D.W. Kisker

Tapered Fabry-Perot waveguide optical demultiplexer

We describe a novel waveguide spectrometer for use in wavelength division multiplexing (WDM) systems. A high refractive index resonator is coupled to a waveguide and extracts light from the waveguide at its resonance frequency. A taper in the thickness of the resonator allows light of different wavelengths to be extracted at different positions. A structure is fabricated for use at 750 nm with a measured dispersion of 29 nm/cm and a wavelength resolution of 1 nm. The device operation is in good agreement with first-order coupled-mode calculations.<<ETX>>

Atomic scale characterization of organometallic vapor phase epitaxial growth using in-situ grazing incidence X-ray scattering

Abstract In this work, we present the results of the application of grazing incidence X-ray scattering to the study of organometallic vapor phase epitaxial (OMVPE) growth of GaAs. We determined that, although several types of reconstructions may be present under steady-state conditions both prior to and after growth, these reconstructions disappear during growth. In addition, by monitoring the intensity of the surface-sensitive crystal truncation rod, we have been able to establish that layer-by-layer growth occurs under typical OMVPE growth conditions, while the growth mode changes to step-flow at high temperatures. Finally, by observing diffuse scattering near the truncation rods, we have been able to estimate the average island spacing, its temperature dependence, and the anisotropic nature of the nucleation and growth of GaAs.

Microstructure of AlGaAs‐oxide heterolayers formed by wet oxidation

We have carried out a transmission electron microscopy based study of AlGaAs–Al(oxide) heterolayers created by lateral sidewall wet oxidation and identify the oxide phase formed as a consequence of the oxidation of AlAs to be γ‐Al2O3, with the cubic Fd 3m structure. The oxide‐semiconductor interface is weak and porous, possibly due to the high stress loads developed during oxidation, and we propose that the fast oxidation rates are a consequence of reactants transported to the oxidation front along the porous interface.

Surface lifetimes of Ga and growth behavior on GaN (0001) surfaces during molecular beam epitaxy

We report a study of the desorption behavior of Ga on the GaN(0001) surface and the growth behavior of GaN during molecular beam epitaxy. A desorption activation energy of 2.2±0.2 eV is measured for Ga adatoms. Porous columnar features in the GaN microstructure are observed that are enhanced by higher growth temperatures and eliminated by growing Mg or Si doped GaN. We propose a model for this observation.

Vertical cavity devices as wavelength selective waveguides

We show that novel wavelength-sensitive devices can be fabricated by coupling a semiconductor vertical cavity resonator to a low index waveguide. The optical mode in the resonator propagates at an angle, and the resonator resembles a high index waveguide. A taper in the thickness of the resonator allows different parts of the waveguide to operate at different wavelengths. These structures are analyzed using both thin film equations and waveguide normalism. Concentrating on a waveguide demultiplexer, simple design equations are derived, and a demonstration device is fabricated for TE mode at 0.75 /spl mu/m operation. Using AlGaAs/AlAs multilayers and a polymer top waveguide, the spectrometer exhibited a dispersion of 29 nm/cm, a wavelength resolution of better than 1 nm, and an intrinsic device efficiency of about 90%. A similar structure containing a light-emitting quantum well operated as a multiwavelength light source by modifying the spontaneous emission into the polymer waveguide. >

Gallium arsenide surface reconstructions during organometallic vapor‐phase epitaxy

In situ surface x‐ray scattering studies of the GaAs(001) surface were used to determine whether specific surface reconstructions occur during organometallic vapor‐phase epitaxy. Prior to growth, we find that surfaces heated in the presence of As form a c(4×4) structure, while those heated in the absence of organometallics or in Ga form two similar fourfold reconstructions. We find no evidence for the presence of any surface reconstruction during the actual layer‐by‐layer growth process.

Optical characterization of surfaces during epitaxial growth using RDS and GIXS

Structures of (001)GaAs surfaces during organometallic vapor phase epitaxy (OMVPE) have been studied by simultaneous measurements using reflectance difference spectroscopy (RDS) and grazing incidence X-ray scattering (GIXS). The data obtained by the two techniques are strongly correlated. We find that RDS, although not a diffraction probe, can distinguish most surface reconstructions of (001)GaAs. The information obtained by these two probes is complementary, so by combining RDS and GIXS data obtained from surfaces under static and dynamic conditions we can gain detailed information about atomic arrangements and growth mechanisms. Under static conditions, we find that RD spectra under OMVPE conditions are similar to those previously observed in ultrahigh vacuum (UHV). By simultaneously monitoring the surface reconstructions using GIXS and RDS, we have established that RDS accurately reflects the local structural configurations but is not uniquely sensitive to changes in long range order or subtle differences in symmetry. Growth oscillations were also measured simultaneously by RDS and GIXS under various conditions. The observed oscillation periods agree with each other, but the details are condition dependent, thus providing important clues concerning the surface processes involved. For example, the one-to-one correlation observed between RDS and GIXS oscillations suggests that the RDS oscillations are related to island formation.

Atomic scale studies of epitaxial growth processes using X-ray techniques

In this work we have applied grazing incidence X-ray scattering to the study of nucleation and growth of GaAs by organometallic vapor phase epitaxy. Using the sensitivity of scattering at the 110 crystal truncation rod (CTR) position to the smoothness of the surface, we are able to monitor the crossover from layer-by-layer to step-flow growth. More detailed information about the nucleation process was determined by using diffuse scattering near the CTR position to measure the spacing of two-dimensional islands during growth as a function of flux and deposition temperature. Application of standard nucleation theory to the analysis of this data suggests that the critical island size during OMVPE growth is likely to be much larger than a single adatom, in contrast with what is usually assumed for semiconductor systems.

Multiple wavelength light source using an asymmetric waveguide coupler

We obtained multiple wavelength light emission from a coupled waveguide structure formed from a conventional waveguide and a vertical cavity resonator. The cavity and quarter‐wave mirrors are optimized to act as another waveguide, thus forming an asymmetric directional coupler. This asymmetry between the waveguide and the vertical cavity causes wavelength selectivity in the cavity light emission. By tapering the cavity, the phase‐matched wavelength becomes a function of position, and pumping different regions along the length of the waveguides causes different wavelengths to be emitted from the device. We obtained narrow‐band luminescence with a minimum width of 1 nm using a 75‐A quantum well in a GaAs/AlGaAs vertical cavity structure coupled to a low index polymer waveguide.

Characterization of vapor phase growth using X-ray techniques

Abstract Chemical vapor deposition environments, while technologically quite important, are difficult to study using traditional analytical probes, such as electron-based techniques and optical tools. In this work, we will describe some of the ways in which X-rays can be applied to understand not only the gas phase composition through fluorescence, but also surface processes such as nucleation and diffusion.