Luminescent Properties and Energy Transfer in Pr3+ Doped and Pr3+-

Article pubs.acs.org/JPCC

Luminescent Properties and Energy Transfer in Pr3+ Doped and Pr3+Yb3+ Co-doped ZnO Thin Films M. Balestrieri,*,† M. Gallart,† M. Ziegler,† P. Bazylewski,§ G. Ferblantier,‡ G. Schmerber,† G. S. Chang,§ P. Gilliot,† D. Muller,‡ A. Slaoui,‡ S. Colis,*,† and A. Dinia† †

Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, B.P. 43, F-67034 Strasbourg Cedex 2, France ‡ Laboratoire ICube, Université de Strasbourg, CNRS UMR 7357, 23 rue du Loess, B.P. 20, F-67037 Strasbourg Cedex 2, France § Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, SK S7N 5E2, Canada S Supporting Information *

ABSTRACT: Embedding luminescent rare earth ions into transparent oxides such as ZnO is a well-known approach to functionalize the material by adding photon-management properties. In this paper we present a detailed study of the luminescence properties and energy level structure of Pr3+ and Yb3+ ions embedded in ZnO thin films deposited by magnetron reactive sputtering. Careful study of the photoluminescence and excitation spectra allowed identifying and locating almost all excited levels of Pr3+ and Yb3+ ions. Thus, an almost complete electronic energy level diagram of these ions in the ZnO crystal lattice can be drawn for the first time. In particular, we show that the crystal field of ZnO strongly modifies the energy level structure of the Pr3+ and Yb3+ ions creating energy mismatches between the transitions useful for cooperative down conversion. Finally, we demonstrate that light emission from Pr3+ ions can be induced both by direct excitation of the ions and indirectly by energy transfer from the ZnO matrix.

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INTRODUCTION In recent years, ZnO has been investigated as a low-cost replacement of GaN in optoelectronic devices and ITO in transparent electronic devices such as solar cell, LCD screens, etc. This II−VI semiconductor combines several advantages such as a large direct band gap of 3.37 eV,1 high exciton binding energy of 60 meV,2 and good conducting properties based on oxygen vacancies. When ZnO is used as a transparent conductive oxide (TCO) for solar cell applications, the addition of luminescent rare earth (RE) ions can further functionalize the film and photon-management properties can be obtained. In particular, down-conversion (DC) could help overcome the thermalization losses related to absorption of energetic photons in low band gap semiconductor solar cells.3,4 The DC process consists of splitting one high-energy photon into two photons with lower energy with a theoretical quantum yield up to 200%. In their trivalent form, almost all RE ions have an incomplete 4f shell with a rich electronic level structure (the available electronic configurations of an atom with n electrons can be labeled as 2S+1LJ in the spectroscopic notation, where S, L, and J are the spin, the momentum, and the total angular momentum of the n electron system around the nucleus). Transitions between these levels are forbidden, unless a perturbation is introduced that breaks the spherical symmetry and creates some wave function mixing. Therefore, the transition © 2014 American Chemical Society

probability strongly depends on the electronic environment of the host matrix. The energies of the excited states of Pr3+ are potentially well suited for DC in silicon solar cells. While Pr-doped crystals have already been widely investigated5−9 and found their main application in lasers, to the authors’ knowledge, only a few reports exist on Pr-doped ZnO.10−12 In particular, no report has been found on Pr-doped ZnO thin films. In this context, the present paper presents a detailed study of the luminescence properties and energy level structure of Pr3+ in 100 nm thick ZnO:Pr thin films deposited by magnetron reactive sputtering on silicon substrates. Although Pr3+ ions could lead alone to useful DC, the radiative transition probability might be weak for one or both transitions. One possible solution is to combine Pr with a second RE having levels that are in resonance with two Pr3+ transitions and potentially lead to DC through a cooperative process. With reference to Dieke’s diagram,13 Yb3+ is one of the best candidates and many research groups devoted much effort to the study of the Yb−Pr couple. Interesting results have been obtained in ceramic glasses and other materials. 14−24 Consequently, co-doping with Pr and Yb also has been tested Received: March 6, 2014 Revised: May 23, 2014 Published: May 28, 2014 13775

