Effectiveness of multiple-pair buffer layer to improve the GaN layers grown by metalorganic chemical vapor deposition

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Abstract
High-quality GaN epitaxial layers with a multiple-pair buffer layer have been grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. Each pair of buffer layer consists of a 300 Å thick GaN nucleation layer grown at a low temperature of 525 °C and a 1–4 μm thick GaN epitaxial layer grown at a high temperature of 1000 °C. The GaN samples with a multiple-pair buffer layer are characterized by double-crystal x-ray diffraction (DC-XRD), Hall method and photoluminescence (PL) at 300 K, and etch-pitdensity measurements. The optimized condition to obtain the best quality of GaN epitaxial layers is to grow the four-pair buffer layer with a pair thickness of 4 μm. The GaN samples with the optimized buffer layer exhibit a narrow full width at half maximum (FWHM) of 150 arcsec and a strong intensity in DC-XRD, a high electron mobility of 450 cm2/V s, a low background concentration of 3×10 17 cm −3 , a low etch-pit density of mid- 10 5 cm −2 , and a narrow FWHM of 56 meV in PL spectrum.