Chemical vapor deposition of hexagonal boron nitride for the development of neutron detectors

Abstract

Chemical vapor deposition (CVD) processes were developed for hBN growth on sapphire, (111)Si, and AlN/(111)Si substrates, and (111)Si vertical sidewalls of parallel trenches. Detailed characterizations of the crystalline, chemical composition, optical, and transport properties were performed for the grown films. hBN films grown on sapphire were highly crystalline. hBN films were turbostratic or nanocrystalline hBN, when grown on (111)Si and other substrates.

All fabricated hBN detectors showed strong responses to deep UV excitation. hBN-Si template detectors are the first reported neutron detectors with hBN grown on Si. Excellent neutron response was also observed for hBN/sapphire vertical MSM detectors, but their large device capacitance (~0.2 nF/cm²) limits area scalability. Lateral detectors based on thick (up to 15µm) and good crystalline quality hBN showed the best response to thermal neutrons. In-plane µτ products for hBN/sapphire films are large enough to get good charge collection and generate neutron-indicating pulses above low electronic noise which arises from low leakage current (~0.1 nA/cm² at 100V) and low device capacitance (~10-14 F/cm²). Low electronic noise also enables scaling to large detector areas (tens of cm²). These lateral detectors also showed excellent scalability with hBN thickness. 2.5 to 15 µm thick hBN based detectors could count most (≥ 92%) of the absorbed neutrons, yielding close to theoretical detection efficiencies for respective hBN thicknesses. A 15 µm thick natural hBN (with 20% ¹⁰B) based lateral MSM detector showed an efficiency of 4.7%, which is equivalent to 21.4% for enriched hBN (with 100% ¹⁰B). Fabricated hBN SSNDs are also inexpensive and relatively insensitive to gamma rays. These results demonstrate the promise of hBN based SSNDs as viable alternatives to the 3He based neutron detection technology.

hBN based SSNDs, with both in-plane and out-of-plane charge collection, were designed, fabricated, and characterized. The highest mobility path in hBN, a layered semiconductor, is the in-plane direction which is used for hBN/sapphire based lateral (in-plane) metal-semiconductor-metal (MSM) detectors. Growth of highly crystalline thick hBN, required for achieving high efficiency for such detectors, represents a considerable challenge. hBN-Si template detectors are based on an innovative design, where thick hBN growth problem was addressed by growing hBN on the (111)Si vertical sidewalls of trenches and collecting charges along the in-plane direction. Required hBN growth is only a few microns (the half-width of the trench), but the neutron interaction length equals tens of microns (trench depth or vertical hBN thickness) potentially yielding high detection efficiency for these detectors.

Nuclear weapons and dirty bombs contain special nuclear materials (SNMs) such as uranium-233, uranium-235, and plutonium-239. SNMs produce suspect signatures which can be detected, including fast neutrons (~1 MeV), thermal neutrons (~0.0259 eV), and gamma rays. Thermal neutron detection is one of the methods of detecting SNMs and nuclear weapons in places where nuclear activities can take place. ³He gas based neutron detection technology is considered the gold standard for thermal neutron detection, but is expensive and requires high pressure and high voltage. With large demand for neutron detectors, the Department of Homeland Security (DHS) is looking to replace ³He based detectors with solid-state neutron detectors (SSNDs) that are inexpensive and possess characteristics such as high neutron detection efficiency, large detection area, and low gamma sensitivity. Large thermal neutron capture cross-section of ¹⁰B isotope makes hexagonal boron nitride (hBN) a promising material for SSND fabrication. hBN SSNDs are homogeneous detectors where both neutron conversion and charge collection occur in the same material (hBN). Thicker hBN increases neutron interaction probability and good crystalline quality hBN has high mobility-lifetime products enabling good charge collection. Growth of thick hBN films (tens of microns) of good crystallinity was therefore targeted in this work for achieving high detection efficiency hBN SSNDs.