# GaN diodes comparative study for high energy protons detection

Matilde Siviero, Maxime Hugues, Lucas Lesourd, Eric Frayssinet, Shirley Prado de La Cruz, Sebastien Chenot, Johan-Petter Hofverberg, Marie Vidal, Jean-Yves Duboz

► To cite this version:

Matilde Siviero, Maxime Hugues, Lucas Lesourd, Eric Frayssinet, Shirley Prado de La Cruz, et al. GaN diodes comparative study for high energy protons detection. Journal of Semiconductors, 2025, 46 (9), pp.092503. 10.1088/1674-4926/25020014. hal-05343726

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Submitted on 7 Nov 2025

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# GaN diodes comparative study for high energy proton detection

Matilde Siviero¹, Maxime Hugues¹, Lucas Lesourd¹, Eric Frayssinet¹, Shirley Prado de la Cruz¹, Sebastien Chenot¹, Petter Hofverberg², Marie Vidal², Jean-Yves Duboz¹

¹ Université Côte d'Azur, CNRS, CRHEA, 06560, Valbonne, France

² Université Côte d'Azur, Fédération Claude Lalanne; Cyclotron Biomédical, Centre Antoine Lacassagne, Nice, France

## Abstract

GaN diodes for high energy (64.8 MeV) proton detection were fabricated and investigated. A comparison of the performance of GaN diodes with different structures is presented, with a focus on sapphire and on GaN substrates, Schottky and pin diodes, and different active layer thicknesses. Pin diodes fabricated on a sapphire substrate are the best choice for a GaN proton detector working at 0V bias. They are sensitive (minimum detectable proton beam <1pA/cm²), linear as a function of proton current and fast (<1s). High proton current sensitivity and high spatial resolution of GaN diodes can be exploited in the future for proton imaging of patients.

Proton therapy is an alternative to classical x-ray radiotherapy commonly used for the treatment of cancers. The unique property of protons is their steep stopping profile in matter (Bragg peak) granting to use higher energy doses than in conventional X-ray radiotherapy while strongly limiting the collateral damage of healthy cells during irradiation ¹ ²,³. Nonetheless, in proton therapy and high energy physics, there is a need for proton detectors with high sensitivity and good spatial resolution. At the moment, high energy protons (typically in the range 60 - 230 MeV for proton therapy) can be detected by various techniques, such as ionization chambers (e.g. MatriXX by IBA⁴), silicon detectors (faraday cups, silicon strips ⁵,⁶, silicon pin diodes) or scintillators coupled to CCDs (e.g. Lynx by IBA⁷). However, all these approaches have some drawbacks. For example, the ionization chambers have a good dose resolution but their spatial resolution is limited to a minimum of 500 µm (most often 1 mm) and require a bias of few kV. On the other hand, the scintillators have a limited dynamic range and rapidly saturate at high proton currents. Finally, silicon has a small displacement energy (21 eV) making the silicon-based devices fragile under irradiation by energetic protons. During conventional radiotherapy treatment the patients are irradiated with dose rates of 0.01-0.05 Gy/s while a promising procedure called FLASH therapy using dose rates higher than 40 Gy/s is currently arousing a strong interest of the medical community. Consequently, the detectors daily used for beam calibration have to withstand several Gy of irradiation each day. As the number of proton therapy centres and patients is continuously increasing the development of new proton detectors made of SiC ⁸,⁹, amorphous silicon ¹⁰, and hybrid materials scintillators ¹¹ is currently emerging.

Thanks to its higher displacement energy (109 eV for N and 45 eV for Ga), GaN material is very robust to high energy particle irradiation. The work of A. Floriduz et al. ¹² had previously shown a resistance to degradation of GaN LEDs up to doses of 10¹⁴ protons/cm², while the degradation fluence in Si is 10¹⁰ - 10¹² protons/cm² for proton energies of 50 MeV ¹³. Other studies of Pearton et al.¹⁴ and Logan et al. ¹⁵ clearly showed how GaN is far more robust to particle irradiation than Si or GaAs. Finally, since the fabrication of GaN diodes is already a mature technology being used in visible and UV LEDs, ¹⁶, but also alpha particle ¹⁷ and neutron ¹⁸ detectors, it can be easily developed and scaled to large sizes (wafers up to 6" or more).

