The Surface Morphology Evolution of GaN Nucleation Layer during Annealing and Its Influence on the Crystal Quality of GaN Films

Round 1

Reviewer 1 Report

The manuscript is of great scientific interest, but:

1. Introduction is poor. It could be improved by the description of the articles reporting on the dislocation evolution mechanism related to the crystal quality of the GaN epitaxial layer in terms of NL annealing time. It will help to show the problem.
2. It is unclear from the article what kind of morphology is more preferable.
3. Application of GaN coatings with preferable morphology should be shown.

Author Response

1、Introduction is poor. It could be improved by the description of the articles reporting on the dislocation evolution mechanism related to the crystal quality of the GaN epitaxial layer in terms of NL annealing time. It will help to show the problem.

After the anealing of NL, the GaN NL will transform to three-dimensional (3D) nucleation islands (NIs). TDs are usually generated at the boundaries of NIs and inside the NIs. Thus, low density NIs are beneficial to reduce the TD density in GaN film during the lateral growth of 3D islands. During my experiments, the NIs size and density have great relationship with annealing time, so it is necessary to investigate the 3D NIs evolution under different situation.

2、It is unclear from the article what kind of morphology is more preferable.

TDs are usually generated at the boundaries of NIs and inside the NIs. Thus, low density NIs are beneficial to reduce the TD density in GaN film during the lateral growth of 3D islands.

 

3、Application of GaN coatings with preferable morphology should be shown.

TDs are known to play detrimental effects on device performance, such as non-radiative recombination centers and reduce the lifetime of devices by acting as diffusion channels between the electrodes. Thus, high performance GaN coatings with low dislocation density are used as high-efficiency light emitting diodes and laser diodes. Flat surface morphology of GaN film is good at obtaining multiple quantum well structure with abrupt interface.

Reviewer 2 Report

The authors in their manuscript entitled "The surface morphology evolution of GaN nucleation layer during annealing and its influence on the crystal quality of GaN films", presented research on the impact of annealing time on the surface morphology of the GaN seed layer and thus on the crystal quality of GaN layers . The manuscript contains interesting research results that allowed the authors to present the structure evolution of GaN nucleation and 3D growth mechanism. According to the Reviewer, it is a valuable asset of this work.

I have only one question:

Line no 121 – it is written „… the GaN NLs before and after different annealing time are characterized by HRXRD (002) plane rocking curve scan, as shown in Fig. 5.” Annealing time is 0, 2 and 4 min. What does it mean 0 min of annealing time and before different annealing time? Does before different annealing time mean the moment before the substance is heated to 1080^oC? I suggest clarification.

Author Response

1、Line no 121 – it is written „… the GaN NLs before and after different annealing time are characterized by HRXRD (002) plane rocking curve scan, as shown in Fig. 5.” Annealing time is 0, 2 and 4 min. What does it mean 0 min of annealing time and before different annealing time? Does before different annealing time mean the moment before the substance is heated to 1080^oC? I suggest clarification.

After nucleation growth, the chamber temperature was ramped from 530℃ to 1080 ℃ and then maintained at this temperature for 0, 2 and 4 min for annealing treatment. We called the maintenance time at high temperature 1080 ℃ is NL annealing time.

Reviewer 3 Report

The GaN films grown on sapphire and annealed at high temperature with different times are analyzed. Three different time of annealing are considered. Two methods of analysis AFM and XRD are applied, that in fact is not much. Obtained results are interesting. The schematic model based on obtained experimental results of GaN film growth is proposed and well described. The model is realistic and interesting, gives impact on understanding about kinetics and mechanisms of processes taking place in this system.

Some questions and remarks

1. Why in the step 4 the reflectance has oscillatory character?
2. Reference [14] is cited but it is not clear for what reason, what information authors want to confirm by this citation, because sentence looks trivial.
3. Authors must clearly write which step 2 or 3 correspond with annealing time. If t is step 2, it becomes not clear for other curves: 2 min, and 0 min. If it is step 3, then it is something wrong with time scale. Authors must clarify this problem, it must obvious for readers. It would be good to put the step intervals onto each curve of fig. 1 (like it is done for curve 4 min.)
4. Talking about fig. 2 authors state “For 4-min annealing time sample, the NIs’ size becomes smaller…” But from given figure it is seen opposite. Isn’t it? What means this statement?
5. Authors must clearly define the term “coalescence time”. Does the coalesce takes place not the whole annealing period? What is “coalescence boundary”? It need to define also.
6. Can authors clarify what is difference between AFM images given in fig. 2 and fig.3? Both are AFM but looks very differently. What authors mean “AFM images (5×5 μm) of bulk GaN” in this case? Which mode is in fig. 6 where AM is also? Why only before annealing are presented and not after?
7. The sentence in line 113-114 is almost the same as in line 119-120 i.e. one chapter ends with the same sentence as second starts.
8. Can authors clearly explain the difference between bulk GaN and GaN NL which XRD results are presents in fig. 4 and fig. 5.
9. Talking about XRD results, how thick is the GaN film. Can signal come from substrate? The XRD spectra of substrate (without film) is not included. Why? It is usual practice to include spectra of references.
10. The introduction is very short. Just two chapter, where in one of them is the description of presented work. Introduction must be extended. Moreover, authors present the model of growth, so the existing models must be reviewed. Introduction must be extended.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript could be accepted

Reviewer 3 Report

Authors made corrections according my remarks and answered to all my questions.