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ABSTRACT: By employing an insulating zinc oxide (i-ZnO) as an electron accelerating layer, and an n-type ZnO as an active layer, ultraviolet (UV) emissions at 385 nm caused by the excitation of the n-ZnO layer by the accelerated electrons from the i-ZnO layer have been realized. By replacing the active layer with larger bandgap Mg0.39Zn0.61O and properly optimizing the structure, shorter wavelength emissions at around 328 nm have been obtained. Considering that the p-type doping of wide bandgap semiconductors is still a challenging issue, the results reported in this Letter may provide a promising alternative route to UV emissions.
Optics Letters 05/2012; 37(9):1568-70. · 3.40 Impact Factor
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ABSTRACT: By proper controlling the carrier generation and multiplication processes, an Au/MgO/Mg0.52Zn0.48O/MgxZn1−xO/n-ZnO structure has been designed and fabricated. In this structure, holes are multiplied via an impact ionization process in the MgO layer and injected into the Mg0.52Zn0.48O layer, and electrons are injected into the Mg0.52Zn0.48O layer from the n-ZnO layer through a composition-gradient MgxZn1−xO bridging layer. With the injection of electrons and holes, a deep ultraviolet emission at around 276 nm, coming from the Mg0.52Zn0.48O active layer, has been observed. The results reported in this letter may provide a promising route to high performance deep ultraviolet light-emitting devices.
Applied Physics Letters 09/2011; 99(10):101110-101110-3. · 3.84 Impact Factor
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ABSTRACT: Although great efforts have been made, reproducible p-type doping is still one of the largest hurdles that hinders the optoelectronic applications of ZnO. In this Letter, a reproducible route to p-type ZnO films employing lithium-nitrogen as a dual-acceptor dopant has been demonstrated, and p-i-n structured light-emitting devices (LEDs) have been constructed. Obvious purple emissions have been observed from the LEDs, confirming the applicability of the p-type ZnO films in optoelectronic devices. The results reported in this Letter provide a reproducible route to p-type ZnO films, and thus may lay a solid ground for future optoelectronic applications of ZnO.
Optics Letters 02/2011; 36(4):499-501. · 3.40 Impact Factor
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ABSTRACT: The ZnO nanocombs were synthesized by chemical vapor deposition method, which the uniform and dense nanotips were along one side of the comb ribbon. The growth mechanism was described as free catalyst self-assembled and vapor-solid model. The optical properties related with the surface states were investigated by Raman and PL spectra. The large redshift of 1LO phonon peak was attributed to the surface and interface states. The normalized PL spectra showed that the deep-level emission decreased for the sample to be annealed in O2 ambient at 500 degrees C. On the contrary, the deep-level emission increased while for the annealed sample in O2 ambient at 600 degrees C. The deep-level emission was attributed to the transition from the shallow donor to the deep acceptor. The XPS analysis showed the existence of oxygen rich and Zn deficient in the ZnO nanocombs annealed at 600 degrees C. The abnormal temperature dependence of integrated PL intensity was attributed to the abundant surface states.
Journal of Nanoscience and Nanotechnology 04/2010; 10(4):2370-4. · 1.56 Impact Factor
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ABSTRACT: The ZnO nanocombs were synthesized by chemical vapor deposition method, which the uniform and dense nanotips were along one side of the comb ribbon. The growth mechanism was described as free catalyst self-assembled and vapor-solid model. The optical properties related with the surface states were investigated by Raman and PL spectra. The large redshift of 1LO phonon peak was attributed to the surface and interface states. The normalized PL spectra showed that the deep-level emission decreased for the sample to be annealed in O2 ambient at 500 °C. On the contrary, the deep-level emission increased while for the annealed sample in O2 ambient at 600 °C. The deep-level emission was attributed to the transition from the shallow donor to the deep acceptor. The XPS analysis showed the existence of oxygen rich and Zn deficient in the ZnO nanocombs annealed at 600 °C. The abnormal temperature dependence of integrated PL intensity was attributed to the abundant surface states.
