Comparison of semi-insulating GaAs and 4H-SiC-based semiconductor detectors covered by LiF film for thermal neutron detection

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Comparison of semi-insulating GaAs and 4H-SiC-based semiconductor detectors covered by LiF film for thermal neutron detection ⁎

Katarína Sedlačkováa, , Bohumír Zat'kob, Andrea Šagátováa, Vladimír Nečasa, Pavol Boháčekb, Mária Sekáčováb a

Slovak University of Technology in Bratislava, Faculty of Electrical Engineering and Information Technology, Institute of Nuclear and Physical Engineering, Ilkovičova 3, 812 19 Bratislava, Slovakia b Slovak Academy of Sciences, Institute of Electrical Engineering, Dúbravská cesta 9, 841 04 Bratislava, Slovakia

A R T I C LE I N FO

A B S T R A C T

Keywords: Semiconductor detectors GaAs detectors SiC detectors Thermal neutrons MCNPX Simulation

In this contribution we have focused on comparison of spectroscopic properties of semi-insulating (SI) GaAs and 4H-SiC detectors of thermal neutrons fabricated at the Institute of Electrical Engineering SAS in Piešťany. 6LiF reactive film has been applied on Schottky contact as a convertor of thermal neutrons to detectable charged particles (tritons and α particles). Optimal thickness of the 6LiF film has been determined for front side irradiation using MCNPX code to be ca 25 μm. From the energies deposited by secondary charged particles in the active volume of a detector, corresponding responses have been calculated for different thicknesses of the 6LiF conversion layer. The calculated responses have been compared with those collected by measurements using thermal neutrons generated by 239Pu-Be neutron source. Thermal neutron spectra have been recorded by SI GaAs and 4H-SiC detectors using different 6LiF film thicknesses. Generally, in spite of limited spectroscopic performances of the studied detectors, two hills related to tritons (2.73 MeV) and α particles (2.05 MeV) are discernible in all spectra beside a noise peak positioned in the low energy region. The specific differences between SI GaAs and 4H-SiC detector response to thermal neutrons will be closely discussed in the paper.

1. Introduction There is a variety of high quality semiconductor materials providing a broad range of their utilization as detectors for different kind of ionizing radiation. Semiconductor neutron detectors represent an attractive alternative to other types of detectors due to their compact size, fast charge collection and lower operational voltage [1]. The choice of a detector material is usually subjected to specific application demands. Nowadays, materials like Si, SiC, GaAs and CdTe, are the most commonly employed to fabricate radiation detectors. In our recent research, we have focused our interest on Schottky barrier detectors based on semi-insulating (SI) GaAs- and 4H-SiC-epitaxial layer [2,3]. Detectors based on SI GaAs have high reaction rates ensured by high mobility of charged carries (µelectrons > 8000 cm2 V−1 s−1 and 2 −1 −1 µholes = 400 cm V s at room temperature (RT)), good spectrometric properties, high efficiency for X-ray and γ-ray detection due to relatively high atomic number of Ga and As, high resistance against radiation damage [4,5] and are able to operate reliably at RT due to quite wide bandgap (1.43 eV) [6,7]. Other important features are a good technology base and low production and operating costs.

⁎

Similarly, SiC-based detectors are characterized by fast reaction rates and high spectrometric performance. The 4H–SiC polytype is perspective due to its high breakdown voltage of about 2 × 106 V/cm, electron mobility of about 900 cm2V–1s–1, and saturation drift velocity of 2 × 107 cm/s [8]. Compared to Si detectors, SiC-based detectors posses much better radiation hardness and temperature resistance due to wide bandgap of the base material (3.26 eV at RT) and high displacement threshold energy (EdSi = 35 eV and EdC = 25 eV) [1]. SiC detectors are therefore demanded in harsh environments, where the ability to operate at much higher temperatures (up to several hundreds of degrees Celsius) or under strong radiation is asked [9–11]. But for the present, their drawbacks are still small active areas, low thicknesses of epitaxial layers with relatively high concentration of impurities requiring high voltages to reach a full depletion (up to 1 kV for 100 μm) and high costs for epitaxial layer fabrication [12]. Regardless of the choice of the semiconductor material, registration of thermal or fast neutrons requires additional medium serving as a converter of neutral particles to easily detectable, usually charge-carrying particles [13]. In the case of thermal neutrons, the conversion layer of 6LiF is often used to produce α particles and tritons via nuclear

