On the structure and ultraviolet emission of terbium doped zinc oxide thin films on silicon after high temperature treatment

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Results in Physics 32 (2022) 105121

Available online 13 December 2021

On the structure and ultraviolet emission of terbium doped zinc oxide thin films on silicon after high temperature treatment

L. Zhang

^a

, C.L. Heng

^a^,^*

, C.N. Zhao

^a

, W.Y. Su

^a

, Y.K. Gao

^b

, P.G. Yin

^b

, T.G. Finstad

^c^,^*

aSchool of Physics, Beijing Institute of Technology, Beijing 100081, PR China

bKey Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, PR China

cPhysics Department, University of Oslo, PO Box 1048, Blindern, N-0316 Oslo, Norway

A R T I C L E I N F O Keywords:

ZnO Thin films UV emission Tb doping TiN nanoparticles

A B S T R A C T

Terbium (Tb) doped zinc oxide (ZnO) thin films were deposited on Si substrates by magnetron sputtering in an oxygen containing plasma. The Tb doping concentration was varied from 0 to 0.38 at.%. The effect of heat treatment at high temperature on the ultraviolet (UV) emission was studied along with the structure of the films.

Photoluminescence (PL) spectra show that the UV intensity increases much by heat treatment and after 1000 ^◦C annealing the doped films have stronger UV emission than that of an un-doped film. The enhanced UV emission is related to crystallinity improvements of the films. After heat treatment at 1000 ^◦C Zn (Tb) silicates had formed.

The film prepared with a Tb concentration of 0.029 at.% showed the best crystallinity and highest UV intensity.

Finally, it was shown that the UV PL intensity can be increased further with TiN nanoparticle capping on the surface of the films.

Introduction

Strong and efficient ultraviolet (UV) light emission from ZnO has attracted considerable interest for a wide variety of applications in photonics, non-line-of-sight communication, biosensing, and so on [1–7]. ZnO has a bandgap of 3.37 eV and a high exciton binding energy allowing near band-edge (NBE) emission in the UV by exciton recombination at room temperature. The most basic strategy to enhance the UV emission of ZnO is to minimize competing recombination routes which include recombination via defect levels in the bulk and on the surface [8]. Various techniques to enhance the UV emissions have been reported, such as using buffer layers [9,10], surface decoration [11], controlling the post-treatment ambient [12], utilizing quantum well structures to acquire UV lasing [13], using surface plasmon resonance and dielectric microspheres [14–21] and so on. The enhancements themselves range from a few percent to six orders of magnitude, and the suggested mechanism range from subtle point defect modifications to drastic nanostructure modifications of several systems involved with ZnO. It has also been reported that rare earth (RE) doping of ZnO can yield enhanced NBE photoluminescence (PL) [22–28], while the mechanisms are generally not unambiguously identified.

Previously, we reported that RE doped ZnO films on Si showed a very

large enhancement of NBE PL after high temperature (1100 ^◦C) annealing, compared to un-doped ZnO [24–26]. These films were sputter deposited under conditions giving material that is rich in oxygen, which means low concentration of oxygen vacancies. Later, we investigated the NBE PL of ZnO films doped by Eu [27] or Yb [28] with various concentrations, and found that the dependence of UV PL on the RE concentration was quite different for Yb and Eu.

We presently report on Tb doped ZnO (ZnO:Tb) films. There have been many reports on Tb doping of ZnO inspired by the wide use of Tb³⁺ as a green phosphor. Thus, transmittance of excitation energy from ZnO to Tb³⁺ and structure have been addressed. [29–32]. However, the detailed effects of Tb doping concentration on the UV PL from ZnO have not been investigated. So, in this work, we have studied whether Tb doping has similar effects on the UV PL as those observed for Eu or Yb doping in sputtered ZnO films on Si subjected to high temperature treatment. This work addresses the relationship between the structure and UV PL enhancement of ZnO by the Tb doping to reveal possible PL enhancement mechanisms. We have additionally examined titanium nitride, TiN, as a plasmonic capping material [33], to further increase the NBE PL of ZnO. The work has importance for potential applications of ZnO-based UV photonics.

* Corresponding authors.

E-mail addresses: [email protected] (C.L. Heng), [email protected] (T.G. Finstad).

Contents lists available at ScienceDirect

Results in Physics

journal homepage: www.elsevier.com/locate/rinp

https://doi.org/10.1016/j.rinp.2021.105121

Received 24 September 2021; Received in revised form 16 November 2021; Accepted 11 December 2021

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Experimental details

The Tb-doped ZnO films were made by similar techniques and identical deposition conditions found in our previous work [27,28] on RE-doped films. In the present case the composite target was made by hot pressing ZnO and Tb4O7 powder and the second target was pure ZnO (purity N4.5). Deposition with different Tb doping concentration were made by controlling the power of the two targets. Eight depositions giving films with different Tb concentrations were made. The resulting sample films were named as S0, S1, S2, S3, S4, S5, S6 and S7. The film named S0, having a [Tb] =0, was made by sputtering from the ZnO target only.. After deposition the Si substrates with films were cleaved into small sample pieces (~1 cm²), and heat treated for 30 min in a high temperature furnace in a flow of nitrogen gas of purity N5.0.

