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Ab-initio calculations of electronic, transport, and structural properties of boron phosphide
We present results from ab-initio, self-consistent density functional theory calculations of electronic and related properties of zinc blende boron phosphide (zb-BP). We employed a local density approximation potential and implemented the linear combination of atomic orbitals formalism. This technique follows the Bagayoko, Zhao, and Williams method, as enhanced by the work of Ekuma and Franklin. The results include electronic energy bands, densities of states, and effective masses. The calculated band gap of 2.02 eV, for the room temperature lattice constant of a=4.5383 Å, is in excellent agreement with the experimental value of 2.02±0.05 eV. Our result for the bulk modulus, 155.7 GPa, agrees with experiment (152–155 GPa). Our predictions for the equilibrium lattice constant and the corresponding band gap, for very low temperatures, are 4.5269 Å and 2.01 eV, respectively.
Preparation, properties, and characterization of boron phosphide films on 4H- and 6H-silicon carbide
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Boron phosphide as a p-type transparent conductor: Optical absorption and transport through electron-phonon coupling
Boron phosphide has recently been identified as a potential high-hole-mobility transparent conducting material. This promise arises from its low hole effective masses. However, boron phosphide has a relatively small, 2 eV, indirect bandgap which will affect its transparency. In this study, we computationally study both optical absorption across the indirect gap and phonon-limited electronic transport to quantify the potential of boron phosphide as a p-type transparent conductor. We find that phonon-mediated indirect optical absorption is weak in the visible spectrum and that the phonon-limited hole mobility is very high (around 900 cm 2 /Vs) at room temperature. This exceptional mobility comes from the combination of a low hole effective mass and very weak scattering by polar phonon modes. We rationalize the weak scattering by the less ionic bonding in boron phosphide compared to oxides. We suggest that this could be a general advantage of nonoxides for p-type transparent conducting applications. Using our computed properties, we assess the transparent conductor figure of merit of boron phosphide and show that it exceeds by one order of magnitude that of established p-type transparent conductors, confirming the potential of this material.
Materials Development for Boron Phosphide Based Neutron Detectors: Final Technical Report
The project goal was to improve the quality of boron phosphide (BP) by optimizing its epitaxial growth on single crystal substrates and by producing bulk BP single crystals with low dislocation densities. BP is potentially a good semiconductor for high efficiency solid state neutron detectors by combining neutron capture and charge creation within the same volume. The project strategy was to use newly available single crystal substrates, silicon carbide and aluminum nitride, engineered to produce the best film properties. Substrate variables included the SiC polytype, crystallographic planes, misorientation of the substrate surface (tilt direction and magnitude) from the major crystallographic plane, and surface polarity (Si and C). The best films were (111)BP on silicon-face (0001) 4H-SiC misoriented 4° in the [1-100] direction, and BP on (100) and (111) 3C-SiC/Si; these substrates resulted in films that were free of in-plane twin defects, as determined by x-ray topography. The impact of the deposition temperature was also assessed: increasing the temperature from 1000 °C to 1200 °C produced films that were more ordered and more uniform, and the size of individual grains increased by more than a factor of twenty. The BP films were free of other compounds such as icosahedral boron phosphide (B 12 P 2 ) over the entire temperature range, as established by Raman spectroscopy. The roughness of the BP films was reduced by increasing the phosphine to diborane ratio from 50 to 200. Bulk crystals were grown by reacting boron dissolved in nickel with phosphorus vapor to precipitate BP. Crystals with dimensions up to 2 mm were produced.
Strain tunable electronic and magnetic properties of pristine and semihydrogenated hexagonal boron phosphide
Tunable electromagnetic properties of pristine two-dimensional boron phosphide (h-BP) nanosheet and its semihydrogenated structure were studied by density functional theory computations. In sharp contrast to previously reported tensile strain-induced red shift in two-dimensional semiconductors, the direct gap of h-BP undergoes blue shift under biaxial tensile strain. Once semihydrogenated, the h-BP not only transform from the nonmagnetic semiconductor into metal which is spin-resolved but also exhibits linear response between the magnetic moment and biaxial strain with a slope up to 0.005 μB/1%. These findings provide a simple and effective route to tune the electronic and magnetic properties of h-BP nanostructures in a wide range and should inspire experimental enthusiasm.
