Low temperature depositions of GaN thin films by plasma-enhanced atomic layer deposition

Metalorganic chemical vapour deposition and molecular beam epitaxy have already been demonstrated to be successful techniques for obtaining high-quality epitaxial GaN layers with low impurity concentrations and pretty good electrical properties. However, high growth temperature employed in both of these methods give rise to some intrinsic defects of the thin films, such as high background-carrier concentrations. As a low-temperature thin film deposition method, plasma-enhanced atomic layer deposition (PE-ALD) has more unique advantages compared to both methods for epitaxial growth of GaN. In this paper, the polycrystalline GaN thin films were fabricated on Si (100) substrates at 150-300℃ by PE-ALD. Trimethylgallium and N2/H2 plasma gas mixture were used as the Ga and N precursors. The growth rate of the thin films was demonstrated by the spectroscopic ellipsometer. The crystal structrue and composition of the GaN thin films were characterized by X-ray diffractometer and X-ray photoelectron spectrometer (XPS). It is showed that the growth window for PE-ALD grown GaN thin films is 210-270℃, where the growth rate remains constant at 0.70 /cycle. And it is known that it is the self-limiting nature of PE-ALD that is ascribed to the plateau of the growth rate. Films grown at relatively higher temperature are polycrystalline with a hexagonal wurtzite structure, while films grown under relatively lower temperature are amorphous. The grazing incidence X-ray diffraction (GIXRD) patterns of the polycrystalline thin films reveal three main peaks located at 2=32.4, 34.6 and 36.9, which are corresponding to the (100), (002) and (101) reflections. It is showed that the Ga, N atoms would get higher energy for more effective migration to positions with lowest energy to form ordered crystalline arrange at higher growth temperature. The XPS results show that all the N elements of the as-grown thin films are in the form of NGa bond, indicating that all the N elements are formed into GaN thin films; and there is a little amount of the Ga elements that exist in GaO bond. The fact that there is no Ga2O3-related peaks in the GIXRD pattern suggests that there is small amount of amorphous Ga2O3 dispersed in the polycrystalline GaN thin films. In the future work, reducing the concentration of the C and O impurities may be achieved by increasing the time of the reaction and plasma pules in the process formula and replacing the inductively coupled plasma with the hollow cathode plasma, respectively.