Browsing by Author "Ahn, H."

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Anisotropic Structure Induced Electrical Properties of A-plane InN
(2013-01-01)
Chang, P. -H.
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Chia, J. -W.
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Gwo, S.
;
Ahn, H.
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物理研究所
Terahertz spectroscopy reveals that the anisotropic electrical properties of nonpolar InN film along in-plane c-axis and in-plane m-axis are determined by the orientation of narrow and thin stacking faults, not by the density of defects.
Carrier Dynamics in InN Nanorod Arrays
(2013)
Su, S. -H.
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Yu, C. -C.
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Gwo, S.
;
Ahn, H.
;
光電工程學系
A fast initial decay was observed from InN nanorods with similar to 30 nm diameter, the lifetime of which is much shorter than the carrier cooling time, demonstrating the substantial surface-associated influence on carrier relaxation in nanorods.
Carrier dynamics of Mg-doped indium nitride
(2010-08-09)
Ahn, H.
;
Yu, K. -J.
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Hong, Y. -L.
;
Gwo, S.
;
光電工程學系
Recently, we have reported a significant enhancement (>500 times in intensity) in terahertz emission from Mg-doped indium nitride (InN:Mg) films compared to undoped InN. It was found that the intensity of terahertz radiation strongly depends on the background electron density. In this letter, we present the results on ultrafast time-resolved reflectivity measurement employed to investigate the carrier dynamics of InN:Mg. We find that the decay time constant of InN:Mg depends on background electron density in the same way as terahertz radiation does. The spatial redistribution of carriers in diffusion and drift is found to be responsible for the recombination behavior as well as terahertz radiation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3479523]
Carrier dynamics of Mg-doped indium nitride
(2011)
Ahn, H.
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Hong, C. -C.
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Hong, Y. -L.
;
Gwo, S.
;
光電工程學系
We report the carrier density dependence of carrier dynamics of Mg-doped InN (InN:Mg) films. Recently, we have demonstrated a significant enhancement of terahertz emission from InN: Mg, which is due to the temporal evolution of drift and diffusion currents depending on the background carrier density. We studied the details of carrier dynamics of InN: Mg which is crucial for the clarification of the terahertz emission mechanism by performing the time-resolved optical reflectivity measurement on InN: Mg films grown with different Mg-doping levels. Experimental analysis demonstrates that the initial sharp drop and recovery of reflectivity response of InN: Mg films are dominated by photocarrier-dependent bandgap renormalization and band filling processes, whereas the slow decay time constant (tau(2)) of reflectivity of InN: Mg has the strong dependence on the background carrier density. As the carrier density decreases from that of undoped InN, tau(2) of InN: Mg continuously increases and reaches the maximum value at a critical value of similar to 1x10(18) cm(-3). Interestingly, the strongest terahertz radiation was observed at this carrier density and it keeps decreasing with the increase of carrier density. Intense terahertz radiation corresponds to the fast and large spatial separation of charged carrier density through diffusion and drift. Large spatial separation results in the longer decay time for charged carriers to reach equilibrium after strong emission of terahertz waves, and it explains the similar carrier density dependence of terahertz emission and tau(2).
Efficient defect healing of transition metal dichalcogenides by phthalocyanines
(2019-01-01)
Ahn, H.
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Lee, G. -H.
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光電工程學系
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Department of Photonics
We present that a thin layer of phthalocyanine (Pc) molecules can efficiently improve the optical quality of defective MoSe2 grown by CVD method. Since MoSe2 flakes are coupled with Pc molecules through van der Waals force, the structural change can be minimized, but the pristine properties can be significantly recovered. This defect healing effect can be achieved via ultrathin coverage of Pc molecules using a simple solution-mediated process. We also observed that Pc molecules with nanometer-scale lateral dimensions is useful in suppression of point defects commonly observed in mechanically exfoliated 2D TMDCs when it is improperly annealed at sub-decomposition temperatures.
Free carrier dynamics of InN nanorods investigated by time-resolved terahertz spectroscopy
(2009-01-15)
Ahn, H.
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Chuang, C. -H.
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Ku, Y. -P.
;
Pan, C. -L.