Boron phosphide under pressure: In situ study by Raman scattering and X-ray diffraction
Cubic boron phosphide, BP, has been studied in situ by X-ray diffraction and Raman scattering up to 55 GPa at 300 K in a diamond anvil cell. The bulk modulus of B{sub 0} = 174(2) GPa has been established, which is in excellent agreement with our ab initio calculations. The data on Raman shift as a function of pressure, combined with equation-of-state (EOS) data, allowed us to estimate the Grüneisen parameters of the TO and LO modes of zinc-blende structure, γ{sub G}{sup TO }= 1.26 and γ{sub G}{sup LO }= 1.13, just like in the case of other A{sup III}B{sup V} diamond-like phases, for which γ{sub G}{sup TO }> γ{sub G}{sup LO }≅ 1. We also established that the pressure dependence of the effective electro-optical constant α is responsible for a strong change in relative intensities of the TO and LO modes from I{sub TO}/I{sub LO} ∼ 0.25 at 0.1 MPa to I{sub TO}/I{sub LO} ∼ 2.5 at 45 GPa, for which we also find excellent agreement between experiment and theory.
Boron Phosphide Films by Reactive Sputtering: Searching for a P‐Type Transparent Conductor
Abstract With an indirect band gap in the visible and a direct band gap at a much higher energy, boron phosphide (BP) holds promise as an unconventional p‐type transparent conductor. This work reports on reactive sputtering of amorphous BP films, their partial crystallization in a P‐containing annealing atmosphere, and extrinsic doping by C and Si. The highest hole concentration to date for p‐type BP (5 × 10 20 cm −3 ) is achieved using C doping under B‐rich conditions. Furthermore, bipolar doping is confirmed to be feasible in BP. An anneal temperature of at least 1000 °C is necessary for crystallization and dopant activation. Hole mobilities are low and indirect optical transitions are stronger than that predicted by theory. Low crystalline quality probably plays a role in both cases. High figures of merit for transparent conductors might be achievable in extrinsically doped BP films with improved crystalline quality.
Sintered Cr/Pt and Ni/Au ohmic contacts to B{sub 12}P{sub 2}
Icosahedral boron phosphide (B{sub 12}P{sub 2}) is a wide-bandgap semiconductor possessing interesting properties such as high hardness, chemical inertness, and the reported ability to self-heal from irradiation by high energy electrons. Here, the authors developed Cr/Pt and Ni/Au ohmic contacts to epitaxially grown B{sub 12}P{sub 2} for materials characterization and electronic device development. Cr/Pt contacts became ohmic after annealing at 700 °C for 30 s with a specific contact resistance of 2 × 10{sup −4} Ω cm{sup 2}, as measured by the linear transfer length method. Ni/Au contacts were ohmic prior to any annealing, and their minimum specific contact resistance was ∼l–4 × 10{sup −4} Ω cm{sup 2} after annealing over the temperature range of 500–800 °C. Rutherford backscattering spectrometry revealed a strong reaction and intermixing between Cr/Pt and B{sub 12}P{sub 2} at 700 °C and a reaction layer between Ni and B{sub 12}P{sub 2} thinner than ∼25 nm at 500 °C.
Hydride CVD Hetero-epitaxy of B 12 P 2 on 4H-SiC
Icosahedral boron phosphide (B 12 P 2 ) is a wide bandgap semiconductor (3.35 eV) that has been reported to “self-heal” from high-energy electron bombardment, making it attractive for potential use in radioisotope batteries, radiation detection, or in electronics in high radiation environments. Our study focused on improving B 12 P 2 hetero-epitaxial films by growing on 4H-SiC substrates over the temperature range of 1250–1450 °C using B 2 H 6 and PH 3 precursors in a H 2 carrier gas. Furthermore, XRD scans and Laue transmission photographs revealed that the epitaxial relationship was (0001)<11$\bar{2}$0> B 12 P 2 || (0001)<11$\bar{2}$0> 4H-SiC . The film morphology and crystallinity were investigated as a function of growth temperature and growth time. At 1250 °C, films tended to form rough, polycrystalline layers, but at 1300 and 1350 °C, films were continuous and comparatively smooth (R RMS ≤7 nm). At 1400 or 1450 °C, the films grew in islands that coalesced as the films became thicker. Using XRD rocking curves to evaluate the crystal quality, 1300 °C was the optimum growth temperature tested. Finally, at 1300 °C, the rocking curve FWHM decreased with increasing film thickness from 1494 arcsec for a 1.1 μm thick film to 954 arcsec for a 2.7 µm thick film, suggesting a reduction in defects with thickness.
