PECVD Synthesis and Thermoelectric Properties of Thin Films of Lead Chalcogenides (PbTe)_1−x(PbS)_x

1. In introduction, the reason of choose the Al2O3 and Si substrate should be given.

An important condition when studying transport and thermoelectric characteristics of thin-film samples is the absence of electrical conductivity of the substrate. Due to some greater thickness relative to the film, the substrate is capable of making a decisive contribution to the properties under study. To avoid shunting of the thin-film layer during heating, it is necessary to use a dielectric substrate, such as sapphire. Note that these considerations were given in section «3.3. Thermoelectric properties of the films».

The lattice constants for PbS, PbTe, Si, and c-Al₂O₃ are 5.936 A, 6.460 A, 5.430 A, and 12.991 A, respectively. Note that, as shown later in the article, a two-phase system is formed in the process of synthesis in which the exact composition is not predetermined. Under such conditions, it is incorrect to match the film and substrate with respect to the lattice parameter. For that reason, two situations are studied in the work: a Si substrate with a strong lattice mismatch with both PbS and PbTe (over 10 %) and a sapphire substrate with half lattice parameter (6,496 A) almost matching PbTe but having large mismatch with PbS. The difference in film formation on selected substrates was demonstrated by analyzing SEM images. The reasoning described above was omitted to avoid overextension of the paper. However, the authors agree that the choice of substrates is to be better discussed, therefore, in the "Introduction" section, reasoning was added in a shorter form than given here.

2. The authors should briefly explain why the PECVD power and substrate affect the film composition and the surface morphology.

The precursor components are supplied into the reaction zone in the form of molecules (Pb, S₈, Te₂) The plasma discharge leads to precursor atomization resulting in better homogenization of a grown film. The homogenization efficiency in turn depends on the discharge power.

Since the dissociation energies of Pb, S8, and Te2 are different, the discharge power also affects the proportion of elements in the deposited films although the observed modulation is rather small.

The substrate effect on composition is probably due to the difference in Pb, Te and S incorporation efficiency in growing film driven by different lattice mismatch between Si(sapphire) and (PbTe)1-x(PbS)x.

The corresponding discussion has been added into the text.

3. In XRD section, the standard pdf card should be cited in figure4. Why PbS(111) disappeared in figure 4b? PbS(111) is the most preferred orientation. Moreover, the preferred orientation of film grown on different substrates changed, which probably related to the mismatch of film and substrate. It is best to simple calculate the mismatch value.

The citation on a pdf card was added at Fig.4.

According to paper (https://doi.org/10.1016/0379-6787(80)90050-2) the preferred orientation of PbS on Si (100) is [100] and indeed the preferred orientation is strongly dependent on the type of substrate. The PECVD technique we used is a highly nonequilibrium method for the film deposition. The preferred orientation might be dependent on type of a substrate, interaction between PbS and PbTe phases and also on the growth parameters. Deriving the contribution of each of the factors is a difficult task that could be the subject of another paper. In the present work we have have decided to limit ourselves to short discussion on thin film crystallographic orientation which was added into the corresponding part of the paper.

The discussion on the mismatch was also added on page 4.

4. The reason of the authors ignored the thermoelectric performance of film grown on silicon substrate may not correct since many papers have stuided the thermoelectric performances of other films grown on silicon substrates

The overall silicon substrate resistance is of the same order of magnitude as the film resistance (about 10³ Ohm/□). In such case the substrate gives the contribution to both resistivity and Seebeck coefficient. The contributions from the film and the substrate are inseparable. For that reason, correct measurement of thermoelectric properties of investigated films grown on Si substrates are hardly possible. The reason of using Si was only studying the peculiarities of the (PbTe)_1-x(PbS)_x growth when using different substrates.

5. The equations (1 and 2) and thermoelectric mechainism explaination should be cited the related reference.

We suggest that the most appropriate and exhaustive references are the works of L. D. Hicks and M. S. Dresselhaus "Thermoelectric figure of merit of a one-dimensional conductor" and Mona Zebarjadi et al. "Power Factor Enhancement by Modulation Doping in Bulk Nanocomposites". References have been added to the bibliography.

