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Luminescence Properties of ZnO Nanostructures and Their Implementation as White Light Emitting Diodes (LEDs)
Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, luminescence properties of ZnO nanostructures (nanorods, nanotubes, nanowalls and nanoflowers) are investigated by different approaches for possible future application of these nanostructures as white light emitting diodes. ZnO nanostructures were grown by different growth techniques on different p-type substrates. Still it is a challenge for the researchers to produce a stable and reproducible high quality p-type ZnO and this seriously hinders the progress of ZnO homojunction LEDs. Therefore the excellent properties of ZnO can be utilized by constructing heterojunction with other p-type materials.

The first part of the thesis includes paper I-IV. In this part, the luminescence properties of ZnO nanorods grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) grown on the same substrate were investigated. The effect of the post-growth annealing of ZnO nanorods and nanotubes on the deep level emissions and color rendering properties were also investigated.

In paper I, ZnO nanorods were grown on p-type GaN and 4H-SiC substrates by low temperature aqueous chemical growth (ACG) method. The luminescence properties of the fabricated LEDs were investigated at room temperature by electroluminescence (EL) and photoluminescence (PL) measurements and consistency was found between both the measurements. The LEDs showed very bright emission that was a combination of three emission peaks in the violet-blue, green and orange-red regions in the visible spectrum.

In paper II, different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) were grown on p-GaN and the luminescence properties of these nanostructures based LEDs were comparatively investigated by EL and PL measurements. The nanowalls structures were found to be emitting the highest emission in the visible region, while the nanorods have the highest emissions in the UV region due to its good crystal quality. It was also estimated that the ZnO nanowalls structures have strong white light with the highest color rendering index (CRI) of 95 with correlated color temperature (CCT) of 6518 K.

In paper III, we have investigated the origin of the red emissions in ZnO by using post-growth annealing. The ZnO nanotubes were achieved on p-GaN and then annealed in different ambients (argon, air, oxygen and nitrogen) at 600 oC for 30 min. By comparative investigations of EL spectra of the LEDs it was found that more than one deep level defects are involved in the red emission from ZnO nanotubes/p-GaN LEDs. It was concluded that the red emission in ZnO can be attributed to oxygen interstitials (Oi) and oxygen vacancies (Vo) in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) and 690 nm (1.79 eV) to 750 nm (1.65 eV), respectively.

In paper IV, we have investigated the effect of post-growth annealing on the color rendering properties of ZnO nanorods based LEDs. ZnO nanorods were grown on p-GaN by using ACG method. The as grown nanorods were annealed in nitrogen, oxygen, argon, and air ambients at 600 oC for 30 min. The color rendering indices (CRIs) and correlated color temperatures (CCTs) were estimated from the spectra emitted by the LEDs. It was found that the annealing ambients especially air, oxygen, and nitrogen were found to be very effective. The LEDs based on nanorods annealed in nitrogen ambient, have excellent color rendering properties with CRIs and CCTs of 97 and 2363 K in the forward bias and 98 and 3157 K in the reverse bias.

In the 2nd part of the thesis, the junction temperature of n-ZnO nanorods based LEDs at the built-in potential was modeled and experiments were performed to validate the model. The LEDs were fabricated by ZnO nanorods grown on different p-type substrates (4H-SiC, GaN, and Si) by the ACG method. The model and experimental values of the temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared. It was found that the series resistance has the main contribution in the junction temperature of the fabricated devices.

In the 3rd part of the thesis, the influence of helium (He+) ion irradiation bombardment on luminescence properties of ZnO nanorods based LEDs were investigated. ZnO nanorods were grown by the vapor-liquid-solid (VLS) growth method. The fabricated LEDs were irradiated by using 2 MeV He+ ions with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2. It was observed that the He+ ions irradiation affects the near band edge emissions as well as the deep level emissions in ZnO. A blue shift about 0.0347 eV and 0.082 eV was observed in the PL spectra in the near band emission and green emission, respectively. EL measurements also showed a blue shift of 0.125 eV in the broad green emission after irradiation. He+ ion irradiation affects the color rendering properties and decreases the color rendering indices from 92 to 89.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2011. , 108 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1378
Keyword [en]
Nanotechnology, Zinc oxide, nanostructures, luminescence, LEDs, color rendering
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-69155ISBN: 978-91-7393-139-7OAI: oai:DiVA.org:liu-69155DiVA: diva2:424214
Public defence
2011-08-26, K2, Kåkenhus, Campus Norrköping, Linköpings universitet, Norrköping, 10:15 (English)
Opponent
Supervisors
Available from: 2011-06-17 Created: 2011-06-17 Last updated: 2014-01-15Bibliographically approved
List of papers
1. Fabrication and characterization of high-brightness light emitting diodes based on n-ZnO nanorods grown by a low-temperature chemical method on p-4H-SiC and p-GaN
Open this publication in new window or tab >>Fabrication and characterization of high-brightness light emitting diodes based on n-ZnO nanorods grown by a low-temperature chemical method on p-4H-SiC and p-GaN
Show others...
2010 (English)In: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 25, no 6, 065004- p.Article in journal (Refereed) Published
Abstract [en]

