Elsevier

Applied Surface Science

Volume 528, 30 October 2020, 146913
Applied Surface Science

Redistribution of Tb and Eu ions in ZnO films grown on different substrates under thermal annealing and its impact on Tb-Eu energy transfer

https://doi.org/10.1016/j.apsusc.2020.146913Get rights and content

Highlights

  • Annealing at 900 °C of Tb,Eu-ZnO/Si films causes formation of Tb oxides and silicates.

  • RE ion enrichment of n-ZnO/p-Si interface is explained by ion drift in electric field.

  • Annealing at 900 °C of Tb,Eu-ZnO/Al2O3 films results in formation of Tb oxides and ZnAl2O4.

  • Energy transfer from Tb3+ to Eu3+ is observed in Tb2O3 and TbxEuySiO7 phases.

  • A theoretical approach for Tb3+ and Eu3+ energy spectra calculation is developed.

Abstract

In (Tb, Eu) co-doped ZnO films grown on p-Si and Al2O3 substrates, the annealing at 900 °C was found to result in the diffusion and segregation of rare earth (RE) dopants to ZnO grain boundaries and film surface. The segregation of RE dopants to film/substrate interface was observed in the film on p-Si and explained by the drift of positively charged RE ions in the electric field of n-ZnO/p-Si junction. The annealing promoted also formation of additional crystal phases, namely Tb2O3, TbO2, TbxEuySiO7, Zn2SiO4 and ZnAl2O4. In the photoluminescence spectra, it caused the appearance of narrow and broader Eu3+ emission lines. The broader lines emerged only for the film on p-Si substrate and were ascribed to Eu3+ in TbxEuySiO7 inclusions formed at the ZnO/p-Si interface. The narrow lines were attributed to Eu3+ ions in Tb2O3 inclusions located at ZnO grain boundaries and film surface. The energy transfer from Tb3+ to Eu3+ was demonstrated in the annealed films only. A theoretical approach that allowed to describe the RE ion surrounding and to calculate the energy spectra of Tb3+ and Eu3+ in various site positions was developed. The crystal field parameters were also estimated.

Introduction

Zinc oxide is the wide band gap semiconductor which has unique physical properties and is related to the group of transparent conducting oxides [1]. Doping of ZnO modulates its characteristics expanding its scope. For example, doping of ZnO layers with Al increases their electrical conductivity and extends their application prospects in solar cells [2], [3] and thermoelectric devices [4], [5]. Doped ZnO can be also used in photodetectors [6], ligh-emitting diodes [7], gas sensors [8], varistors [9], biosensors [10], etc.

Besides, zinc oxide is considered as a good host for RE elements owing to its wide band gap. In particular, lanthanide-doped ZnO is receiving great attention for potential applications in down and up energy converting layers for the incident photons in photovoltaic structures [11], [12], [13]. Another possible applications are in solid state lighting, display technology and various other areas due to multicolour emissions of rare-earth (RE) ions resulting from their intra-shell electronic transitions [14], [15], [16]. The specific photoluminescence (PL) of RE ions observed under UV and visible optical sources is appropriate for producing white light emitting devices (LED) on the basis of LEDs-excited inorganic phosphors [17]. Electroluminescence of RE ions, which can be obtained under impact excitation and indirect excitation, gives the opportunity to produce thin film electroluminescent devices [18], [19]. The use of RE element could be considered as an irrelevant choice in terms of cost but the RE quantity involved in the present films (a few at.%) is very small.

Among the various RE elements, trivalent terbium (Tb3+) ions are considered as one of the most important sources of green emission [20], [21], while europium (Eu3+) ions can be used for obtaining the red one [22], [23]. Simultaneous doping of solid state matrix by these ions is also used for enhancement of Eu3+ emission because of efficient energy transfer from Tb3+ to Eu3+ [24], [25].

However, to obtain Tb3+ and Eu3+ emissions in ZnO, it is necessary to overcome some issues. One of them is the difficulty to incorporate RE ions in +3 oxidation state into Zn2+ place caused by the large difference in the ionic radius and different charge state. It is known that RE element incorporation affects essentially structural properties of the films resulting in the appearance of mechanical stress and defect formation [26], [27]. These defects are supposed to play an important role in charge balance, excitation energy transfer and formation of RE ion luminescent complexes [28], [29], [30]. In thin films, the latter can depend on the type of substrate [31]. In particular, the intensity of Tb3+ luminescence was found to be higher in the film on Al2O3 and SiO2 substrates compared to that of the film grown on Si [31], [32], [33]. The incorporation of Eu ions into ZnO host also faces an additional problem. Specifically, the Eu element was reported to exist in ZnO in two charge states: Eu2+ and Eu3+ [34]. The former is due to Eu substituting Zn2+ site ions. The latter was supposed to be due to Eu localization at the surface of ZnO grains or in the near surface regions [34], as well as in the interstitial position or in the precipitate phases appearing in some local areas [29].

