Cap Layer Effect on Key Features of Persistent Photoconductivity Spectra in HgTe/CdHgTe Double Quantum Well Heterostructures

Round 1

Reviewer 1 Report

 

The  results reported in the manuscript and the conclusions of the authors are quite reasonable.  However, I believe that  some information on the hall measurements and a comment on how these properties depend on the structural quality and characteristics of the sample could improve the relevance of the findings

Author Response

Thank you for your comment. We have added information about characteristic mobilities to the text. However, the question of the relationship between these values and the quality of the structures and the material of the cap layer is very extensive. We believe that this is beyond the scope of this work.

Reviewer 2 Report

 

 

The paper is devoted to studying the influence of the cap layer material on persistent photoconductivity in structures with tunnel-coupled HgTe double quantum wells. Persistent photoconductivity is one of the most technologically demanded properties of semiconductor materials, which has found application in a variety of electronic devices, such as photoresistors, photodetectors, including night vision devices, as well as in copiers and printing devices, which makes it absolutely necessary to study this remarkable phenomenon on new groups of materials. The electrical resistance of structures based on HgTe/CdHgTe has been studied as a function of the incident radiation energy in the range of 0.62–3.1 eV. It is found that two of the three main spectral features are associated with the cap layer, the third - with the buffer layer. It is shown that when a narrow-gap material is used as a cap layer, the position of the mentioned features shifts to the long-wavelength region. The authors emphasize that the qualitative form of the spectra of persistent photoconductivity and the quantitative features that determine it are essentially formed by the material of the cap layer.

In addition, in the persistent photoconductivity spectra of these heterostructures, oscillating resistance dependences are observed in the energy range of 0.7–1.6 eV, the occurrence of which apparently is not related to the cap layer. The nature of these remarkable oscillations is not yet clear, and the report of their observation is certainly of considerable interest.

A question should be asked to the authors of the article about the error in determining the values of the period of oscillations as a function of the energy of their minima (Fig. 2). What can the spread in values of the period of oscillations of more than 10 meV between neighboring minima mean?

The work seems to be very interesting for a wide range of researchers and is recommended for publication without re-reviewing.

Comments for author File: Comments.pdf

Author Response

Thank you for your question. The spread of values, first of all, is determined by a random error that occurs when subtracting the baseline (especially for sample A) and choosing the position of a specific minimum. When the minimum is not clearly expressed, the error in determining its position increases. Since we simply calculate a period as the difference between adjacent minima, this can cause the value of the “current” period to be, for example, underestimated, and the “next” period, respectively, overestimated. This can lead to a fairly large periods difference of 10 meV. That is why we do not discuss in the paper the dependence of the period on the energy of the incident quantum in detail, but only talk about the general trend, which is clearly the same for all samples.

We have added a sentence to the article regarding the spread of values.

Reviewer 3 Report

The article is of interest to the relevant community. The material of the article is presented quite clearly and structured. Minor comments are more of a recommendation.

I think it's not entirely correct to introduce abbreviations in the abstract. As a rule, this is done if it is used further in the abstract itself. Otherwise, it is more correct to enter abbreviations directly in the text of the article.

In Section 2.1, the authors write that the active region of the structure consists of HgTe QWs, and then indicate that one structure contained a nonzero Cd value. Please provide a clearer description of the heterostructures used.

Author Response

Thank you for the appreciation of our work. We have changed the Abstract to avoid unnecessary abbreviation and provided a clearer description of the the active region of heterostructures.

Reviewer 4 Report

In this paper, the author demonstrates the effect of cap layer on HgTe/CDHgTe spectrum by studying the PPC heterojunction of double quantum wells with different cap layers. Through experiments, it is proved that the main characteristics of PPC spectra are determined by the hat layer of heterostructures. And the abnormal oscillation behavior of PPC spectrum was observed in the experiment. This behavior has been proved to be independent of both the caprock and the barrier.This is a well-written paper containing interesting results which merit publication. This is a carefully done study and the findings are of considerable interest. A few minor revisions are listed below:

1.    Improve the level of English.

2.    The main feature of the PPC spectrum is what contribution this discovery has made to scientific research, which is determined by the hat layer of the heterostructure.

3.    The author should pay attention to typesetting to make it easier for readers to watch.

4.    The format of references should be unified.

5.    Is there any special significance for the temperature and wavelength range selected in the measurement of PPC spectra?

6.    With the increase of photon energy, the most obvious oscillation change of resistance is observed in ZnTe. Does this material have any special properties?

7.    Is there any advantage in using Lorenti's formula to calculate the band gap of Cd_xHg_1-xTe?

Comments for author File: Comments.pdf

No comment.

Author Response

Thank you for the appreciation of our work.

We made the following corrections.

• Improve the level of English.
• The main feature of the PPC spectrum is what contribution this discovery has made to scientific research, which is determined by the hat layer of the heterostructure.
• The author should pay attention to typesetting to make it easier for readers to watch.
• The format of references should be unified.

We carefully re-checked the manuscript and eliminated the found typos, formatting inaccuracies, and also improved English.

• Is there any special significance for the temperature and wavelength range selected in the measurement of PPC spectra?

The chosen wavelength range was determined, firstly, by the characteristic interband energies of the heterostructures under study (see energy diagrams below) and, secondly, by the 600 lines/mm diffraction grating used. The two temperatures at which the measurements were made are those of liquid helium and liquid nitrogen, the standard temperatures for measurements using liquid gases under normal conditions.

We have included relevant comments in the text.

• With the increase of photon energy, the most obvious oscillation change of resistance is observed in ZnTe. Does this material have any special properties?

Presumably, we are talking about a strong change in resistance from the ‘dark’ value (120 kOhm at T = 4.2 K) to the ‘illuminated’ values (~ 2 kOhm).

We do not think that ZnTe itself has any special properties, however, as stated in the paper, in the specific case of sample B, we believe that the cap layer is quite defective and inhomogeneous. It is known [A. V. Ikonnikov et al. Origin of structure inversion asymmetry in double HgTe quantum wells // JETP Letters 116, 547 (2022)] that most charge carriers in DQWs come from the surface. In the case of a defective ZnTe surface layer with a large bandgap, it can be assumed that as the temperature decreases in ‘dark’ conditions, “possible” free charge carriers remain in the near-surface layer (the temperature is not enough to transfer them above the ZnTe layer itself). At the same time, the energy of visible or near-IR light is sufficient to “throw” charge carriers from the near-surface layer into the DQW, where they remain.

However, it seems to us that these thoughts are beyond the scope of the work under consideration, so we do not present them in the manuscript.

• Is there any advantage in using Lorenti's formula to calculate the band gap of CdxHg1-xTe?

Currently, the main empirical formulas used to determine the band gap in CdxHg1-xTe are the Lorenti [1] and Hansen [2] formulas. In general, they give fairly close values (differences are no more than 10%), but, for example, for pure CdTe at T = 4.2 K, the Hansen formula gives Eg = 1.65 eV, which is farther from the position of feature 1 (1.55 eV) observed in our PPC spectra, and more than the “traditional” value of Eg of 1.6 eV. Therefore, we used the Lorenti formula.

However, this does not affect the interpretation of the observed results, nor the conclusions of the article.

[1] J. P. Laurenti et al. Temperature dependence of the fundamental absorption edge of mercury cadmium telluride // J. Appl. Phys. 67, 6454 (1990).

[2] G. L. Hansen et al. Energy gap versus alloy composition and temperature in Hg1−xCdxTe // J. Appl. Phys. 53, 7099–7101 (1982).