Optical characterization of damage and concentration profiles in H ion implanted amorphous silicon

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Abstract
190 keV H ions are implanted with doses ranging from 1 × 1015 to 1.4 × 1018 cm−2 into pure amorphous silicon layers prepared by 2 MeV Silicon irradiation of crystalline silicon. Depth profiles of the changed complex refractive index (n + ik) are determined from high-resolution reflection and transmission measurements across bevelled surfaces. The increase of the reflectivity of annealed amorphous silicon (a-Si) resulting from collision induced Si bonding defects is used to determine the nuclear stopping power depth profile. A nuclear energy density of 3.1 eV per Si atom has to be deposited by the H projectiles in order to transform the relaxed structure of a-Si into the state of saturated disorder which is characteristic for irradiation amorphized silicon (ia-Si). On account of the saturated damage in pre-amorphized ia-Si targets the observed reduction of n and k is associated with the implanted H concentration c H. Projected range, range straggling, and skewness are evaluated from the optically determined range distribution. The comparison of calculated high-dose concentration profiles with corresponding optical depth profiles yields the optical constants as well as the atomic H and Si densities as functions of C H up to 66 at. %. A model for the incorporation of the implanted H atoms into the amorphous bonding network accounts for the removal of Si bonding defects and for the formation of H2 bubbles. A corresponding analysis of the K(CH) curve yields a density of 4 × 1021 Si bonding defects per cm3 in ia-Si. In accordance with the experimental results the permittivity ε1 is calculated in dependence on CH by mixing the optical polarizabilities of Si-H bonds, H2 molecules, Si bonding defects, and partially distorted Si-Si bonds.