High-resolution electron microscopy of diamond hexagonal silicon in low pressure chemical vapor deposited polycrystallin
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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 {0110} reflections of a diamond hexagonal (dh) Si phase, which occurs as small inclusions with the orientation relationship (Oil) || (0001), [011] || [2110] 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 (111) || (0001), [011] || [2TT0]. 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 {0110} 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: [2110], (0116), 0 = 35° with a translation vector t = l/2[0331] 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.
I. INTRODUCTION Thin polycrystalline silicon (poly-Si) films form major elements in silicon integrated circuit devices: they are used as transistor gate material, interconnects, and as high-ohmic resistors.1 The microstructure of these films is of utmost importance to their physical properties such as sheet resistance, stress, oxidation behavior, etc., and depends strongly on deposition conditions as well as doping. Today poly-Si layers are routinely fabricated by low pressure chemical vapor deposition (LPCVD) from silane gas at pressures on the order of 1CT4 bar and at temperatures ranging from 520 °C to 650 °C. For these conditions films are amorphous below 600 °C and crystalline above 600 °C. Further annealing and/or doping of such layers may result in changes of the microstructure. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) have provided much information on grain size, preferred orientation, stress, dopant segregation, and others.1"3 A detailed XRD study by Hendriks et al.A revealed that additional x-ray peaks can be observed in amorphous-deposited and crystallized as well as in crystalline-deposited poly-Si films. The most prominent additional reflection appears on the lowangle side of the {lll} S i reflection at approximately 0.334 nm, and can also be observed by selected area electron diffraction (SAD).2'4 Hendriks et al.A worked out that certain additional peaks are not due to im2324
J. Mater. Res., Vol. 6, No. 11, Nov 1991
purities or double diffraction but can be explained by a diam