Post growth thermal annealing is usually performed in order to enhance RE ions luminescence. It can promote RE ion activation due to the formation of emitting clusters or incorporation of RE ions in crystal lattice, strain relaxation and defect annealing. Specifically, the increase of the intensity of RE ion emission in ZnO films was observed after post growth thermal annealing at about 500–700 °C [35], [36], [37]. This was ascribed to activation of RE ions [37] and to the reduction of concentration of the non-radiative recombination centers upon annealing [36]. Another reason of this improved RE emission upon annealing could be the redistribution of RE ions in the matrix. In particular, more homogeneous RE ion distribution was considered as the main reason of the increase of Tb3+ emission intensity in Tb-doped ZnO films annealed at 600 °C [35]. On the other hand, the stimulated by thermal treatment accumulation of RE ions in specific regions of matrix [30] as well as the formation of additional phases [35], [38] can affect the energy transfer between different RE ions and serve as a source of emission [39].

Recently, in Tb and Eu co-doped ZnO films grown on Si and Al2O3 substrates, we revealed the intensive energy transfer from Tb3+ to Eu3+ ions that appeared after conventional thermal treatment of the films at 900 °C [40]. This effect was not observed in the as-deposited films and those annealed at 600 °C. The annealing at 900 °C also produced strain relaxation and decreased defect concentration in ZnO matrix [40]. However, the Eu3+ emission spectra was found to depend on the type of substrate and supposed to originate from some RE enriched phases formed upon annealing. The X-ray diffraction (XRD) patterns of the 900 °C annealed films showed the additional peaks that can be ascribed to Tb2O3 and Zn2SiO4 crystal phases. However, clear identification of the additional phases formed under annealing in the films on different substrates was not done and their relation to Eu3+ emission was not established.

In this work, the redistribution of the components of the film and substrate stimulated by thermal annealing at 900 °C in thin films of Tb and Eu co-doped ZnO grown on Si and Al2O3 substrates was investigated by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS) and luminescence methods with the aim to elucidate the nature of emergent additional phases depending on the type of substrate and their role in the emission process. We hope that the results of our study could be of some interest for the development of ZnO based electroluminescent devices.

Section snippets

Experimental details

ZnO films doped with 3 at% of Tb and less than 1 at% of Eu were grown by bottom-up radio frequency (RF) magnetron sputtering on p-Si (1 0 0) and c-cut Al2O3 substrates at 100 °C under argon plasma. The dopant concentrations were measured from the as-grown films by EDX. More details about the deposition procedure can be found elsewhere [30], [35]. The thicknesses of the films grown on Si (Tb,Eu-ZnO/Si) and Al2O3 (Tb,Eu-ZnO/Al2O3) substrates, were about 840 and 680 nm, respectively. Afterwards,

TEM observations and chemical analysis of Tb, Eu-ZnO/Si films

TEM bright and dark field images of the as-deposited and 900 °C annealed films on Si substrate are shown in Fig. 1. The corresponding SAED patterns are depicted in Fig. 1(e) and (f), respectively.

Both as-deposited and annealed films demonstrate the columnar growth with the columns perpendicular to the Si substrate. The column width varies from 15 up to 50 nm in different regions of the film, but their width is rather constant from the bottom to the top of the film and no change in this

Cluster approximation for calculation of energy spectra of Tb, Eu doped ZnO films

The understanding of the optical properties of RE ions in crystalline surrounding requires the knowledge of the rare-earth ion to nearest-ligands coupling. In the pioneering work of Brecher and Riseberg [58] a simple structural model of the rare-earth environment has been proposed to describe the behavior of the first coordination shell of the Eu3+ ion in glasses. This model predicts crystal-field parameters whose behavior agrees well with the experimentally derived values. Detailed structure

Conclusions

The effect of conventional thermal annealing on impurity redistribution, additional phase formation and luminescence of Tb and Eu co-doped ZnO films grown by magnetron sputtering on p-Si and Al2O3 substrates were investigated by TEM, EDX, ToF-SIMS, XPS and PL methods. The films were annealed in nitrogen flow during 1 h at 900 °C. The annealing of the film on Al2O3 substrate resulted in the formation of 10–20 nm in size inclusions of crystalline Eu-doped Tb2O3 and TbO2 phases and a 60 nm thick

CRediT authorship contribution statement

N. Korsunska: Conceptualization, Methodology, Project administration, Validation, Visualization, Writing - original draft, Writing - review & editing. L. Borkovska: Conceptualization, Methodology, Project administration, Validation, Visualization, Writing - original draft, Writing - review & editing. L. Khomenkova: Validation, Visualization, Writing - review & editing. T. Sabov: Data curation, Investigation, Validation, Writing - review & editing. O. Oberemok: Data curation, Investigation,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

XP and LB are grateful to the French-Ukrainian program “DNIPRO” (project #37884WC). XP acknowledges the “Agence Nationale de la Recherche” (ANR) for the EQUIPEX “GENESIS” grant “ANR-11-EQPX-0020” in the frame of the “Investissements d’avenir”. He wants to thank the “Fonds Européens de Développement Régional” (FEDER) and the Normandie Region. The fundings of all these institutions allowed the purchase and the use of the FIB system for TEM sample preparations. XP also thanks F. Lemarié for the

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