Neutron detectors are very important for several nuclear and particle physics, biomedical, aerospace, and defense applications. This project is focused on investigation of two novel concepts for the development of high efficiency, temperature and radiation hard, long lifetime neutron detectors. Molecular beam epitaxy and reactive magnetron sputtering of highly efficient and stable Nitride materials, and advanced laser ablation process will be used to incorporate high neutron capture efficiency gadolinium atoms in the working nano and micro device structures that will be fabricated on commercial silicon wafers. Complete chemical, structural, and electrical in situ and post growth characterization, as well as testing of the working structures under exposure to high energy charged particles, and neutrons will be performed in order to prove the feasibility of the investigated concepts.

Low energy neutron (En<1eV) detection plays an increasingly critical role in many areas including biomedical [1], high energy physics (HEP)[2], home land security, and defense [3] applications. Current neutron detector efficiencies are reasonable for detection of thermal neutrons in high flux environments, such as in HEP applications, however they become impractical in low flux environments, such as in detecting presence of radioactive material for homeland security and in biomedical applications. The promising 3He based neutron detectors are not able to meet the demand, since United States have consistently reduced their nuclear stockpile making the helium isotope scarce. The most promising alternative approach has been the use of scintillator, water-based, and semiconductor type detectors [4]. Thus far the major challenge with semiconductor detectors is the ability to fabricate very thick layers of semiconducting materials that incorporate high efficiency neutron detection materials and fabricate efficient devices from them. However, in contrast to current neutron detectors, used for nuclear physics, biomedical, defense, and space applications, semiconductor based devices can have a low operating voltage, miniature size, and excellent stability [4,5].

Naturally occurring Gd has a neutron capture cross section of 49700 [6] barns, which is much higher than those of the most materials. Compared to 6Li, the capture cross-section is more than 50 times higher for naturally occurring Gd, and it is more than 500 times higher if 157Gd is used. Of high interest is that Gd easily forms GdN, which is a semiconductor with an experimental bandgap of ~1eV. GdN (111) plane has a hexagonal symmetry that matches with the 0001 plane of III-Nitride materials. Several groups have grown GdN on GaN [7] and AlN [8] and have demonstrated low quality polycrystalline GdN materials.

Our achievements in III-Nitride semiconducting material growth (Figures 1a and 1b) [9,10, 12-17] and availability of high quality bulk GaN wafers [11], can provide for seamless integration of a high neutron capture cross-section material, such as Gd [18] within active nano- and micro structured III-Nitrides based detector devices [19] for the development of highly efficient, high temperature and radiation hard, long lifetime neutron detectors that can be used in a very large variety of applications, including health care, homeland security, high energy physics research, aerospace, and military applications. Employment of advanced material growth methods, such as Radio Frequency Molecular Beam Epitaxy (RFMBE) and reactive high temperature and RF assisted Magnetron Sputtering (MS), combined with novel nanocolumn growth (Figure 2a) and laser ablation based micro column array (MCA) technology (Figure 2b) [20-22] will result in building of advanced neutron detectors based on low cost conventional silicon (Si) wafers. Employment of Si wafers as substrates for fabrication of the neutron detector devices introduces an important advantage of their

compatibility and simple integration with standard control, readout, and analysis interfaces and circuitry.

compatibility and simple integration with standard control, readout, and analysis interfaces and circuitry.

The proof of feasibility of two neutron detector concepts is the main goal of this project. The first concept (Figure 3a) is based on integration of a lattice matched GaN/InGaN/InN heterostructure grown by plasma assisted molecular beam epitaxy with a lattice matched multiple ~10 µm thick InN/GdN heterostructures capped with ZrN [23], all grown by RF and high temperature assisted reactive magnetron sputtering [24]. The second concept (Figure 3b) is based on using a Si wafer based nano and micro column array structures with GdN nonocolumns grown by RFMBE; or Gd atoms diffused into the micro-cones during the pulsed laser ablation process. A semiconducting polymer layer [25] will be used as both, filler and window material. In situ RHEED and Spectral Ellipsometry characterization will be used during the growth. Post growth analysis, such as SEM, XRD, and TEM will be performed on structures fabricated by using both concepts.  Electrical characterization, testing under the high energy charged particles and neutron expose will be performed after processing of the fabricated structures into devices with contacts for biasing and signal acquisition

In order to investigate the feasibility of the described above concepts the following has to be accomplished:

  1. Optimize the RFMBE growth of lattice matched GaN/InGaN/InN heterostructures and GdN nanocolumn arrays. The goal of this optimization is to achieve high crystalline quality, targeted thickness, doping type and level device structures.
  2. Characterize the crystalline quality of the grown materials by using SEM, XRD, and PL. This characterization is important, as the crystalline quality e.g. level of defects greatly affects the future device performance.
  3. Fabricate micro column arrays of Si/Gd structures by using pulsed laser ablation. This task has a bi-fold goal of forming micro column arrays with drastically (~10 times) increased specific surface area, and infusing the neutron capturing Gd in to the Si micro columns.
  4. Characterize chemical composition by using XPS and SIMS. The chemical composition of the layers is important, as the device performance is highly dependent on the purity of the materials and their doping levels.
  5. Fabricate experimental device structures with contacts by using methods of: photolithography, wet and reactive ion etching (RIE), metal deposition; polymer spin coating, and ashing. Employment of standard semiconductor device processing methods will result in working device structures.
  6. Perform initial testing of the detection efficiency under charged particles exposure. As the neutron detection mechanism involves charge particle detection by the diode structure. This testing will be performed first.
  7. Perform testing under neutron exposure at the CERF facility (CERN, Switzerland). This testing will be performed in order to determine specifically neutron detection efficiency.

The details on the realization of the above devices are disclosed in a recently accepted US Patent  [26].


[1] Polaczek-Grelik, K., Kozlowska, B., Dybek, M., Obryk, B., Ciba, A. Assessment of radiation exposure outside the radiotherapeutic room during medical accelerator beam emission with the use of TL detectors (radiation exposure outside a linac room). Radiation Protection Dosimetry, volume 156, Issue 3, September 2013, Article number nct077, Pages 268-276.

[2] Aza, E., Caresana, M., Cassell, C., Charitonidis, N., Harrouch, E., Manessi, G.P., Pangallo, M., Perrin, D., Samara, E., Silari, M. Instrument intercomparison in the pulsed neutron fieldsat the CERN HiRadMat facility. Radiation Measurements, volume 61, February 2014, Pages 25-32.

[3] Mukhopadhyay, S., Mitchell, S. Hard X-Ray, Direction sensitive neutron detectors in near field measurements. Gamma-Ray, and Neutron Detector Physics XIV; San Diego, CA; United States; 13 August 2012 through 15 August 2012.  Proceedings of SPIE – The International Society for Optical Engineering, volume 8507, 2012.

[4] “Neutron Detectors – alternative to using Helium-3” US Government Accountability Office – Technology assessment (http://www.gao.gov/assets/590/585514.pdf)

[5] McGregor, D.S., Hammig, M.D.; Yang, Y.-H.; Gersch, H.K.; Klann, R.T., Design considerations for thin film coated semiconductor thermal neutron detectors – I: Basics regarding alpha particle emitting neutron reactive films, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, v 500, n

1-3, p 272-308, November 3, 2003.

[6] http://www.ncnr.nist.gov/resources/n-lengths/elements/gd.html

[7] M. A. Scarpulla et al. “GdN (111) heteroepitaxy on GaN (0001) by N2 plasma and NH3 molecular beam epitaxy”, Journal of Crystal Growth Vol. 311, 5, (2009), pg. 1239–1244.

[8] H. Yoshitomi et al “Optical and magnetic properties in epitaxial GdN thin films”, Phys. Rev. B, 83,155202 (2011)

[9] J.W. Ager III, J. Wu, K.M. Yu, R.E. Jones, S.X. Li, W. Walukiewicz, E.E. Haller, H. Lu, and

W.J. Schaff, Proc. SPIE 5530, p. 308 (2004).

[10] S. X. Li, K. M. Yu, J. Wu, R. E. Jones, W. Walukiewicz, J. W. Ager, W. Shan, E. E. Haller, H. Lu, and W. J. Schaff, Phys. Rev. B 71, 161201 (2005).

