Redistribution of Tb and Eu ions in ZnO films grown on different substrates under thermal annealing and its impact on Tb-Eu energy transfer
Graphical abstract
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
References (68)
- et al.
Rear-emitter silicon heterojunction solar cells with atomic layer deposited ZnO: Al serving as an alternative transparent conducting oxide to In2O3:Sn
Sol. Energy Mater. Sol. Cells
(2019) - et al.
Joint improvement of conductivity and Seebeck coefficient in the ZnO: Al thermoelectric films by tuning the diffusion of Au layer
, Mater. Des.
(2018) - et al.
Structure and thermoelectric properties of Al-doped ZnO films prepared by thermal oxidization under high magnetic field
Superlattices Microstruct.
(2017) - et al.
Temperature dependent selective and sensitive terbium doped ZnO nanostructures
Sens. Actuators, B
(2016) Electrically active interfaces in ZnO varistors
Sol. St. Ionics
(1995)- et al.
Materials for downconversion in solar cells: perspectives and challenges
Sol. Energy Mater. Sol. Cells
(2017) - et al.
Enhanced red upconversion emission of Er3+-doped ZnO by post-annealing
J. Lumin.
(2017) - et al.
Deep level defect correlated emission and Si diffusion in ZnO:Tb3+ thin films prepared by pulsed laser deposition
J. Colloid Interface Sci.
(2016) - et al.
Energy transfer mechanism between terbium and europium ions in zinc oxide and zinc silicates thin films
Thin Solid Films
(2014) - et al.
Comparative study on gas sensing properties of rare earth (Tb, Dy and Er) doped ZnO sensor
J. Phys. Chem. Solids
(2017)
Photoluminescence, conductivity and structural study of terbium doped ZnO films grown on different substrates
Mat. Sci. Semicond. Process.
Annealing effects on photoluminescence of terbium doped zinc oxide films
Thin Solid Films
Structural and optical properties of Yb-doped ZnO films deposited by magnetron reactive sputtering for photon conversion
Sol. Energy Mater. Sol. Cells
Influence of annealing on luminescence and energy transfer in ZnO multilayer structure co-doped with Tb and Eu
Thin Solid Films
Intense ultraviolet emission from Yb doped ZnO thin films on Si after high temperature annealing
J. Alloys Compd.
Investigation on direct-current-driven inorganic electroluminescence in rare-ears and Li co-doped ZnO pellets
Mater. Sci. Eng. B
Transformations in the photoluminescent, electrical and structural properties of Tb3+ and Eu3+ co-doped ZnO films under high-temperature annealing
J. Lumin.
Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn
Appl. Surf. Sci.
Optical and photocatalytic properties of rare earth metal-modified ZnO quantum dots
Appl. Surf. Sci.
Structural and spectroscopic characteristics of terbium hydroxide/oxide nanorods and plates
Ceram. Int.
Influence of annealing atmosphere and temperature on photoluminescence of Tb3+ or Eu3+-activated zinc silicate thin film phosphors via sol–gel method
Chem. Phys. Lett.
Line-shape analysis of optical spectra in metaphosphate glasses doped with erbium ions
Chem. Phys.
Interpretation of europium(III) spectra
Coordination Chem. Rev.
ZnO devices and applications: a review of current status and future prospects
Proc. IEEE
ITO-free silicon heterojunction solar cells with ZnO:Al/SiO2 front electrodes reaching a conversion efficiency of 23%
IEEE J. Photovoltaics
ZnO-based ultraviolet photodetectors
Sensors
Antireflective downconversion ZnO:Er3+, Yb3+ thin film for Si solar cell applications
J. Appl. Phys.
Direct growth of freestanding ZnO tetrapod networks for multifunctional applications in photocatalysis, UV photodetection, and gas sensing
ACS Appl. Mater. Interfaces
Optically pumped lasing of ZnO at room temperature
Appl. Phys. Lett.
Room-temperature ultraviolet nanowire nanolasers
Science
Chapter 8. Electroluminescent Thin Film Phosphors
Cited by (13)
Ho and Nd ion beam modifications of ZnO thin films
2023, Materials Chemistry and PhysicsLine-shape analysis and simulation of Er<sup>3+</sup> photoluminescence spectra in erbium-stabilized nanocrystalline zirconia structures
2023, Journal of LuminescenceCitation Excerpt :It means that at high temperature Er3+ ions are subjected to the same symmetry restrictions formed by similar local environment for the crystal structures with different weights of monoclinic, tetragonal and cubic phases. In order to describe the observed PL spectra and determine actual local symmetry of environment we apply the technique of molecular orbitals in cluster approximation developed and successfully exploited for description of rare-earth ions in various environments [20–22]. The spectra of trivalent Rare-Earth Ions (REIs) in crystalline matrix have peculiar properties.
Influence of post annealing treatments on the luminescence of rare earth ions in ZnO:Tb,Eu/Si heterojunction
2021, Applied Surface ScienceCitation Excerpt :%), Eu (<1 at. %)/alumina films of about 800 nm thick and annealed at 1173 K has been performed [17]. The authors demonstrated the formation of Tb oxide at the grain boundaries between the ZnO columns (columnar growth) within the film and the formation of zinc silicate and rare earth silicate at the bottom of the film in the case of the Si substrate.
Thin film electroluminescent device based on magnetron sputtered Tb doped ZnGa<inf>2</inf>O<inf>4</inf> layers
2020, Journal of LuminescenceCitation Excerpt :Classical photoluminescent (PL) and electroluminescent (EL) powder materials, usually synthesized by chemical procedures, are available in the form of sulphide, oxide, oxi-sulphide, nitride, and selenide compounds [1–9].
- 1
0000-0002-7832-3796.