We thus propose to use GaN to fabricate detectors for high energy protons with the main application of quality assurance procedures (QA) and proton imaging. “Quality assurance” refers to all the beam characterization procedures that take place every day/week/month in proton therapy clinics¹⁹. These procedures comprehend beam shape detection, beam current and Bragg peak measurements. Proton imaging could be used for patient imaging procedures that are required prior to each proton therapy irradiation. ²⁰CT scans and MRIs are the imaging techniques in use, however the conversion from these
images to proton range introduces an error in beam calibration of 3%. Imaging directly with protons would cut this error, but there is no device ready for this task. ²¹⁻²³

Diodes fabricated with microelectronic procedures can be a few µm large, leading to an extreme spatial resolution out of reach for non-semiconductor devices. Arrays of GaN diodes have thus the potential to be used not only for QA procedures but also to directly acquire patient images with protons. We have previously demonstrated that GaN Schottky diodes were sensitive and linear detectors for protons, with a spatial resolution given by the diode size²⁴, and their response was modelled based on the Bethe-Bloch's law ²⁵.

We hereby present the results of a thorough investigation of several GaN diode structures irradiated with a 64.8 MeV proton beam. The study compares the response of pin and Schottky diodes grown on sapphire and GaN substrates with various active layer thicknesses.

Samples were grown by MOCVD on GaN and on sapphire substrates, with an active region thickness varied from 1 to 10 µm. Schottky and pin structures were grown and processed, following a usual process for mesa etching, contact metal deposition and annealing. Details are given in the Supplementary materials.

The proton detection capability of the diodes was assessed at the MEDICYC line of Antoine Lacassagne proton therapy centre in Nice. The proton energy is 64.8 MeV. The proton current was varied from 1 pA to 10 nA. Details of the proton beam and measurements are given in the Supplementary materials.

All the diodes are first characterized outside of the proton beam, and we refer to this measurement as “dark”. To measure the response of a diode the dark current is systematically subtracted from the current signal measured during irradiation. Unless otherwise specified the samples are irradiated with a proton beam current of 100 pA corresponding to a current density of 132 pA/cm², equivalent to 1 Gy/s.

The noise was estimated from time measurements of the diode current. The noise could be fitted to vary as $\sqrt{i_0^2 + 2el\Delta f}$, where *l* is the diode current, either in the dark or under proton irradiation. The residual noise $i_0$ depends on the set up and on the Keithley. This variation shows that the main source of noise for the irradiated diodes is the shot noise and that it increases as $\sqrt{f}$. We did not observe 1/f noise in our experimental conditions. The measurement bandwidth is defined by the integration time, which was set to 20 ms (NPLC parameter of Keithley = 1).

As previously reported ²⁵, when GaN Schottky diodes are irradiated with an energetic proton beam, electron-hole pairs are generated and they can be collected at the contacts. At 0V bias and small reverse biases, the dark current is small and we observe a photovoltaic behaviour characterized by a quantum efficiency smaller than unity and short response times. In large positive biases, the dark current is large and the diode behaves as a photoconductor with gain and long response times.

In figure 1(a) the response of Schottky diodes (800µm × 800µm) with a 10µm active region fabricated on a GaN substrate and on a sapphire substrate (Al₂O₃) are reported. Due to the lattice mismatch between Al₂O₃ and GaN wurtzite structures ²⁶,²⁷ the sample on sapphire has more defects than the one on GaN. Indeed, the dislocation density is in the range of a few 10⁸ cm⁻² on sapphire and 10⁷ cm⁻² on GaN. Nevertheless, I-V curves do not show a significant difference in the response. The smaller current in forward bias in the diode on sapphire is due to larger (lateral) access resistances.