;
光電工程學系
Ultrafast time-resolved terahertz spectroscopy is employed to investigate the carrier dynamics of indium nitride (InN) nanorod arrays and an epitaxial film. Transient differential transmission of terahertz wave shows that hot carrier cooling and defect-related nonradiative recombination are the common carrier relaxation processes for InN film and nanorods. However, the electrons confined in the narrow structure of nanorods are significantly affected by the carrier diffusion process near the surface, which causes the abnormally long relaxation time for nanorods.
Grain-size-related transient terahertz mobility of femtosecond-laser-annealed polycrystalline silicon
(2009-09-01)
Wang, Y. -C.
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Ahn, H.
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Chuang, C. -H.
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Ku, Y. -P.
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Pan, C. -L.
We investigated carrier-relaxation dynamics of femtosecond laser annealed (FLA) polycrystalline silicon (poly-Si). The correlation between morphology and electrical properties of poly-Si after femtosecond laser annealing is elucidated by optical-pump-terahertz-probe and terahertz time-domain spectroscopies. The transient conductivities of FLA-processed poly-Si with large (similar to 500 nm) and small (similar to 50 nm) grain sizes were both well fitted by the Drude model in the terahertz regime from 0.4 to 2 THz. The transient mobilities of these materials were determined to be 175 +/- 19.4 and 94.5 +/- 20.2 cm(2)/V s, respectively. After annealing, reduction of deep-state density rather than tail-state density in large-grain poly-Si is responsible for its higher mobility.
Highly stable, frequency-controlled mode-locked erbium fiber laser comb
(2007-01-01)
Peng, J. -L.
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Ahn, H.
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Shu, R. -H.
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Chui, H. -C.
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Nicholson, J. W.
A mode-locked Er: fiber laser-based optical frequency comb with high stability in the repetition frequency and carrier-envelope offset (CEO) frequency is realized. The CEO beat signal was detected right after the supercontinuum generation by a compact single-beam f-2 f self-referencing interferometer, which does not require further delay compensation. The stabilized repetition frequency has an out-of-loop tracking stability of 1.3 x 10(-13)/root tau for an integration time tau less than 1000 s, which is limited by the stability of the frequency measurement system. The stabilized CEO frequency has a residual fluctuation of 0.52 mHz measured with a 1 s gate time. This is, to our knowledge, the highest tracking stability realized for fiber laser-based optical frequency comb.
Influence of structural anisotropy to anisotropic electron mobility in a-plane InN
(2013-02-11)
Ahn, H.
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Chia, J. -W.
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Lee, H. -M.
;
Gwo, S.
;
光電工程學系
This study reports on the anisotropic electron transport properties and a correlation between the electron mobility (mu) and the stacking faults (SFs) in the a-plane InN film. Electron mobilities measured by terahertz time-domain spectroscopy and Hall effect measurement along the in-plane [(1) over bar 100] (c(perpendicular to)) orientation were much higher than those of the in-plane [0001] (c(parallel to)) orientation. This result shows a sharp contrast to higher defect density for the c(perpendicular to) orientation as measured by x-ray diffraction. The electrons transporting through the planar SFs aligned along the c(perpendicular to) direction are expected to experience more scattering by defects, resulting in lower mu for the c(parallel to) orientation. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792209]
Intense terahertz emission from a-plane InN surface
(2008-03-10)
Ahn, H.
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Ku, Y. -P.
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Chuang, C. -H.
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Pan, C. -L.
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Lin, H. -W.
We report a significant enhancement in terahertz emission from the indium nitride (InN) films grown along the a axis (a-plane InN), relative to the InN films grown along the c axis. The primary radiation mechanism of the a-plane InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field perpendicular to the a axis, which effectively enhances the geometrical coupling of the radiation out of semiconductor. In addition, azimuthal angle dependence measurement shows that the p-polarized terahertz output consists of a large angularly independent component and a weak component with a distinctive fourfold rotation symmetry. (c) 2008 American Institute of Physics.
Mg-induced terahertz transparency of indium nitride films
(2011-12-05)
Ahn, H.
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Chia, J. -W.
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Lee, H. -M.
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Hong, Y. -L.
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Gwo, S.