1  The phase diagram and crystal structure of (PbTe)1-x(PbS)x should be provided. 

Information has been added to the text with references to relevant literature (Phys. Chem. Chem. Phys. 2014, 16, 1835-1840.).

 2 How to get the dislocation density？the details should be given。

The details of dislocation density calculation using the XRD spectra are given in Ref. [https://doi.org/10.1142/S0219581X1450001X]. The corresponding reference has been added in structural investigations section.

3 The predominantly oriented in the (200) direction depending on the type of substrates， more discussion should be added。The crystal structure of substrates should be added。

The crystal structure was discussed at page.4 (sample description).

4 How about the thermoelectric of (PbTe)0.5(PbS)0.5，(PbTe)0.3(PbS)0.7 and PbS ？

The paper studies the modulation of PbTe thermoelectric properties by adding some ratio of PbS phase. The main result of the paper is a PbS ratio providing the highest enhancement of power factor with respect to that of pure PbTe (20 % of PbS phase). Increase of PbS ratio above 20 at.% leads to decrease of a power factor. Thus, we have shown the W optimization in (PbTe)_1-x(PbS)_x mixtures moving from pure PbTe towards higher PbS concentrations which we believe is a completed research. Both the equiatomic (PbTe)_0.5(PbS)_0.5 and PbS with smaller PbTe content are different types of materials with properties more defined by PbS phase. We believe that the investigation of these materials is a separate task which will be the subject of further research.

 5 Check the sentence of “Single phase PbSe Sample 1 was characterized by the lowest power factor among the batch (figure 6d line 1)”， PbSe, Fig.6d? 

The sentence was corrected as follows: “Single phase PbTe Sample 1 was characterized by the lowest power factor among the batch (figure 6c line 1)”

6 Compared with the literature values of thermoelectric properties in the table. 

A table was added to the text of the article with a comparison of the power factor with literature data. Additional references have also been added.

Please keep all the variables in slanted font (i.e. UTE = α ∙ ΔT) for the following equation.

Checked

Keep the consistency in the following sentence:

ρ– resistivity, λ is a thermal conductivity

Checked

 “…..thermoelectric material efficiency, it is preferable to use the power factor (W) along with ZT coefficient:”

 What is “ZT coefficient”

“ZT coefficient” was a translation mistake. The phrase was corrected as follows “thermoelectric material efficiency, it is preferable to use the power factor (W) along with ZT”. The ZT was earlier introduced as formula (2).

Double check if the following statement is true.

“One can cite only few publications, for example, one devoted to investigation of mechanical properties of bulk PbSxTe1-x 35. The thermoelectric properties of such compounds weren’t investigated neither for bulk nor for thin film materials to the best of our knowledge.”

First, the grammar of the sentence was double checked and some corrections were introduced to avoid ambiguities.

Second the authors carefully re-searched for available information from the literature related with the study of the thermoelectric properties of the PbSxTe1-x compositions. The trends of recent years are focused at studying the thermoelectric characteristics of (PbTe)z-x(PbSe)y(PbS)x pseudo-ternary compounds with a high concentration ratio of the PbS phase to the PbTe phase. The addition of a high concentration of the (PbSe)y phase (y ~ 0.35) increases the limit of the equilibrium solubility of the PbS phase in the PbTe. This approach further reduces the thermal conductivity; however, the interfacial scattering of carriers significantly reduces the Seebeck coefficient as well which, in turn, leads to a decrease in the power factor. This limits practical interest of these compositions. All these considerations were discussed in detail in the review part of the article. The authors of course, cannot guarantee the absence of articles devoted to studying thermoelectric characteristics of PbSTe ternary compounds, but no such articles were found during the double search. The authors of the article suggest that the lack of such information is primarily due to the technological difficulties in obtaining such films. The PECVD method described in the paper makes it possible to form such compositions, which was displayed in the article and is, first of all, its novelty.

