| 1. |
- Chen, Shula, et al.
(författare)
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On the origin of suppression of free exciton no-phonon emission in ZnO tetrapods
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 96:3, s. 033108-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Lee, Sun-Kyun, et al.
(författare)
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Long lifetime of free excitons in ZnO tetrapod structures
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 96:8, s. 083104-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
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| 4. |
- Chen, Shula, et al.
(författare)
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Donor bound excitons involving a hole from the B valence band in ZnO: Time resolved and magneto-photoluminescence studies
- 2011
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 99:9, s. 091909-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
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| 6. |
- Chen, Shula, et al.
(författare)
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Dynamics of donor bound excitons in ZnO
- 2013
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 102:12, s. 121103-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
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| 8. |
- Chen, Shula, et al.
(författare)
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Efficient upconvertion of photoluminescence via two-photon-absorption in bulk and nanorod ZnO
- 2012
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Ingår i: Applied physics. B, Lasers and optics (Print). - : Springer. - 0946-2171 .- 1432-0649. ; 108:4, s. 919-924
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Chen, Shula
(författare)
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Excitonic Effects and Energy Upconversion in Bulk and Nanostructured ZnO
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 10. |
- Chen, Shula
(författare)
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Excitonic effects in ZnO
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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