Terahertz time-domain spectroscopy (THz-TDS) has been used to investigate electrical properties of Mg-doped indium nitride (InN). Mg-doping in InN was found to significantly increase terahertz transmittance. THz-TDS analysis based on the Drude model shows that this high transmittance from Mg-doped InN is mainly due to the reduction in mobility associated with ionized dopants. The Hall-effect-measured mobility is typically lower than the THz-TDS-measured mobility for the same samples. However, the results of both measurements have the same slope in the linear relation between mobility and density. By introducing a compensation ratio of similar to 0.2, an excellent agreement in mobilities of two methods is obtained. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3669538]
Nonlinear absorption in InN under resonant- and non-resonant excitation
(2013)
Ahn, H.
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Lee, M-T
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Chang, Y-M
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Peng, J. L.
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Gwo, S.
We report the wavelength-dependent nonlinear absorption (NLA) of InN film grown on an. r-plane sapphire by molecular beam epitaxy technique. In order to understand the nonlinear optical properties of InN, the Z-scan measurement was performed at two different wavelengths, the photon energies of which are near (resonant excitation) and much higher (non-resonant excitation) than the bandgap of InN, respectively. Under non-resonant excitation, band-filling effect leads the dominant saturable absorption, while under resonant excitation, reverse saturable absorption dominates the nonlinear absorption. From the open-aperture Z-scan measurement under resonant excitation, we found that InN exhibits more than one nonlinear absorption process simultaneously. Particularly, under relatively weak resonant excitation, the transformation from saturable absorption to 2PA through the intermediate excitation state absorption was observed as the sample approaches the beam focus. The close-aperture Z-scan signals of InN show valley-peak response, implying that the nonlinear refraction in InN is caused by the self-focusing of the Gaussian laser beam. Using the Z-scan theory, the corresponding nonlinear parameters, such as saturation intensity, 2PA coefficient, and nonlinear refractive index, of InN were estimated.
Oxidation of Ag Nanowires Studied by Terahertz Spectroscopy
(2013-01-01)
Tsai, Y. -J.
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Chang, C. -Y.
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Yu, P.
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Ahn, H.
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光電工程學系
The electrical conductivity of silver nanowire assembly is measured by using terahertz spectroscopy. Growth of the surface-oxidized layer on silver nanowires hinders electron transportation through the nanowire-to-nanowire junction and reduces the conductivity of nanowire network.
Photoluminescence From InN Nanorod Arrays with a Critical Size
(2013-01-01)
Chang, K. -Y.
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Liu, Y. -S.
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Gwo, S.
;
Ahn, H.
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光電工程學系
Abnormal temperature dependence of peak energy and bandwidth of photoluminescence spectra was observed for InN nanorods with a diameter <30 nm, which is of the same order of the thickness of surface electron accumulation layer.
Spectral dependence of third-order nonlinear optical properties in InN
(2014-05-19)
Ahn, H.
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Lee, M. -T.
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Chang, Y. -M.
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光電工程學系
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Department of Photonics
We report on the nonlinear optical properties of InN measured in a wide near-infrared spectral range with the femtosecond Z-scan technique. The above-bandgap nonlinear absorption in InN is found to originate from the saturation of absorption by the band-state-filling and its cross-section increases drastically near the bandgap energy. With below-bandgap excitation, the nonlinear absorption undergoes a transition from saturation absorption (SA) to reverse-SA (RSA), attributed to the competition between SA of band-tail states and two-photon-related RSA. The measured large nonlinear refractive index of the order of 10(-10) cm(2)/W indicates InN as a potential material for all-optical switching and related applications. (C) 2014 AIP Publishing LLC.
Terahertz emission and spectroscopy on InN epilayer and nanostructure
(2009)
Ahn, H.
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Pan, C. -L.
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Gwo, S.
