David D. Tuschel

Characterization of proton exchange lithium niobate waveguides

Proton exchanged samples of LiNbO3 have been profiled by micro‐Raman spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering channeling, and by x‐ray diffraction (XRD). Following proton exchange (PE) there are two different phases in addition to pure LiNbO3 detected by XRD. After successive annealing steps the outermost phase disappears and an interfacial region forms progressively between PE and LiNbO3. Specific vibrational bands are correlated to electro‐optic and nonlinear optical properties of the system, and the recovery of these properties upon annealing is correlated to chemical bonding changes.

Raman scattering study of lattice disorder in 1‐MeV Si‐implanted GaAs

We have used Raman scattering to study the lattice disorder created by the implantation of 1‐MeV Si ions into GaAs. Using the change in the longitudinal optical (LO) phonon‐line position as the signature for lattice damage, combined with chemical etching for controlled layer removal, we monitored the evolution of the disorder depth profile as a function of implantation dose. The shape of the depth profile of the disorder agrees with the theoretical simulation trim for doses of 1×1014 cm−2 or lower. For higher doses a saturation is observed in the amount of residual disorder. This saturation is a manifestation of dynamic annealing occurring during the high‐energy implantations, which we attribute to enhanced defect mobility, induced by the transfer of energy to the lattice, in atomic collision cascade processes. In order to correlate the spectral features in the Raman spectra with structural changes in the ion‐implanted samples, we characterized the implantation‐induced lattice damage using ion‐channeling ...

Time dependence of ferroelectric coercive field after domain inversion for lithium‐tantalate crystal

We found the ferroelectric coercive field of LiTaO3, both in forward and reverse direction, vary with time after the domain is inverted. The coercive field drops when the domain is inverted, then gradually recovers. This phenomenon is light sensitive. The existence of a net time‐varying internal electric field after domain inversion is hypothesized. The internal field is composed of the depolarization field, which is due to the spontaneous electric dipole moments, and an opposite direction time‐varying space‐charge field which is due to the redistribution of free‐carriers transport under the influence of the depolarization field. Electro‐optical effect caused by the internal electric field has been observed by means of an in situ optical monitoring technique for the domain inversion process. The in situ optical monitoring technique is based on using the LiTaO3 thin‐plate crystal as a low finesse Fabry–Perot interferometer.

Depth profiling of proton exchanged LiNbO3 waveguides by micro-Raman spectroscopy

Z‐cut LiNbO3 wafers were proton exchanged (PE) with pyrophosphoric acid. The polished edges provided a side view of the exchanged region. The region was probed by micro‐Raman spectroscopy by stepping a 488.0 nm laser at intervals as small as 0.2 μm across the edge face starting below the exchanged region and moving towards the wafer surface, thereby traversing the PE region. Profiling revealed significant changes, in the 125–800 cm−1 and 3200–3600 cm−1 frequency ranges. The PE region expanded after annealing at 300 °C indicating proton penetration into the wafer. Continued annealing resulted in the progressive recovery of spectral characteristics resembling unexchanged lithium niobate. Micro‐Raman profiles provide spectroscopic information regarding which vibrational modes are affected by the exchange and the annealing processes as a function of depth.

Chemical bonding and atomic structure of Rb+ exchanged KTiOPO4 waveguides probed by micro‐Raman spectroscopy

Channel waveguides of Rb+ exchanged single‐crystal KTiOPO4 were studied by micro‐Raman spectroscopy. Rb+ exchange causes a disruption of the long‐range translational (crystal) symmetry of the lattice and a tilting of the TiO6 octahedra. The ability to nondestructively map the chemical and physical structure related to the optical properties of channel waveguides is demonstrated.

