Influence of Terbium Doping and Annealing on the Structural and Optical Characteristics of Sputtered Zinc Oxide Thin Films

Round 1

Reviewer 1 Report

 

I have read the paper, and my general opinion about this work is positive. ZnO is a very important and interesting material. The paper's general quality is acceptable and all the analysis seems to be correct. The authors analyzed the properties of the obtained material for different compositions and thermal treatments. The analysis is really detailed and contains valuable results. The paper is clearly and carefully written and is well organized. The language and the style in my opinion are OK, did not find mistakes or typo errors. Here are some more comments on this paper:

-        The abstract is too long. The abstract should be conceived and give info about the given work.

- Some figures are just copied and the quality is not the best (Fig 1 for example), if possible they should be replaced with better ones.

-        The only concern is there are many papers on this topic, one can easily find articles about ZnO doped with Tb, even obtained with sputtering technique. Therefore the authors should describe in a more detailed way in the Introduction section what is new in their work, compared with the current state of the art.

In summary, I recommend a minor revision of the paper.

 

 

Author Response

Dear Reviewers,

 

Thank you very much for your attention and time for reading of the manuscript “Influence of Tb doping and annealing on structural and optical characteristics of sputtered ZnO thin films” (crystals-2535680). In accordance with your recommendations, we reduced the Abstract and extended the Introduction, added a new Figure 9d, corrected and added new references and ordered them with respect to the corrected text.

All new revisions are identified in the revised manuscript by highlighting them in yellow.

We hope that our revised manuscript will be recommended for publication in the journal.

 

Best regards,

the authors.

 

Comments of the Reviewer:

1. The abstract is too long. The abstract should be conceived and give info about the given work.

Author response:

The abstract was reduced.

 

2. Some figures are just copied and the quality is not the best (Fig 1 for example), if possible they should be replaced with better ones.

Author response:

We replaced Fig.1, Fig. 4 and Fig.5 by better ones.

 

3. Therefore the authors should describe in a more detailed way in the Introduction section what is new in their work, compared with the current state of the art.

Author response:

We extended the Introduction, added what is new in our work and related references.

Author Response File: Author Response.pdf

Reviewer 2 Report

1) There is an enormous quantity of articles devoted to rear-earth metals doped ZnO films. That is why, please highlight the main advantages and scientific novelty of the current research in the introduction section.

2) How to explain near 8 times lower Tb concentration (0.14 % vs. 1.07 %) in the case of ZnO films being only 3 times thinner (200 nm vs. 600 nm)?

3) How you can garantee that all sputterd Tb is in atomic forms rather than in Tb2O3 and TbO2 phases? The EDX does not answer this quation. In addition, how about the Hall measurements to confirm ZnO doping?

4)  Why Ar-O2 plasma was used in growing a thinner film?

5) Why RTA was applied for processing a thin film and CTA for doing that for thicker ones?

6) Is there an option for a sample holder to be rotated? Since there is a clear non uniform elemental distribution in all grown films.

7) One can see peak assigned as Si(004) reflection in the XRD pattern in Figure 3. However, the position of Si(004) for X-ray from CuK lines lies near 69 degrees rather than 82-83. Please explain this feature.

8)  As soon as annealing at a higher temperature is done, the more Tb atoms should to be incorporated into ZnO lattice resulting in its deformation and changing lattice parameters. Please provide lattice parameter dependence on annealing emperature.

9) Why the presense of some Tb-O phase is non-monothonic against annealing te,perature (Figure 4)?

10) What about electron resonance measurements to check the sites where Tb atoms are most likely to be incorporated?

11) Please explain a phrase: "The presense of Tb ions in the deposited film leads to a more intense interdiffusion".

12) Only one peak assigned for a Zn2SiO4 is not enough, please provide more evidences such as TEM cross sections. All in all you have estimayted its thickness to be in the order of several tens of nanometers.

13) The coefficient of 0.94 in the Scherrer equation is for a cubic crystals rather than hexagonal, which is the case of ZnO. Please, recalculate the grain sizes.

