HIGH-RESOLUTION ELECTRON-MICROSCOPY OF DIAMOND HEXAGONAL SILICON IN LOW-PRESSURE CHEMICAL VAPOR-DEPOSITED POLYCRYSTALLINE SILICON

Thin poly-Si layers deposited at 625-degrees-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 {0110BAR} reflections of a diamond hexagonal (dh) Si phase, which occurs as small inclusions with the orientation relationship (011BAR) parallel-to (0001), [011] parallel-to [2110BARBAR] 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 (111BARBAR) parallel-to (0001), [011] parallel-to [2110BARBAR]. 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 {0110BAR] reflections in common and make a major contribution to the 0.334 nm peak. The medium temperature of 625-degrees-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: [2110BARBAR], (0116BAR), THETA = 35-degrees with a translation vector t = 1/2[0331BAR] parallel to it. The dh-Si phase vanishes in this poly-Si film after annealing at temperatures above 1000-degrees-C due to grain growth by recrystallization.