Surface Photovoltage Method for Photovoltaic Quality Control of GaAs-Based Solar Cells

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the authors have shown the possibility of using surface photovoltage method for express qualitative control of solar cells by example of GaAsN, InGaAsN and GaAsSbN compounds. The work contains new interesting results and has practical significance for the development of methods of characterization of materials for solar energy. There is a clarifying comment for the authors:

In the Section 3 (line 223-224), the authors write: « The bandgap values assessed from Tauc plots are 1.39 eV (892 nm) for GaAsN, 1.33 eV (932 nm) for InGaAsN and 1.19 eV (1042 nm) for GaAsSbN». It would be useful to compare the values of the band gap obtained using the SPV spectra with the values known from the literature determined by the classical optical absorption spectroscopy method.  

I recommend the work for publication after making the appropriate clarification in the text.

Author Response

We are thankful to the reviewer for his/her comments and remarks. 

Concerning the remark about the assessment of the bandgap using SPV and absorption spectroscopy we would like to point out the following:

1) As the SPV arises from the spatial separation of the photogenerated electrons and holes the analysis of the results provides information on both light absorption properties and transport properties of charge carriers in the semiconductor sample [1,2]. In the metal-insulator-semiconductor (MIS) operation mode the SPV amplitude is known to emulate the optical absorption spectrum [1]. In particular, SPV spectroscopy has been used to assess the bandgap of various semiconductor materials including multilayered structures [3–7]. The assessment is based on the large increase in absorption coefficient near the bandgap energy found in most semiconductors. This increase results in a significant change in the SPV signal, which is identified easily as a sharp change in the slope of the SPV curve and often is the most significant one in a given SPV spectrum.

We would like to note that SPV spectroscopy gives more information as compared to optical absorption spectroscopy and contrary to the latter does not require light collection and therefore can be applied to arbitrarily thick samples and structures grown on non-transparent substrates.

In the introduction of the revised version of the paper, we have added a more detailed explanation of the possibilities of SPV spectroscopy.

2) We have added the following text in the revised version of the paper (in the paragraph before Figure 5).

To check whether the incorporation of nitrogen into the crystal lattice affects the bandgap values of dilute nitrides assessed by SPV we have measured the transmission spectra of a few GaAsN layers as reported in [48]. The absorption edges assessed from the transmission spectra are in line with the absorption edges found from the SPV spectra of the same samples using Tauc plots.

3) In our previous works [8–10] we have performed calculations of the bandgap of GaAsN and InGaAs(Sb)N using the empirical tight-binding method. The results are in good agreement with the bandgap values assessed from the corresponding SPV spectra using Tauc plots.

4) There are not many works reporting on optical absorption spectroscopy investigations of dilute nitrides. This is probably related to the need for special sample preparation. Such works (e.g. [11]) report results for dilute nitrides with different compositions as compared to our samples. In addition, there are few works on dilute nitride materials grown by liquid phase epitaxy. We couldn’t find optical absorption spectroscopy studies of dilute nitrides, which are grown by liquid phase epitaxy and have a similar content of nitrogen as in our case. For example, Dhar et al. [12,13] reported transmission spectra of GaAsN with nitrogen content (0.5%), which is much larger than that in our samples, and obtained lower band gap values corresponding to the higher nitrogen content in the layer.

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2. Donchev, V. Surface Photovoltage Spectroscopy of Semiconductor Materials for Optoelectronic Applications. Mater. Res. Express 2019, 6, 103001, doi:10.1088/2053-1591/ab3bf.
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4. Sharma, T.K.; Porwal, S.; Kumar, R.; Kumar, S. Absorption Edge Determination of Thick GaAs Wafers Using Surface Photovoltage Spectroscopy. Rev. Sci. Instrum. 2002, 73, 1835–1840, doi:10.1063/1.1449461.
5. González, Y.; Abelenda, A.; Sánchez, M. Surface Photovoltage Spectroscopy Characterization of AlGaAs/GaAs Laser Structures. J. Phys. Conf. Ser. 2017, 792, 012021, doi:10.1088/1742-6596/792/1/012021.
6. Kumar, S.; Ganguli, T.; Bhattacharya, P.; Roy, U.N.; Chandvankar, S.S.; Arora, B.M. Surface Photovoltage Spectroscopy of N-n⁺ and p-n⁺ AlGaAs/GaAs Heterojunctions. Appl. Phys. Lett. 1998, 72, 3020–3022.
7. Yang, J.; Zidon, Y.; Shapira, Y. Alloy Composition and Electronic Structure of Cd_1-xZn_xTe by Surface Photovoltage Spectroscopy. J. Appl. Phys. 2002, 91, 703–707, doi:10.1063/1.1425071.
8. Donchev, V.; Milanova, M.; Lemieux, J.; Shtinkov, N.; Ivanov, I.G. Surface Photovoltage and Photoluminescence Study of Thick Ga(In)AsN Layers Grown by Liquid-Phase Epitaxy. J. Phys. Conf. Ser. 2016, 700, 012028, doi:10.1088/1742-6596/700/1/012028.
9. Donchev, V.; Asenova, I.; Milanova, M.; Alonso-Álvarez, D.; Kirilov, K.; Shtinkov, N.; Ivanov, I.G.; Georgiev, S.; Valcheva, E.; Ekins-Daukes, N. Optical Properties of Thick GaInAs(Sb)N Layers Grown by Liquid-Phase Epitaxy. J. Phys. Conf. Ser. 2017, 794, 12013.
10. Donchev, V.; Milanova, M.; Asenova, I.; Shtinkov, N.; Alonso-Álvarez, D.; Mellor, A.; Karmakov, Y.; Georgiev, S.; Ekins-Daukes, N. Effect of Sb in Thick InGaAsSbN Layers Grown by Liquid Phase Epitaxy. J. Cryst. Growth 2018, 483, 140–146, doi:10.1016/j.jcrysgro.2017.11.023.
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12. Dhar, S.; Halder, N.; Mondal, A. Nitrogen-Related Deep Levels in Dilute III-V Nitrides Grown by Liquid Phase Epitaxy. 4–8.
13. Dhar, S.; Halder, N.; Kumar, J.; Arora, B.M. Observation of a 0.7 EV Electron Trap in Dilute GaAsN Layers Grown by Liquid Phase Epitaxy. Appl. Phys. Lett. 2004, 85, 964–966, doi:10.1063/1.1779346.

