Structure of Semi-Insulating Polycrystalline Silicon (SIPOS)

Semi-Insulating Polycrystalline Silicon (SIPOS) films were deposited on Si02/Si, Si, and sapphire substrates by the chemical vapor deposition of silane and nitroiJs oxide at atmospheric and low pressures. X-ray diffraction was used to identify the phases present, and to study structural properties like grain size and strain as a function of the flow rate ratio, ~, = [N20]/[SiH4].oBoth APCVD and LPCVD films contained atoleast four phases : polycrystalline silicon (grain size > 300 A), microcrystalline silicon (grain size 15-25 A), amorphous silicon (grain size < 9 A), and an additional phase, as yet unidentified, termed SixOy. The Si(222) reflection was observed, which is forbidden for crystalline silicon. SIPOS deposited on sapphire appeared to have a different structure than that deposited on Si02/Si or on Si. Grain size decreased with increasing-y. As-deposited films were either unstrained or in compression. No major structural differences were observed between APCVD and LPCVD films. On annealing at 550-750 o C, the SilO. phase and amorphous silicon disappeared. The intensity of the forbidden Si(222) reflection decre^as~ed with increasing annealing temperature. No grain growth was observed on annealing. Strain changed from compressive to tensile on annealing. The texture of the films was investigated by the pole figure technique and was found to be different for films deposited on sapphire substrates as compared to films deposited on Si02/Si. The texture also depended strongly on the value of ,/. A structural model was proposed to explain the origin of the forbidden Si(222) reflection. Major contributions are that this is the first observation of the Si(222) reflection and the SixOy phase. supervision. I would like to thank the members of the thesis committee, Drs. A.G. Milnes and S. Mahajan, for their guidance and would also like to express my gratitude to Dr. F.A. Selim for providing the APCVD SIPOS samples and forhis" many helpful suggestions. I appreciate stimulating discussions with S. Figure 1-1: From [Aoki 76]. Electrical shielding of the silicon surface. SIPOS films are superior to conventional SiC 2 passivating layers since they can completely protect the underlying silicon surface from the influence of external electric fields (the screening length is as short as 3 nm [Hamasaki 77]) and equally passivate both n÷-p and p+-n junctions since they have no fixed charges [Mochizuki 76b], [Hamasaki 77]. Interface states between the SIPOS film and the silicon substrate inhibit the formation of an inversion layer in the surface region of both …