Journal of Nanoscience and Nanotechnology 03/2010; 10(4):2370-2374. · 1.56 Impact Factor
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ABSTRACT: A Au/MgO/MgZnO metal−oxide−semiconductor-structured photodetector was fabricated. The responsivity of the photodetector was about 2 orders of magnitude larger than that of the Au/MgZnO metal−semiconductor-structured photodetector fabricated under the same procedure except that no MgO layer was introduced. The detectivity of the photodetector is about 1 order of magnitude larger than the corresponding value of Si photodetector that is widely employed for ultraviolet detection currently. The enhanced responsivity was attributed to the carrier multiplication occurring in the MgO layer via impact ionization. The results reported in this paper may provide a facile route to ultraviolet photodetectors with high internal gain.
03/2010;
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ABSTRACT: ZnMgO nanowalls were prepared by plasma-assisted molecular beam epitaxy without a catalyst on c-Al2O3 substrate. The obtained nanowalls have preferred orientation along c axis. The nanowalls are about 10 to 20 nm in thickness and about 50 nm in height. Only Zn, Mg, O and Al signals are detected in the nanowalls from the energy dispersive spectroscopy (EDS). The Mg content is about 3% in ZnMgO nanowalls. The room temperature photoluminescence (PL) spectra shows the emission peak of the ZnMgO nanowalls at 3.346 eV. The origin of the ultraviolet emission is discussed with the help of temperature-dependent PL spectra. The ultraviolet emission band is free exiton recombination observed in the low temperature PL spectra (at 81 K). We also observe the free-to-acceptor (FA) emission of the ZnMgO nanowalls. The acceptor binding energy obtained from photoluminescence studies is about 123 meV. The results show that Mg doping leads to an increase of the acceptor binding energy. The possible growth mechanism of the ZnMgO nanowall networks was discussed.
Journal of Nanoscience and Nanotechnology 03/2010; 10(3):1681-4. · 1.56 Impact Factor
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ABSTRACT: The electroluminescence from an n-MgZnO/i-ZnO/MgO/p-GaN asymmetric double heterojunction has been demonstrated. With the injection of electrons from n-MgZnO and holes from p-GaN, an intense ultraviolet emission coming from the ZnO active layer was observed. It is revealed that the emission intensity of the diode recorded from the MgZnO side is significantly larger than that from the MgO side because of the asymmetric waveguide structure formed by the lower refractive index of MgO than that of MgZnO. The asymmetric waveguide structure reported in this letter may promise a simple and effective route to light-emitting diodes with improved light-extraction efficiency.
Applied Physics Letters 01/2010; 96(4):041110-041110-3. · 3.84 Impact Factor
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ABSTRACT: By employing a relatively low growth temperature and oxygen-rich conditions, single-crystalline cubic MgZnO films were prepared. A solar-blind deep ultraviolet (DUV) photodetector was finished on the MgZnO film. The maximum responsivity of the photodetector is 396 mA/W at 10 V bias, which is almost three orders of magnitude larger than the highest value ever reported in MgZnO-based solar-blind photodetectors. The dark current density is 1.5×10−11 A/cm2, comparable with the smallest value ever reported in solar-blind photodetectors. The improved performance reveals that the single-crystalline cubic MgZnO films have great potential applications in DUV optoelectronic devices.
Applied Physics Letters 09/2009; 95(13):131113-131113-3. · 3.84 Impact Factor
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ABSTRACT: Stable p-type Sb-doped ZnO (ZnO:Sb) was fabricated reproducibly by sintering mixture of ZnO and Sb2O3 powders under 5 GPa at temperatures of 1100–1450 °C. The best p-type ZnO:Sb with resistivity of 1.6×10−2 Ω cm, carrier concentration of 3.3×1020 cm−3, and mobility of 12.1 cm/V s was obtained by doping 4.6 at. % Sb and sintering at 1450 °C. The p-type conduction is due to complex acceptor formed by one substitutional Sb at Zn site and two Zn vacancies. The acceptor level was measured to be 113 meV. Effect of pressure on formation and electrical properties of the p-type ZnO:Sb is discussed.