14) It is better to provide some up-conversion PL efficiency, that is one of the hotest topics concerning ZnO:Tb.

Author Response

Dear Reviewer,

 

Thank you very much for your attention and time for reading of the manuscript “Influence of Tb doping and annealing on structural and optical characteristics of sputtered ZnO thin films” (crystals-2535680). In accordance with your recommendations, we reduced the Abstract and extended the Introduction, added a new Figure 9d, corrected and added new references and ordered them with respect to the corrected text.

All new revisions are identified in the revised manuscript by highlighting them in yellow.

We hope that our revised manuscript will be recommended for publication in the journal.

 

Best regards,

the authors.

 

Comments of the Reviewer:

1. Highlight the main advantages and scientific novelty of the current research in the introduction section.

Author response:

It has been found that by varying a nature of gases used in deposition process the concentration of native point defects can be changed, and both defect-related emission and electrical conductivity of the film can be tuned. It can be expected that growth ambient can also affect the Tb³⁺ emission intensity in the ZnO film. It has been shown that various post deposition thermal treatments of ZnO films grown by RF magnetron sputtering can improve the PL and structural properties of the films [1, 13, 22]. The annealing treatment of ZnO:Tb films can also be useful to increase Tb³⁺ emission being limited by diffusion of RE ion from ZnO grains and formation of secondary crystal phases at elevated temperatures [13, 22].

Although the influence of atmosphere of post deposition thermal treatment on RE ion emission in ZnO films has been studied [30, 31], the effect of growth ambient on the RE ion PL has hardly been addressed. The authors have used several tools such as XRD, EDX, AFM, SEM, Raman, and PL analysis to determine the structural, morphological and optical properties of the prepared thin films. It is expected that the current research will be a practical roadmap to improve optical characteristics of RE ion doped ZnO structures for various applications.

 

2. How to explain near 8 times lower Tb concentration (0.14 % vs. 1.07 %) in the case of ZnO films being only 3 times thinner (200 nm vs. 600 nm)?

Author response:

Such low concentration of terbium was chosen intentionally and was variated in the films with different thickness by variation of technological regime at sputtering. A minimum number of the samples was selected for analysis to reveal general trends in the effect of terbium doping on the structural and optical properties of zinc oxide films and those caused by growth and annealing conditions, in particular growth ambient.

 

3. How you can guarantee that all sputtered Tb is in atomic forms rather than in Tb2O3 and TbO2 phases? The EDX does not answer this question. In addition, how about the Hall measurements to confirm ZnO doping?

Author response:

Indeed, EDX realizes the elemental analysis, without determination of the form in which these elements are present. To confirm the incorporation of sputtered Tb into ZnO lattice we used a complex of different methods. i) XRD patterns showed an increase of the lattice parameter (tensile strains) in [0001] direction, perpendicular to the substrate surface, for doped films that can be associated with Tb incorporation, and that not all sputtered Tb incorporated, some part of Tb formed oxides; ii) Non-resonance Raman spectra of the undoped and Tb doped films showed the low energy shift of the E2 high mode testifies to tensile strains in the plane of the substrate. Such lattice deformation in two perpendicular directions can be caused by the incorporation of Tb³⁺ ions with radio 92 pm into ZnO crystal lattice substituting Zn²⁺ ions with radio 74 pm. iii) The spectra of spectroscopic ellipsometry showed an increase in the refractive index n in the entire spectral range and a decrease in the extinction coefficient k in the range of band-to-band absorption, and variations of the optical band gap depending on Tb concentration for doped films. These data imply incorporation of Tb ions into ZnO films.

Meanwhile, in this work we did not study electrical conductivity of the films. Study of polycrystalline ZnO films by conventional electrical methods (such as direct current measurements, four probe method, Hall experiments, etc.) can give controversial results because of possible formation of barriers at the grain boundaries [*, **, ***].

