Effect of Substrate Deviation Angle on N-Polarized GaN on 4H-SiC

Effect of Substrate Deviation Angle on N-Polarized GaN on 4H-SiC –

PAM-XIAMEN can supply SiC substrate for GaN film growth. For specific parameters, please consult: [email protected]

1. Abstract

This work comprehensively analyzed how substrate off-axis orientation (direction and angle) governs the properties of N-polar GaN grown via hot-wall MOCVD and multi-step temperature epitaxy on 4H-SiC (000-1), critically evaluating the efficacy of two-step versus multi-step temperature protocols for optimizing crystal quality and surface morphology. The results indicate that substrate orientation significantly affects the formation of step flow patterns and surface defects: substrates with a 4 ° off-angle exhibit significant differences in the a-plane and m-plane directions, while substrates with a 1 ° off-angle m-plane are prone to hexagonal hill like structures during low-temperature growth. The multi-step temperature method can effectively suppress hill like defects, induce step bending growth, and achieve excellent surface flatness (RMS roughness as low as 1.5 nm) on specific angle substrates. This study provides a reliable temperature strategy and orientation optimization basis for the epitaxial growth of high-quality N-polarized GaN.

2. Sample Preparation

N-polarized GaN layers were synchronously epitaxially grown on SiC (000-1) substrates with orientation angles of 4 ° towards the [11-20] direction and 4 ° towards the [1-100] direction using hot wall MOCVD technology. The C-shaped SiC epitaxial substrate used in the experiment was annealed in a hydrogen atmosphere at 1340 ° C, and then pre flowed with NH3 for 10 minutes. The entire growth process was maintained at a reaction pressure of 50 mbar, and metal organic sources were used as precursors for Al, Ga, and N using trimethylaluminum (TMAl), trimethylgallium (TMGa), and ammonia gas (NH3), respectively. Firstly, all samples were deposited with AlN nucleation layer at 1300 ° C to ensure pure N polarity and promote step flow growth mode. Subsequently, a 40nm thick low-temperature GaN buffer layer (LT GaN, 850 ° C) was uniformly grown. On this basis, a two-step temperature process was adopted to grow GaN layers with a thickness of 900 nm on the buffer layer at 950 ° C, 1000 ° C, 1050 ° C, and 1100 ° C. The V/III ratio of all GaN growth stages was fixed at 251.

In the multi-step high-temperature growth, four layers of GaN were sequentially deposited on the LT GaN buffer layer: (i) 1000 ° C, V/III=251; (ii) 1050 ° C, V/III=251. The thickness of each layer is 200 nm. Fig. 1 summarizes the growth conditions and corresponding performance parameters of all N-polarized GaN samples. The surface morphology was characterized using a Bruker Dimension 3100 AFM in tap mode, while the crystal quality was analyzed using high-resolution XRD.

Fig. 1 Growth conditions, RMS surface roughness, and selected XRD FWHM curves of N-pole GaN layers grown on three types of substrates by two-step and multi-step temperature processes

Fig. 2 AFM of N-polarized GaN thin films on different off-angle SiC substrates

Fig. 2 AFM of N-polarized GaN thin films on different off-angle SiC substrates: (a)~(c) two-step heating growth process; (d-f) Multi step heating growth process

Fig. 3 Variation of surface roughness of N-polarized GaN with temperature on different C-plane SiC substrates

Fig. 3 Variation of surface roughness of N-polarized GaN with temperature on different C-plane SiC substrates, with scanning areas of (a) 20µm * 20µm and (b) 5µm * 5µm

Fig. 4 FWHM of : (a) GaN (0002); （b）GaN (10-15); （c）GaN (10-12)

3. Conclusion

This study systematically compared the growth of N-polarized GaN layers on SiC (000-1) substrates using hot wall MOCVD technology, with a focus on analyzing the effects of substrate orientation angles of 4 ° in the a-plane, 4 ° in the m-plane, and 1 ° in the m-plane on material properties. The study investigated the effect of growth temperature on two-step and novel multi-step temperature processes, and found that step flow growth mode can occur on substrates with a 4 ° angle. However, due to surface anisotropy behavior, there are significant differences in step morphology: for substrates biased towards the a-plane ([11-20] direction), both A-type and B-type step edges can exist simultaneously; In substrates biased towards the m-plane ([1-100] direction), typically only A-type or B-type single edges appear. In addition, the surface morphology characteristics of N-polarized GaN were analyzed, including the correlation between step height (ESB) and growth temperature and orientation angle.

On a substrate with a deviation angle of 1 ° towards the m plane, within the growth temperature range of 950 ° C to 1100 ° C, the diffusion length of adsorbed atoms is insufficient, resulting in the formation of hexagonal hill like structures on the surface. Although using a multi-step temperature process can suppress such hill like defects, promote step bending growth, and improve surface morphology, an increase in dislocation density has also been observed.

The study also showed that in the two-step temperature method, as the growth temperature of the top GaN layer increased, all three types of substrates exhibited a decrease in surface roughness and a narrowing of the XRD rocking curve width, indicating an improvement in crystal quality. For substrates with a 4 ° tilt angle, the difference in XRC performance between the multi-step temperature process and the two-step process at the highest temperature is not significant. However, the multi-step method has significant advantages in further reducing surface roughness, especially for N-polarized GaN grown on substrates with a 4 ° angle towards the m-plane, achieving root mean square roughness as low as 1.5nm in the range of 20 µ m × 20 µ m. Combined with the results of reduced dislocation density, it indicates that the multi-step temperature growth strategy is a highly promising epitaxial growth pathway for SiC substrates with a 4 ° angle between the a-plane and m-plane.

Whether you need SiC wafer for research or for industrial applications, please contact us email at [email protected] and [email protected].

Reference:

Zhang, H., Chen, J. T., Papamichail, A., Persson, I., Tran, D. Q., Paskov, P. P., & Darakchieva, V. (2025). Effect of substrate misorientation angle on the structural properties of N-polar GaN grown by hot-wall MOCVD on 4H-SiC (0001̄). Journal of Crystal Growth, 651, 127971.