Second-harmonic Spectroscopy Research Articles

Second-harmonic spectroscopy of bulk boron-doped Si(001)

The effect of bulk boron incorporation on the second-harmonic generation (SHG) spectrum of Si(001) films grown epitaxially by chemical vapor deposition is studied as a function of doping level and temperature. At room temperature, boron doping (NA∼1018 cm−3) strongly enhances and blueshifts the E1 resonance of the second-harmonic generation spectra to 3.4 eV. Surface hydrogen termination reverses this effect. The observed doping and temperature dependence are modeled as electric-field-induced SHG in the bulk depletion region. The results suggest applications of SHG as an in situ, noninvasive probe of electrically active dopants.

Optical second-harmonic phase spectroscopy of the Si(111)–SiO2 interface

The two-photon interband electron transitions in a silicon layer directly underneath the Si(111)–SiO2 interface in the vicinity of the E2 critical point (CP) are probed by optical second-harmonic (SH) spectroscopy. The combination of the SH phase and intensity spectroscopy allows the complete deconvolution of the spectral behavior of the amplitude and phase of the quadratic susceptibility χ(2) near the E2 critical point of silicon.

Erratum: Second-harmonic spectroscopy of interband excitations at the interfaces of strainedSi(100)−Si0.85Ge0.15−SiO2heterostructures [Phys. Rev. B59, 2915 (1999)

Erratum: Second-harmonic spectroscopy of interband excitations at the interfaces of strained<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Si</mml:mi><mml:mo>(</mml:mo><mml:mn>100</mml:mn><mml:mo>)</mml:mo><mml:mo>−</mml:mo><mml:mi mathvariant="normal">S</mml:mi><mml:mi mathvariant="normal">i</mml:mi></mml:mrow><mml:mrow><mml:mn>0.85</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ge</mml:mi></mml:mrow><mml:mrow><mml:mn>0.15</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mo>−</mml:mo><mml:mi mathvariant="normal">S</mml:mi><mml:mi mathvariant="normal">i</mml:mi><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>heterostructures [Phys. Rev. B<b>59</b>, 2915 (1999)

Determination of the magnetic structure of hexagonal manganites RMnO3 (R=Sc, Y, Ho, Er, Tm, Yb) by second-harmonic spectroscopy

Hexagonal manganites RMnO3 (R=Sc, Y, Ho, Er, Tm, Yb) are compounds in which a ferroelectric and an antiferromagnetic order may coexist. The ferroelectric crystal structure is well known but theoretical studies show that there are different possibilities for the spin structure. By means of polarization- and temperature-dependent second-harmonic spectroscopy it is possible to determine the magnetic structure of RMnO3 compounds.

Study of Interfacial Charge-Transfer Complex on TiO2 Particles in Aqueous Suspension by Second-Harmonic Generation

We report the first direct observation of an interfacial charge-transfer complex using second-harmonic spectroscopy. The second-harmonic spectrum of catechol adsorbed on 0.4 micron-sized TiO2 (anatase) colloidal particles in aqueous suspension reveals a charge-transfer band centered at 2.72 eV (456 nm). In addition, the adsorption isotherm of catechol on the colloidal TiO2 suspension was obtained and gave an excellent fit to the Langmuir adsorption model. From this, we infer the free energy of the adsorption to be ΔG° = −6.8 kcal/mol.

Second-harmonic spectroscopy of Ge/Si(001) and Si 1-x Ge x (001)/Si(001)

Ge0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E1 critical point of Si(001) or Si0.9Ge0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH4 or Ge2H6 deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E1 resonant peak position. SH signals from Si0.9Ge0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si0.9Ge0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated.

Interface electronic transition observed by optical second-harmonic spectroscopy inβ−GaN/GaAs(001)heterostructures

Optical second-harmonic spectroscopy was used to probe the interface electronic structure of highly mismatched $\ensuremath{\beta}\ensuremath{-}GaN/GaAs(001)$ heterostructures in the vicinity of the ${E}_{0}$ interband critical point of $\ensuremath{\beta}\ensuremath{-}\mathrm{G}\mathrm{a}\mathrm{N}$. The resonance energy of both bulk and interface two-photon ${E}_{0}$ transitions from layers between 1- and 100-nm thickness are identical, indicating the absence of appreciable amounts of strain and electric fields in this materials system. This finding is in striking contrast to observations made for other materials systems, including ZnSe/GaAs and ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}$, where large shifts of several 10 meV with respect to the bulk values have been found.

Second-harmonic spectroscopy of a Si(001) surface during calibrated variations in temperature and hydrogen coverage