Light emitting diodes (LEDs) based on n-ZnO nanorods (NRs)/p-4H-SiC and n-ZnO (NRs)/p-GaN were fabricated and characterized. For the two LEDs the ZnO NRs were grown using a low temperature (andlt;100 degrees C) aqueous chemical growth (ACG) technique. Both LEDs showed very bright nearly white light electroluminescence (EL) emission. The observed luminescence was a result of the combination of three emission lines composed of violet-blue, green and orange-red peaks observed from the two LEDs. Room temperature photoluminescence (PL) was also measured and consistency with EL was observed. It was found that the green and violet-blue peaks are red-shifted while the orange peak is blue-shifted in the EL measurement. It was also found that due to the effect of the GaN substrate the violet-blue peak in the EL measurement is more red-shifted in n-ZnO (NRs)/p-GaN LEDs as compared to n-ZnO (NRs)/p-4H-SiC LEDs.

Place, publisher, year, edition, pages
Iop Publishing Ltd, 2010
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-57158 (URN)10.1088/0268-1242/25/6/065004 (DOI)000277982700005 ()
Available from: 2010-06-11 Created: 2010-06-11 Last updated: 2014-01-15Bibliographically approved
2. Fabrication and comparative optical characterization of n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN white-light-emitting diodes
Open this publication in new window or tab >>Fabrication and comparative optical characterization of n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN white-light-emitting diodes
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2011 (English)In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 64, no 8, 697-700 p.Article in journal (Refereed) Published
Abstract [en]

White light-emitting diodes (LED) based on ZnO (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN were fabricated and their electrical, optical and electro-optical characteristics were comparatively characterized. All the LED showed rectifying behavior. The nanowalls and nanorods structures have the highest photoluminescence emission intensity in the visible and UV (at 3.29 eV) regions, respectively. The nanowalls have the highest color rendering index, with a value of 95, and the highest electroluminescence intensity with peaks approximately centered at 420, 450 nm and broad peak covering the visible region.

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2011
Keyword
Zinc oxide, Nanostructure, Aqueous chemical growth, Optical properties, Heterojunctions
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-66849 (URN)10.1016/j.scriptamat.2010.11.046 (DOI)000287908300002 ()
Available from: 2011-03-22 Created: 2011-03-21 Last updated: 2014-01-15Bibliographically approved
3. The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes
Open this publication in new window or tab >>The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes
2011 (English)In: NANOSCALE RESEARCH LETTERS, ISSN 1931-7573, Vol. 6, no 1, 130- p.Article in journal (Refereed) Published
Abstract [en]

In this article, the electroluminescence (EL) spectra of zinc oxide (ZnO) nanotubes/p-GaN light emitting diodes (LEDs) annealed in different ambients (argon, air, oxygen, and nitrogen) have been investigated. The ZnO nanotubes by aqueous chemical growth (ACG) technique on p-GaN substrates were obtained. The as-grown ZnO nanotubes were annealed in different ambients at 600 degrees C for 30 min. The EL investigations showed that air, oxygen, and nitrogen annealing ambients have strongly affected the deep level emission bands in ZnO. It was concluded from the EL investigation that more than one deep level defect is involved in the red emission appearing between 620 and 750 nm and that the red emission in ZnO can be attributed to oxygen interstitials (O-i) appearing in the range from 620 nm (1.99 eV) to 690 nm (1.79 eV), and to oxygen vacancies (V-o) appearing in the range from 690 nm (1.79 eV) to 750 nm (1.65 eV). The annealing ambients, especially the nitrogen ambient, were also found to greatly influence the color-rendering properties and increase the CRI of the as - grown LEDs from 87 to 96.

