Polariton effects in the dielectric function of ZnO excitons obtained by ellipsometry

Cobet, Munise; Cobet, Christoph; Wagner, Markus R.; Esser, N.; Thomsen, C.; Hoffmann, A.

doi:10.1063/1.3284656

Cited by 23 publications

(21 citation statements)

References 17 publications

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“…Studying the transmission properties of ZnO, Liang and Yoffe 9 observed sidebands separated about one LO-phonon energy above the main excitonic peak and attributed them to exciton-phonon bound states or so called exciton-phonon complexes (EPC). A recently published highresolution DF of ZnO at low temperature 10 exhibits the sidebands as well, but they where not discussed further. These features are not only observed in ZnO, but have also been found in optical spectra of many partly ionic materials.…”

Section: Introductionmentioning

confidence: 99%

Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

Neumann

Cobet

Esser

et al. 2011

Journal of Applied Physics

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The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg x Zn 1Àx O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x ¼ 0 and x ¼ 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.

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Section: Introductionmentioning

confidence: 99%

Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

Neumann

Cobet

Esser

et al. 2011

Journal of Applied Physics

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“…Here, the energy spacing of 47 meV of the peak at 3.423 eV from the FX ðn¼1Þ A at 3.376 eV matches exactly 3/4 of the exciton binding energy in ZnO. Therefore, the transition at 3.423 eV, also reported in the literature, [14][15][16]19,20 is attributed to the (n ¼ 2) state of the A-exciton. A magnification of the (n ¼ 2) spectrum is shown enlarged on a linear scale in the inset of Fig.…”

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confidence: 53%

Intra-excitonic relaxation dynamics in ZnO

Chernikov

Koch

Laumer

et al. 2011

Applied Physics Letters

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The temperature and carrier-density dependent excitonic relaxation in bulk ZnO is studied by means of time-resolved photoluminescence. A rate-equation model is used to analyze the population dynamics and the transitions between different exciton states. Intra-excitonic (n ¼ 1) to (n ¼ 2) relaxation is clearly identified at low excitation densities and lattice temperatures with a characteristic time constant of 6 6 0.5 ps. V C 2011 American Institute of Physics.[doi:10.1063/1.3668102] Over the last decades, ZnO has attracted much interest in the scientific community mainly due to its potential application for optoelectronic devices in the ultraviolet (UV) spectral regime. In addition, properties associated with the large exciton binding energy of 60 meV and the strong electron-phonon coupling are of fundamental interest. Recently, high quality ZnO samples have become available due to improved growth methods.2-6 These advances allow for accurate studies of the electro-optical properties of the material and are important steps towards the realization of ZnO-based technology. Significant research effort has been directed towards the growth and applications of quantum well structures, 7,8 ternary ZnO-based compounds, [9][10][11][12] electron-phonon interaction, 13 and exciton-polaritons.14 Even steady-state investigations of the excited excitonic states have become possible. 15,16 In this letter, we study the dynamics of the intraexcitonic processes by means of ultrafast emission spectroscopy with sub-picosecond time resolution. We apply a rateequation model to analyze the various relaxation channels and to investigate the influence of temperature and excitation density.All time-resolved photoluminescence (PL) measurements are performed using a streak-camera setup 12 yielding spectral and time resolutions of 0.15 nm and 500 fs, respectively; a pulsed 100 fs-Ti:sapphire laser is used as an excitation source. The investigated sample is a 300 nm thick c-plane ZnO layer grown by plasma-assisted molecular beam epitaxy.11 The emission signal is collected normally to the sample surface in reflection geometry. Structural analysis by high resolution x-ray diffraction was carried out and the lattice parameters were extracted from reciprocal space maps recorded at the asymmetric (20.5) reflex. The c-lattice parameter was determined to (5.2046 6 0.001) and the a-lattice parameter to (3.2515 6 0.001). Comparison to data reported in literature reveals that the present ZnO layer can be considered as unstrained. transition is attributed to the luminescence of the free B-excitons. Also, an additional, distinct emission peak is found at 3.423 eV. In a simplified picture, the excitonic level scheme resembles the energy structure of a hydrogen atom.1 In this case, the energy of an excited states is proportional to E b n 2 , where E b is the binding energy and n is the quantum number of the corresponding state. Thus, the energy of the first excited state with (n ¼ 2) is 1/4 E b and the separation between the (n ¼ 1) and (n ¼ 2) states eq...

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“…On the other hand, the group velocity of that in 200 and 500 nm thin films is estimated 2.9 × 10 7 cm/s and 3.6 × 10 7 cm/s, respectively, from Eq. (2) by using the observed decay time (τ d ) and the reported value of the reflection coefficient (R) of 0.3 [5] and the sample thickness (L). This result demonstrates that the group velocity of the photon-like polariton generated by the exciton-exciton scattering in the ZnO thin film is two orders of magnitude slower than that calculated from the exciton-polariton dispersion in ZnO bulk crystals.…”

Section: -P2mentioning

confidence: 99%

Slow propagation of photon-like polaritons generated by exciton-exciton scattering in ZnO thin films

Ichida

Wakaiki

Kawase

et al. 2013

EPJ Web of Conferences

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Abstract. We report on the first observation of the thickness-dependent photoluminescence-decay time of exciton-exciton scattering in ZnO thin films, which indicates the slower propagation of photon-like polaritons compared to that in bulk by two orders magnitude.The intense emission caused by exciton-exciton scattering, which is called P emission, is an interesting subject from an aspect of ultrafast phenomena leading to stimulated emission. Another attractive aspect of the P emission is that it enables us to access the related photon-like polariton state that can lead to slowdown of light propagation [1]. In the inelastic scattering process of two excitons in the first quantum (n = 1) state, one exciton is scattered into a higher excited state with n ≥ 2, while the other exciton is scattered into a photon-like polariton state whose energy is lower than that of the n = 1 exciton state by the energy difference between the n = 1 and n ≥ 2 states [2]:where Ex(n,k) represents the exciton in n = i exciton state with a wavenumber of k. In the dynamical property of the exciton-exciton scattering, we have to take into account at least two processes on the basis of the exciton-polariton picture. First, in the exciton-exciton scattering process, the two exciton-polaritons are converted into a higher exciton and a photon-like polariton when the exciton density is higher than the threshold density. The rate of this collision process depends on the exciton density. The second process is the transformation of the polaritons into external photons on the crystal surface. The rate of this process is determined by [3] Here, vg is the group velocity of the polaritons and R and L are the reflection coefficient and the film thickness, respectively. From Eq. (2), the lifetime of photon-like polariton is proportional to the group-velocity dispersion of the photon-like polariton branch. In our previous work, we reported that the decay time of the P emission changes with the detection energy and the energy dependence of that could be reproduced directly from the group-velocity dispersion of the photon-like polariton EPJ Web of Conferences

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Polariton effects in the dielectric function of ZnO excitons obtained by ellipsometry

Cited by 23 publications

References 17 publications

Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

Intra-excitonic relaxation dynamics in ZnO

Slow propagation of photon-like polaritons generated by exciton-exciton scattering in ZnO thin films

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