Corresponding author. E-mail address: [email protected] (K. Sedlačková).

https://doi.org/10.1016/j.apsusc.2018.05.121 Received 8 March 2018; Received in revised form 26 April 2018; Accepted 17 May 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Sedlacková, K., Applied Surface Science (2018), https://doi.org/10.1016/j.apsusc.2018.05.121

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Ionization [eV/Angstrom]

reactions, which can be detected with high probability [14,15]. This contribution is dedicated to comparison of detection properties of two semiconductor structures, namely bulk SI GaAs and 4H-SiC high quality epitaxial layer detectors, covered by 6LiF reactive film for thermal neutron registration. Simulations as well as experiments have been employed to design the detectors and to evaluate their response to thermal neutrons. 2. Theory and calculations Neutrons have no electric charge and therefore cannot ionize any detecting medium directly. The primary mechanism for slow (thermal and epithermal) neutron detection is mainly neutron capture by nuclei in the absorbing material followed by emission of secondary ionizing radiation. To convert neutrons to detectable charged particles, an appropriate neutron reactive film, so called conversion layer, is utilized and usually applied as an external coating on the top of a detecting medium. There are many materials, which can satisfyingly facilitate registration of thermal neutrons, like 10B, 6Li, LiH, 157Gd, Li3N, Li2C2, Li2O, 235U or Zr10B2 [9]. Lithiumfluoride, 6LiF, is often used as converter due to relatively high energies of reaction products. In this material, the neutron capture reaction on 6Li results in production of α and triton particles, which are released in opposite direction (Fig. 1) and carry following energies: 6

Li + n → α (2.05 MeV) + 3H (2.73 MeV)

Diode

tritons 2.73 MeV

20 10 0 5

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Fig. 2. The Bragg curves for 2.73 MeV tritons and for 2.05 MeV α particles in 6 LiF.

the detection efficiency of a semiconductor detector is for charged incident particles close to 100%, the resultant detection efficiency in this case will be consistent with the relative neutron sensitivity. The relative neutron sensitivity is defined as the integral flux density of reaction products (3H and α particles) counted in the active region of a detector divided by the integral flux density of incident neutrons. The values of the respective integral flux densities were derived from simulations performed using a point source of thermal neutrons (E = 0.0253 eV) collimated into a cone at a perpendicular distance of 1 cm to a detection structure. The outside medium was air and the number of ionizing particles used in the simulations was 2.108. Relative neutron sensitivities calculated for different conversion layer thickness are depicted in Fig. 3. As obvious, a maximal value of ca 4.76% is reached for a thickness between 25 μm and 30 μm, and then slowly decreases for thicker 6LiF conversion film. The results are in agreement with those presented in Refs. [1,19]. H. Huang at al. [1] calculated conversion efficiency using FLUKA code and its maximal value of approximately 5% was achieved for 25–30 μm thick conversion layer. Uher et al. [19] used a Monte Carlo simulation package combined MCNP-4C, SRIM and an own C++ Monte Carlo code to predict the detection efficiency, which reached for frontal irradiation its maximum of 4.48% for 7 mg/ cm2 of 6LiF (corresponding to a thickness of 26–31 μm, depending on the assumed mass density of the deposited film). MCNPX code has been also used to model the energy deposition by reaction products and thus to evaluate the response of detectors to thermal neutrons by means of the pulse height tally calculation. The simulation details can be found in Ref. [20], which also discusses the option to consider the powder-like nature of the applied 6LiF reactive