Micro-Raman Characterization of Unusual Defect Structure in Arsenic-Implanted Silicon

Raman spectroscopy has often been used to study the damage to semiconductors induced by ion implantation. Off-axis, macro-Raman spectra reveal extensive damage to the silicon lattice, consistent with many literature reports. However, when the same samples were analyzed in the backscattering mode by micro-Raman spectroscopy, evidence was found for orientational dependent lattice damage and an unusual defect structure. P/O micro-Raman spectra reveal the spatially-varying appearance of a band between 505 and 510 cmalways accompanied by that of the silicon optical mode at 520 cm-.

UPCONVERTING TM-DOPED BAYYBF8 OPTICAL WAVEGUIDES EPITAXIALLY GROWN ON GAAS

TM‐doped BaYYbF8 films were epitaxially grown on both (100) and (111) GaAs substrates using CaF2 or LiF as intermediate layers. The BaYYbF8 phase was found to be a previously unreported cubic phase with a lattice constant of 0.5711 nm, which is different from a monoclinic phase reported for bulk crystals. The films produced UV and visible radiation with wavelengths at 360 nm (UV), 450–480 nm (blue), and 500–550 nm (green) when pumped by laser radiation at 647 nm.

Micro-Raman Characterization of Arsenic-Implanted Silicon: Interpretation of the Spectra

Raman spectra were measured on arsenic-implanted silicon with micro-Raman spectroscopy in the backscattering mode and with macro-Raman spectroscopy. A peak is observed between 505 and 510 cm-I with 488 and 514.5 nm excitation. This peak and a related peak from the substrate at about 520 cm-1 are seen in selected regions of the implanted samples when the implant dose is above 2 x 1014 As/cm2. These features may be due to a long room temperature anneal, as they are absent in recently prepared samples. Possible explanations for the features are presented.

Phase Transformations Induced by Arsenic Implants into Silicon

Micro-Raman spectroscopic investigations of arsenic-implanted silicon show lines characteristic of silicon crystallites even at implant doses above the amorphization threshold. The intensity and frequency of occurrence of the lines increase with the implanted dose. Polarization/orientation Raman studies indicate the crystallites are silicon in the hexagonal phase (Si-IV) and silicon in the diamond phase (Si-I). The latter are oriented differently than the substrate silicon. Monte Carlo simulations of the arsenic ion energy loss and published molecular dynamics studies suggest that each arsenic ion deposits sufficient energy to locally melt the silicon lattice. This is taken as the basis of the present attempt to explain the origin of the crystallites. A one-dimensional numerical model is developed to determine the time scale for the liquid silicon to solidify. The effect of amorphous silicon on the solidification is also investigated.

Micro-Raman and luminescence spectroscopic techniques for the characterization and process control of Rb+ exchanged KTiOPO4 waveguides

The fabrication of ion-exchanged waveguides with high-frequency doubling conversion efficiency requires high-quality crystalline substrates, an understanding of the effects of partial cation exchange on the optical properties of the waveguide, and control of the degree and effects of ion-exchange. To address these needs we have developed micro-Raman and luminescence spectroscopic techniques for the characterization and process control of Rb+ exchanged KTiOPO4 (R/KTP) waveguides. We report on the use of laser excited luminescence to screen device substrates for unacceptable levels of impurity transition metals, which contribute to photorefraction and optical losses due to absorption. Micro-Raman spectroscopy has been used to probe types of R/KTP channel waveguides for the degree and effects of Rb+ exchange. The high spatial resolution and nondestructive nature of micro-Raman spectroscopy make it suitable as a probe for in situ characterization of photonic devices. Specifically, micro-Raman spectroscopy can detect cation-exchange induced changes in the polarizability, reduction of crystal symmetry, and changes in the chemical bonding and orientation of TiO6 octahedra, the anionic groups primarily responsible for the nonlinear properties of the materia. Individual R/KTP waveguides from different devices have been studied by micro-Raman spectroscopy and structural differences have been detected. The uniformity of a channel waveguide is another quality that can be readily probed and quantified by micro-Raman spectroscopy.