Due to unique properties nitrides of metals of the third group are very promising for the development of optoelectronicand electronic devices based on them. Most popular substrates for GaN growth and for growth of heterostructures based on GaN such as sapphire, SiC or Si substrates. SiC substrates have best characteristics for growth, but Si substrates have lowest price and widely used technology for treatment. Using Si substrates it is the way to reduce the price of GaN technology. Therefore development of methods for the forming of transitional and buffer layers and precise with low defect density heterointerfaces with the absence of discontinuities in nanoscale layers and with a minimum of impurity is actual problem of research laboratories working in this direction. In this work we investigated the conditions of creation SiC layers on Si (111) substrates and also researched growth features of the layers AlN, GaN and AlGaN on created transition layers by molecular beam epitaxy with the ammonia source. Substrates Si (111) are cleaned by RCA clean, diping in HF, rinsing in dionized water and blow dry. Carbidization surface of Si substrates was carried out at temperatures from 1000 to 1400 °C in an atmosphere of C3H8 with Ar in the vacuum furnace. Modified surface was cleaned and researched by methodes of atomic force microscopy, electron microscopy and Raman spectroscopy.
After substrate analysis 500 nm metal film was deposited on back side of Si. It is necessary for efficient heating by IR radiation in the epitaxy. AlN 400 nm buffer layers was formed on modified Si surface in MBE process after one more cleaning in liquids and annealing in MBE chamber. The formed heterostructure also consisted of 280-nm-thick AlGaN/AlN supperlattice layers, a 1-μm-thick GaN layer, and a 10-nm-thick AlN layer. AlN/GaN structures were formed on Si wafers with/without carbonization to investigate the effect of carbonization. The surface roughness of the AlN/GaN structure was characterized by optical microscopy and AFM.
Carbonized substrate roughness was about 20 nm. It was because of on the surface the three-dimensional silicon and SiC grains were present. Raman spectroscopy showed difference particle composition. These particles could be deposited after evaporation from surface at temperatures more than 1000 °C and when silicon vapour did not react with hydrogen for creating silane it was condensed as grains on the surface. Some of grains were carbonized. SiC layer covered and surface of Si and surface of nanoparticles. After carbonization also a lot of amorphous carbon was present on a surface, which needed to be washed away.
Carbonized surface of Si helped to get rid of a big difference of parameters of a lattice. Also the thick layer of nitride of aluminum performs this function and relieves stress in the crystal structures. AlN the best layer for constructing heterostructures based on GaN and the roughness of this layer in the future determines the quality of the whole heterostructure. In this case, the surface roughness of AlN was from 5 to 10 nm. Superlattice layers allowed greatly reduce the amount of growth defects. Morphology of GaN layer have rouhgness about 3-5 nm and grown at temperature 930 °C. AlN barrier layer was grown at the same temperature.
In conclusion, AlN/GaN heterostructures were successfully grown on Si substrates. Carbonization Si surface offer an efficient growth process for GaN-on-Si wafers. Results show the possibility to combine the silicon technology and technology of nitride heterostructures in electronics.