Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy

Round 1

Reviewer 1 Report

Dear Authors,

this is a very good article!

Congratulations on your work.

Author Response

The authors would like to thank the reviewer for the positive feedback.

Reviewer 2 Report

This review article describes several advantages of the epitaxial GaN thin film and nanorod were processed by magnetron sputter epitaxial (MSE) method compared to alternative methods such as MOCVD, MBE and HVPE. The article provides many good examples as references shown in the text based on authors' previously published works (except ref. 56). It is quit good technical values and information regarding  magnetron sputter epitaxial system potentially will be used in the industry.  Suggest to add one or two published examples shown in the text from other different research groups to show the technical merits of MSE even better.    

Author Response

The authors thank the reviewer for the positive feedback and suggestions. The following paragraphs have been added to section 3.2

Although the growth of GaN film using sputtering has been reported from as early as 1972 [48], early works typically report the growth of polycrystalline GaN films [49,50]. In order for MSE to compete with established epitaxial growth techniques such as MOCVD and MBE, the GaN film must have high crystallinity, an acceptable level of impurities, and contain a low number of threading dislocations. This section describes the progress for the epitaxial growth of single crystal GaN films using MSE.

Singh, et al. [51] has epitaxially grown single-crystal GaN on (0001) sapphire substrate using DC MSE with Ar and 99.9995% pure ammonia as the working gas. Under this configuration, the growth at higher temperature (>850 °C) is limited by the Ga flux due to additional thermal cracking of ammonia. The growth was performed at 4 mTorr with two growth steps consisting of the initial GaN buffer layer growth and the GaN epilayer at 900 °C. X-ray diffraction (XRD) measurement using rocking curve and ω-2θ scan gives an FWHM of 620 and 32 arcsec respectively, indicating high degree of crystal quality, but with low angle tilt between the crystal grains. Low temperature PL measurement on a 1.7 µm thick GaN epilayer gives a strong band-edge emission at 3.48 eV, with an FWHM of 8 meV. In this ammonia-based technique, the supply of active nitrogen species is sensitive to the substrate temperature as the temperature may affect the thermal cracking of ammonia.

Daigo, et al. [41] utilized RF MSE to produce single crystalline c-plane GaN film on (0001) sapphire. In this work, only pure N₂ gas is used as the working gas. By adjusting the growth temperature and total working pressure, the crystalline quality of the GaN can be improved. By using an optimized growth condition, 1.5 µm thick GaN film was grown at 800 °C substrate temperature, 100 W RF power, and 2 mTorr total pressure. XRD rocking curve measurement gives an FWHM of 2106 arcsec while Raman spectroscopy measurement gives a E₂ (high) phonon mode peak width that is close to bulk single crystal GaN (3.5 cm^-1). The film surface shows a faceted pyramid structure with six {10-11}, which can be attributed to the limited Ga adatom diffusion under N-rich growth conditions.

Reviewer 3 Report

This paper reviewed the progress made in MSE growth of GaN thin film and  Nanorods.  MSE can produce high quality GaN thin films and Nanorods within a large substrate area makes it an interesting alternative for GaN epitaxy, Although the article lacks novelty point, but it is a complete review.

Some suggestions for author:

1.Please improve the clarity of the Fig2/5/8/12..

2. It is recommended to add a table to summarize the advantages and disadvantages of MOCVD CVD MBE PVD MSE.. to improve the readability.

Author Response

The authors thank the reviewer for the suggestions. All the mentioned figures have been modified to improve clarity, and Table 1 has been added to summarize the key differences between MOCVD, MBE, and MSE.