Q = 4.78 MeV

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6 LiF conversion film proved advantageous also due to satisfying thermal neutron absorption cross-section (σ = 942 b at a neutron energy of 0.0253 eV) and acceptable chemical properties. The thickness of the applied reactive film plays a crucial role in detector design. For front irradiation (i.e. a conversion layer is placed between a source of radiation and a detector), the detection efficiency is increasing with conversion layer thickness, levels off at a certain thickness as a result of the finite range of reaction products and after reaching its maximum value starts to decrease due to thermal neutron absorption in the conversion layer [16]. The ionization properties of tritons and α particles traveling through 6LiF conversion film have been simulated using SRIM code [17] and the resultant Bragg ionization distributions for the 2.73 MeV - tritons and for the 2.05 MeV - α particles shown in Fig. 2 manifest that the average range of the heavier α particles is remarkably lower as compared to lighter tritons; i.e. 6.1 and 33.7 μm, respectively. The transport of reaction products through 6LiF layer was simulated also using MCNPX code in order to follow the above described mechanism and to determine the optimal thickness of the conversion layer. MCNPX is a widely spread calculation code based on Monte Carlo algorithms used to simulate interaction of radiation with matter. It contains high-quality physics and has access to the most up-to-date cross-section data [18]. The code enables to follow the transport of neutrons and also products from neutron interactions like α particles and tritons, whereby the nuclear data tables beside model physics are employed. An optimal thickness of the conversion film is its thickness, for which the highest detection efficiency can be reached. Assuming that

V

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Fig. 1. Illustration of the thermal neutron detection principle [16]. 2

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(a) Gauss energy broadening (GEB) Corresponding LiF thickness:

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Fig. 4. Detector responses to thermal neutrons calculated by MCNPX code using powder-like model for different 6LiF conversion layer thicknesses, (a) using Gauss energy broadening (GEB) function, (b) without GEB function.

film, especially for smaller thicknesses. The pulse height tally assuming full absorption of reaction products is shown in Fig. 4. A special tally treatment function GEB (Gaussian Energy Broadening) has been applied to pulse height tallies to take into account the observed energy broadening of the physical radiation detector. Resultant responses are shown in Fig. 4(a) and are better fitting to responses of a detector with worse energy resolution (e.g. GaAs, as will be shown in Section 4). Fig. 4(b) depicts pulse height tallies calculated without energy broadening function, where particularly sharper edge of triton peaks is pronounced. According our results, this simulation is better matching to a detector with higher energy resolution (SiC in this case). Depicted spectra include responses from charged reaction products, i.e. from tritons and α particles. The contribution from tritons manifests as a dominant peak positioned at the energy light above 2.5 MeV, as expected due to the presence of the most energetic tritons created in the vicinity of the 6LiF-detector interface. Analogously, the contribution from produced α particles to the total response is shifted to lower energies below 2 MeV. Its lower amplitude is caused by the fact that α particles undergo during interactions fewer steps to deposit the whole carried energy as compared to lighter tritons. With increasing thickness of the 6LiF film, the contribution from tritons is getting broader and predominates in the pulse height tally. The total response height apparently tends to decrease for reactive film thicknesses above the optimal value. Finally, using SRIM code we have calculated the ranges of 2.73 MeV - tritons and 2.05 MeV - α particles in GaAs and in SiC in order to know the minimal thickness of the active region ensuring full reaction product absorption. These are for tritons and α particles 32.0 μm and 6.1 μm in GaAs and 28.1 μm and 4.9 μm in SiC, respectively.

