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Ni, Wei-Xin
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Publications (10 of 56) Show all publications
Adnane, B., Karlsson, F., Hansson, G., Holtz, P.-O. & Ni, W.-X. (2010). Spatially direct and indirect transitions of self-assembled SiGe/Si quantum dots studied by photoluminescence excitation spectroscopy. Applied Physics Letters, 96(18), 181107
Open this publication in new window or tab >>Spatially direct and indirect transitions of self-assembled SiGe/Si quantum dots studied by photoluminescence excitation spectroscopy
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2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 18, p. 181107-Article in journal (Refereed) Published
Abstract [en]

Well-resolved photoluminescence excitation (PLE) spectra are reported for selfassembled SiGe dots grown on Si(100) by molecular beam epitaxy. The observation of two excitation resonance peaks is attributed to two different excitation/de-excitation routes of interband optical transitions connected to the spatially direct and indirect recombination processes. It is concluded that two dot populations are addressed by each monitored luminescence energy for the PLE acquisition.

Keywords
Ge-Si alloys, luminescence, molecular beam epitaxial growth, self-assembly, semiconductor quantum dots
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-53204 (URN)10.1063/1.3424789 (DOI)000277422000007 ()
Note
Original Publication: Bouchaib Adnane, Fredrik Karlsson, Göran Hansson, Per-Olof Holtz and Wei-Xin Ni, Spatially direct and indirect transitions of self-assembled SiGe/Si quantum dots studied by photoluminescence excitation spectroscopy, 2010, Applied Physics Letters, (96), 18, 181107. http://dx.doi.org/10.1063/1.3424789 Copyright: American Institute of Physics http://www.aip.org/ Available from: 2010-01-19 Created: 2010-01-19 Last updated: 2017-12-12
Zhou, Y., Tvingstedt, K., Zhang, F., Du, C., Ni, W.-X., Andersson, M. R. & Inganäs, O. (2009). Observation of a Charge Transfer State in Low-Bandgap Polymer/Fullerene Blend Systems by Photoluminescence and Electroluminescence Studies. ADVANCED FUNCTIONAL MATERIALS, 19(20), 3293-3299
Open this publication in new window or tab >>Observation of a Charge Transfer State in Low-Bandgap Polymer/Fullerene Blend Systems by Photoluminescence and Electroluminescence Studies
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2009 (English)In: ADVANCED FUNCTIONAL MATERIALS, ISSN 1616-301X, Vol. 19, no 20, p. 3293-3299Article in journal (Refereed) Published
Abstract [en]

The presence of charge transfer states generated by the interaction between the fullerene acceptor PCBM and two alternating copolymers of fluorene with donor-acceptor-donor comonomers are reported; the generation leads to modifications in the polymer bandgap and electronic structure. In one of polymer/fullerene blends, the driving; force for photocurrent generation, i.e., the gap between the lowest unoccupied molecular orbitals of the donor and acceptor, is only 0.1 eV, but photocurrent is generated. It is shown that the presence of a charge transfer state is more important than the driving force. The charge transfer states are visible through new emission peaks in the photoluminescence spectra and through electroluminescence at a forward bias. The photoluminescence can be quenched under reverse bias, and can be directly correlated to the mechanism of photocurrent generation. The excited charge transfer state is easily dissociated into free charge carriers, and is an important intermediate state through which free charge carriers are generated.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-51897 (URN)10.1002/adfm.200900702 (DOI)
Available from: 2009-11-23 Created: 2009-11-23 Last updated: 2015-06-01
Adnane, B., Karlsson, F., Zhao, M., Hansson, G., Holtz, P.-O. & Ni, W.-X. (2009). Origin of photoresponse at 8-14 μm in stacks of self-assembled SiGe/Si quantum dots. Physical Review B. Condensed Matter and Materials Physics
Open this publication in new window or tab >>Origin of photoresponse at 8-14 μm in stacks of self-assembled SiGe/Si quantum dots
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2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235XArticle in journal (Other academic) Submitted
Abstract [en]