The epitaxial growth of silicon films by chemical vapor deposition (CVD) is strongly affected by temperature and hydrogen (H) termination. We report measurements of $p$-polarized optical second-harmonic (SH) spectra generated in reflection from clean $2\ifmmode\times\else\texttimes\fi{}1$-reconstructed and H-terminated epitaxial Si(001) surfaces with no intentional doping by Ti:sapphire femtosecond laser pulses for SH photon energies $3.0l~2\ensuremath{\Elzxh}\ensuremath{\omega}l~3.5\mathrm{eV}$ near the bulk ${E}_{1}$ resonance. Temperatures were varied from 200 to 900 K and H coverages from 0 to 1.5 monolayers (ML). Increases in temperature at fixed H-coverage redshift and broaden the ${E}_{1}$ resonance, as observed in linear bulk spectroscopy. Increases in H coverage from 0 to 1 ML at fixed temperature strongly quench, redshift, and distort the lineshape of the ${E}_{1}$ resonance even though reflection high-energy electron diffraction shows that the surface maintains the dimerized $2\ifmmode\times\else\texttimes\fi{}1$ reconstruction. The latter spectroscopic variations cannot be explained by vertical strain relaxation in the selvedge region, nor by bulk electric-field-induced SH (EFISH) effects. We instead attribute these variations to a monohydride-induced surface chemical modification, which we parametrize as a surface EFISH effect because submonolayer H strongly alters surface electric fields by redistributing charge from surface dimers into the bulk. The effects of vertical strain relaxation are weakly evident as a blueshift of the ${E}_{1}$ resonance accompanying dihydride termination (1.0--1.5 ML), which breaks the surface dimer bond. This modification is parametrized as a separate field-independent alteration to the surface dipole susceptibility ${\ensuremath{\chi}}_{\mathrm{surface}}^{(2)}.$ Finally, guided by these SH spectroscopic studies, we demonstrate dynamic real-time (100-ms resolution) SH monitoring of H coverage (5% accuracy) during temperature programmed hydrogen desorption and CVD epitaxial growth of silicon from disilane.

Femtosecond carrier-induced screening of dc electric-field-induced second-harmonic generation at the Si(001)–SiO_2 interface

Carrier-induced screening of the dc electric field at the Si(001)-SiO(2) interface is observed by intensity-dependent and femtosecond-time-resolved second-harmonic spectroscopy. The screening occurs on a time scale of ~?(p)(-1) , the reciprocal plasma frequency of the generated carriers.

Nonlinear optical spectroscopy of Si–heterostructure interfaces

Strain, dislocations, and electrically active defects at and near the interface of Si/SiO2 and Si/GaP heterostructures are analyzed by optical second-harmonic spectroscopy. For plasma oxides deposited on Si(001) and Si(111), time-dependent second-harmonic experiments reveal that near-interface oxide defects trap charge by the tunneling of photoexcited electrons from the Si conduction band. The space-charge field-induced second-harmonic transients are resonantly enhanced by two-photon E1 transitions in silicon. In GaP epilayers grown on Si(001) the bulk dipole-allowed electro-optical effect is suppressed by the formation of antiphase domains. In contrast, in GaP films grown on Si(111) and vicinal Si(001) the density of antiphase domains is considerably reduced yielding an enhancement of the second-order nonlinear optical response by two orders of magnitude.

Electronic transitions at Si(111)/SiO2 and Si(111)/Si3N4 interfaces studied by optical second-harmonic spectroscopy.

The existence of both strain and disorder at the interface of thermally grown ${\mathrm{SiO}}_{2}$ or plasma-deposited ${\mathrm{Si}}_{3}{N}_{4}$ films on vicinal Si(111) was ascertained unambiguously by frequency, polarization, and crystal orientation dependent studies of optical second-harmonic generation. The strain was seen to cause a redshift of 40 and 70 meV of the interband critical points ${E}_{0}^{\ensuremath{'}}$ and ${E}_{1}$ compared with the bulk silicon values. The disorder is observed by the perturbation of states out of the silicon bulk bands into the gap. Both at terraces and at steps, the density of these charge traps is found to be considerably reduced after rapid thermal annealing.

Oxygen chemisorption on Cu(110): A combined study by second-harmonic spectroscopy and photoemission

We have studied the formation of the addedrow (2×1)O overlayer on Cu(110) using Second-Harmonic Generation (SHG). To characterize the electronic properties of the surface, simultaneous observations with LEED and angle-resolved photoemission were performed. We are able to interpret our results in terms of transitions between surface bands of Cu(110) and Cu(110)-(2×1)O, respectively.

Oxygen chemisorption on Cu(110): A combined study by second-harmonic spectroscopy and photoemission

Oxygen chemisorption on Cu(110): A combined study by second-harmonic spectroscopy and photoemission

Electroreflectance from Gallium Nitride Using Second-Harmonic Generation

ABSTRACTThe optical second-harmonic (SH) response of a reverse biased gallium nitride (GaN) film was investigated for SH photon energies near the fundamental absorption edge. With the application of a DC electric field (∼ 100 to 220 kV/cm) along the optical axis of the sample, a strong two-photon resonance was observed in the specular reflected SH signal. This resonance was attributed to electric-field induced SH generation, EFISH, a third-order nonlinear response which arises from an induced polarization that is linearly dependent on the amplitude of the DC field. The EFISH contribution was spectrally localized at the bandedge, demonstrating the potential of SH spectroscopy for analysis of critical points in the band structure of semiconductors.

Influence of heterointerface atomic structure and defects on second-harmonic generation.

Second-harmonic spectroscopy is shown to be sensitive to interfacial electronic traps, lattice relaxation and buried surface reconstruction in ZnSe/GaAs(001) heterostructures. Newly developed photomodulation-second-harmonic-generation experiments reveal that the interfacial region contains predominantly hole traps, and that the density of these traps is substantially lower for 3\ifmmode\times\else\texttimes\fi{}1 buried surface reconstructed samples.

Orientational fluctuations and phase transitions of long chain molecules at the air/water interface.

Orientational fluctuations in long chain molecules at the air/water interface were detected by second-harmonic spectroscopy. The flucutations depend on the polarization of the incident (\ensuremath{\omega}) and detected (2\ensuremath{\omega}) light showing that the fluctuations are orientational rather than density in origin. The phenomenon is attributed to an orientational phase transition that is dependent on chain length, head group, and charge. Scaling experiments based on increasing the laser beam area indicated a correlation length (domain size) greater than 6 \ensuremath{\mu}m near the transition point.