Daniel W. Thompson

Giant photoresistivity and optically controlled switching in self-assembled nanowires

We report the observation of giant photoresistivity in electrochemically self-assembled CdS and ZnSe nanowires electrodeposited in a porous alumina film. The resistance of these nanowires increases by one to two orders of magnitude when exposed to infrared radiation, possibly because of real-space transfer of electrons from the nanowires into the surrounding alumina by photon absorption. This phenomenon has potential applications in “normally on” infrared photodetectors and optically controlled switches.

Application of IR variable angle spectroscopic ellipsometry to the determination of free carrier concentration depth profiles

Abstract Free carrier concentration profiles were determined by Fourier Transform Infrared (FTIR) variable angle spectroscopic ellipsometry. The technique exploits carrier absorption in the mid-infrared spectral range and combines the sensitivity of ellipsometry with a simple Drude free carrier absorption model to determine the carrier profile. In this study, the carrier profiles were modeled as graded multilayers that were constrained to a specific functional form (e.g. Gaussian, complementary error function) when appropriate. Carrier profiles from boron and arsenic ion-implanted that had been subjected to furnace or Rapid Thermal Annealing (RTA) annealed silicon wafers were compared to Spreading Resistance Probe and Secondary Ion Mass Spectrometry profiles. p−p+doped epitaxial silicon samples (before and after annealing) were also measured and the results were compared to theory.

Determination of the optical dispersion in ferroelectric vinylidene fluoride (70%)/trifluoroethylene (30%) copolymer Langmuir–Blodgett films

We report measurements of the optical dispersion in ferroelectric Langmuir–Blodgett films of polyvinylidene fluoride (70%)-trifluoroethylene (30%) copolymer, using variable-angle spectroscopic ellipsometry over a wide spectral range from infrared to ultraviolet. Film thickness averaged 1.78±0.07 nm per deposition layer for films ranging from 5 to 125 deposition layers as determined from multi-sample analysis. This deposition rate was consistent with capacitance measurements, yielding a dielectric constant of 9.9±0.4 normal to the film, by quartz microbalance measurements, and by atomic force microscopy.

Optical characterization of porous alumina from vacuum ultraviolet to midinfrared

Porous alumina was fabricated and optically characterized over a wide spectral range. Layers were formed electrochemically in oxalic acid solution from 10-μm-thick aluminum films evaporated onto silicon wafers. The layer formation was monitored with in situ spectroscopic ellipsometry in the visible and near-infrared wavelength range to accurately determine the thickness and dielectric functions. Anisotropy due to the columnar nature of the porous structure was determined using optical modeling. The porous alumina layer was found to have a small but significant absorption tail throughout the visible region. Atomic force microscopy and scanning electron microscopy were used throughout the process to assess the quality of pore formation. The mean pore center-to-center spacing was approximately 100 nm with thicknesses up to 5μm. The infrared spectra revealed absorption peaks previously seen in ceramic alumina and peaks not associated with bulk alumina.

Determination of the mid-IR optical constants of water and lubricants using IR ellipsometry combined with an ATR cell

Abstract The mid-IR bulk optical constants for the lubricants Fomblin, Demnum S200, 2001A and Krytox 16256N, as well as water, are determined by infrared ellipsometric analysis using an Attenuated Total Reflection (ATR) cell. The water optical constants closely match those found in other studies. In addition, the real and imaginary parts of the dielectric functions for all the fluids are self-consistent under Kramers–Kronig transformation.

Determination of optical anisotropy in calcite from ultraviolet to mid-infrared by generalized ellipsometry

Abstract Generalized (Jones matrix) ellipsometry is gaining considerable interest because of its ability to determine properties of anisotropic samples. Here, the strong uniaxial anisotropy of calcite (calcium carbonate) was investigated using generalized ellipsometry. The wavelength range from 0.73 to 6.5 eV (190 nm to 1.7 μm) was covered using a standard variable angle spectroscopic ellipsometer; from 0.089 to 0.68 eV (1.8–14 μm), using a similar instrument based on a Fourier transform spectrometer. Measurements were made on a single air–calcite interface for which the optic axis lay nominally in the plane of the surface. To determine the optical constants and orientation of cut, both the angle of incidence and rotation of the sample about its surface normal were varied. Properties of the sample were arrived at by optimizing the parameters of a material model such that the calculated normalized Jones matrix elements best matched the measured ones. Localized spectral regions of absorption due to the internal vibrational modes of the carbonate ions were observed in the infrared at energies which differed for the ordinary and extraordinary optical constants. The certainty to which sample properties could be determined was maximized by judicious choice of measurement configurations.

Optical determination of shallow carrier profiles using Fourier transform infrared ellipsometry

Dopant profiles were determined by ex situ Fourier transform infrared variable-angle spectroscopic ellipsometry. The technique exploits carrier absorption in the mid-infrared spectral range and combines the sensitivity of ellipsometry with a simple Drude free carrier absorption model to determine the carrier profile. The noncontact, nondestructive nature of the measurement suggests both ex situ and in situ monitoring and control applications. In this study, the carrier profiles were modeled as graded multilayers that can be constrained to a given functional form (Gaussian, erfc, etc.) when desired. Boron and arsenic implanted silicon wafers that were rapid thermal anneal and furnace annealed were measured and compared to spreading resistance probe data.

Infrared emittance modulation devices using electrochromic crystalline tungsten oxide, polymer conductor, and nickel oxide

Abstract A prototypical small area electrochromic device was fabricated, and emissivity was measured from 1 to 30 microns. The devices show change in emissivity from about 0.60 to about 0.68, that is a total modulation of 13%. The emittance performance was calculated, based on the reflectivity modulation. One difference between these devices and the more frequently explored visible light transmission devices is the utilization of crystalline tungsten oxide instead of highly disordered amorphous tungsten oxide. The crystalline tungsten oxide and nickel oxide charge storage films are characterized by IR transmission/reflection, and spectroscopic ellipsometry. A theoretical model has been developed which describes the device performance to within 10% of experimental results.

Preparation of high Tc Tl‐Ba‐Ca‐Cu‐O thin films by pulsed laser evaporation and Tl2O3 vapor processing

Tl‐Ba‐Ca‐Cu‐O superconducting thin films with zero‐resistance temperatures up to 115 K have been prepared using a Tl2 O3 vapor process on Ba‐Ca‐Cu‐O precursor thin films. The Ba‐Ca‐Cu‐O thin films were made by laser deposition on Y‐stabilized ZrO2 substrates. This technique minimizes problems caused by the toxicity of Tl2 O3, and its subsequent decomposition to the volatile and toxic Tl2 O upon heating. Therefore, it may have practical application in the fabrication of high Tc Tl‐Ba‐Ca‐Cu‐O superconducting thin‐film devices.

Optical constants of crystalline WO3 deposited by magnetron sputtering

Crystalline WO3−x is an infrared (IR) electrochromic material having possible applications in satellite thermal control and IR switches. Optical constants of electrochromic materials change upon ion intercalation, usually with H+ or Li+. Of primary concern for device design are the optical constants in both the intercalated and deintercalated states. In situ and ex situ ellipsometric data are used to characterize both the deposition process and the optical constants of the films. Ex situ data from a UV-Vis-NIR ellipsometer are combined with data from a mid-infrared Fourier-transform-infrared-based ellipsometer to provide optical constants over a spectral range of 0.031–6.1 eV.