C. Tarrio

A surface recombination model applied to large features in inorganic phosphor efficiency measurements in the soft x‐ray region

The photoluminescent quantum efficiencies of the inorganic phosphors Y2O2S:Eu, Y2O3:Eu, La2O2S:Tm, Gd2O2S:Tb, and Sr5Cl(PO4)3:Eu have been measured in the range 17 to 450 eV. The optical properties of these phosphors from 2 to 160 eV have been determined from inelastic electron scattering measurements. Using a model which involves nonradiative recombination at the surface of the material, we relate photoluminescent efficiency to optical absorption properties, and find that surface recombination is the predominant source of efficiency loss for these materials in the soft x‐ray range. From the model, we obtain values for the diffusion length, surface recombination velocity, and bulk quantum efficiency of these materials.

Optical properties of silicon and its oxides

We have measured the inelastic electron scattering spectra of a variety of Si and SiO2 thin films from the fundamental absorption threshold to well above the L-shell thresholds. We have used Kramers–Kronig analyses and sum rules to obtain the dielectric and optical response functions. We compare the optical properties of crystalline and evaporated and hydrogenated amorphous Si, amorphous evaporated SiO and SiO2, and chemical-vapor-deposition SiO2 in both the interband and the L-shell absorption regions. The interband structure in crystalline Si shows three sharp peaks that are blended into a single broad peak in the amorphous samples. At the L threshold crystalline Si also shows more structure than amorphous Si, however, the overall shape in the region well above the threshold is quite similar in the three samples. Above the SiO2 band gap the three oxide samples show strikingly similar behavior.

Response of photodiodes in the vacuum ultraviolet

We have measured the responses of four commercial photodiodes in the vacuum ultraviolet from 20 to 600 eV and have also measured the inelastic‐electron‐scattering spectra of the materials contained in the diodes from 0 to 260 eV. Three of the diodes are silicon: an enhanced channel device, an x‐ray‐stabilized silicon diode, and a p‐i‐n diode. The fourth is a gallium arsenide phosphide Schottky diode. The diode response has been modeled by considering absorption through the surface layer and inelastic surface recombination. The model produces an excellent description of the measured responses. From our analysis we have obtained reasonable values for the number of electrons produced per eV of incident radiation, the thicknesses of the surface layers, the surface recombination velocities, and the average diffusion lengths of the minority carriers. The highest efficiency is obtained for a silicon x‐ray‐stabilized diode followed by the gallium arsenide phosphide diode. We find that both of these diodes make ex...

Absolute photoluminescent efficiency and photon damage of sodium salicylate in the soft-x-ray regime

We have measured the absolute photoluminescent efficiency of sodium salicylate from 70 to 400 eV, as well as the inelastic electron-scattering spectrum from 0 to 200 eV. We find that features in the luminescence spectrum can be described well by using a model incorporating absorptance, reflectance, carrier-diffusion length, and reduced surface-recombination velocity. We have also measured the dose (total photons/unit area) dependence of the model’s efficiency at several photon energies. Results indicate a decrease in efficiency when dose and x-ray energy are increased, which makes this material a nonideal choice for long-term stable detection of high-intensity soft x rays. The decrease in efficiency is caused by a bulk-damage process that is second order in the number of electron–hole pairs produced by the incident x rays, as well as by changes in surface-recombination velocity.

Surface recombination effects in soft x‐ray efficiencies

We have measured the soft x‐ray efficiencies of a silicon p‐i‐n photodiode and a La2O2S:Tm phosphor over a broad energy range. We have also measured the inelastic electron scattering spectra of the constituent materials and obtained values of optical absorption coefficients versus energy. The energy dependence of the efficiencies is well explained by a model based on surface recombination of electron hole pairs, and the quality of data which can now be obtained from synchrotrons makes possible quantitative fits from which we obtain diffusion length, surface recombination velocity, and bulk quantum efficiency.

Observation of second‐order kinetic damage in sodium salicylate due to soft x rays

We have observed the dose dependence of the bulk quantum efficiency for luminescence of sodium salicylate as a function of the photon energy from 7 to 150 eV. We show that the damage is a second‐order or higher kinetic process in the number of electron‐hole pairs and is not reversible. We predict that the threshold for damage occurs at 7.2 eV, or twice the band gap of sodium salicylate.

Optical absorption measurements in the soft-x-ray range

We report measurements of optical-absorption coefficients obtained by techniques not requiring free-standing films. The results of these methods are compared with data obtained by inelastic electron scattering. We find that, as expected, absorption data obtained with smoother substrates yield results with more detail in the regions with rich structure. The spectra resulting from samples evaporated onto a Si diode vacuum-UV detector with a KCI buffer layer yield results that are quite similar to the results for inelastic electron scattering.

A low-temperature sample mount for an inelastic electron scattering spectrometer

A continuously operable low‐temperature (10–20 K) sample mount for a solid‐state inelastic electron scattering spectrometer is described. The cooling is achieved by a closed‐cycle gas phase He refrigerator. Because the entire sample chamber is at a potential of 300 kV, it must be isolated from ground, requiring computer automation for positioning, and insulating plumbing for the helium. The motion control has a detachable coupling that allows for complete thermal isolation from room temperature. Details and problems encountered in the design are described.

Local fields in high-Tc materials

Abstract Most high-temperature superconductors exhibit two-dimensional conductance; therefore the conduction electrons are localized in the third dimension, and experience the local electric field rather than the macroscopic applied field in that direction. We report model calculations which indicate that the local field leads to enhanced electron-phonon coupling in these materials which may play a role in determining the high transition temperatures.

Effective masses of the 4p excitons in solid Kr

Abstract We have measured the momentum dependent electron energy loss spectrum of solid Kr and evaluated the effective masses of the excitons for the first two principal quantum numbers of each excitonic series. We find that n = 1 excitons disperse more slowly than the n = 2 excitons of the same series indicating greater effective masses for n = 1 excitons than for n = 2 excitons within the effective mass model. For intermediate wave vectors, where the splitting of the heavy hole and light hole bands should be observable, we find no evidence for a splitting in the two bands, in conflict with band structure calculations.