Defect-Selective Etching of Icosahedral Boron Arsenide (B 12 As 2 ) Crystals in Molten Potassium Hydroxide
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Defect-Selective Etching of Icosahedral Boron Arsenide (B12As2) Crystals in Molten Potassium Hydroxide 1
C.E. Whiteley, 1A. Mayo, 1J.H. Edgar, 2M. Dudley, and 2Y. Zhang. 1 Department of Chemical Engineering, Kansas State University, Manhattan KS, 66506. 2 Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY, 11794.
ABSTRACT The present work reports on the defect-selective etching (DSE) for estimating dislocation densities in icosahedral boron arsenide (B12As2) crystals using molten potassium hydroxide (KOH). DSE takes advantage of the greater reactivity of high-energy sites surrounding a dislocation, compared to the surrounding dislocation-free regions. The etch pits per area are indicative of the defect densities in the crystals, as confirmed by x-ray topography (XRT). Etch pit densities were determined for icosahedral boron arsenide crystals produced from a molten nickel flux as a function of etch time (1-5 minutes) and temperature (400-700°C). The etch pits were predominately triangle shaped, and ranged in size from 5-25μm. The average etch pit density of the triangle and oval etch-pits was on the order of 5x107cm-2 and 3x106cm-2 (respectively), for crystals that were etched for two minutes at 550°C.
I. INTRODUCTION The high neutron absorption capture cross-section (~3800Barns) of 10B allows for even thin layers of compounds rich in 10B to absorb all incident thermal neutrons (100% efficiency). If the radiation interaction occurs in a semiconductor, for each neutron captured, ~1.5 x 106 electronhole pairs are produced as the energetic 7Li and 4He ions pass through the material [1,2]. This ionization generates a current that can be detected directly without further amplification. Thus, a boron-rich semiconductor radiation detector could be based on a Schottky, pn, or pin diode.
Figure 1 - Common orientations of facets found in the B12As2 crystals. There are two rhombohedral boron-rich III-V wide bandgap semiconductors: B12As2 (3.24eV) and B12P2 (3.35eV) [3]. The structure of the two borides consists of 12-boron-atom icosahedra located at the corners of a rhombohedral (trigonal) unit cell and a two-atom As-As or P-P chain spanning the unit cell body diagonal aligned with the c-axis, as seen in Figure 1 [3]. For the boride semiconductors to work well as radiation detectors, the crystals must be excellent
quality and have favorable electrical properties, i.e. a large mobility and charge carrier lifetime (a large μτ product, ~10-3 cm2/V) [1,2]. The electrical properties of a semiconductor are heavily influenced by the density of defects (imperfections in the crystal lattice structure) on the surface and within the material. The study of such imperfections or defects is important for understanding the influence of imperfections on crystal-growth processes, and conversely, such feedback can be used to optimize the process to produce higher quality crystals [4]. Hence, it is essential for process engineers and crystal growers to characterize and identify the density, distribution
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