulse the internal build-up field is not affecting the photoresponse.

The same study was then conducted using a VUV flash lamp, delivering 5µs duration pulses at tunable frequencies, here kept at 10Hz. The top contact through which the impinging photons are interacting in the device is made from a thin semitransparent metallic contact. The main difference with the preceding experiment is that when VUV photons are used, the penetration depth of the photons below the device surface remains shallow. As opposed to the preceding experiment, it therefore implies that only one type of carriers created at the vicinity of the surface will contribute to the signal while travelling through the device volume. Hence, if the carrier drift length is shorter than the device thickness, only one type of carriers can here be trapped in shallow levels, and namely carriers of the same sign than that of the semitransparent electrode. Shallow level transient trapping will therefore here imply a local increase of the electric field seen by the device with respect to that of applied. The effect of shallow carrier trapping is therefore expected to be not affecting the device response as it was in the preceding experiment under volume excitation.

The progressive evolution of the transient photoconductivity pulses under the VUV surface excitation is probed in figure 4 at a constant bias voltage of 1V/µm. Here the number of photons available after filtering is much lower than in the earlier experiment and the PHS exhibit counts at lower channels. Nevertheless, when a DC bias is used, we see in figure 4-a- that the signal is very stable with time. Also, when a pulsed bias is used, in other words when the device is biased during half a period that matches the VUV pulse frequency, no significant difference is observed as displayed in figure 4-b-.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Stability of the signal photoresponse under VUV photon pulses: The PHS display stable response.</figcaption></figure>
CONCLUSIONS

The aim of this communication was to discuss on the characteristics of the signal
formation in diamond detectors, and particularly on the influence of trapping levels, deep or
shallow, and on their influence on the device response. Although deep trap effects have been
very well documented over the years through a profusion of literature, only the presence of
shallow levels which are progressively emptied from thermal agitation can explain several
features observed. From the non-homogeneous trapping of shallow defect levels, one can explain
the frequently observed “overshoot” responses when a diamond detector is exposed to irradiation
that is interacting in the device volume, as well as the non reproducible response according to the
time during which the device was left unirradiated. Nevertheless, we have shown that even in
devices made from polycrystalline materials which more frequently are considered too defective
for providing a reliable measurement, we can use a pulsed bias in order to stabilise the signal
photoresponse with time. We claim this has a high interest for the detection of pulsed line
sources, i.e. where one can trigger and synchronise the bias applied to the device on the event to
detect. For example, this can be of high interest when diamond is to be used for the monitoring
of pulsed signals. This accounts e.g. radiotherapy [11], synchrotron light sources [12], or even
DT confinement fusion plasma [13]. We believe that these techniques can benefit for the
monitoring of several pulsed accelerators, which therefore can then rely on the use of cheap and
easy to fabricate polycrystalline materials, with improved signal to noise ratios from lock-in
detection techniques.

ACKNOWLEDGEMENTS

We wish to acknowledge the partners that were involved in those measurements, and
namely the SAPHIR team of CEA-LIST and namely Medhi Gmar and Frédéric Lainé. Moreover,
Hassen Hamrita acknowledges the SOLEIL Synchrotron facility for grant funding and Caroline
Descamps acknowledges the European Commission for student funding from the MAESTRO
project (IP CE503564)

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