Time-dependent Second-harmonic Generation Research Articles

Second-harmonic spectroscopy of two-dimensional Si nanocrystal layers embedded in SiO2 films

We present observations of optical second-harmonic generation (SHG) from dense (1010 or 6×1011 cm−2) layers of 5 or 8 nm average diameter silicon nanocrystals (NCs) embedded in thin (6–15 nm) SiO2 films on silicon substrates. Time-dependent SHG monitors optically-driven electrostatic charging of the Si NC layer as well as subsequent charge leakage, and thus provides noncontact electrical characterization of Si-NC-based device structures. SHG intensity and phase spectra of Si NCs are distinguished from contributions of the Si substrate by polarization-dependent and frequency-domain interferometric SH spectroscopy, which reveal a NC-size-dependent blueshift of the E1 resonance consistent with quantum confinement.

Characterization of charge-carrier dynamics in thin oxide layers on silicon by second harmonic generation

First measurements of time-dependent second-harmonic generation (SHG) at a $\mathrm{S}\mathrm{i}/({\mathrm{ZrO}}_{2}{)}_{x}({\mathrm{SiO}}_{2}{)}_{1\ensuremath{-}x}$ interface show a behavior that is drastically different from similar measurements at ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interfaces. We suggest that in ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ only electron injection is important, while both electrons and holes contribute to the dynamics at the $\mathrm{S}\mathrm{i}/({\mathrm{ZrO}}_{2}{)}_{x}({\mathrm{SiO}}_{2}{)}_{1\ensuremath{-}x}$ interface. Multiphoton excitation occurs in Si for all oxides, and involves direct interband transitions. The marked difference between the two systems is related to the population of multiphoton excited states in Si, the corresponding conduction- and valence-band offsets, and trapping/detrapping processes in the oxides. Our measurements confirm the existence of an initial built-in field at the interface.

Nonlinear optical diagnosis of oxide traps formed during reactive ion etching

Oxide traps generated by reactive ion etching are studied using a pulsed femtosecond laser. The second harmonic generation (SHG) signal from the Si/SiO2 interface is sensitive to charged traps in the oxide. The time evolution of the SHG signal indicates that positive traps predominate. The angular dependence of the polarized signal shows that the electric field generated by the oxide traps alters the symmetry of the sample. The damage is greatest for an oxide thickness of 13 nm (for a plasma dc bias of 300 V). Thicker oxides have smaller SHG signals, presumably because the Fowler–Nordheim tunneling currents induced by plasma charging of the oxide surface are smaller. Very thin oxides also exhibit reduced damage. The time dependent SHG signals depend on the temperature of the samples; these data provide information on the trapping and detrapping of substrate electrons by oxide holes.

Time-dependent second-harmonic generation from the Si–SiO_2 interface induced by charge transfer

We have observed that the second-harmonic signal generated from oxidized Si(001) varies on a time scale of several seconds in experiments involving a fundamental beam of lambda = 770 nm, 110-fs pulses at 76 MHz. We suggest that the temporal behavior arises from absorption of weak (<100-fW average power) third-harmonic light generated in air or in the sample, inducing charge transfer across the Si-SiO(2) interface and trapping in the oxide layer. Detrapping has been determined to take several minutes.

The possibility of Li+ off-centre ion tunnelling in K1-xLixTaO3 studied by second-harmonic generation

Time-dependent second-harmonic generation experiments in K1-xLixTaO3 crystals for the first time yield a Li concentration dependence of the activation energy of the Arrhenius-like reorientation process of the Li+ ions. The experimental data support the assumption of a Li+ off-centre ion tunnelling via excited and ground vibrational states. With increasing Li concentration, the tunnelling reorientation is suppressed in favour of a thermally activated over-barrier hopping. The suppression of the tunnelling reorientation can be explained by a model of cooperative tunnelling in a percolation dipole cluster and destruction of the tunnelling resonance condition by internal electric dipole fields.