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Excitonic effects in ZnO
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Zinc Oxide (ZnO) is an extensively researched II-VI wide bandgap semiconductor material. As a promising material for future optoelectronic and spintronic applications, it continues to attract enormous amount of interest. Though over the past decades extensive experimental and theoretical work has been done to deepen the understanding of its fundamental material properties, there are still controversial and unexplored areas. The research work summarized in this thesis was aimed at clarifying and exploring some of these areas, as will be introduced below.

One of attractive properties of ZnO is a very large binding energy of free excitons (FX), which makes excitonic effects of particular importance in this material. The excitons couple with other elementary excitations inside the material such as longitudinal optical (LO) phonons or photons. The former leads to the intense LO phonon-assisted radiative transitions, while the latter causes formation of the exciton-polariton.

The exciton-phonon coupling was suggested to be enhanced in ZnO-based nano- and microstructures. This conclusion was based on the prevalence in these structures at room temperature of LO phonon-assisted FX transitions, which is in contrast with bulk ZnO photoluminescence (PL) where the no-phonon (NP) FX emission dominates. The exact mechanism for this effect, however, was not clear. In paper 1, we have clarified these issues by employing PL and cathodoluminescence (CL) measurements performed for bulk ZnO material and ZnO tetrapods. From spatially resolved CL studies, we have shown that the suppression of the NP FX emission strongly depends on structural morphology of the ZnO tetrapods and becomes most significant within areas with faceted surfaces. The effect is interpreted using a model based on re-absorption due to multiple internal reflections in the vicinity of the FX resonance.

As to the exciton-photon coupling, it usually leads to formation of mixed or coupled states of excitons and photons known as exciton-polaritons. The exciton-polariton formation has been demonstrated to lead to slow-down of light in several semiconductor materials such as CdZnTe, GaN, etc. Due to the strong exciton-photon coupling in ZnO, the polariton formation may also affect light velocity in this medium. To explore this effect, we have performed timeof-flight measurement using pulsed laser light. Our studies that are summarized in paper 2 have shown that the group velocity of light in bulk ZnO could be decreased down to 2044km/s and the magnitude of this decrease depends on light polarization. The main physical mechanism responsible for this effect was singled out as being due to the formation of free exciton-polaritons that propagate coherently via ballistic transport. Based on the experimentally determined spectral dependence of the polariton group velocity, the polariton dispersion was also determined.

Excitonic effects in ZnO could also be utilized to investigate fundamental properties of ZnO. For example, previous magneto-optical studies of donor bound excitons allowed to establish ordering of valence band (VB) states and also provided consistent information on the sign and g-factor of holes from the upper A-valence subband. On the other hand, properties of the higher lying B-VB subband were not fully understood. To clarify this issue, we have performed time-resolved and magneto-PL studies for the so-called I6 B and I7 B excitonic transitions which involved a hole from the B-VB subband as summarized in paper 3. From the magneto-PL measurements, values of effective g-factors for conduction band electrons and B valence band holes were determined as ge =1.91, gh =1.79 and gh =0, respectively.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. , 27 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1534
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-77170Local ID: LIU-TEK-LIC-2012:19ISBN: 978-91-7519-874-3 (print)OAI: oai:DiVA.org:liu-77170DiVA: diva2:525313
Presentation
2012-05-31, Schrödinger E324 , Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2017-03-27Bibliographically approved
List of papers
1. On the origin of suppression of free exciton no-phonon emission in ZnO tetrapods
Open this publication in new window or tab >>On the origin of suppression of free exciton no-phonon emission in ZnO tetrapods
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2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 3, 033108- p.Article in journal (Refereed) Published
Abstract [en]

Temperature dependent photoluminescence and cathodoluminescence (CL) spectroscopies are employed to investigate free exciton (FX) emissions in ZnO tetrapods. The intensity of the no-phonon line is found to be largely suppressed as compared with longitudinal optical phonon assisted transitions, in sharp contrast to bulk ZnO. From spatially resolved CL studies, this suppression is shown to strongly depend on structural morphology of the ZnO tetrapods and becomes most significant within areas with faceted surfaces. A model based on reabsorption due to multiple internal reflections in the vicinity of the FX resonance is suggested to account for the observed effect.

Keyword
cathodoluminescence, II-VI semiconductors, nanostructured materials, phonon-exciton interactions, photoluminescence, wide band gap semiconductors, zinc compounds
National Category
Engineering and Technology Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-53938 (URN)10.1063/1.3292027 (DOI)000273890500049 ()
Note
Original Publication: Shula Chen, Sun Kyun Lee, Weimin Chen, H X Dong, L Sun, Z H Chen and Irina Buyanova, On the origin of suppression of free exciton no-phonon emission in ZnO tetrapods, 2010, APPLIED PHYSICS LETTERS, (96), 3, 033108. http://dx.doi.org/10.1063/1.3292027 Copyright: American Institute of Physics http://www.aip.org/Available from: 2010-02-12 Created: 2010-02-12 Last updated: 2017-12-12Bibliographically approved
2. Slowdown of light due to exciton-polariton propagation in ZnO
Open this publication in new window or tab >>Slowdown of light due to exciton-polariton propagation in ZnO
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 24, 245212- p.Article in journal (Refereed) Published
Abstract [en]

By employing time-of-flight spectroscopy, the group velocity of light propagating through bulk ZnO is demonstrated to dramatically decrease down to 2044 km/s when photon energy approaches the absorption edge of the material. The magnitude of this decrease is found to depend on light polarization. It is concluded that even though the slowdown is observed in the vicinity of donor bound exciton (BX) resonances, the effect is chiefly governed by dispersion of free exciton (FX) polaritons that propagate coherently via ballistic transport. Based on the experimentally determined spectral dependence of the polariton group velocity, the polariton dispersion is accurately determined.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-69847 (URN)10.1103/PhysRevB.83.245212 (DOI)000292254000003 ()
Note

Original Publication: Shula Chen, Weimin Chen and Irina Boyanova, Slowdown of light due to exciton-polariton propagation in ZnO, 2011, Physical Review B. Condensed Matter and Materials Physics, (83), 24, 245212. http://dx.doi.org/10.1103/PhysRevB.83.245212 Copyright: American Physical Society http://www.aps.org/

Available from: 2011-08-10 Created: 2011-08-08 Last updated: 2017-12-08
3. Donor bound excitons involving a hole from the B valence band in ZnO: Time resolved and magneto-photoluminescence studies
Open this publication in new window or tab >>Donor bound excitons involving a hole from the B valence band in ZnO: Time resolved and magneto-photoluminescence studies
2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, no 9, 091909- p.Article in journal (Refereed) Published
Abstract [en]

Time-resolved and magneto-photoluminescence (PL) studies are performed for the so-called I(6)(B) and I(7)(B) excitonic transitions, previously attributed to neutral donor bound excitons involving a hole from the B valence band (VB), D(0)X(B). It is shown that PL decays of these emissions at 2 K are faster than that of their I(6) and I(7) counterparts involving an A VB hole, which is interpreted as being due to energy relaxation of the hole assisted by acoustic phonons. From the magneto-PL measurements, values of effective Lande g factors for conduction electrons and B VB holes are determined as g(e) = 1.91, g(h)(parallel to) = 1.79, and g(h)(perpendicular to) = 0, respectively.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-70746 (URN)10.1063/1.3628332 (DOI)000294489300024 ()
Note

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Available from: 2011-09-16 Created: 2011-09-16 Last updated: 2017-12-08

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