The diode response at zero bias, as a function of the proton current, is reported in figure 1(b) in log scale. The slope close to unity shows that the response varies linearly with the proton current. A linearity over 5 decades has been observed (not shown here). Linearity is a fundamental parameter of a detector, especially for medical applications requiring high precision. Some deviations of the exponent are
observed, with values smaller (0.99, sapphire) or larger than unity (1.03, GaN). A first possible
explanation to this small disparity could be that the proton beam shape slightly changes with the current
so that the measured linearity depends on the exact position of the diode in the beam. This explanation
is reinforced by measurements with a diffuser (homogeneous beam) in a lower range of proton current
density (not shown here) as in that case a perfect linearity with a coefficient of 1 is determined. A second
contribution to the disparity of the slope coefficient is the vicinity of some diodes to the edge of the chip
carrier where the diodes are bonded. In close proximity of the metal, we have previously observed a
parasitic contribution to the signal due to external electron emission from the metal; this is also the
reason why the linear regression of figure 1(b) for sapphire does not take into account the two points at
low proton beam current: the signal is too small and the parasitic contribution is too important. When
operated at negative biases a small decrease of the linearity coefficient is observed, typically in the 0.85-
0.9 range. On the other side, when operated at large positive biases, in the photoconductive regime,
diodes show a non-linear variation of the response with proton current, with an exponent around 0.5,
typical for a photoconductor.

Overall using a freestanding GaN substrate does not provide major benefits in terms of response and linearity compared to using a sapphire substrate. The detrimental effect of dislocation on the response is not visible in this range. It is interesting to recall that the efficiency of blue LEDs is also comparable on sapphire and on GaN substrates, again in this dislocation density range. Additionally using a sapphire substrate is convenient since it is cheaper and makes scalability to big wafers easier.

After the substrate, the second step is the choice of the detector structure, Schottky versus pin diodes. In figure 2(a) we compare the response and dark current of 10µm thick pin and Schottky diodes (800µm×800µm) on GaN, although the same experiment was repeated on sapphire substrate diodes with analogue results. As expected, the turn on bias of pin diodes is larger and the response in positive biases is larger in Schottky diodes than in pin diodes. However, at 0V or small reverse biases, which is the preferred regime for detection (better linearity), pin diodes have a higher response than Schottky diodes. Pin diodes also show a linear dependence on proton current, although the slope coefficient here slightly deviates from unity (figure 2(b)). In addition to the explanation already given for the dispersion of the slope coefficients, we observe that the slope coefficient tends to be slightly smaller for pin than for Schottky diodes. We do not have any explanation for this. The time-response measurements for pin and Schottky diodes (figure 2(c-d)) gives a further insight into the effect of the bias on the proton detector performance. These measurements were carried out with a proton current of 100pA, but similar results are expected for different proton currents due to the detector linearity. Biasing the diodes with a small reverse voltage provides a stable response with a relative standard deviation of the current of 1.31% at -2V and 0.63% at -1V for the Schottky diodes and of 1.75% at -2V and 1.28% at -1V for the pin structures. For these biasing conditions, the switching time from ON to OFF state is fast, less than 1s, corresponding to the time resolution of the experimental setup. The small and fast transient at the pulse beginning is due to the proton beam opening and not dependent on the set-up. After a fast (less than 1s) turn-off transient, a slower transient is observed, smaller on pin than on Schottky diodes, and likely due to charge de-trapping. This effect decreases at smaller reverse bias and disappears at 0 bias. The response / dark current ratio is smaller at reverse bias than at 0V due to leakage current. This can be clearly observed by the increase of the dark current (before and after the pulse) as negative bias is applied. In addition, the more stable and fast response (no slow transient) is at 0V with a relative standard deviation of 0.81% for pin diodes and of 0.82% for Schottky diodes. At 0V the response has a linear dependence on proton current, with linearity over 3 decades, with a slope of 1.03±0.1 for Schottky on GaN, and of 0.89±0.01 for pin on GaN. The time response measurements for forward biases (supplementary material S1) show that both Schottky and pin diodes have larger and slower response transients, which is explained in terms of photoconductive behaviour: charge trapping and de-
trapping effects are amplified by the photoconductive gain. Similar measurements were made on pin and Schottky diodes with a thinner active region (2µm). While both diodes show a similar time response at zero bias and small reverse biases, response instabilities are observed in Schottky diodes at reverse biases larger than 3V while pin diodes keep a stable response (supplementary material S2), again showing that the effect of traps is larger in Schottky diodes than in pin diodes. All these results indicate that a bias close to 0V leads to the best performance in terms of response time and stability, and pin diodes are preferred to Schottky diodes.