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光電工程學系
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Department of Photonics
We report a comprehensive study on THz emission and spectroscopy of indium nitride (InN) films and its nanorod arrays grown by plasma-assisted molecular beam epitaxy technique. For the enhancement of THz emission from InN, we demonstrated two method; firstly using nanorod arrays, which have large surface area for optical absorption and THz emission, and secondly using nonpolar InN film, of which the electric field is along the sample surface. We propose that a "screened" photo-Dember effect due to narrow surface electron accumulation layer of InN is responsible for the nanorod-size-dependent enhancement from InN nanorods. The primary THz radiation mechanism of nonpolar InN is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field. THz time-domain spectroscopy has been used to investigate THz conductivity and dielectric response of InN nanorod arrays and epitaxial film. The complex THz conductivity of InN film is well fitted by the Drude model, while the negative imaginary conductivity of the InN nanorods can be described by using a non-Drude model, which includes a preferential backward scattering due to defects in InN nanorods, or a Coulombic restoring force from charged defects.
Terahertz emission from Mg-doped a-plane InN
(2011)
Ahn, H.
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Yeh, Y. -J.
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Gwo, S.
;
光電工程學系
;
Department of Photonics
We report terahertz (THz) emission from magnesium doped a-plane indium nitride (a-InN:Mg) films with different background carrier density, relative to the Mg-doped InN films grown along the c-axis (c-InN: Mg). Due to its high electron affinity, as-grown InN film is typically n-type and it has extremely high background carrier density, which causes much weaker THz emission than that from other semiconductors, such as InAs. The background carrier density of Mg-doped InN can be widely changed by adjusting the Mg doping level. For c-InN: Mg, THz emission is dramatically enhanced (x500 than that of undoped c-InN) as the background carrier density decreases to a critical value of similar to 1x10(18) cm(-3), which is due to the reduced screening of the photo-Dember field at the lower carrier density. For a-InN, however, intense THz emission (x400 than that of undoped c-InN) is observed for both undoped and Mg-doped a-InN and the enhancement is weakly dependent on the background carrier density. The primary THz radiation mechanism of the a-plane InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field perpendicular to the a-axis, which effectively enhances the geometrical coupling of the radiation out of semiconductor. The weak dependence of THz radiation on the background carrier density for a-InN shows that in-plane surface field induced-terahertz emission is not affected by the background carrier density. Small, but apparent azimuthal angle dependence of terahertz emission is also observed for a-InN, indicating the additional contribution of nonlinear optical processes on terahertz emission.
Terahertz emission from vertically aligned InN nanorod arrays
(2007-09-24)
Ahn, H.
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Ku, Y.-P.
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Wang, Y.-C.
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Chuang, C.-H.
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Gwo, S.
Terahertz emission from indium nitride (InN) nanorods and InN film grown by molecular-beam epitaxy on Si(111) substrates has been investigated. Terahertz emission from InN nanorods is at least three times more intense than that from InN film and depends strongly on the size distribution of the nanorods. Surface electron accumulation at the InN nanorods effectively screens out the photo-Dember field in the accumulation layer formed under the surface. The nanorods with considerably large diameter than the thickness of accumulation layer are found to be dominant in the emission of terahertz radiation from InN nanorod arrays. (c) 2007 American Institute of Physics.
Terahertz emission mechanism of magnesium doped indium nitride
(2009-12-07)
Ahn, H.
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Yeh, Y. -J.
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Hong, Y. -L.
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Gwo, S.
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光電工程學系
We report carrier concentration-dependence of terahertz emission from magnesium doped indium nitride (InN:Mg) films. Near the critical concentration (n(c) similar to 1 x 10(18) cm(-3)), the competition between two emission mechanisms determines the polarity of terahertz emission. InN: Mg with n > n(c) exhibits enhanced positive polarity terahertz emission compared to the undoped InN, which is due to the reduced screening of the photo-Dember field. For InN: Mg with n < n(c), the polarity of terahertz signal changes to negative, indicating the dominant contribution of the surface electric field due to the large downward surface band bending within the surface layer extending over the optical absorption depth. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3270042]
Terahertz Emission of Magnesium doped Indiu
(2010)
Yeh, Y. -J.
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Ahn, H.
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Hong, Y. -L.
;
Gwo, Shangjr
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光電工程學系
Significant THz power enhancement and polarity reversal were observed from Mg-doped InN. The carrier concentration-dependent THz polarity reversal reflects the interplay between the surface-electric-field and the photo-Dember field for THz emission from InN:Mg. (C)2010 Optical Society of America