After treatment at 1000 ^◦C, some of the samples were capped with a layer of TiN nanoparticles to explore the plasmonic effects. Here, the used TiN nanoparticles had an average size of either 20 ±10 nm or 40 ± 10 nm for two different capping layers. The preparation procedure started by filling two beakers with 150 mL anhydrous ethanol in each.

Then, five grams of TiN nanoparticles were dissolved into the anhydrous ethanol solution. After ultrasonic vibration for one hour to dissolve the nanoparticles, the solutions were left standing for four hours. Next, 5 μL supernatants with the TiN (~20 nm) and TiN (~40 nm) were extracted respectively using a pipettor, and dripped onto the surface of the S0-S7 samples after annealing at 1000 ^◦C for 30 min and then air dried natu- rally. As such, two batches of the samples capped by TiN (20 nm) and TiN (40 nm) nanoparticles were prepared.

The composition of the film samples was measured by Backscattering spectrometry (BS) using 4.5 MeV ⁴He particles from the HVEC EN tan- dem accelerator of the Peking University and the data analyzed by comparison with theory using the simulation software SIMNRA v7.02 together with SimCalc for handling non-Rutherford nuclear scattering cross sections. The samples were analyzed by X-ray diffraction (XRD) using a D8 Bruker Discover diffractometer with Cu Kα1 radiation, in Bragg-Brentano configuration (θ-2θ) to monitor the structural evolution of the films for each Tb doping concentration after each annealing step.

The full width at half maximum (FWHM) of diffraction peaks was used as a measure of the crystal quality, which in this work is termed crystallinity and is quantified by the equivalent minimum crystallite size using the Scherrer formula [34] for easy comparisons. It is well known that the FWHM has contributions from coherence length limitations as well as micro-strain [35]. Both are related to a reduction of the crystal perfection. Scanning electron microscopy (SEM, JSM-7500F) was used to study the surface morphology of the films. The crystal quality of the films was further examined by high-resolution transmission electron microscopy (HRTEM) with a FEI TECNAI F30 microscope. The valence state of the elements in the annealed films was examined by X-ray photoelectron spectroscopy (XPS) by using a Thermo ESCALAB 250XI system with monochromatic Al Kα X-ray radiation (150 W). PL from the samples was excited by a 30 mW He-Cd laser (325 nm) and analyzed and collected by a confocal Raman spectrometer (LabRAM HR800). The PL was further studied by excitation spectroscopy using an F-380 fluorescence spectrophotometer and time resolved PL was recorded with a FLS 920 fluorescence spectrophotometer using a Xe lamp as excitation source. All the measurements were done at room temperature (RT).

Results and discussion BS characterization

The thickness and composition of the as-deposited (A.D.) ZnO:Tb films were measured by BS and the results are shown in Table 1.

Different from our previous work [28], the energy of the alpha particles was chosen as high as 4.5 MeV in order to have a good separation between the Zn signal and the Si signal so that inter-diffusion is detectable for these relatively thick films (~1 µm). This high energy also results in

non-Rutherford cross sections for both Si and oxygen. The results in Table 1 reveal that all the A.D. samples are oxygen-rich, as expected from the deposition conditions. The calculated deposition rate of the films is in a range of 2.3–2.7 nm/min. Fig. 1(a) shows the BS spectra of the S6 film A.D. and after heat treatment at 700 ^◦C and 1000 ^◦C respectively. The A.D. sample has a spectrum that is consistent with a uniform Zn element distribution with depth as expected. For an increase in temperature to 700 ^◦C and then to 1000 ^◦C, the spectra indicate inter- diffusion between the film and the Si substrate (actually formation of silicates as we will show later). That can be seen from the rounding of the low energy side of the Zn signal (ch.320–355) and the high energy side of the Si signal (ch.200–235). The other clear difference between the spectra of the annealed samples and the A.D is the decrease of counts from Tb atoms in most of the film. The Tb concentration decrease from about 0.14 at.% for the A.D film to 0.02 at.% for the film heat treated at 1000 ^◦C. This Tb part of the spectra is shown in Fig. 1(b). Unfortunately, the yield from the Tb distribution closest to the substrate overlaps the yield from Zn and thus vanishes in the counting statistical variation. The Table 1

The composition and thickness of the as-deposited films.

Sample Zn (at.%) O (at. %) Tb (at.%) Thickness (nm)

S0 48 52 0 1020

S1 48 52 0.15 1160

S2 48 52 0.014 1190

S3 46 54 0.074 1120

S4 47 53 0.018 1300

S5 47 53 0.029 1040

S6 47 53 0.14 1000

S7 47 53 0.38 1110

Fig. 1.(a) The BS spectra of a Tb doped ZnO sample, S6 A.D. and after heat treatment at 700 ^◦C and 1000 ^◦C; and (b) The Tb part of the spectra and their simulation . The arrows labelled with element symbols indicate the energy positions from the elements at the surface.