dx.doi.org/10.1021/jp502311z | J. Phys. Chem. C 2014, 118, 13775−13780

The Journal of Physical Chemistry C

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Table 1. Lattice Parameter, Crystallites Size, Band Gap Values, and 630 nm Peak Area and Width (FWHM) of Selected ZnO:Pr (0.9%) Samples

a

deposition T

annealing T/gas

c (Å)

15 °C 200 °C 400 °C 15 °C 15 °C

/ / / 500 °C/O2 500 °C/Ar

5.2761(11) 5.2253(6) 5.2065(3) 5.225(3) 5.217(2)

cryst size (nm)

Tauc gap (eV)

peak areaa

± ± ± ± ±

3.349(5) 3.300(5) 3.289(5) 3.277(5) 3.282(5)

1 1.44 1.89 1.76 0.44

18.3 26.8 29.0 18 18

1.3 1.3 1.3 3 3

peak width (meV) 11.6 9.7 8.0 8.8 8.8

± ± ± ± ±

0.3 0.3 0.3 0.3 0.3

The peak area of the sample deposited at 15 °C is taken as reference.

environment and from the excitation by using a set of filters. The spectra presented here have been corrected for the filter and lamp emission, as well as for the CCD response. Element-specific near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was employed to investigate the Pr valence in Pr-doped ZnO and Pr,Yb-co-doped ZnO films. Spectroscopic measurements at Pr M4,5 edge were performed at the Resonant Elastic-Inelastic X-ray Scattering (REIXS) beamline of the Canadian Light Source. The Pr M4,5 NEXAFS spectra were obtained in total electron yield mode at a 45° angle of incidence and were normalized to the incoming photon flux recorded by Au mesh. Measurements of the Pr M4,5 edge were intensity normalized to a constant background at 980 eV.

on ZnO thin films and the results are briefly presented in this work. For a detailed study of the structure and luminescence of Yb-doped ZnO thin films, please refer to our previous work.25,26

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EXPERIMENTAL SECTION Pr-doped and Pr,Yb-co-doped ZnO thin films were deposited on p-type Si(100) and glass substrates by radio frequency (RF) magnetron reactive sputtering using an Orion 3 device from AJA International Co. Doping was carried out by placing several pure RE metal discs on the surface of pure zinc targets (sputtering with substrate-on-top configuration and one target per RE). Oxygen was added in the sputtering chamber to form ZnO. Although Pr can be easily oxidized to its 4+ state,27 the Ar/O2 gas flow ratio could not be decreased below a value of 4 due to plasma stability issues related to the RE discs. The RF power and deposition pressure were optimized to 50 W and 3.4 mTorr, respectively. The substrate-to-target distance was kept constant at about 12 cm. Two different Pr concentrations were tested (0.3% and 0.9% with respect to the Zn content). The codoped sample contained 0.8% Pr and 0.4% Yb. Rutherford backscattering spectroscopy (RBS) measurements indicated that the concentration is uniform along the growth direction before and after annealing (within the limits of the RBS technique). The samples with higher concentration exhibit higher luminescence and will therefore be considered below. Since the deposition temperature is known to be a key parameter for dopant insertion and activation, it was varied between 15 °C (environment of the substrate holder cooled with water) and 400 °C. Postdeposition annealing for 10 min was also performed in a tubular quartz furnace under both argon and oxygen atmospheres up to 500 °C on the sample deposited at 15 °C. The structural properties of the films were analyzed in the 20°−80° 2θ range by means of a Rigaku SmartLab X-ray diffractometer equipped with a monochromatic source (Ge(220)×2) delivering a Cu Kα1 incident beam (45 kV, 200 mA, λ = 0.154056 nm). The film thickness and optical constants were measured with a HORIBA UviselTM Lt M200 FGMS (210−880 nm) spectroscopic ellipsometer. Transmittance was obtained by means of a Perkin-Elmer Lambda 950 spectrophotometer. Photoluminescence (PL) measurements were performed in order to have insight on the film defects and on the energy transfer from ZnO to Pr3+. The excitation was provided both by the 325 nm line of a He−Cd laser and a xenon lamp equipped with a monochromator. The signals were recorded by means of a cooled CCD camera. Low-temperature PL was measured at about 5 K on an Oxford MG11 cryostat. Particular attention was paid to eliminate parasitic luminescence coming from the

