in the bulk utter unncaling in forming gas but not after high tempemture ( 1100"C) anneaIs in Ar. The presence of hydrogen dmmatical[y incrcascs the broad PL band centered in the near-infmred after tinncaling Lit1100"C but htis almost no effect on the PL spectral distribution. Hydrogen is found to sclccti~cly trap in the region where Si nanocrystais are formed, consistent with a model of H ptissivatin: surfucc states tit the Si/SiO: interface that leads to enhanced PL. The thermal stabiIity of the tmppul H and the PL yield observed ufter a high temperature anneal have been studied. The hydrogen concentration and PL yield are unchanged for subsequent anneals up to 400"C. However, ubovc -IO(YCthe PL decreases tind a more complicated H chemistry is evident. Similar concentmtions of H or D ure tmpped after annealing in HJ or Dj forming gas; however, no differences in the PL yield or spcctrtil distribution tire observed, indicating that the electronic transitions resulting in luminescence are not dependent on the moss of the hydrogen species.

Nanocrystals of group-IV semiconductor materials (Si, Ge, and SiGe) have been fabricated in SiO2 by ion implantation and subsequent thermal annealing. The microstructure of these nanocrystals has been studied by transmission electron microscopy. Critical influences of the annealing temperatures and implantation doses on the nanocrystal size distributions are demonstrated with the Ge-implanted systems. Significant roughening of the nanocrystals occurs when the annealing temperature is raised above the melting temperature of the implanted semiconductor material.

Optical second-harmonic generation (SHG) is used as a noninvasive probe of the interfaces of Si nanocrystals (NCs) embedded uniformly in an SiO2 matrix. Measurements of the generated SH mode verify that the second-harmonic polarization has a nonlocal dipole form proportional to (E x Delta inverted) E that depends on inhomogeneities in the incident field E, as proposed in recent models based on a locally noncentrosymmetric dipolar response averaged over the spherical NC interfaces. A two-beam SHG geometry is found to enhance this polarization greatly compared to single-beam SHG, yielding strong signals useful for scanning, spectroscopy, and real-time monitoring. This configuration provides a general strategy for enhancing the second-order nonlinear response of centrosymmetric samples, as demonstrated here for both Si nanocomposites and their glass substrates.

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