Please explain why Si and c-sapphire substrates were used. The above explanation is not satisfactory. I think it is related to the lattice matching and coefficient of thermal expansion of both the film and the substrate.

An important condition when studying transport and thermoelectric characteristics of thin-film samples is the absence of electrical conductivity of the substrate. Due to some greater thickness relative to the film, the substrate is capable of making a decisive contribution to the properties under study. To avoid shunting of the thin-film layer during heating, it is necessary to use a dielectric substrate, such as sapphire. Note that these considerations were given in section «3.3. Thermoelectric properties of the films».

The lattice constants for PbS, PbTe, Si, and c-Al₂O₃ are 5.936 A, 6.460 A, 5.430 A, and 12.991 A, respectively. Note that, as shown later in the article, a two-phase system is formed in the process of synthesis in which the exact composition is not predetermined. Under such conditions, it is incorrect to match the film and substrate with respect to the lattice parameter. For that reason, two situations are studied in the work: a Si substrate with a strong lattice mismatch with both PbS and PbTe (over 10 %) and a sapphire substrate with half lattice parameter (6,496 A) almost matching PbTe but having large mismatch with PbS. The difference in film formation on selected substrates was demonstrated by analyzing SEM images. The reasoning described above was omitted to avoid overextension of the paper. However, the authors agree that the choice of substrates is to be better discussed, therefore, in the "Introduction" section, reasoning was added in a shorter form than given here.

In 2. Materials and methods, explain where the substrates were placed and the deposition was carried out.

The plasma-chemical reactor is a pear-shaped quartz vessel connected to a vacuum pumping system by a stainless-steel flange. An external inductor was placed on the narrow part of the plasma-chemical reactor. The reactor was equipped with a substrate holder made in the form of a cooled/heated pedestal for substrate positioning prior to the film deposition. The substrate temperature in all experiments was about 15°C.

The corresponding description has been added to section 2.

“The deposition of (PbTe)1-x(PbS)x films was carried out in following conditions: lead source temperature – 700 °С, sulfur temperature – 130 °С, tellurium temperature – 440 °С, total pressure in the reactor – 0.01 Torr, total flow rate – 30 ml/min.”

The – sign may bring the impression of -ve temperatures. Please omit the – sign.

Checked

“The power factor was calculated by formula Error! Reference source not found..”

Checked

Fix the typo in the label of y-axis of figure 6.

The authors have checked carefully the labels at y-axis of Fig.6, from our point of view all of the labels are correct including the scaling. Please specify the mistake we could have made at Fig.6.

The lattice constants of PbS and PbTe were found to be 5.93 and 6.46 Å. How are these values estimated? The authors should give detail about it.

The lattice constants of PbS and PbTe were calculated from the position of corresponding peaks using some standard tool the details are given in  Ref. [https://doi.org/10.1142/S0219581X1450001X] – formula 3. The corresponding reference has been added in structural investigations section.

Note that these values are in good agreement with well known literature data.

The authors should give the EDAX spectrum for at least one of the samples. Only then results will be confirmed.

The EDAX spectrum has been added at Fig.4a. We also have performed some additional investigation of the composition using X-ray photoelectron spectroscopy technique. The results are presented at Fig.4b.

Some discussion has been added.

As Te and S both have the same number of valence electrons, carrier concentration should not change much. However, why the carrier concentration for samples increased with increase in PbS phase concentration.

In case of the low bandgap semiconductors, the carrier concentration is strongly dependent on the concentration and ratio of point defects. The most detailed study of the defect influence on the electronic properties in PbS, PbSe and PbTe was presented in [doi:10.1088/0953-8984/27/35/355801].

We believe that the system of electrically active defect is attributive of the PbS phase is responsible for the concentration increase with the increase of the corresponding phase concentration.

A brief discussion of this was added at page 9.

The authors can also compare their results with the bulk values for these similar kind of materials if available in the literature.

The authors have failed to find a direct bulk analogue of the investigated films in the literature. However the comparison of the thermoelectric characteristics of the PbTe films with that of analogues have been made the results are presented in Table 3.