University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Theses, Dissertations, and Student Research: Department of Physics and Astronomy Physics and Astronomy, Department of 4-2012 An Exploration of Neutron Detection in Semiconducting Boron Carbide Nina Hong University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/physicsdiss Part of the Physics Commons Hong, Nina, "An Exploration of Neutron Detection in Semiconducting Boron Carbide" (2012). Theses, Dissertations, and Student Research: Department of Physics and Astronomy. 20. https://digitalcommons.unl.edu/physicsdiss/20 This Article is brought to you for free and open access by the Physics and Astronomy, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Theses, Dissertations, and Student Research: Department of Physics and Astronomy by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. An Exploration of Neutron Detection in Semiconducting Boron Carbide by Nina Hong A DISSERTATION Presented to the Faculty of The Graduate College of the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy Major: Physics & Astronomy Under the Supervision of Professor Shireen Adenwalla Lincoln, Nebraska April, 2012 An Exploration of Neutron Detection in Semiconducting Boron Carbide Nina Hong, Ph.D. University of Nebraska, 2012 Advisor: Shireen Adenwalla The 3He supply problem in the U.S. has necessitated the search for alternatives for neutron detection. The neutron detection efficiency is a function of density, atomic composition, neutron absorption cross section, and thickness of the neutron capture material. The isotope 10B is one of only a handful of isotopes with a high neutron absorption cross section—3840 barns for thermal neutrons. So a boron carbide semiconductor represents a viable alternative to 3He. This dissertation provides an evaluation of the performance of semiconducting boron carbide neutron detectors grown by plasma enhance chemical vapor deposition (PECVD) in order to determine the advantages and drawbacks of these devices for neutron detection. Improved handling of the PECVD system has resulted in an extremely stable plasma, enabling deposition of thick films of semiconducting boron carbide. A variety of material and semiconducting characterization tools have been used to investigate the structure and electronic properties of boron carbide thin films, including X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, infrared/Raman spectroscopy, current-voltage measurements and capacitance-voltage measurements. Elemental concentrations in the boron carbide films have been obtained from Rutherford backscattering and elastic recoil detection analysis. Solid state neutron detection devices have been fabricated in the form of heterostructured p-n diodes, p-type boron carbide/n-type Si. Operating conditions, including applied bias voltage, and time constants, have been optimized for maximum detection efficiency and correlated to the semiconducting properties investigated in separate electronic measurements. Accurate measurements of the neutron detection efficiency and the response of the detector to a wide range of neutron wavelengths have been performed at a well calibrated, tightly collimated, ―white‖ cold neutron beam source using time-of-flight neutron detection technique. Because neutron detection measurements indicate that charge capture in boron carbide is affected by the nanocrystalline/amorphous nature of the semiconductor, the effects of incomplete charge collection efficiencies on the neutron detection efficiencies and pulse height spectra in heterostructured p-n diode neutron detectors have been modeled using a Monte Carlo GEANT4 simulation. The dissertation ends with suggestions for devices with improved neutron detection efficiencies. Acknowledgements First and foremost, I thank God for raising and cheering me up along this long journey. My sincerest appreciation to my advisor, Professor Shireen Adenwalla for her guidance, encouragement, and valuable discussions throughout my graduate work. She has been my mentor and role model. In completing this dissertation, I have had the benefit of constructive criticism from my committee members: Professor Brian Robertson, Professor Peter Dowben, and Professor Kirill Belashchenko. I greatly appreciate their time and efforts. I would like to thank my research mentors and colleagues: Ellen Day, Andrew Baruth, Abhijit Mardana, Ravi Billa, Sam Davis, Dhairyashil Aher, Keith Foreman, Uday Singh, John Mullins, A. Dwayne Riley, Michael Chu, Anil Rajapitamahuni, and Wei Xiong. In addition special thanks to Professor Mathias Schubert and his students for their fruitful discussions and interest in this project. I am especially indebted to my previous advisor, Professor Y.P. Lee of Hanyang University who has broadened my horizons as a physicist. Finally, I want to thank my family for their everlasting trust in me. i Table of Contents Chapter 1 Introduction..................................................................................................................1 Chapter 2 Preparation of PECVD Grown Boron Carbide Thin Films ....................................3 2.1 Introduction ............................................................................................................................3 2.2 PECVD system .......................................................................................................................4 2.3 Sample depositions .................................................................................................................5 2.3.1 Source molecules .............................................................................................................5 2.3.2 Substrates .........................................................................................................................7 2.3.3 Deposition conditions ......................................................................................................8 REFERENCES ...........................................................................................................................12 Chapter 3 Stoichiometry of PECVD Grown Boron Carbide Films ........................................19 3.1 Ion Beam Analysis (IBA) .....................................................................................................19 3.2 Rutherford Back Scattering (RBS) .......................................................................................20 3.3 Elastic Recoil Detection Analysis (ERDA) .........................................................................26 REFERENCES ...........................................................................................................................36 Chapter 4 Vibrational and Rotational Spectra of PECVD Grown Boron Carbide ..............37 4.1 Introduction ..........................................................................................................................37 4.2 Experimental details .............................................................................................................40 ii 4.3 Results and discussion ..........................................................................................................41 REFERENCES ...........................................................................................................................52 Chapter 5 Ni Doping of Semiconducting Boron Carbide.........................................................56 5.1 Introduction ..........................................................................................................................56 5.2 Experimental details .............................................................................................................57 5.3 Characterization of undoped B5C1H2 films ..........................................................................59 5.4 Ni doping ..............................................................................................................................65 5.5 Conclusions ..........................................................................................................................77 REFERENCES ...........................................................................................................................78 Chapter 6 An Introduction to Neutrons, Neutron Capture, Neutron Detectors, and Moderators .......................................................................................................................82 6.1 Introduction: The shortage of 3He ........................................................................................82 6.2 Currently available alternative neutron detectors .................................................................83 6.2.1 BF3 filled gas proportional neutron detectors ................................................................83 6.2.2 Boron-lined proportional counters ................................................................................84 6.2.3 Lithium-6 loaded glass fibers doped (coated) with scintillating materials ....................84 6.3 Thermal neutrons ..................................................................................................................85 6.3.1 Energy distribution of slow neutrons ............................................................................86