Applied Physics Letters 07/2009; 95(2):022101-022101-3. · 3.84 Impact Factor
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ABSTRACT: Temperature-dependent photoluminescence of MgZnO alloy film has been studied, and it is found that the emission intensity increases significantly at a certain temperature range and then decreases when increasing temperature further. The anomalous increase is resulted from the localized excitons in MgZnO alloy, as revealed by the enhanced second-order longitudinal optical phonon in the Raman spectrum of the MgZnO film. A schematic model was suggested to depict the carrier transportation process in the MgZnO film considering the existence of localized exciton states. The results reported in this paper indicate that localized excitons in MgZnO alloy can result in greatly enhanced emission efficiency, which is eagerly wanted for the application of ZnO-based materials in high-efficiency light-emitting devices.
Journal of Applied Physics 06/2009; · 2.17 Impact Factor
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ABSTRACT: The phase stability of cubic Mg0.55Zn0.45O thin film grown by metal-organic chemical vapor deposition was studied through continuous thermal annealing. The crystal quality and surface smoothness were greatly improved after a continuous thermal annealing at 750 °C. It is attributed to the reducing of interstitial Zn by thermal annealing. However, phase segregation occurred when the sample was annealed at a higher temperature (850 °C), which is identified from both x-ray diffraction patterns and optical transmission spectra.
Applied Physics Letters 03/2009; 94(10):101902-101902-3. · 3.84 Impact Factor
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ABSTRACT: We report on the fabrication of an n-Mg0.12Zn0.88O/p-GaN heterojunction light-emitting diode with an MgO dielectric interlayer by plasma-assisted molecular beam epitaxy. The current−voltage curve of the heterojunction diode showed obvious rectifying characteristics with a threshold voltage of about 8 V. Under forward bias, an ultraviolet electroluminescence (EL) emission located at about 374 nm coming from the Mg0.12Zn0.88O layer was observed at room temperature. This is one of the shortest EL emissions observed in ZnO-based pn junctions to the best of our knowledge. The origin of the EL emission was elucidated in terms of the carrier transportation process modulated by the MgO interlayer in the heterojunction.
02/2009;
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ABSTRACT: Undoped n-ZnO films have been deposited onto p-GaN to form a pn heterojunction, and the current−voltage curve of the heterojunction shows obvious rectifying behaviors. A photodetector is fabricated from the heterojunction. Under back-illumination conditions, the GaN layer on one hand acts as a p-type counterpart for the n-ZnO layer, on the other hand as a “filter” that is transparent to the illumination light with wavelength longer than 360 nm. Because of the GaN “filter”, the photodetector shows a narrow band-pass response of only 17 nm in width. The results reported in this paper may provide a facile route to photodetectors with high spectrum selectivity.
11/2008;
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ABSTRACT: A series of MgxZn1−xO thin films has been prepared by metalorganic chemical vapor deposition and metal-semiconductor-metal structured ultraviolet photodetectors are fabricated from these films. The cutoff wavelengths of the photodetectors can cover the whole solar-blind spectrum range (220–280 nm) by varying Mg content in the MgxZn1−xO thin films. As a representative, the photodetector fabricated from Mg0.52Zn0.48O shows an ultraviolet/visible rejection ratio of about four orders of magnitude, and the dark current is 15 pA at 10 V bias. These results demonstrate that high-performance photodetectors operating in the whole solar-blind spectrum range can be realized in MgxZn1−xO films.
Applied Physics Letters 10/2008; 93(17):173505-173505-3. · 3.84 Impact Factor
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Z. G. Ju,
Y. M. Lu,
J. Y. Zhang,
X. J. Wu,
K. W. Liu,
B. S. Li,
D. X. Zhao,
Z. Z. Zhang,
B. H. Li,
B. Yao, D. Z. Shen
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ABSTRACT: Diluted magnetic semiconductors Cd1−xFexSe were grown by metalorganic chemical vapor deposition. Their structure was studied by X-ray diffraction and selected area electron diffraction. The effect of Fe doping on the morphology and photoluminescence of CdSe was investigated. The photoluminescence study indicated that the crystalline structure of the Cd1−xFexSe films changed from a mixture of cubic and hexagonal phase to a single hexagonal phase with increasing Fe content.
