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TECHNOLOGY GaN SUBSTRATES


Fig. 5: X-ray rocking curves of the symmetric (0 0 2), and skew-symmetric (3 0 2) and (1 0 2) reflections for the freestranding HVPE-GaN (courtesy of M. Bobea, NCSU).


Making great use of these seeds, the part of our team based at the IHPP PAS has developed a combination of mechanical and chemo-mechanical processes for transforming the surfaces of these wafers into an epi-ready state. Using this technique, one can obtain surfaces with uniform bilayer steps and a root-mean- square (RMS) surface roughness below 0.1 nm (see Figure 1b).


To open the door to a new era of GaN bulk crystal and substrate production, the hybrid HVPE-ammonothermal growth technique has to succeed on two fronts: the perfection associated with the ammonothermal GaN seeds must be maintained in the HVPE-grown material; and it must be possible to grow thick HVPE boules to multiply the ammonothermal GaN crystals. IHPP PAS and Ammono S.A. have triumphed in both areas while working on a two- year project that finishes this July and has been backed by $1 million by the Polish National Center for Research and Development.


One of the highlights of this effort has been the formation of 1 mm-thick HVPE-


GaN crystals, which were crystallized on 1-inch Ammono-GaN wafers in a few hours (see Figure 2). This additional GaN grown by HVPE is macroscopically flat and free from cracks or pits. HVPE conditions governed the growth rates, which ranged from 120 μm/h to 240 μm/h. As desired, only one growth center on the entire 1-inch crystal surface after the growth process was observed (see Figure 3).


By slicing the freestanding wafers of HVPE-grown GaN from the ammonothermally grown seeds, members of our team were able to form about 300 μm-thick GaN substrates that replicated the crystalline quality of the original Ammono seed, and were free from cracks and pits (see Figure 4a). Note that the Ammono-GaN seeds that were separated could be used for further HVPE crystallization without any limitations.


IHPP PAS has scrutinized the quality of the freestanding, HVPE-grown GaN substrates with a variety of techniques. Using a molten KOH-NaOH eutectic revealed an etch pit density, and thus a


Table I. Deviation of the surface offcut as a function of bowing radius for wafers of various sizes.


Calculated miscut deviation over the wafer for different bow radii Wafer Dia


1” 2” 3” 4”


5 m 0.3° 0.6° 0.9° 1.6°


Lattice Bow Radius


10 m 0.15° 0.3° 0.5° 0.6°


50 www.compoundsemiconductor.net June 2014


20 m 0.07° 0.15° 0.2° 0.3°


30 m 0.05° 0.1°


0.15° 0.2°


threading dislocation density, of 5 x 104


cm-2


, the same as the original


seed. This effort also revealed three types of etch pits: large etch pits, which were correlated to the screw dislocations; small pits that were associated with threading edge dislocations; and medium-sized pits that originated from threading mixed dislocations (see Figure 4b). These insights showed that the structural properties of the freestanding HVPE-GaN do not differ from the structural properties of the Ammono-GaN seeds. However, IHPP PAS still needs to conduct further work to directly correlate the defects in Ammono-GaN with those found in HVPE-GaN.


Team members at NCSU have determined the structural and optical properties of the freestanding, HVPE- grown GaN. X-ray rocking curves of the symmetric (0 0 2) and skew-symmetric (3 0 2) and (1 0 2) reflections produced very intense, narrow Bragg peaks, indicative of excellent crystallinity and low dislocation density (see Figure 5). The full-width at half-maxima values of the (0 0 2), (1 0 2), and (3 0 2) reflections were 22, 17 and 35 arcsec, respectively; these values were close to the theoretical values for a perfect GaN crystal.


Meanwhile, low-temperature photoluminescence spectra acquired from the (0001) surface exhibited several well-defined bound and free exciton peaks. Dominating the spectra were two sharp donor-bound exciton emission lines at 3.471 eV and 3.472 eV that have values for the full-width at half maximum of 127 μeV and 167 μeV, respectively (see Fig. 6a). This width for the bound exciton peaks is the narrowest ever reported for GaN, confirming not only the


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