[11] http://ammono.com/products

[12] P. Misra, C. Boney, D. Starikov, and A. Bensaoula. M-Plane III-Nitride Materials for Polarization Sensitive Devices Grown by PAMBE with Real Time Analysis by Spectroscopic Ellipsometry. Phys. Stat. Sol. (c) 5, No. 6, 2286–2289 (2008). http://onlinelibrary.wiley.com/doi/10.1002/pssc.200778702/abstract

[13] P. Misra, C. Boney, D. Starikov, A. Bensaoula. Gallium adlayer adsorption and desorption studies with real-time analysis by spectroscopic ellipsometry and RHEED on A-, M-, and C- plane GaN grown by PAMBE. Journal of Crystal Growth, 311 2033–2038, (2009). http://www.sciencedirect.com/science/article/pii/S0022024808011986

[14] “Growth and Characterization of InGaN for Photovoltaic Devices”, C. Boney, I. Hernandez , R. Pillai , D. Starikov , A. Bensaoula , M. Henini , M. Syperek , J. Misiewicz , and R. Kudrawiec. Phyica Status Solidi C 8,2460 (2011). http://onlinelibrary.wiley.com/doi/10.1002/pssc.201000993/abstract

[15] “Molecular Beam Epitaxy  Growth of InGaN-GaN Superlattices for Optoelectronic Devices”; C. Boney, D. Starikov, I. Hernandez, R. Pillai, and A. Bensaoula,, J. Vac. Sci. B 29(3), 03C106-1, (2011). [10] “Growth and Characterization of InGaN for Photovoltaic Devices”; Boney, C., Hernandez, I., Pillai, R. ,Starikov, D., Bensaoula, A., Henini, M., Syperek, M., Misiewicz, J. and Kudrawiec,. 35TH IEEE Phot Spec Conf: 003316 -003321 (2010). http://avs.scitation.org/doi/10.1116/1.3549887

[16] “InGaN Devices for High Temperature Photovoltaic Applications”; Boney, C., Hernandez, I., Pillai, R., Starikov, D. and Bensaoula, A.. Ieee Phot Spec Conf: 002522 – 002527 (2010). http://ieeexplore.ieee.org/document/5614645/?reload=true

[17] R. Pillai, D. Starikov, C. Boney, J. Gandhi, A. Debnath, R. Li, A. Bensaoula; “High Indium InGaN/Silicon Heterojunction Device for Narrow Band Near-Infrared Detectors”. Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 33, 011205 (2015).  http://avs.scitation.org/doi/abs/10.1116/1.4904760?journalCode=jvb

[18] R.L. Varner et al. “Gadolinium Thin Foils in a Plasma Panel Sensor as an Alternative to 3He” Nuclear Science Symposium Record, pg 1130-1136 (2010).

[19] McGregor, D.S., McNeil, W.J.; Bellinger, S.L.; Unruh, T.C.; Shultis, J.K.                               Source: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, v 608, n 1, p 125-131, September 1, 2009

[20] E.G. Baburaj, D. Starikov, J. Evans, G.A. Shafeev, and A. Bensaoula. “Enhancement of adhesive joint strength by laser surface modification.” International Journal of Adhesion & Adhesives 27, 268–276 (2007). http://www.sciencedirect.com/science/article/pii/S0143749606000686

[21] M. Adjim, R. Pillai, A. Bensaoula, D. Starikov, C. Boney, A. Saidane. Thermal Analysis of Micro-Column Arrays for Tailored Temperature Control in Space. Transactions of the ASME, Journal of Heat Transfer, Vol. 129, 798-804 (2007). http://heattransfer.asmedigitalcollection.asme.org/article.aspx?articleid=1448752

[22] A. Bensaoula, C. Boney, R. Pillai, G.A. Shafeev, A.V. Simakin, and D. Starikov “Arrays of 3D micro-columns generated by laser ablation of Ta and steel: modeling of a black body emitter”. European J. Appl. Phys” A: 00, 1–3 (2004). https://link.springer.com/article/10.1007/s00339-004-2588-z

[23] S. Schleussner, T. Kubart, T. Törndahl, M. Edoff. Reactively sputtered ZrN for application as reflecting back contact in Cu(In,Ga)Se2 solar cells. Thin Solid Films 517 (2009) 5548–5552. [24] Ruiteng Li, Jateen S. Gandhi, Rajeev Pillai, Rebecca Forrest, David Starikov, Abdelhak BensaoulaEpitaxial growth of ZrTiN on c-plane Al2O3 as buffer layer for III-nitrides.

[25] A. N. Caruso, The physics of solid-state neutron detector materials and geometries, Journal of Physics: Condensed Matter, 22 (44), p.443201, Oct 2010.

[26] Abdelhak Bensaoula, David Starikov, Rajeev Pillai. Compact solid-state neutron detector. US Patent No. US20150276950. https://www.google.com/patents/US20150276950