A normal incidence photodetector operating at 8-14 μm is demonstrated using p-type δ-doped SiGe dot multilayer structures grown by molecular beam epitaxy on Si(001) substrates. Based on the experimental results of photoluminescence and photoluminescence excitation spectroscopies together with numerical analysis, the origin of the measured photocurrent was attributed to intersubband optical transitions between the heavy hole and light hole states of the valence band of the self-assembled SiGe dots and subsequent lateral transport of photo-excited carriers in the conduction channels formed by Ge wetting layers.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-53203 (URN)
Available from: 2010-01-19 Created: 2010-01-19 Last updated: 2017-12-12
Adnane, B., Karlsson, F., Hansson, G., Holtz, P.-O. & Ni, W.-X. (2009). Photoluminescence excitation spectroscopy of self-assembled SiGe/Si quantum dots.
Open this publication in new window or tab >>Photoluminescence excitation spectroscopy of self-assembled SiGe/Si quantum dots
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2009 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Photoluminescence excitation (PLE) experiments are reported for various self-assembled SiGe/Si dot samples grown on Si(001) by molecular beam epitaxy at substrate temperatures ranging from 430 to 580 C. Two excitation peaks were observed, and the characteristics of the involved optical transitions were studied in detail by PLE (in one case implemented together with selective photoluminescence, SPL) on different samples containing either only one SiGe dot layer or multiple SiGe-dot/Si stacks. The temperature- and power-dependence of the excitation properties together with the results of six-band k.p calculations support the assignment of the observed PLE peaks to spatially direct and indirect transitions collected from two different SiGe dot populations.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-53205 (URN)
Available from: 2010-01-19 Created: 2010-01-19 Last updated: 2015-01-23
Adnane, B., Lai, Y.-F., Shieh, J.-M., Holtz, P.-O. & Ni, W.-X. (2009). Photoluminescence study of nanocrystalline-Si(Ge) embedded in mesoporous silica. Solid-State Electronics, 53(8), 862-864
Open this publication in new window or tab >>Photoluminescence study of nanocrystalline-Si(Ge) embedded in mesoporous silica
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2009 (English)In: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 53, no 8, p. 862-864Article in journal (Refereed) Published
Abstract [en]

Photoluminescence (PL) properties of mesoporous silica (MS) samples incorporated with Si or Ge nanocrystals (nc) have been investigated with various excitation powers and post-RTA processes. The analysis of experimental results revealed a superlinear intensity dependence (m = 1.7) in the MS reference sample without nanocrystals, while a sublinear behavior (m = 0.8) is observed for the nc-Si in MS. It thus suggests the same recombination responsible for the luminescence at similar to 2.75 eV for both samples, but different kinetic limitations for the carrier transfer processes. Si nanocrystals play in this case an important role in generating more photo-excited carriers, enhancing the PL intensity.

Keywords
Mesoporous silica, Nanocrystals, Photoluminescence, Power dependence, Thermal annealing
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-19884 (URN)10.1016/j.sse.2009.04.014 (DOI)
Note
Original Publication: Bouchaib Adnane, Yi-Fan Lai, Jia-Min Shieh, Per-Olof Holtz and Wei-Xin Ni, Photoluminescence study of nanocrystalline-Si(Ge) embedded in mesoporous silica, 2009, SOLID-STATE ELECTRONICS, (53), 8, 862-864. http://dx.doi.org/10.1016/j.sse.2009.04.014 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/ Available from: 2009-08-14 Created: 2009-08-14 Last updated: 2017-12-13
Zhao, M., Hansson, G. & Ni , W.-X. (2009). Strain relaxation of thin Si0.6Ge0.4 grown with low-temperature buffers by molecular beam epitaxy. JOURNAL OF APPLIED PHYSICS, 105(6), 063502-
Open this publication in new window or tab >>Strain relaxation of thin Si0.6Ge0.4 grown with low-temperature buffers by molecular beam epitaxy
2009 (English)In: JOURNAL OF APPLIED PHYSICS, ISSN 0021-8979 , Vol. 105, no 6, p. 063502-Article in journal (Refereed) Published
Abstract [en]

A double-low-temperature-buffer variable-temperature growth scheme was studied for fabrication of strain-relaxed thin Si0.6Ge0.4 layer on Si(001) by using molecular beam epitaxy (MBE), with particular focuses on the influence of growth temperature of individual low-temperature-buffer layers on the relaxation process and final structural qualities. The low-temperature buffers consisted of a 40 nm Si layer grown at an optimized temperature of similar to 400 degrees C, followed by a 20 nm Si0.6Ge0.4 layer grown at temperatures ranging from 50 to 550 degrees C. A significant relaxation increase together with a surface roughness decrease both by a factor of similar to 2, accompanied with the cross-hatch/cross-hatch-free surface morphology transition, took place for the sample containing a low-temperature Si0.6Ge0.4 layer that was grown at similar to 200 degrees C. This dramatic change was explained by the association with a certain onset stage of the ordered/disordered growth transition during the low-temperature MBE, where the high density of misfit dislocation segments generated near surface cusps largely facilitated the strain relaxation of the top Si0.6Ge0.4 layer.

Keywords
buffer layers, dislocations, Ge-Si alloys, molecular beam epitaxial growth, order-disorder transformations, semiconductor growth, semiconductor materials, semiconductor thin films, surface morphology, surface roughness
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17893 (URN)10.1063/1.3091266 (DOI)
Note
Original Publication: Ming Zhao, Göran Hansson and Wei-Xin Ni, Strain relaxation of thin Si0.6Ge0.4 grown with low-temperature buffers by molecular beam epitaxy, 2009, JOURNAL OF APPLIED PHYSICS, (105), 6, 063502. http://dx.doi.org/10.1063/1.3091266 Copyright: American Institute of Physics http://www.aip.org/ Available from: 2009-05-05 Created: 2009-04-24 Last updated: 2009-05-05Bibliographically approved
Cheng, M., Ni, W.-X., Luo, G., Huang, S., Chang, J. & Lee, C. (2008). Growth and characterization of Ge nanostructures selectively grown on patterned Si. Thin Solid Films, 517(1), 57-61
Open this publication in new window or tab >>Growth and characterization of Ge nanostructures selectively grown on patterned Si
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2008 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 1, p. 57-61Article in journal (Refereed) Published
Abstract [en]

