High-resolution electron microscopy of diamond hexagonal silicon in low pressure chemical vapor deposited polycrystalline silicon

Thin poly-Si layers deposited at 625 °C by LPCVD that are used in silicon technology for microelectronics exhibit a pronounced additional x-ray diffraction peak at about 0.334 nm. High-resolution electron microscopy (HREM) reveals that this peak stems from {010} reflections of a diamond hexagonal (dh) Si phase, which occurs as small inclusions with the orientation relationship (01) ‖ (0001), [011] ‖ [20] to the diamond cubic (dc) Si matrix. Due to the high density of planar faults on {111}, the dh-Si phase also exists in the form of the 2H silicon polytype with the orientation relationship (1) ‖ (0001), [011] ‖ [20]. In the first case the formation of the dh-Si phase may be understood by a multiple twinning transformation process, and in the second case by glide of Shockley partial dislocations on {111} planes. Various other hexagonal polytypes occur, which have all the {010} reflections in common and make a major contribution to the 0.334 nm peak. The medium temperature of 625 °C for layer deposition leads to a 〈011〉 preferential orientation and a high density of twins as well as to high compressive stress in the poly-Si layer itself. This seems to promote the formation of dh-Si. The strong twinning behavior produces a typical tilt grain boundary between adjacent dh-Si grains: [20], (016), Θ = 35°with a translation vector t = 1/2[031] parallel to it. The dh-Si phase vanishes in this poly-Si film after annealing at temperatures above 1000 °C due to grain growth by recrystallization.