07/2008;
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ABSTRACT: α - and β- Fe Se thin films were grown by metal organic chemical vapor deposition. Compared to the other parameters, the growth temperature shows decisive influence on the phase transition of the FeSe samples. In temperature-dependent electrical measurements, n -type to p -type reversion was observed for both the α - or and β- Fe Se samples. Furthermore, the p -type character of the films becomes more and more obvious with increasing the Se / Fe atomic ratio in the samples. Ferromagnetism was observed in the α- Fe Se films although which is not supported by calculation on density of states. The ferromagnetic character shows significant dependence on Se / Fe atomic ratio in the films and was attributed to the Fe vacancies or Fe clusters in the α- Fe Se thin films. The magnetic domain and hysteresis loop of the β- Fe Se thin films are also studied.
Journal of Applied Physics 07/2008; · 2.17 Impact Factor
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ABSTRACT: This paper describes ZnO nanocrystals embedded in BaF2 matrices by the magnetron sputtering method in an attempt to use fluoride as a shell layer to embed ZnO nanocrystals core. BaF2 is a wide-band gap material, and can confine carriers in the ZnO films. As a result, the exciton emission intensity should be enhanced. The sample was annealed at 773 K, and X-ray diffraction (XRD) results showed that ZnO nanocrystals with wurtzite structure were embedded in BaF2 matrices. Raman-scattering spectra also confirmed the formation of ZnO nanoparticles. Abnormal longitudinal-optical (LO) phonon-dominant multiphonon Raman scattering was observed in the sample. Room-temperature photoluminescence (PL) spectra showed an ultraviolet emission peak at 374 nm. The origin of the ultraviolet emission is discussed here with the help of temperature-dependent PL spectra. The ultraviolet emission band was a mixture of free exciton and bound exciton recombination observed in the low temperature PL spectra (at 77 K). Abnormal temperature dependence of ultraviolet near-band-edge emission-integrated intensity of the sample was observed. The band tail state was observed in the absorption spectra, illustrating that the impurity-related defects were caused by the shell of the BaF2 grain layer. For comparison, ZnO films on BaF2 substrates were also fabricated by the magnetron sputtering method, and the same measurement methods were used.
Journal of Nanoscience and Nanotechnology 04/2008; 8(3):1160-4. · 1.56 Impact Factor
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ABSTRACT: Cadmium selenide (CdSe) thin films were fabricated on c-sapphire and GaAs (1 0 0) substrates by low pressure metal organic chemical vapour deposition. The structural properties of these CdSe thin films were investigated by x-ray diffraction and photoluminescence spectroscopy. The effects of growth temperature (Tg) on the structural phase of the CdSe thin films were discussed. Phase transition of CdSe thin films grown on sapphire substrates from zincblende (ZB) to wurtzite (W) was observed with increasing Tg, and the transition critical temperature is 440 °C. However, the CdSe films grown on GaAs (1 0 0) keep ZB form in the whole range of Tg investigated from 300 to 500 °C.
Journal of Physics D Applied Physics 12/2007; 41(1):015304. · 2.54 Impact Factor
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ABSTRACT: ZnO nanocrystals (NCs) embedded in a BaF2 matrix have been prepared by a radio-frequency magnetron sputtering technique followed by a thermal annealing process from 400 to 1100 °C. X-ray diffraction results show that ZnO NCs with a hexagonal wurtzite structure are formed in a BaF2 matrix. The average grain sizes are estimated ranging from 7.5 to 33 nm, corresponding to fabricated samples annealed at temperatures from 400 to 800 °C. Photoluminescence showed a very strong ultraviolet near-band-edge (NBE) emission located in the 364–383 nm spectral region. The emission intensity is enhanced and the linewidth is narrowed as the annealing temperature increases. The intensity ratio of ultraviolet NBE emission to deep-level visible is also increased with annealing temperature indicating that deep-level defects in ZnO are suppressed.
Journal of Physics D Applied Physics 08/2007; 40(18):5598. · 2.54 Impact Factor