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Tb atoms in the film diffuse and enters into silicates forming close to the Si substrate. Thus, there is a net transport from the bulk of the film. The formation of silicates is revealed by the XRD and TEM analysis presented in Section 3.3 and 3.5 respectively.

PL study

Light emission properties of the ZnO:Tb films have been studied as function of annealing temperature and Tb doping concentration. Fig. 2 (a) shows the PL spectra of the S5 samples for increasing temperature from A.D. to 1000 ^◦C. The spectral contents are typical for ZnO where the UV peak is from NBE radiative recombination of ZnO and the broad visible band has contributions from various defects. The intensity of the spectra appears modulated by a pronounced wavy structure typical for interference effects [36]. When it comes to emission from Tb, it is known that Tb³⁺ions are optically active and have sharp stable emission in the visible region centered at 487, 546, 588, and 620 nm, which are from

5D₄ㄧ⁷F_j(j =6, 5, 4, 3) intra-4f transitions of Tb³⁺[29]. These sharp lines cannot be detected here. This is related to that we have chosen to

use oxygen-rich ZnO films to maximize the UV PL. This minimizes the concentration of defects related to oxygen-vacancies. These defects have been considered to facilitate excitation energy transfer from ZnO to RE ions [37,38] and in particular to Tb [39]. Our focus is on the UV emission, and it is maximized by reducing other competing recombination paths. The NBE PL intensity increases much with increasing temperature from A.D. to 800 ^◦C and becomes the dominant emission after heat treatment at 900 ^◦C, while the contribution from the visible bands decreases. After heat treatment at 1000 ^◦C, the PL peak intensity is about eight times stronger than that after heat treatment at 900 ^◦C. However, the PL intensity was completely quenched after heat treatment at 1100 ^◦C (not shown). The inset in Fig. 2(a) depicts the integrated NBE PL intensity (INBE) and the ratio of the INBE to the visible PL intensity (IVIS) as a function of heat treatment temperature. It is noticed that the NBE PL intensity and the ratio have nearly the same variation with increasing temperature. In general, the INBE /IVIS ratio is regarded as an indication of the film quality. So, NBE PL enhancement and the film quality are correlated, and the strong NBE PL enhancement after high temperature treatment can be due to the improvement of the film quality.

Fig. 2(b) shows the UV PL spectra of samples S0-S7 after an identical heat treatment at 1000 ^◦C. The UV peaks have different skewness and the peak position varies between 386 nm and 393 nm, which are associated with the transitions of different excitons [40] within the ZnO. The different transitions often involve the states of excitons bound to defects.

Their contributions vary for samples with different overall doping concentration. Here the NBE band shapes are also modulated by the interference effect. The samples with different Tb doping concentration have different ZnO crystallite size and thus the surface to volume ratio and the importance of interface states. The size variation of ZnO crystallites for different Tb concentrations will be revealed by XRD and SEM observation in Section 3.3 and 3.5. The detailed shape of the NBE UV peak can thus effectively vary with Tb doping, as seen in Fig. 2(b). Note that the PL peak intensity of the film with nominal Tb concentration of 0.029 at.

% is about 6.8 times higher than that of the un-doped one. The inset of Fig. 2(b) shows the integrated NBE PL intensity and the ratio of IUV/IVIS

as a function of Tb concentration. Both the PL intensity and the IUV/IVIS

ratio increase for increasing Tb concentration from 0 to about 0.029 at.

%, and then decreases roughly with increasing concentration up to 0.378 at.%. These observations indicate again that the NBE PL is closely related to the crystalline quality of the ZnO films. Compared to the cases of Eu- and Yb-doped ZnO [27,28], the dependence of NBE PL intensity on doping concentration is similar to the case of Eu doping, while different from that of Yb doping. We attribute the difference to the different structural variation by the RE doping and subsequent high temperature treatment, which influences the quality of the ZnO films.

The dependence of the crystal quality upon annealing temperature for the different Tb concentrations will be discussed further in Sections 3.3–3.6, and correlated with the NBE PL results.

XRD study

Structural evolution of the ZnO:Tb films with heat treatment temperature has been investigated by XRD. Fig. 3(a) shows the diffraction patterns of the S5 samples after treatment at the annotated temperatures. The A.D. film gave a dominant peak at 2θ =33.85^◦, which cor- responds to (002) ZnO, showing the preferential orientation of the film.

Small peaks are also present at 2 θ =30.49^◦and 47.51^◦, which are from the (100) and (102) diffraction respectively of ZnO (JCPDS PDF No. 30- 1438 and 21-1486). The peaks with peak positions at 2 θ =51.50^◦, 54.33^◦ and 56.10^◦ are likely due to the allowed 400 reflections from weak Lβ1, Lβ2 and Lγ1 X-rays originating from tungsten (W), which contaminates the Cu anode of X-ray sources after long time use [41].

