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Excitonic Effects and Energy Upconversion in Bulk and Nanostructured ZnO
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Zinc Oxide (ZnO), a II-VI wurtzite semiconductor, has been drawing enormous research interest for decades as an electronic material for numerous applications. It has a wide and direct band gap of 3.37eV and a large exciton binding energy of 60 meV that leads to intense free exciton (FX) emission at room temperature. As a result, ZnO is currently considered among the key materials for UV light emitting devices with tailored dimensionality and solid state white lighting. Full exploration of ZnO in various applications requires detailed knowledge of its fundamental and materialrelated properties, which remains incomplete. The research work summarized in this thesis addresses a selection of open issues on optical properties of ZnO based on (but not limited to) detailed time-resolved photoluminescence (PL) and magneto-optical studies of various excitonic transitions as specified below.

Papers 1 and 2 analyze recombination dynamics of FX and donor bound excitons (DX) in bulk and tetrapod ZnO with the aim to evaluate contributions of radiative and nonradiative carrier recombination processes in the total carrier lifetime. We show that changes in relative contributions of these processes in “bulk” and near-surface areas are responsible for bi-exponential exciton decays typically observed in these materials. The radiative FX lifetime is found to be relatively long, i.e. >1 ns at 77 K and >14 ns at room temperature. In the case of DX, the radiative lifetime depends on exciton localization. Radiative recombination is concluded to dominate the exciton dynamics in “bulk regions” of high-quality materials. It leads to appearance of a slow component in the decays of no-phonon (NP) FX and DX lines, which also determines the dynamics of the longitudinal optical (LO) phonon-assisted and two-electronsatellite DX transitions. On the other hand, the fast component of the exciton decays is argued to be a result of surface recombination.

Paper 3 evaluates exciton-phonon coupling in bulk and tetrapod ZnO. It is found that, in contrast to bulk ZnO, the NP FX emission in ZnO tetrapods is weak as compared with the LO phonon assisted transitions. We show that the observed high intensity of the FX-1LO emission does not reflect enhanced exciton-phonon coupling in nanostructured ZnO. Instead, it is a result of stronger suppression of the NP FX emission in faceted regions of the tetrapods as revealed from spatially resolved cathodoluminescence (CL) studies. This is attributed to enhanced re-absorption due to multiple internal reflections, which become especially pronounced in the vicinity of the FX resonance.

Effects of exciton-photon coupling on light propagation through the ZnO media are studied in Papers 4 and 5. By employing the time-of-flight spectroscopy, in Paper 4 we demonstrate that the group velocity of laser pulses propagating through bulk ZnO can be slowed down to as low as 2044 km/s when photon energies approach the optical absorption edge of the material. The magnitude of this decrease can be manipulated by changing light polarization. In Paper 5 we show that the observed slow-down is caused by the formation of free exciton-polaritons and is determined by their dispersion. On the other hand, contributions of DX polaritons become important only in the proximity to their corresponding resonances.

Excitonic effects can also be utilized to investigate fundamental properties and defect formation in ZnO. In Paper 6, we employ DX to study magneto-optical properties of the B valence band (B-VB) states as well as dynamics of inter-VB energy relaxation. We show that PL decays of the emissions involving the B-VB holes are faster than that of their counterparts involving the A-VB holes, which is interpreted as being due to energy relaxation of the holes assisted by acoustic phonons. Values of effective Landé g factors for the B-VB holes are also accurately determined. In paper 7, we uncover the origin of a new class of bound exciton lines detected within the nearband-edge region. Based on their magnetic behavior we show that these lines do not stem from DXs bound to either ionized or neutral donors but instead arise from an exciton bound to an isoelectronic center with a hole-attractive local potential.

In Paper 8, DX emissions are used to monitor energy upconversion in bulk and nanorod ZnO. Based on excitation power dependent PL measurements performed with different energies of excitation photons, the physical processes responsible for the upconversion are assigned to two-photon-absorption (TPA) via virtual states and twostep TPA (TS-TPA) via real states. In the former case the observed threshold energy for the TPA process is larger than half of that for one-photon absorption across the bandgap, which can be explained by the different selection rules between the involved optical transitions. It is also concluded that the TS-TPA process occurs via a defect/impurity with an energy level lying within 1.14-1.56 eV from one of the band edges, likely a zinc vacancy.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 57 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1560
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-102594DOI: 10.3384/diss.diva-102594ISBN: 978-91-7519-464-6 (print)OAI: oai:DiVA.org:liu-102594DiVA: diva2:679518
Public defence
2014-01-31, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-12-16 Created: 2013-12-16 Last updated: 2017-03-27Bibliographically approved
List of papers
1. Long lifetime of free excitons in ZnO tetrapod structures
Open this publication in new window or tab >>Long lifetime of free excitons in ZnO tetrapod structures
Show others...
2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 8, 083104- p.Article in journal (Refereed) Published
Abstract [en]

Time-resolved photoluminescence (PL) is employed to characterize optical quality of ZnO tetrapods. PL decay of free excitons (FE) is concluded to contain two components with time constants of 1 and 14 ns at room temperature. The fast PL decay is attributed to nonradiative recombination whereas the slow decay is suggested to mainly represent FE radiative lifetime, based on correlation between thermally induced increases in the PL linewidth and FE lifetimes. The results underline superior optical quality of the tetrapods as the decay time of the slow PL component is comparable to the longest lifetimes reported to date for ZnO.