Reviewer 2 Report

Comments and Suggestions for Authors

Donchev and Milanova report on the potential of the contactless surface photo-voltage method for fast and reliable control of GaAs-based solar cells directly on the epitaxial hetero-structures prior to metallization and photolithography processes. The manuscript constitutes an excellent contribution to a hot field, it is easy to read, well referenced and the experimental part was competently executed and thus, it is my pleasure to recommend it for publication in Coatings.

Author Response

We are thankful to the reviewer for his/her comments.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript is based on surface photovoltage methodology, a non-destructive technique that is very useful to understand the photovoltaic quality of grown solar cells. 

In my opinion, the manuscript is worthy of being published.

I have a few comments: 

1. In the abstract, the authors state "SPV methods for fast and reliable control of GaAs-based solar cells." Can you clarify what types of control are being referred to? 

2. Currently, CdTe-based solar cells are being intensively developed and have demonstrated above 20% efficiency (https://doi.org/10.1038/s41467-022-35442-8). Therefore, references should be added for CdTe-based solar cells in line 32 of the Introduction section. 

3. In the Introduction section line 61, please add references.

4. In the introduction section, line 69, please explain about the original interpretation 

5. In line 101 of the Materials and Methods section, the authors stated that GaAs and InGaAsN are nearly lattice-matched. However, they did not provide any experimental basis or references to support this claim.

6. Please check for typos, such as Line 86, and improve the readability of graphs (inset figures, Figure 4 and 8).

Author Response

We are thankful to the reviewer for his/her comments and remarks. Below are our answers to the remarks.

1. In the abstract, the authors state "SPV methods for fast and reliable control of GaAs-based solar cells." Can you clarify what types of control are being referred to? 

We mean that by using the SPV method it is possible to obtain trustworthy information about key parameters of the solar cells.  We do not have in mind a control of the reliability.

2. Currently, CdTe-based solar cells are being intensively developed and have demonstrated above 20% efficiency (https://doi.org/10.1038/s41467-022-35442-8). Therefore, references should be added for CdTe-based solar cells in line 32 of the Introduction section. 

We have added a new reference (No.2) on line 32 and changed the sentence to: ”Perovskites, Cd(Se, Te) thin-film, dye-sensitized, polymer and organic solar cells have been intensively developed in recent years [1–5] .”

3. In the Introduction section line 61, please add references.

We have added references [21-23] in line 61.

4. In the introduction section, line 69, please explain about the original interpretation 

Following the reviewer’s remark we made the following change to better explain the possibilities of SPV spectroscopy:

We have changed the sentences:

“In the metal-insulator-semiconductor (MIS) operation mode the SPV amplitude provides information on the optical absorption spectrum [24]. Due to an original interpretation of the SPV amplitude and phase spectra [26,30] information can be obtained about the energy band bending in the heterostructures and therefore about the direction of the photocarrier transport.”

to:

“As the SPV arises from the spatial separation of the photogenerated electrons and holes the analysis of the results provides information on both light absorption properties and transport properties of charge carriers in the semiconductor sample [24,26]. In the met-al-insulator-semiconductor (MIS) operation mode the SPV amplitude provides information on the optical absorption spectrum [24]. In particular, SPV spectroscopy has been used to assess the bandgap of various semiconductor materials including multilayered structures [30–34]. It has been shown that the SPV phase values are in the IV^th (II^nd) quadrant, when photogenerated electrons (holes) move to the bulk and/or photogenerated holes (electrons), move to the surface [26,35]. Applying the combined analysis of the SPV amplitude and phase spectral behaviours underpinned by the vector model for the SPV signal [26,35] it is possible to obtain information about the energy band bending in the heterostructures and therefore about the direction of the photocarrier transport.“

In the manuscript, we have presented SPV phase spectra of GaAs and GaAsSbN solar cells in Figures 3  and 6, respectively. It is seen that the phase values are in the IV^th quadrant in line with the direction of the built-in electric field in the structures.

5. In line 101 of the Materials and Methods section, the authors stated that GaAs and InGaAsN are nearly lattice-matched. However, they did not provide any experimental basis or references to support this claim.

We have added ref. 21 at that place. In this work (ref. 21), we have performed structural characterization of the grown InGaAsN epitaxial layers using different characterization methods: XRD, XPS, and Raman spectroscopy. The lattice mismatch between the InGaAsN layer and GaAs substrate Δa/a₀ determined from XRD spectra is 0.16%, indicating good lattice matching.

6. Please check for typos, such as Line 86, and improve the readability of graphs (inset figures, Figures 4 and 8).

Thank you for this remark.

The typo at line 86, as well as other typos, were corrected.

The insets of the figures 4 and 8 were made readable.