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RESULTS AND DISCUSSION The structural and morphological investigations showed that highly oriented and uniform ZnO:Pr layers have been obtained. All films have the expected wurtzite structure with a strong texture along the [002] direction. Neither secondary phase nor Pr oxide peaks have been observed in the detection limit of the X-ray diffraction (XRD) technique. Crystallites size (obtained using Scherrer’s formula) and lattice parameter along the growth direction, calculated on the (002) peak, are reported in Table 1 for a selected set of samples. These data are accompanied by the band gap values obtained from a Tauc plot of ellipsometric data and some quantitative analysis on the main Pr-related PL emission peak. These data will help us interpret the PL emission spectra. Figure 1 presents the UV-to-NIR PL emission spectrum (under laser excitation at 325 nm) of the same set of samples. A small peak related to excitonic recombination is found at 379 ± 1 nm (3.272 ± 0.008 eV). The fringes superimposed with it, sometimes wrongly attributed to the free exciton phonon replicas,25,28 are in fact the Raman replicas of the excitation line, separated by the typical longitudinal-optical (LO) phonon energy in ZnO, i.e., 71.6 meV.29−35 The large number of replicas and their intensity indicate the good crystalline quality of the films. The increase of the excitonic peak with the deposition temperature is consistent with the improvement of crystalline order, as indicated by the c value approaching the theoretical value of 5.2066 Å. The crystallites size values prove that the lattice defects avoided using high deposition temperatures or healed by annealing are probably point defects and have little influence on the X-ray coherence length. Comparison between the excitonic emission intensity and the band gap values of Table 1 indicates clearly that the optical absorption is dominated by excitonic features at high deposition temperatures. This means that the Tauc law is no longer valid to obtain the band gap. However, from a DC point 13776



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mechanisms can be at the origin of the emission. The first mechanism involves electron−hole pairs recombination in the vicinity of Pr3+ ions transferring part of their energy to the 4f electron. Recombination could be induced by the perturbation of the crystal field due to the Pr ion itself or some interstitial oxygen. In fact, it cannot be excluded that some interstitial oxygen surrounds the Pr3+ ions in order to balance the extra charge. In the second mechanism, Pr4+ ions act as acceptors and are temporarily reduced to Pr3+.41 In this case, the levels responsible for the observed emission peaks must be situated in the band gap. However, charge transfer to the fundamental state of Pr3+ is more probable than to an excited state. NEXAFS spectroscopy was employed to investigate the Pr valence in Pr-doped ZnO and Pr,Yb-co-doped ZnO films. The Pr M4,5 spectra are reported in Figure 2. Non-negligible amounts of Pr4+ have been detected and the proportion relative to Pr3+ seems to be the same in all samples. Figure 1. Room temperature emission spectra under He-Cd 325 nm laser excitation of ZnO:Pr (0.9%) films prepared at different deposition temperatures and after annealing. The inset provides a closer picture of the main Pr3+ emission peak, without the contribution of the defect band.

