† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 11675198, 11875097, 11975257, 61774072, 61574026, and 61971090), the National Key Research and Development Program of China (Grant Nos. 2016YFB0400600 and2016YFB0400601), the Fundamental Research Funds for the Central Universities, China (Grant No. DUT19LK45), the China Postdoctoral Science Foundation (Grant No. 2016M591434), and the Science and Technology Plan of Dalian City, China (Grant No. 2018J12GX060).
Nowadays, the superior detection performance of semiconductor neutron detectors is a challenging task. In this paper, we deal with a novel GaN micro-structured neutron detector (GaN-MSND) and compare three different methods such as the method of modulating the trench depth, the method of introducing dielectric layer and p-type inversion region to improve the width of depletion region (W). It is observed that the intensity of electric field can be modulated by scaling the trench depth. On the other hand, the electron blocking region is formed in the detector enveloped with a dielectric layer. Furthermore, the introducing of p-type inversion region produces new p/n junction, which not only promotes the further expansion of the depletion region but also reduces the intensity of electric field produced by main junction. It can be realized that all these methods can considerably enhance the working voltage as well as W. Of them, the improvement on W of GaN-MSND with the p-type inversion region is the most significant and the value of W could reach 12.8 μm when the carrier concentration of p-type inversion region is 1017 cm−3. Consequently, the value of W is observed to improve 200% for the designed GaN-MSND as compared with that without additional design. This work ensures to the researchers and scientific community the fabrication of GaN-MSND having superior detection limit in the field of intense radiation.
Due to the shortage and rising cost of 3He gas, the application of 3He proportional counters for detecting neutrons is restricted.[1] Si-based semiconductor neutron detectors have been utilized extensively as an alternative method for detecting neutrons during the past decade.[2–5] Nevertheless, they suffer radiation damage due to the small bandgap and displacement energy of Si.[6] With the development of wide bandgap semiconductors, GaN is considered to be the potential candidate for neutron detection under strong irradiation and high temperature conditions[7,8] due to its wide band gap (3.39 eV), high critical breakdown electric field (3 × 106 V/cm), large displacement energy (22 eV), high density (6.2 g/cm−3) and good thermal stability (melting point: 2500 °C).[9–11] However, the detection performance such as detection efficiency and energy resolution of the neutron detector based on GaN is still not comparable to those based on Si,[12–14] which is due mainly to two reasons. (i) The planar structure is commonly employed in GaN neutron detectors, which cannot avoid the low detection efficiency restricted by the conflict between the long neutron mean free path and the short-charged particle range in neutron conversion layer.[15] (ii) The unintentionally doped GaN is usually of n-type with the background carrier concentration of ∼ 1016 cm−3. Additionally residual shallow donors generated during material growth in the metal–organic chemical vapor deposition (MOCVD) or the hydride vapor phase epitaxy (HVPE),[15,16] bring difficulties for the enlargement in the width of depletion region (always only few micrometers).[17,18] Thus, it is urgently needed to find a solution for GaN-based neutron detectors that could overcome the shortcomings of the planar architecture and achieve a larger depletion region at the same background carrier concentration.
A novel GaN micro-structured neutron detector (GaN-MSND) is proposed in this paper. The advantages of GaN-MSND over the planar structure are analyzed and the significance of the wide depletion region for the detection performance is discussed in detail. Moreover, Sentaurus-TCAD software is utilized to investigate three methods such as the method of modulating trench depth, the method of introducing dielectric layer and p-type inversion region to improve the width of depletion region of GaN-MSND. The mechanism and comparison of these methods are analyzed. It is expected that the GaN-MSND could enhance the detection efficiency of the planar neutron detector and achieve the superb detection performance with the improvement of the depletion region width. Consequently, such a novel strategy will be used in future to make the neutron detectors capable to cope with the extreme conditions.
Figure
When the detector is operated at a reverse bias, the depletion region (near the p-GaN/i-GaN junction) is expanded obviously due to the large difference in concentration between p-GaN and i-GaN as marked in Fig.
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The simulations are performed by using the Sentaurus-TCAD. The doping-dependence and high-field-saturation models are employed for mobility. The Shockley–Read–Hall model is utilized to calculate the effect of charging and discharging of trap. To investigate the avalanche breakdown effect in the detector, the Selberherr’s impact ionization model is used to simulate the reverse characteristics.
Figure
![]() | Fig. 2. (a) Electric field distribution in GaN-MSND at Vr = 150 V, and (b) electric field profile of GaN-MSND extracted along vertical direction (x-cut). |
![]() | Table 1. Structural parameters of GaN-MSND adopted in the work. . |
The trenches filled with neutron convert material 6LiF which plays a role of transferring the neutrons into the charged particles are crucial for the GaN-MSND. It should be noted that the modulation of the trench depth d (as shown in Fig.
![]() | Fig. 3. (a) Reverse characteristics of GaN-MSND versus voltage for various d values, and (b) W and Vr extracted from the simulation results versus d. |
To explain the changing trend of Vr, the distribution of electric field of the device is analyzed. Figure
Based on the optimized results, a possible method to further improve Vr and W is to introduce a dielectric layer on the trench surface as shown in Fig.
Figure
Conversely, the electron accumulation region is formed near the trench surface due to the positive Qf at the SiO2/GaN interface, which is the main factor of increasing the surface leakage along the sidewalls and reducing the Vr as shown in Fig.
The introducing of p-type inversion layer is another way to increase Vr and W as shown in Fig.
Figure
The GaN-MSND with p-type inversion region can significantly improve Vr and W, but the vertical p-type inversion region on the sidewall of GaN-MSND is not easy to realize in the device process. Considering the achievability of the process, the sidewalls are further etched into trapezoids so that the p-type region could easily be formed by ion implantation. The GaN-MSND, of which Np is 1 × 1017 cm−3, with the trapezoidal sidewalls is simulated and its Vr and W are improved to 1342 V and 13.6 μm as shown in Fig.
The transport distance of the charged particles in GaN depends on their energy. Upon absorbing a neutron, the 6LiF fissions into two reaction products, i.e. an alpha particle (2.06 MeV) and a 3H particle (2.73 MeV).[12] Since the energy loss per unit distance of alpha particle in GaN is larger than that of 3H, only the alpha particles incident into the detector is considered, while 3H is supposed to go throughout the device. The electron–hole pairs are generated as the alpha particles pass through the depletion region and drift towards respective electrodes by the electric field. The alpha particles with 2.06 MeV have a transport distance of approximately 5 μm in GaN. Therefore, for a given d of 5.5 μm, W should be at least 10.5 μm so that the detector can collect the alpha particles generated at the bottom of the trenches. As mentioned above, the GaN-MSND without additional design has a maximum W value of 4.7 μm by modulating the trench depth, while adding Al2O3 layer could increase W value to 6.2 μm. In contrast, the GaN-MSND with p-type inversion region has a largest W value of 12.8 μm and thus, it could be the most appropriate method to fabricate the detector with high detection performance.
In this work, three methods of improving W value of GaN-MSND is proposed and analyzed. For GaN-MSND with d value of 5.5 μm, W value increases to 4.7 μm at a Vr value of 226 V. The W value is further improved to 6.2 μm for GaN-MSND with the Al2O3 layer at Vr of 270 V. Moreover, GaN-MSND with the p-GaN inversion region can have a W value of 12.8 μm when Np is 1 × 1017 cm−3 at Vr of 1112 V. This study not only provides a series of pathways to obtaining GaN-MSND with better detection performance through the enhancement of the depletion region but also points out the GaN-MSND applications in the field of neutron detection in intense radiation environment.
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