After heat treatments in the range 600–700 ^◦C, the (002) peak position has shifted to a larger angle and is at 34.27^◦after heat treatment at 700 ^◦C (bulk ZnO has the (002) peak position at 34.42^◦). Similar shifts with temperature were reported by Ziani et al. [29], and were attributed Fig. 2.(a) The PL spectra of S5 film A.D. and after heat treatment at the an-

notated temperatures. The inset shows, as a function of the treatment temperature, the ratio between the integrated intensities of the UV and VIS regions, IUV

and IVIS respectively. Here, IUV is integrated between 350 and 430 nm, while for IVIS the range is in 430 to 700 nm. (b) The NBE UV PL spectra of the S0-S7 samples after heat treatment at 1000 ^◦C in N2 for 30 min. The inset shows the NBE PL peak intensity and the ratio of INBE/IVIS of the spectra for S0-S8 samples after identical heat treatment at 1000 ^◦C.

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to a fraction of the Tb³⁺ions precipitating from the ZnO crystal lattice by annealing [42]. Here we observed by BS that the Tb concentration in the film is decreased by annealing. The origin of the sharp peak at 32.7^◦in the case of 700 ^◦C annealing is not very clear. After the 800 ^◦C and 900 ^◦C heat treatment the peaks at 2 θ =30.83^◦and 37.45^◦can be seen in the pattern, which is likely due to the formation of Zn silicates (such as ZnSiO3, JCPDS PDF No. 27–1495 and/or Zn2SiO4, PDF No. 19-1479) [43–45]. After heat treatment at 1000 ^◦C, the ZnO (002) peak together with the peak at 2 θ =30.83^◦become the most pronounced, which infers the film crystallinity has improved further. Besides indi- cating the formation of Zn silicates, the 30.83^◦and 37.45^◦peaks can also infer the formation of Tb2SiO5 silicate (JCPDS PDF No. 52-1806 and 31- 1380) [46]. Notice that the (002) peak decreases in intensity after heat treatment at 1100 ^◦C, and more silicates (such as Tb2Si2O7) have formed besides the Zn2SiO4 and Tb2SiO5. Therefore, the large decrease of NBE PL we reported in Section 3.2 after heat treatment at 1100 ^◦C, is likely due to a large portion of the ZnO transferring into Zn silicates. It could also be associated with other defects providing nonradiative recombination.

Fig. 3(b) shows the crystallinity indicator of the S5 film after heat treatment at different temperatures. As stated in Section 2 its value is the equivalent crystallite size calculated by the Scherrer formula:

D= Kλ

Bcosθ (1)

Here K is 0.89, λ = 0.15406 nm, B is the FWHM of the (002) diffraction peak corrected for system broadening, and θ is the diffraction angle. This crystallite size parameter of the A.D. ZnO film was about 30.2 nm, while decreases to 23.4 nm after heat treatment at 600 ^◦C. The

effects of strain/stress within the films should be considered when dealing with the true crystallite size. This will be discussed further together with the SEM observations (see Section 3.4). For increasing treatment temperature above 600 ^◦C, the crystallite size parameter increases gradually to 43.7 nm at 1000 ^◦C, and then increases slightly to 45.9 nm by the 1100 ^◦C heat treatment. Obviously, the film crystallinity has improved gradually with increasing heat treatment temperature.

Fig. 3(b) also shows the lattice constant c determined from the ZnO (002) diffraction peak. The value of c for the ZnO powder standard (JCPDS sheet No. 5-664.) is also shown. The lattice c-parameter is 0.5206 nm for the powder standard, while for S5 the c-parameter (determined by using the formula: c=0.15406/sinθ) decreases from 0.5292 nm for A.D. to 0.5224 nm for heat treatment at 1000 ^◦C, and to 0.5229 nm after treatment at 1100 ^◦C. The variation of the c-parameter can be attributed to several sources including the decrease of the dissolved concentration of Tb in the ZnO lattice with the increase of heat treatment temperature [42]. We did not observe a transition from compressive to a tensile stress within the films with increasing the temperature as reported in Ref. [29].

Fig. 3(c) compares the XRD spectra of the S0-S7 films after heat treatment at 1000 ^◦C. The diffraction patterns do not show significant differences in shape except for the sample S7 with nominal Tb concentration of 0.38 at.%. As mentioned above, the peaks at 2 θ =30.9^◦and 37.5^◦in the patterns of S1-S6 can correspond to the diffractions of (300) and (3 1 2) planes of Tb2SiO5. Those two peaks are absent in the patterns of S7, the (002) peak intensity decreases much and several new peaks emerge with peak positions at 2 θ =32.7^◦, 43.5^◦and 44.3^◦, respectively, due to the formation of Tb and Zn silicates. The crystallinity indicator of the films is shown in Fig. 3(d). The size varies between 37.2 nm and 43.7 Fig. 3. (a) The XRD spectra of the S5, A.D., and after different heat treatments as indicated; (b) The crystallinity of the heat-treated films, and the lattice parameter c.

