Optically based radiation detectors in various fields of science still suffer from low resolution, sensitivity and efficiency that restrict their overall performance. Quantum dots (QD) are well-suited for such detectors due to their unique optical properties. CdTe QDs show fast luminescence decay times, high conversion efficiencies, and have band gaps strongly dependent on the particle radius. Since QD particle sizes are well below the wavelengths of their emissions, they remain optically transparent when incorporated in both polymer and sol-gel based silica glass due to negligible optical scattering. In addition, as these composite materials can greatly improve the mechanical robustness of alpha-particle detectors, conventionally known to have delicate components, CdTe QDs show high promise for radiation sensing applications. These properties are especially advantageous for alpha-particle and potentially neutron detection. In this work, CdTe QD-based glass or polymer matrix nanocomposites were synthesized for use as alpha-particle detection scintillators.. The fast photo-response and decay times provide excellent time resolution. The radiation responses of such nanocomposites in polymer or glass matrices were investigated.
The use of light emitting nanoparticles in polymer and glass matrices was studied for the detection of radiation. These
nanocomposite scintillators were produced by various approaches including quantum dot/polymer, fluoride
nanophosphor/epoxy and halide nanophosphor containing glass-ceramic composites. The synthesis and characterization
of these nanoparticles as well as their incorporation into composites is discussed. Further, the application of these
composites for radiation detection and spectroscopy is described.
Nuclear radiation detection is continuously becoming more important for today's society. Conventional scintillator
based gamma-ray detectors use single crystal materials such as NaI:Tl, LaBr<sub>3</sub>:Ce, which provide excellent radiation
detection properties, but suffer from their environment-related fluctuation, high cost and size limitation. The
incorporation of nanophosphors or quantum dots (QD) into a transparent host matrix has been studied recently as a cost-saving
alternative that may solve the scalability and stability problems while still providing considerable optical
performance. In this work, a new glass based detecting material with promising gamma-ray detection performance is
reported. Transparent alumino-silicate glasses containing cerium-doped gadolinium halide nanocrystals were prepared by
a melt-quench method followed by annealing to form nanocrystal precipitates. Samples were cast and polished for
optical and radiation characterization. The preliminary results indicated a similar gamma-ray detection efficiency
compared to a conventional NaI:Tl detector and a gamma-ray peak resolution of ~27% at 662 KeV from some of these
samples. By controlling elemental composition and ratio of the in-situ precipitated nanoparticles, radiation detection
performance is expected to be improved.
The properties of several solid state lighting phosphor compounds have been enhanced through the application of
advanced material processing and particle encapsulation techniques. Also higher light extraction efficiencies from the
pump LED were achieved by index-matching the refractive index of the phosphor to that of the InGaN chip. New solid-state-
reaction protocols have produced a significant increase in the intrinsic efficiency of the orthosilicate yellow
phosphor compared to the traditional YAG:Ce phosphor. In addition, the stability of phosphor materials in high humidity
environments was increased significantly using metal oxide coatings applied by vapor deposition.
Intense visible blue to red emissions were obtained from SiN<sub>x</sub> thin films prepared by plasma enhanced chemical vapor deposition (PECVD) using SiH<sub>4</sub> and NH<sub>3</sub> as the source gases. A continuous blue shift of the photoluminescence (PL) peak from 660nm to 440nm was observed by increasing the NH<sub>3</sub> flow rate from 20 to 150sccm, while the flow rate of N<sub>2</sub> diluted 2% SiH<sub>4</sub> was fixed at 650sccm. This controllable PL was attributed to the quantum confinement effect of Si quantum dots (QDs) which were formed during the deposition process and embedded in the SiN<sub>x</sub> films. White photoluminescence with multiple emission peaks was achieved for potential solid state lighting applications from multi-layered SiN<sub>x</sub> films by changing the SiH<sub>4</sub>/NH<sub>3</sub> ratio during the deposition process. This was attributed to a combination of Si quantum dots with different sizes within the different layers. Surface texturing of the thin film samples was conducted by potassium hydroxide (0.56%) etching the (100) Si substrate for 3~40 min at 80°C before deposition. The reflectivity of the etched samples decreased with increasing etch time due to increased surface roughness. The extraction efficiency of light emission from the textured SiN<sub>x</sub> thin films was significantly improved, owing to a depression of the internal reflection and interference effects. In addition, the elimination of the multiple emission peaks by surface texturing significantly affected the color coordinates of the output spectrum.
Silicon quantum dot (QD) based luminescent structures can emit throughout the visible region by controlling their size and/or the host matrix. Consequently, multiple sized Si QDs embedded in thin films could be used to produce efficient white light sources, when integrated with a blue/UV LED, and film structures designed for high light extraction. In this paper, we report strong red photoluminescence from Si QDs embedded in films prepared by thermal evaporation of SiO in vacuum or an O<sub>2</sub> atmosphere. The SiO<sub>x</sub> film composition (1.0< x <1.9) was controlled by varying the deposition rate and the oxygen flow rate in the chamber. After annealing at 1100°C, silicon nanocrystals of 20nm to 2nm in size were formed in films with different stroichiometry, as indicated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization. Red photoluminescence was observed from films with Si QDs smaller than ~5nm, and attributed to confined carrier radiative recombination in the Si QDs. The emission peak shifted from 840nm to 745nm with increasing O<sub>2</sub> flow rate due to a decrease in the size of the Si QDs.