By utilizing different distribution of strain fields around the edges of oxide, which are dominated by a series of sizes of oxide-patterned windows, long-range ordered self-assembly Ge nanostructures, such as nano-rings, nano-disks and nano-dots, were selectively grown by ultra high vacuum chemical vapor deposition (UHV-CVD) on Si (001) substrates. High-resolution double-crystal symmetrical omega/2 theta scans and two-dimensional reciprocal space mapping (2D-RSM) technologies employing the triple axis X-ray diffractometry have been used to evaluate the quality and strain status of as-deposited as well as in-situ annealed Ge nanostructures. Furthermore, we also compare the quality and strain status of Ge epilayers grown on planar unpatterned Si substrates. It was found that the quality of all Ge epitaxial structures is improved after in-situ annealing process and the quality of Ge nano-disk structures is better than that of Ge epilayers; on planar unpatterned Si substrates, because oxide sidewalls are effective dislocation sinks. We also noted that the degree of relaxation for as-deposited Ge epilayers on planar unpatterned Si substrates is less than that for as-deposited Ge nano-disk structures. After in-situ annealing process,all Ge epitaxial structures are almost at full relaxation whatever Ge epitaxial structures grew on patterned or unpatterned Si substrates.

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2008
Keywords
Self-assembly Ge nanostructures; Selectively epitaxy; Oxide-patterned substrate; Planar unpatterned substrate; Chemical vapor deposition; Strain degree; Electron beam lithography
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-59151 (URN)10.1016/j.tsf.2008.08.149 (DOI)000261510700017 ()
Available from: 2010-09-10 Created: 2010-09-09 Last updated: 2017-12-12
Gomes, P. F., Cerdeira, F., Larsson, M., Elfving, A., Hansson, G., Ni, W.-X., . . . García-Cristóbal, A. (2008). Large optical emission blue shift in Ge/Si quantum dots under external biaxial strain. In: ICPS 29th International Conference on the Physics of Semiconductors,2008.
Open this publication in new window or tab >>Large optical emission blue shift in Ge/Si quantum dots under external biaxial strain
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2008 (English)In: ICPS 29th International Conference on the Physics of Semiconductors,2008, 2008Conference paper, Published paper (Refereed)
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-44291 (URN)76182 (Local ID)76182 (Archive number)76182 (OAI)
Available from: 2009-10-10 Created: 2009-10-10
Zhao, M., Karim, A., Hansson, G., Ni, W.-X., Townsend, P., Lynch, S. A. & Paul , D. J. (2008). Molecular beam epitaxy growth of Si/SiGe bound-to-continuum quantum cascade structures for THz emission. Thin Solid Films, 517(1), 34-37
Open this publication in new window or tab >>Molecular beam epitaxy growth of Si/SiGe bound-to-continuum quantum cascade structures for THz emission
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2008 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 1, p. 34-37Article in journal (Refereed) Published
Abstract [en]

A Si/SiGe bound-to-continuum quantum cascade design for THz emission was grown using solid-source molecular beam epitaxy on Si0.8Ge0.2 virtual substrates. The growth parameters were carefully studied and several samples with different boron doping concentrations were grown at optimized conditions. Extensive material characterizations revealed a high crystalline quality of the grown structures with an excellent growth control. Layer undulations resulting from a nonuniform strain field, introduced by high doping concentration, were observed. The device characterizations suggested that a modification on the design was needed in order to enhance the THz emission.

Keywords
Molecular beam epitaxy (MBE), Si/SiGe, Quantum cascade, X-ray diffraction, Transmission electron microscopy (TEM)
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16177 (URN)10.1016/j.tsf.2008.08.091 (DOI)
Available from: 2009-01-09 Created: 2009-01-09 Last updated: 2017-12-14
Chen, P., Chen, C., Wang, H., Tsai, J. & Ni, W.-X. (2008). Synthesis design of artificial magnetic metamaterials using a genetic algorithm. Optics Express, 16(17), 12806-12818
Open this publication in new window or tab >>Synthesis design of artificial magnetic metamaterials using a genetic algorithm
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2008 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 16, no 17, p. 12806-12818Article in journal (Refereed) Published
Abstract [en]

In this article, we present a genetic algorithm (GA) as one branch of artificial intelligence (AI) for the optimization-design of the artificial magnetic metamaterial whose structure is automatically generated by computer through the filling element methodology. A representative design example, metamaterials with permeability of negative unity, is investigated and the optimized structures found by the GA are presented. It is also demonstrated that our approach is effective for the synthesis of functional magnetic and electric metamaterials with optimal structures. This GA-based optimization-design technique shows great versatility and applicability in the design of functional metamaterials. © 2008 Optical Society of America.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-45467 (URN)10.1364/OE.16.012806 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13
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