of view, these values provide valuable information about the effective absorption cross section of ZnO. The wide emission band between 500 and 800 nm originates from radiative recombination through deep level traps in ZnO.25,36−40 The increase in the defects band and excitonic peak with the crystalline order is probably due to a reduction of nonradiative recombination paths.25 The consequently increased exciton lifetime indicates higher chances for excitons to recombine or reach radiative defects. The narrow intense peaks superimposed to the wide visible band are attributed to radiative transitions of excited Pr3+ ions. This proves the optical activation of Pr inside the ZnO matrix and the feasibility of the transfer. A closer look on the main Pr3+ emission peak is provided in the inset of Figure 1 and the integrated area of the peak is reported in Table 1. It can be noticed that the area almost doubles from 15 to 400 °C and by annealing at 500 °C in oxygen atmosphere, but barely decreases to half of its value due to annealing in argon atmosphere. The Pr emission changes could be due to different transfer efficiency from ZnO and/or different amount of active Pr3+ ions. However, the interpretation of these changes is not straightforward. The good crystalline quality and the low Prrelated emission efficiency of samples annealed in Ar show that the peak intensity is not strictly related to the crystallites size and the lattice parameter. On the other hand, it is important to note that the peak width always decreases together with the stress, as indicated by the c values. In the coming paragraphs, the attention will be focused on the Pr-related emission and the luminescence of the ZnO host will be neglected. It is important to point out that the position and the relative intensity of the different Pr emission lines are the same before and after annealing and independent of the deposition temperature. Since electronic configuration and transition probability strongly depend on the crystal field, the active site environment must be quite insensitive to the preparation conditions. This means that our results are potentially good for all preparation methods of ZnO:RE. So far, only indirect excitation of Pr ions was considered. Although the emission is that characteristic of Pr3+ ions, it does not mean that Pr is in its 3+ form in ZnO. In fact, two

Figure 2. Pr M4,5 NEXAFS spectra of ZnO:Pr and ZnO:Yb,Pr samples. A spectrum of Pr(III,IV) oxide is added for comparison purposes.

To attempt direct excitation of Pr ions and thus have deeper insight in the transfer mechanisms, the PL spectrum was recorded as a function of the excitation wavelength before and after annealing. The excitation was varied between 350 nm (above the ZnO band gap) and 515 nm (below the expected value of the 1H4 → 3P0 transition of Pr3+, see ref 13). Measurements have been performed at low temperature in order to reduce phonon-related widening of the lines and phonon-assisted nonradiative recombination. Narrow and intense absorption lines were therefore observed, corresponding to direct excitation of Pr3+. The emission mechanism based on Pr3+ ions is then confirmed, although some emission due to the reduction of Pr4+ to Pr3+ cannot be excluded. Figure 3 shows the low-temperature PL excitation spectrum (PLE) of the main emission line (630 nm) before and after annealing. The other lines present exactly the same excitation spectrum and are therefore not reported. Pr3+ excited levels are found at 505, 492, 488, and 465 nm, identified as transitions from the fundamental state to the 3PJ multiplet and 1I6 level. Direct excitation activates the same Pr3+ emission lines observed in Figure 1 with the same relative intensity. This can be understood only by assuming that all 13777



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Figure 4. PL spectra of ZnO:Pr (0.9%) samples under direct and indirect excitation at low T and RT. The spectra are normalized with respect to the intensity of the 630 nm emission line.

Figure 3. Low-temperature excitation spectra of the main emission line of Pr3+ in ZnO before and after annealing. The integrated emission intensity was considered. The sample annealed in O2 is underestimated due to slightly smaller excitation power.

