New Insights Reached via Graded-Interfaces Modeling: How High-Power, High-Efficiency Mid-Infrared QCLs Work

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1, page 2 line 86: Cite figure 1 a here to clearly illustrate the states. Also, please briefly introduce which state/curve in the conduction-band diagram mean to

2, The curves for g-g3 in Figs. 1–2, 6, and 10, as well as states 1–3 in Fig. 3, are difficult to distinguish. Please use colors other than grey for these curves.

3, In Figs. 8a, 12a, and 13a, it is difficult to distinguish which curve corresponds to voltage and which to power. Please clarify or adjust the labeling for better readability.

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

The authors develop Graded-Interfaces Modeling to get High-Power, High-Efficiency Mid-Infrared QCLs.

 

Recorded high device performance is achieved The theory and experiment are in good agreement. Their results are impressive.

There are only a few minor issues that should be discussed in the article:

The article mainly uses IFR to characterize interface properties, which is also in good agreement with the results. But will this simplification's effectiveness be restrained to certain structures or wavelengths? Besides IFR, factors such as interface interdiffusion can also bring performance degradation or non idealization. Should those also been taken into account?

Under the conditions of MOCVD and GSMBE, the dynamics of the interface are different throughout the entire process. Will these material differences be considered? It should be explained in the text.

 

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

The MOVPE method allows for the mass growth of a wide class of optoelectronic heterostructures. At the same time, during epitaxy of multi-period (about 400–900 layers in the active region) heterostructures of quantum cascade lasers (QCLs), an effect associated with the temporal instability of precursor flows is observed. The use of ultrasonic systems for monitoring the precursor concentration in real time along with the use of QCL active regions with low sensitivity to layer thickness fluctuations in different cascade periods [DOI: 10.1109/jqe.2010.2048015] makes it possible to compensate for this effect. When growing QCLs by the MOVPE method with an active region based on strain-compensated heteropairs, the effects of phase separation, surface roughness, and changes in the InAlAs composition are observed. As a result, the output characteristics of these QCLs in some cases are significantly inferior to their analogs grown by molecular beam epitaxy (MBE).

Before a manuscript is accepted, authors are asked to answer the following questions:

1.     In the introduction section, the modeling of gradient interface was mentioned. At the same time, the previous results devoted to the modeling and practical implementation of MOVPE-grown structures with gradient interface are not summarized. The following papers should be added: 10.1021/acsphotonics.7b00133, 10.1016/j.jcrysgro.2016.11.029. 10.1109/JSTQE.2017.2677899. The latter papers also reflect the APT characteristic of QCL, which should also be mentioned in the introduction section.

2.     MS is devoted to modeling, but to explain the results of modeling, practical results should be added to MS.

3.     In the second and third section, the authors analyzed the design in Ref. 3. At the same time, this structure was based on a simple heterostructure with strain compensation. The authors compared these results with the step-tapered design developed for MOVPE growth technique. In fact, the most advanced design is the step-tapered, which allowed to reduce the leakage into continuum. Can the authors add the simulation of the design presented in Ref. 10.1038/srep23595? This design is the most suitable instead of the design discussed in Ref. 3, since it is based on composite QWs.

4.     Page 9: " We attribute the difference to two issues: (a) the difference in 325 αw values (i.e., 1.3 cm-1 vs. 0.5 cm-1 ) which gives an αm/(αm + αw) ratio of 1.29;"

Please clarify the background doping level of the MOCVD system used and the type of doping (p or n-type). If it is p-type doping due to TMG-induced carbon incorporation etc., please clarify its effect on the waveguide loss and band diagram.

5.     The authors previously mentioned the difference between the doping level and the position of the flip-flop current [10.1515/nanoph-2023-0687]. The same case in section 4.2.1. What is the reason for this? Background doping? How does the group calibrate this level? Bulk layer growth or SL test structure?

6.     The authors noted that the threshold current density is the same as in MBE growth. But the output characteristics are modest (5.5 W) compared to Professor Razeghi's results (11 W). The roll-over current is about 1.5 A instead of 3 A, but the doping level is 1.4 times different. What is the reason for this?

7.     In fact, the thickness of the AlAs inserts is about 3 ML. Such a small value can be well controlled by MBE. The roughness of the grown MOVPE layers remains an open question. Do the authors evaluate this using XRR analysis?

8.      It is known that the maximum value of mechanical stress of the active region layers, achieved by the MBE method, is significantly higher compared to MOVPE. Given the modeling performed, what are the prospects for growing by MOVPE the design presented earlier in Ref. 2, and, moreover, an optimized design taking into account the thickness gradient?

9.      Previously, Professor Strasser's group demonstrated a significant difference in the lasing wavelength of gradient-free and optimized designs. As a possible confirmation of the modellings performed, the authors can grow simple short-period SLs to show the position of the EL peak etc.

Author Response

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Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for your detailed answers.

I would disagree with the authors' answer to the question about comparison with the DFG design discussed in Ref. 10.1038/srep23595. DFG also requires high output optical power along with nonlinearity to implement THz radiation. In suppl. mat. the authors provide a characterization of uncoated FP lasers on a doped substrate, where an output power of 5 W is shown for a 12-μm-wide, 4.9-mm long device. However, given the lack of data on buried lasers, an exact comparison is not possible with Ref. 3. 

In any case, even if Ref. 3 represents a more advanced design and simple from the point of view of practical implementation due to the use of a single heteropair, then along with the theoretical comparison, the growth of the structure from Ref. 3 by the MOCVD and comparison with the simulation results suggests itself. Of course, the originality of the research will be lost, but it will become clear to what extent the use of the STA-region gives an advantage. A separate question is whether it is possible to implement the STA-region on industrial MOCVD reactors.

 

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