Growth of high-quality nitrogen-polar GaN film by two-step high-temperature method.

• A two-step high-temperature growth method was adopted to grow N-polar GaN. • The influence of growth parameters on the properties of N-polar GaN was analyzed. • N-polar GaN with both smooth surface and high crystalline quality was achieved. Normally, nitrogen-polar (N-polar) GaN films with better crystalline quality tend to have poor surface morphology, whereas those with good surface morphology suffer from degraded crystalline quality. In this work, we use a two-step high-temperature (HT) growth method to grow N-polar GaN films on vicinal c-plane sapphire substrates by metalorganic chemical vapor deposition, which can effectively solve the irreconcilability between surface morphology and crystalline quality. The GaN film consists of two GaN layers (HT-GaN 1 and HT-GaN 2), which were grown at different growth parameters, respectively. The 300-nm-thick HT-GaN 1 layer was directly grown on the low-temperature GaN buffer layer under the higher V/III ratio and pressure at 1040 °C to reduce dislocation density. The 1.7-μm-thick HT-GaN 2 layer was grown under the lower V/III ratio and pressure to improve surface morphology. Besides, the growth temperature of HT-GaN 2 layer was ramped up to 1070 °C to suppress the yellow-band luminescence. Finally, high-quality N-polar GaN films with smooth surface morphology and high crystalline quality were obtained, for which the full-width at half-maximum values of (0002) and (10 1 ¯ 2) planes X-ray diffraction rocking curves are 131 and 390 arcsec, respectively, the root mean square roughness over an area of 20 × 20 μm2 is 1.88 nm, and the residual compressive stress is as low as 0.025 GPa. It is reasonably believed that this work provides an effective approach to preparing high-quality N-polar GaN films for the application of N-polar nitride devices. [ABSTRACT FROM AUTHOR]