An experimental and theoretical study on the dielectric-response function of icosahedral B 12 As 2 in the spectral region between 1.24 and 9.8 eV is presented. Comprehensive experimental information on the energy band structure from the analysis of features in the optical dispersion was complemented by spin-orbit firstprinciples calculations. The lowest indirect band gap width is 3.2 eV; the two lowest direct interband transitions are at 3.46 and 3.9 eV. High-energy critical points are assigned to specific electron transitions in the Brillouin zone and their dimensionality was determined. The static dielectric constant of B 12 As 2 is uniaxially anisotropic with values of 7.84 and 9.02 for polarization perpendicular and parallel to the trigonal axis. Hole and electron effective masses are derived from the band dispersions.

B 12 As 2 / SiC pn heterojunction diodes based on the radiation-hard B 12 As 2 deposited on ͑0001͒ n-type 4H-SiC via chemical vapor deposition were demonstrated. The diodes exhibit good rectifying behavior with an ideality factor of 1.8 and a leakage current as low as 9.4 ϫ 10 −6 A / cm 2 . Capacitance-voltage measurements using a two-frequency technique showed a hole concentration of ϳ1.8-2.0ϫ 10 17 cm −3 in B 12 As 2 with a slight increase near the interface due to the presence of an interfacial layer to accommodate lattice mismatch. Band offsets between the B 12 As 2 and SiC were estimated to be ϳ1.06 eV and 1.12 eV for conduction band and valance band, respectively. © 2010 American Institute of Physics. ͓doi:10.1063/1.3443712͔ Icosahedral boron arsenide ͑B 12 As 2 ͒ is a wide band gap boron-rich semiconductor ͑E g Ϸ 3.20 eV at room temperature 1 ͒. Its structure is based on 12-boron-atom icosahedra residing at the lattice points of a rhombohedral unit cell, with an As-As chain lying along its ͓111͔ axis ͑the body diagonal͒ ͑Refs. 2 and 3͒ ͓Fig. 1͑a͔͒. The boron atoms in the icosahedra are bonded by so-called three-centered bonds, i.e., a pair of electrons is shared among three boron atoms. As a consequence of this unique structure and bonding, B 12 As 2 has extraordinary radiation tolerance via self-healing mechanisms. In addition, it has a high melting temperature, and excellent mechanical properties, making B 12 As 2 highly attractive for applications under harsh conditions. Further details on the material properties of B 12 As 2 can be found in Refs. 1-7, and reference therein.A key potential application of B 12 As 2 is for compact solid-state thermal neutron detectors because 10 B has one of the highest thermal neutron ͑0.0259 eV͒ capture crosssections, ϳ3840 b of all elements. 4 He ions passing through the material. Compared with the present mature neutron detection technology employing gas-filled, proportional counters, or doped scintillating plastic fibers, solidstate neutron detectors would have reduced size and weight features that are beneficial for applications in portable devices or space. A further potential application is for betavoltaics, converting nuclear energy into electrical power, for small-scale long-lifetime batteries, e.g., remote sensors and peacemakers, where present materials are susceptible to radiation damage.9 Despite this great potential, no success has been achieved in fabricating B 12 As 2 based devices up to date, presumably due to the difficulty in growing high crystalline quality B 12 As 2 . In this paper, we report the fabrication of B 12 As 2 / SiC pn diodes, and present a detailed analysis of their electrical properties.Vertical heterojunction diodes were fabricated by epitaxially depositing B 12 As 2 on 300 m thick silicon-face ͑0001͒ n-type 4H-SiC ͑n Ϸ 10 18 cm −3 ͒ using chemical vapor deposition ͑CVD͒ at 1350°C under a constant total pressure of 100 Torr using 1% B 2 H 6 in H 2 and 2% AsH 3 in H 2 in hydrogen atmosphere. More details on the growth and its microstru...

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