Figures

Figure 1

Structure of the patterned GaN on sapphire template typically used to study epitaxial lateral overgrowth of GaN. A 30 Å amorphous silicon nitride film is formed in situ immediately after growing the GaN epilayer. Windows in the mask are opened by reactive ion etching.

Figure 2

HR-TEM view of a cross section of the GaN overgrown on a masked and patterned GaN layer. The silicon nitride mask is about 30 Å thick.

Figure 3

SEM top view of a star pattern mask. No parasitic nuclation is observed in the inner masked area whose diameter is 650 µm nor between the stripes. The marker is 100 µm.

Figure 4

Schematic view of the GaN stripes at the beginning of the regrowth on the patterned substrates of figure 1.

Figure 5

Schematic cross section of the GaN stripes for longer growth time. Dashed lines are different intermediate stages of the development of these stripes. During the growth, the two edges of the top C facet are moving along linear trajectories (provided constant growth rates) indicated by arrows. For undoped GaN, the top C facet vanishes above the crossing of the two trajectories. Note that if depends only on the structural properties of GaN, is determined by the anisotropy of the growth i.e. the ratio of vertical to lateral growth rates.

Figure 6

SEM cross section of lateral overgrowth of undoped GaN in standard conditions. Though perfect coalescence is obtained (without voids at the coalescence boundaries due to very slow growth rates), no smoothing is achieved.

Figure 7

SEM cross section on Mg doped GaN overgrown at 1080°C on patterned substrate. Flat surface, contrasting with figure 6, is obtained due to the favorable growth anisotropy induced by the Mg doping. The mask appears as a very thin line above the dashed filled rectangle.

Figure 8

Schematic cross section of the Mg doped GaN. During the growth, the two edges of the top C facet still move along linear trajectories but due to doping, these trajectories are now diverging and the two slants are vanishing during coalescence as shown by the intermediate shapes (dashed lines).

Figure 9

Schematic principle of the two step GaN lateral overgrowth process. In step (1), undoped GaN is grown at 1080°C. In these conditions, the top C facets vanish. In step (2), under the same growth conditions but due to Mg doping, the C facets become the slow planes therefore reappearing and leading to planarization by expansion. The arrows represent the trajectories of the C facet edges.

Figure 10

SEM cross section view of lateral overgrown GaN following the two step process. The sample was not metallized in order to observe resistivity contrast between (1) undoped (dark gray) and (2) Mg doped (light gray) GaN. In (1) a peaky surface as in figure 6 is obtained, the dashed line was drawn as a guide for the eye. In (2) the top C facets have reappeared and expand following the arrows. Note that voids are created in the deep valley of (1) during (2) mainly due to diffusion limited growth.

Figure 11

SEM tilted view of the cross section, perpendicular to the stripes direction, of GaN overgrown using the two steps process on patterned GaN/Saphir substrate. Very smooth surfaces are obtained. The arrow points to an hexagonal pit formed at the coalescence boundary. Along the cross section a periodic grating of voids is seen.

Figure 12

SEM cross section of two-step GaN overgrowth on GaN masked layer observed without metallization at low temperature. Dashed vertical white lines are the coalescence boundaries. Plain white lines localize the border between steps 1 and 2. The volume filled during step 2 appears to be inhomogeneous, the volume labeled 2S and 2T are filled by expansion of the slants and the top C facets respectively. This is confirmed by cathodoluminescence spectra plotted in the inset. Volume 1 has a typical spectrum of undoped GaN whereas spectra of the volumes 2S and 2T correspond to GaN with low and high concentration of Mg respectively.

Figure 13

TEM cross section view of GaN overgrown with the two-step process.

Figure 14

Schematic view of the two-step process. At the end of step 1 with essentially no lateral expansion, the template for the next growth step is formed by section of the misoriented columns by the slants. During step 2 where anisotropy force mainly lateral expansion, the misorientation of the template could be replicated in stacked lamellae.

Figure 15

Low temperature luminescence spectrum measured on undoped GaN grown on top of a pseudo GaN substrate obtained by the to two-step overgrowth process. Bound excitons give very sharp (FWHM<1 meV) and intense peaks. Free exciton A, B and lines attributed to excited states of A are resolved.

© 1998 The Materials Research Society