Origin of lattice bowing of freestanding GaN substrates grown by hydride vapor phase epitaxy

Abstract

This paper describes a mechanism to explain the lattice bowing of freestanding GaN substrates grown by hydride vapor phase epitaxy on sapphire substrates. The freestanding GaN substrates typically exhibit a concave shape. It is revealed that the radius of curvature and lattice constant of the top surface are almost the same as those of the bottom surface. This is indicative of the complete relaxation of the GaN lattice, even though the freestanding GaN substrate exhibited a curvature. It is shown that dislocations are present in a plane normal to the growth direction in addition to conventionally known threading dislocations; these are referred to as in-plane dislocations. Based on these results, it is proven quantitatively that the extra-half planes related to the in-plane dislocations are primarily responsible for the phenomenon of lattice bowing.

Defects in Single Crystalline Ammonothermal Gallium Nitride

“…This is most likely due to unstable growth of the a-planes which have been reported to convert to inclined planes during growth. A post growth analysis of the a-wing side wall angle suggests that the growth direction transforms to [11][12][13][14][15][16][17][18][19][20][21][22] [40] (as shown in Figure 4) or [10][11] [74] at some point during the growth process. The presence of several growth facets leads to non-uniform growth and impurity incorporation in the a-wing region.…”

“…[11,12] This subsequently limits the size of nonpolar and semipolar substrates obtained by HVPE grown crystals. [9] The origin of bowing is linked with in-plane dislocations, [13] inclination of dislocations, [4] and non-uniform growth at wafer edges. [14,15] The resulting radius of curvature of HVPE grown quasi-bulk substrates is commonly below 10 m and results in inclined lattice planes and complicates subsequent polishing and epitaxial steps.…”

Visualization of dislocation behavior in HVPE‐grown GaN using facet controlling techniques

“…On the other hand, dislocations propagated in the c ‐plane (hereafter, in‐plane dislocations) are observed as dark lines . To more accurately evaluate the in‐plane dislocation density, we observed in‐plane dislocations as dark spots in CL images having the same observation area as c ‐plane‐like from two nearly orthogonal cross‐sections ( a ‐plane and m ‐plane‐like).…”

Effect of InGaN/GaN Superlattice on Lattice Curvature of GaN Layers Grown on Sapphire Substrates

“…Most nitride semiconductor devices are grown on foreign substrates such as sapphire substrates. When GaN is grown on a sapphire substrate, an upward convex warp is generated; this is because of the difference in their thermal expansion coefficients . After the separation of the sapphire substrate, the freestanding GaN substrate can be obtained, which exhibits the concave lattice bowing after polishing.…”

“…However, the other factors causing the bending of a freestanding GaN wafer also have to be considered because defect density distribution alone cannot explain the bending mechanism of the freestanding GaN substrate . Therefore, we have proposed the insertion of the in‐plane extra half plane as a possible mechanism for the origin of the bending of the GaN substrate. Furthermore, we believe that the optimization of the interlayers may improve the bending of the GaN substrate.…”