Maskless Lateral Epitaxial Overgrowth of GaN on Sapphire
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315 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
EXPERIMENT Bulk (>2jtm) GaN layers were grown on sapphire using a standard two-step process. 2 1 These layers were coated with photoresist (PR) and patterned using standard UV photolithography, such that 5 pim wide PR stripes were formed, with 20 gim period and oriented in the GaN direction. Samples were etched using C12 reactive ion etching, resulting in all GaN being removed except the material beneath the PR stripes. Etching in this manner resulted in stripes with some etch damage on the upper surfaces, and occasionally areas between stripes where material ('grass') remained on the sapphire. Examples of both of these features are shown in Fig. 1. It has been found in the present study that the selectivity of the LEO growth is not affected by the features in Fig. 1(b).
(a)
Figure 1. Scanning electron micrographs of as-processed GaN stripes, showing (a) etch damage on upper comers of stripe; (b) 'grass', or post-etch residual material. The LEO GaN was deposited at 76 Torr in a H 2 carrier, with an NH3 flow fixed at 1.77 slpm. Two trimethylgallium (TMG) flows (52 and 104 gmol/min) and three surface temperatures (10 15°C, 1060'C, and 1100*C) were studied. The approximate surface temperature was calibrated using an optical pyrometer. Growth durations of 15 min and 30 min were used for fTMG = 104 jimol/min and 52 jimol/min, respectively, to supply equimolar amounts of Ga to both series of samples. Uncoated samples were characterized in cross section by scanning electron microscopy (SEM) using a JEOL 6300F field emission microscope operating at 15 kV. Specimens for transmission electron microscopy (TEM) were prepared by wedge polishing followed by standard Ar+ ion milling and images were recorded on a JEOL 2000FX microscope operated at 200 kV. Atomic force microscopy (AFM) of LEO GaN surface morphology was carried out on a Digital Instruments D3000 scanning probe microscope. X-ray diffraction was performed using four-bounce Ge(220)-monochromated Cu-Kc radiation in a four-circle diffractometer operating in receiving slit mode, with a 1.0 mm slit on the detector arm. Specifically, rocking curves of the GaN 0002 peak were measured, with the scattering plane perpendicular to the stripe direction. In this orientation, the rotation (rocking) axis is parallel to the stripe direction. RESULTS The maskless LEO stripe morphologies that result from 15 min of growth at a TMG flow of 104 jmol/min (nominal V/II = 750) are shown in Fig. 2. At the lowest temperature, the LEO GaN is bound by the (0001) basal plane and both inclined {112 n} (where n m2) and vertical {11 20) sidewalls. As the growth temperature is increased, the {11 20) sidewalls dominate and the lateral growth rate increases, which has been observed previously on LEO over an Si0 2
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mask. 7 At all three temperatures studied, the LEO GaN grows vertically and laterally from the upper part of the 'seed' GaN stripe, and not from the base of the {11 20) seed sidewalls. This is believed to be due
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