Nanoscale EELS analysis of dielectric function and bandgap properties in gaN and related materials

The low loss region of an EEL spectrum (<50 eV) contains information about excitations of outer shell electrons and thus the electronic structure of a specimen which determines its optical properties. In this work, dedicated electron energy loss spectroscopy (EELS) methods for the experimental acquisition and analysis of spectra are described, which give improved information about the electronic structure near the bandgap region at a spatial resolution in the range of nanometers. For this purpose, we made use of a cold field emission scanning transmission electron microscope (STEM) equipped with a dedicated EELS system. This device provides a subnanometer electron probe and offers an energy resolution of 0.35 eV. Application of suitable deconvolution routines for removal of the zero loss peak extracts information on the bandgap region while the Kramers-Kronig transformation deduces the dielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap materials for the case of III-nitride compounds, which are currently the most promising material for applications on optoelectronic devices working in the blue and ultraviolet spectral range. The obtained results are in excellent agreement with experimental measurements by synchrotron ellipsometry and theoretical studies. The potential of the superior spatial resolution of EELS in a STEM is demonstrated by the analysis of dielectric properties of individual layers of heterostructures and individual defects within wurtzite GaN.

Reflectance measurements and optical constants in the extreme ultraviolet for thin films of ion-beam-deposited SiC, Mo, Mg2Si, and InSb and of evaporated Cr

Reflectance measurements and optical constants of thin films of ion-beam-deposited SiC, Mo, Mg(2)Si, and InSb and of evaporated Cr have been measured in the extreme-ultraviolet (EUV) spectral region from 49.0 to 200.0 nm. In this spectral region no optical constant data were available for materials deposited by ion-beam deposition. We compared our data with those for bulk samples and for thin films prepared by different techniques. The goal of this research has been to study candidate materials for multilayer coatings in the EUV.

Optical properties of thin-film silicon-compatible materials

The optical performance of photodetectors in silicon strongly depends on the transmission of incident light into Si and the charge collection efficiency therein. Consequently, improving the transmission efficiency of light into Si by means of an interference antireflectant (AR) filter can lead to improved optoelectric conversion efficiency. However, the design of these filters requires the availability of data on the optical characteristics of the materials used. Furthermore, for the realization of such filters for light detectors realized in silicon, the required compatibility with standard microelectronic processing implies that only Si-compatible materials should be used. The range of such Si-compatible materials available for fabricating photoelectric devices in silicon includes thermally grown SiO(2), low-pressure chemical-vapor deposited polysilicon, silicon nitride (low stress and stoichiometric), and oxides (low-temperature oxide, phosphosilicate glass, borosilicate glass, borophosphosilicate glass) as well as plasma-enhanced chemical-vapor deposited oxynitrides. We present the refractive index and the extinction coefficient of these materials in the 380-800-nm range. The data presented enhance the accuracy and applicability of simulation and design tools used to design photodetectors in silicon for the visible range.

XUV optical constants of single-crystal GaAs and sputtered C, Si, Cr(3)C(2), Mo, and W

The optical constants for sputtered films of C, Si, Mo, and W are presented for photon wavelengths from 13.3 to 304 A, and for single-crystal GaAs and sputtered Cr(3)C(2) from 13.3 to 1302 A. The best-fit values of the optical constants, and also the surface roughness parameter sigma, are obtained from reflectance vs incidence angle measurements using a nonlinear, least-squares curve fitting algorithm. These optical constants are significantly different from previously reported data in portions of this spectral region. The new optical data result in good agreement between theory and reflectance measurements of soft x-ray multilayer coatings made from the sputtered materials.

Second-harmonic radiation from an AlGaAs laser and its application for absorption spectroscopy

Second-harmonic radiation of several picowatts from various commercial laser diodes with several milliwatts of output power has been observed at various temperatures from 130 to 310 K. No temperature dependence of the conversion efficiency was observed. The spectral width of the second-harmonic radiation was limited by that of the fundamental radiation. An application of the internal second-harmonic radiation to a spectroscopic light source allowed observation of some absorption lines of potassium and aluminum.