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  • 1
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    Carbon-rich hexagonal (BN)C alloys (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-06-02
    Description: Thin films of hexagonal boron nitride carbon, h- (BN) 1− x (C 2 ) x , alloys in the C-rich side have been synthesized by metal-organic chemical vapor deposition (MOCVD) on c -plane sapphire substrates. X-ray diffraction measurements confirmed single hexagonal phase of h -(BN) 1−x (C 2 ) x epilayers. Electrical transport and Raman spectroscopy measurements results revealed evidences that homogenous h- (BN) 1− x (C 2 ) x alloys with x  ≥ 95% can be synthesized by MOCVD at a growth temperature of 1300 °C. The variable temperature Hall-effect measurements suggested that a bandgap opening of about 93 meV with respect to graphite has been obtained for h -(BN) 1−x (C 2 ) x with x  = 0.95, which is consistent with the expected value deduced from the alloy dependence of the energy gap of homogenous h- (BN) 1−x (C 2 ) x alloys. Atomic composition results obtained from x-ray photoelectron spectroscopy measurements revealed that the carrier type in C-rich h- (BN) 1−x (C 2 ) x alloys is controlled by the stoichiometry ratio between the B and N via changing the V/III ratio during the growth. The demonstration of bandgap opening and conductivity control in C-rich h- (BN) 1−x (C 2 ) x alloys provide feasibilities for realizing technologically significant devices including infrared (IR) emitters and detectors active from near to far IR and multi-spectral IR emitters and detectors.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 2
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    High-efficiency, flexible CdTe solar cells on ultra-thin glass substrates (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-03-31
    Description: Flexible, high-efficiency, low-cost solar cells can enable applications that take advantage of high specific power, flexible form factors, lower installation and transportation costs. Here, we report a certified record efficiency of 16.4% for a flexible CdTe solar cell that is a marked improvement over the previous standard (14.05%). The improvement was achieved by replacing chemical-bath-deposited CdS with sputtered CdS:O and also replacing the high-temperature sputtered ZnTe:Cu back contact layer with co-evaporated and rapidly annealed ZnTe:Cu. We use quantum efficiency and capacitance-voltage measurements combined with device simulations to identify the reasons for the increase in efficiency. Both device simulations and experimental results show that higher carrier density can quantitatively account for the increased open circuit voltage ( V OC ) and Fill Factor ( FF ), and likewise, the increase in short circuit current density ( J SC ) can be attributed to the more transparent CdS:O.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 3
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    Realization of highly efficient hexagonal boron nitride neutron detectors (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-08-17
    Description: We report the achievement of highly efficient 10 B enriched hexagonal boron nitride ( h- 10 BN) direct conversion neutron detectors. These detectors were realized from freestanding 4-in. diameter h- 10 BN wafers 43  μ m in thickness obtained from epitaxy growth and subsequent mechanical separation from sapphire substrates. Both sides of the film were subjected to ohmic contact deposition to form a simple vertical “photoconductor-type” detector. Transport measurements revealed excellent vertical transport properties including high electrical resistivity (〉10 13 Ω cm) and mobility-lifetime (μτ) products. A much larger μτ product for holes compared to that of electrons along the c-axis of h- BN was observed, implying that holes (electrons) behave like majority (minority) carriers in undoped h- BN. Exposure to thermal neutrons from a californium-252 ( 252 Cf) source moderated by a high density polyethylene moderator reveals that 43  μ m h- 10 BN detectors possess 51.4% detection efficiency at a bias voltage of 400 V, which is the highest reported efficiency for any semiconductor-based neutron detector. The results point to the possibility of obtaining highly efficient, compact solid-state neutron detectors with high gamma rejection and low manufacturing and maintenance costs.
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  • 4
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    The origin of deep-level impurity transitions in hexagonal boron nitride (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-01-14
    Description: Deep ultraviolet photoluminescence (PL) emission spectroscopy has been employed to investigate the origin of the widely observed deep level impurity related donor-acceptor pair (DAP) transition with an emission peak near 4.1 eV in hexagonal boron nitride ( h -BN). A set of h -BN epilayers were grown by metal-organic chemical vapor deposition (MOCVD) under different ammonia (NH 3 ) flow rates to explore the role of nitrogen vacancies (V N ) in the deep-level transitions. The emission intensity of the DAP transition near 4.1 eV was found to decrease exponentially with an increase of the NH 3 flow rate employed during the MOCVD growth, implying that impurities involved are V N . The temperature-dependent PL spectra were measured from 10 K up to 800 K, which provided activation energies of ∼0.1 eV for the shallow impurity. Based on the measured energy level of the shallow impurity (∼0.1 eV) and previously estimated bandgap value of about 6.5 eV for h -BN, we deduce a value of ∼2.3 eV for the deep impurity involved in this DAP transition. The measured energy levels together with calculation results and formation energies of the impurities and defects in h -BN suggest that V N and carbon impurities occupying the nitrogen sites, respectively, are the most probable shallow donor and deep acceptor impurities involved in this DAP transition.
