| Gallium nitride(GaN)has been widely used for fabricating light-emitting diodes(LED),high-power semiconductor lasers,microwave power devices and so on due to its superior properties such as the wide band gap of 3.4 eV,high electron mobility,excellent thermal conductivity,and high temperature or high pressure resistance.An undesired high dislocation density caused by the lattice mismatch and thermal expansion coefficient mismatch between GaN and heterogeneous substrates deteriorate the electrical and optical properties of semiconductor devices.So it is significant to modulate the crystal quality of GaN films.This paper aims to modulate microstructures of GaN epitaxial films by the in-situ silicon nitride(Si N_x)interlayer.GaN epitaxial films with a SiN_x interlayer were prepared by metal-organic chemical vapor deposition(MOCVD).Effects of deposition times and locations of the SiN_x interlayer on the crystal quality of GaN epitaxial films were investigated based on which a modulation model was proposed.Meanwhile,owing to the excellent stability of SiN_x,the modulation of nano-porous GaN by the in-situ deposited SiN_x mask layer was studied.A method of fabricating nano-porous GaN by chemical corrosion was explored,as well as the relationship of corrosive durations and corrosive temperatures to the morphology of nano-porous GaN.Specific results are as follows:The dislocation density of GaN films can be reduced effectively by the SiN_x interlayer.When the SiN_x interlayer deposited for 120 s,the dislocation density of the GaN film is lower than those of the SiN_x interlayer deposited for60 s and 180 s.In fact,the SiN_x nucleates at the dislocation free regions of the GaN surface.In the subsequent GaN growth,dislocations below the Si N_x layer will continue extending upward and bending to the horizontal direction as the GaN islands growing up.Some of them who have opposite Burgers vectors will be annihilated by forming dislocation loops during the coalescence period of those GaN islands.Simultaneously,some new dislocations originating from boundaries between neighbor GaN islands will also extend upward.When the SiN_x interlayer deposited for 120 s,new dislocations are much less than those ones annihilated by forming dislocation loops.So a lower dislocation density and better crystal quality are achieved.Moreover,When the Si N_x interlayer deposited on GaN nucleation islands after high temperature annealing,the GaN film has the lowest dislocation density and the optimal crystal quality due to the SiN_x interlayer covers the surface of the sapphire substrate and the side walls of hexagonal pits resulting from decomposition on the top plane of GaN nucleation islands.As a result,some of screw dislocations originate from lattice mismatch are resisted by the SiN_x interlayer who can’t extend upward.Meanwhile,at the high temperature growth process of the SiN_x interlayer,some of GaN nucleation islands decomposed quickly due to their worse crystal quality or too tiny sizes.So a smaller number of edge dislocations can be achieved due to the reduced number of GaN nucleation islands.The GaN film without the SiN_x mask layer presents lots of dark pits in various depths after corrosion.By contrast,the GaN film with the SiN_x mask layer presents porous morphology after corrosion,whose corrosive degree is becoming more aggravating as the duration is prolonging and the temperature rising.When the corrosive duration and corrosive temperature are 15 min and200℃,respectively,the optimal diameters and depths of nano-pores can be achieved with the most uniform size distribution.During the process of corrosion,the SiN_x mask layer acts as a protective layer,allowing the area covered by it to be free from acid corrosion and forming side walls of the nano-porous structure. |