3

H4 level could be very close to the valence band of ZnO, as predicted by Dorenbos and van der Kolk. 42 At high temperatures, the extension of the band tail states might create a competitive recombination path. No emission line could be attributed to the 3P0 → 3H5 transition or to the emission from the 1D2 intermediate level. Two small emission peaks (situated at 1.53 and 1.61 eV and labeled with an asterisk), together with one of the peaks in the 1 G4 multiplet, could not be identified. The peak at 1.53 eV is visible on all samples, but only at RT, while that at 1.61 eV is only visible at low T. The degeneracy of each level is 2J + 1 and can be totally or partially lifted by an electric field (Stark effect) such as the crystal field of the host material. The lift can be total only if J is an integer and the symmetry of the environment very low. It can be noted from Figure 4 that each level is indeed composed of a group of emission lines. Since the 3P0 level is nondegenerate, the number of lines in each group at low T must correspond to the number of the 2J + 1 levels of the destination level for which the degeneracy is removed and for which the radiative transition probability from the 3P0 level is non-negligible. The large Stark splitting of some levels indicates that the crystal field is quite strong. This can be understood if we keep in mind the large ionic radius of the rare earth compared to that of Zn. In fact, doping ZnO with Pr3+ means that the large Pr3+ ions, with an effective ionic radius (Pauling) of 99 pm in octahedral coordination43 such as in Pr2O3, should enter the tetrahedral sites of Zn2+ (60 pm43) in ZnO. Besides the large splitting of the levels, the ionic radius difference could also induce oxidation to the Pr4+ state, which has a radius of 85 pm in octahedral coordination.43 Due to the small distance between the levels in the crystal field of ZnO, radiative transitions from the 3P1,2 and 1I6 levels to lower levels are encompassed by non-radiative phonon-related transitions. For this reason, it is not possible to construct a complete energy diagram from the emission spectrum alone. The information contained in the excitation spectrum of Figure 3 is essential to draw a more complete energy level diagram. Figure 5 presents for the first time the 4f states energy level diagram of Yb3+ and Pr3+ ions in ZnO (see Table S1 in the

3

radiative transitions take place from the P0 level. This conclusion is supported by the fact that direct excitation of this level generates the most intense emission. Room temperature PLE has been attempted and weak absorption from the 3P0 level has been detected at 505 nm. Under indirect excitation, the evolution of the emission intensity with annealing is compatible with the results reported in Figure 1. On the other hand, the emission under direct excitation improves for both annealing atmospheres. This means that the reason behind the weak emission of samples annealed in Ar under indirect excitation must be ascribed to weaker transfer and not to fewer active sites. The fact that an oxygen-rich atmosphere is necessary for high transfer efficiencies suggests that the transfer might be due to some oxygen-related defects in ZnO. The smooth increase of the Pr3+ emission intensity at the ZnO absorption edge might appear in contrast with the steep absorption of a direct semiconductor like ZnO, especially at low T. A possible explanation could be that weak absorption due to band tail states and exciton creation is compensated by higher transfer efficiency to Pr3+ ions compared to excitation of free electrons with high k value. Unfortunately, quantitative estimation of the absorption edge or of the quantum efficiency from PLE data is not possible due to the fact that only radiative recombination is detected. Qualitative information can be obtained by comparison with the transmittance spectrum of ZnO:Pr (0.9%) deposited on glass (not reported here). The fact that no appreciable absorption from Pr3+ was detected in the transmittance spectrum when the absorption of the 100 nm thick ZnO was close to 100% indicates that the transfer efficiency must be very low. In Figure 4, the low-temperature emission spectrum under direct excitation is compared with that under indirect excitation at low T (curve ii) and RT (curve iii) for the sample annealed in O2. The purpose is to tentatively identify the Pr3+ levels at the origin of the emission lines. Transitions from the 3P0 excited level to the 1G4, 3F4, 3F3, 3F2, 3H6 underlying levels and to the fundamental state 1H4 could be easily identified. Contrary to all other emission lines, the 3P0 → 1H4 emission line is only visible at low temperature. This suggests that the 13778



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optically active in the presence of Pr. This hypothesis is ruled out by the fact that Yb emission similar to that previously reported25 is detected following indirect excitation of the host (see Figure S1 in the Supporting Information for more details). If on the one hand the efficiency of phonon-assisted transfer is low, on the other hand multi-phonon relaxation might explain why some transitions between Pr3+ energy levels are suppressed. As a rule of thumb, transitions as large as five times the phonon energy can be bridged by multi-phonon relaxation.44 This can explain why the highest excited levels, 3 P1, 1I6, and 3P2, do not radiatively relax to some lower-lying level state when directly excited, but rather relax to the 3P0 that can further radiatively emit. This can also explain the absence of emission related to the 1G4 transition.

