Hisham Menkara

CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imaging.

Investigations are reported on the x-ray scintillation and imaging application of CdTe quantum dots (QDs) and their polymer nanocomposites. Aqueous CdTe QDs with emissions ranging between 510 and 680 nm were prepared and incorporated into polyvinyl alcohol or polymethyl methacrylate polymer matrices. The x-ray luminescent properties were evaluated and a resolution of 5 lines∕mm was obtained from the nanocomposite films. Additionally, the fast decay time, nonafterglow, and superior spectral match to conventional charge coupled devices, show that CdTe QD nanocomposites have high promise for x-ray imaging applications.

ZnTe:O phosphor development for x-ray imaging applications

An efficient ZnTe:O x-ray powder phosphor was prepared by a dry synthesis process using gaseous doping and etching medias. The x-ray luminescent properties were evaluated and compared to standard commercial phosphors exhibited an x-ray luminescent efficiency equivalent to 76% of Gd2O2S:Tb and an equal resolution of 2.5lines∕mm. In addition, the fast decay time, low afterglow, and superior spectral match to conventional charge-coupled devices-indicate that ZnTe:O is a very promising phosphor candidate for x-ray imaging applications.

Solid state lighting: diode phosphors

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.

Lanthanum halide nanoparticle scintillators for nuclear radiation detection

Nanoparticles with sizes <10 nm were fabricated and characterized for their nanocomposite radiation detector properties. This work investigated the properties of several nanostructured radiation scintillators, in order to determine the viability of using scintillators employing nanostructured lanthanum trifluoride. Preliminary results of this investigation are consistent with the idea that these materials have an intrinsic response to nuclear radiation that may be correlated to the energy of the incident radiation.

Gain properties of doped GaAs/AlGaAs multiple quantum well avalanche photodiode structures

A comprehensive characterization has been made of the static and dynamical response of conventional and multiple quantum well (MQW) avalanche photodiodes (APDs). Comparison of the gain characteristics at low voltages between the MQW and conventional APDs show a direct experimental confirmation of a structure‐induced carrier multiplication due to interband impact ionization. Similar studies of the bias dependence of the excess noise characteristics show that the low‐voltage gain is primarily due to electron ionization in the MQW‐APDs, and to both electron and hole ionization in the conventional APDs. For the doped MQW APDs, the average gain per stage was calculated by comparing gain data with carrier profile measurements, and was found to vary from 1.03 at low bias to 1.09 near avalanche breakdown.A comprehensive characterization has been made of the static and dynamical response of conventional and multiple quantum well (MQW) avalanche photodiodes (APDs). Comparison of the gain characteristics at low voltages between the MQW and conventional APDs show a direct experimental confirmation of a structure‐induced carrier multiplication due to interband impact ionization. Similar studies of the bias dependence of the excess noise characteristics show that the low‐voltage gain is primarily due to electron ionization in the MQW‐APDs, and to both electron and hole ionization in the conventional APDs. For the doped MQW APDs, the average gain per stage was calculated by comparing gain data with carrier profile measurements, and was found to vary from 1.03 at low bias to 1.09 near avalanche breakdown.

Nanocrystalline Phosphors for Lighting and Detection Applications

We report the development of new nanoparticle phosphors and quantum dot structures designed for applications to enhance the color rendering and efficiency of high brightness white LEDs, as well as for bio-sensing applications. The intrinsic problem of self-absorption, high toxicity, and high sensitivity to thermal quenching of conventional quantum dot systems has prevented their adoption to LED devices. Doped Cd-free quantum dots may circumvent these issues due to their distinct Stokes shift and improved stability at high temperature. We report on the modification of Mn-doped ZnSe/ZnS core-shell quantum dots for application to the (blue diode + yellow emitter) white LED system. Band gap tuning for 460 nm excitation, inorganic shell growth and in-situ monitoring for enhanced efficiency, and analysis of thermal stability will are reported.

Effect of variations in the doping profiles on the properties of doped multiple quantum well avalanche photodiodes

Abstract The purpose of this study is to use both theoretical and experimental evidence to determine the impact of doping imbalance and symmetry on the physical and electrical characteristics of doped multiple quantum well avalanche photodiodes. Theoretical models have been developed to calculate the electric field, valence and conduction bands, capacitance-voltage ( CV ), and carrier concentration versus depletion depth profiles. The models showed a strong correlation between the p- and n-doping balance inside the GaAs wells and the number of depleted stages and breakdown voltage of the APD. A periodic loping imbalance in the wells has been shown to result in a gradual increase (or decrease) in the electric field profile throughout the device which gave rise to partially depleted devices at low bias. The MQW APD structures that we modeled consisted of a 1 μm top p + - doped (3 × 10 18 cm −3 ) GaAs layer, followed by a 1 μm region of alternating layers of GaAs (500 A) and Al 0.42 Ga 0.58 As (500 A), and a 1 μm n + back layer (3×10 18 cm −3 ). The GaAs wells were doped with p-i-n layers placed at the center of each well. The simulation results showed that in an APD with nine doped wells, and where the 50 A p-doped layer is off by 10% ( p = 1.65×10 18 cm −3 , n = 1.5×10 18 cm −3 ), almost half of the MQW stages were shown to be undepleted at low bias which was result of a reduction in the electric field near the p + cap layer by over 50% from its value in the balanced structure. Experimental CV and IV data on similar MBE grown MQW structures have shown very similar depletion and breakdown characteristics. The models have enabled us to better interpret our experimental data and to determine both the extent of the doping imbalances in The devices as well as the overall p- or n-type doping characteristics of the structures.

Enhanced performance of solid state lighting phosphors

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.

4.1: Invited Paper: Low‐Temperature‐Process Development of SrS Electroluminescent Phosphors

A comprehensive study is reported of the influence of the growth technique, flux, co-activator, and charge compensation element selection in optimizing the performance of SrS: Cu thin film electroluminescent phosphors. These studies have shown that low temperature (<400 C) elemental evaporation followed by a post growth sulfur anneal dramatically enhances the grain growth of SrS and its electroluminescent properties. Most recently, ion-assisted deposition from compound sources has produced good EL performance. For this large-area process, as-deposited SrS:Cu samples had a luminance of 18 cd/m2, an efficiency of 0.15 lm/W with chromaticity coordinates of x=0.198, y=0.330 at 60 Hz operation

Photonic crystal structures for improved scintillator performance

The ability to extract scintillation light from a crystal with efficiency and speed has great potential to realize the much needed gains in the performance of numerous radiation detection and imaging instruments. Such high performance spectrometers/imaging instruments are vital in medical imaging, industrial, and homeland security applications for the detection, localization, and energy classification of X-rays, γ-rays or neutrons. We report on the development of photonic crystal structures for improving the performance of scintillators. Photonic crystals permit effective utilization of the scintillation light which is otherwise lost when a high refractive index scintillator is coupled to a photodetector with low refractive index window, thereby substantially improving the light extraction efficiency, and energy and timing resolution. We have fabricated these photonic crystal structures using electron-beam lithography technique. Details of the design, fabrication and characterization of the photonic crystals, and their impact on scintillator performance are described here.