wafer. For SI GaAs detectors, the depletion region sensitive to ionizing radiation is penetrating into the depth of substrate with increasing applied reverse voltage linearly for a voltage higher than 20 V [21]. As the ranges of tritons and α particles arising from neutron interaction in 6 LiF are in GaAs rather short, even small applied reverse voltages will create enough thick active volume region to stop all neutron products. A reverse voltage of 50 V resulting in depletion of a region having thickness of ca 60 μm was used for our experiments. The choice of low voltages proved advantageous also in suppression of γ rays from neutron source background [3]. 3.2. 4H-SiC detectors Several 4H-SiC-based detector structures were prepared at the Institute of Electrical Engineering SAS in Piešťany from a 70 μm thick nitrogen-doped 4H-SiC layer (with donor doping of about 1 × 1014 cm−3) grown by LPE (liquid phase epitaxy) on a fragment of a 3″ 4H-SiC wafer (donor doping level ∼2 × 1018 cm−3, thickness 350 μm), by growing a 0.5 μm thick n++-SiC buffer layer with donor concentration of 1 × 1018 cm−3. The radiation detector surfaces were prepared by evaporation of a double layer of Au-Ni/4H-SiC with a thickness of 90/40 nm on both sides of the wafer fragment using a high vacuum electron gun apparatus. The Schottky barrier contact with a diameter of 4.5 mm was formed on the epitaxial layer using photolithography masking. Around the Schottky contact two guard rings were also created. A full area contact of Ni/Au double layer was evaporated on the other side (substrate). Prior to evaporation, the sample was cleaned in boiling acetone and isopropyl alcohol, washed in deionized water and dried by nitrogen flow [2]. For SiC detectors, a higher reverse voltage has to be applied to reach the required thickness of the active region. In our experiments, a bias of ca 200 V resulting in depletion of a region with thickness of ca 35–45 μm was used. Analysis of the active region thickness of a 4H-SiC detector can be found in [22].

3. Materials and experimental details 3.1. SI GaAs detectors SI GaAs detector structures have been prepared from bulk VGF (Vertical Gradient Freeze) SI (semi-insulating) GaAs wafer (producer: Wafertech UK) at the Institute of Electrical Engineering SAS in Piešťany. SI GaAs is overcompensated material and its typical resistivity is of 1 × 107–1 × 108 Ω cm. The galvanomagnetic measurements showed the resistivity of 9.1 × 107 Ω cm and the electron Hall mobility of 6285 cm2/Vs at RT. The wafer was polished from both sides to the thickness of 350 µm. On the top side of GaAs wafer the circle blocking Ti/Pt/Au (10 nm/30 nm/90 nm) Schottky contact, of 6 mm diameter, was evaporated. A whole area quasi ohmic contact from Ni/AuGe/Au (30 nm/50 nm/90 nm) multilayer was formed on the back-side of the

3.3. 6LiF conversion layer Reactive films were prepared from a 6LiF powder (enriched in 6Li isotope to 95%) mixed with distilled water and a glue solvent and deposited on the top Schottky contact of a detector using a micropipette. The coats were dried using infrared light. Different quantities of the mixture were applied in order to achieve thicknesses of the film from ca 5 to 50 µm (recalculated roughly from a 6LiF mass density of 2.54 g/ cm3). This procedure did not ensured, however, satisfying uniformity of 3

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Fig. 5. Reverse current-voltage characteristics of the studied SI GaAs (a) and 4H-SiC. (b) detectors before and after 6LiF film deposition measured at RT.

5.48 MeV-peak corresponding to α particles from 241Am source, its slight broadening and a small shift to the lower energies has to be counted with, as the measurement was performed in air (distance between the source and the top contact of the detector was about 1–2 mm). The energy resolution of the tested detectors can be therefore only roughly derived from the full width at half maximum (FWHM) of this monoenergetic 5.48 MeV 241Am peak and reaches for a GaAs detector approximately 670 keV (Fig. 6(a)) and for a SiC detector 180 keV (Fig. 6(b)). The obtained calibration curves show a good linearity and are presented in Fig. 7. Detection tests with monoenergetic protons from Van de Graaf accelerator in the energy range from 0.45 MeV up to 1.9 MeV were also performed and used to support the calibration process [2].

