This paper reports that the GaN thin films with Ga-polarity and high quality were grown by radio-frequency molecular beam epitaxy on sapphire （0001） substrate with a double A1N buffer layer. The buffer layer consists of a high-temperature （HT） A1N layer and a low-temperature （LT） A1N layer grown at 800℃ and 600℃, respectively. It is demonstrated that the HT-A1N layer can result in the growth of GaN epilayer in Ga-polarity and the LT-A1N layer is helpful for the improvement of the epilayer quality. It is observed that the carrier mobility of the GaN epilayer increases from 458 to 858cm^2/V.s at room temperature when the thickness of LT-A1N layer varies from 0 to 20nm. The full width at half maximum of x-ray rocking curves also demonstrates a substantial improvement in the quality of GaN epilavers by the utilization of LT-A1N layer.
This paper reports on N-, mixed-, and Ga-polarity buffer layers are grown by molecular beam epitaxy （MBE） on sapphire （0001） substrates, with the GaN thicker films grown on the buffer layer with different polarity by hydride vapour epitaxy technique （HVPE）. The surface morphology, structural and optical properties of these HVPE-GaN epilayers are characterized by wet chemical etching, scanning electron microscope, x-ray diffraction, and photoluminescence spectrum respectively. It finds that the N-polarity film is unstable against the higher growth temperature and wet chemical etching, while that of GaN polarity one is stable. The results indicate that the crystalline quality of HVPE-GaN epilayers depends on the polarity of buffer layers.
GaN layers with different polarities have been prepared by radio-frequency molecular beam epitaxy （RF-MBE） and characterized by Raman scattering. Polarity control are realized by controlling A1/N flux ratio during high temperature A1N buffer growth. The Raman results illustrate that the N-polarity GaN films have frequency shifts at A1 （LO） mode because of their high carrier density; the forbidden A1 （TO） mode occurs for mixed-polarity GaN films due to the destroyed translation symmetry by inversion domain boundaries （IDBS）; Raman spectra for Ga-polarity GaN films show that they have neither frequency shifts mode nor forbidden mode. These results indicate that Ga-polarity GaN films have a better quality, and they are in good agreement with the results obtained from the room temperature Hall mobility. The best values of Ga-polarity GaN films are 1042 cm^2/Vs with a carrier density of 1.0× 1017 cm^-3.