The lattice parameter of the ZnO powder standard is indicated by a dashed line; (c) The XRD spectra of the S0-S7 films after heat treatment at 1000 ^◦C, and (d) The crystallinity of the film and the lattice parameter c, as a function of Tb concentration.

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nm and seems independent of the Tb concentration, where the S5 film with nominal Tb concentration of 0.029 at.% has the largest ZnO crystallite size parameter and best crystallinity. In Fig. 3(d) is a comparison of the c-parameter for the annealed films. Except for the S7 film, the values of the c-parameter for the doped films are smaller than that of the un-doped one (S0). This is consistent with RE doping mending defects in ZnO as suggested in Ref. [47], if the lattice expansion is related to those defects.

SEM study

Structural evolution of the ZnO:Tb films were further demonstrated by SEM. Fig. 4 shows the surface morphology of the S5 film after heat treatment temperatures presented above. In Fig. 4(a), the A.D. S5 film is made up of closely packed tiny particles with a size in the range 15–50 nm. After heat treatment at 600 ^◦C, the nano-particles have started to aggregate and left nanoscale voids within the film (see Fig. 4(b)), and the particle size appears larger than that of the A.D. case. The particle size from SEM does not equal the crystallinity size parameter determined by XRD and shown in Fig. 3(b). One reason for the sizes being different is that the particles seen in SEM are aggregates of smaller crystallites. Further, the FWHM of XRD peaks has contributions from strain and from the limited coherency size. The contribution of strain to the FWHM is mainly from inhomogeneous micro-strain. The average strain within the films could be reflected by the c parameter. From Fig. 3 (b), the c parameter of the A.D. S5 film is larger than that of the 600 ^◦C treatment case and larger than the powder ZnO reference standard, which infers that the compressive strain within the A.D film is larger than that after 600 ^◦C treatment. The particle size has increased further after heat treatment at 800 ^◦C (see Fig. 4(c)), the film becomes denser and less nano-voids are observed compared with that in Fig. 4(b). Fig. 4 (d) and 4(e) shows the particle sizes after increasing the heat treatment temperature to 900 ^◦C and 1000 ^◦C. It is seen that the particle sizes have increased from dozens to several hundred nanometers. This means the crystallinity has improved by increasing the heat treatment temperature, which is consistent with the XRD analysis. The film structure has become quite different after heat treatment at 1100 ^◦C (see Fig. 4(f)).

The film has many light grey micro/nanostructures that are well sepa- rated. This represents the situation for an annealing temperature higher than the optimum. For thin films the limiting high temperature may correspond to all the ZnO being consumed by the silicate reaction with the substrate, while for thicker films the limit may be associated with the stress caused by the large difference in thermal expansion coefficient for Si and ZnO resulting in cracking, fracture and/or island formation. The

XRD of the sample showed presence of silicates and ZnO (Fig. 3(b)), while there was no significant PL. We have considered this temperature as beyond the range of parameter space we concentrate on for analyzing the present depositions. We may explore this further in the future. A nominally repeated annealing yielded a film with no indications of grains, confirming that 1100 ^◦C is above the limit for PL enhancement.

Fig. 5 exhibits the surface morphology of the films after identical heat treatment at 1000 ^◦C, to examine the influence of Tb concentration on the film structure. In Fig. 5(a), the un-doped film (S0) is made up of ZnO particles with different shapes and sizes in hundreds of nanometers.

There is a “step” structure on some particle surface, which could be stacks of crystallites [39]. Fig. 5(b) shows the surface of S2. There the Tb doping (0.014 at.%) yields a more uniform particle size, that is seem- ingly smaller than that in Fig. 5(a) for S0. With increasing Tb concentration to 0.018 at.% (S4) and then to 0.029 at.% (S5), the particles become larger as is seen in Fig. 5(c) and Fig. 5(d). This is consistent with the XRD analysis as seen in Fig. 3(d). While with the Tb concentration increasing to 0.074 at.% (S3), the film surface becomes uneven and the particles size decreases (see Fig. 5(e)), and many step structures are present on the surface of the particles. Further increasing the Tb concentration to 0.142 at.% (S6) and then to 0.151 at.% (S1), the particles size increases again (see Fig. 5(f) and Fig. 5(g)) compared to Fig. 5(e), while some nano-voids structures appear especially in the S1 film.

However, as the Tb concentration reaches 0.378 at.% (S7,see Fig. 5(h)), the film surface has many irregular “hole” structures, where many white tiny clusters have precipitated inside. The phase of these clusters is unknown.