Thermal expansion coefficients of high thermal conducting BAs and BP materials
Recently reported very high thermal conductivities in cubic boron arsenide (BAs) and boron phosphide (BP) crystals could potentially provide a revolutionary solution in the thermal management of high power density devices. To fully facilitate such an application, the compatible coefficient of thermal expansion (CTE) between the heat spreader and the device substrate, in order to minimize the thermal stress, needs to be considered. Here, we report our experimental CTE studies of BAs and BP in the temperature range from 100 K to 1150 K, through a combination of X-ray single crystal diffraction and neutron powder diffraction. Here, we demonstrated that the room temperature CTEs, 3.6 ± 0.15 × 10 –6 /K for BAs and 3.2 ± 0.2 × 10 –6 /K for BP, are more compatible with most of the semiconductors including Si and GaAs, in comparison with diamond, and thus could be better candidates for the future heat spreader materials in power electronic devices.
B 12 P 2 : Improved Epitaxial Growth and Evaluation of Alpha Irradiation on its Electrical Transport Properties
The wide bandgap (3.35 eV) semiconductor icosahedral boron phosphide (B 12 P 2 ) has been reported to self-heal from radiation damage from β particles (electrons) with energies up to 400 keV by demonstrating no lattice damage using transmission electron microscopy. This property could be exploited to create radioisotope batteries–semiconductor devices that directly convert the decay energy from a radioisotope to electricity. Such devices potentially have enormous power densities and decades-long lifetimes. To date, the radiation hardness of B 12 P 2 has not been characterized by electrical measurements nor have B 12 P 2 radioisotope batteries been realized. Therefore, this study was undertaken to evaluate the radiation hardness of B 12 P 2 after improving its epitaxial growth, developing ohmic electrical contacts, and reducing the residual impurities. Subsequently, the effects of radiation from a radioisotope on the electrical transport properties of B 12 P 2 were tested.
Silicon-Lattice-Matched Boron-Doped Gallium Phosphide: A Scalable Acousto-Optic Platform
The compact size, scalability, and strongly confined fields in integrated photonic devices enable new functionalities in photonic networking and information processing, both classical and quantum. Gallium phosphide (GaP) is a promising material for active integrated photonics due to its high refractive index, wide bandgap, strong nonlinear properties, and large acousto-optic figure of merit. Here, this study demonstrates that silicon-lattice-matched boron-doped GaP (BGaP), grown at the 12-inch wafer scale, provides similar functionalities as GaP. BGaP optical resonators exhibit intrinsic quality factors exceeding 25,000 and 200,000 at visible and telecom wavelengths, respectively. It further demonstrates the electromechanical generation of low-loss acoustic waves and an integrated acousto-optic (AO) modulator. High-resolution spatial and compositional mapping, combined with ab initio calculations, indicate two candidates for the excess optical loss in the visible band: the silicon-GaP interface and boron dimers. These results demonstrate the promise of the BGaP material platform for the development of scalable AO technologies at telecom and provide potential pathways toward higher performance at shorter wavelengths.
Phosphides or nitrides for better NLO properties? A detailed comparative study of alkali metal doped nano-cages
Highlights: • Alkali metal doped boron and aluminum phosphide nano-cages are evaluated for NLO properties. • The NLO response of phosphide nano-cages is compared with nitride nano-cages. • Phosphide nano-cages show better NLO properties than the corresponding nitride nano-cages. • Conductivity of doped nano-cages is much higher than pristine nano-cages. • MAl{sub 11}P{sub 12} and MB{sub 11}P{sub 12} show monotonic increase in hyperpolarizability with size of alkali metal. - Abstract: Phosphide nano-cages of MX{sub 12}P{sub 11} and MX{sub 11}P{sub 12} (where X = B or Al, M = Li, Na, K) are investigated theoretically for geometric, electronic and nonlinear optical (NLO) properties. Alkali metal is doped by replacing (a) aluminum or boron atom and (b) phosphorus atom. Alkali metal doping can remarkably alter the electronic and NLO properties of the nano-cages, regardless of doping site. HOMO-LUMO gap is decreased whereas first hyperpolarizability is enhanced significantly in doped AlP and BP nano-cages. PDOS (partial density of states) analysis suggests that diffuse excess electrons from alkali metal play important role in boosting NLO properties of phosphide nano-cages. TD-DFT calculations are performed to study the effect of alkali metal doping on absorption spectra. Red shift is observed in the UV–vis spectra of all considered nano-cages. Comparison of phosphide nano-cages with nitride nano-cages indicated that doping nitride nano-cages with alkali metal leads to greater hyperpolarizability and NLO response than doping phosphide nano-cages.