Plasma-enhanced Atomic Layer Deposition Of High-quality Gallium Nitride Thin Films On Different Substrates

The third-generation wide bandgap semiconductor materials,represented by gallium nitride(GaN),due to their excellent optical(adjustable bandgap:0.67-6.2 eV)and electrical(high breakdown electric field,high electron saturation drift rate)characteristics,have been widely used in the devices of light-emitting diodes,lasers and high electron mobility transistors.Recently,atomic layer deposition(ALD)and plasma-enhanced ALD(PE-ALD)technology,based on a chemical vapor deposition,have the merits of low deposition temperature(<400?)and good coating properties,and are attracting attention in the preparation of GaN thin films.The research is in full swing,so as to develop the application of ?-nitride semiconductors and their alloys in new electronic and optoelectronic devices.According to the low-temperature growth mechanism of PE-ALD,the decomposition of Group ? metals and nitrogen sources,and the surface activity of different substrates,the impurities(carbon,oxygen,etc.)and GaN/Si interface properties of GaN,and the issue of epitaxial growth of GaN on sapphire by PE-ALD were studied in this paper.In addition,high quality GaN thin films were deposited on quartz substrates,and the following results were obtained:(1)PE-ALD deposition of AlN thin film as an intermediate layer was proposed to reduce the oxygen impurity content of GaN film grown on Si substrate.GaN films deposited directly on Si substrates have a?18 nm thick amorphous GaN transition layer,and it is found that there is a?2 nm thick amorphous interfacial layer between GaN and Si substrates.In these two amorphous layers,the content of oxygen impurities is very high.When the AlN buffer layer was inserted,the crystallinity of GaN is enhanced due to the elimination of the amorphous interfacial layer,and the content of oxygen impurities in GaN film near the interface is reduced by?22%.(2)The content of carbon and oxygen impurities in GaN films can be reduced by using nitrogen plasma pretreatment of Si substrate.After pretreatment of the substrate surface with nitrogen plasma,the thickness of the amorphous interfacial layer between GaN and Si substrate is decreased by?0.4nm,and the amorphous GaN transition layer is thinned by 12.7nm.Furthermore,the hydroxyl(-OH)groups on the surface are partly nitrified into-NH groups.The decreased number of oxygen sites and increased nitrogen sites on the substrate surface not only greatly reduces the oxygen impurity concentration in the GaN film also improves the GaN/Si interfacial properties.(3)High-quality single-crystalline GaN films were obtained by baking and nitrogen plasma pretreatment of sapphire substrates.After baking treatment,the activity of oxygen sites on the surface of the sapphire substrate is enhanced.In addition,nitrogen plasma treatment of the substrate surface is performed at the baking temperature.The highly active nitrogen plasma fully replaces the oxygen on the substrate surface and promotes the initial nucleation of GaN,resulting in the single crystal GaN.The deposited GaN film exhibits c-axis oriented epitaxial growth.The X-ray rocking curve full width at half maximum height of the 24 nm thick GaN(002)plane is?666 arcsec,which indicates that its crystal quality is comparable to that of GaN grown by some high-temperature deposition techniques.The obtained high-quality GaN epilayer provides a basis for subsequent preparation of related devices.(4)High-quality GaN thin films were grown by PE-ALD directly on quartz substrate.The deposited GaN films grown at 300? are poly crystalline,with a sharp GaN/quartz interface,and the contents of carbon and oxygen impurities of the films are 6.34%and 0.64%,respectively.The polycrystalline GaN films obtained by Hall measurement show good electrical properties.When GaN grown under this condition is used in planar perovskite solar cells,the conversion efficiency under AM1.5 sunlight increases from 10.38%to 15.18%,and the relative efficiency increases by nearly 50%.