TEM study

The crystal quality of the films after high temperature treatment was studied further by TEM. Fig. 6(a) shows a bright-field TEM image of the S3 film (with nominal Tb concentration of 0.074 at.%) after heat treatment at 1000 ^◦C. From the image the film can be divided into three regions with different thickness. Fig. 6(b) shows the grey/white region 1 close to the Si substrate with a thickness of dozens of nanometers. The composition of the region is not very clear at the moment. The layer shows amorphous nature by the diffraction pattern being a halo, but the element mapping indicates the region is rich in carbon and has less oxygen, Zn and Si (not shown). We suspect the region was contaminated during the TEM specimen preparation. Fig. 6(c) shows the image of region 2 with a thickness about 200–300 nm. The region shows poly- crystalline nature from the electron diffraction pattern (not shown), and HRTEM observation indicates that Zn silicates (Zn2SiO4) and Tb silicates

Fig. 4.Surface morphology of the S5 films after heat treatment at the temperatures: (a) A.D., (b) 600 ^◦C, (c) 800 ^◦C, (d) 900 ^◦C, (e) 1000 ^◦C and (f) 1100 ^◦C.

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(Tb2SiO4, Tb2SiO5 and Tb2SiO7) have formed in the region besides the ZnO nanocrystals. Region 3 is made up of columnar ZnO crystals along the growth direction with thickness about several hundred nanometers.

The region shows a single crystal nature from the electron diffraction pattern (not shown). Fig. 6(d) shows the HRTEM image of the region 3 close to the film surface. There are large-scale clear lattice fringes of ZnO (002) planes which shows the region has very fine crystallinity. In addition, there are fringes from Zn silicates (Zn2SiO4) and Tb silicates (Tb2Si2O7) also present. This is consistent with the RBS and XRD analysis.

XPS study

Fig. 7 shows XPS spectra of the S1 samples after heat treatment at 700 ^◦C or 1000 ^◦C for 30 min. These were collected before and after Ar^± etching removing about 20 nm of the films. This is common procedure to compare the chemical specifics at the surface with that inside the film.

Fig. 7(a) and 7(b) show Zn 2p spectra before and after etching. They contain two peaks ascribed to Zn-2p3/2 (~1022 eV) and Zn-2p1/2

(~1045 eV) with an energy separation about 23.0 eV, which is close to the standard reference values for ZnO where the Zn atoms are in the Zn²⁺state [32]. Fig. 7(c) and 7(d) show the O 1 s spectra before and after etching. The O^2–peaks are asymmetric and have for simplicity been Fig. 5. The SEM pictures of the S0-S7 films after heat treatment at 1000 ^◦C. (a) 0 at.%, (b) 0.014 at.%, (c) 0.018 at.%, (d) 0.029 at.%, (e) 0.074 at.%, (f) 0.14 at.%, (g) 0.15 at.%, (h) 0.38 at.% .

Fig. 6.(a) The overall TEM picture of S3 film after heat treatment at 1000 ^◦C. (b), (c) and (d) are the high resolution TEM images taken at different regions of the film.

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curve-fitted by using two Gaussian peaks (O1 and O2 [27] with peaks at 529.5 eV and 531.3 eV respectively) as seen by the stippled curves. Note that the ratio of O2/O1 in Fig. 7(c) increases from 1.20 to 1.45 with increasing heat treatment temperature from 700 to 1000 ^◦C for spectra taken before etching. While for the spectra taken after etching, the O2/

O1 ratio in Fig. 7(d) varies little only from 0.188 to 0.189 for these temperatures. This infers that the annealed sample films remained in its oxygen-rich nature, with no indication of oxygen-vacancy concentration change. The oxygen-vacancy is normally regarded to be mediator for energy transfer from ZnO matrix to Tb ions [39]. If we compare the spectra after the identical treatment at 700 or 1000 ^◦C, both the O2

peaks decrease much by the etching. These results indicate that the variation of O2 peak is mainly due to change of chemisorbed oxygen rather than from defects (oxygen vacancies) [18].

Fig. 7(e) and (f) show Tb 4d spectra of S1 samples before and after the etching. In Fig. 7(e), the spectrum of S1 after heat treatment at 700 ^◦C has been curve-fitted with four Gaussian peaks with binding energy at 150.80 eV, 155.52 eV, 158.93 eV and161.45 eV, respectively.

The 150.80 eV and 155.52 eV peaks can be attributed to Tb³⁺ions, while the 158.93 eV and 161.45 eV peaks are attributed to Tb⁴⁺ions [48]. The ratio of Tb³⁺/Tb⁴⁺is calculated to be about 24 at.% : 76 at.%. After heat treatment at 1000 ^◦C, the Tb spectrum can also be fitted with four Fig. 7.The XPS spectra of S1 film before and after etching. The samples are annealed at 700 ^◦C and 1000 ^◦C, respectively. Fig. 7(a) and 7(b) are Zn 2p spectra, Fig. 7 (c) and 7(d) O 1 s, Fig. 7(e) and 7(f) Tb 4d, and Fig. 7(g) and 7(h) Si 2p.

