AMORPHOUS HYDROGENATED BORON CARBIDE FOR SOLID-STATE DIRECT-CONVERSION THERMAL NEUTRON DETECTION A DISSERTATION IN Physics and Chemistry Presented to the Faculty of the University of Missouri-Kansas City in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY by GYANENDRA BHATTARAI B.Sc., Tribhuvan University, 1998 M.Sc., Tribhuvan University, 2007 M.S., University of Missouri-Kansas City, 2015 Kansas City, Missouri 2021 © 2021 GYANENDRA BHATTARAI ALL RIGHTS RESERVED AMORPHOUS HYDROGENATED BORON CARBIDE FOR SOLID-STATE DIRECT-CONVERSION THERMAL NEUTRON DETECTION Gyanendra Bhattarai, Candidate for the Doctor of Philosophy Degree University of Missouri-Kansas City, 2021 ABSTRACT Amorphous hydrogenated boron carbide (a-BC:H) is one of a very limited number of neutron-sensitive materials. It has been studied over the last three decades for solid-state thermal neutron detection due to its high resistivity, moderate bandgap, high neutron absorption cross-section, and high stability under harsh physical and chemical environment. However, its success has been hindered by its poor and/or not well understood charge transport properties as well as fabrication challenges. This study focuses on obtaining thick and stable a-BC:H films using plasma-enhanced chemical vapor deposition (PECVD) and optimizing their charge transport properties for potential application in solid-state direct-conversion thermal neutron detection. We have investigated the effect of single-carrier transport and low charge carrier mobility, which are expected in a-BC:H films, on detection efficiency and spectral performance of a thin-film B4C detector using numerical Monte Carlo calculations. Experimentally, we have used space-charge-limited current (SCLC) analysis to extract iii the charge carrier mobility in a-BC:H thin films. To better describe the extracted charge carrier mobility, we have presented an extension of SCLC theory to include negative field-dependence in mobility as well as a theory to check self-consistency of the extracted mobility. Toward obtaining stable thick films and optimizing the charge transport metrics, we have deposited multiple series’ of a-BC:H thin films using an ortho-carborane precursor with varying PECVD process parameters, including substrate temperature, RF power, process pressure, carrier gas flow rate, and partial precursor flow. Films grown using higher substrate temperature and higher RF power demonstrated higher charge carrier mobility. Partial precursor flow was found to correlate significantly with film growth rate and film properties. The precursor flow rate was found to be very sensitive to the precursor bubbler temperature, which allowed us to fine-tune the precursor flow and produce films with predictable growth rate and film properties. With optimized growth conditions, we demonstrated a 3 μm thick and stable film deposited on copper foil exhibiting a carrier mobility value of 8×10–6 cm2⋅V–1⋅s–1 and a resistivity value of ∼1012 Ω⋅cm. With this film thickness and carrier mobility, our Monte Carlo calculations suggested that a neutron detector using an integration time of ≥5 μs can detect neutrons for a carrier lifetime >10 μs, and the intrinsic detection efficiency saturates to ∼10% for a carrier lifetime ≥100 μs. iv APPROVAL PAGE The faculty listed below, appointed by the Dean of the College of Arts and Sciences have examined a dissertation titled “Amorphous Hydrogenated Boron Carbide for Solid-State Direct-Conversion Thermal Neutron Detection,” presented by Gyanendra Bhattarai, candidate for the Doctor of Philosophy degree, and certify that in their opinion it is worthy of acceptance. Supervisory Committee Anthony N. Caruso, Ph.D., Committee Chair Department of Physics and Astronomy Michelle M. Paquette, Ph.D., Research Advisor Department of Physics and Astronomy J. David Van Horn, Ph.D. Department of Chemistry Paul M. Rulis, Ph.D. Department of Physics and Astronomy Nathan A. Oyler, Ph.D. Department of Chemistry v TABLE OF CONTENTS ABSTRACT .................................................................................................................. iii LIST OF ILLUSTRATIONS ....................................................................................... xii LIST OF TABLES ................................................................................................... xxvii ACKNOWLEDGEMENTS ..................................................................................... xxxii CHAPTER 1 INTRODUCTION .................................................................................... 1 1.1. Motivation and Objectives ............................................................................. 4 1.1.1. Applications of Neutrons and Neutron Detectors .............................. 4 1.1.2. Need for Helium-3 Alternatives ........................................................ 6 1.1.3. Boron-10 Compounds for Direct-Conversion Semiconductor Neutron Detection ...................................................................................................... 9 1.1.4. Amorphous Boron Carbide Thermal Neutron Detectors ................. 10 1.2. Fundamentals of Neutron Detection ............................................................ 12 1.2.1. Interaction of Neutrons with Matter ................................................ 12 Neutron Scattering ..................................................................................... 14 Neutron Absorption ................................................................................... 18 1.2.2. Neutron Detection ............................................................................ 20 1.2.3. Solid-State (Semiconductor) Neutron Detectors ............................. 22 1.3. Amorphous Hydrogenated Boron Carbide (a-BC:H) for Direct-Conversion Thermal Neutron Detection: A Brief History ..................................................... 27 vi 1.4. Scope of Dissertation ................................................................................... 31 CHAPTER 2 DEVICE FABRICATION AND CHARACTERIZATION: THEORY AND EXPERIMENTS ................................................................................................. 33 2.1. Device Fabrication ....................................................................................... 33 2.1.1. PECVD Chamber ............................................................................. 33 2.1.2. Substrate Temperature Calibration .................................................. 38 2.1.3. Precursor Bubbler ............................................................................ 40 2.1.4. Substrate Preparation ....................................................................... 42 2.1.5. Film Growth..................................................................................... 44 2.1.6. Electrical Contact Deposition .......................................................... 47 2.1.7. Device Wiring .................................................................................. 49 2.2. Basic Characterization.................................................................................. 52 2.2.1. Spectroscopic Ellipsometry ............................................................. 52 2.2.2. UV-Visible Reflection and Transmission Spectroscopy ................. 55 2.2.3. Film Stress ....................................................................................... 59 2.3. Charge Transport Characterization .............................................................. 62 2.3.1. Dielectric Constant .......................................................................... 62 2.3.2. Steady-State Current–Voltage Measurements ................................. 64 2.3.3. Transient Photoconductivity ............................................................ 80 vii CHAPTER 3 SINGLE-CARRIER CHARGE COLLECTION IN THIN DIRECT- CONVERSION SEMICONDUCTOR NEUTRON DETECTOR: A NUMERICAL SIMULATION .............................................................................................................. 88 3.1. Introduction .................................................................................................. 88 3.2. Interaction of Thermal Neutrons with B4C .................................................. 93 3.3. Theory: Direct-Conversion Neutron Detection ............................................ 96 3.4. Simulation Setup ........................................................................................ 101 3.4.1. Detector Geometry, and Contacts .................................................. 101 3.4.2. Energy Deposition by Primary Reaction Products ........................ 102 3.4.3. Charge Carrier Excitation and Collection (Signal Generation) ..... 103 3.4.4. Simulation Process......................................................................... 106 3.5. Results and Discussion ............................................................................... 108 3.5.1. Total Energy Deposition: The Case of 100% Charge Collection of Both Types of Charge Carriers ................................................................ 108 3.5.2. Charge Collection Efficiency: Effect of Charge Transport Properties and Device Thickness .............................................................................. 111 3.5.3. Detection Efficiency: Effect of Charge Transport Properties and Device Thickness ..................................................................................... 116 3.5.4. Pulse Height Spectra: Effect of Charge Transport