We present a review of recent studies into the tunability of 2D PC slab waveguides designs. The properties of dynamic, static and hybrid superlattice photonic crystals are reviewed and the mechanism of tunability and its impact on tuning the refractive and dispersion and propagation properties are presented.
A brief review is given of the development of phosphors for solid-state lighting and the properties of new materials that are being developed for this emerging technology to achieve higher efficiencies, full color rendition and ultra-long life characteristics.
To achieve the DoD objective of low cost high performance infrared focal plane arrays a manufacturing technique is required which is intrinsically flexible with respect to device configuration and cutoff wavelength and easily scaleable with respect to volume requirements. The approach adopted is to fully develop the technology of molecular beam epitaxy (MBE) to a level where detector array wafers with a variety of configurations can be fabricated with first pass success at a reduced cost. As a vapor phase process, MBE lends itself directly to: (1) the inclusion of real-time monitoring and process control, (2) a single or multiple wafer growth mode, (3) nearly instantaneous changes in growth parameters. A team has been assembled to carry out the program. It is composed of four industrial organizations -- Rockwell International, Hughes Aircraft Company, Texas Instruments, and Lockheed-Martin, and a university -- Georgia Tech Research Institute. Since team members are committed suppliers and users of IRFPAs, technology transfer among team members is accomplished in real-time. The technical approach has been focused on optimizing the processes necessary to fabricate p-on-n HgCdTe double layer heterostructure focal plane arrays, reducing process variance, and on documenting flexibility with respect to cutoff wavelength. Two device structures have been investigated and fabricated -- a 480 by 4 and a 128 by 128.
The electrical and optical properties of iodine doped n-type HgCdTe alloys and superlattices grown by metalorganic molecular beam epitaxy using ethyliodide are reviewed. The rationale for the use of iodine rather than indium as the dopant species and the incorporation kinetics of iodine at the growth surface are discussed. The electrical and optical properties of iodine- doped CdTe and HgCdTe (x equals 0.24) are presented for carrier concentrations between 10<SUP>15</SUP> and 10<SUP>18</SUP> cm<SUP>-3</SUP>, as determined by Hall effect measurements and low- and room-temperature photoluminescence spectroscopy. These samples show strong room temperature excitonic effects due to free exciton and band to band recombination as determined by photoluminescence excitation spectroscopy. The electrical and optical properties of iodine-doped HgCdTe-CdTe superlattices also are discussed based on magnetoluminescence measurements in tilted magnetic fields of up to 7 Tesla in Voigt and Faraday geometry.
We present the results of a systematic photoluminescence study of ZnGa<SUB>2</SUB>O<SUB>4</SUB>:Mn phosphor powder. This phosphor exhibits bright green luminescence with a spectral peak at 2.46 eV and CIE chromaticity coordinates of x equals 0.073 and y equals 0.696 at room temperature. At low temperatures the luminescence consisted of three components assigned to the <SUP>4</SUP>T<SUB>1-</SUB><SUP>6</SUP>A<SUB>1</SUB> inner transition of the 3D electrons of Mn<SUP>2+</SUP> ions located on different sites of the host crystal. Selective excitations were used to validate the assignment of these features based on a strong-field scheme. The photoluminescence lifetime showed a single exponential decay of about 4 ms and at T equals 1.6 K an optical phonon related fine structure [E<SUB>phonon</SUB> equals (8.2 plus or minus 0.2) meV] of the main photoluminescence line was observed. These results indicate that Mn-doped ZnGa<SUB>2</SUB>O<SUB>4</SUB> has the potential to serve as a green phosphor for field emission display (FED) devices. The CIE coordinates of the green emission measured for this phosphor powder are also sufficient to produce a wide color gamut and a true white color when combined with other red and blue phosphors in FED display devices.
Thin film luminescent layers have applications in electroluminescent devices and potential application to other displays with high brightness and high resolution requirements. Displays such as field emission devices where high current densities, high electric fields, and small dimensional tolerances are present require phosphors which have high maintenance qualities and good adherence to the face plate. To increase the luminance of EL displays high quality phosphor layers are necessary. In this study, we report the growth kinetics studies of ZnS and the properties of high quality ZnS layers grown by metalorganic molecular beam epitaxial deposition for display applications. The layers' structural and optical properties have been characterized by x-ray diffraction, electron and optical microscopy, and photoluminescence spectroscopy. These measurements were made as a function of the growth and process conditions such as growth temperature, flux ratios, different sulfur precursors, etc. PL spectra exhibited free- and bound-exciton transitions indicative of high quality ZnS material.
A brief review is given of recent results to assess the capability of metalorganic molecular beam epitaxy for the low-temperature growth of high-quality low-carrier-concentration CdTe and HgCdTe alloys. In particular, studies of this technique to produce highly uniform HgCdTe material and the extension of the gas source doping of CdTe and HgCdTe with ethyliodide so as to obtain back-doped electron concentrations from 10<SUP>15</SUP> to 10<SUP>18</SUP> cm<SUP>-3</SUP> are reported. Some preliminary results on the growth of ternary CdTe/HgCdTe superlattices and the p-type doping of CdTe with As will also be presented. The electrical and optical properties of these materials were determined by resistivity and Hall effect, photoluminescence, and IR transmission measurements between 300 and 10 K.
A brief review is given of the development of a metalorganic molecular beam epitaxial system for Hg-based II-VI semiconductors. Recent results on the growth of HgZnTe, HgCdTe, and iodine-doped CdTe epitaxial layers are presented and demonstrate the potential of this technique for the growth of high-quality materials.