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CONCLUSIONS High-quality, uniformly doped ZnO:Pr and ZnO:Yb,Pr thin films were deposited by magnetron reactive sputtering. Luminescent transitions of Pr3+ ions could be excited directly and indirectly through the ZnO matrix, indicating optical activation of the RE. By combining the information obtained from study of the PL and PLE spectra, the energy level diagram of Pr3+ and Yb3+ ions in the ZnO crystal field was drawn for the first time. For ZnO:Pr films, indirect excitation is expected to be more favorable due to the wide absorption cross section of ZnO and low concentration of Pr. However, the emission intensity under direct excitation is higher, suggesting that the transfer efficiency is low. The weak emission intensity of the 3P0 → 1G4 transition indicates that the DC efficiency of ZnO:Pr3+ films is low. Codoping with Yb, which could potentially lead to cooperative DC, did not provide better results due to the low resonance between the energy levels induced by the crystal field of ZnO.

Figure 5. Energy level diagram of Pr3+ and Yb3+ in ZnO. Only the center of each emission line is plotted, regardless of the line width and intensity. The brackets group the lines attributed to a specific level and the arrows indicate the principal emission line. The energy levels of the ions in LaCl3 are reported for comparison. The thickness of each level represents the total crystal field splitting in LaCl3.

Supporting Information for the numeric values). The simple level diagram of Yb3+ has been obtained by PL measurements at RT. The energy levels of Pr3+ and Yb3+ in LaCl313 are reported for comparison. Due to the half-integer nature of the total spin of Yb3+, each level can be split at most into (2J + 1)/2 sublevels. It can be seen that the degeneracy is fully lifted by the ZnO crystal field. If the luminescent ions occupy a substitutional position, then the energy diagram of Figure 5 is that of Yb3+ and Pr3+ in tetrahedral Zn2+ site. Depending on the type of energy transfer (exchange, radiative, ...), the distance between the ions can be an important parameter. The low rare earth concentration, in particular that of Yb that should be twice that of Pr for good DC properties, might therefore contribute to decrease the DC efficiency. In cooperative transfer, resonance between transitions is also important, although phonon-assisted nonresonant transfer may occur. Figure 5 clearly shows the poor resonance conditions between the 2F7/2 → 2F5/2 transition in Yb3+ and the 3P0 → 1G4, 1G4 → 1H4 transitions in Pr3+. It turns out that in ZnO, 146 meV separate the 2F5/2 → 2F7/2 transition from the 1G4 → 3H4 transition (corresponding to approximately two times the optical phonon energy of 72 meV) and phonon-assisted energy transfer may occur. At the same time, the energy of only one phonon separates the Yb transition from the upper 3P0 → 1G4 transition. However, the probability of these two multi-step processes should be rather low. To test the cooperative down-converting properties of the Yb−Pr couple, direct laser excitation of the 3P0 level of Pr3+ was used on ZnO:Yb,Pr films at room temperature. A tunable laser provided the necessary 505 nm excitation beam. As expected, only the emission of Pr3+ has been detected, which means that either there is no transfer to Yb3+ or the transfer efficiency is very low. An alternative explanation could be that Yb is not

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 with energy level values of Pr3+ and Yb3+ in ZnO and Figure S1 with emission spectrum of ZnO:Pr,Yb thin films under UV laser excitation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: +33 3 88 10 71 29. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Rhin Solar INTERREG project n° C25 and the French Ministry of Education and Research for financial support.

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Luminescent Properties and Energy Transfer in Pr3+ Doped and Pr3+-

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