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Gaussian peaks ascribed to the Tb 3 +and 4 +states (not shown), but the Tb³⁺/Tb⁴⁺ratio has increased to be 41 at.% : 59 at.%. This infers that more Tb³⁺-related material is present on the film surface for increasing heat treatment temperature. In Fig. 7(f), the Tb³⁺/Tb⁴⁺ratio is calculated to be 44 at.% : 56 at.% for heat treatment at 700 ^◦C, and increases to 55.2 at.%: 44.8 at.% for treatment at 1000 ^◦C. Tb ions in ZnO occupy the Zn²⁺site or interstitial sites [32]. The increase of the Tb³⁺/Tb⁴⁺ratio with increasing temperature from 700 to 1000 ^◦C, infers that some Tb⁴⁺ions have transformed to Tb³⁺ions in phases on the film surface. It could be from Tb segregated out by reaction with other elements for example forming Tb-oxide precipitates or it reacted with Si to form silicates, as we will discuss in the following paragraph.

Fig. 7(g) and (h) exhibit Si 2p spectra of the S1 samples before and after the etching. In Fig. 7(g), the spectra have peak position at around 109 eV, and there is a small shoulder present on the spectra with binding energy at around 103 eV. The shoulder is more evident for the 1000 ^◦C treatment than the 700 ^◦C treatment. Normally this shoulder infers Si-O bond formed. Then the above-mentioned increase of Tb³⁺on the film surface could be due to Tb silicates rather than Tb oxide. In Fig. 7(h), the peaks become apparent with peak position at around 108 eV, but the shoulder are not evident. These results are consistent with the RBS and XRD analysis that Si atoms have entered into the sample films upon the high temperature treatment, and Zn/Tb silicates have formed at a temperature possibly as low as 700 ^◦C.

Compared to the cases of Eu- [27] and Yb- [28] doped ZnO, the mismatch between the ionic radii of Zn²⁺(0.74 Å) and Tb³⁺(0.92 Å) is similar to that between the Zn²⁺(0.74 Å) and Eu³⁺(0.95 Å), but both are larger than that between the Zn²⁺(0.74 Å) and Yb³⁺(0.86 Å). So, a similarity in the dependence of NBE PL with doping concentration for Tb and Eu could be anticipated.

PLE, PL decay and pump-power study

The enhancement of the UV emission has been examined further by PLE. Fig. 8(a) shows the PL excitation spectra of S1, S5 and S0, which

was recorded with monitor wavelengths practically at their NBE peaks.

The emission intensity is larger for the Tb doped samples (S1 and S5) than that for the undoped (S0). It is larger than S0 by a factor of about 1.74 and 1.52 times at the excitation wavelength (325 nm). So, the highest NBE PL of S5 (see Fig. 2(b)) is correlated with this higher PLE.

One could have expected that excitation via the Tb-O charge transfer state could contribute to absorption and excitation transfer to ZnO, however the shape of the PLE spectrum for S0 and S5 are very similar giving no indication of that. Fig. 8(b) displays the time resolved PL decay spectra of the NBE UV emissions for the S1, S5 and S0 samples after heat treatment at 1000 ^◦C. The decay curves are not simple exponentials with a single decay time. Therefore, an average lifetime (τ_ave) has been calculated for each of the spectra by the following equation [49]:

τave=

∫∫t*I(t)dt

I(t)dt (2)

where I(t) is the emission intensity of the UV PL as a function of time t.

The calculated τave for the S1, S5 and S7 are 0.78 ns, 1.01 ns and 0.42 ns, respectively. Our results are consistent with the increased lifetime in S5 is related to the best improved crystal quality, which is also correlated with the best crystallinity parameter (see Fig. 3 (b)) and highest PL intensity (see Fig. 2(b)). The average lifetime value is larger than the longest lifetime constant in bi-exponential decay of the highest quality epitaxial ZnO layers [50]. The contribution to the average lifetime from surface trapping states is much smaller here than that observed for Eu doping [27].

The UV PL intensity versus the excitation laser power is plotted in Fig. 8(c) for the samples S1, S5 and S0 after heat treatment. All the curves exhibit two characteristic regions. The region with lowest intensity could correspond to spontaneous emission in ZnO due to exciton recombination under the influence of loss to some non-radiative mechanism, and the region with a strong increase in the intensity could correspond to stimulated emission due to electron-hole plasma recombination [22]. By fitting the experimental data to a simple power law,

Fig. 8. (a) PL excitation spectra of the S1, S5 and S0 samples which was recorded with monitor wavelengths at their UV PL peaks. (b) Time resolved PL decay spectra of the NBE emissions for the S1, S5 and S0 samples. (c) The NBE PL intensity of the S1, S5 and S0 samples as a function of laser excitation power. (d) The NBE PL spectra of the S1, S5 and S0 samples under their critical excitation power. The samples are annealed at 1000 ^◦C in N2 for 30 min.