 

*. Michał A. Borysiewicz, ZnO as a Functional Material, a Review // Crystals 2019, 9, 505; doi:10.3390/cryst9100505

**. Tsuji T and Hirohishi M, Influence of oxygen partial pressure on transparency and conductivity of RF sputtered Al-doped ZnO thin films // Appl. Surf. Sci. 2000 15747–51, 27

***. Korsunska N et al, Photoluminescence, conductivity and structural study of terbium doped ZnO films grown on different substrates // Mater. Sci. Semicon. Process. 2019, 9451–56

 

4. Why Ar-O2 plasma was used in growing a thinner film?

Author response:

Please see our response on the comment 2.

 

5. Why RTA was applied for processing a thin film and CTA for doing that for thicker ones?

Author response:

Please see our response on the comment 2.

 

6. Is there an option for a sample holder to be rotated? Since there is a clear non uniform elemental distribution in all grown films.

Author response:

Yes, the sample holder was rotated. The origin of some non-uniform elemental distribution in the films is not clear now.

 

7. One can see peak assigned as Si(004) reflection in the XRD pattern in Figure 3. However, the position of Si(004) for X-ray from CuK lines lies near 69 degrees rather than 82-83. Please explain this feature.

Author response:

As the structural characteristics of the films were investigated by using of two diffractometers: Panalitycal Xpert PRO MRD diffractometer with Cu K_α radiation and Bruker D8 Advance diffractometer with Co-K_α radiation, Figure 3 shows XRD patterns measured using Bruker D8 Advance diffractometer with Co-K_α radiation (figure caption). So, the position of 004 reflection of Si for Co-K_α radiation is 2θ = 82.49 degrees, which coincides with the measured position.

 

8. As soon as annealing at a higher temperature is done, the more Tb atoms should to be incorporated into ZnO lattice resulting in its deformation and changing lattice parameters. Please provide lattice parameter dependence on annealing temperature.

Author response:

We added a new Figure 9d that presents temperature dependence of lattice parameter in the [0001] direction for ZnO as only one peak was registered. The figure caption was corrected.

 

9. Why the presence of some Tb-O phase is non-monotonic against annealing temperature (Figure 4)?

Author response:

The presence of some Tb-O phases is revealed in XRD patterns both by small peaks on the scattering curve (for as-deposited film) and by a hump in the angle range of 2θ = 27-32 degrees, which includes the peaks positions of terbium oxides. After CTA at 300 °C the intensity of scattering curve decreased considerably, but a small hump still was observed that indicates further crystallization of ZnO amorphous phase and the decrease of volume fraction of Tb-O nanoparticle due to an incomplete terbium incorporation. The best crystal structure was observed for ZnO:Tb film annealed at 600 ^oC, when only one peak for 0002 reflection of ZnO was revealed and a hump disappears completely. So, the film consists only of ZnO:Tb crystals and terbium completely incorporated. After CTA at 900 °C, the peak position for ZnO:Tb was almost the same as for as deposited film. The obtained result can be associated with the partial Tb escape from crystal lattice and formation of Tb oxides nanocrystals (Figure 4) that is revealed by appearance of a small hump in the angle range of 2θ = 27-32 degrees once more. Inclusions of Tb oxides of 10-20 nm on the grain boundaries and film surface were observed in ZnO:Tb, Eu films after annealing at 900 ^oC [34].

 

10. What about electron resonance measurements to check the sites where Tb atoms are most likely to be incorporated?

Author response:

We did not measure electron resonance and assumed that Tb must substitute Zn as a substitutional solid solution.

 

11. Please explain a phrase: "The presence of Tb ions in the deposited film leads to a more intense interdiffusion".

Author response:

In XRD patterns measured using of Panalitycal Xpert PRO MRD diffractometer with Cu K_α radiation for as deposited undoped and doped films only peak for 0002 reflection of ZnO was detected. After the annealing a new peak at the angle 2θ = 32.9 degrees appeared (Figure 6 a,b) in the patterns for both films, but its intensity, that depends on diffraction volume, was lower for undoped film than for the doped one. A new peak indicates the formation of a new crystalline phase that can be attributed to Zn₂SiO₄ phase, formed by interdiffusion between ZnO film and Si substrate. As diffraction volume (the thickness of a new Zn₂SiO₄ layer) was larger for doped film annealed at the same temperature, it can be concluded that “The presence of Tb ions in the deposited film leads to a more intense interdiffusion”.

