VOLUME 79, NUMBER 15 PHYSICAL REVIEW LETTERS 13 OCTOBER 1997 Formation Mechanism of Nanotubes in GaN Z. Liliental-Weber, Y. Chen, S. Ruvimov, and J. Washburn Lawrence Berkeley National Laboratory 62/203, Berkeley, California 94720 (Received 10 February 1997) A formation mechanism for so-called nanotube defects in GaN is proposed. It is shown that two related types of defects are formed: nanotubes and pinholes. Both start with V shaped facets on 10 11 polar planes. Slow growth rate on these polar planes and impurity poisoning of growth steps are suggested as being responsible for initiation of these defects. [S0031-9007(97)04246-4] PACS numbers: 61.72.Qq, 68.55.Ln, 78.55.Cr Thin film epitaxy of polar materials grown by molecular beam epitaxy (MBE) is frequently associated with the formation of structural defects. One such defect is the so-called “microtube” observed in SiC, Al 2 O 3 , and ZnO [1–3]. These defects are empty hollows extending along the growth direction. Their size in SiC is in the range of a fraction of a micrometer to several micrometers, and their density is in the range of 100 to 1000 cm 22 . Recently, characteristic defects were found in epitaxially grown GaN, which are smaller in diameter than the microtubes in SiC and, therefore, have been called nanotubes [4,5]. They always extend along the c-axis growth direction of the film. Their density was estimated to be in the range of 10 5 10 7 cm 22 : They have radii in the range 3 – 1500 nm. Frank [6] suggested that dislocations of large Burgers vector would have lower total line energy if the core was empty compared to a core filled with the highly strained lattice. According to Frank, the total energy is minimized when the empty core radius r  mb 2 8p 2 g, where b is the Burgers vector of the dislocation, m is the shear modu- lus, and g is the specific surface energy. However, this model does not fit the experimental observations on GaN [4,7]. This Letter presents a possible formation mecha- nism of these nanotubes and related “pinhole” defects. GaN samples grown by MBE and metal-organic chemi- cal vapor deposition (MOCVD) on SiC, Al 2 O 3 , and on bulk GaN crystals were studied by plan view and cross- section transmission electron microscopy (TEM). The latter is critical to distinguish shallow indentations from defects that extend deeply into the layer. Both kinds of “holes” when studied in plan view have a perfect or slightly elongated hexagonal shape [Fig. 1(a)]. It is likely that some defects that have been assumed to be “nanotubes” do not have a substantial penetration along the growth direction and should not be called nanotubes. In general, this work shows that holes which have a hexagonal shape in plan view can be located at different depths in the layer: Some start only in the near surface area [Figs. 1(b) and 2(a) and defect A in Fig. 2(b)], some start and terminate within the layer [Figs. 1(c), 1(d), and 2(c)], and some start near the interface with the substrate and extend through the entire layer [defect B in Fig. 2(b)]. To distinguish between these different defects the ones that do not have a substantial length with a constant diameter extending along the growth direction will be called as pinholes [Fig. 1(b) and Figs. 2(a)–2(c)]. Only TEM studies in cross section can clearly distinguish between a pinhole and a nanotube. The usual diameter of a nanotube in GaN is observed to be in the range of 2–40 nm, but pinholes can extend in diameter at the sample surface to a few hundred nanometers (300–800 nm). Both types of defects may FIG. 1. (a) Plan-view TEM micrograph of a pinhole in a GaN sample grown by MOCVD on Al 2 O 3 . ( b) Cross-section TEM micrograph showing a dislocation in a GaN rich in oxygen (with mixed type Burgers vector) attached to a pinhole. (c) Cross-section TEM micrograph showing two nanotubes along the growth direction aligned with a screw dislocation. This dislocation has been attracted to the tube. Note that the nanotube starts from a pinhole with V shape, and then changes to the tubular shape, and that nanotubes also terminate with overgrowth of perfect material on top. (d) Nanotubes formed above a buffer layer attached to the half-loop and nanotubes not related to dislocations (right-hand corner of the micrograph). 0031-9007 97 79(15) 2835(4)$10.00 © 1997 The American Physical Society 2835