The third part of this work is devoted to investigate the influence of the active undoped layer thickness. For that purpose, Schottky diodes on sapphire with active layer thicknesses of 1µm, 2µm, 4µm, and 10µm have been studied. Capacitance versus voltage measurements have shown that the depletion layer thickness W at 0V in the dark is close to the active layer thickness L (L=1µm W=0.62µm; L=4µm W=3.58µm; L=10µm W=9.25µm). From the C-V measurements, the residual doping $N_D$ can be calculated according to the formula $N_D = \left(\frac{C}{A}\right)^2 \frac{2V_0}{e\epsilon}$, where A is the area of the diode, $V_0$ the internal bias corresponding to the Schottky barrier height (1 eV in GaN), e the electron charge and $\epsilon$ is the dielectrical constant of the material. These C-V measurements indicate a very low effective doping, in the 10¹³ cm⁻³ range. As the residual donor (O or Si) density is larger (typically 10¹⁶ cm⁻³ range), a large compensation exists in the undoped GaN layers grown using these MOCVD conditions, explaining the charge trapping observed earlier. Figure 3(a)-(c) reports the response for many diodes in each sample during irradiation with a proton current of 100pA. A large dispersion of the response is observed, especially the sample with L=4µm. Thus, for a qualitative comparison between samples, we show for each sample the median value of the response. Figure 3(d) shows the response of all diodes at 0V as a function of active region thickness. In spite of the dispersion, we observe that the response increases, but only slightly, with thickness. Theoretically, the response is expected to increase with the depletion region thickness where the internal field separates and collects electrons and holes. As C-V measurements show that the whole active regions are fully depleted, one could expect the response to linearly increase with the active region thickness, thus by a factor of 10 from the thinnest to the thickest sample. However, the experimental response does not follow this trend, and only slightly increases with thickness. The detailed explanation will be given elsewhere but can be summarized as follows. During irradiation, some of the generated holes are trapped by the compensating levels in the bandgap. As a result, the positive charge increases in the depletion region which strongly reduces the compensation and thus restores the doping due to residual donors. Consequently, the depletion region during irradiation is reduced and becomes almost independent of the undoped GaN thickness. A model has been developed to describe this effect and the resulting variation of the response with thickness (blue line on Fig.3(d)). A qualitative agreement is observed, thus confirming that a compensation-decompensation phenomenon limits the enhancement of the response when the n-doped layer thickness increases. In addition, increasing this thickness induces technological difficulties: deep mesa etching by ICP and difficult subsequent lithography on a surface with high mesas. These results and technological considerations lead us to the conclusion that an active region of 1-2 µm is an optimum.

To finish, we would like to show the performance of our optimum structure, namely a pin diode on sapphire with a 2µm thick active layer (Fig.4(a)). This diode has a linear response as a function of the proton current, over 4 decades with a slope of 0.91 (Fig.4(a)). Figure 4(b) shows the time response of the diode when the beam is turned on and off, for a proton current of 1pA, corresponding to 1.32 pA/cm². The average signal goes from 4.39 to 10 pA, giving a response of 5.61 pA. From the values of the current in the OFF state, which is similar to the one in the ON state as the response is weak in this low proton beam irradiation, we can calculate a noise of about 2.7pA (Fig 4(a)). The integration time of the Keithley was set to 20 ms. This allows to estimate the minimum detectable proton current density in these
measurement conditions, which is given by $2.7/5.61 \times 1.32 = 0.63 \text{ pA/cm}^2$. In order to compare with other detection systems, it is interesting to estimate this sensitivity at the reference frequency of 1 Hz. Assuming that the noise varies as the square root of the frequency, the sensitivity at 1 Hz would be $0.63/\sqrt{50} \sim 0.09 \text{ pA/cm}^2$. Such a good sensitivity below $100 \text{ fA/cm}^2$ is very promising for proton imaging applications.