the deposited layers. Consequently, a relatively high uncertainty in conversion layer thickness determination has to be taken into account. 3.4. Electrical characteristics of detectors The functionality of the studied detectors after 6LiF film deposition was verified by current-voltage characteristics measurements before and after 6LiF film deposition. Fig. 5 shows the current-voltage curves of representative SI GaAs and 4H-SiC detector samples (6LiF thickness close to its optimal value is declared in the figure) obtained in reverse direction at room temperature. As obvious, no substantial deterioration of electrical properties was detected. In the case of GaAs detectors, a slight increase of the reverse current by 20 nA at 200 V and a small decrease of the breakdown voltage from ca 340 V to 293 V was observed. Reverse current decrease might be, however, the most probably attributed to some temperature fluctuations during measurements. For SiC detectors, conversion layer deposition had no effect on their electrical performance.

4. Results Fig. 8 shows representative examples of thermal neutron spectra collected by SI GaAs at an applied reverse bias of 50 V (a) and by 4HSiC detectors at an applied reverse bias of 200 V (b) using 6LiF films of different thicknesses. Declared 6LiF thicknesses differ slightly for GaAs and SiC detectors. Preparation of equally thick layers was obscured by limitations in the process of 6LiF-powder mixture preparation and deposition. Precision of the 6LiF thicknesses determination will be discussed and expressed numerically at the end of this chapter. The responses of the GaAs and SiC-based detectors to thermal neutrons are very similar; two hills related to tritons (2.73 MeV) and α particles (2.05 MeV) are visible in all measured spectra. The measured responses show that the shape of the spectra is changing with increasing 6LiF thickness. The contribution from tritons to the resultant response is getting broader for thicker conversion layers because the tritons produced immediately after entering the 6LiF layer pass a longer distance and consequently, also particles having higher energy losses enter the active volume of a detector. The height of the corresponding peaks is gradually rising with increasing 6LiF thickness up to reaching its optimal value of 25–30 μm. After that it is slightly falling down (manifest especially in Fig. 8(a)) due to thermal neutron absorption followed by decreased flux density of reaction products born close to the 6LiF-detector interface. The peaks corresponding to α particles display quantitatively lower height due to their greater mass. One can also observe a steeper fall of the response corresponding to SiC detectors as compared to GaAs detectors. This behaviour can be explained by a better energy resolution of SiC detectors (see Section 3.5). A noise peak positioned in the low energy region is more

3.5. Spectra measurement details The response of detectors to thermal neutrons was measured using a PuBe neutron source, which generates beside slow also fast neutrons with a mean energy of 4.4 MeV with a fluency of 1.7 × 107 neutrons per second into a solid angle of 4π. Fast neutrons were moderated by pyrolytic graphite to obtain neutrons of energies yielding requested nuclear reaction in 6LiF film. It has to be noted that like each source of neutrons also 239PuBe source produces γ rays by activation of the source shielding, which can be also registered by radiation detectors with particular efficiency. Concerning the measurement geometry, the source to detector distance in graphite container was about 6 cm. Detectors were connected to CANBERRA charge sensitive preamplifier and portable spectroscopy workstation based on a digital signal processing InSpector 2000 with Genie2000 software. The experiments were performed at RT. Studied detectors were tested using 5.48 MeV-α particles from 241 Am radioisotope source with the aim to allocate peaks from a measured neutron response. Collected spectra are shown in Fig. 6, where the 5.48 MeV-241Am peak is plotted beside a thermal neutron response (detectors with 6LiF film of a thickness declared in the picture) represented by two neutron product peaks (2.73 MeV tritons and 2.05 MeV α particles). The peaks corresponding to 2.73 MeV tritons and 2.05 MeV α particles are broadened due to the fact that reaction products loose a part of their energy when traveling through the 6LiF conversion layer toward the active region of a detector. For the 239

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pronounced for GaAs detectors due to relatively high efficiency for γ ray registration. In spite of using low reverse voltage as discussed in Section 3.1., SiC detectors exhibit still better properties in suppression of

accompanying low energy γ rays. Poor spectra statistics manifested by SiC detectors is caused mainly by the smaller detector size and can be improved by prolonging the time of measurement.

