Materials Science Laboratory
Crystal growth of semiconductor films generally proceeds under a combination
of nucleus and spiral growth modes. In this study, we tried to clarify
the mechanisms of nucleus and spiral growth modes by using selective-area
metalorganic vapor phase epitaxy (MOVPE) of GaN.
First, a SiO2 mask was deposited on a GaN (0001) bulk substrate with low density of threading dislocations. Then, the SiO2 mask was patterned by photolithography so that it would have many hexagonal
openings with the diameter of 16 µm. Finally, selective-area MOVPE
of GaN films was performed on the mask-patterned GaN substrate. Source
gases were ammonia and trimethylgallium. The surfaces of GaN were observed
by atomic force microscopy (AFM).
A step-free GaN surface (an atomically smooth surface without any monolayer step) was formed by nucleus growth within a mask opening having no screw and mixed-type dislocations [1]. On the other hand, growth spirals were observed on a GaN surface formed within a mask opening having screw and mixed-type dislocations (Fig. 1) and the growth mode was spiral growth. The degree of supersaturation, σ, which is the driving force of crystal growth, is generally estimated from the interstep distance of a growth spiral. Figure 2 shows the nucleus and spiral growth rates plotted as a function of σ. The spiral growth rate increases parabolically with increasing σ, while the nucleus growth rate has very low values in the σ range [2]. Solid and dotted lines in Fig. 2 are fitting results we obtained using a crystal growth theory [3]. It can be seen that our experimental results are well explained by the crystal growth theory.
In summary, selective-area MOVPE enables us not only to fabricate step-free
GaN surfaces but also to clarify the fundamental mechanisms of crystal
growth including nucleus and spiral growth.
[1] T. Akasaka et al., Appl. Phys. Express 2 (2009) 191002.
[2] T. Akasaka et al., Appl. Phys. Lett. 97 (2010) 141902.
[3] W. K. Burton et al., Phil. Trans. Roy. Soc. A 243 (1951) 299.
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