In this paper, an excellent Ohmic contact to p-GaN with a low specific contact resistance (ρc) of 2.0 × 10−5 Ω·cm2 is demonstrated using a patterned sapphire substrate (PSS) and oxidized Ni/Au contacts. GaN epitaxy with high crystal quality on the PSS, confirmed by high-resolution x-ray diffraction, played a key role in the improved Ohmic contact to p-GaN. The edge dislocations were annihilated during the epitaxial process on the PSS to afford a low surface dislocation density, which was in accordance with the results of transmission electron microscopy and cathodoluminescence spectroscopy. Furthermore, a reduced Fermi level and enhanced activation efficiency of Mg with suppressed segregation around the dislocations were demonstrated by Kelvin probe force microscopy and contact Hall measurements, respectively. A GaN p-channel metal oxide semiconductor device fabricated on the PSS displayed a twofold higher forward current density and superior gate controllability compared with that fabricated on a conventional sapphire substrate.
Hetero-epitaxial growth of GaN often leads to high density of threading dislocations, which poses a significant challenge to the promotion of the performance of GaN-based devices. In this study, we address this issue by utilizing an Al-ion implantation pretreatment on sapphire substrates, which induces high-quality regularly arranged nucleation and promotes the crystal quality of GaN. Specifically, we demonstrate that an Al-ion dose of 1013 cm-2 leads to a reduction of full width at half maximum values of (002)/(102) plane X-ray rocking curves from 204.7/340.9 arcsec to 187.0/259.5 arcsec. Furthermore, a systematic investigation of GaN film grown on the sapphire substrate with various Al-ion doses is also performed, and the nucleation layer growth evolution on different sapphire substrates is analyzed. As confirmed by the atomic force microscope results of the nucleation layer, the ion implantation induced high-quality nucleation is demonstrated, which results in the improved crystal quality of the as-grown GaN films. Transmission electron microscope measurement also proves the dislocation suppression through this method. In addition, the GaN-based light-emitting diodes (LEDs) were also fabricated based on the as-grown GaN template and the electrical properties are analyzed. The wall-plug efficiency at 20 mA has risen from 30.7% to 37.4% of LEDs with Al-ion implantation sapphire substrate at a dose of 1013 cm-2. This innovative technique is effective in the promotion of GaN quality, which can be a promising high-quality template for LEDs and electronic devices.
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