Chemical Vapor Deposition of Boron Phosphide Thin Films
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Chemical Vapor Deposition of Boron Phosphide Thin Films Julia K.C. Abbott1, J. Daniel Brasfield1,2, Philip D. Rack3, Gerd J. Duscher3, Charles S. Feigerle1 1
Dept. of Chemistry, University of Tennessee, Knoxville, TN, USA Development Division, Y-12 National Security Complex, Oak Ridge, TN, USA 3 Dept. of Materials Science Engineering, University of Tennessee, Knoxville, TN, USA 2
ABSTRACT Boron Phosphide (BP) is a promising material for use as a room temperature semiconductor detector of thermal neutrons. The absorption of a thermal neutron by a 10B nucleus in BP can yield 2.3MeV of energy which in solid state BP can yield ~0.5 million electron-hole pairs that would be detectable with minimal amplification in a device. BP thin films are grown according to the net reaction below in a cold wall chemical vapor deposition (CVD) reactor: B2H6 + 2PH3 Æ 2BP + 6H2 Thin film depositions are performed using diborane and phosphine with a balance of hydrogen gas at near atmospheric pressure with RF induction heating. The resultant BP films are characterized by Raman, XRD, SEM, TEM and TEM-EELS for chemical composition, surface and bulk morphology. BP growths on Si and SiC substrates are compared. SiC provides reduced lattice mismatch for growth of BP and growth of heteroepitaxial BP on SiC will be discussed. INTRODUCTION Boron Phosphide (BP) has potential as an efficient solid-state thermal neutron detector. A III-V semiconductor, BP has a wide band gap (2.3 eV) and reacts with thermal neutrons to form energetic products and release of 2.3 MeV of energy via the following reaction: 10
B + n Æ7Li + α + 2.3 MeV
This reaction can yield ~0.5 million electron-hole pairs in BP that would require only minimal amplification to detect. A 200μm film of BP is needed to react with 74% of incident neutrons, therefore requiring a film of high enough quality to minimize electron-hole pair recombination. While films of BP have been grown by several methods [1–17], chemical vapor deposition (CVD) has proven to be most successful for epitaxial growth of BP. Three types of boron and phosphorus containing gas-phase precursors have been used in CVD of BP: metal organics [12,13], halides [14], and hydrides [5–11,14-16]. The hydride method was chosen in the study described here because the cleanliness of the by-product (H2). In most previous studies, silicon (Si) was the substrate used for deposition. However, films grown on Si were typically highly defective due to the large lattice mismatch (17%) between Si and BP. In addition, significant silicon contamination has been found in BP grown on Si. Silicon carbide (SiC) has a much better lattice match with BP and therefore is a promising
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substrate for BP growth. SiC has many polytypes, but only the hexagonal 4H- and 6H-structures are readily available. The (0001) surfaces of 4H- and 6H-SiC, only have a 4% lattice mismatch with the (111) surface of BP. The challenge is maintaining high quality growth of BP in the direction. The best polytype would be the 3C cubic form of SiC, which has similar
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