starts flowing through the chamber in step 20 and before the subsequent amount of ammonia is flowed through the chamber in step 21 consequently a very small amount of nitrogen is present on the si 111 1 u00d7 1 surface of the silicon substrate 11 before the aluminum from the trimethylaluminum comes into contact with the substrate surface as the first seed crystals of aln are formed the aluminum atoms form bonds to the nitrogen atoms that are present on the si 111 1 u00d7 1 surface as opposed to directly with silicon atoms on the substrate surface the nitrogen from the ammonia preflow step ensures that the polarity of the alternating layers of aluminum and nitrogen in the aln crystals that are formed across the entire silicon wafer will have a nitrogen layer facing the silicon substrate and an aluminum layer on top fig5 is a diagram illustrating the crystal structure 26 of wurtzite aluminum nitride aln the smaller spheres represent aluminum atoms 27 and the larger spheres represent nitrogen atoms 28 the c plane of the aln crystal intersects all six aluminum atoms that form a hexagon on the top surface of the crystal the distance between adjacent aluminum atoms around.