Due to its unique properties GaN is a material useful for a variety of applications but obtaining good quality films is still a challenge. MOCVD is the main method for epitaxial growth of GaN for mass production and the early stages of growth are decisive for the quality of the film. While extensive work has been devoted to GaN in the last decade, the in-situ analysis of MOCVD films has not received the same amount of attention due to experimental difficulties.
Ion beam analysis is a very powerful technique both for bulk and surface analysis that we have used in conjunction with other complementary techniques to look at the early stages of growth of GaN films grown on sapphire substrate. A new MOCVD reactor has been designed built and integrated with the W.M. Keck Thin Film Analysis Facility such that the characterization is performed in-situ without taking the sample out of the ultra high vacuum environment. We have extensively characterized the sapphire substrate after the preliminary preparation that consisted of cleaning and nitridation. After optimizing the growth parameters we have been able to grow wurtzite GaN films in a wide temperature range (450-1050°C) with un-reconstructed surface and of good quality. We have observed the crystalline quality, surface morphology and the growth mode of the films as a function of growth temperature and the film thickness.
Using Ion Channeling we have determined the thickness and temperature range for which the films are either epitaxial single-crystal, polycrystalline with a preferred orientation or polycrystalline randomly oriented. Even though thin films grown at various temperatures have comparable defect densities we observe that when the substrate temperature is higher the rate of defect annihilation as the film grow is high such that we obtain good films within a thickness of approx. 300 nm.
By comparing the channeling and LEED results we observed that samples for which we observe diffraction patterns can in fact be very poor crystals. This is indicative of the fact that surface ordering can be achieved for polycrystalline films and that surface diffraction alone cannot be used as an indication of crystallinity.
The epitaxial films are oriented with the c-axis of the film paralleled with the c-axis of the substrate and the planes rotated by 30° with respect to the substrate. The defects density calculated for these films are similar to the ones obtained by other researcher for much thicker films.
Due to its unique properties GaN is a material useful for a variety of applications but obtaining good quality films is still a challenge. MOCVD is the main method for epitaxial growth of GaN for mass production and the early stages of growth are decisive for the quality of the film. While extensive work has been devoted to GaN in the last decade, the in-situ analysis of MOCVD films has not received the same amount of attention due to experimental difficulties.
Ion beam analysis is a very powerful technique both for bulk and surface analysis that we have used in conjunction with other complementary techniques to look at the early stages of growth of GaN films grown on sapphire substrate. A new MOCVD reactor has been designed built and integrated with the W.M. Keck Thin Film Analysis Facility such that the characterization is performed in-situ without taking the sample out of the ultra high vacuum environment. We have extensively characterized the sapphire substrate after the preliminary preparation that consisted of cleaning and nitridation. After optimizing the growth parameters we have been able to grow wurtzite GaN films in a wide temperature range (450-1050°C) with un-reconstructed surface and of good quality. We have observed the crystalline quality, surface morphology and the growth mode of the films as a function of growth temperature and the film thickness.
Using Ion Channeling we have determined the thickness and temperature range for which the films are either epitaxial single-crystal, polycrystalline with a preferred orientation or polycrystalline randomly oriented. Even though thin films grown at various temperatures have comparable defect densities we observe that when the substrate temperature is higher the rate of defect annihilation as the film grow is high such that we obtain good films within a thickness of approx. 300 nm.
By comparing the channeling and LEED results we observed that samples for which we observe diffraction patterns can in fact be very poor crystals. This is indicative of the fact that surface ordering can be achieved for polycrystalline films and that surface diffraction alone cannot be used as an indication of crystallinity.
The epitaxial films are oriented with the c-axis of the film paralleled with the c-axis of the substrate and the planes rotated by 30° with respect to the substrate. The defects density calculated for these films are similar to the ones obtained by other researcher for much thicker films.