Keyword
excitons, II-VI semiconductors, photoluminescence, radiative lifetimes, time resolved spectra, wide band gap semiconductors, zinc compounds
National Category
Engineering and Technology Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-54407 (URN)10.1063/1.3328099 (DOI)000275027200070 ()
Available from: 2010-03-12 Created: 2010-03-12 Last updated: 2017-12-12
2. Dynamics of donor bound excitons in ZnO
Open this publication in new window or tab >>Dynamics of donor bound excitons in ZnO
2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, no 12, 121103- p.Article in journal (Refereed) Published
Abstract [en]

Comprehensive time-resolved photoluminescence measurements are performed on shallow neutral donor bound excitons (D0Xs) in bulk ZnO. It is found that transients of the no-phonon D0X transitions (I6-I9 lines) are largely affected by excitation conditions and change from a bi-exponential decay with characteristic fast (τf) and slow (τs) time constants under above-bandgap excitation to a single exponential one, determined by τs, under two-photon excitation. The slow decay also dominates transients of longitudinal optical phonon-assisted and two-electron-satellite D0X transitions, and is attributed to “bulk” D0X lifetime. The fast component is tentatively suggested to represent effects of surface recombination.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-92614 (URN)10.1063/1.4798531 (DOI)000316967100003 ()
Note

Funding Agencies|Swedish Research Council|621-2010-3971|

Available from: 2013-05-16 Created: 2013-05-14 Last updated: 2017-12-06Bibliographically approved
3. 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
4. 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
5. Long delays of light in ZnO caused by exciton-polariton propagation
Open this publication in new window or tab >>Long delays of light in ZnO caused by exciton-polariton propagation
2012 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 249, no 7, 1307-1311 p.Article in journal (Refereed) Published
Abstract [en]

We study the propagation of exciton-polaritons through bulk ZnO using time-resolved photoluminescence (PL) complemented by time-of-flight measurements of laser pulses. When the photon energy approaches donor bound exciton resonances, substantial time delays in PL light propagation are observed which reach up to 210 ps for a 0.55 mm thick crystal. By comparing results from time-of-flight measurements performed using PL light and laser pulses, the observed delay is shown to be due to the formation of exciton-polaritons and their spectral dispersion. It is also shown that the main contribution to the slow-down effect arises from free exciton-polaritons, whereas bound exciton-polaritons become important only in close vicinity to the corresponding resonances.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2012
Keyword
excitons; polaritons; slow light; time-of-flight measurements; zinc oxide
National Category
Natural Sciences Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-74670 (URN)10.1002/pssb.201147559 (DOI)000305966600001 ()
Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-12-08
6. 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

|

Available from: 2011-09-16 Created: 2011-09-16 Last updated: 2017-12-08
7. Zeeman splitting and dynamics of an isoelectronic bound exciton near the band edge of ZnO
Open this publication in new window or tab >>Zeeman splitting and dynamics of an isoelectronic bound exciton near the band edge of ZnO
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 23Article in journal (Refereed) Published
Abstract [en]

Comprehensive time-resolved photoluminescence and magneto-optical measurements are performed on a bound exciton (BX) line peaking at 3.3621 eV (labeled as I*). Though the energy position of I* lies within the same energy range as that for donor bound exciton (DX) transitions, its behavior in an applied magnetic field is found to be distinctly different from that observed for DXs bound to either ionized or neutral donors. An exciton bound to an isoelectronic center with a hole-attractive local potential is shown to provide a satisfactory model that can account for all experimental results of the I* transition. DOI: 10.1103/PhysRevB.86.235205

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87247 (URN)10.1103/PhysRevB.86.235205 (DOI)000312365200008 ()
Note

Funding Agencies|Swedish Research Council|621-2010-3971|

Available from: 2013-01-14 Created: 2013-01-14 Last updated: 2017-12-06
8. Efficient upconvertion of photoluminescence via two-photon-absorption in bulk and nanorod ZnO
Open this publication in new window or tab >>Efficient upconvertion of photoluminescence via two-photon-absorption in bulk and nanorod ZnO
Show others...
2012 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 108, no 4, 919-924 p.Article in journal (Refereed) Published
Abstract [en]

Efficient upconversion of photoluminescence from donor-bound excitons is revealed in bulk and nanorod ZnO. Based on excitation power-dependent PL measurements performed with different energies of excitation photons, two-photon absorption (TPA) and two-step TPA (TS-TPA) processes are concluded to be responsible for the upconversion. The TS-TPA process is found to occur via a defect/impurity (or defects/impurities) with an energy level (or levels) lying within 1.14–1.56 eV from one of the band edges, without involving photon recycling. One of the possible defect candidates could be VZn. A sharp energy threshold, different from that for the corresponding one-photon absorption, is observed for the TPA process and is explained in terms of selection rules for the involved optical transitions.

Place, publisher, year, edition, pages
Springer, 2012
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-81306 (URN)10.1007/s00340-012-5138-y (DOI)000310537100033 ()
Note

funding agencies|Swedish Research Council|621-2010-3971|

Available from: 2012-09-11 Created: 2012-09-11 Last updated: 2017-12-07

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