Light Emitting Devices Based on Quantum Well-Dots

Round 1

Reviewer 1 Report

The authors provide a careful review of basic properties of quantum well-dots based light emitting devices and their associated applications. Attention is also given to keys for the epitaxial formation which emerges in the field.

The authors address the reader by following in a proper sequence starting from a comparison between quantum wells and quantum dots, which is a useful and logical way to present the material. This review article is suitable for both experimentalists and theorists working in the field, but it may also be useful for those outside the target audience.

I believe the authors have succeeded in capturing the essence of the topic in this review based on the complete, detailed and objective review of previous works. Thus I recommend this article to be published.

Author Response

Thank you very much for your review!

Reviewer 2 Report

This is an interesting review of the growth and applications of quantum well-dots (QWDs), especially for use in lasers. I think this is an interesting topic and will be a useful paper for others working in this field. That being said, there are several points that the authors need to address.

Line 47: Is there a typo in “up to 1.3xx µm”?

Lines 48–49 “The formation of QDs is accompanied by efficient strain relaxation that enables stacking 20 and more QD layers with high crystalline quality.”

Can the authors explain what they mean by this? QDs are typically compressively strained to the surrounding matrix material, which together with energy quantization, gives rise to the large increase in emission energy from QDs compared to the band gap of the equivalent bulk material. If the strain was relaxed plastically, we would not see this shift in the emission or indeed expect to see bright emission from the QDs at all due to dislocations etc.

Lines 49-52

Presumably the width of the QWs is a factor in how many can be stacked. If just a single QW is grown with a width exceeding some threshold value, it will undergo plastic strain relaxation. The narrower the QW, the less strain energy is built up and the more QWs than can potentially be stacked. The authors should explain how wide these QWs are.

Lines 61–63 “In case of InGaAs/GaAs 61 QWs there are fluctuations of In composition that act as localization potential for electrons and holes [13].”

The authors may wish to note that similar effects of QD-like emission from In-rich regions have also been observed in InAlAs/InP (see for example: https://doi.org/10.1364/OE.24.023198).

Line 118: “digress” should be “degrees”.

Line 153: When the authors first introduce the concept of a QWD, I think a schematic drawing of a QWD would be very helpful in terms of showing the reader what they are talking about. It is hard to imagine how something can have mixed 0D/2D dimensionality. Since this is central to explaining their behavior later in the paper (e.g. on p12 and in other places) it would be good to get this point across at the beginning.

Line 170: The precursors are usually “tri-“ and not “three-“ e.g. trimethylgallium

Line 171: More growth condition details are needed? What is the structure that is grown (layer thicknesses, compositions etc)? At what temperatures are these samples grown? What growth rates and V/III ratios are required for high quality growth?

Figures 1–3: To help the reader, the authors should point to the QWDs in each of these figures. The conventional QDs are obvious but the QWD features are subtle and so should be pointed out. Perhaps a high-resolution image of an individual QWD feature as an inset would also be helpful.

Lines 352–353: “The mesas arrays were formed by electron beam lithography and etching from three types of GaAs/AlGaAs heterostructures.” Is there an error here? Should it be InGaAs/GaAs instead of GaAs/AlGaAs?

Lines 381-384: “The spectrum of the QD sample contains a dominant peak at 1270 nm due to the emission from the ground-state optical transition as well as weaker peaks at lower energies due to the emission from the first and second excited-state transitions.” I think there is an error here. Emission from excited states should be at higher energy than the ground state. Do the authors mean shorter wavelength?

Figure 3: At what temperature were these PL measurements done? Also, in 3a, it seems the spectra are normalized to the maximum intensity spectral feature. It would be interesting to know which of the three samples had the most intense emission. I assume it is the QW sample but this would be interesting information (maybe added as an inset). At what detection wavelength were the time-resolved measurements in 3b done? For example, if the authors chose the peak (i.e. ground state transition) wavelength then why would they expect to see contributions to the lifetime from the excited state to ground state relaxation emission? More detail of this measurement is needed.

The authors should also comment on the fact that the QW lifetime is longer than the QD lifetime – in my experience, QDs usually have longer lifetime than QWs.

Line 425: It would be helpful to have a schematic showing the laser structure, an more detail about facet coatings, heat sinking, waveguide design etc. I assume the waveguide, QWD layers and GaAs spacers were all undoped but this should be clarified. Also, did the authors place a doped GaAs cap on top of the structure to prevent oxidation of the AlGaAs top barrier?

Section 3.4: It would be nice to include the lasing spectra for these samples to allow the reader to see for themselves any differences in spectral shape or peak intensity.

It would also be helpful for the authors to explain their observed results. For example, why does adding more QWD layers produce a red-shift in the emission? Why does increasing the cavity length red-shift the emission? Why does increasing the number of QWD layers increase the threshold current? How do these threshold currents compare to state-of-the-art in QW and QD lasers?

Based on these results, it seems that 2 QWD layers seems to give the best performance. Do the authors agree with this statement? Either way, they should discuss which of their designs is optimal and why based on the data -presented.

Lines 476-480: The authors should include state-of-the-art T₀ values for QW and QD lasers here to allow the reader to compare their measured value for the QWD laser directly. In addition, the authors state that the cause of the “N-shaped” temperature response of the threshold current is due to a bimodal size distribution of the QWDs and then cross-reference Section 3.1 but they did not discuss bimodality in 3.1. They either need to change this discussion or add results showing QWD bimodality to section 3.1.

Lines 487-488: “We note that stacking as many as 10 layers of QWD without any strain compensation does not deteriorate the device performance (Table 2) as could be expected for QWs.” As per my earlier comment, the number of stacked layers of QWs is going to be dependent on how thick they are and hence how much strain accumulates. They need to clarify this point throughout the paper.

Lines 504-505: “3 mm long broad area laser based on 2 QWD layers showed CW output power of 8.8 W at room temperature. Maximal wall-plug efficiency of 37% is achieved at 2.8 А (Figure 13, a).” The authors should put these values in context by comparing them to the state-of-the-art for QD and QW lasers.

Lines 511-512: “The decrease in differential efficiency at high injection currents (starting from ~40 A) is likely due to overheating.” Did the authors have the laser mounted on a heat sink?

Lines 548-552: Again, a schematic of the laser structure together with details of coatings/passivation, heat sinking, etc would be helpful.

Figure 16: The resolution of this figure is poor and should be improved. It is very difficult to read the inset text. The inset should be made larger or extracted as a figure in its own right. Also, what is the significance of the point in yellow for the QWDs at 30 µm?

Section 3.5.2: In the discussion of state-of-the-art microlasers, for the sake of easy comparison to the results presented here, the authors should make it clear in each case which results are from QW, QD or QD or QWD devices.

General comments

In places the quality of the English could be improved.

If any figures are taken from previously published work, those papers need to be referenced and the authors should obtain any copyright permissions required to reproduce the figures here.

Author Response

Please see the attachment

Author Response File: Author Response.docx