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IPL∝Iexm , where IPL is the luminescence intensity, Iex the excitation power, and m is a fitting parameter, the fitted exponents for the stimulated emission region of S5, S1 and S0, are 0.906, 0.737 and 0.623 respectively. It is clear that these exponents are the result of several competing processes occurring. Fig. 8(d) shows the NBE PL spectra of the three samples at the critical excitation power for stimulated emission. The S1 has the lower excitation power (0.03 mW) than that of the S5 (0.04 mW) and S0 (0.09 mW). This is consistent with that the RE doping during sputtering can decrease the critical excitation power of ZnO PL.

Effects of TiN nanoparticle capping

TiN nanoparticles were capped on the surface of the ZnO:Tb films after heat treatment at 1000 ^◦C to increase the UV PL by plasmonic enhancement of the interaction with light. Fig. 9(a) indicates the enhancement ratio of UV PL peak intensity for the films after the capping with TiN nanoparticles (~40 nm). The PL intensities of the films have increased about 5–25% compared to the case without capping. The inset of Fig. 9(a) displays the SEM picture of the S1 film after the TiN nanoparticles capping. The TiN nanoparticles are observed laying randomly on the surface of ZnO particles (in hundred nanometers) and the distribution is not uniform. Fig. 9(b) indicates the PL enhancement ratio for the films after the TiN nanoparticles (~20 nm) capping. The ratio varies quite differently among different samples: the UV PL intensity of the S7 film has increased by 40% while the intensity of the S3 film changes only a little after the capping. The inset in Fig. 9(b) displays the surface morphology of the S5 film after the capping. The tiny TiN particles (~20 nm) are dark spots under the bright field image and its distribution is not uniform. Note that the variation of the ratio is similar to the size of ZnO nanoparticles with Tb concentration (see Fig. 3(d)). This infers that the tiny TiN nanoparticles indeed play a role in enhancing the UV PL of the ZnO nanocrystals.

TiN has been reported as surface plasmonic material to enhance light emission in the visible to near infrared range [33] where it has advan- tages over gold or silver [51]. The localized surface plasmon resonance (LSPR) spectrum of TiN nanoparticles (~40 nm) can cover a wide range from 400 to 700 nm, and even in the short wavelength (~400 nm), the LSPR normalized intensity still has half of the peak intensity for TiN nanoparticles [52]. Therefore, TiN nanoparticles are used here to enhance the UV PL of ZnO. The smaller TiN nanoparticle (~20 nm) capping seems have better effects than the bigger one (~40 nm) in enhancing the UV PL of the films. Further investigation of TiN capping to enhance the UV PL of RE doped ZnO films is in progress.

Summary and conclusions

We have studied the effects of Tb doping on the structure and NBE UV PL of ZnO films on Si after high temperature treatments. We found that after high temperature treatment, Si has interacted with the films and both Zn- and Tb-silicates have formed. The Tb concentration in the film decreases much by increasing the temperature from 25 to 1000 ^◦C.

The XRD analysis indicates that the crystallinity of the films has improved greatly with the increase of temperature, and after heat treatment at 1000 ^◦C the film with nominal Tb concentration of 0.029 at.

% shows the best crystallinity parameterized by the size of ZnO nanoparticles. The SEM observation shows that the size of the nanoparticles varies with increasing temperature, but there is no simple relation between the size and the Tb concentration. The HRTEM observation con- firms the formation of the silicates, and large scale columnar ZnO nanocrystals have formed in the bulk of the films by the heat treatment at 1000 ^◦C. The XPS results indicate Tb ions exist in the films both in the 3 +and the 4 +state, and more Tb³⁺containing silicates have precipitated near the film surface by increasing the temperature from 700 to 1000 ^◦C. The PL results show the film with Tb concentration of 0.029 at.

% has the strongest UV emission and the highest PL excitation efficiency.

Its PL intensity is about seven times stronger than that of the un-doped film. All the results suggest that the enhancement of NBE PL is closely related to the films quality improvement after the high temperature treatment. In addition, the NBE PL intensity can be increased further with TiN nanoparticle capping on the film surfaces. This study has suggested a new way to enhance the UV PL of ZnO films by combination of RE doping, high temperature treatment and surface plasmon resonance, which may broaden the applications of ZnO films in ZnO-based UV photonics.

Author contributions

All authors contributed equally to the work

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

This work is supported by the National Natural Science Foundation of China No. 61775016. This work is also supported by the State Key Laboratory of Nuclear Physics and Technology, Peking University.

Fig. 9.(a) The ratio of UV PL peak intensity of S0-S7 films after TiN nanoparticles (~40 nm) capping. The inset shows the SEM picture of S1 film with the TiN (~40 nm) nanoparticles. (b) The PL enhancement ratio of the films after the TiN (~20 nm) capping. The inset shows the surface morphology of S5 film with the TiN (≃20 nm) nanoparticles. The sample films have been annealed at 1000 ^◦C in N2 for 30 min.

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