Place, publisher, year, edition, pages
Springer Science Business Media, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-68784 (URN)10.1186/1556-276X-6-130 (DOI)000290525700045 ()
Note
The original publication is available at www.springerlink.com: Naveed Ul Hassan Alvi, Kamran Ul Hasan, Omer Nur and Magnus Willander, The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes, 2011, NANOSCALE RESEARCH LETTERS, (6), 1, 130. http://dx.doi.org/10.1186/1556-276X-6-130 Licensee: Springer Science Business Media http://www.springerlink.com/ Available from: 2011-06-08 Created: 2011-06-07 Last updated: 2014-01-15
4. The effect of the post-growth annealin g on the color rendering properties of n-Zn Onanorods /p-GaN light emitting diodes
Open this publication in new window or tab >>The effect of the post-growth annealin g on the color rendering properties of n-Zn Onanorods /p-GaN light emitting diodes
2011 (English)In: Lighting Research and Technology, ISSN 1477-1535, E-ISSN 1477-0938, Vol. 43, no 3, 331-336 p.Article in journal (Refereed) Published
Abstract [en]

The effect of post-growth annealing on the colour properties of the light emitted by n-ZnO nanorods/p-GaN white LEDs has been investigated. The as-grown ZnO nanorods were annealed in nitrogen, oxygen, argon and air atmospheres at 6008C for 30 minutes. The colour rendering indices and correlated colour temperatures were calculated from the spectra emitted by the LEDs. It was observed that the ambient atmosphere used for annealing is very effective for altering the colour properties of the fabricated LEDs. The LEDs annealed in nitrogen have excellent colour rendering properties with a colour rendering index and a correlated colour temperature of 97 and 2363 K, respectively, in the forward bias and 98 and 3157K in the reverse bias.

Place, publisher, year, edition, pages
Sage, 2011
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-69152 (URN)10.1177/1477153511398025 (DOI)000294134400008 ()
Available from: 2011-06-17 Created: 2011-06-17 Last updated: 2014-01-15Bibliographically approved
5. Junction temperature in n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction light emitting diodes
Open this publication in new window or tab >>Junction temperature in n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction light emitting diodes
Show others...
2010 (English)In: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 54, no 5, 536-540 p.Article in journal (Refereed) Published
Abstract [en]

The junction temperature of n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction light emitting diodes (LEDs) at built-in potential was modeled and experiments were performed at various temperatures (15-65 degrees C) to validate the model. As the LEDs operate near the built-in potential thats why it is interesting to investigate the temperature coefficient of forward voltage near the built-in potential (similar to V-o). The model and experimental values of the temperature coefficient of forward voltage near the built-in potential (similar to V-o) were compared. We measured the experimental temperature coefficient of the series resistance. By including the temperature coefficient of the series resistance in the model, the theoretical and experimental values become very close to each other. It was found that the series resistance has the main contribution in the junction temperature of our devices. We also measured the junction temperature above the built-in potential and found that the model deviates at higher forward voltage. From this observation we concluded that the model is applicable for low power devices, operated near the built-in potential.

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2010
Keyword
Junction temperature, ZnO nanorods, Heterojunction LEDs
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-55507 (URN)10.1016/j.sse.2010.01.020 (DOI)000276754100008 ()
Available from: 2010-04-30 Created: 2010-04-30 Last updated: 2014-01-15Bibliographically approved
6. Influence of helium-ion bombardment on the optical properties of ZnO nanorods/p-GaN light emitting diodes
Open this publication in new window or tab >>Influence of helium-ion bombardment on the optical properties of ZnO nanorods/p-GaN light emitting diodes
2011 (English)In: Nanoscale Research Letters, ISSN 1931-7573, Vol. 6, no 628Article in journal (Refereed) Published
Abstract [en]

Light emitting diodes (LEDs) based on zinc oxide (ZnO) nanorods grown by vapor-liquid-solid (VLS) catalytic growth method were irradiated with 2 MeV helium (He+) ions. The fabricated LEDs were irradiated with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2. Scanning electron microscopy (SEM) images showed that the morphology of the irradiated samples is not changed. The as-grown and He+ irradiated LEDs showed rectifying behaviour with the same I-V characteristics. Photoluminescence (PL) measurements showed that there is a blue shift of approximately 0.0347 eV and 0.082 eV in the near band emission (free exciton) and green emission of the irradiated ZnO nanorods, respectively. It was also observed that the PL intensity of the near band emission was decreased after irradiation of the samples. The electroluminescence (EL) measurements of the fabricated LEDs showed that there is a blue shift of 0.125 eV in the broad green emission after irradiation and the EL intensity of violet emission approximately centred at 398 nm was nearly disappeared after irradiations. The color rendering properties shows a small decrease in the color rendering indices of 3% after 2 MeV He+ ions irradiation.

Place, publisher, year, edition, pages
SpringerOpen, 2011
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-69154 (URN)10.1186/1556-276X-6-628 (DOI)000300300100001 ()
Available from: 2011-06-17 Created: 2011-06-17 Last updated: 2014-01-15Bibliographically approved

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