Similar conclusion that “Si diffusion in the films is more pronounced for doped films” was drawn in [****] from the analysis of depth profiles of chemical composition of Tb-doped ZnO films grown by RF magnetron sputtering on Si substrate. The data were obtained by PP-TOFS method (plasma profiling time of flight mass spectrometry).

 

****. A. Ziani, A. Tempez, C. Frilay, C. Davesnne, C. Labbé, Ph. Marie, S. Legendre, and X. Portier, Concentration determination and activation of rare earth dopants in zinc oxide thin films // Phys. Status Solidi C, 2014, 1–4 / DOI 10.1002/pssc.201300636

 

12. Only one peak assigned for a Zn2SiO4 is not enough, please provide more evidences such as TEM cross sections. All in all you have estimated its thickness to be in the order of several tens of nanometers.

Author response:

Indeed, XRD patterns for the annealed films reveal only one new peak of a new crystalline phase that was expected. As this phase was formed by the interdiffusion between polycrystalline ZnO film with a very strong crystal preferred orientation and single crystal Si substrate, the crystalline structure of the layer must have only one orientation. Really, the formation of the same phase was observed in [17, 22] after annealing of Tb doped ZnO films by using TEM. But we suggested to use SEM backscattered electron images of the cross-section to confirm a new phase formation in the film/substrate interface by the changes of the film thickness owing to contrast differences between silicon and the film. These measurements can be realized more simply, and images give possibility to determine the thickness of a new layer depending on annealing temperature. For the films of 600 nm these images were very important because the peak at the angle 2θ = 32.9 degrees after annealing was not detected owing to wide tail of ZnO diffraction peak at lower angles.

13. The coefficient of 0.94 in the Scherrer equation is for a cubic crystals rather than hexagonal, which is the case of ZnO. Please, recalculate the grain sizes.

Author response:

We agree with the comment. The Scherrer equation has shape factor K that depends on crystal structure and reflection index and change in the range of 0.88 – 0.94. It is possible to calculate the grain size without a shape factor using the Seljakow equation (Seljakow N. Zeitschrift fuer Physik, 1925, 516, 439–444):

,

where  is the integral width of the diffraction peak, which is determined by the area under the diffraction peak divided by the height of the maximum. In such a way we calculated the grain size. The difference in the sizes calculated by different equations did not exceed 8%, which is not essential for obtaining trends in temperature dependence. We have given a standard representation of Scherrer's formula to simplify the explanations, since this work is not devoted to the peculiarities of X-ray diffraction. In the text the factor 0.94 was replaced by K without additional explanations.

14. It is better to provide some up-conversion PL efficiency, that is one of the hottest topics concerning ZnO:Tb.

Author response:

In the present work, we did not investigate the up-conversion processes. The up-conversion is usually studied in co-doped films. There is a number of articles devoted to the study of up-conversion in Yb³⁺-Tb³⁺ pair, where Tb³⁺ acts as an acceptor. Tb³⁺ doping is also used to enhance emission of other RE ions. For example, in our previous studies, the transfer of excitation energy from Tb³⁺ to Eu³⁺ ions in ZnO films co-doped with Tb and Eu was observed. Besides, the methodology for estimation of the absolute up-conversion quantum yield is not well established mainly due to issues that arise from the non-linear behavior of the up-conversion process [*****].

***** C. M. S.  Jones, A. Gakamsky, J. Marques-Hueso, The up conversion quantum yield (UCQY): a review to standardize the measurement methodology, improve comparability, and define efficiency standards // Science and Technology of Advanced Materials, 2021, vol.  22, no. 1, 810–848. https://doi.org/10.1080/14686996.2021.1967698

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors responded well all my questions and appropriate changes were added through the new manuscript version