As a conclusion, we have compared various structures based on GaN for the detection of high energy protons, including GaN and sapphire substrates, Schottky and pin diodes and various active layer thicknesses. The use of GaN as a substrate did not prove to bring significant benefits over sapphire. In the perspective of fabricating large size arrays, sapphire is obviously a better substrate choice than GaN. Indeed, the state of the art for MOCVD GaN grown on $Al_2O_3$ substrates up to 6 inches is already established. Pin diodes have proven to have a more stable and higher response than Schottky diodes when working at 0V or close to 0V in reverse bias.

The voltage bias choice is not obvious and it requires to consider the final application of the device. A bias of 0V would provide a detector with good linearity and stability, which is well suited for proton beam characterization in the frame of QA. For applications requiring a larger response, applying a reverse bias leads to a slightly larger response but at the expanse of stability and linearity. For fabricating arrays of GaN diodes with a read out by an external Si circuit, we also have to consider the range of bias voltages that can be applied by these Si circuits.

As a perspective, the next step will be to fabricate 1D and 2D arrays of such GaN detectors, with high spatial resolution (better than ionisation chambers), a large dynamical range (better than scintillators) high resistance to large proton currents (better than Si) for QA on the one hand, and a large sensitivity (better than ionisation chambers) for proton imaging on the other hand. Work is in progress and results will be shown in the coming months. Preliminary results on 128 pixels arrays with an improved process on 3" wafers show a response uniformity very largely improved compared to the one presented here. A response uniformity of a few % has been achieved. Extensive degradation studies are also in progress to assess the durability of pin diodes after irradiation with high doses.
<figure><img src="image_2.png" /><figcaption>Figure 1: (a) Response (log scale) - Voltage curves for Schottky diodes with a 10µm active layer on GaN and on sapphire substrates with a proton beam current density of 132 pA/cm². (b) Diode response (log scale) as a function of proton beam current density at 0V. The log-log fit show a linearity close to 1. The insert shows the cross-section structure of the diodes.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: (a) Current (log scale) - Voltage curves for Schottky and pin diode with a 10µm active layer on GaN substrate at 132 pA/cm². (b) Diode response (log scale) as a function of proton beam current density at 0V. The log-log fit show a linearity close to 1; the insert shows the diodes structure. (c) Time response (Current in log scale) graph for Schottky diode on GaN. The yellow section of the graph refers to the proton beam being ON with a proton current density of 132 pA/cm², in the white part the beam is turned off. (d) same as c for a pin diode on GaN.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 3: (a-c) Response (log scale)-voltage curves for Schottky diodes with 1,2,4µm active layer (respectively figure a,b,c) on a sapphire substrate with proton beam current density of 132 pA/cm². The black curve is the median value of the response. (d) Response at 0V for Schottky diodes on sapphire in function of the thickness of the active layer. Simulation results are compared to the experimental ones, and to the linear increasing trend of the response that could be expected prior to the simulation.</figcaption></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>Figure 4: (a) Response of a pin diode on sapphire with a 2µm active layer as a function of the proton beam current density. The linearity is close to one (0.9). The response values are compared with the noise level of 2.7 pA, measured with an integration time of 20 ms. In the insert is represented the structure of the sample. (b) Time response of the same diode at 0V. The yellow area indicates when the beam is on (with a current density of 132 pA/cm²).</figcaption></figure>

SUPPLEMENTARY MATERIAL
See supplementary material for the sample preparation, proton beam irradiation details, time response graphs at positive and negative bias.

## ACKNOWLEDGEMENTS

We acknowledge financial support from MATRIX (ANR-22-CE92-0047) and with financial support from ITMI Cancer of Aviesan within the framework of the 2021-2030 Cancer Control Strategy, on funds administrated by INSERM through the project NECTAR. Matilde Siviero acknowledges funding from the French-German University/Saarbrücken (contract CDOC-06-2022).

## AUTHOR DECLARATIONS

### Conflict of Interest

The authors have no conflicts to disclose.

