A method of depositing a coating layer comprising gallium nitride on a substrate comprising the steps of: (a) providing the substance having a plurality of side walls and valleys; (b) forming a first layer of gallium nitride deposited on the substrate, by reacting gaseous trimethylgallium and ammonia at a temparature ranging from 400 to 500 degree celcius, such that the first layer is formed on the side walls and the valleys; and (c) forming a second layer of gallium nitride deposited on top of the first layer, by reacting gaseous trimethylgallium and ammonia at a temperature from 1000 to 1200 degree celcius, to obtain the coating layer comprising the first layer of gallium nitride and the second layer to gallium nitride at a thickness from 3.0 to 4.5 um.