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We report on growth and characterization of epitaxial lateral overgrown (ELO) (112-0) a-plane GaN by metalorganic
chemical vapor deposition (MOCVD). The ELO samples were grown using a SiO2 striped mask pattern consisting of 4
&mgr;m wide open windows and 10 &mgr;m or 20 &mgr;m wide SiO2 stripes. Different growth rates in Ga- and N-wings along with
the wing tilt create a major obstacle for achieving a fully coalesced flat surface in ELO-GaN. To address this problem we
have employed a two-step growth method that is able to provide a high height to width aspect ratio in the first growth
step followed by enhanced lateral growth in the second step by controlling the growth temperature. Depending on the
growth conditions, lateral growth rate of the wings with Ga-polarity were from 2 to 5 times larger than that of the N-polarity
wings. We investigated the effects of growth parameters on wing tilt, which was observed to be ~0.25° from the
Kikuchi lines using large angle convergent beam electron diffraction (LACBED) and accompanied by some twist (0.09°)
between the two opposite wings. Transmission electron microscopy (TEM) results showed that the threading dislocation
density in the resulting fully coalesced overgrown GaN was reduced from 4.2×1010 cm-2 in the window area to 1.0×108 cm-2 in the wing area, and that the wing areas contained relatively high density of basal stacking faults, 1.2×104 cm-1.
The recombination of carriers/excitons localized at stacking faults was evident in far-field near bandedge
photoluminescence (PL) measured at 10 K. Moreover, atomic force microscopy (AFM) measurements revealed two
orders of magnitude higher density of surface pits in window than in wing regions, which could be decorating
dislocation termination on surface.
Time-resolved PL measurements for the a-plane ELO-GaN samples revealed biexponential decays. The recombination
times were significantly increased ( &tgr;1
=80 ps and &tgr;2
=250 ps) compared to the standard a-plane epitaxial layers (<45 ps),
and ratio of the slow decaying component magnitude to the fast decaying one was more than 1.5, showing considerable
reduction of nonradiative centers by lateral overgrowth. In addition, room temperature near-field optical microscopy
studies revealed the improved optical quality in the wing regions of the overgrown GaN. As revealed from far-field PL,
the band edge luminescence at room temperature was more than two orders of magnitude weaker than the yellow
luminescence. Therefore, the overall spectrally integrated near field PL was collected, and its intensity was noticeably
stronger in the wing areas with both Ga and N polarity. The much weaker emission at the windows and meeting fronts of
the two opposite wings were consistent with the observations of high density of dislocations in the window regions and
new defects originating at the meeting boundaries from TEM.
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