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satisfying correspondence of calculated efficiency with values acquired using measurements has been obtained. The relatively low value of the corresponding detection efficiency can be slightly enhanced when irradiated from the back side. In this case, the region with highest reaction rate lies close to the detector and only the detector material can interfere with registered neutrons [16]. Other possibility to increase the detection efficiency is creating a 3D microstructure of dips, trenches or pores in the detector and filling it with a neutron converter [19].

Fig. 9(a) compares measured response of the SI GaAs detector covered by a 9.5 μm thick 6LiF film with pulse height tally simulated using GEB function in the MCNPX code. Analogical comparison is presented in Fig. 9(b) for the 4H-SiC detector covered by a 9 μm thick 6 LiF conversion layer, where the pulse height tallies were calculated without assuming Gauss energy broadening, as discussed in Section 2. Apparently, simulated responses fit satisfyingly the real data in the region, where the peak from tritons occurs. The most visible discrepancy as compared to measurement is in the region, where the α particles contribute to the total response. MCNPX simulations underestimate this part obviously. This tendency can result from the fact that the code uses physics models to transport the α particles created in the nuclear interactions, which might have limited accuracy in the inspected energy region. It has to be noted, that 6LiF thicknesses of 9.5 μm and 9 μm, respectively; were chosen in Fig. 9 for comparison only demonstratively and agreement between measured and simulated results differs slightly for other 6LiF thicknesses. Fig. 10 compares simulated detection efficiency (closely described in Section 2) with efficiency values derived from measured spectra for GaAs and SiC detectors (integral of counts in α and triton peaks). Error bars provided in the figure for x-axes (6LiF thickness) reflect limitations in the process of the 6LiF film preparation and deposition (weighting, mixing, spraying, drying, etc.). Error bars for y-values (counts in peaks) were determined mainly by the inaccuracy of detector to source distance adjustment and account for an error of about 3–5%. As obvious,

5. Conclusion Both fabricated SI GaAs and 4H-SiC semiconductor structures proved capable to register thermal neutrons when covered by a 6LiF film having a suitable thickness. The resultant detection efficiency of studied detectors is determined is in this case primarily by the thickness of the 6LiF conversion layer and its maximal value lies at ca 4.8% for 25–30 μm thick 6LiF film. The effect of 6LiF film thickness on spectrometric properties has been simulated using MCNPX code and the obtained pulse height spectra were compared with experimental data. Presented results showed an acceptable similarity. Because of very different production costs of SI GaAs and 4H-SiC semiconductor detectors (GaAs-based detectors are more than 10 times cheaper), it is important to consider the type of application and environment, where the detectors will be utilized. For operation in harsh environments (high radiation, high temperatures), SiC-based detectors 6

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are much more suitable in spite of their higher costs. Furthermore, their spectrometric properties are almost independent of the applied reverse voltage and low efficiency for accompanying γ rays can be also beneficial. Finally, outstanding feature of semiconductor detectors are their small dimensions allowing the possibility to arrange them into twodimensional fields and thus to create a position sensitive sensors enabling spatial monitoring. In addition, their ability to monitor also high neutron fluencies makes them interesting for many applications, especially for digital radiographic imaging.

[9]

[10]

[11]

[12]

Acknowledgement This work was partially supported by the Slovak Grant Agency for Science through grant 2/0152/16, by the Slovak Research and Development Agency under contract No. APVV-0321-11 and by the Project Research and Development Centre for Advanced X-ray Technologies (ITMS code 26220220170) of the Research & Development Operational Program funded by the European Regional Development Fund (0.7).

[13]

[14]

[15]

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