### Author contributions

**Matilde Siviero:** Investigation (equal); Methodology (equal); Resources (equal); Visualization (lead); Writing/original draft (lead). **Maxime Hugues:** Conceptualization (equal); Investigation (equal); Methodology (equal); Resources (equal); Funding Acquisition (equal); Supervision (equal); Writing – review & editing (equal); Validation (equal). **Lucas Lesourd:** Software(lead); Resources (equal). **Eric Frayssinet:** Resources (equal). **Shirley Prado de la Cruz:** Resources (equal). **Sebastien Chenot:** Resources (equal). **Petter Hofverberg:** Resources (equal). **Marie Vidal:** Resources (equal). **Jean-Yves Duboz:** Conceptualization (equal); Project Administration(lead); Methodology (equal); Investigation (equal); Funding Acquisition (equal); Supervision (equal); Writing – review & editing (equal); Validation (equal).

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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# SUPPLEMENTARY MATERIAL

## Sample preparation

The first set of samples are grown on commercially available 350 µm thick GaN substrates (Lumilog) of 2" using metal organic chemical vapour deposition (MOCVD) in an Aixtron 6 Closed Coupled Showerhead. A thin n-doped GaN buffer layer is first deposited followed by a 10 µm not-intentionally-doped (nid) layer of GaN. For the pin diodes, a p-doped GaN layer (0.5µm) followed by a 30 nm p+ GaN layer are added to the structure. An ohmic contact (Ti/Al/Ni/Au) is deposited on the back side and annealed at 750°C. For Schottky diodes, Pt/Au is deposited on top of the nid GaN layer. For pin diodes, Ni/Au is deposited on top the p+ layer and annealed at 500°C for 4 minutes.

The second set of samples are grown by MOCVD but on 2" sapphire substrates. After a 2µm GaN buffer layer, a n-doped GaN layer (4µm) is grown followed by a nid layer of GaN. We varied the thickness of the nid-layer L (layer L= 1µm, 2µm, 4µm, 10µm). For pin diodes; p and p+ GaN layers are added, as for the first set. The mesa is etched using a BCl3 Cl2 inductive coupled plasma (ICP-RIE) and an n contact (Ti/Al/Ni/Au) is deposited on the bottom of the mesa (n-GaN) and annealed at 750°C for 30 seconds. The p contact of (Ni/Au) is deposited on the top of the mesa for pin diodes and annealed at 500°C for 4 minutes. Pt/Au is used for Schottky diodes and is annealed at 300°C for 5 minutes.

The diodes presented in this work are square with dimensions of 800µm×800µm, but we also studied 300µm×300µm and 200µm×200µm.

After cleanroom processing the samples were cut and glued on carrier chips or on a PCB, and finally wired bonded with micrometric gold wires.

## Proton beam and measurement details

The isochrone cyclotron delivers proton pulses with a duration of 7 ns and a frequency of 25 MHz (and thus a 40 ns bunch separation). The line provides protons with a fix energy of 64.8 MeV while the beam can be tuned to specific proton current densities between 10 nA/cm2 (without any diffuser) and 2fA/cm2 (with a diffuser which broadens and homogenizes the beam). The proton beam without diffuser has a Gaussian shape with a width of about 4,2 mm in one direction, and a larger extension (1,5 cm) in the other direction, therefore a proton beam current of 100pA corresponds to current density of 132 pA/cm². During irradiation the diodes are positioned at the centre of the beam using a mechanical translation stage. The measured sample is in the air at room temperature and the diodes are connected to a Keithley 2450 Source-meter, which applies the bias and measures the current. During irradiation the Keithley and the connection cables are placed outside of the irradiation field and are shielded by blocks of PMMA to prevent signal noise and irradiation damage.
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /><figcaption>Figure S0 : Capacitance-voltage measurement of didoes with an active layer thickness of 1,4 and 10 µm.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure S1: (a) Time response (Current in log scale) for Schottky diode on GaN in positive bias (2V). (b) Time response for pin diode on GaN in positive bias (2V).</figcaption></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Figure S2: (a-b) Time response of a Schottky and pin diode ((a) and (b) respectively) on sapphire with an active region of 2µm. The response is measured at different biases and represented as a function of time. The beam is initially off and then turned on, the yellow area indicates when the proton beam is turned on (132 pA/cm²).</figcaption></figure>

<figure><img src="image_16.png" /></figure>