Resolving decay-time dependent photoluminescence induced by phonon-dressed excitons in ZnO

Ryu Yukawa, Susumu Yamamoto, Ren Arita, Yuki Minami, Kohei Yamanoi, Kenichi Ozawa, Kazuyuki Sakamoto, Toshihiko Shimizu, Nobuhiko Sarukura, and Iwao Matsuda

Phys. Rev. Materials 6, 104607 – Published 21 October 2022

Abstract

Time-resolved photoluminescence (PL) experiments on ZnO crystals have been conducted at room temperature to elucidate the origins of a near-band edge (NBE) emission. A temporal profile of the PL spectra exhibits a two-curve structure with different decaying rates and is reproduced reasonably by a biexponential function, indicating that there are at least two origins with different decay times in the NBE emission. By taking free exciton (FX)–longitudinal optical (LO) phonon interactions into account in the analysis of the PL spectra, the faster and slower decaying emissions are found to be associated with stronger and weaker FX-LO phonon coupling, respectively. We propose that the NBE emissions with different decay times are related to crystal imperfections; the fast-decaying emission originates from the region with a higher density of defects or impurities, while the slower one is from the regions with better crystallinity.

Received 2 September 2021
Revised 2 August 2022
Accepted 19 September 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.104607

Physics Subject Headings (PhySH)

Excitons Optical phonons

Oxides Wide band gap systems

Photoluminescence Time-resolved photoluminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ryu Yukawa ^1,2,*, Susumu Yamamoto ^2,3,4, Ren Arita⁵, Yuki Minami⁵, Kohei Yamanoi ⁵, Kenichi Ozawa ^6,7, Kazuyuki Sakamoto ^1,8,9, Toshihiko Shimizu⁵, Nobuhiko Sarukura⁵, and Iwao Matsuda ^2,10

¹Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan
²Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
³International Center for Synchrotron Radiation Innovation Smart (SRIS), Tohoku University, Sendai, Miyagi 980-8577, Japan
⁴Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai, Miyagi 980-8577, Japan
⁵Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871, Japan
⁶Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
⁷SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki 305-0801, Japan
⁸Center for Spintronics Research Network, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
⁹Spintronics Research Network Division, Institute for Open and Transdisciplinary Research Initiatives, Osaka University, Suita, Osaka 565-0871, Japan
¹⁰Trans-scale Quantum Science Institute, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan

^*ryukawa@ap.eng.osaka-u.ac.jp

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Issue

Vol. 6, Iss. 10 — October 2022

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Images

Figure 1
(a) Schematic image for the NBE emission (red arrow) in ZnO and examples of impurity-induced recombinations that do not involve the NBE emission (black arrows). The top blue area and the bottom orange area represent the conduction band and valence band, respectively. In the direct recombination processes, an electron in the FX level directly drops to the valence band and recombines with a hole while emitting an NBE light (energy of about 3.3 eV). In the impurity-induced recombination processes, an electron in the FX level or conduction band drops to the valence band via the impurity and/or defect states (shaded in gray) and finally recombines with a hole. During the relaxation processes, energies are transferred to heats and/or lights with smaller energy ( $\sim 2.4 eV$ or lower) than the NBE emission energy. In a doped crystal, a decay time of NBE PL, i.e. a lifetime of excited electrons in the FX level, depends on the density of the impurity-induced recombination channels. (b),(c) Time-integrated (b) and energy-integrated (c) PL intensity profiles of the NBE emissions taken for the high-doped (blue curves) and low-doped (orange curves) ZnO crystals. The energy-integrated PL intensity profiles are also displayed logarithmically in the inset of (c). The same laser intensity of $1.7 mJ / c m^{2} / pulse$ with the wavelength of 290 nm was used for the excitation of electrons in both the ZnO crystals. The decay profiles for the high-doped and low-doped ZnO are fitted by single-exponential functions (dashed curves) with lifetimes of 50 ps and 290 ps, respectively. The photograph taken for the high-doped (upper) and low-doped (lower) ZnO crystals used for the PL experiments and Hall measurements is shown on the right side of (b). Here, the samples are placed on pulp paper, and the four metallic points seen on each sample have been used for the Hall measurements.
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Figure 2
(a) Time- and energy-resolved PL intensity map taken for the high-doped ZnO which was irradiated with a laser of the intensity of $1.7 mJ / c m^{2} / pulse$ . Its temporal profiles extracted in the energy range $3.04 – 3.36 eV$ with the interval of 0.04 eV are logarithmically displayed in (b). Here, in order to make the comparison of the spectral shapes clearer, the temporal profiles are displayed with intensity offsets. The green lines shown on the temporal profile at 3.36 eV are the guide for the eye to make a bending feature clearer. The curve fitting results are overlaid with the black curves. (c) Time-integrated PL intensity profiles of the fast and slow decays [ $I_{f} (E) τ_{f} (E)$ and $I_{s} (E) τ_{s} (E)$ ].
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Figure 3
(a) Fitting results of intensity profiles of $I_{f} (E)$ and $I_{s} (E)$ . Here, in order to compare the spectral shape, $I_{f} (E)$ and $I_{s} (E)$ are normalized to their peak heights. (b) Schematic image of the NBE emission processes that accompany LO phonon excitations and reabsorption processes held inside the crystal. In the recombination processes of electrons in the FX level with holes in the valence band, photons are emitted while exciting some LO phonons. Photons with zero LO phonon excitation are strongly reabsorbed inside the crystal before they escape to the surface. Here, in order to focus on the NBE emissions processes, other recombinations and reabsorption processes that could be induced by impurity/defect states are not illustrated in this image.
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