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  • 5
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    The origins of near band-edge transitions in hexagonal boron nitride epilayers (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-02-05
    Description: Photoluminescence spectroscopy has been employed to probe the near band-edge transitions in hexagonal BN ( h -BN) epilayers synthesized under varying ammonia flow rates. The results suggest that the quasi-donor-acceptor pair emission line at 5.3 eV is due to the transition between the nitrogen vacancy and a deep acceptor, whereas the 5.5 eV emission line is due to the recombination of an exciton bound to a deep acceptor formed by carbon impurity occupying the nitrogen site. By growing h -BN under high ammonia flow rates, nitrogen vacancy related peaks can be eliminated and epilayers exhibiting pure free exciton emission have been obtained.
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  • 6
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    Superconductivity of magnetic Sm-substituted LaCo2B2 (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-03-12
    Description: In this work, superconductivity with a maximum superconducting transition onset temperature T C of 3.6 K is reported in La 1- x Sm x Co 2 B 2 in the concentration range 0.12 ≤  x  ≤ 0.20. Substitution of magnetic Sm ions with 4 f electrons for La is beneficial for superconductivity in the system. The low upper critical field of La 0.8 Sm 0.2 Co 2 B 2 suggests that the dominant magnetic-field suppressing superconducting mechanism is the orbit pair-breaking effect. A special specific-heat anomaly is observed around T C .
    Print ISSN: 0021-8979
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  • 7
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    Dramatic enhancement of 1.54 μm emission in Er doped GaN quantum well structures (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-03-25
    Description: Erbium (Er) doped III-nitride materials have attracted much attention due to their capability to provide highly thermal stable optical emission in the technologically important as well as eye-safer 1540 nm wavelength window. There is a continued need to exploring effective mechanisms to further improve the quantum efficiency (QE) of the 1.54  μ m emission in Er-doped III-nitrides. GaN/AlN multiple quantum wells (MQWs:Er) have been synthesized by metal organic chemical vapor deposition and explored as an effective means to improve the QE of the 1.54  μ m emission via carrier confinement and strain engineering. The 1.54  μ m emission properties from MQWs:Er were probed by photoluminescence (PL) emission spectroscopy. It was found that the emission intensity from MQWs:Er is 9 times higher than that of GaN:Er epilayers with a comparable Er active layer thickness. The influences of the well and barrier width on the PL emission at 1.54  μ m were studied. The results revealed that MQWs:Er consisting of well width between 1 and 1.5 nm and the largest possible barrier width before reaching the critical thickness provide the largest boost in QE of the 1.54  μ m emission. These results demonstrate that MQWs:Er provide a basis for efficient photonic devices active at 1.54  μ m.
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  • 8
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    Bandgap and exciton binding energies of hexagonal boron nitride probed by photocurrent excitation spectroscopy (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-09-20
    Description: Photocurrent excitation spectroscopy has been employed to probe the band structure and basic parameters of hexagonal boron nitride ( h- BN) epilayers synthesized by metal-organic chemical vapor deposition. Bias dependent photocurrent excitation spectra clearly resolved the band-to-band, free exciton, and impurity bound exciton transitions. The energy bandgap (E g ), binding energy of free exciton (E x ), and binding energy of impurity bound exciton (E bx ) in h- BN have been directly obtained from the photocurrent spectral peak positions and comparison with the related photoluminescence emission peaks. The direct observation of the band-to-band transition suggests that h- BN is a semiconductor with a direct energy bandgap of E g  = 6.42 eV at room temperature. These results provide a more coherent picture regarding the fundamental parameters of this important emerging ultra-wide bandgap semiconductor.
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  • 9
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    Enhancement of 1.5 μm emission under 980 nm resonant excitation in Er and Yb co-doped GaN epilayers (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-10-13
    Description: The Erbium (Er) doped GaN is a promising gain medium for optical amplifiers and solid-state high energy lasers due to its high thermal conductivity, wide bandgap, mechanical hardness, and ability to emit in the highly useful 1.5  μ m window. Finding the mechanisms to enhance the optical absorption efficiency at a resonant pump wavelength and emission efficiency at 1.5  μ m is highly desirable. We report here the in-situ synthesis of the Er and Yb co-doped GaN epilayers (Er + Yb:GaN) by metal-organic chemical vapor deposition (MOCVD). It was observed that the 1.5  μ m emission intensity of the Er doped GaN (Er:GaN) under 980 nm resonant pump can be boosted by a factor of 7 by co-doping the sample with Yb. The temperature dependent PL emission at 1.5  μ m in the Er + Yb:GaN epilayers under an above bandgap excitation revealed a small thermal quenching of 12% from 10 to 300 K. From these results, it can be inferred that the process of energy transfer from Yb 3+ to Er 3+ ions is highly efficient, and non-radiative recombination channels are limited in the Er + Yb:GaN epilayers synthesized in-situ by MOCVD. Our results point to an effective way to improve the emission efficiency of the Er doped GaN for optical amplification and lasing applications.
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  • 10
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    Rhenium-phthalocyanine molecular nanojunction with high magnetic anisotropy and high spin filtering efficiency (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-07-21
    Description: Using the density functional and non-equilibrium Green's function approaches, we studied the magnetic anisotropy and spin-filtering properties of various transition metal-Phthalocyanine molecular junctions across two Au electrodes. Our important finding is that the Au-RePc-Au junction has both large spin filtering efficiency (〉80%) and large magnetic anisotropy energy, which makes it suitable for device applications. To provide insights for the further experimental work, we discussed the correlation between the transport property, magnetic anisotropy, and wave function features of the RePc molecule, and we also illustrated the possibility of controlling its magnetic state.
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