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| 2. |
- Dobrovolsky, Alexandr, et al.
(author)
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Energy Upconversion in GaP/GaNP Core/Shell Nanowires for Enhanced Near-Infrared Light Harvesting
- 2014
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In: Small. - : Wiley-VCH Verlagsgesellschaft. - 1613-6810 .- 1613-6829. ; 10:21, s. 4403-4408
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Journal article (peer-reviewed)abstract
- Semiconductor nanowires (NWs) have recently gained increasing interest due to their great potential for photovoltaics. A novel material system based on GaNP NWs is considered to be highly suitable for applications in efficient multi-junction and intermediate band solar cells. This work shows that though the bandgap energies of GaNx P1-x alloys lie within the visible spectral range (i.e., within 540-650 nm for the currently achievable x < 3%), coaxial GaNP NWs grown on Si substrates can also harvest infrared light utilizing energy upconversion. This energy upconversion can be monitored via anti-Stokes near-band-edge photoluminescence (PL) from GaNP, visible even from a single NW. The dominant process responsible for this effect is identified as being due to two-step two-photon absorption (TS-TPA) via a deep level lying at about 1.28 eV above the valence band, based on the measured dependences of the anti-Stokes PL on excitation power and wavelength. The formation of the defect participating in the TS-TPA process is concluded to be promoted by nitrogen incorporation. The revealed defect-mediated TS-TPA process can boost efficiency of harvesting solar energy in GaNP NWs, beneficial for applications of this novel material system in third-generation photovoltaic devices.
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| 4. |
- Dobrovolsky, Alexandr, et al.
(author)
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Mechanism for radiative recombination and defect properties of GaP/GaNP core/shell nanowires
- 2012
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In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 101:16, s. 163106-1-163106-4
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Journal article (peer-reviewed)abstract
- Recombination processes in GaP/GaNP core/shell nanowires (NWs) grown on a Si substrate by molecular beam epitaxy are examined using a variety of optical characterization techniques, including cw- and time-resolved photoluminescence and optically detected magnetic resonance (ODMR). Superior optical quality of the structures is demonstrated based on the observation of intense emission from a single NW at room temperature. This emission is shown to originate from radiative transitions within N-related localized states. From ODMR, growth of GaP/GaNP NWs is also found to facilitate formation of complex defects containing a P atom at its core that act as centers of competing non-radiative recombination.
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| 7. |
- Dobrovolsky, Alexandr, et al.
(author)
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Signatures of N incorporation in Raman and optical properties of GaP/GaNP core/shell nanowires
- 2013
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In: 2013 MRS Fall Meeting.
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Conference paper (peer-reviewed)abstract
- GaP/GaNP core shell NWs is a novel material system that has been most recently suggested for applications in solar cells. Adding nitrogen not only allow to tune the bandgap energy of GaNP alloy but also causes splitting of conduction band (CB) states, promising for intermediate band solar cells with improved efficiency. The purpose of this work is to investigate effects of N incorporation on band structure of such GaP/GaNxP1-x core/shell NWs using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Structural quality of the wires will be also evaluated from Raman measurements.The GaP/GaN0.009P0.991core/shell NWs studied in this work were grown on Si (111) substrates by gas-source molecular beam epitaxy (MBE). The GaP NW cores were grown under the vapor liquid-solid (VLS) mechanism, whereas the GaNP shell was formed via the step-mediated growth. The resulted core/shell NWs were found to have an axial length of about 2.5 μm, a total diameter of about 220 nm, and a typical diameter of the GaP core of ~110 nm. According to performed TEM measurements, the NWs predominantly have zincblende structure with some inclusions of the wurtzite crystal phase. Excellent structural quality of the wires was concluded based on the performed Raman measurements. The Raman scattering spectra were found to contain several first-order Raman modes including intense and sharp peaks at 366 and 403 cm-1 and weaker modes at 387, 397 and 499 cm-1. The first two modes are typical for zinc-blende GaP and are related to transverse-optic (TO) and longitudinal-optical (LO) phonons, respectively. The spectral positions of these modes were unaffected by the N incorporation indicating that the formed GaNP shell is unstrained. The Raman mode at 499 cm-1 peak is related to the Ga-N bond vibrations, confirming the formation of the GaNP alloy. The 397 cm-1 peak can be identify as a surface optical (SO) phonon mode due to its sensitivity to the dielectric constant of an external medium.It is also found that incorporation of N causes a dramatic increase of the PL intensity, which can be easily detected at room temperature even from a single wire. This is accompanied by a shortening of the PL decay time revealed from the performed transient PL measurements. We attribute these changes to the N-induced transformation of the band gap from the indirect one in GaP to a direct band gap in the GaNP alloy. Secondly, N incorporation causes a red shift of the fundament absorption edge revealed via the PLE measurements due to the bowing effect. The red shift of the conduction band (CB) edge is accompanied by a strong blue shift of the Γ CB state. This is ascribed to the splitting of the host CB states that are strongly perturbed by N. The revealed changes in the band structure are potentially beneficial for the applications of GaNP/GaP NWs in novel intermediate band solar cell structures with high efficiency.
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| 8. |
- Stehr, Jan Eric, et al.
(author)
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Defects in GaNP Nanowires
- 2014
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In: Abstract Book of the 56th Electronic Materials Conference. ; , s. 114-
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Conference paper (peer-reviewed)
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| 11. |
- Sukrittanon, S., et al.
(author)
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Growth and characterization of dilute nitride GaNxP1−x nanowires and GaNxP1−x/GaNyP1−y core/shell nanowires on Si (111) by gas source molecular beam epitaxy
- 2014
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In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 105:7, s. 072107-
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Journal article (peer-reviewed)abstract
- We have demonstrated self-catalyzed GaN xP1−x and GaN xP1−x/GaNyP1−y core/shell nanowire growth by gas-source molecular beam epitaxy. The growth window for GaN xP1−x nanowires was observed to be comparable to that of GaP nanowires (∼585 °C to ∼615 °C). Transmission electron microscopy showed a mixture of cubic zincblende phase and hexagonal wurtzite phase along the [111] growth direction in GaN xP1−x nanowires. A temperature-dependent photoluminescence (PL) study performed on GaN xP1−x/GaNyP1−y core/shell nanowires exhibited an S-shape dependence of the PL peaks. This suggests that at low temperature, the emission stems from N-related localized states below the conduction band edge in the shell, while at high temperature, the emission stems from band-to-band transition in the shell as well as recombination in the GaN xP1−x core.
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